Line - Ground Fault

[Cerkit]

Hi,

Can someone comment on if in the diagram below, if there is a single phase to ground fault is it possible that there will unbalanced fault currents flowing through all three phases of the transformer on the unfaulted line. Both transformers are solidly earthed on the HV side and there is Grid in-feed on the 132kV busbar.

Thanks

 

[JG2828]

Image from previous post

 

[davidbeach]

What is the connection on low-side of the transformers? Is there a source on the low side?

In almost all certainty there will be unbalanced currents on the unfaulted circuit. The nature and magnitude depends on the answers above.

 

[Cerkit]

Hi,

Thank you for the reply. There is no source on the LV side.


Assuming that red phase is the one that has faulted, does it sound right that in the unfaulted circuit I am getting a very small value for red phase current and then higher and equal currents in blue and yellow phase?

Thank you

 

[stevenal]

Still need an answer to the first question regarding connection. Wye or delta on the LV side?

 

[Cerkit]

Hi.

Apologies. The transformers are 132/33kV , Star-Delta, with the Star neutral solidly earthed.

 

[7anoter4]

In my opinion, the low voltage source is the parallel transformer.

 

[davidbeach]

Solidly grounded wye on the high-side and a delta on the low-side means both source zero sequence current for the fault in addition to a he external 132kV source(s). The exact split depends on how the impedances relate.

 

[Cerkit]

@7anoter4 - The lines you have shown do not pass fault current as there is no zero sequence path. This is my understanding.

I suppose what I am trying to clarify is, will there be fault current returning to source via the non faulty transformer neutral, (which yes is the answer I believe, so both transformer neutrals will carry fault current of different proportions and so will the earthing impedance of the grid), but why is there current in blue and yellow phase when it is a red phase line to ground fault? I thought there should only be red phase fault current flowing everywhere only?

Thanks

 

[waross]

The delta winding will transfer energy from the healthy phases to the grounded or open phase.
(In the event that a feeder phase is ungrounded but open at the supply end, any other loads on that phase of the feeder will be supplied by this back fed transfer of energy.
This energy will come from both transformers, hence the increased current in the healthy phases of the second transformer.
Here is a simple example to illustrate the magnitude of the energy supplied by the delta winding.
Consider a single wye delta transformer on a feeder protected by fuses.
"A" phase goes solidly to ground.
The current is limited by the impedance of the source and the impedance of the conductors plus some backfed current from the delta winding.
The fuse clears on "A" phase but the feeder conductor remains solidly grounded.
The delta winding transfers energy from "B" phase and "C" phase back to "A" phase.
This fault current is now limited by the source impedance and by three times the impedance of the wye:delta transformer (bank).
In your case the addition of a second transformer in parallel will increase the current that is back fed into the fault.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

Let's say we have 3 single-phase transformers connected in ∆_Y
Viewed from 33 kV side it will be a line-to-line [A_B] short-circuit.
You have not Zo but you don't need it.

 

[waross]

A grounded wye:delta will transfer energy from the healthy phases through the delta and back into a primary fault.
I spent over 15 years in wye:delta land and became familiar with the many problems of the grounded wye:delta distribution system.
I have seen a wye:delta bank overheated to destruction in a few months due to the failure of one phase of a voltage regulator on the feeder.
In the sketch above, the healthy phases, "A" and "C" form an open delta on the delta secondary.
The virtual transformer formed by the open delta back feeds into "B" phase and the resulting current into the fault on the "B" phase primary is limited by three times the impedance of the transformer bank.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Yes, there is zero sequence current. What happens depends on a bunch of things. If the two transformers are connected on the 33kV side there is a fair amount of negative sequence current flowing between them. The zero sequence current on the unfaulted phases flows back into the larger system and sorts itself out. Put it into a modeling program and play around with it.

 

[Cerkit]

Hi.

Please see attached my solution on the positive and zero sequence diagrams for the network. Can anyone advise if these look correct?

Thanks