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- _______________________________________________________________________ Kabo Ngwanaamotho – [email protected] Kabo NGWANAAMOTHO MEng Hons Project Report Phase 2 HSP 1943 Transient Analysis of Voltage Dips January 2006

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_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

Kabo NGWANAAMOTHO MEng Hons Project Report Phase 2

HSP 1943

Transient Analysis of Voltage Dips

January 2006

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_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

MEng Project Mission Statement

Background:

As a mine, one of the problems that have not been resolved is the performance of motor

drives under different power quality conditions which are also influenced by incremental

weather, faults on the local and external network to the mine.

Project Definition:

This project will focus on carrying a transient analysis of voltage dips with a view to

understanding how they manifest themselves on a network and how to mitigate against

voltage dips on transmission and distribution systems. This will also entail the analysis

and improvement of current methodologies that are used in mitigating against power

quality.

Aims:

• Identify causes of voltage dips in the Mine

• Investigate possible solutions to the problem

Location:

Debswana Diamond Mining Company, Jwaneng Mine, Botswana. The town of Jwaneng

is located about 180km south of the capital, Gaborone.

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_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

Abstract

Voltage dips are increasingly becoming a major power quality problem for Jwaneng

Mine. A dip lasting a few milliseconds can culminate into interruption of critical and

essential processes that could last several hours and the restart process of a plant after a

voltage dip can be very tedious leading to a substantial revenue loss.

This report is Phase 2 of a 2 part research into the causes, effects, characteristics and

ways to mitigate against voltage. Phase 1 report addressed mostly the first three of the

aforementioned parts whereas this report concentrates on solutions.

The first chapters address how well voltage dips are being documented within the entire

mine and goes on to research about the Red Area Plant which was not covered by Phase 1

report. The other chapters analyze solutions in terms of reducing occurrence of voltage

dips or compensating by means of several energy storage systems or smoothing out the

supply voltage using switches or a reactive VAr compensator. Last but not least

recommendations in terms of general power mitigation are discussed to optimize quality

of supply and a method of recording of dips is suggested.

Finally this report reviews the objectives of this research, concludes and specifies what

future work may need to be carried out.

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_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

Declaration of Originality

I declare that this thesis is my original work except where stated

………………………………………………………………………………

Kabo Ngwanaamotho January 2007

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Company Background – Jwaneng Diamond Mine

Jwaneng mine is under Debswana Diamond Mining Company. Debswana is a company

in which the Botswana Government and the De Beers Consolidated Mines Ltd hold a

50% share each with a total of four mines in Botswana at Jwaneng, Orapa, Letlhakane

and Damtshaa. Jwaneng Mine alone employs more than 2289 people of which 296 are

Engineers of various disciplines.1 Jwaneng mine started its operations in 1978. A new

contract that will run for the next 25 years has just been signed this year (2006).2

In 2005, the mine treated 10 006 752 tons of kimberlite ore, yielding a record 15 618 155

carats.1 above for the year Debswana had group total revenue of BWP15.8 billion.

Debswana is a key player in the national economy of Botswana, producing in excess of

70% of Botswana's export earnings, 30% of Gross Domestic Product (GDP) and 50% of

government revenue.

Mining at Jwaneng mine is by open pit. The mining process can be divided into 5

sections;

1. The Mine Pit - Kimberlite is blasted, hauled and crushed to a size of particles less

than 150mm in diameter. This ore is then conveyed to the primary stockpile.

2. MTP - The ore to the MTP is drawn from the primary stockpile and conveyed to

the scrubber feed bins. This ore is then scrubbed and screened. Less than 25mm

ore goes to the feed preparation stockpile. Greater than 25mm ore goes to the

secondary stockpile and is then crushed further. From the feed preparation

stockpile the ore goes to the DMS. Here the ore is separated according to density,

while diamond rich material, being denser sinks in a FeSi medium, and the non-

diamond rich material floats.

1 http://www.jwaneng.debswanaintranet.com/home/fraStart.asp - accessed 7/31/2006 7:40 AM 2 DeBeers Group Annual Review

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3. Recrush Plant - The tailings from the MTP DMS are crushed to less than 8mm

and then scrubbed and screened and then go to the Recrush DMS.

4. Final Recovery (Red Area) - Ore from the MTP and Recrush DMS is

transported to the Aquarium where diamonds are recovered from the concentrate.

5. Tailings - The tailings from the Recrush DMS in the size range of less than

25mm and more than 1mm is finally conveyed through the Recrush tailings

conveyor system to the tailings dumps. This ore is used to facilitate compact

ability and stability of the dumps.

Jwaneng Mine’s Electrical Reticulation

There are two 132kV incomer O/H lines from the BPC substation at Thamaga [Appendix

A6 & A7 Report Phase 1] feeding Jwaneng mine. The average overall power demand for

the mine is about 33.9MW. Two 20MVA 132/6.6 kV transformers (T1 and T2) supply

MTP, Mining Offices and the workshops closest to the switchyard, another two 25MVA

132/33 kV transformers (T3 and T4 (600A on primary)) are for Well fields (50km), Re-

crush No1 (4km), and Village (township) 1 & 2 on Busbar A - Pit O/H line (7.34km), Re-

crush No2 (4km) on Busbar B.

The mine has two standby diesel generators rated 1.725MW and 2.6MW which generate

at 6.6kV and supply Busbar A with 33kV via a 5MVA transformer in case of an outage.

In the standby generator housing there is a 280kW Auxiliary generator which

automatically starts if the supply from BPC goes off. The standby diesel generators are

manually started.

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_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

Acknowledgements

I would like to convey my gratitude to Kenny Sinyinza – Section Engineer electrical

services who my industrial supervisor at Jwaneng Mine for his help in pointing me

towards the right direction in research for this project.

I would also like to thank all the personnel at MTP, RP, Red Area Plant and the Electrical

Workshops in Jwaneng Mine for their continued support and invaluable comments for

this report.

I thank Disang Mongatane at MTP, Isang Gaolebale and Simon Nyirongo at Re-crush

Jwaneng for the daily production reports at their respective plants and their comments for

economic impact of voltage dip for this report. I also thank Nonofo Kebaitse in the

Diamond Value Management for files to assess revenue losses due to voltage dips.

I am grateful to the help of Otto Keitumetse, Section Engineer MTP, Puso Mooketsi,

Section Engineer at the Red Area Plant, Bobotho Ratsoma, Section Engineer at Recrush,

Josiah Keitshokile – Assistant Plant Superintendent at the Red Area Plant.

I would also like to convey my appreciation for answering my questionnaires to Phana

Matale and Patrick Wakatama at EM15, Jonathan Sesinyi at ET13, Victor Bontsi and

Bojosi Tonkope at the Red Area Plant workshop, T.T Badisang, BAHF manager at the

Red Area plant and Calvin at the Recrush workshop for the daily shift reports

The editorial comments from Kelebonye Leduledi and Segale Mangope on the first phase

of this research are highly appreciated.

My gratitude is also due to Mrs. Same Makuku, protection engineer “Operations and

Transmission” at BPC Gaborone , Mr. Wilfred Shereni, metering engineer at BPC

Gaborone and Mr. Chris Ngulube and all personnel at NCC for shedding a light on

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structure of Botswana’s transmission network and allowing me full access to their records

and reports.

I would also like to thank my project supervisor Dr Ewen Macpherson and Dr David

Renshaw at the University of Edinburgh, UK for ensuring that this project is up to

professional standard.

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_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

CONTENTS-----------------------------------------------------------------------------------PAGE

MENG PROJECT MISSION STATEMENT............................................................................................. I

ABSTRACT ................................................................................................................................................. II

DECLARATION OF ORIGINALITY.....................................................................................................III

COMPANY BACKGROUND – JWANENG DIAMOND MINE .......................................................... IV

ACKNOWLEDGEMENTS ....................................................................................................................... VI

LIST OF SYMBOLS AND ABBREVIATIONS...................................................................................... XI

GLOSSARY OF TERMS ....................................................................................................................... XIV

PROJECT PLANNING ............................................................................................................................XV

1 INTRODUCTION............................................................................................................................... 1

1.1 PROJECT OBJECTIVES & SCOPE .................................................................................................... 1 1.2 BACKGROUND INFORMATION....................................................................................................... 1

1.2.1 Power Quality Issues .............................................................................................................. 2 1.2.2 Definition - Voltage Dip ......................................................................................................... 3 1.2.3 Voltage Dip Types .................................................................................................................. 4 1.2.4 Selected Literature Review ..................................................................................................... 6

1.3 PROJECT METHODOLOGY............................................................................................................. 7 1.4 THESIS OUTLINE .......................................................................................................................... 8

2 CURRENT METHODS OF RECORDING OF DIPS................................................................... 11

2.1 BALANCED SCORE CARD – (USE OF POWER QUALITY METERS).................................................. 11 2.2 POWER DIP RECORD SHEET ....................................................................................................... 12 2.3 CCR........................................................................................................................................... 12 2.4 ELECTRICAL WORKSHOP............................................................................................................ 13 2.5 BPC ........................................................................................................................................... 13 2.6 RED AREA PLANT ...................................................................................................................... 13

2.6.1 Treatment.............................................................................................................................. 14 2.6.2 External to the Red Area Plant ............................................................................................. 14 2.6.3 Security ................................................................................................................................. 14

3 EFFECTS OF POWER DIPS IN THE RED AREA...................................................................... 15

3.1 SECURITY................................................................................................................................... 15 3.1.1 Magnetic Locks..................................................................................................................... 15 3.1.2 Central Command Centre..................................................................................................... 15 3.1.3 Back-Up System.................................................................................................................... 16

3.2 ACID HANDLING (BAHF) .......................................................................................................... 16 3.2.1 Scrubber ............................................................................................................................... 16 3.2.2 Neutralization Process.......................................................................................................... 17 3.2.3 Compressor........................................................................................................................... 17

3.3 GENERAL PROCESS..................................................................................................................... 17 3.3.1 Pneumo Blower (Drier System) ............................................................................................ 18 3.3.2 Feed Preparation Conveyors................................................................................................ 18 3.3.3 SICON................................................................................................................................... 18 3.3.4 Perm Rolls ............................................................................................................................ 19

3.4 INTERNET SERVER DOWNTIME................................................................................................... 19 3.5 AIR CONDITIONING .................................................................................................................... 19

4 ECONOMIC EVALUATION (LOSSES DUE TO VOLTAGE DIPS) ........................................ 20

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4.1 LOSSES DUE TO BAD POWER QUALITY....................................................................................... 20 4.2 PRODUCTION DELAY (HEAD FEED DELAYS) .............................................................................. 21 4.3 FESI LOSSES............................................................................................................................... 23 4.4 SECURITY BREACH..................................................................................................................... 23

5 SUPPLY UTILITY CONDITIONS................................................................................................. 24

5.1 STIPULATION.............................................................................................................................. 24 5.2 POWER QUALITY SPECIFICATIONS ............................................................................................. 24

5.2.1 Voltage & Frequency............................................................................................................ 24 5.2.2 Harmonics ............................................................................................................................ 25

5.3 PROPOSED 220KV TRANSMISSION LINE ..................................................................................... 26

6 CRITICAL LOADS.......................................................................................................................... 27

6.1 RED AREA .................................................................................................................................. 27 6.2 MAIN TREATMENT PLANT.......................................................................................................... 28 6.3 RE-CRUSH PLANT ....................................................................................................................... 28 6.4 SERVICES ................................................................................................................................... 29 6.5 OPEN PIT .................................................................................................................................... 29

7 REDUCING OCCURRENCE OF DIPS......................................................................................... 30

7.1 POWER QUALITY METERING ...................................................................................................... 30 7.2 PROTECTION & DISCRIMINATION............................................................................................... 30 7.3 SIMULATE BAD QUALITY POWER .............................................................................................. 31 7.4 BALANCED NETWORK & HARMONICS ....................................................................................... 32 7.5 POWER FACTOR CORRECTION .................................................................................................... 35 7.6 SOFT STARTING LARGE INDUCTION MOTORS ............................................................................ 36 7.7 EARTHING .................................................................................................................................. 36

7.7.1 Neutral Earthing Resistor..................................................................................................... 37 7.7.2 Neutral Earthing Compensator ............................................................................................ 38

7.8 REDUCE EQUIPMENT SENSITIVITY ............................................................................................. 39

8 COMPENSATE MISSING VOLTAGE ......................................................................................... 40

8.1 BESS - BATTERY ENERGY STORAGE SYSTEM ........................................................................... 40 8.1.1 Cost Analysis ........................................................................................................................ 41 8.1.2 Technical Analysis ................................................................................................................ 41 8.1.3 Connection............................................................................................................................ 42

8.2 SMES- SUPERCONDUCTING MAGNETIC ENERGY STORAGE SYSTEM ......................................... 42 8.2.1 Cost Analysis ........................................................................................................................ 43 8.2.2 Technical Analysis ................................................................................................................ 44 8.2.3 Connection............................................................................................................................ 44

8.3 UPS ........................................................................................................................................... 45 8.3.1 Principle of Operation.......................................................................................................... 46 8.3.2 Case Study - Synchrotron Radiation Source......................................................................... 46 8.3.3 Case Study - Uses in the mine............................................................................................... 47

8.4 FLYWHEEL ENERGY STORAGE ................................................................................................... 49 8.4.1 Cost Analysis ........................................................................................................................ 50

8.5 POWER CONVERSION SYSTEM.................................................................................................... 51 8.5.1 Proposed System................................................................................................................... 51 8.5.2 Cost of the PCS..................................................................................................................... 52

8.6 CASE STUDY – COST BREAKDOWN OF SMES AND BES ............................................................ 52

9 SMOOTHING OUT VOLTAGE PROFILE.................................................................................. 54

9.1 SSTS - THE SOLID STATE TRANSFER SWITCH............................................................................ 54 9.1.1 The Hybrid SSTS................................................................................................................... 54 9.1.2 Connection............................................................................................................................ 55 9.1.3 Technical Analysis ................................................................................................................ 56

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9.2 SETC - STATIC ELECTRONIC TAP CHANGER.............................................................................. 57 9.3 SVC - STATIC VAR COMPENSATOR ........................................................................................... 58

9.3.1 Principle of Operation of the SVC........................................................................................ 58 9.3.2 Technical Analysis ................................................................................................................ 60 9.3.3 Connection............................................................................................................................ 61

10 GENERAL POWER QUALITY MITIGATION........................................................................... 63

10.1 FERRANTI EFFECTS .................................................................................................................... 63 10.2 BLACKOUTS (INTERRUPTIONS)................................................................................................... 63 10.3 SURGES (SWELLS) ...................................................................................................................... 64 10.4 OVER VOLTAGES & UNDER VOLTAGES ....................................................................................... 64 10.5 HARMONICS & INTERHARMONICS .............................................................................................. 64 10.6 NOTCHES.................................................................................................................................... 64 10.7 VOLTAGE FLUCTUATIONS (FLICKER) ......................................................................................... 65 10.8 RECORDING & REPORTING OF POWER DIPS (PROPOSED)........................................................... 65

10.8.1 Procedure ........................................................................................................................ 65 10.8.2 Follow up Action/ Feedback ............................................................................................ 66 10.8.3 Responsibilities ................................................................................................................ 66

11 DISCUSSION .................................................................................................................................... 68

11.1 PROJECT OBJECTIVE REVIEW ..................................................................................................... 68 11.2 CONCLUSIONS ............................................................................................................................ 68 11.3 FUTURE WORK ........................................................................................................................... 70

REFERENCES ........................................................................................................................................... 71

APPENDIX A1 – EFFECTS OF CAPACITANCE................................................................................. 75

APPENDIX A2 – EFFECTS OF LOAD................................................................................................... 76

APPENDIX A3 - DELAYS DUE TO DIPS – 2005 .................................................................................. 77

APPENDIX A4 - UTILITIES FAILURES - RED AREA....................................................................... 78

APPENDIX A5 – CARATS STATISTICS & REAGENT CONSUMPTION....................................... 80

APPENDIX A6 – SAG GENERATOR COSTS @ PSL.......................................................................... 81

APPENDIX A7 – EARTHING FOR DIFFERENT SECTIONS............................................................ 82

APPENDIX A8 –FLYWHEEL VS. BATTERY ENERGY .................................................................... 85

APPENDIX A9 - POWER DIP RECORD SHEET................................................................................. 86

APPENDIX A10 - MAXIMUM POWER DEMAND .............................................................................. 88

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List of Symbols and Abbreviations

A – Amperes

AC (ac) – Alternating Current

ARC – Automatic Re-Closer

AVR – Automatic Voltage Regulator

BAHF – Bulk Acid Handling Facility

BPC – Botswana Power Corporation

BSP – Bulk Sampling Plant

BWP – Botswana Pula (Botswana Currency BWP 1 = 100 thebe)

CARP – Completely Automated Recovery Plant

CB – Circuit Breaker

CBEMA – Computer and Business Equipment Manufacturers Association

CCR – Central Control Room

CHF- Swiss Francs (Switzerland Currency)

CT – Current Transformer

DB – Distribution Board

DC (dc) – Direct Current

DPI – Dip Proofing Inverter

DVR – Dynamic Voltage Restorer

ES – Energy Storage

Eskom – South African Electricity Supply Utility

FeSi – Ferro Silicon

FISH – Fully Automated Sort House

FLC – Full Load Current

FLT – Full Load Torque

GTO – Gate Turn off Thyristor

HV - High Voltage3 – A voltage normally exceeding 650V

Hz – Hertz

3 [CAP. 74:01 S.I, Botswana Power Corporation (Electricity) Bye-Laws – under section 28, 21st December 1979

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IDMT – Inverse Definite Minimum Time

IEC: International Electro technical Commission

IGBT – Integrated Gate Bipolar junction Transistor

IPDB- Interconnected Power Distribution Board

ITIC – Information Technology Industry Council

kV – kilo Volts

kVA – kilo Volts Amps

kW – kilo Watts

LTC – Load Tap Changer

LV – Low Voltage3 above - A voltage not exceeding 250V

MCB – Main Circuit Breaker

ms – milli seconds

MTP – Main Treatment Plant

MV – Medium Voltage3 above – A voltage exceeding 250V but less than 650V

MVA – Mega Volts Amps

MW – Mega Watt (1000 000 W)

MWh – Mega Watt hour

NCC – National Control Centre

O/H – Over Head

P – Real Power

p.f – Power Factor

p.u – per unit

PC – Personal Computer – Microsoft Windows based

PCC – Point of Common Coupling

PCCIE - Power Conditioning and Continuation Interfacing Equipment

PCS – Power Conversion System

PLC – Programmable Logic Control

PS – Parallel Switch

PSM – Plug Setting Multiplier

PSU – Power supply Unit

RMS (rms) – Root Mean Square

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RP – Recrush Plant

RPM (rpm) – revolutions per minute

SC – Starting Current

SCADA – Supervisory Control and Data Acquisition

SICON – Conveyor Belt (manufacturer’s trade name)

Sqrt(X) – Square root of the number X

SVC – Static Var Compensator

TD – Time Delay

TH – Thyristor Switch

TSM – Time Setting Multiple

UPS – Uninterruptible Power Supply

US$ - United States Dollar (US Currency)

V – Volts

VAr – Volts Amps reactive

VDC – Voltage Dip Compensator (Series Voltage Restorer – SVR)

VSDs – Variable Speed Drives

VT – Voltage Transformer

Z – Impedance

ZAR – South African Rand (South African Currency ZAR 1 = 100 cents)

ºC – Degrees Celsius

� – Phase

• W� – White Phase

• B� – Blue Phase

• R� – Red Phase

• 1� or S� – Single Phase

• 3� – Three phase

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Glossary of Terms

Opportunistic costs: Apparent financial losses due to production downtime.

Critical Process: Essential processes that cannot be started up in a reasonable time

before a set automatic discontinuation due to a power dip

Essential Process: A process that is required to maintain a plant

Busbar: any electrical conductor that makes a common connection between several

circuits

Autotransformer: A transformer that has several taps that are designed to connect

automatically depending on a set requirement

Red Area (Aquarium): The high security plant at Jwaneng Mine where final recovery of

diamonds takes place.

Concentrate: Material that contains diamond bearing rocks

Cycle: Time taken to complete one full sine wave of the alternating current supply – for

BPC supply with a frequency of 50Hz this is 20ms

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Project Planning

This project was planned as shown in the Gantt chart below. It should however be noted

that most of the tasks were dependent on the cooperation of various mine personnel and

as such the times shown were not strictly adhered to and furthermore most of the tasks

were carried out simultaneously.

Phase 1 of this project involved establishing what the problem is – voltage dips. A report

was given out on the 15th of August 2006.

The following Gantt chart shows planning of work for Phase 2 of the research on voltage

dips which is mainly concerned with offering recommendations to the mine.

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1 Introduction

This report is phase 2 of a two part report on the research of voltage dips at Jwaneng

mine in order to suggest ways in which to mitigate against this problem. The first report

analyzed the problem mainly at the MTP and Recrush plants whereas this report will be

more focused on the Red Area, which is the final plant in the diamond recovery process,

and will also include a full analysis of the recommendations which were mentioned in

phase 1 report.

Also discussed is background information on general power quality problems which can

be experienced by the mine. A more specific definition of voltage dip is suggested and

voltage dip types are investigated for different phases using a software model in which

parameters such as line impedance, dip duration/magnitude, AVR efficiency, parallel

capacitance and total load can be varied to analyse what happens to the sine waveform

of the ac signal under different conditions.

The research for this project was carried out at Debswana Diamond Mine in Jwaneng,

Botswana.

1.1 Project Objectives & Scope

Even though voltage dips are not new to the mining industry and have been known by

most industries as a major power quality problem, very little has been done to try and

guard against this problem. There have been a few researches on this subject to analyse

causes and effects of voltage dips but they rarely ever offer mitigation choices.

This report aims to go one step ahead to recommend the way forward in minimising or

eradicating the undesirable effects of voltage dips.

1.2 Background Information

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1.2.1 Power Quality Issues

The main power quality problem that is encountered by the mine is voltage dips (sags) as

defined in Section 1.2.2 below which is the main focus of this report. Other power

quality issues which this report shall not dwell much upon but may directly or indirectly

affect the occurrence or severity of voltage dips and hence shall only be discussed briefly

for that sake as shown in Figure 1.1 are;

Figure 1.1

[Note that for cross referencing to Figure 1.1 the name indicated in the picture is given

in square brackets below]

Interruptions (blackout) [Voltage Dropout]

Complete loss of voltage on one or more of the phase conductors,

Swells (surges) [Surge]

Temporary increase in rms voltage or current of more than 10% of the

nominal value at a power system frequency which lasts 0.5 cycles to 1

minute,

Transients [Negative & Positive Impulse Spike]

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Unidirectional impulse of either polarity or a damped oscillatory wave

with the first peak occurring in either polarity,

Over voltages [Over voltage]

Where the voltage has a greater value than nominal for a period of time

greater than 1 minute,

Under voltages [Under voltage]

Where the voltage has a lower value than nominal for a period of time

greater than 1 minute,

Harmonics

Sinusoidal voltages or currents having frequencies that are multiples of the

fundamental power frequency,

Interharmonics

Sinusoidal voltages or currents having frequencies that are NOT integer

multiples of the fundamental power frequency,

Notches

Periodic voltage disturbances lasting less than 0.5 cycles,

Voltage fluctuations (flicker)

Systematic variations in the envelope or a series of random voltage

changes with a magnitude which does not exceed the voltage range of

0.9pu to 1.0pu.

1.2.2 Definition - Voltage Dip

Voltage dips/sags usually carry several definitions according to different authors but most

usually have a similar basis a shown in phase 1 report of this research. However for the

purpose of this report a voltage dip shall be defined as a reduction in nominal voltage to a

value less than 90% up to the time when it goes above 90% but for a period more than

20ms and not exceeding 3s.

In this report a voltage dip to X% means that the voltage dropped by X% from nominal,

i.e. a 10% dip means that the voltage a test node will be 90% of nominal. In the Figure

1.2 below the allowed voltage levels are ±10%

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Figure 1.2

Disturbances that last less than 1/2 cycle are commonly called "low frequency

transients"; voltage reductions that last longer than a few seconds (i.e. > 3s) are

commonly called "under voltage"4 as defined in Section 1.2.1 above.

When there is a fault on a transmission line or anywhere in the system, excess current is

drawn from the supply and because current (I) is proportional to (V) voltage (V = IZ –

Ohm’s law) this causes a minor drop in the voltage for the duration of excess current

being drawn as impedance (Z) stays constant. If the fault is not cleared within a short

time as determined by protection settings either an under voltage as described above or a

blackout will result.

The terms sag and dip shall be taken to be the same and so shall power dip and voltage

dip and hence will be used interchangeably in this report.

1.2.3 Voltage Dip Types

This was investigated using Alex McEchern’s Concept Teaching Toy obtained from

Power Standards Labs to obtain the following graphs.

4 www.powerstandards.com/tutorials/sagsource.htm, accessed 14 June 2006

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Figure 1.3(a, b, c)

Figures 1.3 above and left shows a

simulation of a circuit which reacts to

changes in the input impedance, the

smoothing capacitance, regulator

efficiency and the load. The simulation

in Figure 1.3a assumes an ideal balanced

network experiencing a three phase

voltage dip due to a fault. The dip lasts a

total of three cycles (60ms on a 50Hz

network). Of most interest is the Volts – regulated line as this indicates the voltage being

fed to machine power electronic equipment.

The simulations show that increasing the capacitance [Appendix A1] and the regulator

efficiency has the desirable effect of smoothing out the regulated voltage. For exactly the

same circuit a phase-phase to phase fault (Figure 1.3b) that experiences a similar fault

current would only result in low frequency transients but no “actual” voltage dip whereas

a single phase fault (Figure 1.3c) would have negligible effect on the terminal voltage.

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Increasing the load increases both the magnitude and duration of the voltage dip

[Appendix A2]. This is due to the fact that a larger load draws more current thereby feels

the onset of the dip immediately when the fault occurs and a higher voltage drop.

Emphasis should be made here that even though this particular voltage dip (Figure 1.3a)

lasted no more than a mere 60ms the corresponding interruption to the diamond mining

process may have been extremely large. For example some drives might have stopped

resulting in Head feed delays that can last for several hours leading to long start-up

procedures both of which translate into appreciable revenue losses.

1.2.4 Selected Literature Review

There have been several studies covering the voltage dip phenomenon. The following is a

selected few of some of the reports that have been published about voltage dips.

G.J COETZEE, Power Quality paper #1 – Causes of voltage dips & resulting problems,

Switching Systems Electronic Engineers

POHJANHEIMO P., A Probabilistic Method for Comprehensive Voltage Sag

Management in Power Distribution Systems, Doctoral thesis, Helsinki University of

Technology, 2003

J. D. MASTERS, Dowding Reynard & Associates (Pty) Ltd, Voltage Dip study at

Jwaneng Mine-addendum, 13 March 1997

ABBAS AKHIL, SHIVA, SWAMINATHAN, RAJAT K. SEN, Cost Analysis of Energy

Storage Systems for Electric Utility Applications, February 1997

MATH BOLLEN, Voltage Sag Indices – Draft 1.2, working document for IEEE P1564

and CIGRE WG 36-07, December 2000

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With exception of the voltage dip study done at Jwaneng mine by J.D Masters, all of the

above researches were more academically inclined and do not offer any recommendation

on what should be done to avoid this unwanted situation. Even J.D Masters offers only

one solution in the form of Dip Proofing Inverters and hence does not really give a broad

choice or reason why DPIs may not the best nor does he state the draw backs of this

particular apparent “panacea” (as is the general feeling in the mine). However all these

shed some very vital information on the causes and effects of voltage dips on power

electronic equipment.

1.3 Project Methodology

Research for this project was carried out as follows;

Initially, prior to research at Jwaneng Mine, an extensive literature review on the subject

of voltage dips was carried out using published journals, textbooks, the internet,

magazines and all publications that mentioned this problem.

Knowledge of the other plants within Jwaneng Mine from Previous attachments to the

following sections in the specific years was used;

• 2002 – EM15 Electrical Workshop

o Shovels & Drills

o Pit electrical reticulation maintenance

• 2003 – Recrush plant

o General mining process flow

• 2005 – EE12 Electrical Services Workshop

o Maintenance of diesel standby generators

o Overall mine’s electrical reticulation

Gathering of information was generally done using questionnaires and short discussion

meetings for the respective mine personnel for Electricians, Section, Engineers, Senior

Plant Metallurgists, Electrical Workshop foremen, BPC & NCC.

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There was attendance of Daily production meetings at the MTP to gain knowledge of

how this issue is tackled and what form of feedback is demanded by the non engineering

management.

Email correspondence of the Daily exception report and the Daily production report both

from the MTP and the RP was used to analyse delays caused by several factors on the

plants.

A general tour of the Red Area Plant was done to get an appreciation of the general

diamond mining process at the final point in the mine and identify where critical loads

are located.

1.4 Thesis Outline

This report is organised into 11 chapters which are as follows;

Chapter 1 - The first chapter offers background information on different power quality

problems that the mine encounters and broadly defines the term voltage dip/sag. A

software model is used in order to investigate the different types of voltage dips that the

mine may experience to establish which ones will trip out drives and damage power

electronic equipment. Also discussed here is a literature review of selected researches

that have been done on this subject.

Chapter 2 defines the methods of recording of voltage dips that have been found at

different sections and plants within Jwaneng mine. The methods here are different

according to who does the recording be it the section engineer at electrical services,

electrical workshops, treatment section, CCR, BPC or the Red Area Plant. This chapter

also analyses probable causes of voltage dips in the red area plant

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Chapter 3 discusses the effects of voltage dips on the red area plant processes at Security,

Bulk Acid Handling Facility, FISH & CARP, internet servers, air conditioning and

general diamond production process.

Chapter 4 presents an economic evaluation of the apparent financial losses encountered

due to head feed delays, FeSi losses and security breach that have been caused

specifically by faults on the electrical reticulation network that led to a voltage dip. The

cost to the mine due to dips has never been documented so this report makes some sound

assumptions on some production figures.

Chapter 5 elaborates briefly what has already been presented in the Phase 1 report on

supply utility (BPC) conditions on the quality of supply. This chapter looks at the agreed

levels of voltage supply on magnitude and balance, frequency deviation and allowed total

harmonic distortion.

Chapter 6 is a list of the most critical loads within Jwaneng Mine as presented by the

engineering manager. However this list is incomplete for the red area on the basis that the

records of the drives/machines that trip due to dips are not listed even though they are

very critical to the entire process. These drives are mentioned in Chapter 3.

Chapters 7, 8, 9 and 10 offer several different recommendations to the mine;

Chapter 7 offers ways in which to reduce the frequency of occurrence of voltage dips by

analysing what happens to the system with power quality meters, establishing proper

protection and grading margins, testing power electronic equipment (contactors) with sag

generator, balancing the network, minimising THD, power factor correction, soft starting

large loads and proper earthing methods. The last of this chapter part suggests reducing

equipment sensitivity in order to fall within the limits of the ITIC curve

Chapter 8 discusses in detail some of the ways of compensating the missing due to a

fault. Compensation is by means of energy storage systems such as the SMES, BESS,

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FES or UPS. Closing this chapter is a case study to compare costs of existing SMES and

BESS. A technical and cost analysis of the above methods discussed therein and a point

of connection to the existing electrical reticulation suggested. If the mine chooses one of

the energy storage systems recommended here then a power conversion system shall be

needed to connect to existing system. This chapter suggest a model for such a conversion

system.

Chapter 9 – Smoothing out Voltage Profiles - Mentioned in this chapter is use of switches

namely the SETC or LTC and the SSTS to transfer load to alternate supply to avoid a

voltage dip. The SVC is also discussed in detail to offer MVAr injection and thus sustain

voltage at nominal level. Also here a technical and cost analysis of the above methods

discussed therein and a point of connection to the existing electrical reticulation

suggested.

Chapter 10 addresses general power quality mitigation against problems such as Ferranti

effects, blackouts, surges, over & under voltages, harmonics & interharmonics, notches

and flicker. This chapter also presents a procedure that Jwaneng mine will be

recommended to use to report and record power dips.

Chapter 11 - The last part of this report reviews the objectives for this project in order to

analyse whether they have been properly met and points out what shortcomings were

encountered. This part also discusses, concludes and suggests future work on mitigation

of voltage dips at Jwaneng mine.

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2 Current Methods of Recording of Dips

These methods of recording of dips were found at the RP, MTP and Mining Electrical

Workshops and include the Balanced Score Card, Power Dip Record Sheet, Electricians

shift reports, CCR reports and BPC’s daily reports. Electricians at the Red Area

workshop have no absolute defined recording method of power dips. The causes of

voltage dips for different sections were discussed in Phase 1 report except for the Red

Area Plant which shall be discussed in the last part of this chapter.

2.1 Balanced Score Card – (Use of power quality meters)

Figure 2.1 - Source: KENNY SINYINZA, Power Quality Metering Output, Jwaneng Mine

The graph above shows data as downloaded from power quality meters that are installed

on the supply utility side at the mine’s main switchyard. It shows a dip that lasted less

than 164ms and the voltage dropped about 4kV from nominal on two phases. The data

collected from the meter is used to compile the following graphs called the Balanced

Score Card.

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IP5: POWER DIP FREQUENCY

5

7.0

4.0

2.0 2

0

2 2

1 1

0

1

2

3

4

5

6

7

8

Jan-06 Feb-06 Mar-06 Apr-06 May-06 Jun-06 Jul-06 Aug-06 Sep-06 Oct-06 Nov-06 Dec-06 Jan-07

Actual Internal External Threshold Target Stretch Target

Figure 2.2 – Source: KENNY SINYINZA, Balanced Score Card, Jwaneng Mine

The balanced score card is kept every year and it can be seen that there were 34 voltage

dips last year [Appendix A3] from January to October 2005 and there has been 26

voltage dips this year (2006) in the same period – Figure 2.2 . This can hardly be called

an improvement as it can also be seen that the frequency of occurrence is spread out over

the months this year than last year where there were no dips between June and

September. For the two years the months in which most power dips are experienced are

from January to March. It can also be noted that for these particular years most

thunderstorms occurred during this period which may account for the higher frequency of

occurrence of voltage dips as corroborated by reports from the mine and BPC – see

Appendices report Phase 1.

2.2 Power Dip Record Sheet

The Voltage dip recording sheet shown in [Appendix A9] had been made for the mine

but it is rarely ever used. Adjustments were made to include feedback and comments

comprising dip magnitude and duration, total load shed and comments from treatment,

BPC and electrical services workshops. This is further discussed in detail in Section 10.8

2.3 CCR

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CCR Operators generally record power dips whenever they see the lights dim and any

drives were interrupted. Any head feed delays will therefore be accounted for as being

caused by a voltage dip.

2.4 Electrical Workshop

None of the electrical workshops in the mine have access to any power quality meters

and personnel are not even aware that there is a power quality meter at the mine’s main

switchyard. The workshops will report a power dip as source of fault if none other can be

seen when any machines/drives need to be reset.

2.5 BPC

BPC does not have any specific record of dips that can be made available for the mine

but all faults that occur on transmission line and on the network are documented daily

and these were obtained from their NCC and comparisons made with dates and times

when the mine reported a voltage dip. See Appendices Report Phase 1.

2.6 Red Area Plant

At the Red Area, CCR usually reports that a machine has stopped and the electrician who

has to reset it will report that as a power dip if not other fault is apparent. The foreman

takes this to the daily production meeting as cause of machine trip. However feedback is

neither given nor demanded by the attendees at the daily production meeting on what was

the cause or what is being done to resolve the problem and hence voltage dips are not

documented anywhere.

.

For a plant that does not document voltage dips it makes it difficult to ascertain what

faults on the transmission system lead to this particular problem. However cross

referencing from the electricians at times when they are called to reset machines some of

the following can be attributed to be probable causes at these respective areas;

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2.6.1 Treatment

The power dips that originate from within the plant are mainly caused by bad weather.

The most frequent occurrences of machines/drives tripping are during the rainy season

and as such only a correlation of coincidence can be made that voltage dips are the cause.

Some of these alleged voltage dips however may have been just another power quality

problem and not a voltage dip.

2.6.2 External to the Red Area Plant

Most of the alleged power dips that affect the Red Area are from faults that are external

to the plant and either originate from mining section or outside the mine altogether as

stated in project report phase 1.

2.6.3 Security

There are no reported cases of a situation whereby a voltage dip was caused by an

individual tampering with the security system of the plant but the fact that it has not

occurred yet does not mean it will never do.

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3 Effects of Power Dips in the Red Area

The start-up procedure of the processes in the Red Area plant is not as tedious as that of

the MTP and the Recrush plants. However appreciable delays are encountered whenever

there is a power dip as certain drives trip and lights go off even creating a security risk.

This chapter analyzes the effects of voltage dips on the security of the plant, the potent

acids being used and the delays on the general process.

3.1 Security

A major impact on the security of both the employees of the company and the diamonds

would be encountered if a power dip caused security system failure and if the lights were

to go off.

3.1.1 Magnetic Locks

Most of the security locked doors in the Red Area use magnetic locks which depend on

the presence of power to be able to work. When a voltage dip occurs at the instance when

the door was open it will not be able to lock back on leaving the area unsecured which is

a very high security risk. The door will have to be manually closed even when power

comes back on as the magnetic locks have a limited operating distance.

3.1.2 Central Command Centre

The cameras on the floors (CARP and FISH) may trip out when there is a voltage dip and

security officers will not be able to monitor what is happening. The protection system is

offered by onsite technicians and they have to be called in to reset but there will be a

definite delay. During this time no one can be allowed in or out of the Red Area plant at

the main access gate. Employees knocking off will not able to out of the Red Area and

those reporting for duty will not be able to come until the system has been brought back

online.

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3.1.3 Back-Up System

Back-up in the Red Area is given by means of a diesel standby generator which monitors

the mains supply and automatically switches on when there is a power failure (or power

dip which causes machine failure and lights to go off). The parameters of the generator

are as follows;

Table 3.0 Red Area Standby Diesel Generator

3 Phase, 50Hz, 0.8 p.f

Power 350kVA, 280kW

Output Voltage 550V

Output Current 367A

Revs 1500rpm

Excitation 26V, 2.6A, 0.9 AVR

As can be noted by the ratings on this generator its only used to power up lighting and

very few critical loads just for the security system to stay online and general production is

not accounted for which means the diamond treating process will have stopped in this

instance.

3.2 Acid Handling (BAHF)

The Red Area plant uses Hydrofluoric (HF) acid and Nitric Acid (HNO3) [Appendix A5]

in the production process to clean the diamonds by removing surface impurities such as

silicates and water stains before delivering to the Size Frequency Distribution Unit

Process for auditing..

3.2.1 Scrubber

If a power dip were to stop the scrubber in the FISH plant then the acid fumes can not be

extracted from the plant which means a build up will result thereby affecting the health

and safety all the personnel who may be in the vicinity. Compensation charges for the

affected employees on the mine could cripple the mine revenue and any safety audit

could ultimately lead to closure of the mine if such a situation could ever occur. The

motor that runs the scrubber has the following details on its nameplate;

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Table 3.1 Acid Fumes Scrubber Motor details

Power 110kW

Voltage 525V

Current(Full Load) 151A

Revs 1480rpm

3.2.2 Neutralization Process

Lime (CaCO3) and Potassium Hydroxide (KOH) are used to neutralize the used acids

before they can be disposed of [Appendix A5]. When there is a power dip and that causes

the neutralization process to stop then the rest of the diamond process has to stop and this

obviously translates into some appreciable revenue losses in terms of the lost hours of

treatment.

3.2.3 Compressor

The compressor in the Red Area sometimes stops when there is a voltage dip. This is

used to supply air for the air valves and therefore because continuous operation is

required backup is provided by 3 other compressors located at the MTP. However the

process of transferring to MTP’s compressors is manual and hence a definite time delay

is experienced due to the unplanned stoppage. The drive for this compressor is rated

110kW.

3.3 General process

Diamond rich material from the MTP and the Recrush is fed to the red area via 3 feed

preparation conveyors whose drives are each rated 11kW. There are also four (4) other

conveyors (named SICON) whose drives are each rated 7.5kW which feed tailings to the

RP and reload from the Red Area old dump for reprocessing when the feed preparation

are not delivering at nominal capacity. Two (2) pneumo blowers both rated 110kW are

used in the diamond treatment process, after the material is heated up using several

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heating elements they blow it up a channel up to the to of the CARP building to the 15th

floor to dry it up.

3.3.1 Pneumo Blower (Drier System)

Even though these are each fitted with DPIs rated

3kVA at the Pneumo Drier 550V MCC AKX-230-09

(the same DPIs installed at MTP DMS as documented

in phase 1 report) they are most prone to tripping when

there is a voltage dip according to reports by the

electrical workshop. The DPIs have been in operation

since the time of commissioning of the plant in 1999

and they were put up by a contracted company. When

the Pneumo blower trips all the material that was in the

vertical section of the pipe falls down and can lead to

blockage and major delays will be encountered whilst

the blockage is cleared before they can be restarted.

3.3.2 Feed Preparation Conveyors

If all the 3 feed preparation conveyors stop, then no material is fed from the blue area via

the 110 Ton Bin. However the drives running these conveyors usually trip due to voltage

dips creating head feed delays in the time when an electrician has to be called to reset

them.

3.3.3 SICON

The drives that run the SICON also usually trip whenever there is a power dip and have

to be reset. However as there is material on the conveyors when they are restarted it

means the drives are starting on full load, high currents result and sometimes a second

voltage dip results.

Figure 3.1

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3.3.4 Perm Rolls

Also affected by voltage dips are Perm Rolls. These are used to separate magnetic feed

from non magnetic to recover diamonds and liberate the waste. When the perm rolls stop

the entire process has to stop otherwise all the diamonds that were in the feed at that

particular time will pass as waste.

3.4 Internet Server Downtime

The servers are generally protected by UPS such that in most cases they are protected

against power dips. However the UPS is connected directly online which means that

during its maintenance or a breakdown when the protected load is connected via the

bypass switch it is left vulnerable to all effects of bad quality power including voltage

dips. A voltage dip can therefore cause bit errors in data transmission and some erratic

telephone switching system performance.

Network downtime can cost in excess of US$50,000 per hour when one megabyte of data

needs to be restored or recreated.5

3.5 Air Conditioning

Voltage dips sometimes cause the air conditioning units to trip out. For the Red Area

planning offices the distribution board is located in a room that is usually left locked and

such that when there is a trip and the holder of the keys is not around they can not be

reset until he/she comes back. In a country where temperatures can soar as high as 40ºC it

makes it unbearable for anyone to do any work in the office which has a serious impact

on productivity.

5 http://www.power-innovations.com/about_power/dyk.html accessed 6 November 2006

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4 Economic Evaluation (Losses due to Voltage Dips)

This chapter presents an economic evaluation of the losses encountered due to voltage

dips outside the mine and within the Red Area by head feed delays, FeSi losses and

security breach. However as this particular plant is the last point in the treatment

process and encompasses both the RP and MTP reference shall be made to revenue

losses for the entire mine as shown by head feed delays on the balanced score card.

4.1 Losses due to Bad Power Quality

Power Innovations Inc, a technology company located in Lindon, Utah, has the following

facts and figures published on their website about the financial losses that are

encountered due to voltage dips;

A recent rolling blackout in the greater San Francisco Bay Area

caused an estimated US$75 million in losses in the Silicon Valley.

According to Larry Owens of Silicon Valley Power, a blackout

costs Sun Microsystems up to US$1 million per minute

Every hour of downtime for a typical mid-sized network costs its

owner US$18,000.

Poor power quality costs U.S. businesses more than $26 billion

each year!

80 to 90 percent of power glitches come from voltage sags, which trip motors and

microprocessors, according to Gregory J. Yurek, CEO of American Superconductor

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Corp. of Westborough, MA, another superconducting technology leader. Yurek estimates

that sags cost U.S. industries about US$12 billion a year in downtime.6

4.2 Production Delay (Head Feed Delays)

IP4: Head Feed Delay due to Power Interruptions and Dips

3.3

6.5

2.2

0.85 0.83

0.0

0.90.5

1.3

0.5

0

1

2

3

4

5

6

7

Jan-06 Feb-06 Mar-06 Apr-06 May-06 Jun-06 Jul-06 Aug-06 Sep-06 Oct-06 Nov-06 Dec-06 Jan-07

Actual Internal External Threshold Target Stretch Target

Figure 4.1 – Source: KENNY SINYINZA, Balanced score Card, Jwaneng Mine

The graph above shows head feed delays that occur due to power dips for the entire mine.

However any head delays encountered by MTP and RP will directly be transferred to

delays in the Red Area plant as well.

The balanced score card above shows that the mine lost 15.08 hours from January 2006

to October 2006 due to power dips. Except for the month of February the amount of

delays is generally below both the threshold target and the stretch target of acceptable

amount of delays that the mine can tolerate. However these targets are artificial and were

computed looking at past trends and can be further analysed such that they can be

brought closer to the zero line which would be the most desirable condition.

For the same period last year (2005) the balanced score card shows that the mine had

delays amounting to 19.83 hours due to voltage dips. Again from this data there is hardly

any improvement and averages verify this;

6 http://www.pur.com/pubs/1251.cfm accessed 7 November 2006

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From Section 2.1 there were 36 voltage dips in 2005 – this gives an average of;

19.83/34 = 0.58 hours/dip………………………Equation 4.0

And there were 26 voltage dips this year (2006) giving;

15.08/26 = 0.58 hours/dip……………………Equation 4.1

If the data from last the years 2005 and 2006 can be taken as to reflect a general trend on

voltage dips at Jwaneng mine there on average there is a delay of 0.58 hours for every dip

– Equations 4.0 and 4.1 and it has not changed from the year 2005 to 2006.

The Red Area experienced a total of 24 hours of head feed delays due to power dips

alone in the period between the 20th of December 2005 and the 10th of April 2006 as

shown in [Appendix A4].

In the year 2005 alone the mine produced 15 618 155 carats. This was a record figure for

the mine but if this production figure was achieved again in the year 2006 then on

average it can be assumed that the mine achieves;

15618155 carats / (1year*365days/year*24hours/day) = 1782carats/hour…Equation 4.2

And if the cost per carat bought from the mine was assumed constant at an average of

US$140 as mentioned in Phase 1 report then the profits foregone by the mine due to these

delays would be;

US$ 140/carat*24 hours*1782 carats/hour = US$ 5 987 520 …Equation 4.3

Which is equivalent to BWP 37 402 241.11 as computed from the currency converter at

http://www.xe.com/ucc/convert.cgi accessed 1 November 2006.

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However if the current carats production statistics are assessed for the period from

January 2006 to September 2006 [Appendix A5] then on average Jwaneng mine has been

producing 1 289 758.28 carats per month from the red area plant. If this is extrapolated to

the end of the year (December 2006) the mine’s carat production for the year 2006 will

be 15 477 099.33 which is about 1766 carats/hour which is still close to the value

calculated in Equation 4.2 of 1782 carats/hour.

4.3 FeSi Losses

Ferro Silicon (FeSi) is used in the mining process to aid in separating the diamond rich

material from the waste – a process referred to as Dense Medium Separation. Magnetic

rollers are used to recover the FeSi so that it can be re-used. If the drives running the

magnetic rollers stopped due to a voltage dip then all the FeSi in that particular stream

would be lost.

In the red area FeSi is only used at the BSP which is a small plant used for research

purposes [Appendix A5]. In contrast to the MTP and the Re-crush plants loss of FeSi in

the red area is very minimal. At any instance no more than 1 bag of FeSi would be in

circulation in the process cycle. That means a loss due to a stoppage of the plant by a

power dip would amount to about ZAR 6000 which equates to about BWP 5 064 – see

currency converter section 4.2.

4.4 Security Breach

There are no definite economic losses that can be attached to a security breach caused by

a voltage dip. Exit points of the red area are via search points which have X-Ray

machines. If a voltage dip affected these machines it would make it possible for any

individual to exit the plant with diamonds or other valuables.

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5 Supply Utility Conditions

Supply Utility Conditions were mentioned in Phase 1 report in more detail but Sections

5.1 on supply clause/provision and 5.2 on power quality specifications are mentioned

here for reference and for more elaboration. Also harmonic compatibility levels as

stipulated by BPC are discussed in this chapter and compared with actual (measured)

levels seen at Jwaneng mine’s power quality meters. Also assessed here is effect on

quality of supply of the proposed 220kV line from Morupule to Thamaga substation.

Jwaneng mine is supplied at 132kV and hence is classified by BPC as being on the

transmission network rather than the distribution network. The two 20MVA transformers

at the mine’s switchyard step this down to 6.6kV and two lines both of which are rated

1250A supply the Red Area.

5.1 Stipulation

The Corporation shall not be liable for damages, expenses or costs caused to the

consumer from any interruption in the supply, variation in voltage, variation of

frequency, any failure to supply a balanced three phase current or failure to supply

electricity whether the said interruption is or is not due to the failure of the Corporation to

carry out its obligations or to any other cause whatsoever.7

5.2 Power Quality Specifications

5.2.1 Voltage & Frequency

The fundamental specifications as mentioned in phase 1 report for the quality of supply

as stipulated by NCC are;

Voltage: ±10%

Frequency: ±2.5%

Voltage (Phase) Unbalance shall be maintained at ±2%

7 Bye Law #31, [CAP. 74:01 S.I, Botswana Power Corporation (Electricity) Bye-Laws – under section 28, 21st December 1979

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The highest unbalance measured for the month of December 2005 was 0.63% whereas

the highest frequency was 100.17% and the lowest was 99.82%. Table 7.0 below shows

the highest voltage deviation from stated nominal by % for the same month.

Table 5.0 – Operating voltages per phase

Voltage Phase Highest (%) Lowest (%)

Red 104.74 100.51

White 104.04 101.31

Blue 105.23 102.02

5.2.2 Harmonics

Table 5.1 below was extracted from BPC’s daily Operations and Transmission December

2005 Jwaneng Power Quality Report. The power quality standard used by BPC was the

NRS 048 - 2(2003). The table shows the targets that the supply utility stipulates to be

able to meet everyday.

Table 5.1 – Harmonic Compatibility Levels (according to NRS 048-2)

Harmonics Order Harmonic Voltage (%) 3 5

5 6

7 5

9 1.5

11 3.5

13 3

15 0.5

Table 5.2 – Measured Highest Harmonic Voltages

Harmonic Order

3 5 7 9 11 13 15

Red � (%) 0.65 3.70 0.36 0.01 0.07 0.10 0.03

White � (%) 0.80 3.51 0.32 0.00 0.08 0.07 0.02

Blue � (%) 1.23 3.80 0.25 0.01 0.08 0.10 0.02

Harmonic order 5 seems to be the most dominant but it is still within BPC’s acceptable

limits.

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NRS 048 – 2 specifies that the THD should be less than 8%.

Table 5.3 – Total Harmonic Distortion (THD)

Voltage Phase % THD Red 3.99

White 3.78

Blue 3.98

For the month of December 2005 the THD was within acceptable limits across all phases

as defined by the supply utility.

5.3 Proposed 220kV Transmission Line

The BPC is currently undergoing a project erecting a 220kV transmission line from the

generating power station at Morupule to the substation at Thamaga from which Jwaneng

is fed. The table below shows the improvement on the fault level ratings. This project is

scheduled do be finished by December 2006.

Table 5.4: Comparison of fault level ratings with and without 220kV line

Without Line

With Line

Busbar (kV) MVA KA MVA KA

132 438 1.92 493 2.158

33 231 4.43 270 4.73

6.6 208 19.96 242 21.14

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6 Critical Loads

Companies that sell power quality products require that the customer specify the kind of

equipment that is most vulnerable or most critical/essential to operations. This chapter

shows a list of Jwaneng mine’s most critical loads. The list was obtained from Jwaneng

mine’s intranet pages and was made in June 1996. The intranet pages however indicate

that it was last modified on 24th January 2005.

Some of the equipment may have since then “evolved” and more equipment may have

been added as well but this is the only list available from the company’s intranet pages.

These are the loads that always have to be working to avoid a major production

downtime or breach of security within the mine premises both of which could lead to

substantial revenue loss and even hamper safety of personnel.

Some of the loads such as general plant lighting and low power control equipment have

always had a back up system for the case of any power fluctuations. Back-up is in the

form of UPS for some control equipment and small auxiliary generators (350kVA at the

main switchyard). The larger loads such as the slurry pumps require more power and can

not be compensated by UPS or the small auxiliary generators. The DMS streams at MTP

and some critical drives have protection against dips in the form of DPIs.

6.1 Red Area

Some of the most critical loads in the high security plant of Jwaneng mine as listed on the

intranet are as follows. However it should be noted that Chapter 3 details some other

machinery that is not listed here but has been more prone to tripping during voltage dips

and quite essential to the diamond recovery process

• Heating elements for grease applicators 200 kW

o This is only used in the small Bulk Sampling Plant within the Red Area

• Security / lighting150 kW

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• CAC150 kW

Sub Total 500 kW

6.2 Main Treatment Plant

Most of the following critical drives correlate well with the ones discovered and reported

in appendices in Phase 1 report.

• Thickener rake drives (6)180 kW

• Thickener underflow pumps (1 set)112 kW

• Submersible pumps 40 kW

• Slurry pumps (2 of)220 kW

• One (1) DMS stream400 kW

• Plant lighting250 kW

• Compressor (1 of)160 kW

• Fire pump75 kW

• Jet sump pump and sump pumps260 kW

• Degrit stream (1 of)100 kW

• JX118 and node conveyor165 kW

Sub Total 1962 kW

6.3 Re-crush Plant

The re-crush plant being a recovery plant with operation very similar to the MTP ideally

has similar critical drives but the following are those obtained from the intranet site.

• Ensure that the BPC 132/33kV transformers and the 33kV township feeders have

been isolated by BPC prior to supplying power through the standby emergency

step-up transformer (5MVA 6.6kV/33kV).

• Thickener rake drives (5)150 kW

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• Sump pumps in thickeners200 kW

• Lighting250 kW

Sub Total 600 kW

6.4 Services

Services entail general provision of electricity and maintenance and this is located outside

the mining area and hence has not drives directly linked to the diamond mining process.

• There are no critical loads

6.5 Open Pit

The open pit is where the kimberlite is blasted. It should be noted that the 205 Shovel

which has an induction motor rated 2MW discussed in section 3.3.1.3 of report phase 1

is located in the open pit but the report on critical loads for this area is as follows

• There are no critical loads

The total power requirement for critical loads is: 500+1962+600 = 3062 kW 8

8 DEBSWANA DIAMOND COMPANY (Pty) Ltd, Jwaneng Mine, Engineering Electrical Services Standard Procedure, Procedure No: EE-E1-06, Critical Loads to be supplied from the Power Station Diesel Generators during a BPC Power Supply Failure, Revision 1, 7 June 1996

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7 Reducing Occurrence of Dips

This chapter was briefly introduced in phase 1 report. It has been established in previous

chapters that voltage dips are an inevitable phenomenon and will always occur due to

some unavoidable faults on the system. However some of the artificial faults can be

minimised at least to a considerable level. The different methods of reducing the

frequency of occurrence of voltage dips are discussed in more detail here.

7.1 Power Quality Metering

This is perhaps the most vital aspect of voltage dip mitigation. The best way to solve any

problem is to know what exactly the problem is and validate whether indeed a problem

persists. This should be installed on the mine’s side switch yard and then at points of

connection to only the most critical processes. With a Power Quality meter in place one

can compare against the voltage dip window as mentioned in Section 5.2 of the report for

Phase 1 and thus know how best to protect equipment with knowledge of the most

prevalent type of dips according to magnitude and duration.

Quote requested from Haefely Technology on 12/07/06 was CHF 18 600 for the PLINE

1610 and CHF 19 500 for the ECOMPACT4 testers. CHF 19 500 equates to about BWP

93 135.84 (www.xe.com/full)

7.2 Protection & Discrimination

This is another power quality aspect that may often be overlooked but if properly done

the effects of some voltage dips arising from faults on the network can be reduced such

that only part of the network with the fault that caused a voltage dip is disconnected. This

should be done in a clear and concise way such that any electrician can be able to

pinpoint which timers may need to be changed when a new machine is added into the

system.

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The current criteria within the mine is not clearly defined in the sense that sometimes

grading margins are arbitrarily chosen with no proper calculation or template in the form

of software package such that every time a new unit is incorporated into the system

discrimination becomes a big hassle. Most of the settings have to be changed every time

there is a fault after realization that circuits closest to the fault that were supposed to trip

stayed online an only distant circuit breakers operated (geographical grading) thereby

posing risk of damage to drives nearest the fault.

Relay timers should be set such that they can ride through a dip on a site with DPI

installed at least for the duration of the time setting on the DPI.

ARC dead time should be incorporated into any protection and discrimination settings as

otherwise this could lead to loss of supply and thus render any voltage restorers useless

In order to set proper protection and discrimination the following information should be

sought out and known;

• A single line diagram showing;

o Load current for each connected machine

o Maximum fault current at each busbar

• Impedance seen at each busbar

• Line voltage

• Fault level MVA ratings at each busbar

• PSM as a percentage for each relay

• CT ratio of each relay

• Required grading margin and TMS plug setting increment value.

7.3 Simulate Bad Quality Power

This is just as useful as metering. It is necessary to simulate bad quality power so as to

fully understand what goes wrong with equipment when there is a power dip and how

best to provide a solution. This would aid in certifying whether equipment used in the

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mine conforms to standards such as the SEMI F47, whether it lies in the required

tolerance envelopes of the CBEMA or ITIC curves

Simulating bad quality power may even be necessary after purchase and installation of

voltage restorers to test whether they are indeed in good working order and what up to

what level/severity of a voltage dip the would be able to provide a reliable compensation.

Figure 7.1 shows a sag generator being

used to test the operation of a contactor

when there is a voltage dip. The sag

generator produces a dip of known

duration and magnitude. It can also be

used to choose the point-on-wave (sine

wave angle) of voltage dip initiation to

see what effect this will have on the

contactors.

An alternative to buying the sag

generators is requesting that all suppliers

of power electronic equipment to the mine satisfy a pre-determined tolerance to voltage

dips. See reducing equipment sensitivity - Section 7.8

A Quotation was requested [Appendix A6] on June 20th 2006 from Power Standards

Labs via email – IPC rated 240V ac, 25A max sells @ US $31 500 (equivalent to BWP

189,072.48 from www.xe.com on 7/4/2006 @ 8:20:55 AM) package including IPC

software.

7.4 Balanced Network & Harmonics

The magnitude and the duration of a dip is dependant on the phase of the voltages at the

time of occurrence such that if large negative phase sequence currents are present at onset

of a dip then it very unpredictable events on the voltage profiles may result. The

magnitude of unbalance in the three phase supply voltage will affect the magnitude of a

Figure 7.1

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voltage dip as the torque of induction motor drives is dependant on the positive phase

sequence component and thus the presence of a negative phase sequence will reduce the

motor applied torque.9

A jump in the phase angle of the voltage at the start of a dip can cause some line

commutated variable speed drives to trip even if the dip magnitude is relatively small.

Depending on the phases that are involved in the fault, the negative sequence voltage will

be multiplied by a factor �X where � = ej120° and X = 0, 1, 2 giving a characteristic voltage

of;

V1 + �XV2 …….Equation 7.0

where V1 and V2 are the involved phase voltages.10

Figure 7.1(a, b)

On an ideal 3-phase system, the voltages are balanced (the 3 voltages are exactly equal

and exactly 120º apart) as shown on the Figure 7.2a above. Unbalanced voltages as

shown on the Figure 7.2b above make transformers and motors overheat because part of

9 G.J COETZEE, Power Quality paper #1 – Causes of voltage dips & resulting problems, Switching Systems Electronic Engineers 10 M SCHILDER & RG KOCH, Evaluation of a new 3 Phase dip definition, Energize – Power Journal of the South African Institute of Electrical Engineers, June 2006

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the voltage or current is trying to rotate the system backwards. The phase voltages

resolve as follows to give the characteristic voltages shown in Equation 7.0;

Vo = 1/3(Vred + Vyellow + Vblue)............. (zero sequence) Equation 7.1

V1 = 1/3(Vred + aVyellow + a2Vblue).......... (red positive) Equation 7.2

V2 = 1/3(Vred + a2Vyellow + aVblue).......... (red negative)Equation 7.3

For the Equations 7.1, 7.2 & 7.3 shown above;

• a - rotates 120 º clockwise

• a2 - rotates 240º counter clockwise

The approach of the equations above is only valid in absence of harmonics - i.e.

fundamental is not distorted, but more often than not voltages are not pure sine waves on

the ac system. See Figure 7.3 below simulated using Alex McEchern’s Concept Teaching

Toy obtained from Power Standards Labs in which the fundamental sine wave is what is

transmitted and the Sum of the waves including harmonics is what the mine may receive.

Figure 7.2

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It should be ensured that all three phases are balanced to avoid negative phase sequence

currents flowing in the connected drives. Harmonics can be filtered out of the system by

using appropriate filters. A private company called Netlab™ Group has been contracted

by Jwaneng mine to address this issue. Netlab Group initial recommendation is

installation of filters and capacitor banks at the Recrush plants and in the mining section

latest by mid next year (2007)

7.5 Power Factor Correction

It was mentioned in report phase 1 that the mine does not incur any charges whatsoever

for a bad power factor. Even though this may be true as it is not reflected in any of the

BPC bills, further research into this issue has however revealed that the mine is

contractually bound by BPC to keep the power factor at least above 0.8 lagging.

The power factor observed at the Red Area plant was 0.40 lagging on Incomer No 1

delivering 241kVA and 0.69 lagging on Incomer No 2 delivering 672kVA at the

Aquarium’s 6.6kV switchboard AKX-221-01.

Loads with poor power factor increase reactive power losses on the lines and hence could

induce a voltage drop along the transmission lines. That means sudden start-up of

induction loads could cause a transient drop in voltage but if power factor correction is

properly done there would be less reactive power loss and hence no voltage drop.

Also knowledge of the power factor is vital when calculating the minimum up time of a

DPI as shown in Phase 1 report. If the power factor were to suddenly drop or increase

then the up time of the DPI would obviously change and put drives that are assumed to be

protected under risk of damage.

The mine has two capacitor banks connected to the 11kV busbar at the Northern Well

fields where it pumps its own water supply. The primary reason of the installation of such

was the fact that a lot of induction motors are used to pump the water and this in turn

made the power factor lagging and hence needed some form of correction.

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The SVC can also aid in correcting the power factor – see Section 9.3

7.6 Soft Starting Large Induction Motors

The use of soft starting methods such as autotransformers can reduce the inrush current

which can be up to 7 times full load current which would otherwise cause a drop in

voltage. Soft starters control the current delivered to a motor, which then controls the

torque and hence the motor accelerates smoothly.

Where constant speed is required during operation of a motor then soft starter is a better

choice over VSDs as they are much cheaper and less complex. Moreover VSDs may

introduce harmonic distortion on the power network.

Selective start of critical loads should be done both at plant start up and after a voltage

dip/failure to ensure that minimum current is drawn from the supply to avoid a condition

whereby several voltage dips ensue one after the other indefinitely.

7.7 Earthing

Earthing is another aspect of power quality that needs to be fully addressed to limit the

occurrence of voltage dips within the mine. Earthing is useful as it means that current

flow shall be limited to pre-defined nominal levels even in the case of a fault (maximum

fault current) and that no voltage dip shall be encountered in the most ideal operation of

the earth fault protection system employed.

The earthing requirements as stipulated by the Anglo American Corporation of RSA for

Debswana Diamond Company (Pty) LTD in April 1997 are as shown in [Appendix A7].

The document however, does not explicitly lay out the type of earthing to be used or the

reason of choice thereof. Two types of possible earthing methods are hereby discussed.

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7.7.1 Neutral Earthing Resistor

This is usually for protection against S� to

neutral faults – Figure 7.4. NERs are used for

resistance grounding of industrial power

systems. They are usually connected between

earth ground and the neutral of power

transformers, power generators or artificial

neutral transformers. Their main purpose is to

limit the maximum fault current to a value

which will not damage generating, distribution

or other associated equipment in the power

system, yet allow sufficient flow of fault

current to operate protective relays to clear the fault.

The following is needed to specify a neutral earthing resistor:

1. The line-to-neutral RMS voltage

2. The initial current in amperes.

3. Allowable "time on" - defined as the length of time that the line-to-neutral

voltage can be applied without exceeding the allowable temperature rise.

For extended time ratings (greater than 10 minutes) the allowable temperature rise is

610°C. For continuous ratings the allowable temperature rise is 385°C.11 A Neutral

Earthing Resistor restricts the flow of current during an earth fault on an ac distribution

system. Although a neutral earthing resistor will probably be active for just a few seconds

during its operational life, it must offer dependable protection at all times in case of

fault.12

11 www.fortressresistors.com/neutral_earthing.htm, accessed 11-Aug-06 12 http://www.irescoindia.com/neutral.htm, accessed 11-Aug-06

Figure 7.4

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7.7.2 Neutral Earthing Compensator

This is for protection against phase

to phase faults – Figure 7.5. The

NER could also be connected via

reactances in a delta system, which is

part of the NEC network. Intuition

says that when there is a fault, then

the voltage should collapse, but often

faults can cause voltage increases of

up to many times operating voltage,

especially when reactive NECs are

used.13

This type of grounding is often referred to as resonant grounding system. Figure 7.5

shows one way of connecting a resonant grounding system. When the system capacitance

is matched by the inductance of the coil, the system is fully compensated, or at 100%

tuning. If the reactor inductance does not match the system capacitance, the system is off

tuned. It can be over- or under compensated, depending on the relationship between

inductance and capacitance.14

Resonant grounding a system can reduce the ground fault current to about 3 to 10 percent

of that for an ungrounded system. For 100% tuning, the active coil losses, system

harmonics, and system active leakage current determine the fault current magnitude.

Residual current compensation methods inject a current through the reactor to the system

during the fault, reducing the fault current almost to zero14

13 NEIL JEFFREY, Dynamic Range Requirements of Capacitor VT Sensors for Protection Systems, SURE Engineering CC, 23 March, 1999 14 JEFF ROBERTS, DR. HECTOR J. ALTUVE, AND DR. DAQING HOU, Review of Ground Fault Protection Methods For Grounded, Ungrounded, and Compensated Distribution Systems, Schweitzer Engineering Laboratories, Inc.

Figure 7.5

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7.8 Reduce Equipment Sensitivity

Figure 7.6

In most cases the Mine can not do much as far as making its equipment less sensitive to

dips is concerned but it can ensure that it buys equipment that has a higher sensitivity

margin. For example semiconductor device should conform to the SEMI F47 standard

and should be able to tolerate levels indicated in the ITIC curve. This will ensure that at

least most power electronic equipment within the mine can ride through less severe

voltage dips. In Figure 7.6 the lower graph indicates tolerable levels for voltage dips –

Power Electronic Equipment should not be affected by dips that last less than 0.5s if the

voltage does not falls below 70% off nominal or dips that last more than 0.5s but less

than 10s if the remaining voltage stays above 80% of nominal. Note that if the voltage

stays at 80% of nominal for more than 3s then the condition is an under voltage.

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8 Compensate Missing Voltage

This chapter analyses dip mitigating options by means of compensating the voltage with

energy storage devices. A full analysis is discussed in terms of principle of operation,

Cost analysis, Technical analysis and a possible connection on the mine site. The energy

storage systems discussed in this section all need some power electronic circuitry for

switching to the network. This chapter suggests one possible way of doing such a

connection.

8.1 BESS - Battery Energy Storage System

A battery modules’ basic building

block is the electrochemical cell as

shown in Figure 8.1. A number of

electrochemical cells are packaged

together to form a battery module. The

battery modules are connected in a

matrix of parallel-series combination

to form a string. A string may be

formed to deliver the required voltage

which may range from a few hundred

volts up to approximately 2,000 volts.

A 1-MW/l –MWh BESS discharged at

1MW will be able to supply the entire 1MWh of stored energy over a 1-hour period.

However, if discharged at a 2-MW rate, the battery will operate for less than half an hour,

delivering less than 1MWh of energy in the process. The life of a battery is affected by

the manner in which it is operated. The cycle life (the number of charges and discharges

it can perform) of a battery is highly dependent on the depth of discharge, with deep

discharges (>70-80 percent) significantly reducing its cycle life. Batteries also have shelf-

life limitations.

Figure 8.1

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8.1.1 Cost Analysis

The current cost of one-to two-hour BESS ranges from US$1200 to 1500/kW.15

However, for the purpose of mitigating against voltage dips compensation of up to 1 hour

is not required. A 2MW - 10s BESS designed specifically as a power quality system is

estimated at US$450/kW which is BWP 2,892.5116 on 3 October 2006.

8.1.2 Technical Analysis

Advantages:

• Cheaper than a comparative SMES unit but as batteries require regular

maintenance (similar way to UPS) it may prove more costly to maintain that a

DPI of similar compensation time/magnitude.

• Capable of high power output for a short time.

Disadvantages

• Requires further electronic components after installation for a faster switch over

time (such as the SSTS as outlined below).

• Might not be able to sustain power for repetitive voltage dips due to sustained

faults whereby load is reconnected by ARC.

15 ABBAS AKHIL, SHIVA, SWAMINATHAN, RAJAT K. SEN, Cost Analysis of Energy Storage Systems for Electric Utility Applications, February 1997 16 http://www.xe.com/ucc/convert.cgi, accessed 3 October 2006

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8.1.3 Connection

Figure 8.2

Small BESS units could be installed at the most critical processes in the mine just as the

current UPS systems which are used to compensate voltage for measuring instruments

and PLCs.

The BESS should be used in conjunction with the SSTS (Figure 8.2) as defined in

Section 9.1 to reduce the time it takes to switch from normal supply in the event of a dip.

8.2 SMES- Superconducting Magnetic Energy Storage System

A SMES system, designed to improve

the power quality for critical loads,

provides carryover energy during

voltage sags and momentary power

outages. The system stores energy in a

superconducting coil immersed in liquid

helium.

The superconducting device stores

energy within a magnet created by the

flow of direct current in a coil of

superconducting material. To maintain the Figure 8.3

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coil in its superconducting state, it is immersed in liquid helium contained in a vacuum-

insulated cryostat. In standby mode the current continuously circulates through the

normally closed switch. The power supply provides a small trickle charge to replace the

power lost in the non superconducting part of the circuit. When voltage on the capacitor

bank on the dc side of the inverter drops during a dip, the normally closed switch opens

and current from the coil immediately flows into the inverter – Figure 8.3.

When the voltage across the capacitor returns to a preset level, the switch closes. The

sequence repeats until voltage from the utility feeder is restored. In many cases the device

can be connected to the dc bus of an existing VSD thus eliminating the need to provide a

rectifier and an inverter15 above

Test data by The Materials and Manufacturing Directorate and the Power Conditioning

and Continuation Interfacing Equipment showed over 100 events caused by lightning or

power dips that resulted in voltage sags greater than 10%. It has also shown that SMES

systems can effectively protect against dips. Field testing of the Intermagnetics General

Corporation (IGC) SMES by the PCCIE Office indicated that this unit is capable of

providing conditioned power with an output voltage and current total harmonic distortion

of less than 4% 17

8.2.1 Cost Analysis

A commercial 2.2-kWh SMES unit developed by Superconductivity, Inc., suited for

industrial power quality applications, is estimated to cost US$2.4 million – equivalent to

BWP 15,426,720.0018 on 3 October 2006. It has the ability to protect customers from

momentary outages, voltage dips/surges, and its ability to correct harmonic distortions

and power factors. The cost of the storage component is $700,000 and the Power

Conversion System (PCS) cost is estimated at $300/kW.19

17 http://www.afrlhorizons.com/Briefs/Dec01/ML0009.html, accessed 7 September 2006 18 http://www.xe.com/ucc/convert.cgi, accessed 3 October 2006 19 Abbas Akhil, Shiva, Swaminathan, Rajat K. Sen, Cost Analysis of Energy Storage Systems for Electric Utility Applications, Sandia Report, February 1997

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A cost analysis could be done such that if smaller units are more economical than one

large unit then several units could be installed at appropriate plants only to the most

critical machines in much the same way as the current scenario with DPI that are already

installed in the mine.

8.2.2 Technical Analysis

Advantages:

• Time delay between charge and discharge is short

• High power output for a brief period of time

• Low resistance hence low power loss

• Main components motionless hence increased reliability20

Disadvantages:

• High cost of superconducting wire

o US$1920/kW for an SMES rated at 1000MW21

• Health hazard –

o Possible leak of the liquid nitrogen and

o Extremely High magnetic field that would probably require a buffer zone

to protect mine personnel and wildlife

8.2.3 Connection

A SMES unit could be installed in much the same way as the current diesel standby

generators at the BPC main switchyard to Jwaneng mine. The function would be almost

the same as well, except the SMES would be capable of faster switch over times (unlike

with the generators which are manually started), it would compensate to the entire mine

supply grid (the current generators only supply critical processes) but with the setback of

higher capital costs.

20 http://en.wikipedia.org/wiki/SMES, accessed 31 August 2006 21 http://www.parcon.uci.edu/OLD_WEBSITE/paper/eeenergy.htm, accessed 7 September 2006

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Figure 8.4 – SMES Connection

Figure 8.4 shows a possible connection of the SMES unit feeding on to busbars A at the

main switchyard. Only critical loads will be left online when the SMES unit is operating

to ensure a longer operating time and to avoid overloading.

The South African supply utility has an SMES installed at the South Africa Pulp and

Paper Industry (SAPPI) paper mill located in Stanger, South Africa mainly for the

purpose of eradicating the bad effects of voltage dips.

The SMES has been successful in 16 months of use, giving 100% protection against 139

sags, half or more of which previously would have shut down the plant.22

8.3 UPS

UPS may be used to protect low power sensitive microprocessor based equipment such as

computers (including mail/web servers), PLCs, meters and SCADA systems against

minor (both magnitude and duration) power dips.

UPS can be connected to a network as Standby (offline), Delta conversion online, Dual

conversion online or Ferro-resonant. In standby mode the UPS may drain the battery and

switch off even when line voltage is still present whereas the dual and the delta

22 http://findarticles.com/p/articles/mi_qa3739/is_199903/ai_n8850296 accessed 7 November 2006

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conversion online are not very efficient. Connection under Line Interactive mode is the

most efficient.23 below

8.3.1 Principle of Operation

Line interactive UPS units are designed so that the inverter is always connected to the

output of the UPS. When line power is present, the inverter operates in reverse to charge

the battery. When utility power fails, the UPS reverses the power flow from the inverter

and provides power to the load. This design provides better filtering than a standby unit

because the inverter is always connected to the load. Line interactive units typically will

incorporate an automatic voltage regulator. AVR allows the UPS to effectively step-up or

step-down the incoming line voltage without switching to battery power. This allows the

UPS to correct most long term over-voltages or under-voltages without draining the

batteries. Another advantage is that it reduces the number of transfers to battery which

extends the lifetime of the batteries.

Line-interactive UPS units are the most common design for units in the 0.5 kVA to 5

kVA range. They are typically used in small server environments.23 The following are

some of the factors to consider when choosing a UPS;

• VA and wattage ratings - type of load and maximum

• Runtime – amount of load and battery

• Battery replacement

8.3.2 Case Study - Synchrotron Radiation Source

The Synchrotron Radiation Source (SRS) is the UK’s only dedicated source of high

energy synchrotron radiation. It is required to run 24 hours a day. At the heart of the

system are two 300kVA UPS of the continuous on-line type. The UPS allows a

maximum output current of 417 Amps. If however the load had a unity power factor, this

would have de-rated the UPS to a maximum current of 334 Amps. To optimize the size

23 http://en.wikipedia.org/wiki/Uninterruptible_power_supply, accessed 17 August 2006

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of UPS it was necessary to balance the load

wherever possible and to use the highest

phase current measured.24 This system is

shown in Figure 8.5

When the commercial supply is interrupted,

the internal batteries sustain the power to the

inverter until either the mains returns or the

Diesel generator operates. If the Diesel fails

to start then the mains must return within 6

minutes on maximum load before UPS

shutdown occurs. In the event of this happening the battery contactor will be opened and

a clean break achieved.24

Figure 8.6

8.3.3 Case Study - Uses in the mine

Present use of UPS within Jwaneng Mine is for protection of instrumentation for control

electronics including PLCs, PCs, Power meters and other sensitive small power

electronic equipment. The nameplate ratings and the connection of the UPS units

installed at the DMS in the MTP are as shown in the table and Figure 8.7.

24 S A Griffiths, D E Poole, AN UNINTERRUPTIBLE POWER SUPPLY FOR THE SRS, CLRC Daresbury Laboratory

Figure 8.5

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Table 8.0 - Nameplate Ratings of UPS at MTP DMS

INPUT – 3 Phase ac OUTPUT – 3 Phase ac

Voltage (V) 380 ± 15% 220 ± 1%

Power (kVA) 12.4 10

Frequency (Hz) 50 50

Current (A) 19 45

The bypass switch shown in Figure 8.7 will disconnect the supply to allow the IPDB and

the rectifier to be supplied by the UPS in the event of a voltage dip or surge. The UPS

will therefore compensate voltage in the event of a dip as long as the magnitude of the

dip is not below 15% of nominal voltage as shown in Table 8.0. If the power dip is severe

and the magnitude of the incoming voltage falls below 15% of nominal then the UPS trip

and hence cut off supply to its connected loads and a manual switch over has to be

carried out once all faults have been cleared.

UPS requires regular maintenance of its

battery packs but every time this is done

its loads are left vulnerable directly

connected to the supply. This could

however be ratified by connection in what

can be termed the master and slave system

using two similar UPS units such that one

will be on standby to supply the load when

the other unit fails or is on maintenance.

A larger UPS could be used to keep contactors closed in the event of short, low

magnitude power dips which would not cause any damage on the protected loads

especially on the most critical ones. The contactors will therefore stay closed for a

predefined time and dip magnitude as long as the connected drives are still functioning

normally until the voltage stabilizes to nominal.

Figure 8.7

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However with a larger unit and the current diesel generators that is at the main

switchyard and connection could be made for the most critical processes in the same way

as shown in Figure 8.5 and 8.6 above. This setup could help critical loads ride

uninterrupted through short power dips.

UPS can however cause a power quality problem referred to as flat

topping as shown in Figure 8.8. This normally occurs with

transformer-based UPS units that overload in current.

Standby UPS systems can also cause a frequency shit in the voltage

sine wave as shown in Figure 8.9 at the time of switching on.

8.4 Flywheel Energy Storage

FES works by accelerating a rotor to a very high speed and maintaining the energy in the

system as inertial energy. The rotors normally operate at 4000 RPM or less. Quick

charging is done in less than 15 minutes. Long lifetimes of most flywheels, plus high

energy densities and large maximum power outputs are positive attributes [Appendix

A8]. The energy efficiency of flywheels can be as high as 90%. Since FES can store and

release power quickly, they have found a niche providing pulsed power.25 Energy is

stored in the rotor in proportion to its momentum, but the square of the angular

momentum. The kinetic energy stored in a rotating flywheel is:

Where

• � is the angular velocity, and

• I is the moment of inertia of the mass about the centre of rotation. (dependant on

the shape)

25 http://en.wikipedia.org/wiki/Flywheel_energy_storage, accessed 28 September 2006

Figure 8.8

Figure 8.9

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Figure 8.9a

The dc-voltage output of the motor/generator set has to be conditioned by a typical power

conversion system to interface with the external supply and load.

8.4.1 Cost Analysis

American Flywheel Systems Inc. has estimated the direct cost (excluding overheads) of a

1000kWh/l00kW flywheel system at $200/kWh equivalent to BWP 1,285.5626 on 3

October 2006. This estimate includes the cost of the rotor, shaft structure,

motor/generator, bearing, cooling, vacuum assist, containment, and system

assembly/installation.27

Advantages: Flywheels store energy very efficiently (high turn-around

efficiency) and have the potential for very high specific power compared

with batteries. Flywheels have very high output potential and relatively

26 http://www.xe.com/ucc/convert.cgi, accessed 3 October 2006 27 Abbas Akhil, Shiva, Swaminathan, Rajat K. Sen, Cost Analysis of Energy Storage Systems for Electric Utility Applications, Sandia Report, February 1997

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long life. Flywheels are relatively unaffected by ambient temperature

extremes.28

Disadvantages: Current flywheels have low specific energy. There are

safety concerns associated with flywheels due to their high speed rotor and

the possibility of it breaking loose and releasing all of its energy in an

uncontrolled manner. Flywheels are a less mature technology than

chemical batteries, and the current cost is too high to make them

competitive in the market28 above

8.5 Power Conversion System

8.5.1 Proposed System

Figure 8.9b below shows one possible way of connecting the ES to a critical load. The

control is divided into two loops. The inner loop provides high speed regulation and may

contain voltage control circuitry. The outer loop is slower and may just be a time clock

that schedules the charge and discharge times so as to coincide with the system peak and

low load periods respectively

28 http://www.upei.ca/~physics/p261/projects/flywheel1/flywheel1.htm accessed 6 November 2006

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Figure 8.9b

8.5.2 Cost of the PCS

The average cost of a PCS is usually about US $300/kW. The cost of the PCS installed

for the BES project at PREPA shown in Table 8.1 was US$ 5 713 000 and uses self

commutated GTOs which are capable of 4-quadrant operation

8.6 Case Study – Cost Breakdown of SMES and BES

The following table shows existing plants that use SMES and BES showing the cost in

US$.

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Table 8.1 – Cost Comparison of SMES and BES

Project/Product Description of System $/kW $/kWh 000s of $

Prepa 20MW, 40MWh BES 1102 1574 22042

Anchorage

Municipal L&P

30MVA, 375kWh SMES 1467 117333 44000

Source: Abbas Akhil, Shiva, Swaminathan, Rajat K. Sen, Cost Analysis of Energy Storage

Systems for Electric Utility Applications, Sandia Report, February 1997

For Jwaneng mine average demand was 32.34MW [Appendix A10] in the period from

January to October with a peak at 34.52MW.

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9 Smoothing out Voltage Profile

Discussed herein is the use of switches to successfully transfer load to an alternate supply

with SSTS or change taps on a transformer by SETC during a voltage dip. Also discussed

is the SVC which provides continuous reactive power compensation. This chapter

suggests a point a connection on the mine’s electrical reticulation system for both the

SSTS and SVC.

9.1 SSTS - The Solid State Transfer Switch

The SSTS essentially consists of a pair of thyristor switch devices. However, a thyristor

is not a pure conductor and raises some issues in terms of power loss and cooling. In a

conventional SSTS, line current flows in the thyristors continuously, causing a great deal

of power loss and element heating during normal operation. As a result, relatively large

cooling equipment is required which imposes additional operating costs on the user in

order to maintain thyristor cooling. It also results in reduced efficiency and lower. The

conventional SSTS is capable of transferring the load to an alternate supply in 4ms (38kV

phase, 1200A)29

9.1.1 The Hybrid SSTS

The hybrid switch device (Figure 9.1)

essentially consists of a pair of thyristors

and a high-speed mechanical parallel

switch which has an opening time

capability of less than 1 (One)

millisecond.30

The SSTS employs a secondary

29 POHJANHEIMO P., A Probabilistic Method for Comprehensive Voltage Sag Management in Power Distribution Systems, Doctoral thesis, Helsinki University of Technology, 2003 30 MASATOSHI TAKEDA, PH.D. HIROSHI YAMAMOTO GREGORY F. REED, PH.D. TOMOHIKO ARITSUKA ISAO KAMIYAMA, Development of a Novel Hybrid Switch Device and Application to a Solid State Transfer Switch

Figure 9.1

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independent feeder with sufficient capacity in parallel with a primary line that the load is

connected to. When auto reclose is initiated on the primary due to a fault the SSTS

immediately transfers the sensitive load to the secondary supply within milliseconds.31

The SSTS can take about 0.4ms to switch over with a current of 750A. The time it takes

to switch between the PS and the TH is as shown in the Figure 9.2.

Figure 9.2

9.1.2 Connection

With the introduction of the Morupule-

Thamaga 220kV line in December 2006

as shown in Figure 9.3, SSTS could be

used with the existing lines on the

Thamaga-Jwaneng transmission such

that only one line transmitting at 220kV

(and capable of 40MW) is used at any

given time and the other is taken as a standby

supply.

This arrangement has to be negotiated with the supply utility to see if the current lines are

capable of such a revamp. Installation is at the discretion of the supply utility and the

mine should not incur any charges whatsoever.

31 J. D. MASTERS, Dowding Reynard & Associates (Pty) Ltd, Voltage Dip study at Jwaneng Mine-addendum, 13 March 1997,

Figure 9.3

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Figure 9.4 Figure 9.5

9.1.3 Technical Analysis

SSTS generally requires the following;

• Two feeders from different substations (or even transformers on same line)

• Spare distribution capacity in the backup feeder

• Spare distribution capacity in the substation

• Reliable transmission with good power quality

One major disadvantage of this scheme however is that it reduces the contingency plan of

the supply of having more than one line supplying a load such that one is left online if

one goes faulty. Having the SSTS as close as possible to the mine’s main switchyard

would eliminate this problem (Any transmission lines that go beyond the mine should be

connected before the SSTS as illustrated in Figure 9.4). Village 1 & 2 transmission lines

on the diagram refer to the 2 Jwaneng Township 33kV lines.

Another main factor to consider is that it should be insured that the alternate line (supply)

is not at all affected by disturbances in the primary supply. However if the grid is strong

enough it may be adequate to install the SSTS such that it transfers the load from one

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transformer to the other as shown in Figure 9.5. This arrangement will compensate for all

dips whose cause was on the MV feeders. The setup will be rendered useless for any

faults occurring on the HV side (before either one of the transformers).

This setup is applicable at the MTP DMS’ 2MVA Transformers.

9.2 SETC - Static Electronic Tap Changer

It is often useful to have a number of taps on power

transformers such that the secondary voltage can be varied

accordingly. More often than not these transformer taps are

manually set and left at a certain tap where the nominal

secondary voltage will be defined by switch S as shown in

Figure 9.6 The transformer T4 at Jwaneng Mine’s main

switchyard is set on an appropriate tap manually such that the

power generated by the standby diesel generated can be

correctly synchronized to the grid to match the supply utility

(BPC) voltage. The static electronic tap changer can be used on the transformers T1 and

T2 such that secondary voltage is closely monitored and as soon as a dip from nominal by

more than 10% is seen then the taps are changed automatically to increase the secondary

voltage back to nominal until a time when the cause of the dip (fault) has been cleared.

However allowance should be made for some devices such that an expected voltage drop

should not initiate operation of the SETC prematurely. For example if starting of an

induction motor is expected to drop the voltage and recover within 100ms without

affecting any equipment in the vicinity then the SETC can operate only if the voltage

drop goes below 10% (or a predefined limit) for more than 100ms provided this is also

within the required grading margin for protection and discrimination. This is illustrated as

TD (Time Delay) in the diagram below; (LTC is load tap changer, same as SETC)

Figure 9.6

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Figure 9.7

The switch employed by the SETC uses IGBT which can have a switch over time of

about 300 ns32

9.3 SVC - Static VAr Compensator

A static VAr compensator (or SVC) is an

electrical device for providing fast-acting

reactive power compensation on high-voltage

electricity transmission networks.

Figure 9.8 shows a simplified single-line

diagram of a Static VAr compensator

featuring one thyristor-controlled reactor and

three mechanically switched capacitors.

9.3.1 Principle of Operation of the SVC

32GOOGLE CACHE, http://64.233.183.104/search?q=cache:t-Ff7OuPhW0J:www.members.lycos.nl/caspoceducation/0899.pdf+static+electronic+tap+changer&hl=en&gl=uk&ct=clnk&cd=14, accessed 18 September 2006

Figure 9.8

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Figure 9.9a

Typically an SVC comprises a bank of individually switched capacitors in conjunction

with a thyristor-controlled air- or iron-cored reactor. By means of phase angle modulation

switched by the thyristors, the reactor may be variably switched into the circuit, and so

provide a continuously variable MVAr injection (or absorption) to the electrical network.

Coarse voltage control is provided by the capacitors; the thyristor-controlled reactor is to

provide smooth control - Figure 9.9a. Chopping the reactor into the circuit in this manner

injects undesirable odd-order harmonics, and so banks of high-power filters are usually

provided to smooth the waveform. Since the filters themselves are capacitive, they also

contribute to the net MVAr injection. Other arrangements such as a thyristor-switched

reactor and thyristor-switched capacitors are also practical. Voltage regulation is

provided by means of a closed-loop controller. Remote supervisor control and manual

adjustment of the voltage set-point are both possible – Figure 9.9a.

The three phase SVC unit is a continuously variable reactive power compensator that

connects directly to the utility grid, and operates in a stand alone mode without the need

for any external power sources such as a battery bank or diesel generator. It can feed

leading or lagging currents to compensate for the reactive currents generated by such

appliances as grid feeding motors, generators, wind turbines or any other device having a

reactive power component. The SVC can operate in both quadrants and is not limited to

compensating the inductive reactive component of motors or generators.33 below

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The SVC by Advanced Energy Systems uses their current control technology for power

conversion, and draws only a small amount of real power from the grid and outputs either

a positive or negative reactive power. The SVC has its own micro-controller that

performs low-level control and monitoring of the system, and implemented within the

micro-controller is AES's patented "Ramp time" current control algorithm. This

algorithm is used to control the power electronics of the current control mechanism. 33

below

9.3.2 Technical Analysis

Some of the reasons for incorporating SVC in transmission and distribution systems

include:

• To stabilize voltage in weak systems.

• To reduce transmission losses.

• To increase the transmission capacity, thus delaying the need for new lines.

• To increase the transient stability limit.

• To increase damping of small disturbances.

• To improve voltage control and stability.

• To damp power swings. 33

Series compensation is an economical method of improving the power transmission

capability of the lines. Series capacitor banks are useful because they;

• Increase power transfer capability,

• Improve system stability,

• Reduce system losses,

• Improve the voltage profile of lines,

• Optimize current sharing between parallel lines.

33 http://www.aesltd.com.au/whatwedo/docs/svc_over.htm - 1 September 2006

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The cost of a series capacitor bank is about 10% of the cost of a new transmission line.

Thus, the payback time for a series capacitor bank investment is typically only a few

years.34

The SVC can be operated in two different modes either in voltage regulation mode (the

voltage is regulated within limits) or in VAr control mode (the SVC susceptance is kept

constant). For the protection against dips the voltage regulation mode would be the

preferred mode of operation.

The main advantage of SVCs over simple mechanically-switched compensation schemes

is their near-instantaneous response to changes in the system voltage. For this reason they

are often operated at close to their zero-point in order to maximize the MVAr reserves

they can rapidly provide when required. SVCs are in general cheaper and require lower

maintenance than dynamic compensation schemes such as synchronous compensators.35

The SVC can also aids in actively correcting the power factor at the mine to the desired

level.

9.3.3 Connection

Ideally the best point for connection of an SVC is either on

the 132kV Thamaga to Jwaneng or the 220kV Segoditshane

to Thamaga transmission lines. This is where most of the

faults that have led to voltage dips experienced by the mine

originate. It is therefore vital that talks are held with the

supply utility on the quality of the power they supply to

Jwaneng mine and consideration of the supply contract has

to be made such that they can be asked to implement the

SVC on the transmission lines. This would be the best

approach as a solution to the problem of power dips as

34 http://www.power-technology.com/contractors/tandd/nokian/ - accessed 28 August 2006 35 "http://en.wikipedia.org/wiki/Static_VAr_compensator" accessed 11 August 2006

Figure 9.9b

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it means the mine will not incur any costs whatsoever on the installation process.

However as there is an ongoing research on optimization of the mine’s power factor,

SVC could also be considered as one possible solution – killing two birds with one stone.

This would basically call for re-using the capacitor banks already in place on the network

at the Northern well fields but adding on the extra functionality to make an SVC (See

Figure 9.8).

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10 General Power Quality Mitigation

Power Quality issues are mentioned in Section 1.2.1 as being separate entities. On the

other hand these issues can directly affect the Quality of Supply and therefore if not

attended to they can increase the occurrence of voltage dips. In most cases resolving one

power quality issue directly impacts on the other issues thereby optimizing general power

quality. Some power quality aspects are thereby briefly discussed in this chapter. The last

part proposes a new way of recording of voltage dips to improve on the methods

mentioned in section 2.

10.1 Ferranti Effects

A long transmission line draws a substantial quantity of charging current. If such a line is

open circuited or very lightly loaded at the receiving end, the voltage at receiving end

may become greater than voltage at sending end. This is known as Ferranti Effect and is

due to the voltage drop across the line inductance (due to charging current) being in

phase with the sending end voltages. Therefore both capacitance and inductance are

responsible to produce this phenomenon. The capacitance (and charging current) is

negligible in short line but significant in medium line and appreciable in long line.

Therefore this phenomenon occurs in medium and long lines.36

The SETC mentioned in Section 9.2 can be used on the transformers at the mine’s main

switchyard on the 132/33kV and the 132/6.6kV to avoid this effect. During maintenance

when large loads have to be switched off it should be done in a systematic manner and

the supply utility should be informed so that they can regulate their system accordingly.

10.2 Blackouts (Interruptions)

The causes of blackouts are sometimes similar to those that cause voltage dips. To ensure

that some of these unwanted interruptions are avoided generation should be closely

36 http://www.ku.edu.np/ee/rb/Handouts_COEG301&303/Ferranti%20Effect.pdf, accessed 14 August 2006

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matched to demand so as to avoid overloading the transmission network. This requires

close cooperation of both the supply utility and the mine optimising distribution and load

respectively. The supply utility should also arrange for a systematic and controlled load

shedding when there is a fault such that mine’s supply can be sustained at optimal levels

at all times. The mine should ensure that they have a reliable standby supply with spare

distribution capacity.

10.3 Surges (Swells)

This is mostly caused by lightning and can sometimes occur after a voltage dip when the

system tries to recover under very low load. Surge arresters can be used on light loads to

avoid damage. UPS generally protects against this as well because it smoothens out its

output. For larger loads protection settings should be set to trip and isolate appropriately.

10.4 Over voltages & under voltages

The normal operating voltage at Jwaneng Mine is about 105% on the 6.6kV lines and

about 110% on the 525V network. The higher percentage that nominal (10% is the upper

bound of optimal supply) may be caused by miscalculation on transformer tap settings.

10.5 Harmonics & Interharmonics

These are mainly caused by static frequency converters, cyclo converters, induction

motors and arcing devices. Interharmonics have the effect of inducing visual flicker on

display units. Power line carrier signals are also referred to as interharmonics.

Mitigation of harmonics by active filters or a tuned filter as mentioned in Section 7.4 can

prove quite effective

10.6 Notches

This is caused mainly by power electronics devices when the current is commutated from

one phase to another during the momentary short circuit between two phases. Frequency

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components associated with notching can be very high and measuring with harmonic

analysis equipment may be very difficult.

Similarly to mitigation of harmonics, notching can be avoided by using active power

filters or tuned filters.

10.7 Voltage Fluctuations (Flicker)

There is usually a visible impact of voltage fluctuations on lamps as they flicker. One of

the main causes is arc furnaces. This particular problem has not been reported as a cause

of head feed delays at Jwaneng mine probably because the installed DPIs and UPS help

in minimising voltage fluctuations.

10.8 Recording & Reporting Of Power Dips (Proposed)

Power dips need to be recorded and carefully documented in order to establish their

causes, effects on the mine and ways to mitigate against them.

This part hereby outlines a procedure which is a guideline in order to record power dips.

The procedure applies to all Engineers, Planning Foremen, Engineering Foremen,

Operation Foremen, CCR Operators, Supervisors and all to whom power dips is a

problem who are tasked with ensuring that accurate and correct recording is done. The

revised procedure was approved by the mine personnel and has been posted on the

intranet so that it can be used whenever there is a power dip.

10.8.1 Procedure

Whenever a power dip occurs in any plant area the information should be recorded on the

form provided in [Appendix A9]

The information should be forwarded to the Section Engineers both at Electrical Services

and MTP by fax or e-mail at least within twenty-four (24) hours of the power dip and

should conform to the following notes;

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i. Details of Occurrence must be cross referenced to CCR reports

ii. Duration of delays is reported by CCR and should include all production

downtime (including start up time of the plant)

iii. Comment on extent of production loss should be obtained from TREATMENT

and should include tons/hour treated before onset of dip, FeSi loss etc.

iv. If cause of dip is not clearly known then any electrical fault seen on the line (or

plant) at the time of dip can be recorded as probable cause

v. NCC (or protection engineers at BPC) should be contacted for their input (if the

dip registered on their system) any transmission faults should also be noted

10.8.2 Follow up Action/ Feedback

Follow-up action is the most important part of this recording procedure and should be

updated latest by the following day and be reported at the morning production meetings.

Section Engineer at Electrical Services should update form for note (v) below;

i. Dip magnitude and duration can be obtained from BPC power meters at the main

switchyard (NOTE: BPC downloads data every month) if the dip registered on the

transmission network otherwise voltage profiles downloaded from the mine yards

(substations) can be used.

ii. Fault type should also state which protection operated e.g. Current differential, 3�

trip and ARC/Lock Out

iii. Comments on what NCC is doing about the problem should be registered (for

faults on the transmission network)

iv. Total load shed should be cross checked with BPC and follow up can be done

with voltage (power) profiles

v. Treatment Unit should also state if they have done anything to resolve the

problem and where in the plant they are most severely affected.

10.8.3 Responsibilities

It is the responsibility of Section Engineers (EE1 and EE2) to ensure that they follow up

the power dip form immediately after there was a power dip. Engineering foremen and

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Operation foremen should ensure that their respective parts are filled and forward the

form to the appropriate recipient well on time as stated in section 10.8.1. EE1 and

Electrical Services should ensure that the BPC part is filled up.

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11 Discussion

This chapter analyses the research presented in this report and report phase 1 and

thereby concludes Analysis of Voltage dips for Jwaneng Mine and proposes future work

on this field in the quest to exterminate the effects of dips.

11.1 Project Objective Review

The main objectives for the research of this project were to;

• Identify causes of voltage dips in the Mine

• Investigate possible solutions to the problem

The objectives for this project have been successfully achieved, the causes,

characteristics and effects of voltage dips on different operations within Jwaneng mine

were established the bulk of which was presented in report Phase 1 and several possible

solutions were recommended in this report.

11.2 Conclusions

It was reported in the report for this research phase 1 that the recording and reporting of

power dips is inadequate but new light into the research is that there is a balanced score

card being carefully updated and both magnitude and duration of any dips that register at

the main switchyard can be recorded. However this information is not being made

available to electrical workshops. Power Meter readings should be put on the internal

servers such that Electrical Workshop foremen and Section Engineers at various parts of

the mine can monitor general power quality.

Power Factor Correction capacitors and UPS units may sometimes contribute to

Harmonics hence ways to mitigate against this should be investigated.

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There is no general transparency of information that is usually dished out to the mine. For

the mine to validate the quality of supply they would have to use the supply utility’s

records which can be biased. Contracted companies do not usually share with the mine

what they call their “trade secrets”. Some examples are the two following companies;

• Netlab on protection and discrimination settings

o The company comes to the mine, does their feasibility studies, and

recommends new settings which they put up and more often that not the

drives still trip after they leave.

• Switching Systems on DPI settings

o The time settings for the DPI are not set by personnel on the site but these

can not be left at only one setting when the load changes or even when the

power factor changes.

It was indicated in phase 1 report that the mine incurs no charges for a bad power factor,

however, even though this may be true, new light has been shed into this research that the

mine is contractually bound by the supply utility to keep its power factor above 0.85 as

stated in the mine quarries act.

For the DPI currently installed within the mine, MTP and RP have expressed satisfaction

on reliability but Red Area plant still has problems with protected loads tripping.

Comparison of the balanced score card from 2005 with the one for 2006 shows that no

significant improvement has been made in minimising head feed delays due to power

dips or cutting down on the frequency of their occurrence.

More often than not SMES is usually discarded as being way too expensive for the mine

environment – solid evidence needs to be brought up to back this up and compare the

revenue losses that may be encountered to the capital and maintenance costs of the

SMES. The same applies to the other Energy storage devices. The mine should request

quotations for each of these.

Page 87: Voltage Dip

Transient Analysis of Voltage Dips - 70 --

_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

11.3 Future work

Asses how introduction of the 220kV transmission line from Morupule affected the

system – does it improve the fault level rating significantly and does this directly reduce

the occurrence of dips that the mine experiences?

Commissioning of Mmamabule coal plant proposed for 2010 – how will this impact on

the transmission system?

Feedback should be sought on the improvement on the recording procedure as suggested

in this report.

Analysis of reliability and effectiveness of the chosen power dip mitigating solution by

the mine

The list of critical loads mentioned in chapter 8 should be updated to reflect the current

scenario at all times.

There is a new plant to be built to replace the current MTP, Load may change

considerably. A further investigation should be carried out to see how this is going to

affect the quality of supply.

Page 88: Voltage Dip

Transient Analysis of Voltage Dips - 71 --

_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

References

[1] www.powerstandards.com/tutorials/sagsource.htm, accessed 14 June 2006

[2] http://www.power-innovations.com/about_power/dyk.html accessed 6 November

2006

[3] http://www.pur.com/pubs/1251.cfm accessed 7 November 2006

[4] BYE LAW #31, [CAP. 74:01 S.I, Botswana Power Corporation (Electricity) Bye-

Laws – under section 28, 21st December 1979

[5] DEBSWANA DIAMOND COMPANY (Pty) Ltd, Jwaneng Mine, Engineering

Electrical Services Standard Procedure, Procedure No: EE-E1-06, Critical Loads

to be supplied from the Power Station Diesel Generators during a BPC Power

Supply Failure, Revision 1, 7 June 1996

[6] G.J COETZEE, Power Quality paper #1 – Causes of voltage dips & resulting

problems, Switching Systems Electronic Engineers

[7] M SCHILDER & RG KOCH, Evaluation of a new 3 Phase dip definition,

Energize – Power Journal of the South African Institute of Electrical Engineers,

June 2006

[8] www.fortressresistors.com/neutral_earthing.htm, accessed 11-Aug-06

[9] http://www.irescoindia.com/neutral.htm, accessed 11-Aug-06

[10] NEIL JEFFREY, Dynamic Range Requirements of Capacitor VT Sensors

for Protection Systems, SURE Engineering CC, 23 March, 1999

Page 89: Voltage Dip

Transient Analysis of Voltage Dips - 72 --

_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

[11] JEFF ROBERTS, DR. HECTOR J. ALTUVE, AND DR. DAQING HOU,

Review of Ground Fault Protection Methods For Grounded, Ungrounded, and

Compensated Distribution Systems, Schweitzer Engineering Laboratories, Inc.

[12] ABBAS AKHIL, SHIVA, SWAMINATHAN, RAJAT K. SEN, Cost

Analysis of Energy Storage Systems for Electric Utility Applications, February

1997

[13] http://www.xe.com/ucc/convert.cgi, accessed 3 October 2006

[14] http://www.afrlhorizons.com/Briefs/Dec01/ML0009.html, accessed 7

September 2006

[15] http://www.xe.com/ucc/convert.cgi, accessed 3 October 2006

[16] ABBAS AKHIL, SHIVA, SWAMINATHAN, RAJAT K. SEN, Cost

Analysis of Energy Storage Systems for Electric Utility Applications, Sandia

Report, February 1997

[17] http://en.wikipedia.org/wiki/SMES, accessed 31 August 2006

[18] http://www.parcon.uci.edu/OLD_WEBSITE/paper/eeenergy.htm,

accessed 7 September 2006

[19] http://findarticles.com/p/articles/mi_qa3739/is_199903/ai_n8850296

accessed 7 November 2006

[20] http://en.wikipedia.org/wiki/Uninterruptible_power_supply, accessed 17

August 2006

[21] S A GRIFFITHS, D E POOLE, An Uninterruptible Power Supply For The

SRS, CLRC Daresbury Laboratory

Page 90: Voltage Dip

Transient Analysis of Voltage Dips - 73 --

_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

[22] http://en.wikipedia.org/wiki/Flywheel_energy_storage, accessed 28

September 2006

[23] http://www.xe.com/ucc/convert.cgi, accessed 3 October 2006

[24] ABBAS AKHIL, SHIVA, SWAMINATHAN, RAJAT K. SEN, Cost

Analysis of Energy Storage Systems for Electric Utility Applications, Sandia

Report, February 1997

[25] http://www.upei.ca/~physics/p261/projects/flywheel1/flywheel1.htm

accessed 6 November 2006

[26] POHJANHEIMO P., A Probabilistic Method for Comprehensive Voltage

Sag Management in Power Distribution Systems, Doctoral thesis, Helsinki

University of Technology, 2003

[27] MASATOSHI TAKEDA, PH.D. HIROSHI YAMAMOTO GREGORY F.

REED, PH.D. TOMOHIKO ARITSUKA ISAO KAMIYAMA, Development of a

Novel Hybrid Switch Device and Application to a Solid State Transfer Switch

[28] J. D. MASTERS, Dowding Reynard & Associates (Pty) Ltd, Voltage Dip

study at Jwaneng Mine-addendum, 13 March 1997

[29] GOOGLE CACHE, http://64.233.183.104/search?q=cache:t-

Ff7OuPhW0J:www.members.lycos.nl/caspoceducation/0899.pdf+static+electroni

c+tap+changer&hl=en&gl=uk&ct=clnk&cd=14, accessed 18 September 2006

[30] http://www.aesltd.com.au/whatwedo/docs/svc_over.htm - 1 September

2006

[31] http://www.power-technology.com/contractors/tandd/nokian/ - accessed

28 August 2006

Page 91: Voltage Dip

Transient Analysis of Voltage Dips - 74 --

_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

[32] "http://en.wikipedia.org/wiki/Static_VAr_compensator" accessed 11

August 2006

[33] http://www.ku.edu.np/ee/rb/Handouts_COEG301&303/Ferranti%20Effect

.pdf, accessed 14 August 2006

Page 92: Voltage Dip

Transient Analysis of Voltage Dips - 75 --

_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

APPENDIX A1 – Effects of Capacitance

All Three Phases

Phase to Phase

Single Phase

Page 93: Voltage Dip

Transient Analysis of Voltage Dips - 76 --

_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

APPENDIX A2 – Effects of Load

All Three Phases

Phase to Phase

Single Phase

Page 94: Voltage Dip

Transient Analysis of Voltage Dips - 77 --

_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

Appendix A3 - Delays due to Dips – 2005

IP4 : Headfeed delay hours due to power interuptions & dips

5.0

0.5

3.6

2.93

1.8

0.0 0.0 0.0 0.0

6.0

0.00.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

Jan-05

Feb-05

Mar-05

Apr-05

May-05

Jun-05

Jul-05

Aug-05

Sep-05

Oct-05

Nov-05

Dec-05

Actual- Total EXTERNAL INTERNAL Threshold Target Stretch Target Baseline Other

IP5 : Reduction of Power Dips - Frequency of dips affecting plants

7.0

2.0

10.0

5.0 5

0 0 0 0

5

00.0

2.0

4.0

6.0

8.0

10.0

12.0

Jan-05

Feb-05

Mar-05

Apr-05

May-05

Jun-05

Jul-05

Aug-05

Sep-05

Oct-05

Nov-05

Dec-05

Actual EXTERNAL INTERNAL Threshold Target Stretch Target Baseline Other

Objectives - To clearly identify external and internal dips especially after the

commissioning of the NER whose performance must be measured and compared to last

year when there was no NER.

Page 95: Voltage Dip

Transient Analysis of Voltage Dips - 78 --

_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

Appendix A4 - Utilities Failures - Red Area

DATE DELAY(min) STM MACHINE CODE DELAY REPORT

12/20/05 25 PNEUMO AJX0305 POWER DIP ON THE COMPRESSOR AND PNEUMO DRYER

01/08/06 67 PNEUMO AJX0320/0305 POWER DIP ON THE PNEUMO BLOWERS AND THE PERMROLLS.

01/09/06 45 PNEUMO POWER DIP (AFFECTED PNEUMO BLOWER, AND SCRUBBER)

01/09/06 37 PNEUMO PLANT START UP AND RAISING PNEUMO TEMPERATURES

174

01/22/06 30 PNEUMO POWER DIP

02/04/06 58 PNEUMO POWER DIP

02/04/06 40 PNEUMO

START UP AND RAISING SOUTH EAST PNEUMO TEMPERATURE AFTER POWER DIP

02/08/06 48 PNEUMO POWER DIP

02/08/06 14 PNEUMO SE RAISING PNEUMO LINE TEMP AFTER POWER DIP

02/11/06 20 PNEUMO PNEUMO BLOWER POWER DIP

02/12/06 51 PNEUMO POWER DIP (JC NO'S R00767941 AND R00767946)

02/03/06 57 PNEUMO AJX2801 POWER FAILURE .( FEED PREP SECTION) R00765904

02/14/06 94 PNEUMO AJX0305

POWER DIP ( AFFECTED THE PNEUMO , SICON AND COMPRESSOR ) .

412

02/17/06 40 PNEUMO AJX0305 POWER DIP. ( R00769371)

02/20/06 57 PNEUMO AJX0305 POWER DIP TRIPPING THE SE PNEUMO.

02/23/06 25 PNEUMO ALX LT 0801 DUST SCRUBBER RAW WATER STORAGE TANK LEVEL LOW.

02/16/06 41 PNEUMO AJX 0305 POWER DIP

02/22/06 40 PNEUMO AJX 0305 POWER DIP

02/22/06 18 PNEUMO AJX 0305 RAISING SE PNEUMO TEMPERATURE AFTER POWER DIP

02/23/06 38 PNEUMO AJX 0305 POWER DIP

02/23/06 29 PNEUMO AJX 0305 RAISING PNEUMO TEMPERATURE

03/07/06 68 PNEUMO AJX0305 POWER DIP. ( FEED PREP SECTION) R00773141

356

03/18/06 80 PNEUMO AJX0305 POWER DIP

03/19/06 54 PNEUMO AJX0305 POWER DIP

03/19/06 27 PNEUMO AJX0305 POWER DIP. ( R00775898)

Page 96: Voltage Dip

Transient Analysis of Voltage Dips - 79 --

_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

04/09/06 66 PNEUMO AJX0305 Power dip

04/10/06 80 PNEUMO AJX0305 POWER DIP

307

06/01/06 43 PNEUMO AJX8001 GREEN AREA RAW WATER PUMP TRIPPED.

43

07/15/06 21 PNEUMO AJX 0305 POWER DIP

07/09/06 63 PNEUMO AJX 0305 POWER DIP

84

08/01/06 30 PNEUMO AJX 0305 POWER DIP AND START UP

30

09/11/06 16 PNEUMO AJX 0320 NW Pneumo tripped.

09/07/06 61 PNEUMO AJX 0305/0320 POWER DIP (PNEUMO BLOWERS TRIPPED)

09/07/06 38 PNEUMO AJX 2501 RAISNG PNEUMO TEMP AFTER POWER DIP

115

10/04/06 37 PNEUMO AJX 0305 POWER DIP

37

Total Hours 24

Page 97: Voltage Dip

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Page 98: Voltage Dip

Transient Analysis of Voltage Dips - 81 --

_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

Appendix A6 – Sag Generator Costs @ PSL

Page 99: Voltage Dip

Transient Analysis of Voltage Dips - 82 --

_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

Appendix A7 – Earthing For Different Sections

1. EARTHING

The Contractor, in conjunction with the Engineer’s site representative is to propose a

scheme that complies with the requirement set out herein, mark up routing and racking

drawing and submit for the project Engineers approval.

2. Earthing of Racking

2.1.1. Each racking route shall have a minimum of one 70mm2 bare copper earth

run along its route from the substation earth bar. The 70mm2 bare copper

earth shall form a “ring” such that both ends of the ring are connected to the

substation earth bar.

2.1.2. Racking shall be bonded to the 70mm2 bare copper earth ring by means of

a 35mm2 bare copper earth wire and purpose made earth clip at intervals not

exceeding twelve metres. Further, the 70mm2 bare copper earth shall be

bonded to the steel structure at intervals not exceeding twelve metres. This

may be done by means of 35mm2 copper wire being bonded to racking

support steel bolts where it is bolted onto the building or structure.

2.1.3. Racking between substations is to be bonded in the same fashion as

described above except that the ring shall be from one substation earth bar to

the other. Earths required to be run with M.V. and H.T. feeder cables may

be used for this purpose

3. Earthing of Substations

3.1.1. Each substation is to have an earth bar installed in its cable basement or

trench fixed to the side wall.

3.1.2. The substation earth bar is to be connected to the substation earth bar from

which it is fed by means of either a 70mm2 bare copper or green PVC

Page 100: Voltage Dip

Transient Analysis of Voltage Dips - 83 --

_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

insulated earth. In the case of multiple feeds, one earth connection between

substations will suffice.

3.1.3. In substations where earth mats are installed, the earth bar is to be

connected to the earth mat by means of two 70mm2 green PVC insulated

copper cables. This connection is to be to a separate earth bar which has a

removable link connecting it to the substation main earth bar.

3.1.4. Substation earth bars shall be of hard drawn copper, 50x6x1000mm

minimum pre-drilled and fitted with brass nuts, bolts, washers and spring

washers to facilitate all connections.

3.1.5. Substation earth bars when measured to true earth, shall not exceed 1

ohms

4. Earthing of Switchboards and M.C.C.’s

4.1.1. Switchboards and M.C.C.’s earth bars are to be connected to the

substation earth bar by means of two 70mm2 bare copper conductors, one

from either end of the MCC or switchboards earth bar.

5. Earthing of Transformers

5.1.1. Solidly earthed neutral to earth conductors are to be the same size as phase

conductors but, in all cases, not less than 16mm2 and connected from the

neutral terminal directly to the substation earth bar.

5.1.2. Solidly earthed neutral transformers shall have their tank earth connected

to the substation earth bar by the same size conductor as the neutral earth,

but in all cases, not less than 70mm2.

5.1.3. The neutral and tank earth connections may not be connected to each other

and to the substation earth bar by one set of conductors. However, an earth

bar may be established in the transformer bay, both the transformer tank and

the neutral connected to it and a connection from the transformer bay earth

bar to the substation earth bar made in conductors’ sizes as specified above.

5.1.4. Transformers installed in locations that are not in close proximity of

substations containing earth bars may require earth electrodes installed to

Page 101: Voltage Dip

Transient Analysis of Voltage Dips - 84 --

_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

provide a earthing connection of not more than 1 ohms to true earth.

5.1.5. Generally, on 525V transformers, Isoloc or resistive earth systems are

used. The details of the systems will vary for different applications and will

be described in the Project Specific documentation.

6. Earthing of Motors

6.1.1. All motors are to have a separate visible earth, equal in cross-sectional

area to the supply cable conductor, connected from the motor housing to the

70mm2 racking earth ring. Further, a connection of the same cross sectional

area as the supply cable is to be made from the motor housing to the motor

base plate. These connections are to be made in such a manner that the

connection to the base plate will not be broken if the motor is removed.

6.1.2. In installations where the motor base plate is separate from the driven

machinery base plate a connection is to be made bonding the motor base

plate to the driven machinery with a 70mm2 copper earth conductor.

6.1.3. H.V. and M.V. motors will, in addition to the above, be bonded to the

structural steel by means of a 70mm2 copper earth conductor.

7. General

7.1.1. Earth conductors will be fixed to racking by means of U.V. resistant PVC

cable ties or other approved non magnetic means.

7.1.2. Earth conductors will not be run in metal conduit.37

37 ANGLO AMERICAN CORPORATION OF SOUTH AFRICA LIMITED, FOR DEBSWANA DIAMOND COMPANY (PTY) LTD, GENERAL SPECIFICATION, OR2-GS-220, ELECTRICAL INSTALLATION GENERAL, 22/04/97

Page 102: Voltage Dip

Transient Analysis of Voltage Dips - 85 --

_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

Appendix A8 –Flywheel vs. Battery Energy38

38 Source: http://www.activepower.com/index.asp?pg=technology_flywheel_vs_battery, accessed 28 September 2006

Active Power Clean Source �����������

����� �����

�������� ���� ����� ���� ��������

Life Span • Design: 20+

• Operational: since 1997

• Design: 10 in float service

• Operational: 3 to 6, typical

Discharge Cycles • Unlimited for product life span • 1000-2000 shallow

• 100-200 deep

Optimal Temp (�C) -20 to +40 • 25

Maintenance Air filters: as req.

• Oil Change: 12 mos.

• Bearings: 30 mos.

• Preventative maintenance: 3

to 6 mos.

• Jar replacements: as req.

Predictability • Self Diagnostics: standard

• Monitoring software & cards:

optional

• Requires battery monitoring

system

Footprint

(w/ service access

• 500kW =>

• 20 sq.ft.

• 500 kW =>

• 60-80 sq.ft.

"Green"

Technology

• No environmental hazards • HazMat disposal issues

• Spill containment

• Personnel safety

• EPA regulations

Page 103: Voltage Dip

An

aly

sis

of

Vo

lta

ge

dip

s

___

___

___

___

___

___

___

___

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___

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ab

o

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wa

na

am

oth

o

kn

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aa

mo

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deb

swa

na.b

w

Ap

pe

nd

ix A

9 -

Po

we

r D

ip R

ec

ord

Sh

ee

t

1.

Det

ail

s of

occ

urr

ence

Dat

e T

ime

Wea

ther

Con

dit

ion

2.

Rep

ort

on

Pla

nt

aff

ecte

d

H

ead

Fee

d D

elay

s

Are

as A

ffec

ted

L

isti

ng o

f pla

nts

aff

ecte

d

So

urc

e o

f re

port

D

ura

tion

of

Del

ays

Co

mm

ent

on

exte

nt

of

pro

du

ctio

n l

oss

3.C

au

se o

r P

rob

ab

le C

au

se o

f V

olt

ag

e D

ip

C

ause

of

fault

(bir

d,

recl

ose

etc

) S

ou

rce

of

rep

ort

D

ura

tion

of

outa

ge

Rec

ord

of

pro

tect

ion d

evic

es t

ripp

ed

3.1

Min

e

3.2

BP

C

C

om

men

ts f

rom

BP

C:

Page 104: Voltage Dip

An

aly

sis

of

Vo

lta

ge

dip

s

___

___

___

___

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ab

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4.0

Fo

llo

w-u

p A

ctio

n/C

om

men

ts

Ele

ctri

cal

T

rea

tmen

t B

PC

Dip

Ma

gn

itu

de

&

Du

ratio

n

Tota

l L

oa

d S

he

d (

kW

)

Co

mm

en

ts b

y E

lectr

ica

l S

erv

ice

s:

(in

cl. F

au

lt

typ

e)

N

ame:

-

S

ign

ature

:-

Page 105: Voltage Dip

Analysis of Voltage dips

_______________________________________________________________________Kabo Ngwanaamotho – [email protected]

Appendix A10 - Maximum Power Demand

IP5: MAXIMUM DEMAND

31.8032.24

32.76

32.22

30.98

30.48

32.2432.48

34.52

33.72

28

29

30

31

32

33

34

35

Jan-06 Feb-06 Mar-06 Apr-06 May-06 Jun-06 Jul-06 Aug-06 Sep-06 Oct-06 Nov-06 Dec-06 Jan-07

Internal External Actual Threshold Target Stretch Target

IP5: TOTAL MINE POWER CONSUMPTION IN MWh

17254

19306

17576

18879

17028

1873819009 19004

20648

17987

15000

16000

17000

18000

19000

20000

21000

22000

23000

Jan-06 Feb-06 Mar-06 Apr-06 May-06 Jun-06 Jul-06 Aug-06 Sep-06 Oct-06 Nov-06 Dec-06 Jan-07

Actual Threshold Target