stevesummers
Electrical
- Nov 2, 2015
- 26
All,
I have an 11kV site that when in island mode has a handful of EDGs that run unearthed, with an 11kV system earth provided by a zig-zag grounding transformer. the grounding transformer is rated at 131A (142ohms) for 30s.
My understanding of these units is that the grounding transformer impedance should limit the fault current to around about 131A.. in a similar manner to having a neutral earthing resistor in a step-down transformer.
I have modelled the system in ETAP using, their suggested method for creating an equivalent model for the grounding transformer... So all good so far, but when i run the fault study, the earth fault value calculated is in the order of 2.5kA, so much higher than i would expect.
I have been back and forwards with the ETAP tech support guys on this, and they are stating that although the generators are unearthed and thus have no zero sequence impedance, the positive and negative sequence do interact with the zero sequence rating of the grounding transformer making the fault current higher. i.e. so the zero sequence current is given by:
Ia0 = Ia1 = Ia2 = E/(Z1+Z2+Z0)
I also did a similar experiment with a 20MVA, z=10% YNd transformer with the same zigzag and no generators, and got a similar very high earth fault current.
So, while i can see what the ETAP guys are saying, the end resultant figures just don't look correct to me. I would expect some variation, but not a resultant fault current that is out by a factor of 20.
This obviously has a knock on effect to the protection settings, and potential damage to the generators as the earth fault current is much higher. So i am keen to make sure i understand the issue correctly,
Can anyone shed any light on the above. Are the ETAP guys correct, and i am missing something (its a been 20+ years since i did sequence currents in anger) - or is my hunch correct, and ETAP is coming up with a 'wrong' answer for some other reason?
Steve
I have an 11kV site that when in island mode has a handful of EDGs that run unearthed, with an 11kV system earth provided by a zig-zag grounding transformer. the grounding transformer is rated at 131A (142ohms) for 30s.
My understanding of these units is that the grounding transformer impedance should limit the fault current to around about 131A.. in a similar manner to having a neutral earthing resistor in a step-down transformer.
I have modelled the system in ETAP using, their suggested method for creating an equivalent model for the grounding transformer... So all good so far, but when i run the fault study, the earth fault value calculated is in the order of 2.5kA, so much higher than i would expect.
I have been back and forwards with the ETAP tech support guys on this, and they are stating that although the generators are unearthed and thus have no zero sequence impedance, the positive and negative sequence do interact with the zero sequence rating of the grounding transformer making the fault current higher. i.e. so the zero sequence current is given by:
Ia0 = Ia1 = Ia2 = E/(Z1+Z2+Z0)
I also did a similar experiment with a 20MVA, z=10% YNd transformer with the same zigzag and no generators, and got a similar very high earth fault current.
So, while i can see what the ETAP guys are saying, the end resultant figures just don't look correct to me. I would expect some variation, but not a resultant fault current that is out by a factor of 20.
This obviously has a knock on effect to the protection settings, and potential damage to the generators as the earth fault current is much higher. So i am keen to make sure i understand the issue correctly,
Can anyone shed any light on the above. Are the ETAP guys correct, and i am missing something (its a been 20+ years since i did sequence currents in anger) - or is my hunch correct, and ETAP is coming up with a 'wrong' answer for some other reason?
Steve