guide book on custom power devices

Guidebook on Custom Power Devices

Technical Report

EPRI Project ManagerA. Sundaram

EPRI 3412 Hillview Avenue, Palo Alto, California 94304 PO Box 10412, Palo Alto, California 94303 USA800.313.3774 650.855.2121 [email protected] www.epri.com

Guidebook on Custom PowerDevices1000340

Final Report, November 2000

DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIESTHIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS ANACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCHINSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THEORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I)WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, ORSIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESSFOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON ORINTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTYS INTELLECTUALPROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USERSCIRCUMSTANCE; OR

(B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER(INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVEHAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOURSELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD,PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT.

ORGANIZATION(S) THAT PREPARED THIS DOCUMENTEPRI PEAC Corporation

ORDERING INFORMATIONRequests for copies of this report should be directed to the EPRI Distribution Center, 207 CogginsDrive, P.O. Box 23205, Pleasant Hill, CA 94523, (800) 313-3774.

Electric Power Research Institute and EPRI are registered service marks of the Electric PowerResearch Institute, Inc. EPRI. POWERING PROGRESS is a service mark of the Electric PowerResearch Institute, Inc.

Copyright 2000 Electric Power Research Institute, Inc. All rights reserved.

iii

CITATIONS

This report was prepared by

EPRI PEAC Corporation942 Corridor Park Blvd.Knoxville, TN 37932

Principal InvestigatorC. Perry

InvestigatorsA. MansoorW. Sunderman

This report describes research sponsored by EPRI.

The report is a corporate document that should be cited in the literature in the following manner:

Guidebook on Custom Power Devices, EPRI, Palo Alto, CA: 2000. 1000340.

vREPORT SUMMARY

This report provides engineers in the electric power industry with a comprehensive guide toproper selection, specification, operation, and maintenance of custom power devices anddescribes in detail the tools necessary to perform these tasks. For proper application and sizing,the report provides the data equipment vendors require. Possible operation or maintenanceconcerns also are discussed to provide engineers with a heads up for possible problem areas.

BackgroundThe move toward a global economy is forcing companies to improve their product quality andplant efficiency to stay competitive. Electric power disturbances are often seen as one of thelargest causes of equipment shutdown and product defects. Therefore, companies are placingever-increasing demands on their energy suppliers to improve the quality and reliability of theirservice. To answer this call for better power quality, many energy suppliers are turning to custompower devices. However, few engineers have had exposure to custom power devices and theirproper application. Those engineers, who may or may not be well versed in power quality, arethe target audience for this report. Though custom power devices continue to mature, they arenot plug and play devices like equipment traditionally used in power distribution andtransmission systems. Therefore, proper selection, sizing, and specification are not alwaysstraightforward processes. Engineers need to first understand the power quality performance ofthe energy-supply system, then they must identify the customers production process. Finally,they must choose a proper range of custom power devices that addresses customers' needs.

ObjectiveTo provide engineers in electric power companies with the tools necessary to properly assess theapplication of custom power devices.

ApproachThe project team first consulted EPRI reports, scholarly papers, vendor literature, and conferencepresentations. When practical, the team incorporated data from these sources into this report. Asurvey of existing customer power installations was performed to determine any possibleconcerns with regard to installation, operation, and maintenance. The team then determined thetypes of information and calculations that the typical engineer would need when approaching avendor of custom power devices. Finally, team members examined possible system interactionproblems involving custom power devices.

ResultsAlthough some custom power technologies are quite mature, none are as straightforward to applyas traditional devices in the power delivery system, such as voltage regulators, reclosers, andfuses. Engineers need to perform a great deal of up-front work before approaching a vendor of

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custom power devices. This helps ensure selection of the proper device as well as a goodinstallation. Selecting and installing a custom power device that has been forced to fit thesituation by a vendor wanting to push a particular device should be avoided. Proper education ofpower engineers can prevent this from occurring. Many custom power devices require controltuning or modification after installation to guarantee the device's optimum performance in thesystem. A well-designed power quality monitoring system can greatly improve the success ofthis process by providing data about the device's performance and the energy-delivery system inwhich it is installed. With proper engineering, planning, and monitoring, successful applicationof a custom power device is possible.

EPRI PerspectiveBy providing utilities with the tools necessary to evaluate the installation of custom powerdevices, EPRI is enabling utilities to effectively use these devices to meet their customers desirefor better power quality and reliability. Discussion of the power quality environment givesengineers an overview of power quality disturbances in distribution systems as well asinformation on their relative frequency of occurrence. The overview of custom power devicesprovides engineers with a general description of custom power devices available, as well as adiscussion of power quality events they can mitigate. The chapters dealing with individualcustom power devices provide data needed by engineers to properly size, specify, install, andmonitor a custom power device. They also provide information on possible operation andmaintenance concerns for particular devices. It is important that utilities have this information toproperly apply custom power devices to mitigate power quality events affecting customers.

KeywordsPower qualityCustom power

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ABSTRACT

The move toward a global economy is forcing companies to improve their product quality andplant efficiency in order to stay competitive. Electric power disturbances are often seen as one ofthe largest causes of equipment shutdown and product defects. Therefore, companies are placingever-increasing demands on their energy suppliers to improve the quality and reliability of theirservice. To answer this call for better power quality, many energy suppliers are turning to custompower devices. However, many engineers have had little or no exposure to custom power devicesand their proper application. This report addresses the need of engineers in the electric powerindustry for a comprehensive guide to the proper selection, specification, operation, andmaintenance of custom power devices.The objective of this report is to provide engineers in electric power companies with the toolsnecessary to properly assess the application of custom power devices. The target audience forthis report is engineers who may or may not be well versed in power quality. Custom powerdevices continue to mature but are not plug and play devices, unlike much of the equipmenttraditionally used in a power distribution and transmission system. The proper selection, sizing,and specification are often not straightforward processes. An engineer needs to first understandthe power quality performance of the energy-supply system. The phenomena of concern to thecustomers production process must be identified, and then a proper range of custom powerdevices that may address the problem must be chosen. This report gives the engineer the toolsnecessary to perform these tasks. The report then provides the engineer with the necessary datathat must be provided to the equipment vendor for proper application and sizing. Also, possibleoperation or maintenance concerns are discussed to provide the engineer with a heads up forpossible problem areas.

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CONTENTS

1 INTRODUCTION.................................................................................................................. 1-1What is Custom Power? ..................................................................................................... 1-1Why is a Guidebook Necessary?........................................................................................ 1-1Contents ............................................................................................................................. 1-2Major Sources .................................................................................................................... 1-3

2 RANGE OF POWER QUALITY VARIATIONS IN DISTRIBUTION CIRCUITS..................... 2-1Background ........................................................................................................................ 2-1Categories of Power Quality Variations............................................................................... 2-1Voltage Sags, Swells, and Interruptions.............................................................................. 2-2

System Faults ................................................................................................................ 2-3Overvoltages and Undervoltages ................................................................................... 2-4Voltage Flicker ............................................................................................................... 2-5Harmonic Distortion........................................................................................................ 2-5Voltage Notching............................................................................................................ 2-6Transient Disturbances .................................................................................................. 2-7

Ranges of Power Quality Variations from the EPRI DPQ Project........................................ 2-8Characteristics of Voltage Sags........................................................................................ 2-13

Point of Initiation .......................................................................................................... 2-13Point of Recovery......................................................................................................... 2-14Phase Shift .................................................................................................................. 2-15Impact of Phase Shift on Sizing of Static Voltage Compensator (SVC) ........................ 2-16Missing Voltage............................................................................................................ 2-18

3 OVERVIEW OF CUSTOM POWER DEVICES..................................................................... 3-1Introduction......................................................................................................................... 3-1Reactive Power and Harmonic Compensation Devices ...................................................... 3-1

Static Var Compensator ................................................................................................. 3-1

xStatic Shunt Compensation............................................................................................ 3-4Compensation Devices for Voltage Sags and Momentary Interruptions.............................. 3-4

Source Transfer Switch.................................................................................................. 3-4Static Source Transfer Switch (SSTS)....................................................................... 3-4

Thyristor (SCR)..................................................................................................... 3-4Gate-Turnoff (GTO) Thyristor................................................................................ 3-5

Hybrid Source Transfer Switch .................................................................................. 3-6High-Speed Mechanical Source Transfer Switch (HSMSTS)..................................... 3-7

Static Series Compensators........................................................................................... 3-7Static Voltage Regulators............................................................................................... 3-8Backup Energy Supply Devices ..................................................................................... 3-9

Battery UPS .............................................................................................................. 3-9SMES...................................................................................................................... 3-10Flywheel .................................................................................................................. 3-10

Device Matrix.................................................................................................................... 3-12Cost Analysis for Custom Power Devices ......................................................................... 3-13

Equipment Costs vs. System Cost vs. Installation Cost................................................ 3-13Life-Cycle Cost............................................................................................................. 3-13Comparing Life-Cycle Costs for Competing Custom Power Systems........................... 3-14Analysis Method........................................................................................................... 3-15

4 REACTIVE POWER AND HARMONIC COMPENSATION DEVICES ................................. 4-1Fundamentals..................................................................................................................... 4-1

Static Var Compensator ................................................................................................. 4-1Static Var Compensator Topologies .......................................................................... 4-1Direct Connected Static Var Compensation for Distribution Systems ........................ 4-4

Static Shunt Compensator (DSTATCOM) ...................................................................... 4-4Procedure 1 Preliminary Assessment ................................................................................. 4-5Procedure 2 Data Gathering ............................................................................................... 4-7Procedure 3 Technical Analysis.......................................................................................... 4-9

Substituting Into Equation 4-6.................................................................................. 4-12Interaction with Distribution Equipment and System ......................................................... 4-17

Static Var Compensator ............................................................................................... 4-18Static Shunt Compensator (DSTATCOM) .................................................................... 4-18

Installation Considerations................................................................................................ 4-19

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Documentation............................................................................................................. 4-19Set Points .................................................................................................................... 4-19General Issues............................................................................................................. 4-19

Maintenance..................................................................................................................... 4-20Static Var Compensator ............................................................................................... 4-20Static Shunt Compensator (DSTATCOM) .................................................................... 4-20

Operation.......................................................................................................................... 4-21Primary Sensing Devices ............................................................................................. 4-21

Voltage Sampling- High-Voltage.............................................................................. 4-21Current Sampling- Fixed Current Transformers ....................................................... 4-22Current Sampling Clamp-On Devices ................................................................... 4-22Recommended On-Hand Instrumentation ............................................................... 4-22

Operation Experience .................................................................................................. 4-22

5 HIGH-SPEED SOURCE TRANSFER SWITCH DEVICES ................................................... 5-1Fundamentals..................................................................................................................... 5-1

Static Source Transfer Switch ........................................................................................ 5-1Hybrid Source Transfer Switch....................................................................................... 5-2High-Speed Mechanical Source Transfer Switch .......................................................... 5-3

Procedure 1 Preliminary Assessment ................................................................................. 5-3Availability of Sources .................................................................................................... 5-3Independent Feeders ..................................................................................................... 5-4Feeder Capacity for Load............................................................................................... 5-4Synchronization ............................................................................................................. 5-4

Procedure 2 Data Gathering ............................................................................................... 5-5Procedure 3 Technical Analysis.......................................................................................... 5-6

Step 1: Sag Performance at the Proposed Location....................................................... 5-6Step 2: Voltage Drop Due to Transfer ............................................................................ 5-8

Interaction with Distribution Equipment and System ......................................................... 5-10Installation Considerations................................................................................................ 5-11


Maintenance..................................................................................................................... 5-12Static Source Transfer Switch ...................................................................................... 5-13

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Hybrid Source Transfer Switch..................................................................................... 5-13High-Speed Mechanical Source Transfer Switch ......................................................... 5-13


Voltage Sampling - High-Voltage............................................................................. 5-14Current Sampling- Fixed Current Transformers ....................................................... 5-14Current Sampling Clamp-On Devices ................................................................... 5-14

Operation Case Study 1............................................................................................... 5-15Year 1 ..................................................................................................................... 5-15Year 2 ..................................................................................................................... 5-16

Operation Case Study 2............................................................................................... 5-16System Description.................................................................................................. 5-16PQ Monitoring Data................................................................................................. 5-17Lesson Learned....................................................................................................... 5-19

Operation Case Study 3............................................................................................... 5-19System Description.................................................................................................. 5-19PQ Monitoring Data................................................................................................. 5-19Lesson Learned....................................................................................................... 5-22

6 STATIC SERIES COMPENSATORS................................................................................... 6-1Fundamentals..................................................................................................................... 6-1Procedure 1 Preliminary Assessment ................................................................................. 6-2Procedure 2 Data Gathering ............................................................................................... 6-3Procedure 3 Technical Analysis........................................................................................ 6-12Interaction with Distribution Equipment and System ......................................................... 6-17Installation Considerations................................................................................................ 6-18


Maintenance..................................................................................................................... 6-19Operation.......................................................................................................................... 6-19

Primary Sensing Devices ............................................................................................. 6-20Voltage Sampling - High-Voltage............................................................................. 6-20Current Sampling- Fixed Current Transformers ....................................................... 6-20Current Sampling Clamp-On Devices ................................................................... 6-20

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Recommended On-Hand Instrumentation ............................................................... 6-21Case Study Waveform Indicates Interaction between Utility and Static SeriesCompensation Device .................................................................................................. 6-21Case Study - Lessons Learned from Application of Leading Edge Technology ............ 6-25

Overview ................................................................................................................. 6-25Device Operation History......................................................................................... 6-25Lessons Learned..................................................................................................... 6-26

Case Study Failure of SSC Due to Bypass Breaker Problem .................................... 6-27Overview ................................................................................................................. 6-27Operation History .................................................................................................... 6-27

7 BACKUP ENERGY SUPPLY DEVICES .............................................................................. 7-1Fundamentals..................................................................................................................... 7-1

Battery UPS ................................................................................................................... 7-2SMES............................................................................................................................. 7-2Mechanical Energy......................................................................................................... 7-2

Procedure 1 Preliminary Assessment ................................................................................. 7-3Power Quality Events..................................................................................................... 7-3Physical Space .............................................................................................................. 7-3Circuit Protection............................................................................................................ 7-3

Procedure 2 Data Gathering ............................................................................................... 7-4Interaction with Distribution Equipment and System ........................................................... 7-5Installation Considerations.................................................................................................. 7-5

Documentation............................................................................................................... 7-5Set Points ...................................................................................................................... 7-6General Issues............................................................................................................... 7-6

Maintenance....................................................................................................................... 7-7Operation............................................................................................................................ 7-7

Primary Sensing Devices ............................................................................................... 7-8Voltage Sampling - High-Voltage............................................................................... 7-8Current Sampling- Fixed Current Transformers ......................................................... 7-8Current Sampling Clamp-On Devices ..................................................................... 7-8

Operation Experience .................................................................................................... 7-8

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8 STATIC VOLTAGE REGULATORS .................................................................................... 8-1Fundamentals..................................................................................................................... 8-1Procedure 1 Preliminary Assessment ................................................................................ 8-3

Verify the source of the power quality issue ................................................................... 8-3Procedure 2 Data Gathering .............................................................................................. 8-3Procedure 3 Technical Analysis ........................................................................................ 8-10Interaction with Distribution Equipment and System ......................................................... 8-11Installation Considerations................................................................................................ 8-12

Documentation............................................................................................................. 8-12Set Points .................................................................................................................... 8-13General Issues............................................................................................................. 8-13Maintenance ................................................................................................................ 8-13


Voltage Sampling - High-Voltage............................................................................. 8-14Current Sampling- Fixed Current Transformers ....................................................... 8-14Current Sampling Clamp-On Devices ................................................................... 8-15

Operation Experience .................................................................................................. 8-15

9 BIBLIOGRAPHY.................................................................................................................. 9-1

A APPENDIX ..........................................................................................................................A-1Survey Data for Utility Feedback on Custom Power Demonstration Projects ......................A-1

Target 19: Distribution Systems Integrated Custom Power Guidebook ..........................A-1Section 1. Type of Custom Power Technology ...................................................................A-2Section 2. Purpose of Installation........................................................................................A-2Section 3. Customer Description.........................................................................................A-3Section 4. Distribution System Description .........................................................................A-5Section 5. Point-of-Service Monitoring ................................................................................A-7Section 6. Utility-Side Improvements ..................................................................................A-8Section 7. Selection of Specific Mitigation Technology .......................................................A-9Section 8. Employed Mitigation Technology .......................................................................A-9Section 9. Energy Storage Media (If Applicable)...............................................................A-10Section 10. Project Development......................................................................................A-13Section 11. Changes Necessitated by the Installation.......................................................A-14

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Section 12. Project Installation..........................................................................................A-17Section 13. Project Commissioning ..................................................................................A-18Section 14. Project Performance ......................................................................................A-19Section 15: Future Deployment of Future Units ................................................................A-24

B MANUFACTURERS OF CUSTOM POWER DEVICES.......................................................B-1

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LIST OF FIGURES

Figure 2-1 Typical Short Duration RMS Voltage Variations ..................................................... 2-3Figure 2-2 Example RMS Measurement of Undervoltage........................................................ 2-5Figure 2-3 Example of Voltage Flicker Caused by an Arc Furnace.......................................... 2-5Figure 2-4 Example Voltage Waveform with 3rd Harmonics and 17.42% Total Harmonic

Distortion ......................................................................................................................... 2-6Figure 2-5 Example Waveform with Notching.......................................................................... 2-7Figure 2-6 Impulsive Transient Waveform ............................................................................... 2-8Figure 2-7 Oscillatory Transient Waveform Caused by a Capacitor Energizing ....................... 2-8Figure 2-8 Sag and Interruption-Rate Magnitude Histogram, One-Minute Aggregation,

6/1/93 to 6/1/95, Treated by Sampling Weights, All Sites................................................. 2-9Figure 2-9 Sag and Interruption Rate Magnitude Duration Histogram, One-Min

Aggregation 6/1/93 to 6/1/95, Treated by Sampling Weights, All Sites........................... 2-10Figure 2-10 Histogram for Magnitude of Oscillatory Transients Measurement Events,

3/1/95 to 9/1/95, Treated by Sampling Weights, All Sites............................................... 2-10Figure 2-11 Magnitude and Duration of Oscillatory Transients Measurement Events,

3/1/95 to 9/1/95, Treated by Sampling Weights, All Sites............................................... 2-11Figure 2-12 Voltage THD and Individual Harmonics, 6/1/93 to 3/1/95, All Sites..................... 2-11Figure 2-13 Histogram of Point of Initiation for RMS Variations 6/1/93 to 6/1/95, Treated

by Sampling Weights, All Sites ...................................................................................... 2-14Figure 2-14 Histogram of Point of Recovery for RMS Variations 6/1/93 to 6/1/95, Treated

by Sampling Weights, All Sites ...................................................................................... 2-15Figure 2-15 Histogram of Maximum Transition Angle during Event for RMS Variations

6/1/93 to 6/1/95, Treated by Sampling Weights, All Sites............................................... 2-16Figure 2-16 Voltage Injection Required to Correct Voltage Sag Down to 60% of Nominal

with No Phase Shift ....................................................................................................... 2-17Figure 2-17 Voltage Injection Required to Correct Voltage Sag Down to 60% of Nominal

with 60-Degrees Phase Shift ......................................................................................... 2-17Figure 2-18 Missing Voltage - Duration Summary Table: Rate of RMS Variations per 365

days 60-Second Temporal Aggregation 6/1/93 to 6/1/95, Unweighted, All Sites ............ 2-19Figure 3-1 FC/TCR (Fixed Capacitor/Thyristor Controlled Reactor)......................................... 3-2Figure 3-2 TSC (Thyristor Switched Capacitor) ....................................................................... 3-3Figure 3-3 TSC/TCR (Thyristor Switched Capacitor/ Thyristor Controlled Reactor) ................. 3-3Figure 3-4 Thyristor (SCR) ...................................................................................................... 3-5Figure 3-5 Medium-Voltage Static Source Transfer Switch...................................................... 3-6

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Figure 3-6 Hybrid Source Transfer Switch............................................................................... 3-6Figure 3-7 SSC Using Line Energy Supply (LES) .................................................................... 3-7Figure 3-8 Static Voltage Regulator......................................................................................... 3-9Figure 3-9 Medium-Voltage Standby UPS............................................................................. 3-10Figure 4-1 Thyristor Switched Capacitor (TSC) ....................................................................... 4-2Figure 4-2 Typical Static Var System Configuration (FC/TCR) for Arc Furnace

Applications ..................................................................................................................... 4-3Figure 4-3 Thyristor Switched Capacitor/Thyristor Controlled Reactor (TSC/TCR) .................. 4-3Figure 4-4 Example Diagram of Static Shunt Compensator..................................................... 4-4Figure 4-5 Example of Flicker. Note the Correspondence Between the Voltage and

Current ............................................................................................................................ 4-9Figure 4-6 Example Distribution System................................................................................ 4-10Figure 5-1 Preferred/Alternate Configuration ........................................................................... 5-2Figure 5-2 Split Bus Configuration........................................................................................... 5-2Figure 5-3 Hybrid Source Transfer Switch............................................................................... 5-3Figure 5-4 One-Line Diagram of System With Instrument Transformer Locations ................. 5-17Figure 5-5 Plot of Daily Average Even Harmonic Current at the Output of the SSTS............. 5-18Figure 5-6 Normal Voltage and Current Waveforms .............................................................. 5-18Figure 5-7 Voltage and Current Waveforms with Increased Even Harmonics........................ 5-19Figure 5-8 Preferred Source Phase A Voltage and Current ................................................... 5-20Figure 5-9 Preferred Source Phase B Voltage and Current ................................................... 5-20Figure 5-10 Preferred Source Phase C Voltage and Current................................................. 5-21Figure 5-11 Preferred Source and Alternate Source Phase A Current................................... 5-21Figure 5-12 Preferred Source and Alternate Source Phase B Current................................... 5-22Figure 5-13 Preferred Source and Alternate Source Phase C Current................................... 5-22Figure 6-1 Basic Configuration of a Stored Energy Supply Static Series Compensator ........... 6-2Figure 6-2 Voltage Sag to 50% with 0 Phase Shift ............................................................... 6-14Figure 6-3 RMS Plot of Voltage Sag to 50% with No Phase Shift .......................................... 6-15Figure 6-4 Missing Voltage Required for Full Compensation for Voltage Sag to 50% with

No Phase Shift............................................................................................................... 6-15Figure 6-5 Voltage Sag to 50% with 30 Phase Shift ........................................................... 6-16Figure 6-6 RMS Plot for Voltage Sag to 50% with 30 Phase Shift ...................................... 6-16Figure 6-7 Missing Voltage Required for Full Compensation for Voltage Sag to 5-% with

30 Phase Shift............................................................................................................ 6-17Figure 6-8 Diagram of the Medium-Voltage Distribution System, Including Capacitor

Banks ............................................................................................................................ 6-22Figure 6-9 Voltage Captured on the Utility Side Before and During the Event ....................... 6-22Figure 6-10 Voltage Captured on the Load Side Before and During the Event ...................... 6-23Figure 6-11 Voltage Captured on the Utility Side During the Clearing of the Event................ 6-23

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Figure 6-12 Voltage Captured on the Load Side During the Clearing of the Event ................ 6-24Figure 6-13 Increase in Peak Voltage Caused by the Interaction Between the SSC

Device and the Utility System ........................................................................................ 6-24Figure 6-14 Increase in the Sixth Harmonic Voltage Distortion Caused by the Interaction

Between the SSC .......................................................................................................... 6-25Figure 7-1 Typical Medium Voltage Offline UPS...................................................................... 7-1Figure 8-1 Typical Static Voltage Regulator............................................................................. 8-2

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LIST OF TABLES

Table 2-1 Categories of Power Quality Variation IEEE 1159-1995 ....................................... 2-2Table 2-2 Service Conditions for the Voltage Supply at Medium Voltage ............................. 2-12Table 2-3 Voltage Injection Required by an SVC for a Range of Voltage Sag Magnitude

and Phase Shift ............................................................................................................. 2-18Table 2-4 Rate of Occurrence for Various Levels of Cumulative Voltage Compensation

for RMS Variations 6/1/93 to 6/1/95, Unweighted, All Sites............................................ 2-18Table 3-1 Custom Power Device Application Matrix .............................................................. 3-12Table 4-1 Source Characteristics ............................................................................................ 4-7Table 4-2 Calculate Required Size for Voltage Control and Power Factor Control................. 4-13Table 5-1 Source Characteristics ............................................................................................ 5-5Table 5-2 Example Voltage Sag Performance Table ............................................................... 5-8Table 5-3 Voltage Drop Calculation......................................................................................... 5-9Table 6-1 Transmission Line Fault Performance Table ........................................................... 6-4Table 6-2 Area of Vulnerability Calculation Table for the Transmission System ...................... 6-5Table 6-3 Calculating Expected Performance at End User for a Specific Sag Severity............ 6-6Table 6-4 Effect of Transformer Connections Given a Line to Ground Fault on Phase A

of the Primary .................................................................................................................. 6-7Table 6-5 Table for Collecting Data Concerning Distribution Feeder Circuits........................... 6-8Table 6-6 Worksheet for Calculating Voltage Sag Performance for Distribution Faults.......... 6-10Table 6-7 Load Characteristic Data ....................................................................................... 6-11Table 6-8 Source Characteristics .......................................................................................... 6-12Table 6-9 Static Series Compensation .................................................................................. 6-13Table 6-10 SSC Operation Summary(Rose, Thomas, Custom Power Reduces Wafer

Production Losses: A Case Study, Power Quality 2000 Proceedings, October2000) ............................................................................................................................. 6-28

Table 7-1 Source Characteristics ............................................................................................ 7-4Table 8-1 Static Voltage Regulator Compensation Ranges ..................................................... 8-2Table 8-2 Effect of Transformer Connections Given a Line to Ground Fault on Phase A

of the Primary .................................................................................................................. 8-5Table 8-3 Table for Collecting Data Concerning Distribution Feeder Circuits........................... 8-6Table 8-4 Worksheet for Calculating Voltage Sag Performance for Distribution Faults............ 8-8Table 8-5 Load Characteristic Data ......................................................................................... 8-9Table 8-6 Source Characteristics .......................................................................................... 8-10

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Table B-1 Shunt Reactive Compensation Devices .................................................................B-1Table B-2 Source Transfer Switch..........................................................................................B-2Table B-3 Static Series Compensation ...................................................................................B-2Table B-4 Backup Energy Supply Devices .............................................................................B-3Table B-5 Static Voltage Regulators.......................................................................................B-3

1-1

1 INTRODUCTION

What is Custom Power?

Custom power is the employment of power electronic or static controllers in distribution systemsrated at 1 kV through 38 kV for the purpose of supplying a level of reliability and/or powerquality that is needed by electric power customers who are sensitive to power variations. Custompower devices, or controllers, include static switches, inverters, converters, injectiontransformers, master-control modules, and/or energy-storage modules that have the ability toperform current-interruption and voltage-regulation functions in a distribution system.

A custom power device is a type of power-conditioning device. Power-conditioning technology,in general, includes all devices used to correct the problems faced by end-user loads in responseto voltage sags, voltage interruptions, voltage flicker, harmonic distortion, and voltage-regulationproblems. While the term power conditioning has no voltage boundary, the term custompower is bound by the scope of IEEE P1409 to include power conditioners with input and/oroutput voltage ratings between 1 and 38 kV. This scope limits the location of a custom powerdevice to a medium-voltage primary distribution system. However, the technologies used atmedium voltage and low voltage are frequently very similar.

Why is a Guidebook Necessary?

The move toward a global economy is forcing companies to improve their product quality andplant efficiency in order to stay competitive. Electric power disturbances are often seen as one ofthe largest causes of equipment shutdown and product defects. Therefore, companies are placingever-increasing demands on their energy suppliers to improve the quality and reliability of theirservice. To answer this call for better power quality, many energy suppliers are turning to custompower devices. However, many engineers have had little or no exposure to custom power devicesand their proper application

The application of a custom power device is much more complicated than the application oftraditional distribution equipment, such as a voltage regulator. Couple this with the fact thatmany utility engineers are not well versed in power quality and you get a situation in which asuccessful application of a custom power device is difficult. Manufacturers can be a very goodsource of information for the engineer. However, the engineer needs to know what type of devicebests fits a particular situation before contacting manufacturers. Otherwise, a manufacturer maytry to promote a less-than-ideal solution in order to sell a particular device that it produces.

Introduction

1-2

There are also many technical reports and papers written on custom power devices in general, aswell as specific devices, such as static series compensators. However, the average utilityengineer will not have all of these at his or her disposal. Taking the time to research the subjectand acquire the related documents may not be an option when a customer is demanding asolution to a particular problem. Due to the long lead times of custom power devices and thedistribution construction often necessary for their installation, an engineer often has a very shortperiod of time in which to choose a type of device and begin contacting manufacturers. Knowingwhat types of information a manufacturer will need to properly size and specify a custom powerdevice will help the engineer reduce the time needed to get the proper device on order.

This guidebook addresses these issues by collecting pertinent information into a singledocument, providing education to the engineer with regard to power quality disturbances,providing information about the types of disturbances each custom power device can mitigate,and providing information about the information needed for sizing and specification of custompower devices. The guidebook also provides the engineer with any operation and maintenanceconcerns that may be associated with a particular type of device, as well as pointing out the typesof issues than can arise when using a leading-edge device.

Contents

This guidebook begins with a description of power quality events and a discussion of theirprevalence in distribution systems. The purpose is to give someone who is unfamiliar with powerquality the information necessary to determine the types of events that need to be mitigated. Nextis an overview of the types of custom power devices currently available. A brief discussion ofthe operation of each type of device is provided along with information on the types of powerquality events that each can mitigate. Also, a discussion of the relative cost and cost/benefitanalysis of custom power devices is provided.

The available custom power devices are then divided into chapters, with each chapter coveringspecific detail relevant to the particular type of device. Items covered include sizing, specifying,operation, and maintenance. Case studies are also provided to illustrate the possible concerns.These case studies provide the engineer with some insight as to the possible problems that maybe encountered during the installation and operation of the device.

The results of a survey of utilities that have applied custom power devices are provided. Thesurvey results illustrate the fact that custom power devices cannot, at this point, be applied aseasily as traditional distribution equipment. The results also indicate some of the barriersfinancial and technicalto applying custom power devices. It also indicates the need to workwith the manufacturer before and after installation in order to ensure a successful project.

Last, a list of manufacturers of custom power devices is provided, along with some informationon the ratings of the devices they offer. The list of available devices increases every year asadvances in power electronics are made. This list provides a good starting point when looking fora device of a certain type.

Introduction

1-3

Major SourcesThe goal of this guidebook is not to duplicate work currently available, but rather to collectinformation into a single location and then supplement it with the information and proceduresneeded by a utility engineer. To that end, information from the following sources was used whenpossible. Other sources are listed in the Bibliography. However, those sources listed hereprovided the bulk of the currently available information on the application of custom powerdevices.

IEEE P1409 Guide for Application of Power Electronics for PQ Improvement onDistribution Systems Rated 1kV through 38kV.

EPRI, Custom Power Primer: Power Quality Solutions for Energy Delivery Systems. PaloAlto, California, 1999.

EPRI, StaticTransfer Switch Primer. Palo Alto, California, EPRI Report TR-111697, 1998. EPRI, System Compatibility Test Protocol for Static Voltage Restoration Devices, Palo Alto,

California, 1999.

2-1

2 RANGE OF POWER QUALITY VARIATIONS INDISTRIBUTION CIRCUITS

Background

Proper application of custom power devices requires a good understanding of the electricalenvironment in which the custom power device will be installed. This requires an understandingof the characteristics of the power quality events and the range of expected variation of theseevents in a typical distribution circuit. Understanding the electrical environment is critical notonly to properly specify the performance requirement for custom power devices but also toensure that such devices have the proper immunity to survive the electrical environment of thedistribution system.

In this chapter, we review the definition of power quality events as described in IEEE Std. 1159-1995, Recommended Practice for Monitoring Electric Power Quality, review the expectedranges based on the EPRI Distribution Power Quality Project, describe some of the subtlecharacteristics of electrical disturbances (specifically characteristics of voltage sags such asphase shift and point-of-initiation), and describe any standards pertaining to the range ofvariation in electrical environment for distribution-level voltages. The material in this section ofthe report has been compiled from several EPRI reports that have been published over the lastdecade and represent a vast body of knowledge about the range of power quality variations indistribution circuits. For a more detailed treatment of power quality variations, refer to thesources referenced in the Bibliography.

Categories of Power Quality Variations

The recent proliferation of electronic equipment and microprocessor-based controls has causedelectric utilities to redefine power quality in terms of the quality of voltage supply rather thanavailability of power. In this regard, IEEE Std. 1159-1995, Recommended Practice forMonitoring Electric Power Quality, has created categories of power quality disturbances basedupon duration, magnitude, and spectral content. Table 2-1 shows the categories of power qualitydisturbances with spectral content, typical duration, and typical magnitude.

Range of Power Quality Variations in Distribution Circuits

2-2

Table 2-1Categories of Power Quality Variation IEEE 1159-1995

CategoriesSpectralContent

TypicalDuration

TypicalMagnitudes

1.0 Transients1.1 Impulsive

1.1.1 Voltage1.1.2 Current

1.2 Oscillatory1.2.1 Low Frequency1.2.2 Medium Frequency1.2.3 High Frequency

2.0 Short-Duration Variations2.1 Sags

2.1.1 Instantaneous2.1.2 Momentary2.1.3 Temporary

2.2 Swells2.1.1 Instantaneous2.1.2 Momentary2.1.3 Temporary

3.0 Long-Duration Variations3.1 Overvoltages3.2 Undervoltages

4.0 Interruptions4.1 Momentary4.2 Temporary4.3 Long-Term

5.0 Waveform Distortion5.2 Voltage5.3 Current

6.0 Waveform Notching7.0 Flicker8.0 Noise

> 5 kHz> 5 kHz

< 500 kHz3002 kHz> 2 kHz

0100th Harmonic0100th Harmonic0200 kHz< 30 Hz0200 kHz

< 200 s< 200 s

< 30 cycles< 3 cycles< 0.5 cycle

0.530 cycles30120 cycles2 sec2 min

0.530 cycles30120 cycles2 sec2 min

> 2 min> 2 min

< 2 sec2 sec2 min> 2 min

steady-statesteady-statesteady-stateintermittentintermittent

0.11.0 pu0.11.0 pu0.11.0 pu

0.11.8 pu0.11.8 pu0.11.8 pu

0.11.2 pu0.81.0 pu

000

020%0100%

0.17%

Voltage Sags, Swells, and Interruptions

Figure 2-1 shows a typical voltage sag, swell, and interruption. A voltage sag is a short-durationdecrease of the RMS voltage value, lasting from 0.5 cycles to 120 seconds. Sags are caused byfaults on the power system or by the starting of a relatively large motor or other large load. Avoltage swell may accompany a voltage sag.

A voltage swell occurs when a single line-to-ground fault on the system results in a temporaryvoltage rise on the unfaulted phases. Removing a large load or adding a large capacitor bank canalso cause voltage swells, but these events tend to cause longer-duration changes in the voltagemagnitude and will usually be classified as long-duration variations.


2-3

A voltage interruption is the complete loss of voltage. A disconnection of electricity causes aninterruption, usually by the opening of a circuit breaker, line recloser, or fuse. For example, if atree comes into contact with an overhead electricity line, a circuit breaker will clear the fault(short circuit), and the customers who receive their power from the faulted line will experiencean interruption.

Figure 2-1Typical Short Duration RMS Voltage Variations

System Faults

Customers located on a faulted feeder will experience one or more interruptions, depending onthe type of fault and the reclosing practices of the utility. For a temporary fault, one or tworeclosing operations may be required before normal power is restored. For a permanent fault, anumber of reclosing operations (usually no more than three) will occur before the breaker locksout. In this case, the customers will experience a sustained interruption. Note that theinterruptions associated with successive operations of the breaker may be of varying durationdepending on relay characteristics. This gives the fault multiple opportunities to clear. Themultiple operations also give sectionalizers the opportunity to operate. These devices typicallyopen during the dead time after counting a certain number of consecutive incidents of faultcurrent within a short time period. The number of fault-current incidents is typically two,although it could be one if the sectionalizer is at the head of an underground cable, where allfaults are assumed to be permanent.

Reclosing practices vary from utility to utility and, perhaps, from circuit to circuit. Feeders thatare mostly underground will typically not have any reclosing operations because most faults onunderground feeders are permanent. Some utilities are experimenting with faster reclosing times(0.3 to 0.5 seconds) for the first reclosing operation in order to solve residential customerproblems with momentary interruptions. Residential electronic equipment such as clock radios,VCRs, microwaves, and televisions can often ride through a 0.5-second interruption but cannotride through longer-duration interruptions. At medium-voltage levels, it usually takes a minimumof 10 to 12 cycles of dead time to ensure that the ionized gases from faults are dispersed.

Customers located on parallel feeders (that is, feeders that are supplied from the same bus as thefaulted feeder) will experience a voltage sag for as long as the fault remains on the line. Onmedium-voltage systems, nearly all faults are cleared within one second and can be cleared in asshort as three cycles, depending on the magnitude of the fault current and the relay settings. Thismeans that customers on parallel feeders will experience at least one voltage sag lasting from


2-4

three cycles to about one second and, possibly, additional voltage sags if reclosing operations arerequired. Voltage sags are much less severe than interruptions, and the duration of interest isonly the period of time that the fault is on the line.

If there are more than two feeders supplied from a common distribution bus, then voltage sagswill occur more frequently than actual interruptions because a fault on any one feeder will causevoltage sags on all the other feeders.

Customers that are fed directly from the high-voltage system (that is, transmission-fed or largeindustrial customers) usually have more than one line supplying the facility. Therefore,interruptions should be very infrequent for these customers. However, these customers willexperience voltage sags during fault conditions over a wide range of the transmission system.Voltage sags caused by faults in a high-voltage system generally have more consistentcharacteristics. The faults that originate in the medium- and low-voltage systems tend to havemore variation.

Because voltage sags can be much more frequent than interruptions, it is important to considerthe impacts and possible remedies for voltage sags separately from the required solutions forcomplete interruptions.

Overvoltages and Undervoltages

Long-duration voltage variations that are outside the normal magnitude limits are most oftencaused by unusual conditions on the power system. For example, out-of-service lines ortransformers sometimes cause undervoltages, as shown in Figure 2-2. These types of RMSvoltage variations are normally short-term, lasting less than one or two days. Voltage variationslasting for a longer period of time are normally correcting by adjusting the tap on a step-voltageregulating transformer.

The root cause of most voltage-regulation problems is that there is too much impedance in thepower system to properly supply the load. The load draws the current that gives a voltage dropacross the system impedance. The resistive drop is in phase with the current, and the reactivedrop is 90 degrees out of phase. Therefore, the load voltage drops low under heavy load. Highvoltages can come about when the source voltage has been boosted to overcome the impedancedrop and the load suddenly diminishes.


2-5

Figure 2-2Example RMS Measurement of Undervoltage

Voltage Flicker

Voltage flicker is an amplitude modulation of voltage at frequencies less than 25 Hz, which thehuman eye can detect as a variation in the light intensity of a lamp. Voltage flicker, as shown inFigure 2-3, is caused by an arcing condition on the power system. The arcing condition may be anormal part of a production process, such as a resistance welder or an electric arc furnace.Voltage step changes greater than 3% usually caused by the starting of large motors may alsocause light flicker, but these events are better classified as sags. Flicker problems can becorrected with the installation of filters, static VAR systems, or distribution static compensators.

Figure 2-3Example of Voltage Flicker Caused by an Arc Furnace

Harmonic Distortion

The phenomenon known as harmonic distortion is the presence of frequencies in the voltage thatare integer multiples of the fundamental system frequency, which is 60 Hz for the North


2-6

American system. Electronic loads and saturable devices generate harmonic distortion.Computers, lighting, and electronic office equipment generate harmonic distortion in commercialfacilities. In industrial facilities, adjustable-speed motor drives and other power electronic loadscan generate significant amounts of harmonics.

It is generally safe to assume that the sine wave voltage generated in central power stations isvery good. In most areas, the voltage found on transmission systems typically has much less than1% percent distortion. However, the distortion may reach 5% to 8% as we move closer to theload. At some loads, the current waveforms will barely resemble a sine wave. Figure 2-4, forexample, shows a waveform with over 17% harmonic distortion.

Figure 2-4Example Voltage Waveform with 3rd Harmonics and 17.42% Total Harmonic Distortion

Electronic power converters can chop the current into a variety of waveforms. Most distortion isperiodic, or harmonic. That is, it repeats cycle after cycle, changing very slowly, if at all. Thishas given rise to the widespread use of the term harmonics to describe perturbations in thewaveform. However, this term must be carefully qualified to make sense.

Solutions to problems caused by harmonic distortion include the installation of active or passivefilters at the load or bus, or taking advantage of transformer connections that enable cancellationof zero-sequence components.

Voltage Notching

Voltage notching is caused by the commutation of power electronic rectifiers. It is an effect thatcan cause concern over power quality in any installation where converter equipment, such asvariable-speed drives, are connected. The effect is caused by the switching action of a drivesinput rectifier. When the DC-link current in a drive is commutated from one rectifier thyristor tothe next, there is an instant during which a line-to-line short circuit occurs at the input terminalsof the rectifier. The result is a phase voltage with four notches per cycle caused by a six-pulseelectronic rectifier, as shown in Figure 2-5.


2-7

Figure 2-5Example Waveform with Notching

Transient Disturbances

Transient disturbances are caused by the injection of energy by switching or by lightning. Thedisturbance may either be unidirectional or oscillatory. Lightning, electrostatic discharge, loadswitching, or capacitor switching may cause a unidirectional transient, as shown in Figure 2-6,which is characterized by its peak value and rise time. On the other hand, an oscillatory transient,as shown in Figure 2-7, is characterized by its frequency content. It can be caused by a switchingoperation such as the energization of a capacitor bank, distribution line, or cable, or the openingof an inductive current. Low- and medium-frequency oscillations, with principle frequencies lessthan 2 kHz, are normally caused by power system switching. The switching of a load close inproximity to the point of interest may cause high-frequency oscillations with principlefrequencies above 2 kHz. Common solutions to problems caused by transients include theapplication of surge arresters, passive and active filters, and isolation transformers.


2-8

Figure 2-6Impulsive Transient Waveform

Figure 2-7Oscillatory Transient Waveform Caused by a Capacitor Energizing

Ranges of Power Quality Variations from the EPRI DPQ ProjectThe EPRI project RP3098, commonly known as the EPRI Distribution System Power QualityMonitoring Project, or EPRI DPQ Project, consisted of a power quality monitoring survey of 277measurement locations on the primary distribution feeder of 24 electric utilities across thecontinental United States, which provided geographical and operating-practice diversity. Theresult of the site-selection process was a set of 100 distribution feeders in the voltage range of 4kV to 33 kV.

The monitoring sites were determined by using a systematic and controlled selection process toprovide a wide diversity of distribution system conditions. The monitored feeders ranged involtage level from 4.16 kV to 34.5 kV and in length from 1 to 80 km. The 27 months of


2-9

monitoring resulted in a staggering collection of data that was statistically summarized in a three-volume EPRI report1. The data collected during the measurement period provides a statisticallyvalid sample of the range of power quality events in a distribution system, although notnecessarily valid at any given site.

Figures 2-8 through 2-12 provide some results from the DPQ study to quantify the electricalenvironment based on the monitoring results. The data shows the sag and interruption rate,average magnitude and duration of sags and interruptions, oscillatory transient rate, averagemagnitude of oscillatory transients, voltage THD, and individual harmonics from all monitoringsites.

Sag and Interruption Rate Magnitude His togram

0.00

0.25

0.50

0.75

1.00

1.25

0 to

55

to 1

010

to 1

515

to 2

020

to 2

525

to 3

030

to 3

535

to 4

040

to 4

545

to 5

050

to 5

555

to 6

060

to 6

565

to 7

070

to 7

575

to 8

080

to 8

585

to 9

0

RMS Voltage Magnitude (%)

Sags

and

Inte

rru

ptio

ns

per

Site

per

30 D

ays

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Cum

ulat

ive F

requ

ency

Sag and Interruption RateCumulative Frequency

All Sites , One-Minute Aggregate Window

Figure 2-8Sag and Interruption-Rate Magnitude Histogram, One-Minute Aggregation, 6/1/93 to 6/1/95,Treated by Sampling Weights, All Sites

1 An Assessment of Distribution System Power Quality : Volumes 1-3; TR-106294-V1, TR-106294-V2, TR106294-

V3


2-10

1 cy

c3

cyc

5 cy

c

10 to

20

cyc

0.5

to 1

s

2 to

5 s

10 to

30

s

1 to

2 m

in

0 to

10

10 to

20

20 to

30

30 to

40

40 to

50

50 to

60

60 to

70

70 to

80

80 to

90

0.0

1.0

2.0

3.0

4.0

5.0

6.0

Duratio n Vo ltage (%)

S ag and Interruptio n Rate per S ite per 365 Days

RMS Vo ltag e Variatio n S ag and Inte rruptio n Rate

Figure 2-9Sag and Interruption Rate Magnitude Duration Histogram, One-Min Aggregation 6/1/93 to6/1/95, Treated by Sampling Weights, All Sites

Peak Magnitude of Oscillatory Transients

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1.05

- 1.

10

1.10

- 1.

15

1.15

- 1.

20

1.20

- 1.

25

1.25

- 1.

30

1.30

- 1.

35

1.35

- 1.

40

1.40

- 1.

45

1.45

- 1.

50

1.50

- 1.

55

1.55

- 1.

60

1.60

-1.65

1.65

- 1.

70

1.70

- 1.

75

1.75

- 1.

80

1.80

- 1.

85

1.85

- 1.

90

Absolute Peak Magnitude (pu)

Mea

sure

men

ts

per

Site

per

30

Da

ys

0%10%20%30%40%50%60%70%80%90%100%

Cu

mu

lativ

e Fr

eque

ncy

Events per MonthCumulative Frequency

Figure 2-10Histogram for Magnitude of Oscillatory Transients Measurement Events, 3/1/95 to 9/1/95,Treated by Sampling Weights, All Sites


2-11

1 3 5 7 9

11 13 15

1.051.2

1.351.5

1.65

1.80

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Peak Magnitude and Duration o f Os c illatory Trans ients

Mea

sure

men

ts

per

Site

per

30

Day

s

Pe ak Magnitude(pu)

Duration (ms)

Figure 2-11Magnitude and Duration of Oscillatory Transients MeasurementEvents, 3/1/95 to 9/1/95, Treated by Sampling Weights, All Sites

0

0.5

1

1.5

2

2.5

THD 2 3 4 5 6 7 8 9 10 11 12 13

Voltage THD and Individual Harmonics

% o

f Fun

dam

enta

l CP05Me anCP95

Figure 2-12Voltage THD and Individual Harmonics, 6/1/93 to 3/1/95, All Sites

In addition to the DPQ results, Table 2-2 shows the expected electrical environment in thedistribution level as defined by the International Electrotechnical Commission (IEC) in IEC61800-3.


2-12

Table 2-2Service Conditions for the Voltage Supply at Medium Voltage

Phenomenon Referencedocument

Level

Frequency variations IEC 61800-3 fLN 2%

fLN 4% (for separated supply networks)Frequency rate-of-change IEC 61800-3 2% fLN/s

Voltage variations IEC 61800-3 10%

+ 10%, -15% 1 min

Voltage fluctuations IEC 61800-3 Maximum step amplitude:

12% within the tolerance band

minimum interval between steps: 2s

rise time: 5 periods of the supply

Voltage dips IEC 61800-3 10 50% t 300 ms

(noticeable changes of operatingcharacteristics, self recoverable)

Voltage unbalance IEC 61800-3 HV supply:

2% (zero- and negative sequence components)LV supply:

3% (zero- and negative sequence components)Voltage harmonics:

steady state

transient

IEC 61800-3 THD 8% steady state

THD 10% t 15 s

Voltage interharmonics:

steady state

transient

IEC 61800-3 THD 0,5%

THD 0,75% 5 15s

Commutation notches IEC 61800-3 depth: 40% ULWM

area: 120% x degrees


2-13

Characteristics of Voltage Sags

Voltage sags are the principal power quality concern for process industries. Typically, thesedisturbances are described in terms of RMS magnitude and duration. However, compatibilitytesting has shown that other characteristics of a disturbance play a role in whether a process willbe affected. In order to develop effective mitigation methods and better define power qualitycontracts between a power provider and a power consumer, we need better information on howthese characteristics impact process industries and statistically how much the characteristics varyin magnitude, duration, and frequency of occurrence.EPRI initiated a project under WO5460 to identify the characteristics of voltage sags, developmathematical definitions to quantify these characteristics and determine statistics describing therange of values and frequency of occurrence for certain waveform characteristics. Thesecharacteristics were described by EPRI project WO5460 through the use of the database ofvoltage waveforms collected during EPRI DPQ project. Key findings of the project that impactproper application of custom power devices are summarized in the following sections. For acomplete assessment of the statistics for waveform characteristics, refer to EPRI TR- 112692,Waveform Characteristics of Voltage Sags: Statistical Analysis.

Point of Initiation

Figure 2-13 shows statistics related to the point of initiation for the RMS variation measurementsrecorded during the EPRI DPQ Project. The point of initiation indicates where the disturbancebegins to deviate from the expected waveform in the measurement. The point of initiation valueis reported in degrees with a range from -180 degrees to 180 degrees. The reference sine wavefor this characteristic has zero values at -180, 0, and 180 degrees; a negative peak is at -90degrees, and a positive peak is at 90 degrees. It is interesting to note that the sag initiation wasnot biased to voltage peaks. That is to say, sags are no more likely to occur at a voltage peak thanany other location on the waveform. Figure 2-13 shows that the characteristic known as point ofinitiation is fairly random. However, the eight tallest columns in the histogram suggest that thereare points in a waveform where a voltage sag seems slightly more likely to initiate. These eightcolumns represented 36.7% of the weighted samples and represent these four ranges: from -180to -160 degrees, from -70 to -50 degrees, from 0 to 20 degrees, and from 110 to 130 degrees.


2-14

Histogram of Point of Initiation

0%

1%

2%

3%

4%

5%

6%

-18

0

-16

0

-14

0

-12

0

-10

0

-80

-60

-40

-20 0 20 40 60 80 10

0

120

140

160

Point of Initiation (degrees)

Rela

tive

Freq

uenc

y

0%10%

20%30%40%50%

60%70%80%

90%100%

Records: 101779Minimum: -178.594Average: -6.617Maximum: 180.0

Cum

ula

tive

Freq

uenc

y

Figure 2-13Histogram of Point of Initiation for RMS Variations 6/1/93 to 6/1/95, Treated by SamplingWeights, All Sites

Point of Recovery

The point of recovery indicates where the disturbance begins to return to the expected waveformbased on a pre-disturbance phase-locked loop in the measured waveform. It estimates where thevoltage disturbance ends. Figure 2-14 shows a histogram of the point of recovery for the RMSvariation measurements from the EPRI DPQ Project. In creating the plot, an additional filter wasapplied that ignored records with a point of recovery less than -180 degrees or greater than 180degrees. It is interesting to note that nearly 60% of the measurements showed a point of recoverybetween two ranges: from -150 to -80 degrees and from 30 to 100 degrees. The two favoredrecovery ranges are separated by 180 degrees and suggest that recovery occurs as the voltageapproaches a positive or negative peak.


2-15

Histogram of Point of Recovery

0%

1%

2%

3%

4%

5%

6%

-18

0

-16

0

-14

0

-12

0

-10

0

-80

-60

-40

-20 0 20 40 60 80 10

0

120

140

160

Point of Recovery (degrees)

Rela

tive

Freq

uenc

y

0%10%

20%30%40%50%

60%70%

80%

90%

100%


Cum

ula

tive

Freq

uenc

y

Figure 2-14Histogram of Point of Recovery for RMS Variations 6/1/93 to 6/1/95, Treated by SamplingWeights, All Sites

Phase Shift

Figure 2-15 shows a histogram of the maximum phase shift experienced during each RMSvariation event of the EPRI DPQ Project. Interestingly, a greater number of samples exhibited anegative initial transition angle. This tendency may be explained through the inductive nature offault currents. However, this is speculation because no in-depth analysis of this relationship wasperformed.


2-16

Histogram of Maximum Phase Angle

0%

5%

10%

15%

20%

25%

30%

35%

40%

-45

-40

-35

-30

-25

-20

-15

-10 -5 0 5 10 15 20 25 30 35 40

Maximum Phase Angle (degrees)

Rela

tive

Fre

quen

cy

0%

10%

20%

30%

40%

50%60%

70%

80%

90%

100%


Cum

ula

tive

Fre

quen

cy

Figure 2-15Histogram of Maximum Transition Angle during Event for RMS Variations 6/1/93 to 6/1/95,Treated by Sampling Weights, All Sites

Impact of Phase Shift on Sizing of Static Voltage Compensator (SVC)Because an SVC injects voltage on an instantaneous basis, the amount of required boost tocorrect a given voltage sag depends not only on the magnitude of the sag but also on the phaseshift. This situation can be explained using the phasor diagram and waveforms shown in Figure2-16 and Figure 2-17. If phase shift is ignored for a 60% voltage sag, an SVC needs thecapability of injecting 40% voltage, as shown in Figure 2-16.

However, as shown in Figure 2-17, if the voltage sag down to 60% is associated with a 60-degree phase shift, then the required boost capability of the SVC must be greater than 40%.


2-17

Voltage Sag Down to 60% of Nominal for 5 Cycles Without any Phase Shift

-100%

-80%

-60%

-40%

-20%

0%

20%

40%

60%

80%

100%

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

Voltage Injection Capability Required by the SSC to Correct Voltage Sag Down to 60% of Nominal for 5 Cycles Without any

Phase Shift

-100%-80%-60%-40%-20%

0%20%40%60%80%

100%

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

Vnom = 100%Vsag = 60%

Vinj = 40%

Figure 2-16Voltage Injection Required to Correct Voltage Sag Down to 60% of Nominal with No PhaseShift

Voltage Sag Down to 60% of Nominal for 5 Cycles with 60 Degrees Phase Shift

-100%

-80%

-60%

-40%

-20%

0%

20%

40%

60%

80%

100%

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

Injection Capability Required by the SSC to Correct Vltage Sag Down to 60% of Nominal for 5 Cycles with 60 Degrees

Phase Shift

-100%

-80%

-60%

-40%

-20%

0%

20%

40%

60%

80%

100%

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

Vnom = 100%

Vsag = 60%

Phase Shift = 60oVinj = sqrt{V2nom + V2sag - 2 Vnom VsagCos ()} = 56%

Figure 2-17Voltage Injection Required to Correct Voltage Sag Down to 60% of Nominal with 60-Degrees Phase Shift

The phase shift at any given site will vary based on the fault characteristics and the systemcharacteristics of the site. If statistical data for phase shift at the SVC site is available, therequired boost capacity for the SVC should be sized based on this information. Table 2-3 showsthe required voltage injection capability of an SVC for different phase shifts. If site-specificphase-shift data is not available, then data based on the DPQ results shown in Figure 2-17 may


2-18

be used. Based on that, 95% of the voltage sag events during the DPQ project had a phase shiftof 15 degrees or less. As can be seen from Table 2-3, for a 15-degree phase shift, the errorintroduced in the sizing of the SVC is negligible, and the impact of phase shift can be ignored forSVC sizing.

Table 2-3Voltage Injection Required by an SVC for a Range of Voltage Sag Magnitude and PhaseShift

0 15 30 45 60 75 9080% 20% 23% 31% 40% 50% 61% 71%60% 40% 41% 45% 50% 57% 64% 72%40% 60% 61% 62% 65% 68% 72% 77%20% 80% 80% 81% 82% 83% 85% 87%

Voltage Injection Capability in % of RMS nominal Voltage Required by the SSC for Correcting Voltage Sags with Phase Shift in Degrees

Voltage Sag MagnitudeIn % of RMS Nominal

Missing Voltage

Missing voltage is the instantaneous difference between a measured waveform and an idealwaveform based on pre-fault conditions. It is the voltage that is required to be injected to makethe actual voltage waveform become a continuation of the pre-fault waveform. Instantaneousmissing voltage can be computed at any point in the voltage waveform. However, a goodprocedure to summarize the statistics of missing voltage is to examine the duration that themissing voltage is above a given level.

Table 2-4 shows the rate of occurrence required for various cumulative voltage compensationrequirements. In each case, a determination of how many times per 365 days various levels ofmissing voltage occur is made. For example, the average site in the EPRI DPQ Project wouldhave required at least 75% compensation at least 17.35 times per 365 days.

Table 2-4Rate of Occurrence for Various Levels of Cumulative Voltage Compensation for RMSVariations 6/1/93 to 6/1/95, Unweighted, All Sites

Missing Voltage for Full Compensation

RMS Variationsper 365 days

>5% >25% >50% >75% >100% >125% >150% >175%

142.07 142.07 71.42 33.97 17.35 8.50 3.61 1.56 1.05

Figure 2-18 presents a summary of the duration of missing voltage that was measured for RMSvariations from the EPRI DPQ Project. One example interpretation of the graph is that anaverage compensation of at least 25% was required for no more than 30 ms a total of 20 timesper 365 days. The results were computed by using 60-second temporal aggregation, which meansthat measurements that were recorded close chronologically were not counted more than once.


2-19

0 20 40 60 80 100 120 140 160 180

>5%

>25%

>50%

>75%

>100%

>125%

>150%

>175%

Duration (msec)

Mis

sin

g Vo

ltage

Missing Voltage - Duration Requirements (60-Sec Aggregation )

60-7050-6040-5030-4020-3010-200-10

Rate of Occurrenceper 365 Days

Figure 2-18Missing Voltage - Duration Summary Table: Rate of RMS Variations per 365 days 60-Second Temporal Aggregation 6/1/93 to 6/1/95, Unweighted, All Sites

3-1

3 OVERVIEW OF CUSTOM POWER DEVICES

Introduction

When a power-conditioning device is applied in a medium-voltage distribution system of anelectric utility, its purpose is to protect an entire plant, an entire feeder, or a block of customersor loads. These devices generally have voltage input and/or output ratings between 1 kV to 38kV, with load ratings in excess of 500 kVA. The concept of applying a power-conditioningdevice at this level is known as custom power. The following is a brief discussion of the types ofcustom power devices, their application, and economic considerations.

Reactive Power and Harmonic Compensation Devices

Static Var Compensator

There are three basic configurations of static var compensators (SVCs):

1. FC/TCR (Fixed Capacitor/Thyristor Controlled Reactor)

The FC/TCR shown in Figure 3-1 behaves like an infinitely variable reactor. The unit consists ofone reactor in each phase, controlled by a thyristor switch. The reactive power is changed bycontrolling the current through the reactor by means of varying the firing angle on the thyristorvalve--that is, by controlling the duration of the conducting interval in each half cycle by issuinggating pulses to the thyristors. A fixed harmonic filter provides the capacitive VARs necessaryfor voltage regulation under the worst design conditions. With the filter supplying VARs, theTCR controls the amount of reactive power supplied.

Overview of Custom Power Devices

3-2

Figure 3-1FC/TCR (Fixed Capacitor/Thyristor Controlled Reactor)

2. TSC (Thyristor Switched Capacitor)

The TSC shown in Figure 3-2 consists of several sets of thyristor switched capacitor (TSC) steps.The major components include capacitors, thyristor switches, fuses, and possibly a soft-startresistor system. The control valve, or switch, is often an anti-parallel connected thyristor/diode orthyristor/thyristor pair. A parallel diode would keep the capacitors charged while in standby.When the control turns on a capacitor step, the charged capacitor results in no voltage across theclosing thyristor. This is a result of the natural operation of the thyristor in which the device,when gated, waits until the correct forward biasing voltage is applied across its terminals ( fewvolts). This results in no inrush current, no generated harmonics, and no over duty on thecapacitors. A capacitor is switched off at current zero, leaving it charged and ready to beswitched again. The controlled switching allows for thousands of operation per day.

When the TSC is started, a resistor in series with the capacitors can ensure that they are chargedslowly, avoiding high inrush currents and system disturbances. After the capacitors are initiallycharged, a contactor can automatically bypass the resistor.

Documents

guide book on custom power devices