Testing and Maintenance of Electromechanical Protective Relays
The reliability of protective relays in isolating faulted equipment is dependent on correct installation and maintenance. After protective relays are correctly installed and tested, the maintenance testing objective should be to achieve maximum performance with minimum testing. Relays usually operate for an extremely short time during their long life. Therefore, the question arises as to whether the relay will operate under fault conditions. The answer is to rou- tinely test all protective relays. However, overtesting should be avoided, because testing can potentially add more trouble than is corrected. All relay test programs should include tests that simulate normal operating conditions. The test program should include acceptance, installation, routine, and repair. Before meaningful tests can be conducted, advance preparation should be undertaken in order that the testing personnel become familiar with the relays or relay systems.
Relay Inspection and Tests
The installation, maintenance, and small repair testing are done in the field, whereas acceptance and major repair testing are conducted in the laboratory.
To minimize the potential liability of adding trouble to the relays or relay system, the following general procedures are recommended.
• Study the protection scheme (station prints, relay instruction manuals)
• Obtain and review results of previous tests and other pertinent information
• Arrange for test equipment to perform all tests
• Make outage request and switching arrangements
• Schedule remote tripping and load tests, when required
• Set up test equipment. Observe precautions in selection and connec- tion to low-voltage service
• Operating or test personnel perform switching, as arranged, according to approved outage requests
Open and isolate, TEST DEAD, and ground if required; place “Keep Out” tag, and report “On” the circuit; complete operating log entries.
If test personnel are not present when switching is performed, verify the isolation, grounding, tag placement, and TEST DEAD before reporting “On” the circuit.
• Isolate control circuits; that is, remove control fuses, open test switches, and/or operate selector controls as required. Caution: Be aware of overlapping and interconnecting protective circuits associated with operating equipment. Take measures necessary to keep such schemes in operation. Isolate control, current, and voltage transformer secondary circuits to protect against an unintentional operation from tests on the tagged circuit.
Tests and inspections
• Perform and record results of as found tests. Confirm calibrations and settings with a system protection study or relay setting and man- ufacturer’s instructions. Record any defects found; discrepancies should be reported promptly to a supervisor or person in charge and resolved, if necessary.
• Verify printed information on the routine inspection sheet (RIS) test forms from previous tests. Prepare other RIS forms, if required.
• Perform visual and mechanical inspections.
Check tightness, clearance of exposed lugs, and condition of wiring on panels and switchboards. Check clips of fuse holders for tightness and alignment. Inspect and perform minor repairs on relays and auxiliary devices. Observe clearances, mechanical freedom, condition of contacts and control springs, condition of internal insulation, and tightness of internal connections. Clean magnets. Check targets and reset mechanisms. Clean glass covers, inspect and replace cover gaskets as needed.
• Inspect and test CT and VT, related auxiliaries, and associated wiring.
• Inspect for evidence of corona.
• Check nameplate information with test forms and other data sources available on the equipment.
• Perform CT secondary winding impedance and continuity (backfeed test).
Electrical tests on VT secondary windings normally are not necessary. VT performance is assured by in-service observations and primary fuse monitoring schemes.
• Perform “as left” relay tests. Record results on test forms. Make neces- sary electrical and mechanical adjustments to achieve desired results.
• Calibrate local indicating and recording instruments, and adjust as necessary. Record results on instrument test forms. Calibrate and adjust supervisory telemetering transducers.
• Perform complete tripping and operational tests to verify all control and protective functions and alarms. Include supervisory, remote tripping, and bus differential trip circuits.
• Complete test forms. Make necessary corrections for circuit designation changes and other changes that may have occurred but were not previously recorded. Include man-hours required to make the tests.
• Replace covers, switchgear plates, remove test leads, jumpers, and separators, close test switches, replace fuses, inspect circuit equipment, and set up control and selector switches preparatory to switching.
• Make inventory of tools, jumpers, separators, instruments, and other equipment used. When completed, instruct crew members to con- sider the circuit as energized and the tag holder to advise that he is ready to report “Off” the circuit.
• Report “Off” the circuit and arrange for circuit restoration.
• Operating or testing personnel remove protective grounds and per- form switching to restore the circuit to service.
• Perform desired tests under load. Replace covers when completed.
• Restore all station controls to normal, complete station operating log and “Keep Out” tag entries.
After tests completed on all circuits
• Most of the following items may be performed progressively during the total test period.
• Make necessary field corrections to station prints. Arrange for follow up in order that corrections are made in permanent records.
• Arrange for necessary changes or additions to panel or other circuit designations.
• Complete entry of the relay test records in station ledger if such a ledger is available.
• Prepare a list of items that were not complete or tests not performed.
Include items that may need to be referred to other groups. Submit
this list along with completed test forms to your supervisor or persons in charge.
• Inspect station to confirm that prints and records are secured and various equipment accessories and spare parts are correctly stored.
Protective Relay Test Procedures and Circuits
The testing of protective relays and associated circuitry can be carried out by following recommendations outlined in manufacturer’s bulletins or the user’s own test procedures. These procedures should always be updated based upon a review of past relay performance, test equipment evaluation, and testing methods.
The test interval can be adjusted based upon experience. Otherwise, testing of relays on a yearly or two yearly bases is recommended. The test methods used for relay testing consist of relay functional tests (i.e., relay equipment is separated from power equipment) and only secondary tests are made. The following general guidelines are recommended for electrical testing of pro- tective relays, associated instrument transformers, and wiring.
General protective relay calibration and checklist
• Perform insulation resistance test on each relay coil to frame. Do not perform this test on solid-state relays. Check manufacturer’s instruc- tions to verify if any other precautions are required.
• Perform the following tests on the nominal settings specified.
Pickup parameters on each operating element.
Timing tests should be performed at three points on the time dial curve.
Pickup target and seal-in units.
Special tests as required to check operation of restraint, directional, and other elements per manufacturer’s instruction manual.
• A zero check test should be conducted on any relay that has a time dial. The purpose is to determine proper time dial position when the relay is fixed and moving contacts are closed by the manual rotation of the time dial toward zero.
• Perform phase angle and magnitude contribution tests on all differ- ential- and directional-type relays after energizing to vectorially provide correct polarity and connection.
Relay Test Points and Test Circuits
Time Overcurrent Relays
Most of the time overcurrent relays have three torque-producing elements.
They are control spring which restrains the unit from operating and is the fine adjustment for pickup current; the drag magnet which retards the unit’s operating time; the “U” or the electromagnetic which produces operating torque and is the coarse adjustment for pickup. The shape of the time–current curve is essentially a function of the electromagnetic iron circuit. As the cur- rent through the coil increases, the flux increases, thereby increasing the torque and thus decreasing the operating time. However, at current levels above pickup, the iron begins to saturate, resulting in less torque (flux) being produced for a corresponding increase in current. Also, the effect of iron saturation is to produce nonsinusoidal currents. Thus the relay operating time becomes fixed (i.e., time–current curve flattens out) regardless of cur- rent magnitude. The saturation of the iron increases the reluctance of the iron and thus flux is spilled out of the iron. The relay case normally acts as a shunt for this flux and the flux is passed through the case and not through the relay disk. If the relay is tested in its case, the relay published time–current curves will be duplicated and in-service conditions duplicated as well. However, if the relay is tested outside its case, the published time–current curves may not be duplicated including the service conditions accurately. Therefore, from a preventive maintenance point of view, testing the relay out of the case can yield results that will not check the performance of the relay accurately for in-service conditions, or previous and future results.
The first test on the overcurrent relay should be to check minimum pickup. Pickup is defined as that value of current which will just close the relay con- tacts with the relay set at the lowest time dial position. The minimum pickup should be within ±5%.
The next test should be to check the relay calibration at minimum of three timing points, such as at 2 × tap, 4.5 × tap, and 6 × tap settings. The periodic inspection pickup tolerance is ±5% of tap value for nongeared relays and ±7% for geared relays. For new relays, the tolerance is ±1% of tap value. Check the relay for dropout or reset by reducing the current until the relay drops out or fully resets. This test will indicate excessive friction in the jewel bearing. If the relay is sluggish in resetting or fails to reset completely, then the jewel bearing and pivot should be inspected for cracks in the jewel and dirt. If dirt is the problem, the jewel can be cleaned with an orange stick while the pivot can be wiped clean with a soft, lint free cloth.
Check the instantaneous unit pickup by gradually applying the current. Also check the target seal-in unit by blocking the main overcurrent contacts. The testing of an overcurrent relay is done one phase at a time. The ground relay is tested similarly to the phase relays.
Directional overcurrent relays
The overcurrent unit of a directional relay should be checked similarly to the overcurrent relay, with the directional unit blocked closed. The directional relay should be tested for minimum pickup, maximum torque angle, contact gap, and clutch pressure. If the phase power supply is not available, the directional unit can be tested by applying single-phase voltage and current in phase. Usually, this test will give large variations in in-phase pickup, because of in-phase angle being far different from maximum torque angle.
The test conducted on differential relays is to check minimum pickup values using operating and differential currents. The slope (differential characteristic) and harmonic restraint should also be checked. It may also be desirable to trip all circuit breakers from differential relays as a regular testing procedure.
The distance characteristics of the relay are checked near the fault and load angles. Similar to the directional overcurrent relays, the pickup, maximum torque angle, clutch pressure, and contact gap tests should be made.
Pilot wire relays
The pilot wire relay schemes should be tested for shorts, continuity, and grounds in the pilot wires. The operating values are checked along with supervisory and alarm relays used in pilot wire schemes.
These types of relays are instantaneous and/or auxiliary relays, such as PJC, SC, HFA, etc. These relays are tested for operating pickup and dropout values by gradually increasing of decreasing the operating current or voltage.
Check pickup of each coil as explained under section on overcurrent relays. Check for no-trip condition by applying equal amounts of current to opposing coils. Also check operation of the target indicator coil similar to an over-current relay.
Check minimum pickup of overvoltage coil similar to overcurrent relays. Select three timing points on the specified time dial. Pickup and timing points should be within ±1% for new installations and within ±5% on existing installations.Check the instantaneous (if applicable) pickup and target indicator coil.
Check dropout of relay and time relay trip when voltage is suddenly reduced from rated voltage to dropout voltage settings or to zero. Dropout and timing points should be within ±1% for new installations and within ±5% for existing installations. The instantaneous unit should be checked for dropout and target indicator coil.
Thermal overload relays
The thermal overload relays minimum pickup value should be checked using some convenient multiple of tap settings. Because of long time characteristics, the relay pickup point below 200% of tap setting may take a considerable time. Therefore, for test purposes, check pickup at about 200% to 400% of tap settings. Similar to overcurrent relays, the relay time should be checked for several points on the time dial curve. The acceptable time should be within ±10% of specified values. Also check the instantaneous pickup values and target indicator coil.
Voltage-restrained or voltage-controlled overcurrent relays The overcurrent unit is checked and calibrated much the same as a simple time overcurrent relay. In the case of a voltage-restrained relay, the current pickup of the relay will change with the voltage applied to the voltage sensing coil.
In the case of the voltage- or torque-controlled relay, the overcurrent element will not function at all until the voltage element drops out. Care should be taken when working with any of the voltage circuits on switchgear where these relays are applied because loss of sensing voltages will cause the relays to operate on what would otherwise be considered normal current flow.
Under–over frequency relays
The settings for these relays must be derived by a careful, engineering analysis; and not be guessed at or estimated as they will affect the entire system’s continuity of service. The relays generally require three calibration functions:
(1) voltage cutoff or drop out; (2) over or under frequency pickup points; and (3) time delay before trip after frequency set point has been sensed. The delay times are not necessarily equal.
Synchronism check relays
Setting and calibrating these relays requires test equipment similar to that used in distance relaying. The permissible window of the angle between the bus and line voltages must be accurately determined during the calibration or maintenance tests. These relays generally have delay times associated with the angle pickup points, and a set of condition switches that dictate relay action when one or more of the sensing voltages is (are) not present.