Study On Propagation Characteristics And Wavelet-Packet-Based Denoising Of Ultra-High-Frequency Signals Of Partial Discharges In Transformers

Insulation deterioration, which is mainly caused by partial discharge (PD) occurring inside power transformers, is one of the prime reasons to cause transformer faults. In order to prevent accident faults and insure stable performance of power system, it is valuable to judge accurately the condition of transformers through on-line monitoring PD activities of transformers.Ultra-high-frequency (UHF) monitoring approach is focused on recent years because of its effectiveness to avoid low-frequency noises. However, there are still unsolved problems to obstacle the on-site application of UHF on-line monitoring system for PD in transformers. In this paper, UHF on-line monitoring approach for PD in transformers is studied on the basis of concluding and analyzing the research situation of on-line monitoring for PD activities. This paper concentrates on three aspects: the optimized design of UHF sensor (UHF antenna), interference suppression of UHF signal and the propagation characteristics of UHF signal, all of which are shown below.â‘ The basic principles of Two-wire Archimedian spiral antenna is introduced and the optimization and design approaches of Two-wire Archimedian spiral antenna are presented for UHF on-line monitoring for PD in transformers based on receiving principle receiving principle of sensor and fractal and antenna magnetic theories. The performance of Two-wire Archimedian spiral antenna is discussed. Two-wire Archimedian spiral antenn is designed and its standing wave ratio is tested according to the characteristics of partial discharge detection and actual structure of transformer.â‘¡An effective method for analysising the characteristics of partial discharge UHF signal is present and a device for placeding oil-paper barrier is designed, UHF signals of three oil defects are extracted with or without oil-paper barrier respectively, and whose waveform, average amplitude of vibration, and power spectrum are analyzed and compared.â‘¢An improved wavelet packet denoising method is presented to suppress the white noise mixed within the UHF signal generated by PD activities. According to the principle of the sum of wavelet packet coefficients energy is maximum in frequency band of sensor, the method for seeking optimal base wavelet is present. The method is applied to denoise UHF signals generated by four types of classic artificial insulation defects.