Step Distance Transformer

Hv&Lv

Senior Member
SEL487E providing differential and 50/51- soon SEL 311C or 421 for the step distance.

This comes from an overhaul of electromechanical relaying.

When you say look into you mean into the bus or through the transformer and the respected other side bus? That part is what I'm debating.
I mean have the high side 21s look just halfway into the transformer with the high side bushing and coil impedance, and the low side 21s looking back into the secondary side. With that many 21s, my thinking is not to look through the XF.
without pages of calculations, my gut tells me the admittance values will overlap on one or more and you may have nuisance tripping, especially during energizing
 

mbrooke

Senior Member
I mean have the high side 21s look just halfway into the transformer with the high side bushing and coil impedance, and the low side 21s looking back into the secondary side. With that many 21s, my thinking is not to look through the XF.
without pages of calculations, my gut tells me the admittance values will overlap on one or more and you may have nuisance tripping, especially during energizing
I don't want to look through the xformers either- but here is my worse case devils advocate:

CCVTs on BB-A fails and triggers a bus fault. 3 phase voltage is lost. Bus coupler clears in 12 cycles as it still has 3 phase voltage from BB-B, and lines 1 and 2 clear on remote zone 2 in 35 cycles. However, the trafos are left connected with no guarantee of clearing selectively.
 

Hv&Lv

Senior Member
I don't want to look through the xformers either- but here is my worse case devils advocate:

CCVTs on BB-A fails and triggers a bus fault. 3 phase voltage is lost. Bus coupler clears in 12 cycles as it still has 3 phase voltage from BB-B, and lines 1 and 2 clear on remote zone 2 in 35 cycles. However, the trafos are left connected with no guarantee of clearing selectively.
True. But that’s where the secondary level should pickup IMO. It’s the reason we have redundant systems. The 67 50,51, and 87 on the XFs should protect it, and there would be the selective tripping your looking for until the CCTVs are replaced.

If your worried about trying to get the transformers off line to clear everything if a CCTV blows, use a DTT scheme for that.

IMO the POTTS scheme should come into play to clear the bus coupler quicker than 35 cycles.
BB-A and BB-B will both see the fault. BB- A a zone 1, BB-B in zone 2 I’m assuming. Not sure where your zone points are set at.
once all the relays sees the fault, BB-A sends a permissive signal to Bus coupler to immediately trip ~3 cycles if your zone 1 is 0 cycles.
 

mbrooke

Senior Member
True. But that’s where the secondary level should pickup IMO. It’s the reason we have redundant systems. The 67 50,51, and 87 on the XFs should protect it, and there would be the selective tripping your looking for until the CCTVs are replaced.

If your worried about trying to get the transformers off line to clear everything if a CCTV blows, use a DTT scheme for that.

IMO the POTTS scheme should come into play to clear the bus coupler quicker than 35 cycles.
BB-A and BB-B will both see the fault. BB- A a zone 1, BB-B in zone 2 I’m assuming. Not sure where your zone points are set at.
once all the relays sees the fault, BB-A sends a permissive signal to Bus coupler to immediately trip ~3 cycles if your zone 1 is 0 cycles.

My thinking is both zone 2s will start timing, but the bus coupler set at 10 cycles will open first. Once open, lines connected to BB-B will no longer time remotely, but lines connected to BB-A will time until 35 cycles is met.

Granted at 400kv you have POTT, but at 132kv that is not always the case.
 

Hv&Lv

Senior Member
My thinking is both zone 2s will start timing, but the bus coupler set at 10 cycles will open first. Once open, lines connected to BB-B will no longer time remotely, but lines connected to BB-A will time until 35 cycles is met.

Granted at 400kv you have POTT, but at 132kv that is not always the case.
Oh, ok. I thought you said it was there already.
 

Hv&Lv

Senior Member
Question. Will the tranfomer X/R ratio lead to any difficulty? Most lines aren't that inductive, if it all.
Exactly! And especially with those short distances.
thats why I think you need to stay on one side or the other with the look through.
 

Hv&Lv

Senior Member
You mean without the look through?
Yes. Without the look through. Like I stated before, your line length isn’t miles.
The look through the XF to another relay downline works for miles of lines, not a couple of hundred feet. When I suggested it I didn’t realize we were on such small areas.
That’s also why I suggest leaving the transformer 21 off. There seems to be enough 21s there already.
if it weren’t for the expense, bus differential would be the way to go over the station.
 

mbrooke

Senior Member
Yes. Without the look through. Like I stated before, your line length isn’t miles.
The look through the XF to another relay downline works for miles of lines, not a couple of hundred feet. When I suggested it I didn’t realize we were on such small areas.
That’s also why I suggest leaving the transformer 21 off. There seems to be enough 21s there already.
if it weren’t for the expense, bus differential would be the way to go over the station.
Be honest with me- and frank- 21 won't work in this case. At least through the trafo.


My idea is bus + 10-15% of the shortest line.
 

Hv&Lv

Senior Member
Be honest with me- and frank- 21 won't work in this case. At least through the trafo.


My idea is bus + 10-15% of the shortest line.
21 won’t work through the transformer with that many 21s in such a small area. I feel like your going to be nuisance tripping more than protective tripping with all the overlapping zones and tight ohmic values.

+ 10-15% seems to be the way
 

Hv&Lv

Senior Member
Explain.

I'll agree with you on the nuisance tripping due to the X/R and what is a none existent transmission line.
You have so many overlapping zones with such low settings, the transformer X/R through both coils and any distance at all on the other side on the XF relay is going to make that particular function setting useless.
The setting will be so high (by comparison) that it will never see a fault or trip. All the other protections will beat it out.
you can set it and turn it on, I just don’t think it will have any effect at all.
 

mbrooke

Senior Member
You have so many overlapping zones with such low settings, the transformer X/R through both coils and any distance at all on the other side on the XF relay is going to make that particular function setting useless.
The setting will be so high (by comparison) that it will never see a fault or trip. All the other protections will beat it out.
you can set it and turn it on, I just don’t think it will have any effect at all.
My gut feeling confirmed. We can both agree on that here on in.

Which leaves us with the bus coupler opening and the remote ends tripping- is there any way that the trafos can be removed. Even with 67 or 32, I can still see all 4 transformers clearing in that 2000MVA would flow into trafos C and D, into the 132kv bus, up to trafos A and B and into the fault. There is no way discrimination can be achieved.
 

Hv&Lv

Senior Member
My gut feeling confirmed. We can both agree on that here on in.

Which leaves us with the bus coupler opening and the remote ends tripping- is there any way that the trafos can be removed. Even with 67 or 32, I can still see all 4 transformers clearing in that 2000MVA would flow into trafos C and D, into the 132kv bus, up to trafos A and B and into the fault. There is no way discrimination can be achieved.
DTT scheme. Logic would have to dictate which XFs to disconnect based on fault zone.
 

mbrooke

Senior Member
Still looking and thinking...
I’m thinking about the XF relays with a reverse zone and a very small forward zone just through primary coil for an 87 backup...
Brilliant! :cool:

Means the world to me- my brain is stuck in mud so to speak.

As of now the only workable (viable option) for coordination is having the trafo 21 relay look into the protected bus + 10% of the shortest T line. A garden variety bus fault will clear without problem, and I could even add a reverse zone 3 to act as backup for stuck line breakers. Works out perfectly.

Down side will be a CCVT failure induced bus fault, but even then the bus coupler will still open and something like 50/51N could clear out all the trafos. Trafos and half the lines clearing is WAY better then all the equipment clearing.

Actually, now that I think about it... I could rig it so LOP is initiated in milliseconds... reverting to a low level 50... Is this to good to be true? Or will it yield me other issues?

And- How does 67 behave with one phase lost during a fault? I could block 21, maybe have 67 but don't think it would work- I'd be happy with 50/51.
 
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