Study On On-line Monitoring And Condition Evaluation Of500kV CVT

The use of CVT is increasing day by day in recent years, but the changing of divider ratio due to capacitance changing and insulation aging directly affects the measuring of the system parameter and the stability of Relay Protection.’Preventive Test Electrical Equipment Order’formulates that:The capacitance and tan δ (%) of the condenser divider should be measured regular, and because of regular measurement, it’s timeliness has some limitations, on the one hand, it is difficult to find the failure defects of the equipment which is developed in the cycle, it can’t be found in time, and overhaul timely; on the other hand, the actual status is not considered adequately, leading to a lot of unnecessary maintenance, it not only takes a waste of maintenance costs, and may make damage or hidden faults to the equipment.It is inevitable for on-line monitoring to take the place of regular monitoring, the former can shorten the repair hours of the equipment, reduce cost, and prolonging the life of using, is of importance for on-line monitoring of CVT, timely predicting and diagnosing troubles, keeping equipment working safely, improving the reliability of power system, keeping the security of the equipment and field operators.The characters of on-line monitoring are that it can overcome the shortcoming of maintenance routine, monitor the insulation running state of CVT at any time, and can monitor the changing states of the capacitance and dielectric loss at the same time. It is of highly practical and economic value.The paper studied the state and trend of the technology of on-line monitoring and fault diagnosis, summarized the problems in the using of on-line monitoring devices, the means of monitoring which are often used and the problems at present are also researched. In order to eliminate the effect of the harmonics in the power grid, FIR digital filter is designed by using window function, and the effectiveness of the digital filter is tested by MATLAB. Considering the low accuracy and stability of tan δ monitoring due to the existence of the harmonics in power grid, a harmonic analysis method based windowing is raised, comparing to the traditional harmonic analysis methods, it can effectively avoid spectrum leakage and picket fence effect caused by frequency fluctuation, own higher accuracy in the normal range of frequency variation. This paper researched the principle of wavelet analysis, and then applied it to dielectric loss measurement and a check analysis is also made through MATLAB. On account of large amount of data, influencing factors and having no idea about the mechanism of insulation aging and failure occurrence in insulation on-line monitoring, the methods for diagnosis based on expert system and fuzzy set theory are researched, which provide theoretical basis for insulation diagnostic analysis.