| Power electronic transformer is the key equipment to realize flexible interconnection,conversion,and optimal scheduling of future energy network.The dielectric properties of insulating materials directly affect the equipment development to high-voltage level,compactness,and large capacity.Compared with the traditional AC and DC power equipment,the insulation of power electronic transformer is restricted to the special electrical stress of high repetition rate,high amplitude sine wave and square wave waveforms,which further cause the dielectric loss and temperature rise.In particular,the polyimide-coated insulation directly bears the special electro-thermal coupling stresses on the metal winding side of the transformer,and its dielectric properties directly determine the actual operation performance of the equipment insulation system.Under the special electrical stresses of high frequency and high voltage,the discharge development and premature failure of polyimide insulation are prone to occur.However,the microscopic mechanism of these phenomena is still unclear.The key is that the space charge evolution mechanism that causes serious distortion and deterioration of the local electric field inside the material is not clear.The existing charge measurement methods for DC and power frequency AC are difficult to be directly applied to the charge research under special electrical stresses.To solve the above problems,this paper systematically studies the transport characteristics of bipolar carriers under special electrical stress,the high-precision measurement method of space charge and the evolution of space charge under different conditions by using the micro-simulation and experimental observation methods,which can provide theoretical basis and effective methods for the optimal design and performance evaluation of polyimide insulation of power electronic transformer.In view of the transient characteristics of special electrical stress conditions,the bidirectional migration behavior of space charge in the bipolar charge transport model is studied.The applicability of the previous DC Schottky injection principle under special stress is analyzed,and the principle of low electric field charge injection threshold setting is proposed to improve the accuracy of the model calculation.Aiming at the high-precision calculation of model self-consistent equations,different algorithms for solving convection conduction equation and Poisson’s equation are studied based on the injection threshold setting.A highprecision model solution method is established by combining the finite differential weighted essentially non-oscillatory of convection conduction equation and the boundary element method for Poisson’s equation.The new model and algorithm are further employed to study the influence of sample traps and interface characteristics on the evolution of insulating space charge under special stress conditions such as unipolar and bipolar square wave and sine wave.The results show that the charge accumulation and migration under unipolar stresses are coupled by multiple factors such as trap depth,interfacial barrier and charge mobility,while the charge evolution under bipolar condition is mainly determined by injection barrier and charge mobility.To construct the pulsed electro-acoustic(PEA)method suitable for special electrical stress conditions,an equivalent transmission line model of PEA system is proposed based on the mathematical representation of charge elastic wave and voltage wave in distributed parameter transmission line.The applicability of the model is verified based on the consistency of simulation results and theoretical analysis,and the design scheme of hardware system is provided accordingly.Combined with the coupling effect of the special polarization stress on the voltage of the sample and the system components,an improved design scheme of the measurement system suitable for the special electrical stress condition is proposed,and the evaluation criterion of the measurement range affected by the frequency of the electrical stress is established.It is pointed out that the PEA method has the upper limit of application frequency.Aiming at the acoustic impedance matching problem of the system,an improved calculation method of the attenuation coefficient and voltage-charge conversion coefficient of the measured signal is deduced by using the transmission process of the charge elastic wave in each measurement link.The charge results of the polyimide insulation show that the improved algorithm can improve the signal recovery accuracy by 33.5%.With the improved PEA system,the space charge evolution of polyimide insulation under positive/negative square wave,half-wave sine and full-wave sine conditions is studied in the range of 20-60 kV/mm field strength,10-500 Hz frequency and 25-80℃ temperature.Combined with the simulation analysis of bipolar charge transport process,it is pointed out that the accumulated charge under different electrical stress waveforms is derived from the electrode injection at different sides.The positive and negative polarity of the square wave determines the frequency range of the positive charge accumulation,while the space charge in the low-frequency half-wave sine and full-wave sine conditions mainly comes from the upper electrode.At the same time,high temperature conditions will promote the accumulation and evolution of space charge under high-frequency electrical stresses.The polarity,waveform,frequency,and other conditions of special electrical stress will cause the change of polyimide insulation trap and barrier characteristics,thereby affecting the evolution mechanism of space charge.Aiming at the limitation of space charge measurement method in temporalspatial resolutions,the criterion of space charge which can be accurately measured based on hopping conduction model is proposed,and the calculation method of trap depth under the influence of temporal-spatial resolutions of space charge measurement method is established by using the basic relationship between charge hopping migration probability and migration time.The quantitative criterion for evaluating the measurable range of space charge measurement method is proposed.It is pointed out that the previous method can only measure the deep trap charge with depth above 0.68 eV,and the measurable charge range of the method is verified by the current integration charge test results.On this basis,combining the elastic-optical effect and ellipsometry detection technology,a space charge optical measurement method that can realize nanoscale spatial resolution,microsecond-level rapid response and all-optical isolation structure is proposed.A mathematical representation model of the whole measurement process including excitation,signal transmission and final detection is established.Further,using the constructed ellipsometry measurement system,experimental and simulation studies show that the measured signal can accurately reflect the elastic stress on the sensor while maintaining high signal-to-noise ratio,and the change of the refractive index of the sensor can track the deformation change of the picosecond pulse width,which verifies the feasibility of the optical measurement method to achieve high temporalspatial resolutions.The above research can provide theoretical basis and effective method for space charge measurement of polyimide insulation under high-frequency special electrical stresses. |
