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 Post subject: Modeling a NGR on a 2MW 4.16kV GEN in SKM PTW
PostPosted: Thu Apr 15, 2010 1:23 pm 

Joined: Thu Jan 29, 2009 2:06 am
Posts: 29
Does anyone have any experience modeling a Neutral Grounding Resistor (NGR) connected to a MV Generator?

I'm preempting and starting a discussion now, any thoughts would be greatly appreciated.

(Moderators if this is in the wrong forum please move, thanks)


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PostPosted: Thu Apr 15, 2010 5:36 pm 
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Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
If the NGR is sized and rated properly, I would expect no line to ground IE to speak of. But then, the calcs are and assume 3 phase, so does the NGR really come into play?


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PostPosted: Fri Apr 16, 2010 9:01 am 

Joined: Thu Jan 29, 2009 2:06 am
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acobb wrote:
If the NGR is sized and rated properly, I would expect no line to ground IE to speak of. But then, the calcs are and assume 3 phase, so does the NGR really come into play?


Yeah exactly since the NGR limits the ground fault current to minimal levels. I assume it will affect the SLG Available Fault Current.

Would it be appropriate to connect a resistor to the same bus as the generator but I don't think that would work since it would be putting it across the phase conductors as well. Though I haven't really looked at the characteristics of the resistor block (the block in the SKM software)


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PostPosted: Fri Apr 30, 2010 7:29 am 

Joined: Thu Jan 29, 2009 2:06 am
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Silly me, this would be covered under the neutral impedance input section of the medium voltage generator device within the program.

Would this also apply for the primary substation utility transformer NGR. As in would you incorporate this into the transformers impedance?


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PostPosted: Sat May 01, 2010 8:40 am 
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Joined: Sun Dec 23, 2007 1:44 pm
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Location: Charlotte, NC
I am not familiar with the software....expect someone else here is though.


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PostPosted: Sun Sep 19, 2010 6:53 pm 
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Joined: Fri Jan 11, 2008 4:35 pm
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Ngr

SKM's "Plant" Example Project (check it out) shows a Neutral Grounding Resistor graphically tied to a transformer symbolically, but it is obviousy not tied to the 3-phase bus. Running the study calc modules will give you a 'bus not connected' warning, but it will not affect your 3-phase calcs. When you are doing your GND TCC's, any protective relay(s) you have for the NGR will obviously need to show up somewhere in your overall coordination planning. I recently obtained the protective relay settings from a utility for their NGR on their Utility Tie XFMR and superimposed their relay setting curves on the existing curves for the downstream plant ground protective relays and found a pretty major miscoordination. The utility settings were lower (for the applicable Ignd<400A range) than those of the switchgear's group main breaker they fed and many of the plant's feeder breakers on that bus. The right ground fault on several of the plant's 5kV feeders could unfortunately trip the 115kV Utility Circuit Switcher feeding the Utility Tie Transformer's primary (we are straightening that out now)! I got off on a tangent, but that's how I've handled NGR's with SKM.


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