Zero sequence impedance of grounding transformer
Zero sequence impedance of grounding transformer
(OP)
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.
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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It may be like this in theory and practice, but in real life it is completely different.
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RE: Zero sequence impedance of grounding transformer
RE: Zero sequence impedance of grounding transformer
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
RE: Zero sequence impedance of grounding transformer
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!
RE: Zero sequence impedance of grounding transformer
RE: Zero sequence impedance of grounding transformer
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
RE: Zero sequence impedance of grounding transformer
RE: Zero sequence impedance of grounding transformer
if you can limit the fault current by using the GT zero sequence impedance, why you need earthing resistance?
thanx
RE: Zero sequence impedance of grounding transformer
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
RE: Zero sequence impedance of grounding transformer
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