HIGH VOLTAGE TESTS ON INTERNAL INSULATION

In a HV test field, the HV components of test systems are designed with an external indoor insulation usually. When internal insulation shall be tested, the test voltage must be connected to the internal part of the apparatus to be tested. This is usually done by bushings which have an external insulation. For reasons of the insulation co-ordination or of the atmospheric corrections, cases will arise that the HV withstand test level of the internal insulation exceeds that of the external insulation (bushing). Then the withstand level of the bushing must be enhanced to permit application of the required test voltages for the internal insulation. Usually special ‘‘test bushings’’ of higher withstand level, which replace the ‘‘service bushings’’ during the test, are applied. A further possibility is the immersion of the external insulation in liquids or compressed gases (e.g., SF₆) during the test.

In rare cases, when the test voltage level of the external insulation exceeds that of the internal insulation a test at the complete apparatus can only be performed when the internal insulation is designed according to the withstand levels of the external insulation. If this cannot be done, then the apparatus should be tested at the internal test voltage level, and the external insulation should be tested separately using a dummy.

Internal insulation is influenced by the ambient temperature of the test field, but usually not by pressure or humidity of the ambient air. Therefore, the only requirement is the temperature equilibrium of the test object with its surrounding when the HV test starts.