NGR Sizing for Three Parallel Generators

NGR sizing depends on what % of winding you want to protect through differential protection and the remainning by REF,

Hi,

First of all the system protction is need to be performed for your three Generators that are going to be operated in parellel condition. To do so, the basic short circuit current calculations should be performed, then only we can select of the type of NGR to be used and CT ratio.

You have to provide the existing soil resitivity, cable size & type, cable race-way and complete electrical data for all appliences.

And further, the most important is your power reqirement. Total capacity of your generators (6.6 KV) and type of power utalization.

SK Engineer India

Dear Sir,

If I am not mistake you ask about the NGR sizing.

Used this formula: I ground fault = {V/(Z1 + Z2 + Zo + 3R)}

in which Z1 is a positive sequence, Z2 is negative sequence and Zo is Zero sequence impedances. You have to get those data from your generator manufacturer, R is NGR sizing. Assumed fault located at generator feeder.

In common practice we have to decide the ground current current first, let say 40 Amps for 10 seconds (It is implemented on 6.6 kV, 1.8 MVA diesel generator).

For a certain condition the generator impedance is too small compared to R, and the calculation result is only slightly different. So, we can conduct simple calculation by eliminating Z1, Z2 and Zo for estimation purpose.

R = [Vln/I]. Vln = voltage line to neutral = 3.81 kV. I = 40 Amps

R = 3.81kV/40A = 95.25 Ohm.

Note: If three generators are operated in parallel, each generator will contribute 1/3 of groundfault current. So, you have to set generator protection refer this value (lower), if higher than this value, your generator protection cannot see the groundfault.

Where will you put the CT?

You can select the CT ratio 50/1 located between common ground busbar and generator star point (for better coordination).

Also you have to select the relay range setting consider the generator running paralel or stand alone.

If you put the CT between common busbar and NGR you cannot make a good coodination. CT located at this location cannot recognize which the generator fault to ground.

We can implement both locations if it is justified.

You can contact the generator and NGR manufacturer for more comprehensive calculation.

Ok so a guy has three generators all three are different wattages say between 1000 and 1500 watts and 110 to 120 volts.

The cycle is not even the same b/c the rpm of the generators hasn't been set, which is a easy thing to due with two light bulbs in line plugged into the generator and a household outlet.

Anyway if a resistance load device, such as a heating element equivalent or a little less to the generators output is used in line with the three connected to a common bus bar what's the problem??

You might have one heating element slightly warmer then the others.

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