Puncture risks when testing large synchronous machines with direct voltage

The electrostatic field distribution is calculated for the insulation of the coil ends of a winding undergoing a direct-voltage test. It is found that a relatively high voltage can occur across the insulation of the coil ends if these are covered with a layer of dust or moisture, and a puncture may occur. By considering the effect of such a puncture upon the current in the test circuit, it is demonstrated that a puncture at some distance from the stator core may be left unnoticed. Such punctures have been discovered in coils subjected to field tests with direct voltage.