Can anybody tell me the easiest way to test the differential protection function for generator ? I need to see trip of generator ACB by protection relay.
It depends on what type of test you will accept: rated short circuit current or secondary injection.
In the USA we bench test the protective relay with a test set, then verify the CT and trip wiring to the relay. Then we are satisfied with a trip test done by injecting current into the relay to get an output or by forcing the trip manually.
In Europe and Asia, the practice is to apply a short circuit on load side of the generator breaker, sometimes even on the high voltage side of the generator step up transformer. To test the differential system, the short must be placed outside the zone. Then the generator is brought to speed under manual control and the excitation is slowly raised until full load current or some fraction of it flows to the shorting bars. The high current flow allows verification of the entire CT wiring, bus connections and phasing for normal operation.
To test the differential systems, the unit is stopped and a phase-to-phase short is placed inside the zone with jumper cables or bus. Then the generator is rolled and excitation increased until the relay trips.
If you can't do the short circuit test but you can apply load to the generator, shorting our one CT will test the differential relay. The relay will see current on the neutral of one phase but none on the line side and interpret it as a fault.
If you can't run the generator until the differential is active, set up the relay test set to inject current into one input on the relay. Close the breaker in the test position and inject the current, the relay should trip the breaker.
Just for a small clarification to rcwilsons very informative reply, the short circuit test is also performed quite commonly in the US (I've done it many times). Its normally more of a proof test of all the components and always after the relay has been previously tested by secondary injection.
shkim2000 hi , rcwilson gave you a very nice review but I would not recommend any experience that will include creating a short circuit especially on the high voltage side as this request tons of precautions and only I repeat only electrical engineers are alowed to do so by all the regulations I know . One of the things I would recommend is to take a loop test that is able to measure very low impedences (there are a few models) and test the protections , after that take a RCD tester and make the test and at the end make a ground test of a floating system (similar to hospital systems). At the end see that te connection between the ACB trip coil is properly connected to the relay . The only short circuit that should be tested is in the case you have a ground fault system is at the load side connect a regular 100W light bulb between ground and neutral to see if the ground fault trips.
BE VERY VERY CAREFULL !
Avishai
Re-read rcwilson's post: he is not proposing to short out the generator with full excitation applied. It is a standard test where a shorting link is applied prior to the generator being started and the excitation voltage is very carefully raised until rated current flows in the windings. The method is essentially a primary injection test using the generator itself as the injection source. It totally different to a s/c withstand test.
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ScottyUK , thank you for this emphasys . I know this test, being an inspector, but by reading the other remarks I wrote it for pin pointing the safety problem . My feeling was that shkim2000 is not an electrical engineer . I believe that it is safer to use the testing equipment when there is a safety risk .
Avishai
The easiest way to do this is by relay secondary injection from the CT circuits. (It's not necessary to disconnect the CT circuits or short them out. This will activate the relay trip circuits and trip the ACB. If you just want to see if the relay trip to the ACB is working, then you could use the relay internal test function if it has one (normally they do). For secondary injection you need a current source. If you want to time the relay or check the pickup accurately it's best to get a test set. (Doble or Multiamp make good sets - or you could call Electrorent, they will rent them to you for a couple hundred dollars per week)
If you don't feel comfortable using this equipment then contact a relay test company
