Intermittent Line-to-Ground Faults in Generator Stator Windings and Consequences on Neutral Grounding

Line-to-ground faults in the stator windings of unit generators have been simulated on the basis of test results obtained on medium-voltage cables in systems with isolated neutral. Those tests have shown that single-phase faults in insulating materials, such as polyethylene or comparable synthetics, are normally arcing faults and the arcing channel in the insulating material behaves like a restriking switch which is able to interrupt transient fault currents at current zero; however, it restrikes at a certain voltage level due to damaged insulation and small distances between the phase conductor and ground. Similar behavior may be assumed for generator stator windings. The simulations show that for such a fault pattern, resonant grounding is distinctly to be preferred to high resistance grounding. The corresponding standards do not reflect transient and intermittent fault currents in stator windings and, therefore, assess both grounding methods as nearly equivalent.