Currents & Voltages during L-Gnd Fault in HRG System 1

[jy121]

I think this is a simple question, but I can't find any references to determine the answer... perhaps my Blackburn textbook would help... Here goes:
In a low voltage 600/346VAC, 3ph, 3w, high-resistance grounded system, what are the line voltages and line currents during a line-ground fault, in general? I understand from what I've read that the line voltages in the unfaulted phases rise to equal the phase voltage, i.e. 600V L-N, but what does the faulted line voltage look like? How about the currents? Does the system still deliver current to the load on the faulted phase?

 

[tem1234]

There's a lot of info on resistor manufacturer website.

Here's a exemple:

 

[jy121]

Yeah, I looked on I-Gard's website prior to posting my question... I thought of posing a question to their blog, but figured I'd come here first. Your attachment is informative, as is the data on their site, but it doesn't answer my question...

 

[dpc]

Your terminology is confusing to me. The line-to-line voltage doesn't change. The line-to-ground voltage in the unfaulted phases goes to full line-to-line voltage. The fault current in the faulted phase is determined by the resistance in the grounding resistor.

With HRG, there can be no line-to-neutral loads. Everything must be line-to-line. The loads will continue to see line-to-line voltages. A high-resistance grounded wye system behaves similarly to an ungrounded delta system, but eliminates the worst of the transient overvoltages that were such a problem with the old delta systems.



David Castor

http://www.cvoes.com

 

[magoo2]

A high resistance grounded system behaves pretty much like an ungrounded system. Instead of treating it like an ungrounded wye, just draw it as a delta system. Now put a ground on one of the corners. The phase to ground voltage is zero at that point and the other 2 phase to ground voltages are sides of the delta or phase to phase voltage.

You'll typically get zero fault current for the faulted phase and charging current for the other 2 phases.

 

[jy121]

@dpc, Sorry about the confusion... I tried to be too smart with my question by trying to distinguish between line and phase voltages..
So my understanding is this: during a L-G fault in an HRG system, the load is obviously still connected line-line, the line-line voltages remain at nominal, and rated current is delivered to the load... however, in the faulted phase, fault current also flows, the magnitude of which is determined by the size of the neutral resistor. Line to ground voltages in the unfaulted phases are now 1.73 times their pre-fault values, and the line to ground voltage of the faulted phase is now 0. Is this correct?
Thanks to all for the help in understanding.

 

[dpc]

Sounds about right to me. The fault current is typically less than 5 amps.

A great reference is "Industrial Power System Grounding Design Handbook" by Dunki-Jacobs, et al.

David Castor

http://www.cvoes.com

 

[jy121]

Thanks to all...

 

[waross]

With a ground on "A" phase the line to line and line to wye point voltages remain the same. Line to ground voltages on "B" and "C" phases rise to line to line voltage and the wye point voltage rises to 1.73 of line to line voltage.
The charging current on "A" phase drops to zero and the charging currents on "B" and "C" phases is now based on line to line voltage rather than line to wye point voltage. There will now be a charging current on the neutral based on 1.73 time line to line voltage.
The fault current will be based on the NGR impedance and line to wye point voltage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jy121]

@waross, Can you further explain the following part of your response: "the wye point voltage rises to 1.73 of line to line voltage." So for a 600V 3p, 3 wire system, the wye point voltage would be 600*1.73 = 1038V? Is this what you are saying?

 

[waross]

Sorry, should be "the wye point voltage rises to line to line voltage / 1.73."
347 Volts on a 600 Volt system.
Basically, if "A" phase is grounded, the voltages relative to "A" phase and now ground also stay the same. Of course the exception is "A" phase which is now zero volts to ground.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jy121]

@Waross... Thanks for the clarification...