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474 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 23, NO. 2, JUNE 2008 Evaluation of the Degradation of Generator Stator Ground Wall Insulation Under Multistresses Aging Zhidong Jia , Member, IEEE , Xiang Peng, Zhicheng Guan , Member, IEEE , Liming Wang, and Bo Yue Abstract —The actual stator bars of an 18 kV/300 MW genera- tor were used for accelerated multistress aging tests to investigate the degradation of ground insulation. An aging system with com- bined stresses including thermal, electrical, mechanical vibration, and thermomechanical factors was used to test the aging of stator bar specimens. The nondestructive parameters including partial discharge (PD) quantities and dissipation factor parameters were measured during aging test in order to evaluate the degradation of stator ground wall insulation. At the end of the aging test, an ac breakdown test was performed on the bars in order to obtain the remaining breakdown voltage (BDV) after their nondestructive data had been measured. The changes of nondestructive parame- ters indicated that the degradation of ground wall insulation. For example, the changes of skewness of H qn ( ϕ ) indicated the shape of voids in ground insulation changes from ±at to narrow, and even exhibits treeing, which is dangerous to insulation because it short- ens the effective distance of insulation between high voltage and the ground. The results obtained are summarized as follows. The BDV data ²t a Weibull distribution. The correlation between the dissipation factor, the PD parameters, and the breakdown voltage were analyzed, among which the skewness of the average discharge had the most correlation with breakdown voltage. Index Terms —Aging, breakdown voltage (BDV), correlation, discharge, dissipation factor, generator, ground wall insulation, multi-stress, nondestructive, partial discharge (PD) quantity. I. INTRODUCTION R ELIABLE operation of a generator depends critically on the integrity of its stator winding insulation, which is aged by exposure to a combination of thermal, electrical, mechani- cal, and environmental stresses. Continuous deterioration of the stator insulation can ultimately result in failure, and therefore, utility providers have a keen interest in diagnostic measurements to assess the condition of the stator insulation in generators. There are two kinds of methods to assess the condition of stator winding insulation: the direct method and the indirect method. A direct measure of the integrity of an insulation sys- tem may be the breakdown strength under ac stress, as deter- mined through destructive testing, which damages the winding, and making the machine unserviceable. An indirect measure of an insulation system may be nondestructive diagnostic tests, after which the insulation remains serviceable. However, the Manuscript received May 21, 2007; revised May 21, 2007. This work was supported in part by the National Natural Science Foundation of China (No.
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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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