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The generator manufacturer says my generator will only put out so much SLG fault current (8.3kA) but PTW calculates a much higher SLG current, why?

Category: Studies-Short Circuit-Generator Contribution

Answer: PTW is correct. The fault duty does change for the generator as more sources are switched in. The generator vendor is saying that all by itself the generator puts out 8.3kA. If the induction motors, other generator, or utility are switched in - the Thevenin positive/negative/zero sequence impedances are reduced at any given bus in the system. including the generator terminals. And the duty goes up. This affect is negated when we add neutral ground resistors. Why? Because the resistor impedances are much much larger than any other impedances in the system.

Notes: If you simplify your system down to just the Generator as a souce (scheduling it as a swing bus), you’ll see the SLG fault current limited to 8.3kA.