High Impedance Bus Differential Relay (KAB)

[Kyle2022]

I've come across an odd situation in a switchyard outside of a power plant. An electro-mechanical high impedance differential relay (KAB) is used as bus differential protection. As is practice, a lockout contact is used to short-circuit the varistor in order to prevent it from overheating.

I would understand completely, if the KAB tripped this lockout, but the unit differential (a separate device) actually trips the lockout that shorts across the varistor of the KAB. I just can't think of a reason this would be done.

I assume this was done in error long ago, but I would like some confirmation.

 

[jghrist]

Normally, the KAB would trip a lockout that would both trip all bus breakers and short out the varistors. If the generator differential shorted out the varistors, then the bus protection would be disabled when the generator differential operated. Also, the varistors would not be shorted out if only the bus differential operated. This can't be right.

 

[Kyle2022]

Thanks jghrist, I just wanted to be sure I wasn't missing something. I am positive this is just one of those things that has been wrong for years.

It looks like it might have come about from a lack of contacts on the bus differential lockout, but there is no way the unit differential lockout is serving the intended purpose.