Zig Zag Overvoltages
Zig Zag Overvoltages
(OP)
A post a few years ago discussed the calculations associated with zig-zag grounding transformers. In it one user posted the following:
"You might need to reconsider an almost entirely reactive impedance for the grounding. It is generally considered a bit of a no-no to have reactance grounded systems, due to the problems with power frequency overvoltages under arcing earth fault conditions. I don't recall all the details, but the mining industry in South Africa had some problems with it in the 1970s. There were a few papers in the SAIEE journal - people like a Dr Pretorius at the CSIR were involved with it back then, I think. Eastern Natal Region of Eskom used an NEC / NER combination at 11kV to keep earth fault levels to 300A, and the R componenet was to ensure that the system was not reactively earthed. "
We are involved in a project in which a 22.9kV system with a zig-zag grounding transformer has experienced some surge voltage problems. The zig-zag is directly grounded without a resistance so that minimum ground resistance will be leakage inductance only. I have been searching for the above papers describing possible power frequency overvoltages during arcing earth fault conditions. This seems like a plausible cause if there is phase angle displacement on each arc fault. I did search SAIEE (I am in the USA) but could not find anything that old. Can anyone point me to where I might find the above references or any other references that may cover arcing fault transient voltages on zig-zag grounded systems? Much appreciated!
"You might need to reconsider an almost entirely reactive impedance for the grounding. It is generally considered a bit of a no-no to have reactance grounded systems, due to the problems with power frequency overvoltages under arcing earth fault conditions. I don't recall all the details, but the mining industry in South Africa had some problems with it in the 1970s. There were a few papers in the SAIEE journal - people like a Dr Pretorius at the CSIR were involved with it back then, I think. Eastern Natal Region of Eskom used an NEC / NER combination at 11kV to keep earth fault levels to 300A, and the R componenet was to ensure that the system was not reactively earthed. "
We are involved in a project in which a 22.9kV system with a zig-zag grounding transformer has experienced some surge voltage problems. The zig-zag is directly grounded without a resistance so that minimum ground resistance will be leakage inductance only. I have been searching for the above papers describing possible power frequency overvoltages during arcing earth fault conditions. This seems like a plausible cause if there is phase angle displacement on each arc fault. I did search SAIEE (I am in the USA) but could not find anything that old. Can anyone point me to where I might find the above references or any other references that may cover arcing fault transient voltages on zig-zag grounded systems? Much appreciated!






RE: Zig Zag Overvoltages
This points to two effects.
1. An arcing fault may generate high frequencies.
2. The high frequencies will see motors and transformers as very high impedances.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Zig Zag Overvoltages
The condition to limit transient over voltages from arcing faults is Ro/X0> 2.Please check this. The grounding resistance shall be more than 2 times (reactance of grounding transformer +zero sequence reactance of line transformer) Please see above hand book and GE Industrial Power System data book SECTION 0.3- http://digital.lib.uiowa.edu/cdm/ref/collection/ui...
The overvoltages are created when system capacitance to ground reach the grounding reactance value,setting in resonnance.
I doubt whether your problem is due to transients. Recently I came across some grounding transformer failures.Root cause was found to be poor quality of core laminations used in the transformer. Please share failure mode details.
RE: Zig Zag Overvoltages
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Zig Zag Overvoltages