Reverse power protection for a transformer

[Phil Corso]

Who are "they"? Phil

If "they" are the relay mfg, then ask "them" if "they" know about the probability of such maloperation!

Phil

 

[mbrooke]

Who are "they"? Phil

The source you dug through. But it should not matter as the science and theory of application is the same.

 

[Sahib]

Normally a 345kv (or 115kv) bus fault is cleared via bus bar differential protection which opens all the breakers connected to that bus. But picture a scenario where the bus bar protection is out of service (or fails)- you need a last resort backup. For 345kv lines entering the station the last resort for a fault on the 345kv bus bar will be cleared via Zone 2 step distance at the remote substations. However, Id like to have some type of means to remove the transformers.

No back up protection to differential protection? A restricted earth fault protection may so serve. In fact, one is back up protection to the other.

 

[mbrooke]

No back up protection to differential protection? A restricted earth fault protection may so serve. In fact, one is back up protection to the other.

There is over current, about 115 to 125% of the OF-AF rating.

 

[Ingenieur]

Can you sketch a single line and show the fault that would cause this to happen?
thnx

 

[Bugman1400]

I agree that an OL is needed for further discussion since your schemes have been a bit atypical in the past. For a fault on the 345kV t-line, the local distance relays (w/ comm trip) would clear. My guess is that there would also be some type of breaker failure scheme because of the 345kV level. For another backup layer, the xfmrs would have simple overcurrent in addition to the typical differential. For an additional backup layer, the lowside 115kV bank breaker relays could use a reverse distance element to look through the xfmr for faults on the 345kV side and trip after a 60-75 cyc delay. Since this element would be looking trough the xfmr, you could set the MTA to 90 and avoid any load encroachment issues for times when you want the 115kV to source the 345kV side. A lot depends on if most of the gen is on the 115 or 345 side.

 

[Ingenieur]

That is the question
is there ANY generation on the 115?
is it directly on the 115? Gen step up to 115
need a one-line showing combined capacity on each bus

 

[mbrooke]

That is the question
is there ANY generation on the 115?
is it directly on the 115? Gen step up to 115
need a one-line showing combined capacity on each bus

I will draw or post an equivalent single- yes the 115kv is looped with other 345-115kv stations.

 

[mbrooke]

I agree that an OL is needed for further discussion since your schemes have been a bit atypical in the past. For a fault on the 345kV t-line, the local distance relays (w/ comm trip) would clear. My guess is that there would also be some type of breaker failure scheme because of the 345kV level. For another backup layer, the xfmrs would have simple overcurrent in addition to the typical differential. For an additional backup layer, the lowside 115kV bank breaker relays could use a reverse distance element to look through the xfmr for faults on the 345kV side and trip after a 60-75 cyc delay. Since this element would be looking trough the xfmr, you could set the MTA to 90 and avoid any load encroachment issues for times when you want the 115kV to source the 345kV side. A lot depends on if most of the gen is on the 115 or 345 side.

Thank you- and this post is gold. Single line to come.

There is breaker failure- but I don't think it will play much of a role with bus-bar protection disabled as there are no breakers which are failing in the first place. Line Zone 2 can not trip the transformers nor am I familiar with any case that can.

Most generation (around 95% of it) is on the 345kv side.

 

[Bugman1400]

If you're referring to the 345kV bus bar, I would be surprised if there is not a primary and backup bus diff. Per NERC guidelines and most utility policy, I don't think you can simply disable bus diff protection these days. There are many times when I've seen a cleared bus when enabling and disabling the bus diff. For a primary and backup bus diff relay, if the relay needs to be PM'd then one relay can be taken out at a time for testing. An added layer, is the Zone 2 of the remote end. Another added layer, is a Zone 3 reverse of the local line relays. They can be set to see through the xfmr(s) to the lowside.

 

[mbrooke]

If you're referring to the 345kV bus bar, I would be surprised if there is not a primary and backup bus diff. Per NERC guidelines and most utility policy, I don't think you can simply disable bus diff protection these days. There are many times when I've seen a cleared bus when enabling and disabling the bus diff. For a primary and backup bus diff relay, if the relay needs to be PM'd then one relay can be taken out at a time for testing. An added layer, is the Zone 2 of the remote end. Another added layer, is a Zone 3 reverse of the local line relays. They can be set to see through the xfmr(s) to the lowside.

There are two set of BB relays for both the 115 and 345kv bus bar and two DC supplies but I'd still like some type of backup.

In real life cases where Zone 3 looks through the transformer, what are the typical time delays? In most cases I simply disable zone 3 (the reversed zone).

 

[Bugman1400]

Reverse Zone 3 time delay would be a minimum of 60 cycles. It could be more depending on if you have minimum clearing time intervals (CTIs)....say 18 or 24 cycles.

 

[mbrooke]

Can you sketch a single line and show the fault that would cause this to happen?
thnx

Drew up a generic single line of the substation in question:

 

[mbrooke]

I agree that an OL is needed for further discussion since your schemes have been a bit atypical in the past. For a fault on the 345kV t-line, the local distance relays (w/ comm trip) would clear. My guess is that there would also be some type of breaker failure scheme because of the 345kV level. For another backup layer, the xfmrs would have simple overcurrent in addition to the typical differential. For an additional backup layer, the lowside 115kV bank breaker relays could use a reverse distance element to look through the xfmr for faults on the 345kV side and trip after a 60-75 cyc delay. Since this element would be looking trough the xfmr, you could set the MTA to 90 and avoid any load encroachment issues for times when you want the 115kV to source the 345kV side. A lot depends on if most of the gen is on the 115 or 345 side.

Single line:

 

[mbrooke]

The desired outcome is that if a fault developed on either 345kv bus 1 or 345kv bus two with bus protection disabled 345kv lines 1-7 will clear on zone 2 and the autotransformers will clear without taking the 115kv busses out of service. Under most conditions with all 115 kv lines in service the 115kv load can survive without the 345kv support.

 

[Bugman1400]

If you truly have a lowside (115kV) bank bkr on all the xfmrs then you can use a distance relay to provide backup protection for a 345kV bus fault. This relay has the added benefit of using a different set of CTs and PTs than the highside relays.

 

[mbrooke]

If you truly have a lowside (115kV) bank bkr on all the xfmrs then you can use a distance relay to provide backup protection for a 345kV bus fault. This relay has the added benefit of using a different set of CTs and PTs than the highside relays.

I do, the diagram does not lie If I do distance, you said something about the MTA (maximum torque angle)?

Also, should I even consider directional over current or just skip right to a 311C? Preferably less relays would be a good preference.

 

[rian0201]

device 32 and 67 for me are not the same...


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[mbrooke]

device 32 and 67 for me are not the same...


Sent from my iPad using Tapatalk

Thank you for bringing this up, you would indeed be correct. Directional power (32) is not the same as directional over current (67).

So it thickens, what would get the job done better in this case? My money is on directional over current, but folks are welcome to disprove me.

 

[mbrooke]

Bus tie

Bus tie

This should perhaps be in a new thread, but it goes hand in hand with this. I am debating about having one reverse and one forward impedance zone on the 345kv bus tie, both set at 10% of the incoming line impedances and both having a delay of 8 cycles.


The idea, or rather objective, is that when a fault occurs on either 345kv bus section (with the bus protection disabled), the bus tie will trip in about 10.5 cycles splitting the bus. 25 cycles latter the remote line breakers will open on zone 2 and if not having happened already the auto transformer breakers will clear on their chosen protection.


My question is, since there is current in-feed into the faulted bus section, a forward zone will severely under reach, possible not tripping the bus tie before remote zone 2 trips? Correct? But if I have a reverse zone looking into the bus section, I will accurately clear? Same if my voltage quantities/polarization are fed from the in faulted bus?


Just double checking my logic on this.