Analysis of insulation diagnosis and failure in stator windings of air-cooled gas turbine generator

In order to evaluate the insulation deterioration in the stator windings of air-cooled gas turbine generators(119.2 MVA, 13.8 kV) which has been operating for more than 15 years, diagnostic test and AC dielectric breakdown test were performed on phases A, B and C. Diagnostic test included measurements of AC current, dissipation factor, partial discharge (PD) magnitude and capacitance. ΔI and Δtanδ in all three phases (A, B and C) of generator stator windings showed that they were in good condition but PD magnitude indicated marginally serviceable condition. After the diagnostic test, an AC overvoltage test was performed by gradually increasing the voltage applied to the generator stator windings until electrical insulation failure occurred, in order to determine the breakdown voltage. Although phase A of generator stator windings failed at breakdown voltage of 29.0 kV, phases B and C endured the 29.0 kV. The breakdown voltage in all three phases was higher than that expected for good-quality windings (28.6 kV) in a 13.8kV class generator.

I. Introduction

The stator winding of air-cooled gas turbine generators eventually breaks down after gradual insulation deterioration and partial discharge due to creation of internal voids, delamination, and contamination in the end-winding surface caused by thermal, electrical, mechanical, and environment stresses. In the case of the water-cooled generators, dielectric breakdown occurs due to vibration and corrosion of the stator winding, which are caused by leakage of cooling water and absorption of moisture. An unexpected dielectric breakdown in the stator winding insulation of large generators leads to degradation in reliability, and quick restoration is difficult; therefore, significant economic losses are incurred [1], [2].