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Wednesday, 18 April 2012

Relay test

TESTING OF PROTECTIVE RELAYS

            INTRODUCTION

In a given power system, protective relays & relaying systems detect the abnormal conditions & operate the associated switchgear to isolate the faulty section, this limits the damage at the fault location & prevents the effects of the fault spreading in the system.  Hence, the protection system, which comprises of relays, should be able to recognize an abnormal condition in a power system & take the suitable steps to ensure its isolation with the least disturbance in the normal operation. The reference standard is “IS 9124 - Maintenance & field testing of electrical relays”.

To ensure the healthiness of relays and the core element of protection system, its testing is done as per the schedule.

            Periodic Maintenance Tests
a)      The clearance of a fault on the system is correct only if the number of circuit breakers opened is the minimum necessary to remove the fault.  A small number of faults are incorrectly cleared, the main reasons being :
·         Limitations in protection design
·         Faulty relays
·         Defects in the Secondary wiring
·         Incorrect connections and
·         Incorrect settings

b)      The objective of a Relay Testing & Maintenance program is to ensure the integrity of the protection system on a periodic basis after installation.  Calibration testing is required to verify relay testing calibration, configurations and to identify any protection system defect.  Functional testing is required to verify that the intent of the protection system is being carried out.

c)      Protection relay evaluation starts with :
·         First generation – Electro mechanical Relays
·         Second generation – Static relays with Transistors
·         Third generation – Static relays with Integrated Circuits
·         Fourth generation – Processor based relays

d)     Testing features in relays
Most of the relays of the first, second and third generation relays do not have the capabilities of auto testing of internal circuits or providing an alarm in case a failure is detected.  Electro mechanical relays have a lot of mechanical parts, which may become clogged with dirt or corroded due to environmental conditions, affecting both operation calibration and movement of the disks.

Static relays generally employ a lot of electronic components made by other manufacturers.  Most of the static relays in use have in built power supply unit and do not have the means to detect the failure of power supply and initiate an alarm.  If these electronic components are not tested with rigorous quality control, the chances of failure of components during relay life time may exist.

The fourth generation processor based relays do have the watchdog feature which facilitates the checking of power supply rails, clock frequencies, and other patterns.  Most of these relays have auto test features, which test the electronic circuit functions.

               TESTING OF PROTECTIVE RELAYS

             Frequency of Tests & Inspection

The frequency of periodic testing shall depend on the ambience of surroundings in relay room and size & importance of the equipment being protected.

Protective relays shall be tested & inspected as under:

·         Before commissioning of the protection system
·         Once in a year for critical equipment where the surroundings are clean & dry
·         Once in two years for less critical equipment.
·         Opportunity testing & maintenance during unit shutdowns.
·         For microprocessor-based relays, which are self-monitoring type and regular retrieval and analysis of event records, following system faults verifies operability.  The analog metering digital input/output is verified with functional tests. 
·         All protective system trip operations should be analysed for cause and corrective action. Testing for cause is required in the event of an incorrect operation of a protection system.

Tools / Tips for Relay Testing

1.      Over current relay should be made to check the minimum pick up current & operating time at current values of approximately. 2, 5 & 10 times of the tap setting.

2.      Instantaneous element in the over current relay should be checked for minimum pick up current.

3.      Directional elements in the over current relays should be checked for the minimum pick up voltage on the polarized winding with 5A flowing in the current winding at 100% power factor & for the phase angle which gives maximum torque.

4.      The internal wire terminations relay ligaments & coil ends should be examined for sulphation or green spots caused by electrolysis. If found, it should be removed by wiping with suitable solvent.
                       
5.      Relay targets should operate freely without friction & also reset freely.

6.      On draw out type relays, the finger contacts should be examined for sulphation & tracking between the terminals which should be cleaned.

Tests on Relay Circuits

These tests are necessary to check & determine healthiness of the complete protective circuit.  The following tests will be carried out for the said purpose.

1.      General:  Check for the continuity of circuits, loose terminal connections, open circuits, connections in incorrect phases, incorrect panels, incorrect circuits & continuity at all disconnecting type links or junction boxes and precise relay flag operation.

2.      Primary Injection tests: In this test the required current is applied through the primary terminal of the protected equipment or test points so as to enable to check the integrity of circuitry as well as the relay operation.  Hence the actual conditions of the fault can be simulated & the effectiveness of the protection circuit can be checked.

3.      Secondary injection tests: Where the primary injection is not effective & feasible, there secondary injection is done.  Here the current/ voltage is applied to the required terminals (as per the relay manual) in order to actuate the relays. Precise relay calibration is checked at desired settings preferably.

            INSPECTION AND TEST PROCEDURES

The instruction & maintenance manuals issued by the equipment manufacturers and individual testing methods developed should be used as the basic source of information in testing & maintaining protection systems. 


            Protective relays
                       
1.      Visual and Mechanical specification checks:

1.         Compare equipment nameplate data with drawings and specifications.

2.         Inspect relays cases for physical damage. Remove shipping restraint material.
3.         Tighten case connections.  Inspect cover for correct gasket seal.  Clean cover glass.  Inspect shorting hardware, connection paddles and/or knife switches.  Remove any foreign material from the case.  Verify target reset.

4.            Inspect relay for foreign material, particularly in disk slots of the damping and electromagnets.  Verify disk clearance.  Verify contact clearance and spring bias.  Inspect spiral spring convolutions.  Inspect disk and contacts for freedom of movement and correct travel.  Verify tightness of mounting hardware and connections.  Burnish contacts.  Inspect bearings and/or pivots.

5.            Set relays in accordance with protection coordination study analysis.

2.      Electrical tests
                     (* - Optional )
1.            Perform insulation-resistance test on each circuit-to-frame.  Determine from the manufacturer's instructions the allowable procedures for this test for solid-state and microprocessor-based relays.

2.            Inspect targets and indicators.

3.            Set contrast for liquid-crystal display read outs.

3.      Functional operation checks in relays

1.            62 - Timing relay
1.      Determine time delay.
2.      Determine instantaneous contacts.

2.            21 – Distance relay
1.      Determine maximum reach
2.      Determine maximum torque angle
3.      Determine offset.
4.      *Plot impedance circle

3.            24 - Volts/ Hertz Relay
1.      Determine pickup frequency at rated voltage.
2.      Determine pickup frequency at a second voltage level.
3.      Determine time delay.

4.            25 - Sync check relay
1.      Determine closing zone at rated voltage.
2.      Determine maximum voltage differential that permits closing at zero degrees.
3.      Determine live line, live bus, dead line & dead bus set points.
4.      Determine time delay.
5.      Verify dead bus/ live line, dead line/ live bus and dead bus/ dead line control functions.

5.      27 - Under voltage relay
1.      Determine dropout voltage.
2.      Determine time delay.
3.      Determine the time delay at a second point on the timing curve for inverse time relays.

6.            32 - Directional Power Relay.
1.      Determine minimum pickup at maximum torque angle.
2.      Determine closing zone.
3.      Determine maximum torque angle
4.      Determine time delay
5.      Verify the time delay at a second point on the timing curve for inverse time relays.
6.      *Plot the operating characteristic.

7.            40 - Loss of Field (Impedance) relay
1.      Determine maximum reach
2.      Determine maximum torque angle
3.      Determine offset
4.      *Plot impedance circle

8.            46 - Current balance relay
1.      Determine pickup of each unit.
2.      Determine percent slope
3.      Determine time delay

9.            40N - Negative sequence current relay
1.      Relay negative sequence alarm level and trip.
2.      Determine negative sequence minimum trip level.
3.      Determine maximum time delay.
4.      Verify two points on the (I2)2t curve

10.        47- Phase sequence or phase balance voltage relay.
1.      Determine positive sequence voltage to close the normally open contact.
2.      Determine positive sequence voltage to open the normally closed contact (under voltage trip).
3.      Verify negative sequence trip.
4.      Determine time delay to close the normally open contact with sudden application of 120 percent of pickup.
5.      Determine time delay to close the normally closed contact upon removal of voltage when previously set to rated system voltage.

11.        49R - Thermal replica relay
1.      Determine time delay at 300 percent of setting
2.      Determine a second point of the operating curve
3.      *Determine pickup

12.        49T - Temperature (RTD) relay
1.      Determine trip resistance
2.      Determine reset resistance

13.        50 - Instantaneous over current relay
1.      Determine pickup
2.      Determine dropout
3.      *Determine time delay.

14.        51 time over current
1.      Determine minimum pickup
2.      Determine the delays at two points on the time current curve.

15.        55 - Power factor relay
1.      Determine tripping angle.
2.      Determine time delay

16.        59 - Over voltage relay
1.      Determine over voltage pickup
2.      Determine time delay to close the contact with sudden application of 120 percent of pickup

17.        60 - Voltage balance relay

1.      Determine voltage difference to close the contacts with one source at rated voltage.
2.      *Plot the operation curve for the relay.
18.        63 - Transformer sudden pressure relay
1.      Determine rate-of-raise or the pickup level of suddenly applied pressure in accordance with manufacturer's specifications.
2.      Verify operation of the 63 FPX seal-in circuit.
3.      Verify trip circuit to remote breaker.

19.        64 - Ground detector relay
1.   Determine maximum impedance to ground causing relay pickup.

20.        67 - Directional Over current relay
1.      Determine directional unit minimum pickup at maximum torque angle.
2.      Determine closing zone.
3.      *Determine maximum torque angle.
4.      *Plot operating characteristics
5.      Determine over-current unit pickup
6.      Determine over current unit time delay at two pints on the time current curve.

21.        79 - Re-closing relay
1.      Determine time delay for each programmed re-closing interval.
2.      Verify lockout for unsuccessful re-closing.
3.      Determine reset time
4.      Determine close pulse duration.
5.      Verify instantaneous over-current lockout

22.        81 - Frequency relay
1.      Verify frequency set points.
2.      Determine time delay
3.      Determine under-voltage cutoff

23.        85 - Pilot wire Monitor
1.      Determine over current pickup
2.      Determine undercurrent pickup
3.      Determine pilot wire ground pickup level.

24.         87 - Differential Relay
1.      Determine operating unit pickup
2.      Determine the operation of each restraint unit
3.      Determine slope
4.      Determine harmonic restraint
5.      Determine instantaneous pickup
6.      Plot operating characteristics for each restraint.
25.    Thermal Relay
1.      Pick up value
2.      Timing

                   26.  Auxiliary Relays
1.      Operation

27.        Special Relays & Timers
1.      As per manual

4.      Control verification

1.      Verify that reach of the relay contacts performs its intended function in the control scheme including breaker trip tests, close inhibit tests, 86 lockout tests and alarm functions.
2.      For numerical relays, verify all used inputs, outputs and internal logic.

5.      System tests

After the equipment is initially energized, measure magnitude and phase angle of all inputs and compare with expected values.

6.      Test values

1.            Use manufacturer's recommended tolerances when other tolerances are not specified.

2.            When critical test points are specified, the relay shall be calibrated to those points even though other test points may be out of tolerance.
.           Test Procedure of electrical relays  
1.      Refer Annexure-4 for test procedures and circuits.
2.      Refer Annexure-5 for relay testing connection diagram /procedures as per IS-9124. Guide for Maintenance and field testing of electrical relays.
3.        Refer Annexure-6 for relay testing of typical English Electric relays using EE relay testing kit.
4.        Refer Annexure 6-A for Microprocessor relays.
                






ANNEXURE – 1

1.         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 method.
           
            The test interval can be adjusted based upon experience. Otherwise, testing of relays on a yearly basis is recommended. The test methods used for early 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 protective relays, associated instrument transformers, and wiring.

a)                  General Protective relay Calibration and Checklist

i)                    Perform insulation resistance test on each branch circuit to frame. Do not perform this test on solid- state relays. Check manufacturer’s instructions to verify  if any other precautions are required.

ii)                  Perform the following tests on the nominal setting specified.

·         Minimum pickup parameters on each operating element. A pickup test is conducted to determine the minimum or maximum current, voltage, power or frequency that causes closure of relay contacts.
                            
·         Timing test should be performed at three points on the time dial curve  to verify the timing characteristics of the relay.

·         Pickup target  and seal-in units.

·         Special tests as required to check operation of restraint,
directional, and other elements per  manufacturer’s instruction manual.
                       
iii)          
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 towards zero.
iv)                Perform phase angle and magnitude contribution tests on all differential – and directional – type relays after energization to vectorially provide proper polarity and connection.

2.         Relay Test Points and Test Circuits

a)                  Time Overcurrent Relays – Overcurrent relays are checked for minimum pickup. Check a minimum of three timing points at 2 X tap. 4.5 X tap. and  6 X tap settings. The Periodic inspection pickup tolerance is +/-5% of tap value  for nongeared relays and +-7% for geared relays. For new  relay the tolerance is +/-1% of tap value. Pickup is defined a that value of current  that will just close the relay contacts. Check the instantaneous unit pickup by gradually applying the current . Also check the target seal-in unit blocking  the main Overcurrent contacts. The testing of Overcurrent relay is done one phase at a time. The ground relay is tested similarly to the phase relays.
b)                  Directional  Overcurrent Relays – The Overcurrent unit of  directional relays 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 will give large variations in-phase pickup, because of in-phase angle being far different from maximum torque angle.
c)                  Differential Relays – 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.
d)                  Distance Relays -  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 test should be made.
e)                  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.
f)                   
Plunger-Type Relays – In this type, relays such as PJC,SC, and HFA, are included. These relays are tested for tested for operating pickup and dropout values by gradually increasing or decreasing the operating current or voltage.
g)                  Current Balance Relays – 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 in a manner similar to Overcurrent relay.
h)                  Overvoltage 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.
i)                    Undervoltage Relay – Check dropout of relay and  time relay trip when voltage is suddenly reduced from rated voltage to dropout voltage setting or to zero. Select three timing points on the specified time dial. 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.
j)                    Thermal Overload Relays. The thermal overload relays minimum pickup value should be checked using some convenient multiple of tap  setting. 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 setting.
k)                  Voltage Restrained o 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 unless the voltage element drops out.
l)                    Under-Over Frequency Relays – The relays generally require three calibration function: 1) Voltage cutoff or drop out ; 2) Over or under frequency pickup points; 3) Time delay before trip after frequency set point has been sensed. The delay times are not necessarily equal.
m)               
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 calibrating or maintenance test. These relays generally have delay times associated with the angle pickup point.

3 comments:

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