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Neutral Grounding Resistor Dimensioning

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Marvie

Electrical
Dec 15, 2004
14
How do I calculate the neutral grounding resistor value for a 3.2 MW 690V synchronous generator? What is the best way and what applicable International Standards are there?
 
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E/I = R
(690/√3)Volts / ? Amps = R
Ohms Law

Bill
--------------------
"Why not the best?"
Jimmy Carter
Thanks for the heads up xnuke.
 
Use L-N voltage to calculate the resistance required to give you the desired maximum single-phase fault current, not L-L. make sure you choose a maximum current higher than the charging current.

See IEEE Standard 32. Since this is a low voltage machine, IEEE Standards 142 and 3003.2 may apply.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
 
Hi Marvie,
I'd recommend a 5A (continuous rated) 400V NGR for a 4MVA Generator.
GG



"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 
The theoretical answer is to make the resistance equal to the capacitive reactance of the system. This gives a reasonable time constant and limits maximum voltages during ground faults to manageable levels. Trying to compute the capacitive reactance can be difficult. 3-5 amps is a reasonable guess. This assume high resistance grounding. Low resistance grounding is not recommended at 690 V.
 
Systems versus generators.
Low resistance grounding of 600 Volt systems is quite common in Canada and in some instances is required by the electrical code.
However for a generator, high resistance grounding gives much better protection for the generator.
It limits internal fault current and arcing in the event of a ground fault in the generator windings. That may avoid unrepairable iron core damage in the event of an internal fault in the generator or in the GSU.


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

I'm curious about the concept of low resistance grounding at low voltages. I have never seen low resistance grounding of system neutral at 480 or 600 V. I believe this is generally not a safe practice unless ground fault protection is provided for every feeder and branch circuit. Solidly grounding is fine and high resistance grounding is also common. The NEC allows high resistance grounding from 480 V to 1000 V but not low resistance grounding. What are applications for low resistance grounding at 600 V in Canada?

Cheers,

Dave
 
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