Significance of Defects Inside In-Service Aged Winding Insulations

This study aims to gain insight into the consequences of local defects in stator winding insulation; therefore, optical (microscopic) inspections and voltage-endurance tests have been carried out on in-service aged generator bars. In this context, generator bars with mica-bitumen insulations and generator bars with mica-epoxy insulations were investigated. The mica-bitumen insulations had been in service for 43 years in a 12 kV hydro-generator. The generator bars with mica-epoxy insulation had been dismantled from two identically constructed 10.5-kV generators after 35 and 36 years of service. The time to breakdown values obtained from the voltage-endurance tests were evaluated using a Weibull distribution. Apart from the scale parameter, Weibull exponent of this distribution allowed for the identification of early failures. The voltage-endurance tests carried out on replacement bars for these generators did not yield any early failures; such failures, however, were detected on both types of the in-service aged insulations. In particular, these occurred at high levels of electric stress and at delaminations between the copper conductor and the groundwall insulation. It is concluded that the interpretation of the measured time to breakdown values and their scatter depends on the level of electric stress.

[1]  G. Stone,et al.  Problems with modern air-cooled generator stator winding insulation , 2000 .

[2]  K. Shibayama,et al.  Development of new mica paper epoxy insulation systems for high voltage rotating machines , 1973 .

[3]  Robert J. Ross,et al.  Analysis of in-service aged stator bars , 1995 .

[4]  W. McDermid,et al.  Insulation systems and monitoring for stator windings of large rotating machines , 1993, IEEE Electrical Insulation Magazine.

[5]  Curt Brinkmann,et al.  Die Isolierstoffe der Elektrotechnik , 1975 .

[6]  L. Simoni,et al.  A General Approach to the Endurance of Electrical Insulation under Temperature and Voltage , 1981, IEEE Transactions on Electrical Insulation.

[7]  B. Fruth,et al.  Detection of electrical tree propagation in generator bar insulations by partial discharge measurements , 2003, Proceedings of the 7th International Conference on Properties and Applications of Dielectric Materials (Cat. No.03CH37417).

[8]  Y. Inoue,et al.  Statistical Analysis on the Electrical Failure Properties of the Form-Wound Epoxy Micaceous Insulation Systems for Rotating Machines , 1977, IEEE Transactions on Electrical Insulation.

[9]  W. McDermid.  Damage resulting from long term slot discharge activity in a hydrogen environment , 1990, IEEE International Symposium on Electrical Insulation.

[10]  J. Forster,et al.  IEEE Working Group Report of Problems with Hydrogenerator Thermoset Stator Windings Part II-Detection, Correction and Prevention , 1981, IEEE Transactions on Power Apparatus and Systems.

[11]  H. Yoshida,et al.  Rotating Machine Insulation , 1986, IEEE Transactions on Electrical Insulation.

[12]  Y. Kako,et al.  An Analysis of Combined Stress Degradation of Rotating Machine Insulation , 1983, IEEE Transactions on Electrical Insulation.

[13]  J. R. Michalec,et al.  Estimating the remaining service life of asphalt-mica stator insulation , 1994 .

[14]  G. Stone,et al.  Electrical Insulation for Rotating Machines , 2003 .

[16]  Ruben Vogelsang,et al.  Time to breakdown of high voltage winding insulations with respect to microscopic properties and manufaturing qualities , 2004 .

[17]  George M. Wolff,et al.  A New Look at Reliability of Asphalt-Mica Insulation in Large Conventionally Cooled Turbine Generators , 1970 .

[18]  A. Kelen,et al.  Evidence and presumption in PD diagnostics , 1995 .

[19]  W. McDermid,et al.  Significance of defects found during high direct-voltage ramp tests , 1999, Proceedings: Electrical Insulation Conference and Electrical Manufacturing and Coil Winding Conference (Cat. No.99CH37035).

[20]  Y. Inoue,et al.  Thermal Cyclic Degradation of Coil Insulation for Rotating Machines , 1983, IEEE Transactions on Power Apparatus and Systems.

[21]  M. B. Srinivas,et al.  Multifactor aging of HV generator stator insulation including mechanical vibrations , 1992 .

[22]  A. Futakawa,et al.  Interaction Mechanism between Conductor and Ground Insulation of Stator Windings , 1983, IEEE Transactions on Electrical Insulation.

[23]  H. Yoshida,et al.  Influence of Mica Tapr Application on Insulation Characteristice of High Voltage Rotating Machinery Coils , 1985, IEEE Transactions on Electrical Insulation.

[24]  Thomas W. Dakin,et al.  Electrical Insulation Deterioration Treated as a Chemical Rate Phenomenon , 1948, Transactions of the American Institute of Electrical Engineers.