Ye Huisheng

The effect of thermal aging on surface structure and mechanical strength of oil-impregnated pressboard

Oil-impregnated paper is generally used as insulation material in power transformer, which consists of transformer oil and insulating paper (pressboard). And the pressboard made of cellulose is one of important components in transformer insulating system. Oil-impregnated pressboard stays in the environment with high temperature during normal operation of transformer. It is generally recognized that the hot-spot temperature may be the chief factor determining the lifespan of transformer. Furthermore, the inner temperature of transformer will also influence both electrical and mechanical performances of oil- impregnated pressboard. In this paper, the micro surface appearance and mechanical strength of insulating pressboard under different aging time are studied. The sample preparation includes the following steps: vacuum drying for pressboard and transformer oil, impregnating pressboard with transformer oil, and taking oil-impregnated pressboard into vacuum drying oven for thermal aging. When the aging time of oil-immersed pressboard reaches 100h, 200h, 400h and 800h, the pressboard will be taken out of the oven as specimens respectively. After all the specimens under different aging time are obtained, the micro surface appearance of them was observed by scanning electron microscope (SEM) and confocal laser scanning microscope (CLSM). Then the tensile strength of them was tested by tensile strength tester. The results indicate that long cellulose chain in the surface of pressboard is broken into the shorter one or shed directly with the aging time becoming longer, meanwhile the mechanical strength of pressboard becomes weaker.

Severity diagnosis and assessment on defects in GIS based on partial discharge detection

In complex high voltage components local insulation defects can cause partial discharges (PD), especially in highly stressed gas insulated switchgear (GIS) these PD affected defects can lead to major blackouts. PD detection is recognized as one of the most effective and most important methods of insulation condition assessment in GIS. In this paper, we designed a scheme to diagnose and assess the severity levels of the PD provoked by defects in GIS. With the application of gradually increased voltage, long-term tests were conducted on a well-established 252kV GIS experiment platform to observe the entire evolution process of PD from its very initiation till the eventual flashover. Ream-time measurement was undertaken during the tests to capture the trend curve of as a result of test time, including the scatter plot, histogram, grey-scale map, etc. The results indicate that PD initiated by defects in GIS can be classified into three severity levels, namely, petty discharge, medium discharge, and threatening discharge. Moreover, on the basis of the features of phase distribution and the corresponding spectra, a procedure based on k-means cluster analysis are proposed to diagnosis and assess severity of PD automatically.

The application of sweep frequency impedance on the diagnosis of short circuit faults within transformer winding

As a central device of a power system, the safe running of a power transformer is considered to be significance. Through the statistics of onsite faults, the amount of the transformer winding deformation caused by short-circuit faults within transformer winding is maximum, therefore the detecting on the short circuit faults is important to improve the reliability of power transformers in service. In this paper, a data acquisition system has been developed based on a signal source and a power amplifier, and the short circuit faults are simulated by using a three-phase transformer in lab, as well as the sweep frequency impedance (SFI) curves are extracted and analyzed to find out the inner relevancy. The result indicates that the resonance frequencies and their amplitudes of the sweep frequency response curves vary as the degree and type of short circuit faults differ, and by analyzing the relationship between each curve, some preliminary conclusions can be drawn here. This investigation has some degree of guiding significance in predicting structural faults of windings in the transformers and is the basis for further experimental studies.