Zero sequence impedance of grounding transformer

[lz5pl]

For a new industrial installation we have to specify technical data of grounding transformer. The system is following:
110/6,3 kV, 25 MVA, star/delta connected power transformer. 6,3 kV side will be grounded via grounding transformer (to create star point) and neutral earthing resistor. Earth fault current have to be limited to 200 A. For 200 A zero-sequence impedance of the grounding circuit will be about 18 Ohm.
My question is: how much should be zero-sequence impedance of the grounding transformer as a percentage of total grounding circuit? Grounding transformer will be used just to create star point, without any taps for compensation.

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[DanDel]

If the grounding transformer is a zig-zag connection, then the zero-sequence impedance is very low, and the total impedance to ground will be very close to that of the neutral grounding resistor.

 

[lz5pl]

Thank you, DanDel. My problem is that the proposed grounding transformer is ZN connection, but has zero-sequence impedance of about 22 Ohms, what is comparable with NER's resistance - 20 Ohms (designed for 200 A).
It seems that GT is not properly designed and has too big part of earth fault current limiting. Actual earth fault current with that values should be around 87 A only.

Unfortunately I haven't available test records from my commissioning years, when I have field tested such transformers. And I have totally forgotten what should be the typical value. :~/

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[RRaghunath]

Iz5pl,
I have done a quick check following the sizing calculation for Zigzag grounding transformer available in IEEE C62.92.4.

The earth fault current magnitude with Earthing Trafo having Zo of 22 ohm/phase will be nearly 500A.

Additionally, if 20 ohm NER is added in the ZN transformer neutral circuit, the available E/F current magnitude drops to ~132A.

Is this helpful!

 

[DanDel]

A zig-zag grounding transformer is typically modeled as having no zero-sequence inpedance, using just the impedance of the neutral grounding resistor/reactor. As far as the actual zero-sequence impedance of your grounding transformer, if 20 ohms is the tested result, you may want to check the configuration or the testing method.

 

[lz5pl]

Well, the problem is that for that project we will deliver only the NER, but grounding transformer is in the scope of another supplier. And the customer cannot give me any test reports for GT. Actually I doubt that Zo is really 22 Ohms. Another data I have for that trafo are: 250 kVA, 300 A. Using the methodology described on FAQ-page for ETAP software I calculated Zo to be 2,29 Ohms, what is a reasonable value.

The easiest way is simply to deliver the NER rated 20 Ohms, 200 A, as the customer specified it. But we are doing also design of the substation and I would like to be sure in the results.

raghun, could you give me some information about calculation method you have used (acc. to IEEE C62.92.4)? I am not IEEE member (I live in IEC world) and haven't access to their materials.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[DanDel]

I would insist on having the supplier of the grounding transformer explain the test results, since the design spec should require a zero-sequence impedance of nearly zero for the transformer itself.

 

[SMB1]

Iz5pl,i have a question!

if you can limit the fault current by using the GT zero sequence impedance, why you need earthing resistance?

thanx

 

[lz5pl]

That installation is a bit complex case. The customer already has GT and NER installed in 6 kV switchgear of existing co-generation plant. We design now 6/110 kV substation for connection of generation to the national grid. In new substation we must duplicate GT and NER. Only one set will be used at a time. When the generator will be switched-off 6 kV system in the factory will be grounded from our set of GT and NER.
That's the reason our set to be exactly the same as existing one. And here is the problem - there are no any factory test records available ! During the last maintenance body of the GT have been re-painted and the table with factory data also disappeared ! Field testing is not possible now, because the generator is in operation now. We advised the customer not to order new GT before the installation is stopped and field tests are performed on existing GT and NER.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[lz5pl]

Additional explanation regarding these 22 Ohms - that is the information from the customer's personnel. They said that the parameters of the GT are: 250 kVA, rated to 300 A and Zo is 22 Ohms. NER has resistance is 20 Ohms.
I don't trust totally these values, so I insisted on field testing before ordering of new set GT and NER.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant