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# Protection Riddle No.13 – Setting of earth fault relay

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Topic
• #154
Hamid

A 20 KV switchgear have equipped with one zig-zag earthing transformer. As you know, A grounding transformer must handle the unbalanced load on the circuit as well as the duty during line-to-ground faults.
If the circuit has a lot of unbalance load, then we must concern the continuous rating of transformer in addition of its short time withstand currents. The continuous rating power of mentioned grounding transformer is 100 KVA.
The primary current setting of related earth fault relay that installed in neutral point of E.T. is 8 Amp.
This set point current is chosen in base of E.T. continuous rating current and following calculations:

It =100/ (20×1.73) = 2.89 A

In= 3x It = 8.67 A

But the earth fault current calculation and analyses dictate other primary current setting (12.5 A).

Which one current setting is correct? Which one do you choose?

Viewing 9 replies - 1 through 9 (of 9 total)
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• #1303
AMIR KHASHAYAR

For setting of relay 8.67 Amp. is correct . In fact 12.5 Amp. consists of two components resistive and capacitive . 12.5 Amp. is 8.67 *1.44 (root of 2) . 8.67 passes the earthing resistance or earthing transformer and initiates the relay and this is correct for setting of relay.

#1306
Hamid

I think we must think more and more.

#1307
AMIR KHASHAYAR

Of course , we think more. In 20 KV system we don’t have zero phase sequence current as any result of unbalace load . Any unbalance load appears as negative phase sequence , and NPS current doesn’t pass zigzag earthing transformer. This is the question , How 100 KVA or 8.67 Amp. is achieved ?If neutral point of zigzag earthing transformer is grounded solidly , so the earth fault current won’t be 8.67 Amp. It will be much higher. and if it has a secondary wye winding the earth fault depends on probable resistance (if any) and impedance voltage . the earthing scheme of your idea isn’t clear . It’s not clear why the continuous rating is 100 KVA.

#1319
Hamid

Unfortunately, nowadays with our world information transmission system (internet), we usually neglect the basic conceptions and deep thinking about new problems. With more deep thinking, we can find that the mentioned evident equation (S = 1.73 VI) is not correct for zig-zag earthing transformers. Indeed, each leg of a grounding transformer carries one-third of the neutral current and has line-to-neutral voltage. So in a grounded wye

#1321
AMIR KHASHAYAR

20000/1.73/1.73 NAMELY 6666.6 V IS THE VOLTAGE OVER HALF WINDNG ‘ NOT TWO HALVES WIDINGS. IN THIS CASE THE APPARENT POWER OF HALF WINDING IS 16.66 KVA. THE RELATIONSHIP OF S=1.73 VI CAN NOT BE VIOLATED DUE TO ZIGZAG CONFIGURATION . THIS RELATIONSHIP IS STILL VALID . SO THE CURRENT PASSING THE NEUTRAL IS 8.67 Amp. NOT 15 Amp.Sorry I can’t agree to Mr. Hamid’s idea.

#1336
AMIR KHASHAYAR

If we assume , the voltage over winding is half and on the other hand the flux density is half , it means the number of winding is fix and hasn’t been reduced. U=4.44*N*(?)*F In this case we have used 2.3 times of copper in comparison with star winding . On the one hand , we state that the flux density is 1.7 T divided by two , while , this isn’t feasible and economical . If we assume the flux density is 1.7 T (as it is usual) , thus the current in one leg can’t be 5 AMP. It is 2.88 Amp. And if flux density is 0.85 T , so, it isn’t surprising if it can transfer 1.73 S.

#1337
Hamid

Mr. AMIR KHASHAYAR, excuse me I think we must go back to basic conseption of electrical transformer.

#1338
AMIR KHASHAYAR

Both of us should be referred again not me alone. Flux density is very important in economical design and construction of transformer .Normally this is 1.7 T . when you say the voltage over zigzag is divided by 1.73 with the same number of turns, in fact you are saying that flux is also divided by 1.73 . in this case your transformer hasn’t been designed economically . However it can transfer 1.73 times its rated apparent power . I think both of us must come back university.

#1339
Hamid

Yes, I said “we must go back to basic”.

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