| Partial discharge(PD)is a commonly adopted tool for condition monitoring of power equipment to assess their future behavior and operating capability.Several techniques are being adopted,but the ultra-high frequency(UHF)technique is documented among most of engineers due to high sensitivity,less interference from outside.Despite those features,several wrong alarms are reported per year.In parallel,there are also periodic tests to be conducted at power stations for their safety check.We as a PD testing team at substations encountered tons of interference and wrong results problems.These quite wrong measurements at power stations are quite wrong,they raise economic,credibility and safety risk.There are technologies already introduced to mitigate these interference problems,but they are complex to apply to the field and not very much effective.It is studied and concluded that these noise and interference problems are caused due to superimposition of the external signals from the external PD phenomenon.Several substations have been tested by using an Ultraviolet imager and UHF sensors,double ridge cone antenna attached with an oscilloscope to investigate impulse signals in the time domain,changed by Fourier series to the frequency domain and power spectrum to study their properties.It is observed that the weather condition of the substation,as well as rated voltage level,are the driving parameters for PD energy and frequency parameters behavior of signals in substations.Only in the damp creeping discharges produce UHF band signals and surface and corona discharges only produce UHF band signals in 500KV substations.The communication and other broadcasting radio wave signals are investigated in power substations.They also interfere with the measuring system installed in substation equipment for their PD monitoring.For discriminating purpose of these signals,they are statistically analyzed from three aspects:the uplink and downlink of signals are categorized according to their frequency,the power of uplink and downlink signals is analyzed and compared and last,the phase-resolved PD spectrum is also analyzed by using an antenna and a spectrum analyzer-Ubiquitous radio wave signals are in 358-365 MHz with amplitude of-74 to-52 dBm.Mobile communication signals are divided into two frequency bands.825-960 MHz and 1710-1875 MHz with-3dB amplitude.The strength of the downlink signal is significantly higher than that of the uplink signal.Downlink signals are not only large in number but also relatively high in amplitude.The propagation characteristics of those external noise signals in the air and then their behavior upon entering the chosen monitored equipment is additionally studied by making models of GIS equipment in simulation software XFDTD and by practical experiments on GIS models in the laboratory.Propagation characteristics of external interference signals from the casing of the GIS equipment,insulator and nonmetal ring basin have been studied.The bushing of GIS equipment plays a coupling role for external signal upon interaction with it.When the polarization direction of the external signal to bushing is perpendicular,it becomes attenuated.while it is observed that the amplitude of the signal implies 1000 times when its polarization direction is parallel to the bushing.The length of bushing also affects and boosts the wavelength of the signal.It is concluded that the nature of the propagating signals and the angle of inclination with the equipment have deep concerns with the interference properties.The radio wave signals in substations were detected at 200MHz-3GHz.Mobile communication signals of 2G,3G,4G and other communication signals frequency range and PRPD spectrum analysis laid the foundation of dual passing-band filter choice 200MHz～825MHz+960MHz～1710MHz for suppression of mobile communication signals.A set of seven segments of filter modules with components is proposed.They can be used according to the site and nature of interference signals in a substation.This anti-interference scheme is tested in the laboratory for verification,and it can be applied in the station. |
