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Saturday, 4 April 2015

TESTING OF PROTECTIVE RELAYS

"Protective equipments including relays are in-operative most of their useful life." 

They operate only under fault conditions. Testing of protective gears thus pose a problem. Testing of protective relays under normal operating condition normally does not give realistic results. Hence defects may easily pass undetected until revealed by incorrect operation or no operation on occurrence of actual fault.

Question arises why the relays or any other protective devices or gears are tested? 

Testing of relays is usually done to ensure that:
1)      The relay will operate correctly so as to clear a fault, and
2)   The relay will remain in-operative on faults outside its specified zone.

Now let’s talk about the various tests that are necessary for a protective gear and relay. Broadly the tests that are conducted on a relay are:

1)      Factory Test: For testing of a relay, the operational condition of the system during faults must be closely reproduced. The plant supplying the current for these tests particularly for high speed protection must be of sufficient capacity and design. Relay test benches are used for light current tests, and heavy current tests are carried out with the relays and CTs connected operationally.
Factory test also include many other tests such as material investigation, effects of impacts and vibration, resistance to atmospheric corrosion, effects of temperature and dust, tightness of relay case etc.
2)      Installation or Commissioning Test: The main purpose of commissioning tests is to prove that the relay has been properly installed and connected and it functions correctly and is ready for service. Comprehensive tests at sites of many other elements associated with the protective relays and which may have been supplied by different suppliers, are of great importance (Eg. Ratio and polarity test of CTs and PTs).
These tests include the general inspection of the equipment, checking of all connections etc. Insulation resistance of all the circuits is measured and the relay is tested by secondary injection. Further these tests at the site provide a check over the components which have passed factory tests and have been transported safely.

3)      Periodic Maintenance Test: Moisture and dust tend to produce deterioration of insulation, increase resistance of contacts, stickiness of relay bearings and pivots. A general inspection of the physical condition of all equipments, a check of accessible connections including fuses and links, secondary injection tests of relays, functioning test of logic and trip circuits are included in this test. The operation of indicating and alarm circuits should also be checked during this test.