Overvoltage During Earth Fault 1

[newelecteng]

Dears

We are experiencing an issue where, during an earth fault on our 33 kV outgoing feeder, the voltage measured by the ABB REF relay (Vo) on the incomer spikes to around 14 kV. This overvoltage is affecting our 0.4 kV system, which is fed by our ZNyn1 earthing transformer. Is this behavior normal for a system like ours?

 

[Kiribanda]

1) What is the earthing method of your 33kV system?
2) You have mentioned that an earthing transformer ZNyn1 is feeding your 400V system. Bit unusual for me. Could you please upload your SLD?

 

[newelecteng]

Yup we use our earthing transformer ZNyn1 to feed out 0.4kv system

Please see attached SLD

 

[stevenal]

Is the fault location physically distant from the zig-zag ground?

 

[newelecteng]

The fault happend on outggoing feeder h03 or h04

See the attached sld on my previous reply

 

[stevenal]

The SLG does not show fault locations or distances. Please share the distance from a typical fault location and the zig-zag ground.

 

[jraef]

There are circumstances under which voltage escalation can take place in an arcing fault situation on AC lines due in part to constant reignition of the arc acting like a capacitor. I’ve only read about it in regard to ungrounded systems, but maybe it can happen with an impedance grounding system like this. I don’t know for sure, but it might be worth exploring that as a possible cause.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[newelecteng]

the fault was 10km away

plase see attahecd disturbance recoder of the feeder.

 

[newelecteng]

plase see attahecd disturbance recoder of the incomer.

 

[beyond86]

Actually, this is not typical solution for supply auxiliary. You will have overvolatge all time during EF condition because of transformer ZNyn1.

 

[stevenal]

I had thought the distance might add some impedance that would shift the neutral causing overvoltage in un-faulted phases. A simple model in Oneliner says otherwise. I only see voltage sags on the 400 V side.

 

[newelecteng]

Stevenal,
Do you think we could still experience a neutral shift even with an earthing transformer in place?

Beyond8,
Do we need to consider changing the earthing transformers?

 

[beyond86]

I was wrong about overvoltage at LV side of transformer ZNyn1.
I built simple model of your SLD and got these results:

Nothing happens with phase voltage at LV side. Can you please provide more details about what happens at LV side during EF condition (records)?

 

[stevenal]

The 33kV neutral definitely shifts. I see the full 33 kV line to line voltage line to ground on the un-faulted phases with a bolted fault. This overvoltage does not show up on the 400 V side. Tried various fault impedances as well as inserting some non-zero values in the transformer ground, but still fail to get any rise. Oneliner only deals with steady state solutions, and does not simulate transients. Is the overvoltage transient?
Vo on the 33 kV side does rise, but not on the 400 V side.

 

[Kiribanda]

newelecteng:
Few questions to you please.
1)What is the reason for using WYE EARTHED transformer for the 132kV side?
2)Are all feeders H01 to H06 connected to generators exporting power to 132kV side or are they normal feeders connected to typical loads?
3)What is the vector group of the transformers at the other end of all feeders H01 to H06?
Your input is appreciated.

 

[Kiribanda]

newelecteng:
I noted the following.
From the incomer relay, I Found that U0=14407.808(V)=14.4kV
Per your SLD the incomer is solidly grounded. Therefore,U0 cannot be 14.4kV.
It should read 76kV.
Per SLD there is no VT at the 132kV side.
Any comments?

 

[beyond86]

Kiribanda, look at 36 kV side, there is EF condition.

 

[Kiribanda]

beyond86,
Yes I noted it. See the feeder relay results.
Io_calc =643.458(A) meaning 3I0=1929Amps is the earth fault current.
How do you justify that amount of large earth fault current if the 33kV side is DELTA?
If the 33kV system is really a DELTA bus then there is only the charging current during an earth fault.

 

[beyond86]

Kiribanda, yes, you completely right, it's delta connection, but if you be more attentive, you will see transformer ZNyn1 at 33 kV side.

 

[Kiribanda]

beyond86
Yes, but does it mean that the 33kV system has got 643A as charging current?
How do you account for that?
Before I make any further comments I am curious to know,
1) Is the 33kV system a real "DELTA" system?
2) Does the 33kV system really needs an earthing transformer?
3) If yes, how it was sized?