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Generator Neutral grounding system
Engineering and workplace issues. topic
Posted by George on 24 October, 2006 - 7:04 pm
Does anyone knows which are the differences between using a generator neutral grounding resistor (NGR) instead of using a generator neutral grounding transformer (NGT) with a resistor in the secondary winding?


1 out of 1 members thought this post was helpful...
Posted by Phil Corso on 31 October, 2006 - 8:56 pm
Responding to George's Oct 24, 7:04pm query... of course, both are used but the NGT method has a unique advantage:

NGR uses a limited-duty high-voltage resistor. This precludes extended operation under fault-conditions. Typically, then, immediate tripping of the generator is necessary! Conversely, NGT uses a continuous-duty distribution-type transformer whose secondary (eg,120 Volts) is connected to a continuous-duty low-voltage resistor. This, then, extends
fault-condition operation until a safe and orderly shutdown can be executed!

Please understand that this discussion omits other design considerations (eg, over-voltage.) If additional information or details are required,
let me know!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[tal-2@webtv.net] (Cepsicon@aol.com)


Posted by Anonymous on 3 November, 2006 - 1:52 am
How can I choose between these two solutions? Both of them leave only app. 15 A fault current.


1 out of 1 members thought this post was helpful...
Posted by Phil Corso, PE on 5 November, 2006 - 5:19 pm
Responding to Anonymous' Nov 3, 1:52am question... first step, determine if overvoltage due to system capacitance is a problem!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[tal-2@webtv.net] (Cepsicon@aol.com)


Posted by AKM on 2 December, 2006 - 2:08 pm
I require my 415 Volt LT system to be restricte to 750 milli amps earth fault current.Wheather NGR or NGT is suitable,effect of restricted grounding wheather wll raise the neutral potential and how much.The insulation level of existing transformer.O/C ,ground fault relay do need to be further checked.

regards
AKM


Posted by Anup Mahajan Sr. Engineer on 11 May, 2010 - 2:23 pm
Dear Georg,

I have only one query. why the primary side of NGT is always grounded.
Please reply me

Regards


Posted by Abhay on 25 November, 2006 - 2:08 pm
There are two major basic methods used within the industry to connect generators to the power system. They are

1. Direct Connected system - The generator is connected to its load bus without going through a voltage transformation. The gen supplies power directly to the load. Basically this config is for small process industries where the power is utilised locally. This is also called as the low resistance grounding where the fault current is limited to 100-400 Amps.

2. Unit connected system - The generator is connected to the power system through a dedicated step-up transformer (wye-delta) connected to the gen terminals. Most large generator sets are connected to the power system in this manner. This is also called as the high resistance grounding where the fault current is limited in the range of 5-10 Amps typically.
Till here the referene used is Ch1 of IEEE tutorial on the protection of synch gens.

Now your question - using a generator neutral grounding resistor (NGR) instead of using a generator neutral grounding transformer (NGT) with a resistor in the secondary winding.

Using NGR at "unit connected" configuration will only cause a higher value of fault current (100-400Amp depending upon resistance value) to flow through the stator core (hence causing more damage to the core) before the protection system takes over. In case a NGT with a secondary loading resistor would have limited the fault current through the generator neutral to a mere 5-10 Amps which is not detrimental to the stator core.

However using a NGT at a "direct connected" configuration in place of a NGR could create nuissance by allowing spurious triping of the generator.

Hope that the argument fits the forum and takes your question.


Posted by anand on 23 June, 2009 - 1:27 pm
When we are using medium resister grounding NGR in direct connected load, then also 100 or 200A current is flowing through stator core. In this case stator core will also get damaged.

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