Moisture State Analysis Based On The Equivalent Model Of Dielectric Responses And Electrical-thermal Coupling Simulation Of Bushings

Oil-paper insulated equipment serves the fundamental functions of energy conversion,metering and insulation protection in a power system,the operation status of which concerns the overall safety of a power grid.Moisture in oil-impregnated paper?OIP?insulation will cause the degradation of insulation performance,which is one of the main causes of equipment failure.At present,frequency-domain spectroscopy?FDS?technology based on dielectric relaxation theory is widely employed to determine the moisture level of the OIP bodies in equipment due to the advantages of non-intrusive application,strong anti-interference capability and the wide-band frequencies applied for measurement.However,FDS encounters on-site application problems,where only a single characteristic parameter is normally extracted from FDS measurements and thus the resultant evaluation are unreliable.Besides,the existing work of OIP status analysis mainly focuses on the internal problem—dielectric properties of insulation material,while the influence from operational voltage conditions which is a critical external factor on insulation tends to be disregarded.Therefore,it will be of great academic and engineering significance in this work to investigate a quantitative approach for analyzing OIP dampness state,the power loss characteristics and thermal effects of OIP equipment under complex voltage conditions.Taking OIP condenser bushings as typical research objects and frequency-domain dielectric response as theoretical background,the author investigates two specific problems that are corresponding to the lower and higher frequencies of FDS respectively.Based on the dielectric information from lower-frequency FDS,extended Debye model?EDM?is used to analyze the correlation mechanism between the model parameters and moisture content by identifying EDM,and then a new multi-parameter group is put forward to evaluate the insulation dampness state.The influence of moisture on OIP insulation is further extended to the power loss characteristics and thermal effects under real voltage conditions.Based on the high-frequency dielectric characteristics,the calculation of dielectric loss subjected to complex voltages and temperature field simulation considering the non-linear characteristics of the material of condenser bodies are studied.The main research contents and innovative achievements of this work are as follows:?1?EDM is used to model the FDS of OIP insulation,where an improved optimization model considering the low-frequency weighting of FDS and the constraint of insulation resistance is proposed.A combined method of genetic algorithm and Levenberg-Marquardt algorithm is employed to identify the model parameters.The validation results show that the goodness of fit between EDM and 28 groups of complex capacitance C*-f spectra of OIP bushing samples is 0.92⁰.99,which confirms the theoretical applicability of EDM to FDS in a wide band range(10^-4Hz¹0³Hz).It solves the problem that the model can only match time-domain dielectric spectra at present,but has poor reconstructing effect on FDS.The results signify the feasibilities of using EDM to interpret the information of insulation state that is carried by FDS data.?2?A method based on analyzing the sub-spectrum of EDM branches is proposed.The correlation mechanism among the change patterns of dominant branches in EDM,original FDS spectra,and sample moisture state is determined by studying the subspectral peak characteristic.Based on the above anlysis,insulation resistance R₀,the maximum time constants?₁ and R₁^R₂ are dertermined as four time-dominant dielectric characteristic parameters.The relationship between each parameter and moisture content is established.?3?The time-domain dielectric response is transformed by EDM,and a set of mixed dielectric characteristic parameters in both frequency and time domains is proposed for assessing the moisture state of OIP insulation.Based on the identified model,polarization/depolarization currents as well as recovery voltages are calculated.Four time-domain dielectric characteristic parameters are proposed thereby:absorption ratio K,polarization index P.I.,decay time of depolarization current t_‰1,and the peak time of recovery voltage t_p.This set of hybrid characteristic parameters can be used for comprehensive assessment and analysis of the moisture state of OIP equipment.?4?Based on the dielectric properties of OIP insulation,an external factor—applied voltages are furthered considered.For the type of non-sinusoidal periodic waveforms,the harmonic voltages generated by converter valves are used and an index—enhancement coefficient of dielectric loss is proposed to characterize the influence of harmonics on dielectric thermal effect.For the type of non-stationary transient voltages,a fourth-order Runge-Kutta method is used,which calculates the dielectric loss by solving the input-output equation of the EDM.?5?Finally,a full-scale simulation model for analyzing the thermoelectric coupling of a condenser type bushing is established.The spectrum characteristics of the harmonics in periodic voltages and load currents,the non-linear relationship among dielectric properties of OIP insulation,moisture content,temperature and frequency,as well as the non-linear dependence of the thermal parameters of OIP and insulating oil on temperatures are comprehensively involved by developing a joint simulation process using a finite element analysis?FEA?software and a numerical software.Distribution of power loss and temperature along the bushing is obtained,and the effects from applied voltages,loading currents,ambient temperatures and dampness condition of OIP insulation etc.on the temperature field are analyzed.