Two Transformer Operation with One NGR

It may be noted that transformer are not operating in parallel.

01 is kept in service and other kept as reserve

Do not these two statements contradict?

If they are in parallel then it is perfectly acceptable to use single NGR.

No. As stated in my question after load increase it is required to pot both in operation .

Primary of transformer shall be feed from same source where as secondary shall be feeding different section of 33 KV Switch gear.

Out of curiosity, are there one or two zigzag transformers?

I'd rather see someone else answer your question--I think it can be done, but I've never seen it done. Maybe the rating of the NGR would have to be bigger (handle more power) than a similar NGR that could only ever see a ground fault from (to) one transformer.

You may have noted

"The installation has 01 NER which can be connected to any earthing transformers by switches between them"

So there are 02 Zig-zag earthing transformer.

By ignoring the zig-zag impedance, the earth fault NER will be

= [33000/Sqrt(3)]/700 amps = 27.21 ohms.

Note the value of ohms depends purely on 33 kV and 700 ampers for limiting.

Thus you can use two transformers in parallel with a common NER, the current will still be limited to 700 ampers only , irrespective of number of transformer in parallel, as the voltage applied is still only a maximum of 33 kV/sqrt(3.0) under worst condition.

Further the impedance of the zig-zag transformer if any will further reduce the current below 700 A.

Trust this clarifies

Dear Dr.Raghunatha Ramaswamy,

1. What would be value of fault current when two different phases of transformer (say R in Tr-1 and Y in Tr-2) develops Single phase to ground fault.

2. I have not worked on such arrangement with 2 TR and 1 NER. Have you?

Dear Sir,

Considering he is using same NER for both the zig zag transformers. Then in case of earthfault on 33 KV system, total zero sequence thevenin impedance will contain

1. 3 times value of NER plus

2. equivalent impedance which is equal to parallel circuit of both zig zag trafo zero sequence impedance.

This total value will be less than the value which will be obtained considering only one zig zag trafo in line with NER.

Thus, fault current in case of earthfault will increase if both zig zag trafo are connected to same NER. But, It will be limited to 700 amps per NER rating.

Now i have a interesting ques here:

Now since fault current is increasing, fault MVA pf 33 kv bus is also increasing but since NER is limiting this fault current to 700 amps, then is it prudent to consider increased fault MVA ?

Will NER always limit the current to 700 Amps irrespective of whether it is L-G or LL-G fault?

The current is always determined by the voltage and impedance as per Ohm's law.

Consequently as long as the voltage remains same, NER value remains same, The current will remain same.

This has nothing todo with parallel arrangement or single circuit arrangment.

We are only interested in what is the voltage applied across the NER.

We may find out these by calculations.

Since there are 2 transformers in parallel, the positive seq , negative seq and zero sequence impedances of the transformers will become half. But the sizing of the NER has not considered any source impedance. Meaning, irrespective of the source impedance, the current will always remain under 700 Amps. The actual current will depend on source impedance also, which needs to be computed.

The 3 times NER value is in the zero - sequence network. Sizing calculations are done in phase domain assuming a single full phase voltage across the NER.

The increased fault MVA is only due to changes in the source impedance. But the fault MVA will still be restricted to a value limited by this 700 Amperes ceiling.

Then as per your saying,

Can he connect this 2nd trafo to 33 KV bus without any NGR connection through zig zag trafo..?

If this is the case suppose then, doesn't paralleling of these transformers on same 33 KV bus increases the fault MVA of this bus? Since +ve and -ve seq impedance are getting half. (considering identical transformers)

and i am also taking into account buscoupler of 33 KV bus as NC (normally close)

The fault level will increase due to parallel combination but still be less than 700 A. 700 A is the maximum fault level for single line to ground fault.

fault mva on respective bus is limited by ngr rating. I think that is why ner is rated as x amp for y sec. Now don't you think this ner is designed taking into account the fault current when only one transformer is operating? I am of view that ner is needed to be redesigned of different rating considering increased fault mva and fault current when two trafo are operating.

NO- NER is designed considering 0 source impedance or infinite fault MVA for the voltage considered. Hence it does not matter how may transformers are in parallel, the fault current will remain within the limit specified.

ok. now i got it. thanks for clearing my doubt. :)

Note also that the OP has stated that he is not planning to connect the transformers in parallel, they will feed separate (secondary) buses--from the OP:

If load on Utility increases (say 50 MVA) Is it possible to put both transformer in service feed from same Primary source and feeding two section of 33 KV Buses.

With respect to ground faults, I agree with raghunath7 that even paralleled, the ground faults will not increase.

However, if the transformers are paralleled to a common bus (perhaps by use of a NC tie breaker), the available fault current for non-ground faults will increase (the impedance of the transformer will be halved), so the ratings of the secondary bus and breakers would have to be re-considered.

Dear Mr rhkuma,

I have drawn the arrangement with 02 transformer secondaries, 02 Zig-zag transformers and 0ne NER for 02 cases,

1. When transformer operating in parallel i.e. Bus-coupler close.

2. When transformer operating independently i.e. Bus-coupler open.

Intrestingly, In both cases earth fault remain same but is case 1 the total fault current is divided among two Ziz-zag transformers and reach both main Transformers, whereas in case 2, the fault current flows only in circuit (section) under fault.

With such arrangement Earth fault current setting shall be a tricky job.

Assuming normal current through the NER under healthy condition is nearly 0.0, any current in NER that is reasonably beyond normal unbalanced condition can be considered as due to earth fault condition only.

The general practice is therefore, to set earth fault current setting at lowest/near lowest available current/plug setting and do the back up coordination using time dial setting. It is rarely a difficult situation.

Why don't you go for another NGR of identical parameters for ensuring dedicated NGR for each 40 MVA transformer so as to keep the scheme simple and reliable ?

This will also increase the operational flexibility, since failure of the single NGR will force you to switch OFF both the 40 MVA transformers.

Dear spradhan,

As mentioned in question, the facility belongs to Utility Co. So it is there choice to use one NGR.

It is always advantageous to have independent redundant system. In my opinion Saving one NGR and losing 100% redundancy s not a wise decision.

You are right.You may suggest this to the concerned Authority of the Utility Co. as a well wisher .