RELAYS 101

RELAYS 101 Lecture Notes Bus Diff Issues

Shows the wiring of a high impedance bus differential. Works great.

Even with a circuit breaker (CB) out of service still works great.

Care should be taken with shorting CT's on any CB in this scheme. The initial reaction by technicians is that shorting the ct on the out-of-service CB is OK and that shorting the ct on an in-service CB will result in the tripping of the 87B. Nope! Shorting any ct in the string is the same as shorting across anything in the parallel circuit. It shorts across the high impedance within the 87B relay. There will never be a high voltage developed across the 87B to operate the relay. Short the ct in the out-of-service CB and you are disabling the 87B. Short the ct on an in-service CB and you are disabling the 87B. So no trip. But now you have a bus without 87B protection.

This is a diagram that shows the unexpected action of doing everything by the book. Take a CB out of service, ground the bushings for safety work in an energized substation, work on the CB and consequently perhaps you close the CB. This is not only a mere possibility but it truly happens in a high voltage substation every single day somewhere! "Reflected Impedance" is the simple phenomena of an impedance appearing across a transformer. Short across the primary of a transformer and it appears that the secondary has a short across it. Thus, in this example (with bushings grounded and CB closed) then the ct's are effectively shorted and the 87B is disabled. No trip, just the 87B is disabled as long as the situation is there. (Possible Solution: ground the leads connected to the bushings then lift the leads connected to the bushings, thus you are no longer shorting across the CB.) (Another Possible Solution: Leave everything connected to grounds but lift the ct leads leaving the CB heading towards the 87B. The ct secondary is still effectively shorted but the short is removed from the 87B circuit. Just be sure to restore the ct leads before re-energizing the CB!)

A low-impedance differential scheme also works great.

Shorting the ct of the out-of service CB does nothing to the 87B circuit. The 87B is still in service. DO NOT SHORT THE CT'S OF THE IN-SERVICE CIRCUIT BREAKERS AS THAT COULD CAUSE A TRIP (if load on that circuit is high enough)!

You can even ground the ct on the out-of-service CB, go ahead close the CB. The 87B is unaffected. 87B in service and no trip. However, and this is huge. If the out-of-service CB is left in this condition with the CB closed and the CB bushings grounded then an "Outside Line Fault" can cause circulating ground currents that can be an issue. If the circulating current during the duration of the Line Fault flows through the closed CB then now you can have an additional ct secondary that will now show up being added to the 87B calculations. This is another example of how you can do everything right and a trip can still result. This example has happened. It just takes a smart tech to diagnose the situation if it should ever happen again.