Characteristics of Insulation Diagnosis and Failure in Gas Turbine Generator Stator Windings

Abstract

-In order to evaluate the insulation deterioration in the stator windings of five gas turbine generators(137 MVA, 13.8 kV) which has been operated for more than 13 years, diagnostic test and AC dielectric breakdown test were performed at phases A, B and C. These tests 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 No. 1 generator stator windings showed that they were in good condition but PD magnitude indicated marginally serviceable and bad level to the insulation condition. Overall analysis of the results suggested that the generator stator windings were indicated serious insulation deterioration and patterns of the PD in all three phases were analyzed to be internal, slot and spark discharges. 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. The breakdown voltage at phases A, B and C of No. 1 generator stator windings failed at 28.0 kV, 17.9 kV, and 21.3 kV, respectively. The breakdown voltage was lower than that expected for good-quality windings (28.6 kV) in a 13.8kV class generator. In the AC dielectric breakdown and diagnostic tests, there was a strong correlation between the breakdown voltage and the voltage at which charging current increases abruptly (P i1 , P i2 ).

Analysis of Correlationship between Partial Discharge and Ozone Emission in High Voltage Motor Stator Windings as the Index of the Insulation Diagnosis

“…Many electrical diagnostic tests have been performed to evaluate the health of insulation systems for rotating machine stator windings. In general, Insulation Resistance(IR), Polarization Index(PI), Dissipation Factor(DF), AC current test and PD test have been conducted and analyzed to assess the soundness of insulation systems as had been shown (Kim, H. D. (2014), Kim, H. D., Kong, T. S., Ju, Y.H., & Kim, B. H. (2011), Kim, H. D., & Kong, T. S. (2012), Kim, H. D., & Kong, T. S. (2014)). Also, it is well known that the O3 is produced when the electric discharge occurs in air and studies are still going on the correlations between PD and O3 emission as works in this topic (Lépine, L., Lessard-Déziel, D., Bélec, M., Guddemi, C., & Nguyen, D.N.…”

Analysis of Off-Line and On-Line Partial Discharge in High Voltage Motor Stator Windings

“…Therefore, inspection and insulation diagnostic tests are regularly performed to estimate the degree of deterioration in order to prevent insulation breakdown in large-capacity HV motor stator windings. An insulation diagnostic test is provided during the preventive maintenance period for an overall evaluation of the insulation condition, wherein insulation resistance, polarization index, alternating current, dissipation factor, and partial discharge (PD) magnitude are measured [2,3]. Furthermore, the detailed analyses of the AC current, dissipation factor, and PD magnitude can verify the condition of infiltration of foreign objects [4,5].…”