Running 4 Generator Without NGR

Dear Anonymous Poster,

Even when all the four NGRs are available, it is general practice to ground only one generator neutral. There should not be any problem, even if you ground only one generator neutral (normally, the neutral of the largest capacity generator), with all other neutrals kept unearthed. Still, the system can be said to be resistance grounded.

And, I am surprised how residual ground over current protection can be a replacement for failed NGRs. The objectives are entirely different.

If you are still worried, you may opt for bus earthing, with a zig-zag earthing transformer, keeping all the generator neutrals floating.

Earthing via NGR or solid earthing of 3 phase alternator is required:

1. To set reference for Phase Voltage with respect to earth.

2. For safety of Operators and other equipment. Safety of Human - In absence of ngr or solid earth, if any phase of machine develops earth fault - the entire frame of machine shall be at phase voltage. Which can be dangerous if operator touches the frame. Hence in your case NGR is required when the DG Set is running in Island mode.

Once the DG is parallel with other DG Set which has its neutral earthed - above risk is not there.

Safety of equipment -

1. Neutral Voltage will be at phase voltage due to resistance of NGR in earth fault circuit if any one phase develops earth fault - without NGR or Solid earthing - circulating current is Zero in earth circuit and E/F relay will not operate and isolate the fault. In this case because of the earth is at voltage of faulty phase - other phases are subjected to phase to phase voltage and insulation of old machine can fail to earth at weak point - Resulting in phase to phase fault.

2. Since probability of Phase to Earth Fault is many more that Phase to Phase, to avoid damage to equipment at location of earth fault the fault energy is limited to low value but sufficient to be sensed by a protection relay.

Hence NGR is must for all DG Sets for safe running of DG Set before its breaker is closed (synchronized) on 13.8kV system having neutral earth system.

Once the DG CB is closed as above - require only one of several DG to have NGR.

Regarding multiple NGRs (as in your system), please note that if with one NGR Earth Fault Current is I, it will be 4 x I with 4 DG Sets in parallel, each having its neutral earthed through NGR. IN SUCH SYSTEM IT WILL BE DIFFICULT TO ACHIEVE PROPER DISCRIMINATION BETWEEN VARIOUS CBs IN EARTH FAULT CIRCUIT UNLESS THE RELAY SETTINGS ARE REVISED WITH CHANGE IN NUMBER OF DG SETS ON LINE.

Summary:

1. Have NGR in all neutrals for safety before their respective CBs are closed on system having at least one NGR.

2. Have only one NGR closed for multiple sets synchronized on a Bus.

3. Value of Resistance of NGR of all DG Sets (or source) shall be same so that earth fault current does not vary to ensure simple settings of protection relays and discrimination.

Nicely explained.Thanks for the value addition....

Thanks.....