voltage transformer secondary grounding 3

[andross]

Hi,

I have a 11000/110v voltage transformer - wye/wye. The secondary has a 4 wire output. L1 - L2 - L3 - N. The drawings show the secondary as having three windings, one per phase. L2 secondary is grounded, as opposed to the neutral being grounded. Can anyone tell me why?

 

[dpc]

Maybe - what is the application? A generator PT perhaps?

 

[andross]

Hi,

Yes, the VT is on a generator circuit.

 

[rcw retired EE]

Does this VT go to a synchronizing circuit with a synchscope, synch check relay or auto-synchronizer? If so, is there an input from another set of VT's connected open delta with the L2 lead on the secondary grounded?

To prevent circulating currents and blown fuses between the two systems, the L2 phase (B phase) of the wye VT's is grounded. If the N point was grounded on the wye VT secondary, there would be 63 volts between the two L@ voltage signals.

 

[andross]

rcwilson

This makes sense. The VT is connected into the generator control panel, which contains a generator controller and synch equipment. We are not 100% sure of the bus VT configuration, but one drawing does show it as open delta with the L2 grounded.

Many Thanks.

 

[dpc]

This may be done to prevent nuisance tripping of the 59G generator ground relay in a high-resistance grounding system.

If the secondary neutral of the PTs is solidly grounded, an accidental ground on the PT low side can cause the 59G relay to operate. By floating the neutral and grounding one phase as a safety ground, this risk is reduced. It also means that all connections to the PT secondary have to be line to line.

Just saw a presentation at a conference that reminded me about this practice.

 

[davidbeach]

I believe that dpc has the correct answer, although I disagree with the conclusion that all connections to the PT secondary have to be line to line. The four wires can be brought to the relay and accurate measurements of primary phase to ground voltages can be made.

dpc, if you are referencing Mozina's erroneous statement on Tuesday that only phase to phase quantities may be used, consider the measurement of third harmonic voltages on the terminals of the generator. Given the nature of the third harmonic it is impossible to measure third harmonic looking at phase to phase voltages as the third harmonic does not produce any relative change between the phases.

One phase grounded, typically B, and all four wires to the relay. The relay doesn't care which one is grounded, only the relationship between the voltages on the four wires.

 

[andross]

Many Thanks to you all for your replies and assistance with this. I think we will be having an interesting discussion in work this morning.

Thankyou.

 

[asymptote]

This is the normal configuration for metering and protection VT's in the UK

 

[slavag]

Andross, it's interesting not only for you .
I'm so confused.
Old standard configurations was open delta with phase B (S, L2, Y)grounded/earthed. Phase to phase voltages is 110/115/120V.
Part of old relays ( for example 51V) had 67V voltages inputs and we add some inter. VT and conversed 3 phase to phase to neutral voltages ( 4 wire ).
Today all applications ( that I see of course) used wye (Y, star) connection of VT with neutral grounded only!!!.
In RCWilson's cases ( on BB open delta VT ) we add inter. VT ( is prevent all problems).
For metering , as David saied, better used phase to neutral voltages meas.( 4 wire connection).
I see in your case some mix of options.
What connection you have for 59G? May be here my misunderstanding? Actually we used VT with floating neutral and w/o fuses and MCB in the circuit.
Are you have also some back-up 59N relay? We used some inter
VT Y/open delta ( and in open delta we grounded B (S,Y, L2).
Are you have separated VT MCB for each voltage circuit?
Regards.
Slava

 

[dpc]

David,

You're probably right - I was just thinking it would be better to avoid connecting the PT neutral to anything but as long as it isn't grounded, it shouldn't be a problem. If it is grounded, you will find out pretty quickly.

 

[andross]

SLAVAG.

I have attached a quick sketch of the VT layout and protection system. Apologies for the quality, but it was drawn in a bit of a rush. But, hopefully it will clear up your confusion.

The VT outputs are fused seperately, with a solid link in the L2 phase.

I am not 100% sure of the Generator protection set up as I do not have this documentation to hand. But I am sure of the following;

There are two CT's in the Generator ground (neutral) circuit breaker. These provide standby earth fault (connected to the generator neutral CB protection relay. This trips both the neutral CB and the Generator CB. The othe CT is connected to the generator protection relay and provides instantaneous e/f protection.

I hope this goes someway to clearing up your confusion. As stated by ASYMPTOTE, this is a very commonly found connection here in the UK. I have often puzzled over this connection and have never been given what I felt was a realistic answer until asking the question on this forum.

Many Thanks all.

 

[slavag]

Dear Andross.
Thank you for layout. It's clear.
First of all, you don't have any problem with 59G, you haven't it. Your function is 51G.
I think in your case we have some historical issue, nothing technical. I think it's some British standard ( maybe also Indian, Australian, South African), or not standard, some traditional grounding of L2(S, Y, B). It continue from L2 grounding in case of VT delta connection. Actually is not bad, you know L2 is grounded in all applications.
Intresting is also other: CB in neutral of generator.
It's also first time for me, we use grounding switch.
Regards.
Slava

 

[slavag]

Forgot something.
I'm not shure if it's good solution because safety.
It's 3 single phase VT, if I need check something and open link of L2 grounding, my voltages circuit leaving w/o safety grounding.
Regards.
Slava