Short time rated and protected high voltage ac testing of generator stators using parallel resonant circuits
Power station generators require short duration HV ac testing of their insulation as part of their acceptance before being put into service. Depending on the value of the capacitance of the insulation, the rating of the test power supply can be large. One method of reducing this supply requirement is to compensate the capacitance with inductance. The paper describes a parallel resonant compensation test method which initially uses LV inductances to compensate 0.422μF of stator capacitance at 23kV. This equates to 70kvar. In a development of testing equipment, this is replaced by a HV inductance that supplied 115kvar of reactive power compensation at 23kV. As a further development, the inductor was turned into a resonant transformer by the addition of a LV primary. The magnetising reactance was matched to the generator stator insulation capacitance. A laboratory test showed that the required HV of 23kV could be obtained from energising the primary at 285V at 60A or at a rating of about 1/7th the load. A further resonant transformer was then designed for a 334kvar capacitor load to test generator stators at 31.5kV. This transformer was supplied from a nominal 400V supply and gave a gain in kVA of 16. The transformer has a finished weight of approximately 300kg. As a final test, both units were used in parallel to supply and compensate 1.06μF of stator capacitance at 32kV.