Characteristics And Mechanisms Of X-ray Induced Partial Discharge In GIS

The existing methods for detecting partial discharge(PD)in gas insulated switchgears(GIS)are still lack of sensibility and inadequate for discontinuous PD pulses,and partly unable to locate PD sources accurately.Especially,these methods are unsuitable for distinguishing PD and external interference signals in substations when being used in the prospective direct current(DC)voltage GIS.To solve the above problems,an innovative X-ray induced PD technique for defects in high pressure SF6 is proposed,which aims to excite and make PD pulses controllable.This method is of help to improve the accuracy and effectiveness of PD detection in GIS.A highly sensitive mini platform using PD pulse current,light emission and ultra-high frequency(UHF)method was built,which can detect wideband PD pulses with a fast rise-time of 100ps or above in high pressure SF6.Also,a genuine 252 kV GIS PD test platform was built.The UHF system has a bandwidth of 0.3?1.5 GHz and the sensitivity is-75 dBm.The UHF acquisition system is based on high speed field programmable gate array(FPGA),and the max sampling speed is up to 10,000 pulses per second.The tube voltage and tube current of the X-ray sources is 130 kV/0.5-1.0 mA,which can generate X-ray photons with an average energy of 65 keV and the max energy density is about 86 keV.Correspondingly,the X-ray alignment and radiation shielding system was built.Firstly,the influence of X-ray on PD in high pressure SF6 under non-uniform DC field is revealed,the mechanism that ionized space ions by photoelectrons can enhance the DC electric field strength is proposed.The tests showed that X-ray induced partial discharge inception voltage(XPDIV)were lower than partial discharge inception voltage(PDIV)by 28.6%under positive DC voltage,and by 15.6%under negative DC voltage correspondingly.Based on the X-ray ionization process,the mechanism that ionized space ions by photoelectrons can enhance the electric field is proposed.X-ray can supply enough photoelectrons by ionizing SF6 molecules.On one hand,these photoelectrons can trigger mass avalanches and secondary avalanches in gas,and then transfer to streamers below PDIV.On the other hand,the space ions in gas ionized by X-ray will enhance the electric field strength near the needle.For the positive needle,photoelectrons recombine with the positive ions and finally enhance the electric field strength.For the negative needle,photoelectrons can generate more positive ions to enhance the electric field strength.As a result,streamers induced by X-ray can be detected below PDIV under DC voltage.The tests also showed that PD pulse repetition rate under X-ray were much higher than that without X-ray.For example,the positive PD pulse repetition rate under X-ray was 17 times higher than that without X-ray and 166.7 times higher for negative PD pulses.The mechanism is that photoelectrons can promote streamer process by generating enough avalanches and the electric filed is also enhanced by the same process proposed above.Secondly,the influence of X-ray on PD on the gas-solid interface in non-uniform DC field and high pressure SF6 is revealed,the mechanism that ionized space ions by photoelectrons together with ionized surface charges can enhance the DC electric field strength is proposed.The test showed that the PDIV of surface needle defect under positive DC voltage was lower than the needle defect in gas with the same curvature radius.For the negative situation,the PDIV of surface needle defect was higher than needle defect in gas.The mechanism is that surface negative charges on gas-solid interface can enhance the electric field strength under positive DC voltage and reduce the electric field strength under negative DC voltage.X-ray could lower the PDIV of surface needle defect by 53.7%under positive DC voltage and 57.4%under negative DC voltage,which is also much prominent than that of needle defects in gas with same curvature radius(36.1%under positive and 17.2%under negative).Not only X-ray can ionize the molecules in gas as discussed before,but also X-ray can ionize the surface solid by photoionization and secondary ionization process.These additional electrons can move into gas and promote the streamer in gas.Furthermore,the electron emission means the surface charges decrease at the same time,which leads to the PD pulse repetition rate of surface needle defect on gas-solid interface equals to the needle defect in gas.Thirdly,the mechanism of X-ray induced PD under alternative current(AC)voltage is revealed.For the increasing curvature radius from 50 ?m to 200 ?m,the decreases of PDIVs of needle defects in gas were getting prominent under AC voltage,which increased from 2.4%to 20.6%.In the initiation process of PD under AC voltage,the space charges driven by alternative electric field reciprocate in gas,which will enhance the needle electric field strength in the following cycles.For the needles with small curvature radiu,PD initiates in negative half cycle and produces enough space charges and X-ray has limited effects.For the bigger curvature radius,PD initiation in negative half cycle is frustrated by great decrease of field strength.X-ray will trigger avalanches in gas and produce enough space charges to promote PD initiation together with AC electric filed.Finally,for defects that have discharged for a long time under AC voltage,the influence of X-ray on PD characteristics of these defects is revealed.The mechanism of discontinuous UHF signals of these defects and the process of X-ray enhanced UHF signals is proposed.The discontinuous UHF signal sequence was discovered in the test.For surface needle defect which had discharged for 60 hours,no UHF signal was observed in 99%of all positive half cycles and 97%of all negative half cycles.At this time,large amount of PD current pulses could be observed,whose charge quantity was about 0.001?0.01 pC.X-ray could regenerate UHF signals and increase total UHF signal pulses by 6.27 times than that without X-ray.The amount of positive half cycles and negative half cycles that had new UHF signal increased by 130.5 times and 1.6 times respectively.For PD current pulses corresponding to new UHF signal pulses,the charge quantity increased by 5?10 times in positive half cycles and 2?3 times in negative half cycles.Based on scanning electron microscope(SEM)imaging and energy dispersive spectrometer(EDS)analysis,a large amount of discharge byproduct accumulated on the needle tip,which mainly is Aluminum-Fluoride compound.As test time evolves,the insulating compounds gradually cover the high field reign on the tip,which will weaken the streamer process and decrease the discharge quantity.X-ray will enhance the electric field strength and trigger enough avalanches to promote the streamer process in the slightly high-field regions near the covered tip,and eventually regenerate new streamer discharge and emit new UHF signals.