differential protection in 120kV substation

[JLuc]

Hi,

we are designing a new 120kV/25kV substation with 2 (18 MVA) transformers paralleled. we are using only one breaker on HV and another one on the LV side of the transformers. we already have a diff. protection on each transformer + one overcurrent on the HV side and another one on the LV side. Would it be a good idea to add a bus diff. protection covering the entire substation? or would it be better to use 2 or 3 diff. protection for different section of the substation.

thank you.

JL

 

[davidbeach]

It depends. With no sectionalizing breakers on your bus there is no benefit to multiple zones of bus differential.

A single high side breaker and a single low side breaker shared by a pair of transformers sounds penny wise and pound foolish. So you have a transformer fault and you trip what? You trip the whole kit and caboodle. It would be far better to have a couple more breakers.

 

[dpc]

I agree - there's little to be gained in adding relay zones unless you have something to trip.

 

[slavag]

JLuc.
Could you please give us some reason (please not cost) of
such kind of xfr's connection.
Regards.
Slava

 

[JLuc]

The reason of this setup is this: we have two 120 kV lines entering the substation, one main line and one backup line. We know that the main line will be off 18 days per year (2 stops) and we have to be able to switch lines without stopping the plant. So we need one breaker per line. If we want to have one breaker per txfo, we would need two more breakers(4 total), so we decided to be a little less effective on protection to save two 120 kV breakers. Anyway, the load of the plant is to large to be transfered on one txfo.

JL

 

[slavag]

O.K.
Not so clear fo me, but it's not important.
For 120kV bus, you defently don't need any 87B (BBP).
You have only one in and only one out.
Regards.
Slava

 

[slavag]

BTW.
Your case, it's clasical H-connection.
two in, two out and breaker or disconnector in middle of H.
Regards.
Slava

 

[davidbeach]

Bit the bullet and put in four breakers in a ring bus configuration, line - transformer - line - transformer. Run with all four closed except in fault conditions. Your primary 120kV line is going to take other outage than just the planned outages and the ring bus will give you the best reliability at the least cost.

 

[slavag]

Hi David.
Ring connection is best solution.
But it's request more disconnectors and other level of protection scheme.
We use ring connection for EHV 400kV and more.
Best Regards.
Slava

 

[Marmite]

Jluc,
The substation design you have outlined seems very peculiar. Why are you designing a brand new substation with 2 transformers, neither of which is big enough to support the load of the plant on its own? If you don't need duplicated transformers for security of supply, then it must be cheaper to buy 1x 36MVA unit. With 2 transformers you've doubled the risk of a fault or a maintenance issue, but you are not providing means of isolating them independently. Davidbeach has the best suggestion. The price of 120kV SF6 circuit breakers will be a very small part of the overall project cost. If it's cheapness you are after then you could consider installing motorised isolators to open in the dead time for transformer faults and also provide a means of isolating the transformers independently for work. I suspect however that it wouldn't be much cheaper than the properly engineered solution.

Regards
Marmite

 

[jghrist]

We have a similar arrangement with a municipal utility client at their delivery point. The delivery point sub has two breakers connecting a single bus from two separate 100 kV lines, one breaker normally open. Transformers are connected by motor-operated air-break disconnects (MOABs). The difference is that the transformers serve separate distribution buses that are not normally connected.

If one line goes dead, the bus is swapped to the other line. The 100 kV bus is protected by overcurrent relays. Transformers are protected by differential and overcurrent. If one transformer protection operates, the breaker is tripped, the MOAB is opened, then the breaker is reclosed to restore service to the good transformer.

I'm not sure how you would have bus differential to protect the entire substation. It would essentially be a transformer differential covering both transformers because it would have to allow for the transformer ratio and transformer inrush. Then it would have to trip and lockout both transformers for a transformer fault as well as a bus fault.

You could use the breaker CTs and transformer high side CTs to put a differential on the high side bus, and the same thing on the low side bus, but I think the overcurrent relays would suffice. You probably don't have extra sets of transformer CTs anyway. Make sure you use ground relays from a neutral CT to cover low side bus ground faults.

 

[slavag]

O.K.
If we begined with cheaper solution:
Jluc.
Put two CB for the lines, one line (back-up) will open.
As Marmite saied, will bay one xfr 36MVA and connect it
to BB with disconnector.
Use diff protection with 3 sets of 3xCT:
Two sets from CT's of both of lines , one from xfr. LV side.
Now you have diff protection for stub and xfr, it's also overcurrent protection.
As Jghrist posted, "You probably don't have extra sets of transformer CTs anyway. Make sure you use ground relays from a neutral CT to cover low side bus ground faults".
Regards.
Slava.
PS
For my pinion, is not right technical solution for the new industrial substation

 

[JLuc]

Our installation will probably look like the one jghrist described.

The reason why we need 2 txfo is to be able to run the plant (it needs at least 20 MW to run) if one of the txfo fails. But to go from full load (~40MW) to minimum load (20MW) it has to stop completly. So no need to have a ring connection (4 CBs).

two 40 MW txfos + ring connection would increase the cost.

also... about the extra sets of CT, isn't possible to use one set of CT for 2 different relays? we already did that and it worked Ok.

JL

 

[Marmite]

Jluc
I'm not sure what country you are in, but in the UK transformers with a continuous emergency OFAF rating of double the normal ONAN rating are in widespread use in utilities. Obviously assuming a certain ambient temp and accepting some loss of insulation life. If you bought this type of transformer you could maintain full production with one Tx out on fault or otherwise.
Regards
Marmite

 

[jghrist]

also... about the extra sets of CT, isn't possible to use one set of CT for 2 different relays? we already did that and it worked Ok.

That would be a little tricky with two sets of differential relays. You'd have to be real careful to get the polarities correct and I'm not sure how you would connect the CT neutrals. It would be even trickier if the bus differential relay was a high impedance type. I'm not saying it couldn't be done, but I wouldn't.

 

[davidbeach]

Low impedance differential wouldn't be a problem as long as polarities all wind up in the right place. Don't even think about sharing a CT between a high impedance bus differential and any thing else, even including another high impedance bus diff.

 

[bacon4life]

We have a substation somewhat similar. It has two incoming lines with breakers feeding a two transformers with a common bus. One differential relay wraps the two 115 kV breakers and the low side of each transformer. There are also two additional transformer differentials using bushing CT's on the transformers. This was apparently done to identify if faults were internal or external to the transformes.