TB006 -Current Transformer Polarity Markings

SUBSTATION EARTH SYSTEM INJECTION TESTING SWP

Check this is the latest Process Zone version before use. Page 1 of 10 Standard Work Practice SP0510 Ver 2

Ergon Energy Corporation Limited ABN 50 087 646 062 Ergon Energy Queensland Pty Ltd ABN 11 121 177 802

1. PURPOSE AND SCOPE

The purpose of this Standard Work Practice (SWP) is to standardise and prescribe implementing an 'Earth System Injection Test' for the purpose of determining the Current Distribution, Earth Potential Rise (EPR) and Personnel Safety at a particular site. Personnel Safety includes the determination of possible Step, Touch and Transfer voltage hazards.

This applies to the testing of new and existing earthing systems, with the purpose of validating the adequacy of the earthing system to meet design and safety criteria.

2. STAFFING RESOURCES

a) EFM competent in the use of test equipment.

b) Safety Observer (required for all “live work” as defined in the ESO Code of Practice for Electrical Work).

Both are required to -

Be trained in switchboard rescue and resuscitation and manual handling techniques.

Have appropriate Switching & Access authorisations for the roles they are required to perform and have the ability to assess and maintain relevant exclusion zones from exposed live electrical apparatus.

Hold current licences for any vehicles and equipment they may be required to operate.

Required Training and Certificates

Regulatory Training

Course Code Course Description – Regulatory Training 2120 Cardio-Pulmonary Resuscitation

2130 Low Voltage/Switchboard Rescue

2140 Pole Top Rescue (if required)

2160 EWP Rescue and Escape (if required)

3131 Pole Testing for Safe Access (if required)

Additional Training

Course Code Course Description – Additional Training

4445 Switching Operator and Authorised

4440 Switching Operator Assistant and Authorised

4430 Safe Entry to High Voltage Enclosures and Authorised




3. DOCUMENTATION

CS000501F115. Daily/Task Risk Management Plan ES000901R102. Health and Safety Risk Control Guide SP0510R01 Substation Earth Testing Job Safety Analysis

SP0510C01 Earth System Current Distribution Test Report

SP0510C02 Earth System Voltage Gradient Test Report

SP0510C02R01 Earth System Voltage Gradient end point (calculation spreadsheet0

SP510C03 Step and Touch Potential Test Report

SP0506 Carry Out Field Testing SWP Maps and site plans – use these for marking where test readings were taken, these may include but not be limited to:

Local map – Can give useful information on surrounding area

Civil Works - Site Clearing Plan

Civil Works - Earth Grid Plan

Test Reports

Fault levels for all voltages at substation or site – these are used to calculate estimated Step and Touch voltages under fault conditions. For Ergon sites, these are normally obtained from NP&D group.

Clearing times for faults use these to determine 'Allowable Prospective Voltage Criteria'. For Ergon sites, these are obtained from the Protection Group.

Earth Resistivity Test results

Test Equipment Manuals

4. KEY TOOLS AND EQUIPMENT All equipment used is to be calibrated and within test due date.

Current Distribution Frequency tuneable voltmeter or Spectrum Analyser Godowsky coil

Earth Potential Rise Frequency tuneable voltmeter or Spectrum Analyser Spool(s) of cable (distance required will depend on recorded measurements) up to 1km may need to be available. Earth test electrode (stake)

Step and Touch Potential Frequency tuneable voltmeter or Spectrum Analyser Two earth test electrodes (stakes) 5m test leads

Switching and Access Operating Equipment – PEDs, Live Line Tester, Class 0 gloves. All equipment to be inspected and confirmed within test date prior to use Suitable barriers and warnings signs for erection at the source and the remote end to prevent inadvertent contact with 'LIVE' equipment PPE including full-length protective cotton clothing, safety footwear, helmet. Additional PPE as required: brim for safety helmet, leather work gloves, class 00 gloves, hearing protection, safety eyewear, high visibility clothing when working on or near roadways. All PPE to be inspected and confirmed within test date (where applicable) prior to use.

Sun protection to be used when working outdoors.




5. TASK STEPS 5.1. On Site Risk Assessment

Prior to performing this activity any hazards associated with prerequisite tasks at the worksite shall be identified and assessed with appropriate control measures implemented and documented in accordance with Daily/Task Risk Management Plan CS000501F115 and using reference document Health and Safety Risk Control Guide ES000901R102.

If any risks cannot be managed or reduced to an acceptable level, do not proceed with the task and seek assistance from your Supervisor. Measurement of current distribution, earth potential rise and step and touch potential is carried out when current is injected into the earth grid under test. The normal configuration is that an injection source (generator) is connected to the substation earth grid under test and also to an out of service feeder that is shorted to a remote earth grid. In this way a current is circulated through the earth grid under test, through the earth, through the remote earth grid to return along the feeder to the source. The length of the line used for remote injection must be at least 10 times the diagonal length of the substation earth grid to avoid interference to measurements from the remote earth grid. It is preferred that a line length of 50 times the diagonal be used (for a theoretical accuracy of 98.5%)

A generator capable of maintaining a stable frequency off 50Hz is used as an injection source to avoid errors introduced by any standing 50 Hz ground current. Typically 48 or 52Hz is used. Problems have occurred with smaller generators (~7kVA) as they have not been able to be 'tuned' to an off 50Hz frequency successfully. 30 kVA is usually a reasonable size. To achieve a

reasonable current level a step up transformer and/or series tuning capacitors may be required depending on the impedance of the earthmats and the feeder used for injection. The injection voltage may therefore be in the order of 1-2 kV.

Errors are minimised by using an injection current significantly higher than any standing 50 Hz ground current. 20 A to 30 A is preferred, however as low as 10 A is suitable providing a stable frequency injection source (eg solid state) is used with a high selectivity voltmeter/spectrum analyser. 5.2. Hazards and Controls

The feeder connected to the remote earth grid must be either decommissioned / uncommissioned or under a Test Permit as defined in the operate the Network Procedures

The injection source voltage may have to be increased to a high level to get sufficient earth current for accurate measurements. Appropriate barriers and warning signs must be erected at the injection source to prevent inadvertent contact with dangerous voltages.

This voltage will also be transferred to the remote earth grid, and can also be considered dangerous (particularly in the case of a high earth resistance or bad earth connection). Appropriate barriers and warning signs must be erected at the remote earth grid as well as the earth grid under test at the point of connection of the feeder to the earth grid.

Dangerous voltages with respect to remote earths may be transferred out of the substation. Areas to watch are where there is a low impedance connection to a remote earth such as telephone cabling, feeder cable sheaths, overhead earths on other feeders, buried metallic pipes.




The mobile generator and coupling transformer used for current injection may present manual handling hazards. Use appropriate mechanical assistance such as trailer mounted equipment, vehicle loading crane etc.

The injection current should not pass through any current transformers that may inadvertently operate protection (eg line CT bus differential cores)

Three-phase remote injection will cause balanced currents in each phase and negligible earth current.

If one phase of a three-phase generator is used as the injection source, out of balance loading will cause overheating of the generator. A de-rating factor of 10 should be used, although it is preferable to use a single-phase generator instead.

There may be significant current and/or voltage induced in the feeder used for the remote injection from another feeder in the same easement or on the same structures (double circuit line). Always maintain an earth on one end of the injection feeder, and verify that the voltage on the other end is safe for the connection of injection equipment.

Tests should not be carried out in poor weather conditions when a system fault on incoming or outgoing feeders could cause earth potential rise at the earth grid under test. Similarly, tests should not be carried out in poor weather conditions where the feeder being used for injection could be subjected to faults. For long feeders, it is necessary to use aids like the lightning tracker and/or weather radar to ensure good conditions exist for the entire length of the feeder.

5.3. Preliminary Tests

It is assumed at this stage that preliminary tests have already been completed. These are an Earth Grid Continuity Test to prove that all connections to the earth grid under test are sound, and a Soil Resistivity Test. If the injection test is being performed with no prior investigations of the soil structure then the soil resistivity around the installation should be determined at the time of the injection testing.

Current Distribution Determining the current distribution throughout the earthing system under test is vital in determining:

1. What components are independent or dependant of the earthing system under various fault scenarios.

2. Likely voltage hazards.

3. Assessing individual component performance and identification of critical system components.

CAUTION There is a possibility that a system earth fault will occur when tests are carried out, thereby exposing test staff to dangerous step, touch and transfer voltages. Insulated leads and class 00 insulated gloves should therefore be used when carrying out voltage measurements relative to a remote earth. Class 00 gloves are required for step and touch measurements only if the allowable step and touch voltage limits are exceeded.




Measuring the current within a component of the earthing system and tracing that current until it exits the system or passes beyond the system boundary constitutes a current distribution test. Points or paths of consideration are metallic systems passing beyond the boundary of the earthing installation, such as:

Overhead Earth Wires (OHEW)

Cable sheaths, power and communications

Pipelines (water, gas)

Telecommunications systems

LV neutrals

Structures (railway lines, conveyor systems)

Using an existing drawing or one prepared on site, document all possible paths for current to flow (Figure 1). Using the Rogowski Coil and frequency tuneable voltmeter, determine the dispersion of the test current throughout the test site noting current measured and turns used on Rogowski Coil. Results should be recorded on the “Earth System Current Distribution Test Report” Form SP0510C01. From this test you should be able to accurately determine the current flowing into the earth grid via the earth (Igrid)

It may not be practicable to identify every path of current leaving the earthmat, however assuming that 100% of the injected current is exiting via the earthmat under test will result in a calculated earthmat impedance lower than the actual value. It is therefore essential that the current flow in all accessible connections be determined.

Figure 1: Earthing System with Commonly Found Associated Additions Earth Potential Rise (EPR)

The EPR test is used to determine the impedance of the earth grid under test. Using the results of the Current Distribution test we are able to quantify the percentage of injection current actually entering the site earth grid via the ground. With this knowledge and the result from the EPR test we can calculate the earth grid impedance which will be used to find the EPR under fault conditions.

The method that used is called "Fall-of-Potential Method". Figure 2 shows the basic circuit.

OVERHEA

D TRANSMISSION L

INE

O.H.E.W.

SHEATH

UNDERGROUND CABLES

TELECOM C

ABLE

PIPELINE (

WATER/G

AS)

FENCING




Figure 2: Basic Circuit for Fall-of-Potential Method

The EPR is found by measuring the voltage difference between the site earth grid and "Remote Earth". This is achieved by taking measurements between an earth test stake and the site earth grid under test at increasing distances. The stake need only be driven to a depth that will support it. The test lead used to measure the EPR of a system should be run at either 90 or less to the injection loop, as at these angles the induced voltage is additive and hence a conservative result will be obtained. However, leads run at angles greater than 90 are influenced by magnetic end effects and this is not easily calculated.

Measurement distances should increase until a definite plateau in results is observed. No less than three lots of results within a few percent of each other should be interpreted as a plateau. This is where extra reels of cable may be necessary to get the required result. Figure 3 shows a typical EPR curve. At the most remote measurement location it is required to take a reading from a RMS indicating voltmeter (e.g. fluke) to double check the results of

either the spectrum analyser or the frequency tuneable voltmeter. Results should be recorded on the “Earth System Voltage Gradient Test Report” Form SP0510C02. Spreadsheet SP0510C02R01 can be used to estimate the earthmat potential rise at an “infinite” distance, i.e. the true EPR.

Figure 3: Typical Fall-of Potential Curve

Step and Touch Voltages

The safety of a substation earthing system depends on the step, touch and mesh voltages being less than the allowable voltages specified at the end of this SWP. The allowable voltage depends on various parameters such as fault clearing time, permissible

Vm

POTENTIALELECTRODE

EARTHIMPEDANCE

SOURCE

EARTHINGSYSTEM

Is

CURRENTELECTRODE ORREMOTEEARTHINGSYSTEM

Zg

INJECTED CURRENT If

Vm

x(m)

X

X

X

XX

X X X




current by body weight, the earth resistance between hand and two feet in parallel (touch voltage) and the earth resistance between feet (step voltage).

Figure 4: Step, Touch, Mesh and Transfer Voltages

Possible step, touch and transfer voltage hazard locations associated with metalwork in or adjacent to an earthing system, must be identified and measured to ensure safety for power authority personnel and the public. Recommended locations to conduct touch, step and mesh voltage measurements include:

Within Substation

Within the substation the following voltages should be measured:

In the centre of large grid meshes (mesh voltage)

Corner meshes (adjacent to corner at 1 and 2 m spacing)

Adjacent to equipment that may be operated (water taps, operating handles etc). It is required to obtain at least one measurement from every outdoor bay in the substation.

External to Substation

Design verification can be made at a number of locations external to the substation including:

External perimeter fence corners

Frequented areas (such as fence gates opened out if possible)

Nearby towers (if connected to the grid)

Any section of the perimeter fence that the public may come into contact with.

Any metallic pipelines or fences in close proximity to the earthing system. These can often be the worst measurements recorded during a test because a fence or pipeline can be insulated from the ground for a significant distance and therefore transfer a remote earth potential into (or adjacent to) the substation under test.

With the 'Earth Grid Plan' and Step and Touch Potential Test Report SP0510C03, measurements will be taken using two potential electrodes (1m apart) to determine step potential. Touch potential is taken by having a potential electrode 1 m from the plant or apparatus that will be touched. It is important that the potential electrode is NOT driven into the earth more than a few centimetres, merely enough to support the electrode. Results should be recorded on the Step and Touch Potential Test Report SP0510C03.

V step

V Touch V Mesh

V Transfer

Grid Conductors

Surface Voltage Gradient

External Wire Fence Insulating Posts

Substation Fence




To check that your whole site would be safe under fault conditions, find the highest Touch, Step and Mesh voltages from your measurements and use the following formula to determine actual voltages.

Using Igrid (i.e. the proportion of the injection current that enters the earth grid under test) from the 'Current Distribution' test and the measured EPR find Zgrid.

gridgrid I

EPRZ

Using the Fault Level (IFL ) and the ratio of Igrid and Iinject you can find the current into the grid under fault conditions (If).

inject

gridFLf I

III

EPRf is the EPR under fault conditions and is calculated by:

gridff ZIEPR

Substituting for If and Zgrid.

inject

FL

gridinject

gridFL

gridff

IEPRI

IEPR

II

I

ZIEPR

To find the highest step, touch or mesh voltage (Vstm) under fault conditions use the highest voltage for each from the tests in the formula:

measured

inject

FL

measuredf

VII

EPRVEPRVstm

If the highest is below the calculated 'Allowable Prospective Voltage Criteria' then no further investigation is required.

Always use the appropriate Fault Level (IFL) in the above calculation. For example, a fault at a step-down substation on the HV system will cause significant earthmat potential rise, whereas an LV fault at the same substation, even though the fault level may be higher, will cause negligible earthmat potential rise because the fault is contained within the substation. An external LV fault will cause earthmat potential rise, however the fault level decreases rapidly as the distance to the fault increases.

Note that the Powerlink test procedure divides EPRf and Vstm calculated above by K2 (shielding factor of the injection feeder). This assumes a worst case scenario that the feeder used to inject the test current does not have any earth current return via it’s overhead earth during a fault at maximum fault level IG.

If the Allowable Prospective Voltage Criteria are exceeded, measurements should be repeated using the simulated personnel method instead of the open circuit measurements described above. For the simulated personnel method, the body impedance is simulated by a 1000 ohm resistor, and the foot to ground contact resistance by a 20 kg 8 cm radius disc weight. The ground contact area should be wetted down to simulate worst case conditions.




REFERENCES ESAA EG1 – 2000 "Substation Earthing Guide"

ANSI/IEEE Std 80 – 1986

For co-ordination of earthmat injection testing with Telstra, contact David Lister from Telstra Power Co-ordination [email protected]

Useful Formula Allowable Prospective Voltage Criteria (Open Circuit)

Prospective Touch Voltages

Prospective Step Voltages

50 kg body weight (to be used in areas with public access)

tC ss174.0116

tC ss696.0116

70kg body weight (to be used in restricted areas within a substation)

tC ss236.0157

tC ss942.0157

where

Cs = Derating factor relating to surface layer thickness and resistivity (= 1 when crushed rock resistivity is equal to soil resistivity).

s = Resistivity of surface material (m)

t = Duration of shock current (seconds). [Clearance time of protection plus circuit breaker opening time]

(m)layer rock crushed of thicknessm)(y resistivit soil

m)(y resistivitrock crushed

factor reflection

08.0/2121

96.01

12

s

s

s

s

n s

n

s

h

K

hn

KC

As an alternative simplification (as per IEEE Std 80 Eq 27):

09.02

109..01

s

ss h

C

These formulas are available in the spreadsheet “SP0510R02 Earth System IEEE 80 Formula”. This spreadsheet is an extract from Design Standard SS-1-7.1 Substation Earthing and uses the following guidelines:




Inside the substation: 70kg person, primary protection time

Outside the substation: 50 kg person, backup protection time

Surface layer resistivity (saturated): 3000 ohm-m

Allowable Simulated Personnel Voltage Criteria (Loaded)

Allowable Body Current (amps)

Maximum Simulated Personnel Voltage (V) (assuming 1000 ohm resistor, valid for step or touch measurements)

50 kg body weight (to be used in areas with public access)

t116.0

t0.116

70kg body weight (to be used in restricted areas within a substation)

t157.0

t0.157

Documents

TB006 -Current Transformer Polarity Markings