Electrical motor insulation conditions dc testing

Abstract

This paper presents one of the maintenance test methods and the results of its application in the determination of the insulation condition of HV electric motors. Testing and data processing were carried out on site using a PC. In our tests the concept of the Hofler method is used. Highly stabilized dc voltage as a series of voltage steps at 0.3 U/sub n/, 0.6 U/sub n/, 0.9 U/sub n/, 1.2 U/sub n/, and 1.5 U/sub n/ is applied to the motor windings, while at the same time the leakage currents are being measured. In the case of 'ideal insulation', leakage currents are proportional to the applied dc voltage, i.e. the insulation resistance is independent of the applied voltage. In reality, insulation resistance and leakage currents depend on the applied voltage. The ways the leakage current depends on voltage and its magnitude can be used to determine the insulation condition, based on the nonlinearity. In the last five years over 100 on-site tests have been performed on motors with nominal voltage >6 kV and power ratings to 7 MW. The selected test voltage is 1.5 U/sub n/ with the consideration that testing should be virtually non-destructive and still provide a reliable assessment of the insulation condition. The test voltage may not exceed 1.5 U/sub n/, and the test shall be terminated at any time in case of sudden increase in leakage current indicative of a serious risk of breakdown. Depending on the nonlinearity at a particular dc voltage, the insulation condition is assessed. Motors, /spl sim/4%, with nonlinearity coefficient >2 at test-voltage 1.5 U/sub n/ (at ambient temperature) were considered to be in non-satisfactory condition and have been sent for further testing and any necessary repairs. Cracks, filled with contaminant in aged micaceous insulation of stator windings and slip ring rotor windings, have been detected. Although there is a decrease in popularity of dc testing world wide, we hope that the presented method can be an improvement of conventional dc test methods.