I think students leave already with a set in stone opinion.
@marks1080, do these tap substations have their own circuit breakers or circuit switchers? In theory if you have a protective device on the transformer and secure communication chasing low side faults through the line breaker is unnecessary.
Most of our stations have line taps terminate directly to a transformer with low side breakers. A companion transformer would tap to a companion line. We call these DESN's (not sure if that term is used industry wide). We do use HV line blocking out of the transformer protection when necessary. This requires coordination of course.
In such a case it would make sense to coordinate through the transformer. Around here these substations are being retrofitted with circuit switchers. BTW, I will probably start a new thread soon regarding coordinating through a transformer as this issue is about to come up for me.
I'm not sure I know what you mean by coordinate through the transformer... if a line Z2 terminal sees a LV fault the only thing a transformer protection can do is send a blocking signal (coordinating the blocking signal may cause other problems, but those are other problems...).
In that if a fault occurred on the secondary MV side of the transformer, the HV line protection relays would pick it up on either impedance or negative sequence over current and trip the HV line. This is done as a last resort backup in the event communication fails where normally a DTT would be sent from the tap substation.
Sorry Mbrooke, I think there's something I'm missing. I'm very interested in what you are saying, just need a little more help getting there...
For a low side fault on a tapped station (which does not have it's own line protection for the HV line its connected to) you do not want your HV line protections tripping. Now we DO use something called 'line back-up' in our tapped transformer stations, but that's a different topic and I don't want to get distracted. The goal is that any fault on the LV side of the tapped station would be cleared by an element on the low side. The only equipment in a tapped station that would trip a HV line for a fault would be a fault in the transformer. And then only because we do not use high side breakers for tapped stations. I am thinking that we typically work on stations with different configurations? Is that why I am still confused?
What if that fault is between the transformer secondary bushings and your first breaker? Or, What if you have a buss fault with a stuck MV transformer breaker?
And no need to apologize, I am sure I am stating/asking things a bit skewed on my part.
Ahhh yes I see what you are saying now. Makes sense
You're absolutely right, a transformer fault would send a TT signal to trip the line. I've never looked at that as being a 'line protection' though... which it technically is. I think my confusion was more a matter of semantics. Thanks for the info!
