Study On UHF Method And Sleeve Monopole Antenna For Partial Discharge In Transformer

Partial discharge (PD) is one of the important reasons causing power transformer insulation degradation and breakdown. It is meaningful to detect and diagnose partial discharge in power transformer. Based on microwave and antenna theory, electrical measurement and electromagnetic theory, the ultra-high frequency (UHF) method used in power transformer for PD detection was studied in this paper.Propagation of PD signals in power transformer was studied using electromagnetic theory. Many factors affect the propagation of UHF signals caused by PD on the waveforms and energy of signals. To investigate the relationship of factors and UHF signals, simulation model of transformer for PD study are established. UHF signals in simulation model caused by PD are calculated using finite difference time domain method. By mean of amplitude and cumulative energy plot of simulation signals, and combing the measured result, the following factors: distances between PD source and detection position, different metallic conductor position around detection position and iron core and winding are researched. These results and analysis procedures show that iron core and winding can affect UHF signals, especially the wavefront. Furthermore, signals amplitude detected by sensor can decrease far from discharge source because of refraction and reflect of UHF signals on the interface. It is also suggested that metallic conductor around detection position affect the signals, and compared with detection distance, influence of position is larger.Based on the structure of power transformer, experience of antenna design and simulation technique, a sleeve monopole antenna for detecting the partial discharge in transformer has been designed. Through optimizing structure parameters, which change the input impedance of antenna, the good VSWR characteristic, high gain and steady pattern can be obtained. According to analysis and measurement, the bandwidth of the antenna is 350-525MHz, the gain reach 2.3dBi at center frequency and variety is small in whole bandwidth. Whenθ=900andθ=2700, the antenna has the best performance.Compared with antenna performance in air by simulation and measurement, the antenna in oil still has good performance except having lower resonant frequency. The result of measurement in laboratory show that sleeve monopole antenna can satisfy the need of detection of partial discharge.Physical model of four defects in simulation of internal defects are developed according to PD signal feature in power transformer. In the laboratory, resistance, current transformer, sleeve monopole antenna sensor and high-speed PD acquisition system is adopted to access a large numbers of PD patterns and current pulse from various insulation defects and construct the ?- q -n distributions images of three test methods for PD in power transformer, which are used to analyze the discharge characteristic of four defeats.UHF signals of four insulation defects were obtained using a sleeve monopole antenna and recorded using an oscilloscope. In terms of the spectrum of UHF signals, some factors affecting signals spectrum were studied, including voltage, different time, antenna performance parameter and fixed position.