Sizing Generator Neutral Conductor

[duki]

We have 4-13MW, 13.8kV generators that are cabled to a 13.8kV switchgear. Each generator has its own breaker. The generators are grounded via a resistor rated at 200a for 10s. The problem comes in sizing the cable from the star point at the generator to the resistor located outside the substation. Since we have the 200A resistor, I says we only have to size the conductor for 200A. Another engineer is saying we should size it for 200A then double it for a 3rd harmonic current. The resistor manufacturer is running 4/0 awg from the resistor to ground. Any opinions??

 

[dpc]

If you size the conductor for 200 A you should be fine. The resistor will burn up long before the conductor if excessive current is present. The fundamental and the harmonic currents add as the square root of the sum of the squares when considering the rms heating effect, so it takes a fair amount of third harmonic current to make a lot of difference under fault conditions.

BUT....if you are expecting any appreciable 3rd harmonic current, you really need to evaluate the resistor's continuous rating. Typically, a 10-sec rated resistor has no specified continuous rating. Of course, it does have some continuous capability, but it is not stated or guaranteed. You need to satisfy yourself that the resistor is going to be able to handle the expected continuous third harmonic current. Most conservative approach is to specify appropriate continuous rating for the resistor.