Cause of Distance Relay Incorrectly Tripping 1

[mesutphen]

Recently, while performing a reactive test on our generator, our 40-2 loss of excitation relay tripped before we reached our generator capability curve. We backed off the MVAR, the relay cleared. We increased the MVA, and the relay tripped again at approximately the same point (0.828 pu). This relay is an old Westinghouse KLF distance relay.

We checked the calibration on the relay, and it checked good at 2.07pu. We checked four points on the curve, and the circle appears to be circular and centered on the y axis. At the time, I did not think to input the conditions which caused the trip.

If the relay calibrated good, and the mho circle is circular, what else could cause the relay to trip incorrectly? What could we check?

 

[oldfieldguy]

Check your current circuits thoroughly. It seems you have been successful using this equipment in the past. Look for what may have changed.

If you're absolutely sure that your generator is operating properly, take readings of currents and potentials at the relay while you're approaching the problem point to make sure your relay is monitoring good signals.

I had a 600 MW unit that wouldn't take a load after a maintenance effort. It would start up correctly, but as it loaded up, the currents went unbalanced into a current-balance relay. Like yours, the relay tested perfectly. The problem was ultimately traced to a terminal block where a contractor had shorted CT's during testing. He used a small strand of copper wire. At low currents, the normal current path flowed enough current for things to look normal, but as the generator tried to pick up load, increasing amounts of current flowed through the shorting jumper, resulting in unbalance.

I don't have a manual handy for the KLF, but you might want to bench test it under load for an extended period to see if you might have a capacitor breaking down, causing its characteristic to shift after components heat up.

old field guy

 

[karls2]

I am not use with distance relay, but have you any other back up relay in the protection panel who can help you to understand ? Maybe you can also try to have any OPG (curve capture) in order to see whats really happen at the trip time ? May be you can also test it with injection box to secure the relay side , and maybe cross the informations with the protection relays include in the generator panel ?

 

[slavag]

You have check current AND voltage wiring, polarity and phasing.
Seems as mistake in wiring.