Study On Inner Sensors And UHF Detection For Partial Discharge In Cable Accessories

Partial Discharge (PD) is the main apparent forms of early insulation faults in cable accessories, it is not only one of the primary reason to cause the insulation degradation of cable, but also is the characteristic scalar to describe the insulation state of cable. The inner insulation condition of cable accessories can be evaluated timely and accurately by developing deeply the research of on-line monitoring and fault diagnosis for PD in cable accessories, which is valuable to prevent accident faults of cable lines and insure stable performance of power system.From the UHF (Ultra High Frequency) detecting approach for partial discharge, based on the 110 kV high voltage cross-linked polyethylene (XLPE) cable accessories structure and operating characteristics, combined with microwave and antenna theory, electrical measurement techniques and theory of electromagnetic field analysis, ultra high frequency detection technology research and analysis for PD in the cable accessories was carried out in this paper. The main research work is as follows:The model characteristic equation of coaxial line filled with multilayer dielectric is derived, combined with the dielectric waveguide structure of power cable, the propagation model and field structure as well as its characteristics of electromagnetic wave signal in cable is analyzed theoretically, and the influence to field structure by semiconducting layer is represented. In order to investigate the distribution regularity, attenuation characteristic of electromagnetic wave in cable and accessories, and also the influence factors of detecting UHF electromagnetic signal, numerical simulation model of cable and accessories are constructed by FDTD (finite difference time domain method) . Furthermore, through the introduction semiconducting layer weight factor to transmission line model of cable, attenuation spectra and upper cut-off frequency of PD impulse are analyzed and calculated under different distances in the cable, which provides theoretical reference for frequency selection of high frequency sensor.Broadband inner ring capacitive coupler sensor and miniaturized spiral antenna sensor with loading a shorting pin applied to PD detection in cable accessories are developed and its characteristics are overall analyzed. Through field measurement, The results show that coupler whose band reaches 500MHz work well and can detect nanosecond transient pulse signal effectively, and its sensitivity is nearly 4~6pC in the laboratory; The bandwidth of antenna sensor is about 353MHz~ 380MHz when the voltage standing wave ratio (VSWR) is less than 3, which lay the foundation of UHF detecting for PD in cable accessories.According to typical fault type and characteristics of PD in 110 kV cable accessories, physical model of four defects in simulation of internal defects are developed. In the laboratory, using current sensor capacitance sensor, antenna sensors, large number of partial discharge samples of four kinds of insulation defects are acquired by high-speed data acquisition system, single waveform are gained and ? ? q ? n distributions images of four typical defects are constructed. Different discharge characteristics against four kinds of defects are studied, and the three detecting methods are compared and analyzed.