We have two identical generator sets. 1.5MVA, 4.16KV, 60Hz. We put 10ohm NGR connected at the neutral of each of the generators.. We haven't paralleled the generators yet. How should the NGRs be connected when the generators are already paralleled??

Also, in my other project, they only have one NGR for the two generators but they do have two disconnect switches.. Can anyone explain to me how the NGR be connected to the generator??

 

If you have two generators n parallel you want only one of them to have a grounded neutral - this provides the neutral reference point for the whole system.

So you have two disconnect switches, presumably one to each alternator neutral fr4om the NGR.

If Gen A is running on its own, Gen A neutral is grounded, Gen B neutral is disconnected.

If Gen B is running on its own, Gen B neutral is grounded, Gen A is disconnected.

If both generators are running, only one neutral is connected. It does not matter which one, but since one of the machines will be running when the other is started, close the neutral on an incoming alternator only if the other neutral contactor is open.

If the machine with the grounded neutral trips, you then need to close the open neutral contactor.

Bruce

 

Blackstoneboy: a) the NGR has no effect on synchronization of generators; b) it is used to reduce phase-to-ground fault current magnitude and eliminate system over-voltages; and, c) for the system you submitted, both NGRs can remain connected at all times.

The reason for switching of NGR circuits is maintain a zero-sequence impedance greater that of the generators' equivalent zero-sequence impedance. It should be high enough in magnitude so that ground-fault current doesn't exceed 3-phase fault-current. In your case, the resistors are large enough to insure the situation never arises, thereby eliminating the need for switching NGRs.

I suggest you search the Control List Archives to acquaint yourself with earlier topics related to the grounding of multiple gen-set systems as well as the mathematical tool called Symmetrical Components.

If additional detail is required, contact me off- or on-list.

Regards, Phil Corso (cepsicon [at] aol [dot] com)

 

Dear Phil,

Analyzing the circuit, if the two NGRs are connected when the generators are in parallel, the zero sequence impedance will be less than of that only one. Meaning the phase-ground fault will be higher if the two NGRs are connected. Please correct me if i'm wrong, so i will know what to do.

Thanks a lot Phil.
Regards,
Blackstoneboy

 

Blackstoneboy, of course the current increases, but that's not the point. A major problem is to protect the generator against a ground-fault current which will, in general, exceed its 3-ph fault current. It has nothing to do with synchronization or anything else.

I will repeat what had been presented in earlier threads. 3-ph fault current is determined solely by the generator's positive-sequence impedance, X1 as shown in the following equation:

I(3)=Ep/X1, where Ep is the generator's phase voltage. But, 1-ph fault current is limited by the sum of its positive-sequence impedance, X1, its negative-sequence impedance, X2 (about equal to X1) and its zero-sequence impedance, Xo (usually less than X1). The resultant equation is:

I(1)=(3xEp)/(X1+X2+Xo) representing a higher 1-ph fault current.

The NGR adds an additional impedance, Rn, to the sum above, substantially reducing 1-ph fault current. The resultant equation is:

I'(1)=(3xEp)/(X1+X2+Xo+3xRn) reducing not only current magnitude but changing the subsequent arc-characteristic from inductive to resistive.

In answer to your comment about doubling of fault current... if the generators are equal, then both are protected, thereby eliminating additional hardware to switch NGRs out or in!

Regards, Phil Corso

 

Blackstoneboy: a) the NGR has no effect on synchronization of generators; b) it is used to reduce phase-to-ground fault current magnitude and eliminate system over-voltages; and, c) for the system you submitted, both NGRs can remain connected at all times.

The reason for switching of NGR circuits is maintain a zero-sequence impedance greater that of the generators' equivalent zero-sequence impedance. It should be high enough in magnitude so that ground-fault current doesn't exceed 3-phase fault-current. In your case, the resistors are large enough to insure the situation never arises, thereby eliminating the need for switching NGRs.

I suggest you search the Control List Archives to acquaint yourself with earlier topics related to the grounding of multiple gen-set systems as well as the mathematical tool called Symmetrical Components.

If additional detail is required, contact me off- or on-list.

Regards, Phil Corso (cepsicon [at] aol [dot] com)
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Dear Phil,
I am learning about NGR usage because I am working on installation of 20MW gas- generator power plant.
Let me explain you about my power plant.
- we have 10 unit Gas generator (11KV, 50Hz,0.8Pf)
-we will produce the power by those gen-sets and distribute to the city.
-our gen-sets will run continuously in parallel as per load demand.

As there we had use one NGR (613Ohm, 6.36Kv L-N, 20A 10 sec) and before this NGR we have NGS ( Netural Ground Switch) 10 units (400A/1P , vacuum contactor).

- Netural point of each gen-sets will connect to respetive NGS incoming and outgoing will combine and connect to the NGR.

My question is
1)all NGS should be close all the time in parallel opertion and this is only for isolaton purpose?
2) Do we need to close only one NGS when using in parallel operation?
3) could you please guide line to me for requirement operation?

This is my very first job for MV gen-sets and I had experienced only in LV generators. So, I didn't use NGR before.

Please kindly reply to me

Best Regard,