400 kV Transformer neutral reduced insulation. 2

[rcw retired EE]

We are bidding a spec for a plant in the Mid-east that requires a disconnect switch in the neutral ground of the 400 kV transformers, 375 MVA, 400 kV-18.5kv, YNd11. The intent is to allow 400 kV system operation grounded or ungrounded. No reason given.

Can the neutral insulation level be reduced? If so, is there a good resource to calculate the suggested Neutral Point BIL? Full level is about 1425 kV, minimum for an impedance grounded neutral is 350 kv. (C57.12.00)

At 400 kV, full insulation appears impractical. Is this a common requirement? Is 400 kV ungrounded an actual operating condition? We think it may be a specification carry over from 220 kV systems where ungraded insulation is practical.

 

[sk201]

If you are requested to have a neutral disconnector on the neutral ground but without having the Surge arrestor, it is no way to reduce the BIL of the equip neutral. In most case, customer has their min. system BIL requirement. You may ask them to have that figure, choose a appropriate rating Surge arrestor and connect parallel to the disconnector, as such your equip' neutral BIL can much reduce.
Ungrounded system is for limiting the system fault level...
*it is the first time I come here and share my view. if you find my reply is not appropriate/ incorrect, let me know and let' check out the "TRUE" together

 

[odlanor]

You cannot reduce BIL of ungrounded neutral 400kV system.
400kV transformer winding will be connected in Y but with full insulation.
Besides, you must install surge arrester and disconnector at neutral, adequated for ungrounded system.
An EHV transformer is very expensive. You must protect it consider multiple contingencies during a short circuit.

I guess 345kV-Y winding transformer of Korean System are connected to ground by surge arrester and disconnect switch in parallel. That means, transformer can operate ungrounded or solidly grounded.

 

[FPelec]

rcwilson,
I think that in all 400 kV systems (even in middle-east) neutral is efficacely grounded. The purpose of disconnecting from ground the neutral of some transformsers could be to increase the Xo/X+ ratio, i.e. to reduce 1-phase short circut current in the network. Actually you will not change the system neutral status from grounded to ungrounded, so neutral insulation can be reduced by installing propers surge arresters. In some cases I've seen internal surge arresters.
However in the 60's some 400 kV step-up transformed with fully insulated neutral (1425 kV) were produced in Italy .

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[odlanor]

Fpelec,
...Actually you will not change the system neutral status from grounded to ungrounded,...
this is true if transformer is only supplied remotely by 400kV grounded.(X0/X1 < 3)
Let´s assume:
- there are source at 11kV side too.
- fault-to-ground at phaseA-400kV transformer terminal
- remote 400kV circut breaker opened
- stuck breaker at 400kV transformer side.
Your 400kV phaseB/phaseC windings will be submitted to high voltage throughout winding of ungrounded system.(X0/X1 > 10)

 

[rcw retired EE]

Thanks for the information. The application is power plant GSU transformers. The specification also requires a surge arrestor on the neutral bushing.

We wanted to reduce the neutral end of the winding below the phase terminals' 1425 kV impulse level and then protect the neutral with appropriately sized arrestors. Saves costs and is a more practical winding design. I have to do more analysis to convince myself that a surge arrestor won't get destroyed by a close-in fault if the neutral switch is open.

In my opinion, the system should never be operated with a gounded phase. The additional risk and the cost of GIS switchgear and cabling to handle continuous elevated phase-to-ground voltages is prohibitive. Am I missing something?

 

[ScottyUK]

rcwilson,

Is this a Kuwait project?

The surge arrestor + neutral earthing breaker combination is common in Kuwait and I believe is fairly successful in allowing the GSU transformer HV winding to operate without a solid connection to earth. I don't recall the GSU transformer being anything unusual over and above the normal requirements for GSU duty, although it is a little while ago so don't take my word as gospel truth!


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If we learn from our mistakes I'm getting a great education!

 

[rcw retired EE]

Scotty - this is not in Kuwiat. I'm not supposed to say where at this time, even though it is a public RFP.

I appreciate the feedback. My engineering/operating instincts tell me it will be fine as long as the 400 kV ground fault relaying clears quickly and the arrestor and neutral insualtion zare propeply coordinated. But with the neutral ungrounded, the GSU contribution to a ground fault will be small and may not be detectable by the relaying.

 

[odlanor]

we had the idea duplicate surge arrester at neutral and take BIL reduced in the winding of transformer.
It is against standards to operate an ungrounded system with Reduced BIL at neutral.
In case of an accident the insurance company could not pay the transformer.

 

[ScottyUK]

odlanor,

In a typical power plant the GSU transformers of several generators will be paralleled. A normal mode of operation would be to have one or two GSU transformers operating with the HV neutral switch open to reduce the earth fault current while the remainder of the GSU transformers operate with the normal connection to earth. This would would not make the system 'ungrounded' in the conventional sense.


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If we learn from our mistakes I'm getting a great education!

 

[odlanor]

ScottyUK
my viewpoint is: if there is a contingency where you submit your transformer to a ungrounded condition In case of an accident the insurance company could not pay the transformer.

 

[prc]

wilson, I have seen similar arrangement in Syria too in step down transformers. Transformers are with non-uniform insulated HV windings ie with reduced neutral insulation for solid grounding.I thought the isolator was to by pass the surge arrester and not for an operation with floating neutral.

 

[FPelec]

If "island" operation of the transformer i.e. the situation evidenced by odlanor is possible, proper protection as the 59N relay should be foreseen in order to limit the duration of the temporary neutral overvoltage.
As a consequence you have to verify that the windings are able to endure the neutral temporary overvoltage (230 kV), for example by means of ACSD test performed on neutral side. This would probably bring something around 170 kV or 245 kV class insulation level for neutral, according to EN 60076-3.
Surge arresters, if necessary, will thus be dimensioned in order to protect the neutral and to endure the TOV (e.g. using Uc=156 kV / MCOV=170 kV)

However, if the plant is composed of more GSU, I fully agree with ScottyUK, as the operating rules will always foresee a part of the step up transformers solidly grounded and then you could use a significantly reduecd neutral insulation level.
BTW, in Turkey 123 kV is often used as neutral insulation level for step-up transfomers of that size.

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[odlanor]

FPelec
...If "island" operation of the transformer i.e. the situation evidenced by odlanor is possible, proper protection as the 59N relay should be foreseen in order to limit the duration of the temporary neutral overvoltage....

What's happened if 59N does not operate?

 

[FPelec]

odlanor,
you will have a similar problem in case the maximum current or differential relays do not operate.
Generally speaking the usual practice in my company is to use two different protections by different manufacturers, fed by different cores of the same CT so that a failure of both protection system is really a remote possibility.
However, assuming only one 59N relay, in case this does not operate you will have in a few seconds a termal runaway on neutral surge arrester and then a second (low impedance) fault to ground; thus the neutral voltage will decrease and the fault current will increase; maximum current relays will then identify the fault and will open the circuit breakers.
The ACSD test should verify the capability to endure the TOV.


Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[odlanor]

FPelec,
I never saw insulation coordination project to be involved with relaying. Maybe it's because they are produced in different phase of the project, experts from different areas. The insulation coordination is part of the sizing equipment. The relaying is a complement, in order to prevent the events that occur resulting in sizing.
It is not uncommon operation remove 59 for maloperation.
I would never do this reasoning!