Field experiences using a prototype open core resonating transformers for A.C. high potential testing of hydro-generator stators
When generator stators are rewound or undergo major repairs, there is often a requirement to perform a high potential test. These tests can be completed using d.c., very low frequency or power frequency a.c. test voltages. A.c. power frequency tests are becoming common, but such tests are difficult because of the large kVAr requirement to charge stator insulation capacitance. This is particularly a problem with hydro generator stators because of the physical size of the generator. In many cases, the reactive power and hence the supply current cannot be provided at a local distribution board available within a power station. Resonant circuits are normally employed to minimise the distribution board loadings. In traditional a.c. high potential test sets, large variacs are employed with a high voltage transformer and multiple compensating coils. The U of C has tested many hydro generators in New Zealand. In this paper an open core inductor and transformer is described that can provide high voltage and inductive compensation in an original and most compact manner. In one case the required reactive power was supplied by an open core resonant inductor for testing a 55.5 MVA, 11 kV rewedged hydro-generator. In a second case, an open core high voltage resonant transformer (which functions as both a high voltage transformer and compensating inductor) was used to test a 120 MVA, 15.4 kV rewedged hydro-generator. In a third case, the authors were required to test a stator with just over 1 [micro]F in capacitance at a voltage of 32 kV. This required a single-phase controllable high voltage source rated at 286 kVAr prior to considering losses. Under such a situation, the large reactive power demand was fulfilled by employing both the open core high voltage resonant transformer as well as the open core inductor. The U of C open core high voltage testing kit not only performed well but also withstood severe electric disturbances and flashovers when multiple stator failures were experienced.