Step Distance Transformer

[mbrooke]

I have a single breaker double bus station. Same for both high and low side voltages, 345kv to 115kv, 4 autotransformers.

The bus coupler has two MHO distance elements one looking in each direction; both set to reach at 10% of the shortest line and delayed by 8 cycles.
The transmission lines have an:


1. 80% Zone 1 set at 0 cycles
2. 125% Zone 2 at 35 cycles
3. A reverse zone 3 set at 16 cycles at 10% reach of the shortest line
4. A forward Zone 4 set at 65 cycles looking well beyond into the system.


I'm unsure what to do with the transformer's distance elements or how they will behave.


My desired outcome is that should a bus fault occur, the bus coupler will open about 10 cycles and then all the line breakers connected to the effected bus will trip in about 18 cycles in reverse zone with re-close blocked. I'd like the transformers connected to the effected bus to do the same via step distance elements; leaving the unaffected remaining bus, lines and trafos energized.

Here is a generic example of the scheme:

 

[Hv&Lv]

I’m not by no means an expert at this, but I’ll play for awhile...
this is usually thrown off on an AE...
Couple of questions here.

You do have 87 protection, I assume?
You want your 21 to be primary, does Zone 1at each end reach into the transformers? I realize this is a Substation, but that single breaker- double bus takes up quite a bit of real estate.
and do you want your transformer 21 to cover far overreaching to react in case of a failed operation of a transmission breaker?
as far as the tripping, you said you wanted all this through distance relays with logic to block reclosing. So no communications at all.

 

[mbrooke]

I’m not by no means an expert at this, but I’ll play for awhile...
this is usually thrown off on an AE...
Couple of questions here.

You do have 87 protection, I assume?

Yes, each transformer has differential protection.

You want your 21 to be primary, does Zone 1at each end reach into the transformers? I realize this is a Substation, but that single breaker- double bus takes up quite a bit of real estate.

Transmission line zone 1 does not reach into any transformer.

Transformer zone is up for debate. Ideally I'd like the transformer step distance to look forward into the bus being the element to react for a bus fault, however if the bus fault is caused by a failed CCVT, I am inclined to believe that reaching through the transformer and using the unaffected side CCVTs is better.

and do you want your transformer 21 to cover far overreaching to react in case of a failed operation of a transmission breaker?

Not a requirement, but if it can be done that would be a nice feature.

as far as the tripping, you said you wanted all this through distance relays with logic to block reclosing. So no communications at all.

Yes and Yes

 

[Hv&Lv]

More questions. Helps you think and educates me as well on your situation...

I’m assuming one Mho element on each side of the transformers, so therefore you basically have to go through two transformers.

I see a Wye wye in the drawing,(I realize it’s a representation). Using a distance ground element? What effect will zero sequence currents have on these elements if you look through them.

Would you want the distance element reaching all the way through the transformer or just to the low side.

normally the transformer far reaching element would be for longer lines. In a sub I’m assuming your not over a mile of line?
I don’t understand why there isn’t a comm scheme available in the sub, POTT or some other scheme?

 

[Hv&Lv]

Not a requirement, but if it can be done that would be a nice feature.

It can be done, and is done frequently at different installations. The difference is it’s generally done for miles of line.
I'm still thinking about this....

It almost seems like this would be a coordination nightmare trying to get the right balance of dependability and security. Trying to make sure the relay operates when there is an in zone fault and making sure it doesn’t operate with an out of zone fault with the short lengths that are inside a Substation may be a mathematical impossibility.

wait... did I just use those last two words together??

 

[mbrooke]

More questions. Helps you think and educates me as well on your situation...

I’m assuming one Mho element on each side of the transformers, so therefore you basically have to go through two transformers.

There is an multi function microprocessor step distance relay on each transformer breaker- meaning both sides have multiple distance elements that can be set forward or reverse.

I see a Wye wye in the drawing,(I realize it’s a representation). Using a distance ground element? What effect will zero sequence currents have on these elements if you look through them.

Would you want the distance element reaching all the way through the transformer or just to the low side.

If reaching through, I'd like them to see the busbar and maybe 5-10% of the shortest line so that a bus fault is "seen" by the relay.

normally the transformer far reaching element would be for longer lines. In a sub I’m assuming your not over a mile of line?
I don’t understand why there isn’t a comm scheme available in the sub, POTT or some other scheme?

There is POTT on the primary, but the secondary doesn't have it despite being transmission since the critical clearing time is less of a concern.

 

[mbrooke]

It can be done, and is done frequently at different installations. The difference is it’s generally done for miles of line.
I'm still thinking about this....

It almost seems like this would be a coordination nightmare trying to get the right balance of dependability and security. Trying to make sure the relay operates when there is an in zone fault and making sure it doesn’t operate with an out of zone fault with the short lengths that are inside a Substation may be a mathematical impossibility.

wait... did I just use those last two words together??

Only thing that I think will make it work is timing. If the distance elements covering a near in zone fault are set to operate faster then those simultaneously seeing it from afar coordination may just be possible.

 

[Hv&Lv]

Only thing that I think will make it work is timing. If the distance elements covering a near in zone fault are set to operate faster then those simultaneously seeing it from afar coordination may just be possible.

I agree with this. Your timing seems to be fine with the four Zones.
Im trying to think about you original question. What would happen with an additional 21 added looking both ways, along with the 87 protection as backup...

 

[Hv&Lv]

If reaching through, I'd like them to see the busbar and maybe 5-10% of the shortest line so that a bus fault is "seen" by the Relay.

I’m thinking there is some compensation setting for a Wye Primary transformer that has to be done to account for the zero sequence currents for the ground distance element.
We were discussing this at an SEL conference. Never thought I would use it, so I wasn’t paying close attention...

 

[mbrooke]

I agree with this. Your timing seems to be fine with the four Zones.
Im trying to think about you original question. What would happen with an additional 21 added looking both ways, along with the 87 protection as backup...

One thing that has me hung up is that in some cases with minimal generation or ties, its possible that during the bus fault little current will flow through the transformers until the bus couplers open.

 

[mbrooke]

I’m thinking there is some compensation setting for a Wye Primary transformer that has to be done to account for the zero sequence currents for the ground distance element.
We were discussing this at an SEL conference. Never thought I would use it, so I wasn’t paying close attention...

Never ignore SEL, they know what they're talking about :happyyes: I'm confident the 421 would have some trick up its sleeve, not sure about 311Cs.

 

[Hv&Lv]

Here is some light reading...

https://www.eiseverywhere.com/file_...5cf6b23f35b6_5_SEL_421_UG_20111215.pdf#page50

https://www.eiseverywhere.com/file_uploads/f1d0745d49127515761a86dd426a1b28_SEL311C.pdf#page50

 

[Hv&Lv]

One thing that has me hung up is that in some cases with minimal generation or ties, its possible that during the bus fault little current will flow through the transformers until the bus couplers open.

I think that would depend on which side your on.
line side, I could see that. Load side would be maximum fault levels.
in either case, in something like the 311C the directional 67 would supervise your distance

 

[mbrooke]

Here is some light reading...

https://www.eiseverywhere.com/file_...5cf6b23f35b6_5_SEL_421_UG_20111215.pdf#page50

https://www.eiseverywhere.com/file_uploads/f1d0745d49127515761a86dd426a1b28_SEL311C.pdf#page50

Yup- I have those- including the latest 487B user manual which is going to remain in mint condition

 

[mbrooke]

I think that would depend on which side your on.
line side, I could see that. Load side would be maximum fault levels.
in either case, in something like the 311C the directional 67 would supervise your distance

Makes sense.

I feel as though coordination looking into the bus will not present any problem, but going through the unit might causing over or under reaching... am I correct to think this?

 

[Hv&Lv]

Makes sense.

I feel as though coordination looking into the bus will not present any problem, but going through the unit might causing over or under reaching... am I correct to think this?

That’s going to depend on your impedance values used in your calculations.
The transformer impedance values used in the calculations shouldn’t overreach or under reach if entered correctly. A transformer should be treated like a line section for a last ditch effort.
Other zones should have half the impedance set so they don’t see all the way through.

That was my question earlier about stopping at the high side.
Again, with 87, 21, 50/51and 67 supervision, coordination is of course, going to be the biggest challenge with these short distances in the Substation.
finding the balance of making sure it trips when in zone, and not trip out of zone is critical.
I have heard horror stories of engineers with pages of calculations and settings only to find out the relays won’t even let the transformer energize without tripping...

 

[mbrooke]

That’s going to depend on your impedance values used in your calculations.
The transformer impedance values used in the calculations shouldn’t overreach or under reach if entered correctly. A transformer should be treated like a line section for a last ditch effort.
Other zones should have half the impedance set so they don’t see all the way through.

That was my question earlier about stopping at the high side.
Again, with 87, 21, 50/51and 67 supervision, coordination is of course, going to be the biggest challenge with these short distances in the Substation.
finding the balance of making sure it trips when in zone, and not trip out of zone is critical.
I have heard horror stories of engineers with pages of calculations and settings only to find out the relays won’t even let the transformer energize without tripping...

Hadn't thought about energizing inrush and harmonics.

 

[mbrooke]

That’s going to depend on your impedance values used in your calculations.
The transformer impedance values used in the calculations shouldn’t overreach or under reach if entered correctly. A transformer should be treated like a line section for a last ditch effort.
Other zones should have half the impedance set so they don’t see all the way through.

That was my question earlier about stopping at the high side.
Again, with 87, 21, 50/51and 67 supervision, coordination is of course, going to be the biggest challenge with these short distances in the Substation.
finding the balance of making sure it trips when in zone, and not trip out of zone is critical.
I have heard horror stories of engineers with pages of calculations and settings only to find out the relays won’t even let the transformer energize without tripping...

As a frame of reference so we are both on the same page in discussion. I'm sure you already have this in your head or have it drawn out- but to clear up any confusion a single line equivalent diagram showing all the step distance relaying involved.


Each circle is a microprocessor based relay with at least 4 zones of MHO or Quad any of which can be set forward or reverse.

 

[Hv&Lv]

I was wondering if every breaker in the station had 21 or if it was tie breaker only..
What relay do you have on the transformer?
For transformer only I’m debating why even turn it on? ( if the relay has that capability? )
Make the 21s there now look just into their respective sides of the transformer. Let the transformer relay protect just the transformer differential zone.

 

[mbrooke]

I was wondering if every breaker in the station had 21 or if it was tie breaker only..
What relay do you have on the transformer?
For transformer only I’m debating why even turn it on? ( if the relay has that capability? )
Make the 21s there now look just into their respective sides of the transformer. Let the transformer relay protect just the transformer differential zone.

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.