Generator capacitance and neutral earthing resistor

[veritas]

I saw in a text on the subject recently that the neutral earthing resistor (NER) on the secondary side of the neutral earthing transformer (in the neutral of a generator)reflected to the primary should be around the same magnitude as the generator capacitance to earth.

In my application, the NET = 20kV to 415V, NER = 0.9ohms leading to an "effective" primary impedance of 2.1kohm.

My question is

1. why does the "effective" R have to match the capacitive impedance?

2. What exactly is meant by the generator capacitive impedance? Is it the value of Xc to ground per phase?

Thanks.

 

[Parchie]

Lemme try:
[ol 1]
[li]The idea is to make sure that the power dissipated in the effective resistance should be equal to or greater than the three-phase zero-sequence reactive voltampere loss in the zero-sequence capacitance of the generator system. The practice is to choose an effective resistance equal to 1/3 of the system capacitance to ground! (because the capacitive charging current of each phase adds up in the neutral, hence I_N = 3 x I_c)[/li]
[li]You have to consider the whole system capacitance to ground: generator winding capacitance to ground; generator connections capacitance; generator output transformer capacitance to ground; transformer cable capacitance to ground; surge arresters capacitance; other elements connected in the system[/li]
[/ol]
Hope that helps

 

[veritas]

Parchie

I think you have switched the light on for me but there some way to go yet as there are still many things unclear to me.

1. Is what you are saying that the resistance should be able to handle the standing current to ground which is capacitive? but this is more to do with the resistor rating than its ohmic value?

2. If this be the case then do we not have a series R-Xc circuit then where R = effective resistance and Xc = total capacitive reactance? I agree that In = 3 x Ic where Ic is the capacitive current per phase. These impedances are 90degrees out of phase so introducing R should decrease IN (from the case where the generator is solidly earthed).

3. A comment though is that I do not see this as zero sequence current as the current per phase to ground is (theoretically) equal in magnitude but 120degrees apart. Remember the currents are source from the generator itself which is three-phase (there is no zero sequence source).

4. Also the resistance is intend to limit the EF current to < 10A, so surely this also has a bearing on its ohmic value?

Thanks.