| With the development of ultra high voltage direct current transmission(UHVDC),the converter transformer,which is the key equipment of UHVDC,takes increased responsibility to the security and stability of power grid operation.In recent years,converter transformer faults occur frequently,and the proportion of insulation faults is high.The measures to reduce the insulation faults are to optimal design the insulation structure(active measures)and to improve the on-line assessment technology of insulation state(passive measures).Partial discharge(PD)is one of the main reasons of insulation faults,and also one of the important means to detect the deterioration degree of insulation,its characteristics and mechanism have attracted much attention:the study of PD characteristics is helpful to establish a impeccable and effective PD feature database,and to promote the development of on-line assessment technology of insulation state;the study of PD mechanism is helpful to find the internal relation between the PD phenomenon and the charge distrnbution,and to promote the insulation structure optimization.However,the oil-pressboard insulation on the valve side of converter transformer mainly withstands the AC-DC combined voltage,which makes its PD characternstics and mechanism more complex.The needle-plate PD(typical representative of PD under extremely uneven electric field),in particular,o.f which the dispersion,randomness and polarity effect are more obvious,is easy to cause sudden and unpredictable insulation faults.Therefore,the characteristics and mechanisms of needle-plate PD in oil-pressboard insulation under the DC component of converter transformer need to be further studied.Aim at above problem,the experimental platform and detection system of PD in oil-pressbaord insulation are established in this paper,the needle-plate PD model with oil gap,the needle-plate PD model with oil gap,and the needle-plate creeping PD model are designed to generate PD signals.The PD signals are collected via the impulse current method,then denoised by using the estimation algorithm for adaptive threshold of hybrid particle swarm optimization wavelet.The simulation model of electric field and charge distribution in oil-pressboard insulation is established baesd on thehydrodynamic drift-diffusion theory and the bipolar charge transport theory,through which the PD characteristics and PD mechanisms of needle-plate PD in oil-pressboard insulation under the DC component of converter transformer are studied.For different PD models,similarities and differences of PD characteristics compared AC electric field with DC electric field are studed,including time-variation trend of PD typical features,distribution and evolution rules in PD patterns and PD pulse waveforms.On this basis,the effects of the DC components proportion and polarity on PD characteristics are studied.Besides,(1)For the needle-plate model with oil gap,the positive PD under the effect of DC component is classified according to the shapes of the discharge channel in oil gap,and the classification mechanisms are analyzed by simulating the effect of interfacial charge on the streamer in oil gap;based on the evolution rules of streamer and the characteristics of PD pulse,the equivalent circuit model of PD under highly uneven electric field is established,and the internal relation between the PD pulses and the streamers is found.(2)For the needle plate model without oil gap.the electrical aging stage under the effect of DC component is divided based on fuzzy clustering analysis,and the physical meaning of stage division is explained;the effective identification method of electrical aging stage is established based on the PD typical features and the characteristics of PD patterns;the distribution of electric field and charge is studied,and the PD mode and mechanisms of each electrical aging stage are explained.(3)For the needle-plate creeping PD model,the influence of strong vertical component on PD characteristics is studied,as well as the distribution of electric field and chargeis,and the discharge mode and mechanisms of each electric aging stage are explained.For the needle-plate model with oil gap,the research shows that:(1)The electrical aging process of oil-pressboard insulation is slow,the PD typical features are almost invariant with time,but have obvious polarity effect.The average apparent discharge amplitude Qave,the PD frequency Fne,and the average equivalent frequency of PD pulse Feqa,ve decrease with the increase of DC component,while the average equivalent time of PD pulse Teqa,ve,the average rise edge of PD pulse Tua,ve and the the average falling edge of PD pulse Tdave have opposite rules.(2)The interface charge induced by the DC electric field component has shielding effect on the streamer development.(3)Under the effect of DC electric field component,the positive PD can be classified as corona,tree-like PD,single-streamer PD,or multi-streamer PD,and the type of PD is mainly related to the amount of oil-pressboard interfacial charge and the air bubbles in oil.(4)The rising edge of a PD pulse corresponds to the process of streamer propagation in the oil gap,the falling edge of the PD pulse corresponds to the process of space charge accumulation at the pressboard surface;the peak time of PD pulse corresponds to the time when streamer reaches the pressboard surface.For the needle plate model without oil gap,the research shows that:(1)Based on fuzzy clustering analysis,the electrical aging process of oil-pressboard insulation can be divided into five stages:initiation,transition,development,stabilization and pre-breakdown.(2)With the increase of DC component,the PD layering distribution phenomenon in the TRPD pattern is more obvious,and even the whole arch shape appears in the equivalent frequency-time pattern.(3)Two effective methods for recogniting the electrical aging stages are proposed:a visual recognition method based on PD radar maps and a high-precision recognition method based on wavelet moment invariant of PD patterns.(4)Qave,Fnc and Feqave decrease with the increase of DC component,Teqave and Tuave increase with the increase of DC component,the polarity effect of PD typical features is obvious.(5)The radial-direction(transverse)creeping discharge is dominated in the initiation stage,the small bridge discharge is dominantn in the transition stage,the streamer discharge in oil gap is dominantn in the development stage and stabilization stage,the axial-direction(lengthways)creeping discharge is dominated in the pre-breakdown stage.For the needle-plate creeping PD model,the research shows that:(1)Under the strong vertical component,the deterioration traces of pressboard can be divided into internal breakdown traces,flashover traces and composite traces;under the weak vertical component,only flashover traces can be produced.The proportion of trace type is different under different DC component.(2)The electric aging process is divided into three stages:initiation,development and breakdown.The DC component has more influence on the discharge characteristics under the strong vertical component of electric field and in the development stage and breakdown stage.(3)Under the effect of DC electric field component,stratification and segmentation phenomenon occurs in the TRPD pattern,and the residual charge discharge occurs after flashover.(4)The development of positive surface streamer can be divided into three stages:strong self-sustaining stage,weak self-sustaining stage and stabilization stage.Under the strong vertical component of electric field,the oil-pressboard interface can keep the retained charge,which can form the surface streamer trace;under the weak vertical component of electric field,the multiple streamers(under positive voltage)and the reverser streamers(under negative voltage)could be generatedThe streamer evolution rules and PD mechanisms obtained in this study can provide theoretical basis for the optimization of converter transformer insulation structure.Meanwhile,the highly efficient electrical aging identification method proposed also promotes the development of oil-pressboard insulation state assessment technology. |
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