Dielectric Properties Analysis of Gas Turbine Synchronous Generator by Polarization and Depolarization Current Measuremen

Abstract

Insulation of stator winding in large electric rotating machines mostly consists of mica and epoxy resins with a number of junctions inside the insulation. The diagnostic test method for monitoring the insulation condition of these machines every specified period of operation is very important. The polarization and depolarization current (PDC) test method is one of the potential test techniques that can be used to evaluate the polarization process of the insulation by measuring the charging and the discharge currents of the insulation under the DC high voltage test. The PDC test experiments was done with the stator winding insulations of four identical gas turbine synchronous generators rated at 58,824 kVA 11 kV and 3,000 rpm (manufactured in 1997). From the results, it was found that increase in ambient relative humidity and operation winding temperatures increased the polarization and depolarization currents of the stator winding insulation. Besides, the in-serviced operation hour of the investigated machine had an effect on the tan δ. The tan δ increased with increase in the degree of insulating deterioration and the in-service operation hour also. Increasing of tan δ is associated with increasing of imaginary capacitance of the tested insulation. The analyzed parameters can be taken into account for evaluating the aging and deterioration conditions of the insulating material of the synchronous machine which are useful for a maintenance strategy of the rotating machines.