Prophylactic measurements on insulating system of high voltage measuring transformers are of great importance for reliable operation of electrical utilities and for safety of the personnel. These measurements are preferably made on site. For this reason, the High Voltage Laboratory of the Faculty of Electrical Engineering in Skopje, Macedonia, a few years ago began systematic on site measurements on the measuring transformers for voltage level of 110 kV. They are with paper-oil insulation. The following two measuring methods were applied: 1. Power Loss Factor (tg/spl delta/) measurements of the entire insulating system between the high voltage winding and ground; 2. Insulation Resistance measurements between the high voltage winding and ground. Both measuring methods have difficulties for on site application. The main reason is the very small capacitance of the measured objects and the presence of strong electrostatic interference from the high voltage conductors in the vicinity of measuring arrangement. This problem is solved in both measuring methods. The measuring technique used is fully described. The results obtained with both methods are compared with each other and good correlation is found. The results are very useful for discussing criteria for insulation evaluation.
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