ECP TESTING AND COMMISSIONING GUIDANCE NOTES

Transcription

1 Document Number: ECP Network(s): Summary: ENGINEERING COMMISSIONING PROCEDURE ECP TESTING AND COMMISSIONING GUIDANCE NOTES EPN, LPN, SPN This document provides guidelines and preferred practices for the safe and effective testing, pre-commissioning and commissioning of electrical plant and associated protection and control equipment installed on UK Power Networks distribution networks. Author: Stephen Tucker Approved By: Paul Williams Approved Date: 16/02/2017 This document forms part of the Company s Integrated Business System and its requirements are mandatory throughout UK Power Networks. Departure from these requirements may only be taken with the written approval of the Director of Asset Management. If you have any queries about this document please contact the author or owner of the current issue. Applicable To UK Power Networks External Asset Management G81 Website Capital Programme Contractors Connections ICPs/IDNOs Health & Safety Meter Operators Network Operations Procurement Technical Training UK Power Networks Services THIS IS AN UNCONTROLLED DOCUMENT, THE READER SHALL CONFIRM ITS VALIDITY BEFORE USE

2 Revision Record Version 4.0 Review Date 10/02/2022 Date 10/02/2017 Author Stephen Tucker Why has the document been updated: Alignment with other referenced documents. What has changed: Document references updated (Sections 4, 4.4, 8 and 20) Renumbered from ECS to ECP Document reviewed and review date extended Version 3.0 Review Date Date 18/03/2014 Author Philip Bennett Renumbered from ECP to ECS Reviewed and reformatted for G81 website Version 2.0 Review Date Kevin Burt Date 19/06/2012 Author Modified to include requirements for checking site and plant cleanliness Version 1.1 Review Date John Lowe Date 11/03/2011 Author Document rebranded Version 1.0 Review Date Date 17/11/2004 Author Stephen Tucker Original UK Power Networks 2017 All rights reserved 2 of 48

3 Contents 1 Introduction Scope Glossary and Abbreviations Co-ordination and Delegation of Commissioning Work Commissioning Personnel Commencement of Testing Quality Control and Attention to Detail Safe Working Practices Documentary Records of Protection and Control Equipment Recording of Test Results Inspection and Testing Order Guidance on Measuring Instrumentation Practice Choice and Use of Test Instruments General Measuring Instrument Practice Measuring Large Quantities Safety Precautions when using Instruments Inspection of Plant Switchgear and Conductors Resistance Measurements Distribution Transformers Primary System Transformers High Voltage Pressure (Dielectric) Testing Wiring Insulation Resistance Tests Circuits to be Tested Precautions Test Voltage and Results CT Secondary Circuits Loop Resistance Tests Current Transformer Secondary Circuits Pilot Wires Other Circuits Current Transformer Magnetisation Curves Secondary Injection and Protection Relay Tests Secondary Injection Applied to Current Operated Measuring Relays IDMT Relay Operation and Timing Tests UK Power Networks 2017 All rights reserved 3 of 48

4 12.3 IDMT Timing Accuracy Start Timing Function Electromagnetic Relay Electronic Relays Instantaneous Relays Complex Protection Relays Voltage Operated Relays in High Impedance Schemes All or nothing Auxiliary Relays Other Devices Current Transformer/Voltage Transformer Polarity and Phasing Tests Primary Injection Tests General Requirements Current Primary Injection of Simple Schemes Current Primary Injection of More Complex Schemes Voltage Primary Injection Voltage Primary Injection of 33kV Neutral Voltage Displacement Protection Functional Checks and DC Operations General Requirements for Functional Testing Tests Full Load Acceptance Test Telecontrol Commissioning Final On-Load Tests Re-commissioning after Fault Repair or Modification References UK Power Networks Standards National Standards Appendix A Diagrams Appendix B Overcurrent Relay Characteristics UK Power Networks 2017 All rights reserved 4 of 48

5 Figures Figure A-1 Test Circuit for CT Magnetisation Curves Figure A-2 CT Magnetisation Curve Example Figure A-3 Flick Test for CT Polarity Figure A-4 CT ratio and Polarity Check by Primary Injection Figure A-5 Typical Bias Differential and Earth Fault Protection Scheme for Yd1 Transformer (+90 Connection) Figure A-6 Typical Scheme for Yd1 Transformer Proof for Phase through Fault Stability Figure A-7 Typical Scheme for Yd1 Transformer Proof for Earth through Fault Stability 44 Figure A-8 Typical Scheme for Yd1 Transformer Simulated Internal Earth Fault (Operate) Conditions Figure A-9 Typical Scheme for Yd1 Transformer Simulated Internal Earth Fault (Stability) Conditions Figure A-10 CT Relay Secondary Injection Tests Tables Table 12-1 Accuracy of IDMT Relays UK Power Networks 2017 All rights reserved 5 of 48

6 1 Introduction This document provides guidelines and preferred practices for the safe and effective testing, pre-commissioning and commissioning of electrical plant and associated protection and control equipment installed on UK Power Networks distribution networks. This document complements, and should be read in conjunction with the references in Section Scope This document provides general guidance on the aims, methods and practice of performing pre-commissioning and commissioning of the following plant and associated systems: 11kV, 33kV and 132kV transformers, switchgear, overhead lines and cables. Current and voltage transformers associated with the above. Protection and control systems associated with the above. Supporting auxiliary systems associated with the above. It is not feasible to cover every configuration of plant and equipment but the guidance and practices outlined are generic to those most likely to be encountered. Some equipment and systems, which require more specialised procedures, may be outside the scope of this document but the philosophy described within and the other referenced engineering standards, should also be applied to such systems wherever practicable. Guidance notes on such systems will be referenced where these are available. The guidance applies to existing plant and systems as well as new plant and systems. Typically, such testing would be required following installation, maintenance, overhaul, relocation, modification or to satisfy reliability targets. While plant inspection is not wholly a subject associated with commissioning it is considered an important aspect which should be completed before commencement of comprehensive electrical testing. This will ensure that the plant has been correctly erected and is ready to progress to the testing stage. Some typical guidance is included with respect to switchgear and transformers; more detailed inspection schedules can be found in the respective test forms. UK Power Networks 2017 All rights reserved 6 of 48

7 3 Glossary and Abbreviations Term AVC AVO CT E/F EDS ENA Fluke IDMT IR LEM NORMA NVD O/C OHL REF RMU SBEF SCADA SEF TCS VT XLPE Definition Automatic Voltage Control Product trade name for analogue multimeter manufactured by AVO International Ltd, Dover Current Transformer Earth Fault Engineering Design Standard Energy Networks Association Digital Multimeter manufactured by Fluke Ltd Inverse Definite Minimum Time (Relay) Insulation Resistance Instrument manufactured by LEM-Norma Ltd Neutral Voltage Displacement Overcurrent Overhead Line Restricted Earth Fault Ring Main Unit Standby Earth Fault Supervisory Control and Data Acquisition Sensitive Earth Fault Trip Circuit Supervision Voltage Transformer Cross Linked Polyethylene UK Power Networks 2017 All rights reserved 7 of 48

8 4 Co-ordination and Delegation of Commissioning Work Reference should be made to ECP for the quality control, planning and programming of commissioning activities. Some general aspects in connection with commissioning activities, outlined in the following sections, augment the requirements of ECP Commissioning Personnel It is desirable that the Commissioning Engineer should remain the same throughout all commissioning activities. Delegation of work may be necessary on larger schemes where the Commissioning Engineer may require assistance. The Commissioning Engineer's responsibility will be to co-ordinate the work and the results obtained, and should be present at all tests where special care has to be taken. However, for smaller or one-off installations, it is accepted that the same person may have to be responsible for both construction and commissioning. 4.2 Commencement of Testing Testing should always be carried out in a logical and efficient order at a relatively late stage in the construction work to ensure that nothing that has already been proved is disturbed by subsequent construction or testing. 4.3 Quality Control and Attention to Detail In all situations, suitable standards should be maintained in terms of: Care. Co-ordination. Completeness. The Commissioning Engineer should make no assumptions and should not accept any circuit as proven until this has been demonstrated in both a positive and negative sense and documented in approved records. A methodical and structured approach together with awareness of what is occurring around the immediate place of testing is essential. For example, the testing process in progress on a different feeder circuit may have caused an alarm received on an apparently unrelated feeder circuit, and all such incidents should be fully investigated. Co-ordination should include a reappraisal of existing network conditions before connecting new extensions. Any necessary revisions to the protection settings elsewhere should be made, to allow for the new plant. Any interim settings applied when setting up the new relays should be confirmed with the planning or protection engineer before the plant is finally put into service. UK Power Networks 2017 All rights reserved 8 of 48

9 4.4 Safe Working Practices All commissioning work shall be carried out in accordance with the requirements of the Distribution Safety Rules, Health and Safety at Work Act and associated regulations using safe procedures. Network Control should be kept fully informed of progress. Where auxiliary supplies are employed, for example, to prove control/tripping, tap changer or pumps and fans, due care shall be taken, and other personnel warned that the equipment may be LIVE. Before the date at which high voltage plant could be made LIVE, a written energisation notice (refer to HSS ) shall be issued by those responsible for making LIVE and given to all parties involved in the construction work, and a written acknowledgement obtained in reply. At this stage the work becomes subject to operational discipline and safety procedures and permit-to-work/sanction-for-test procedures shall then be adopted. 4.5 Documentary Records of Protection and Control Equipment Before commencing testing of control and protection circuits, it is essential to fully record details of the types, ratings and serial numbers of all relays, CTs, VTs and other equipment in the scheme. This provides a check for compatibility, and allows for entry onto the asset database. Future type defects, modifications or replacements of equipment may then be identified and managed. 4.6 Recording of Test Results The cost and effort of carrying out commissioning tests can be wasted if the results are not recorded in a consistent and usable manner. Testing during maintenance or fault finding is a follow-on to the commissioning tests, and deterioration of insulation levels, relay performance etc. can be identified by reference back to the earlier results. Approved test forms are available which cover most usual applications are available from the Intranet. (Standard maintenance forms, generally comprising a shorter sequence of tests than those carried out at commissioning, are available separately). The tests stated in the test forms are the minimum required to fully cover the requirements, and have been set out in a logical sequence. Wherever practicable, the test forms are designed to cover several possible alternative schemes of a similar type. Test items should be crossed through wherever they are not applicable in a particular scheme, and any special tests required should be included under 'other tests/comments', or else additional sheets may be attached to the basic form. After installation and commissioning activities have been completed all relevant schematic and wiring drawings shall be marked-up with any revisions necessary and copies sent to Capital Programme Delivery for modifications. The relevant protection should also be recorded and submitted. Refer to EDS for further details on maintaining records and 'as built' information. UK Power Networks 2017 All rights reserved 9 of 48

10 5 Inspection and Testing Order The following generic list of tests and checks covers most equipment, and provides the recommended order in which the tests are normally performed. The order is chosen to provide as much self-checking advantage as possible. If, for instance, the current transformer circuits are to be disconnected to obtain magnetisation curves, then the primary injection tests carried out at a later stage will prove that the connections have been restored correctly. Removal or disconnection of wiring associated with previously tested circuits shall be discouraged, otherwise all testing to fully prove the disturbed circuit should be repeated. Generic inspection and test list in recommended order of execution: 1. Inspection of the plant. 2. High voltage pressure tests of primary plant. 3. Resistance measurements. 4. Insulation tests of ac and dc control and protection circuits. 5. Secondary circuit insulation resistance. 6. CT magnetisation curves.relay secondary injection tests. 7. CT and VT ratio and polarity tests by primary injection.control circuit wring checks. 8. Functional tests of control, closing, tripping and protection, including Inter-tripping if provided. 9. Mechanical/electrical interlock checks. 10. Functional tests of telecontrol controls, alarms, indications and analogues. 11. CT primary injection and wiring loop resistances. 12. Back-tripping and blocking circuit tests. 13. Pre-energisation checks. 14. Post-energisation checks - on-load checks of relays and directional elements, instruments, analogues etc after making LIVE. UK Power Networks 2017 All rights reserved 10 of 48

11 6 Guidance on Measuring Instrumentation Practice 6.1 Choice and Use of Test Instruments All test equipment used during commissioning of plant should have a current 'tested and/or calibrated' certificate. All test equipment used during a particular test should be recorded on the relevant test form. When using a ready-made test set (such as a Megger, Ductor or Pressure Test set) including a built-in meter designed for the purpose, the integral meter may be used for measurement, subject to calibration as above. For most routine testing jobs, an ordinary multimeter such as an 'AVO' or a 'Fluke' are acceptable. However, when using multi-purpose instruments, it is important to appreciate that different types of voltmeter, ammeter and ohmmeter work in different ways, and can give misleading readings in some situations. The most suitable instrument available should always be chosen, depending on the measurement you want to make. For example: 'True RMS' ammeter for waveforms that are not sine waves (e.g. CT Magnetising Current). High impedance voltmeter (e.g. Fluke, not AVO) when reading voltage in electronic equipment, so that the circuit is not loaded by the meter. Traditional analogue ohmmeter with a battery source (AVO or similar) for continuity through electronic devices (which may not conduct below a minimum applied voltage). 'Bridge' type ohmmeter for any continuity value less than about 10 (where an AVO will be inaccurate, and some 'Flukes' may display only one or two Figures). Ohmmeters, particularly, can give very different results depending on what current and voltage values they are using to make the test. If in doubt, check again with a different type of meter, or swap the leads over (in the case of polarity-conscious electronic devices). Diode-resistor series circuits may be impossible to test with a normal multimeter, because of the minimum voltage that has to be applied across the diode before it will conduct. Testing by 'Ohm's Law' using a separate battery, voltmeter and ammeter will overcome this problem. Severe oxidisation or corrosion of relay or switch contacts can cause defective operation in normal service, but even with contacts in good condition a thin surface layer may exist that requires a minimum test-voltage to break down, and a minimum test-current to produce a consistent resistance reading. Resistances of primary busbar connections or switchgear contacts need to be measured with a specially designed test-set capable of producing an adequate current output. AC voltage measurements in high-impedance circuits isolated during testing (e.g. the VT voltage measuring input to a microprocessor directional relay) can be greatly influenced by stray voltage 'pick-up', which can occur via the inductance or capacitance of adjacent live wiring. In some cases it may be necessary to artificially load a 'disconnected' VT input circuit with an external resistor during testing, to collapse any unwanted voltage pick-up. The same type of problem can give rise to spurious voltage readings from high-impedance digital voltmeters, if the clip leads are not making proper contact. If in doubt, the reading may be rechecked with an AVO or similar. UK Power Networks 2017 All rights reserved 11 of 48

12 6.2 General Measuring Instrument Practice Consider the effect of connecting your test meter to the circuit under test. What is the impedance of the meter? Will it disturb the values you are trying to measure? If using more than one test meter at once in a circuit (e.g. ammeter, voltmeter), will the order they are connected in matter, e.g. is the ammeter reading the current drawn by the voltmeter as well as the load? Always select the multimeter range switch or test-lead socket before connecting to the circuit. For accurate readings, select the meter range where the pointer is in the top half of the scale, or there are the most digits displayed. Don't always believe a digital meter! Check that the connections are on firmly, so you don't get stray 'pick-up.' If you suspect a problem, check with an electromechanical type meter instead. If you are trying to measure something that is varying slowly, an analogue meter (pointer-scale or bar graph) will be easier to read than a digital display. 6.3 Measuring Large Quantities Always consider how big the reading is going to be before connecting the meter. Particularly with currents; could the current be bigger than the meter can withstand? AVOs can usually read up to 10A; some digital meters are limited to 2A. For large currents (above about 10A) use either a clip-on ammeter, or better, use a clip-on current transformer to enable a multimeter to read higher ranges. Remember to select the right meter range, and allow for the multiplying factor in the readings. For high voltages (above normal Mains 230/400V), readings from the network are usually taken off the protection or metering voltage transformers, assuming the VT ratio to be exact. For special injection testing (e.g. high knee-point-voltage CTs) then range extenders are available for AVO or similar meters. 6.4 Safety Precautions when using Instruments Always connect the 'earthy' lead first, and disconnect it last. When using a switch or plug-selected multimeter, always start on the highest range. Never change Range while the meter is connected (some modern digital meters can do this automatically). When taking 'split-plug' readings of CT current on-load from a protection relay or test block, ensure that the meter is on the correct ampere range and that the split-plug, leads, and connections to the meter are all in good order. Open circuited CTs are dangerous! When using current or voltage injection from a portable test set or Variac, remember that one side of the output may be LIVE, and its 'earthy' side may be connected to mains Neutral. If using any special mains powered instrument in a metal case (such as an oscilloscope) remember that one side of its input leads will probably be connected to earth. UK Power Networks 2017 All rights reserved 12 of 48

13 7 Inspection of Plant On acceptance of the equipment from the construction engineer, the equipment should be inspected for damage and for conformity to the scheme details, e.g. equipment type, ratings. The equipment should have been inspected for conformance to the Equipment Specification for its type, and if this evidence is not available from the construction stage, then the inspection shall be carried out at the commissioning stage. It may be convenient to delegate some items of inspection to the engineer supervising construction, providing confirmation is obtained and recorded. For example, some wiring items, components and busbars are inaccessible after they are boxed-in, and limited precommissioning (e.g. Ductor tests) may be required at the construction stage. Some general guidance is given in the following sections. For full checklist, reference should be made to the appropriate test form. 7.1 Switchgear and Conductors Check for filling medium quantity, SF 6 gas pressure (or oil level) as appropriate, in the circuit breaker and voltage transformer tanks etc. Where the equipment is fitted with gas pressure switches, or lock in/out device, the Contractor installing the equipment shall have proved the correct operation. When inspecting switchgear, particular attention shall be paid to the cleanliness of all spouts or bushings which may be inaccessible after the gear has been made LIVE. Solid insulation should be cleaned with dry lint-free wipes. Ensure that all switchgear frame earthing and bonding connections are correctly in place and the connections tight. While disconnected from the system, operate the switchgear to all positions. At each position, also check locking points for application of operational and/or safety locks, and correct function. Check mechanical interlocks in all positions. Prove mechanical and electrical position indicators for correct operation and legend. Ensure all panels and covers are correctly refitted before energisation Ensure that all equipment labels, both primary and panel, are in place and correct. Any apparent defect that is discovered on one switch or panel should be thoroughly investigated on all other similar items in the switchboard, and where appropriate reported to Asset Management Resistance Measurements Perform contact resistance measurements, using a test instrument capable of passing 10A test-current, to prove contact integrity at all points. The testing shall include: 1. All site assembled busbar joints. 2. Contact resistance of all circuit breakers and disconnectors. 3. All joints in phase conductors. 4. All joints in earthing conductors. UK Power Networks 2017 All rights reserved 13 of 48

14 Item 1 should be checked immediately after the joint is made and before replacing covers in the case of metal clad switchboards. After switchboards are completely assembled this test should also be performed by measuring between cable box connection of adjacent panel. Inconsistent results shall be investigated, rectified and retested. For item 3 and 4 the resistance shall not be less than an un-jointed section of conductor. 7.2 Distribution Transformers Ensure that all earthing and bonding connections are correctly in place and the connections tight. Ensure that the oil drain valve is closed, and the drain-plug in place. Inspect for signs of oil leakage, especially around bolt-on type radiators, and switchgear/cable box flanges. Check the transformer oil level is visible in the sight glass, allowing for the ambient temperature. The level should correspond to the marked 15 C point. Ensure the transit cap has been removed from the breather pipe. (In rare instances, a dryer may be fitted, check or fill with dry silica gel as applicable). Check free movement of the tap change handle, set to required position (1-5, nominally position 2 or +2.5% HV volts). With system voltage applied to the primary side, check open circuit secondary voltage (250/433V ± 1.25%, or as noted in the project file) and adjust if necessary. Prove phase rotation, and phase-in with adjacent low voltage network. Note: Any change of tapping position is to be made with the transformer dead only. 7.3 Primary System Transformers Ensure that all frame earthing and bonding connections are correctly in place and the connections tight. Check the conservator oil level is visible in the sight glass, allowing for the ambient temperature. Sample and test the oil. The level should correspond to the marked 15 C point. Inspect for signs of oil leakage, especially around bolted connections and bushing/cable box flanges. Carry out functional and secondary wiring tests, on all control circuits, e.g. pumps and fans, oil and winding temperature devices, voltage control, and Buchholz (oil and gas operated) relay. Follow the basic procedures described in Section 9 onwards as applicable. Carry out functional tests on voltage control system, pumps and fans etc. Ensure the pump filters are fitted correctly, and the pump flow direction is correct. Operate Buchholz alarm and trip by injection of air (where possible), and prove operation of panel alarm and trip flag relays. UK Power Networks 2017 All rights reserved 14 of 48

15 Verify accuracy of winding and oil temperature devices by secondary injection and suitable simulation. Perform voltage testing to establish transformer winding connections (vector group). Verify on-load tap changer operation from all locations (local, remote and telecontrol, if applicable) and also on automatic. Verify correct operation and voltage ratio with three phase test voltages over full range. Ensure there is no break in continuity through each three poles of the tap changer across its entire tapping range. This check should be performed using an analogue 1 (AVO) meter setup to measure the magnetising current of each winding using a 400/230VAC test voltage. Any short break in magnetising current during a tap operation can be seen readily using an analogue meter. Verify any parallel running tap change controls. Perform secondary injection to automatic tap changing relays to verify operations at preferred settings. 1 The response time of a digital (fluke) meter is not fast enough to ensure any short breaks in tap changer continuity are seen during the tests. UK Power Networks 2017 All rights reserved 15 of 48

16 8 High Voltage Pressure (Dielectric) Testing All high voltage plant shall be tested 'to confirm' its ability to withstand full system voltage, and to identify any incipient breakdown of insulation. This section outlines the criteria for the onsite insulation testing of cables, switchgear and transformers. Insulation tests shall be carried out before any new, modified or repaired equipment is energised from the power network. Before re-commissioning cables or plant that has been deenergised for an extended period of time, but has not been worked upon, a risk assessment shall be made to decide if insulation testing is required. This assessment shall take into account factors such as the activity of third parties in the vicinity of the cable or plant. The purpose of site insulation testing is to demonstrate that cable and plant can be safely connected to the system. On-site insulation testing is also required to ensure that UK Power Networks complies with its statutory duties under current legislation. Insulation testing of overhead lines is not considered practical. Therefore, before commissioning, a visual inspection shall be made of new lines and the modified parts of previously energised lines. Before re-commissioning overhead lines that have been deenergised for an extended period of time, a risk assessment shall be made to decide if a visual inspection is required and to decide on the best course of action before returning to service. This assessment shall take into account factors, such as the activity of third parties in the vicinity of the circuit or severe weather conditions. All equipment shall pass an insulation resistance test before and after any high voltage dielectric tests. A record of all tests shall be made on the approved test form. The ac and/or dc pressure test values which shall be applied to both old and new plant shall be in accordance ECP UK Power Networks 2017 All rights reserved 16 of 48

17 9 Wiring Insulation Resistance Tests 9.1 Circuits to be Tested Insulation resistance tests are to be carried out on all ac and dc protection, control, alarm and indication circuits to ensure that wiring is in satisfactory condition before the circuit is put into service. It is desirable to measure the insulation of all circuits before proceeding with other tests, and it is essential that all ac and dc wiring associated with protective gear is proved, relay contacts and auxiliary contacts, etc being closed as necessary, to ensure this. When carrying out insulation resistance tests on secondary wiring it is advantageous to earth all other associated secondary circuits other than the circuit under test. This approach provides the additional benefit of detecting any inadvertent connection between circuits indicating a wiring error. Also, this enhances the insulation resistance test because all wiring on the other circuits will be at earth potential rather than floating. Therefore, any insulation damage between loomed secondary wiring will be more readily identified in the readings obtained. The following tests should be carried out: Insulation resistance of current transformer circuits. Insulation resistance of voltage transformer circuits. Insulation resistance of dc circuits. Insulation resistance between CT and VT circuits. Insulation resistance between dc and VT circuits. Insulation resistance between dc and CT circuits. When measuring the insulation resistance to earth of an individual circuit, all the other circuits should be normal, e.g. earth links closed and dc circuits normal. This will ensure that the insulation of this circuit is satisfactory, both to earth and to all other circuits. It should be noted that in some installations the battery may run 'un-earthed' and its insulation-to-earth may not be monitored by a 'battery earth fault' alarm relay, with centre-point earthed through a high resistance to the dc wiring measured. Therefore a complete insulation test of an individual circuit to earth and relative to battery circuit wiring will not be obtainable unless arrangements are made to temporarily earth the battery via a high resistance, e.g. AVO, during testing. Bus-wiring insulation should be checked between wires, and between each wire and earth. It is particularly important in the case of circuits connected to the protection battery that there are no 'sneak circuits' between positive and negative, or between positive and the trip wiring, which could cause mal-operation or battery failure. Individual circuits, such as TRIP, CLOSE, and ALARM, should be tested to earth and between poles before fuses and links are inserted. At this stage fuses and links can also be checked for size, rating, and function. Inspection of the schematic diagrams will best show how switch wires and other parts of a circuit, such as operating coils and contactors, may be included in the test and not overlooked. It may be necessary to repeat the tests a number of times with switches or relays in different positions or operating states. Insulation tests across open contacts of panel switches and plant/equipment auxiliary contacts shall also be carried out to prove a sufficient gap exists between the contacts. UK Power Networks 2017 All rights reserved 17 of 48

18 9.2 Precautions Before carrying out any insulation resistance test using applied voltage, care shall be taken to ensure that any electronic relays and telecommunications equipment are either disconnected or protected against over-voltage damage, which can be costly. Floating battery chargers should be considered in this category. Modular type electronic relays can usually be withdrawn from their cases during insulation testing, if necessary, but care shall be taken not to damage the electronic components inadvertently through 'static' discharge. To guard against this, always observe anti-static precautions when withdrawing or replacing a relay module from its case. Before withdrawing the relay, first 'earth' yourself by touching the relay case or panel metalwork, and then put the withdrawn relay module down onto a conducting surface that you have touched beforehand. Do not shuffle your feet during this process because more static electricity can be introduced dependent on the footware being worn. If using a earthing wristband ensure it has a one megaohm resistor fitted between the wristband and earth. This reduces the risk of increasing the current that could potentially travel through the chest if a live part is inadvertently touched with the other hand whilst wearing the anti-static wristband. 9.3 Test Voltage and Results 1000V dc is the preferred test voltage. A Megger tester or a test set that can provide alternative output voltages and currents. The values of insulation resistance to be expected cannot be predicted. They may vary in practice from tens of thousands of megaohms down to say 0.2M, depending on humidity and whether plant has been stored etc. Humidity in new substation buildings is often very high. All test values should be recorded and low tests (e.g. less than 1 M) should be investigated and rechecked at the earliest opportunity. A retest may prove that the low readings were not due to an incipient fault. Where low readings are obtained on wiring located within outside compounds the reason for this can often be due to tracking occurring at the points where the wiring is terminated and not necessarily the multi-core cables themselves. The tracking can result though an accumulation of damp grime that has become deposited in termination boxes and marshalling cabinets over a long period of time. Dirty terminations shall be cleaned using an approved cleaning spray intended specifically for this purpose while ensuring compliance with the safety measures identified on the COSHH sheet for the product. 9.4 CT Secondary Circuits CT secondary ac circuits should be tested to Earth with the star-point earth link removed (this should never be done unless the primary circuit is dead). A second test should be made to prove that the replaced earth link is effective. UK Power Networks 2017 All rights reserved 18 of 48

19 10 Loop Resistance Tests 10.1 Current Transformer Secondary Circuits Loop resistance measurements are to be made on all CT secondary circuits. Those associated with circulating-current type protection are necessary to establish the operating characteristics of the protective scheme, and should be checked against the Manufacturer's calculated Figures. Separate values are required for CT and lead burdens, and all measurements are to be recorded on a lead-resistance diagram (refer to the appropriate test form for the format of recording results). The loop resistance of each CT secondary circuit should be measured with a multimeter having a low resistance measuring resolution of no greater than 0.1 ohms. Insulating plugs may be inserted in the plug-bridge or test-plug block of the protection relays, so that the relay input windings are not included in the test. The test should measure the resistive burden of the CT secondary winding plus the phase wiring from the relay panel. The burden of the relay is not included at this stage as it may vary depending upon relay type, setting, saturation etc. Where the total burden of the circuit is required for the assessment of CT performance, typical values for the relays may be added to the CT loop value for calculation purposes. Any variation in resistance between phases should be investigated and explained by the presence of ammeters, metering etc. Loose wire connections show up at this stage by giving abnormally high, or varying, readings Pilot Wires Pilot impedance and phase angle measurement of rented British Telecom pilots is necessary when these pilots are to be used with unit type protection (not intertripping or acceleration etc). When necessary the same measurements shall be made on private pilot cables Other Circuits Within the limitations of the design of the switchgear the loop resistance of trip coils, closing coils and contactors etc should also be measured where practical from the supply fuse and link, with the appropriate function set up. The test will show that the coils etc are of the correct value and are wired correctly with no short circuits. The recorded value of loop resistance can be of use during maintenance. For example, circuit breaker auxiliary switches in a reclose circuit may deteriorate through oxidisation where they are left open for extended periods with the circuit breaker normally closed. This condition would show up as an increase in the loop resistance, and action could be taken to prevent mal-operation. UK Power Networks 2017 All rights reserved 19 of 48

20 11 Current Transformer Magnetisation Curves The importance of adequate CT magnetisation characteristics in relation to protective schemes shall always be stressed. During commissioning, the magnetisation curves of CTs are checked to: Prove that the CT is healthy and without shorted turns. Provide an initial record of the characteristic, from which the 'knee point' and saturation values can be obtained to assess protection performance. Prove in the case of Neutral CTs (e.g. in REF and SBEF schemes) that fortuitous earth loops are not present at insulated cable glands etc. Prove that the characteristic of a CT matches the function of the circuit to which it is connected. It is essential that: Existing or re-used CTs, or new CTs without Manufacturer's curves, should be tested up to and beyond the knee-point, and where practicable up to a maximum saturated magnetising current of (typically) 1A, for which the applied voltage should be recorded. The magnetisation characteristic of all new Current Transformers is to be checked at the minimum number of points necessary to identify the CT with reference to the Manufacturer's estimated design curve, and to determine the suitability of the CT for its intended duty. Because a CT is built around an iron core, the relation between voltage and magnetising current is nonlinear, and harmonic distortion will be always present in the voltage or current waveforms. By plotting the terminal voltage of a CT against the magnetising current in discrete steps, a magnetisation curve will be obtained which will approximate to the hysteresis loop for the iron. It is usual to record the CT 'mag curve' as a table of results, rather than plotting the actual curve on graph paper each time. If it is required to find the exact 'knee point' for the CT, then the curve can be sketched out as necessary later (e.g. see Figure A-2). The 'knee-point' is defined as the position on the curve where a 10% further increase in voltage results in a 50% increase in current drawn. Typically at least 8 or 10 points should be taken on the curve, starting by raising the applied voltage in moderate steps, and then in smaller steps once the current drawn begins to increase rapidly as the knee-point is reached and saturation approaches. The knee-point is normally reached before the magnetising current exceeds 20% of the CT rating. The slope of the curve depends on the quality of the iron core. A CT having a highquality iron or mu-metal core may saturate at a very much lower magnetisation current. Because the repeatability of the curves will be affected by remnant magnetism in the iron core, the readings will depend on whether the current is rising or falling. For consistent results, it is first necessary to de-magnetise the CT and to plot the magnetisation curve for currents increasing from zero. De-magnetisation can be achieved by raising the injected current briefly to fully saturate the iron core, and then reducing the current smoothly to zero by turning down the Variac/Test Set control (and not by switching off suddenly). This procedure takes the iron core through progressively smaller hysteresis loops, until finally the current is zero and there is no magnetism left. UK Power Networks 2017 All rights reserved 20 of 48

21 Figure A-1 shows a common test circuit. The isolating transformer, if used, prevents the supply neutral Earth from influencing the test, and avoids the need to lift the CT star-point earth link. Alternatively the integral variable ac Voltage output on a secondary injection test set can provide a convenient test supply. Both the Variac and the isolating transformer (if used) should only be operated up to 50% of their nominal ratings, to ensure that they do not influence the readings by causing waveform distortion. The voltage applied to the CT is sinusoidal and therefore a rectifier-moving coil type instrument such as an AVO meter, or a basic Fluke, can be used without errors. The current waveform drawn by the CT is distorted by the harmonic current, and strictly speaking a moving-iron or 'true RMS' type meter should be used to record the magnetising current. Other methods of taking magnetisation curves by using a fixed voltage and a variable resistance can give an optimistic result, and the CT may appear better than it is. Magnetisation curves can be taken after the CT Primary circuit has been jointed, provided that earths do not exist on both sides to create a current loop or 'shorted turn'. This occurrence can be identified by a lower, flatter curve (more current being drawn for the same applied voltage) than expected. On large schemes a number of CTs performing different functions may be fitted in one circuit breaker. For example: Overcurrent and earth fault protection, restricted earth fault protection. Unit protection. Ammeter and instruments, metering. AVC. Fault recorder etc. The Metering and Instrument CTs are usually designed to saturate at a much lower voltage value than say the REF CTs, to give protection to the instruments under fault conditions. Although the ratios of protection and instrument/metering CTs may be the same, to install them transposed would result in possible mal-operation of protection on the one hand and damage to instruments on the other. It is therefore essential to positively identify CT type with application. UK Power Networks 2017 All rights reserved 21 of 48

22 12 Secondary Injection and Protection Relay Tests 12.1 Secondary Injection Applied to Current Operated Measuring Relays Relays are essential building blocks that make up protective systems. In all cases, all designs of relay shall be given a minimum amount of testing after connection, to prove their function and settings. These tests should show up any handling or installation faults that may have occurred since manufacture, and are also intended: To check that the relay has not been damaged in transit. To check the relay against its specification. To check the relay as part of a complete scheme. In the case of a user-programmable electronic relay, to confirm that the correct software 'configuration and logic' files have been installed and proven to the intended scheme design. The electrical tests should be preceded by a thorough visual inspection, when any packing material is removed and the relay is made ready for operation. In the case of an electromechanical IDMT relay for instance, it is possible for swarf to be attracted to the disc and brake magnets. So the full travel of the disc should be checked and smooth operation observed. AC secondary injection tests are carried out with the dc tripping fuse and links removed and with the tripping contacts connected to a time interval meter (see Figure A-10). If the relay is an electronic type requiring a permanent auxiliary supply, then the alarm/auxiliary supply fuse and link will need to be left in, or an external supply injected to power the relay. When setting-up the ac test set, it is important (particularly with electronic relays) to ensure that the injected current through the test ammeter and relay is an undistorted sine wave, otherwise apparent current setting and timing errors will occur. As a general rule, always use the lowest-current (highest-voltage) output range or terminal selection on the test set that is compatible with the required test current, and if the current is then difficult to control, add a series resistance, or a filter unit, to the circuit. During test procedures, wiring should be disturbed as little as possible, and where modern high quality CTs are fitted then the relay test current may be injected at a convenient terminal block in the relay panel, or at the scheme test block if fitted. Although the CTs will be in parallel with the relay coils during the secondary injection, their magnetising current will be negligibly small compared with the relay test current. There shall be no short-circuit across any CT primary (e.g. via earths on both sides) otherwise significant current will be shunted through the CT secondary. Where several relays exist on one panel and are powered by the same CT secondary circuit, take care that the test current does not flow through relays in series so as to exceed their rating. Vulnerable relay elements should be shorted out during such testing. UK Power Networks 2017 All rights reserved 22 of 48

23 All relays are to be examined, care being taken before opening relay cases to ensure that no foreign matter can fall inside. Note is to be taken, where applicable, of the following points: Relay mechanical movement is free. Magnet gap and induction disc are clean. Gear teeth are clean. Contacts are clean and have adequate wipe. All contacts make simultaneously, or in a preferred order. Contacts make when time multiplier setting is zero. Resetting times are within limits. Flag mechanism operates in correct sequence with respect to contacts. Flag, indication and contact-reset knobs/buttons operate with relay cover on. Relay cover glass and gasket provide effective seal. Labelling and phase colours are correct. CT shorting and dc isolating contacts or switches in withdrawable relay cases operate satisfactorily IDMT Relay Operation and Timing Tests It is usual to commence testing a relay having an IDMT type characteristic using a plug setting of 100%, to provide a convenient range of test currents, and a time multiplier setting of 1.0 to give full disc travel (if electromechanical) and a convenient operating time. It is only necessary to check the shape of the IDMT curve at this one current setting, since the other settings are simply related by the number of turns on the coil, or in the case of an electronic relay, by scaling resistors or by multiplying factors in the software. It is good practice to also inject the relay at its intended final in-service setting, when known, and confirm that sample current/time values are as expected. A final timing test at setting can be carried out at twice and four times the plug/base setting. For the usual standard inverse characteristic, the relay operating times should be ten times and five times the applied time multiplier respectively. Testing at two points on the inverse/time curve then proves the relay is operating to the correct standard inverse characteristic. A fault in the relay input coil will show up on all settings under ac injection test conditions. Electrical timing tests should be carried out to prove the shape of the IDMT curve at convenient points. The definition of the characteristic and the limits for errors are given in IEC Part 3. There are four curve characteristics normally encountered for IDMT relay and these are: Normal inverse. Very inverse. Extremely inverse. Long time. The formula defining the first three of these curves is derived from BS EN Part 3 and times deduced for each characteristic are shown in Appendix B. UK Power Networks 2017 All rights reserved 23 of 48

24 12.3 IDMT Timing Accuracy Start Timing Function IDMT relays can be tested for 'creep' by slowly increasing the test current up to the setting value. At the relay setting current, the disc should be stable and not move, or for an electronic relay the 'I > Is' or equivalent indication should not show. The disc should commence turning, or the 'I > Is' indication should pick up at above the setting current but not greater than 1.3 times the current setting. In electromagnetic relays, at this point operating torque just balances friction and other restraint forces. Any undue restraint, such as bearing friction, will cause operation to occur at a larger current. For 'long time earth fault' elements the minimum operating current should not exceed 110% of the relay setting and the timing function should not start at a current equal to or less than 75% of the relay setting Electromagnetic Relay At the highest time multiplier setting, and at a current setting of 100% for relays with a 50%- 200% range, 40% for relays with a 20%-80% range, and 25% for relays with a 10%-40% range, the operating time limits of electromechanical induction disc relays are shown in the following table: Table 12-1 Accuracy of IDMT Relays Time Multiplier (TM) Reference Plug Setting % Range 20-80% Range 10-40% Range Multiple of Current (PSM) Acceptable Limit On Normal Operating Inverse Time-lag Elements Normal Inverse Very Inverse Extremely Inverse Long Time Earth Fault % 40% 25% 2 ±16.7% (i.e s) ±17.6% ±18.3% ± 17.6% % 40% 25% 4 ± 9.7% (i.e s) ±11.5% ±14.0% ± 11.5% Timing range acceptable is given, by way of example, for the normal inverse characteristic. Other times may be obtained using the values derived in B Electronic Relays Electronic relays may be expected to operate within a narrower tolerance range than mechanical relays. However some designs are more sensitive to non-sinusoidal waveforms and a current filter unit may be required to obtain credible results. 'True RMS' ammeters should be used for preference when testing electronic ac measuring relays. UK Power Networks 2017 All rights reserved 24 of 48