ADB9464Bd01 - IEEE Transactions on PowerApparatus and S y s...

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IEEE Transactions on Power Apparatus and System, Vol. PAS-96, no. 1, January/February 1977 ACCELERATED VOLTAGE ENDURANCE TESTING OF MICACEOUS INSULATION SYSTEMS FOR LARGE TURBOGENERATORS UNDER COMBINED STRESSES Arnold Wichmann Kraftwerk Union AG, MuelheidRuhr, West Germany ABSTRACT The electrical life of micaceous insulation for high-voltage electri- cal machines was investigated by means of accelerated voltage endur- ance tests under carefully controlled operating conditions. The aging mechanism is assumed unchanged when the insulation is subjected to dielectric field strengths up to ten times normal and test frequencies up to 4000 Hz. A test program was developed whereby the useful electrical life of a particular test specimen can be determined as influenced by temperatures up to 130°C and mechanical stresses approaching the ul- timate strength. INTRODUCTION The dielectric strength of an insulation system is considered the most important criterion governing its life expectancy. The useful life must not be appreciably reduced by any normal operating conditions, especially by exposure to the continuous electrical stress in service. Con- sequently, the basic accelerated tests are applied primarily to deter- mine the actual life expectancy of the insulation in normal operation by subjecting it to higher electrical field strength and frequency for a pre- determined much shorter period. The method selected must not result in changes to the authenticat- ed aging mechanism. Moreover, the effect of each influence should be measurable separately and in combination with others. In addition to the electrical stress (E), the IEC Guide for Evaluation of Insulation Systems,' also lists temperature (T), mechanical influence (M), and ambient influ- ence (A) as decisive factors in the aging behavior of an insulation system as defined in the TEAM-Code of that Guide. The present investigation includes the combined influence of temp- erature and mechanical stresses on the life expectancy of micaceous in- sulation of high voltage windings of large electrical machines.Theacce1- erated voltage endurance tests were carried out on various micaceous insulation systems,but mainly on a system of vacuum-impregnated epoxy-bonded mica splittings termed Micalastice, which is used ex- clusively in all large turbogenerators manufactured by Kraftwerk Union AG and Allis-Chalmers Power Systems, Inc. Fundamentals of Accelerated Electrical Aging To determine the life expectancy of electrical insulation whose ag- ing is affected by partial discharges at internal voids, hairline cracks or at the boundaries of the component materials, accelerated aging can be accomplished by intensifying the field strength, or by increasing the test frequency. This may be done separately or at the same time. Both methods result in multiplication of partial discharges per unit of time as shown in Figure 1. An internal void in which the ignition voltage is barely F 76 439-0.
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This note was uploaded on 06/08/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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ADB9464Bd01 - IEEE Transactions on PowerApparatus and S y s...

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