Advantages of high voltage DC dielectric tests compared with AC tests

Electricite de France (EDF) generators undergo dielectric tests at acceptance or after repair. One of these tests is an overvoltage test following the IEC 34-1 standard. EDF specifications leave manufacturers free to choose either AC or DC voltage to perform this test. Experience acquired in this field seems to confirm that tests under high voltage DC give more information than AC on the hidden weaknesses of the winding. The use of DC limits the consequences of test on insulation aging or of a possible dielectric breakdown while voltage is applied. The advantages and drawbacks of both DC and AC are discussed. Various cases dealing with operating generators are described.