Protection Philosophy for Grounding Transformer : Wye-Broken Delta 2

[SudionoSu]

I tried to post this question in another thread but did not receive any response. So I am starting a new thread for this.

We have three 11KV Switchboards interconnected together by bus tie breakers. Each 11KV Switchboard is earthed through a grounding transformer. The transformer is wye connected with neutral direct earthed on the primary side (11KV) and the secondary side is a broken delta with resistor across it. There are two questions I would like to ask as follow:-

1. What is the protection philosophy for this type of grounding system. We have provided zero sequence CT for the generator, tie breaker ans outgoing feeders. The NGT feeder is protected by three CT connection. I have tried to illustrate the current flow during various fault condition (see attached). Would appreciate if anybody can help to verify that our current protection scheme is adequate.

2. What is the operational philosophy for the three NGTs when two or three switchboards are interconected together? Previous projects seem to show that only one NGT should be in service when two or three switchboards are interconnected. However, I find this scheme risky if the NGT in service fails and the whole system will be floating. Is there any standards or guidelines governing this?

Thanks in advance for your patience with my long story and would appreciate any contribution from any forum members.

Regards,

Su

 

[davidbeach]

I think you are trying to mix ANSI and IEC notations and coming up with a horribly confusing mix.

Am I seeing two generators that only have 67N from 2 CTs? I know you can cheap out and do phase overcurrent on high impedance grounded/ungrounded systems with only two CTs, but I have no idea how a 67N could ever work with only 2 CTs. 50A at 11kV is less than 1MW which seems to be way too small to be worrying about anything above 480V. Or are those supposed to be some form of window CT, and if so why call them out as 2 CTs?

Generally when ground fault currents are limited to very low values the intent is to alarm on ground fault, allow the process to continue, track down the ground fault and schedule a repair. If you are trying to trip on ground fault current you probably want some more current so you can get some selective coordination.

 

[SudionoSu]

Hi David,

I think the handwriting in my sketch is giving the misunderstanding. It shouuld be ZCT (Zero Sequence CT) not 2 CT.

We have calculated the capacitive charging current of each 11KV switchboard to be around 9A. To cater for failure of one NGT when two switchboards are parallelled, we have decided to size the NGT to be 18A. Hence, the minimum fault current with one NGT in operation is 18A. The worst case with three switchboard in operation is 54A. As our application is for offshore, IEC 61892-2 dictates that automatic tripping is required where system earth fault current exceeds 5A.

Hope the above provide more details.

Thanks.

 

[waross]

A grounded wye delta transformer bank will try to balance the phase voltages. It has the abilty to transfer energy from the higher voltage phases to the lower voltage phase. The current is limited by the impedance of the transformer bank.
The broken delta raises the impedance of the delta circuit and limits the maximum current to a low, engineered value.
I am confused by your small red lines. I was under the impression that the current from/to the wye/delta xfrmr would reverse on the faulted phase.

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

 

[SudionoSu]

Hi Waross,

Are you referring the current flow in the generator? The fault current has to flow from the source to the fault. Thats what we think and it seems to agree with some of the technical document from I-Gard.

 

[waross]

Hi;
I may be confused by your conventions.
What I was referring to is the action of a grounded wye/delta transformer bank under conditions of voltage unbalance.
When the wye voltages are balanced, the only current into the grounded wye/delta bank is the magnetizing current.
However, when the voltage on one transformer goes low, energy will flow into the bank on the healthy phases and out of the bank on the phase with the lower voltage.
(I have seen voltages and loads on a distribution line inadvertently balanced and stabilized by a grounded wye/delta bank. When the bank was disconnected, the metering at the sub showed the feeder loads to actually be quite unbalanced.)
Back on topic, the current flow is limited only by the transformer impedances. An per unit unbalance of the order of magnitude of the transformer impedances can cause full load current to flow in the delta. This is energy in on the healthy phases and energy out on the low phase.
Ground fault currents are limited by the impedances of the transformers. The broken delta with a resistor inserted limits this current but gives a dependable indication of ground faults.
Under any fault conditions other than a fault in the grounded wye/delta bank, I expect to see energy (current) in on the healthy phases and energy out on the faulted phase.
I have probably misunderstood your drawing convention.

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