Research On Key Techniques Of Current-based Electronic Voltage Transformer And Their Application

Electronic Voltage Transformer (EVT) has a variety of advantages. It is small,light, non-ferroresonant, has a good insulation performance and a wide frequencyresponse range, as well as it can provide digital output. Therefore, EVT has beenconsidered as an appropriate substitute of the traditional voltage transformer and itrepresents the development trend of the high performance voltage transformertechnology. Many researchers from all over the world have made lots of progress inthe theories and applications of EVT. However, the techniques of EVT have not beenwell developed. The current operating EVTs have some common problems, such astemperature influence, electromagnetic interference, and high failure rate, etc. Aimedat the development of stable and reliable EVTs, this dissertation, firstly, bringsforward a design concept of the directly detecting current-based EVT, then, it studiesthe composition principles and key techniques of the EVT systemically andthoroughly, and finally researches and manufactures a110kV current-based EVTsuccessfully. The major achievements of this dissertation are described as follows:(1) To deal with the problems of the divider-based EVT’s electromagneticinterference during transmission of partial voltage signal, a current-based EVTscheme is proposed, which can realize primary voltage sensing and measurement bydirectly detecting capacitive current. Specifically, it, firstly, transforms the highvoltage, which needs to be measured, into capacitive current signal by usingcapacitive sensor, then re-transforms the current signal into voltage signal for voltagemeausurement. Since current transmission is hardly affected by electromagneticinterferences, a good anti-interference performance of the current-based EVT can beguaranteed from the sensing principles.(2) High-voltage capacitive sensor is a key component of the current-based EVTfor the signal sensing. This dissertation proposes a freestanding SF6coaxial cylinderhigh-voltage capacitive sensor with integrated construction, to deal with the problemsof being much affected by the stray capacitance and high dielectric loss of thecommon oil-immersed high-voltage capacitor. Here, the mathematical and electricfield models of the capacitive sensor are established, and the simulation analysis ofelectric field distribution and the stray capacitance influence are done by using finiteelement method. It also studies systemically the performance of the capacitive sensor as the change of parameters, including SF6gas pressure, temperature, and theelectrodes’ geometric parameter, then based on that, it introduces correspondingimproved methods. The results of simulation analysis and capacitive sensor prototypetests demonstrate that the proposed sensor is almost not influenced by straycapacitance, and has low dielectric loss, uniformity of electric fields between theelectrodes, as well as high insulation performance.(3) Signal integration is a key part of the current-based EVT to realize voltagere-transformation, whose operating characteristics have direct influence on the steadyand transient performances of the EVT. To deal with the problem that the commonused integral circuits cannot pay attention to both of the wide-band frequencyresponse characteristics and the rapid transient response characteristics, thisdissertation proposes a system state-based adaptive integral circuit. The circuit canidentify the steady and transient states of the power system, and then automaticallyswitch the working states of two integrators with different performances. The circuitcan achieve both wide-band frequency response characteristics in steady state andrapid transient response characteristics, so it can meet the requirements of thecurrent-based EVT on precise power-frequency signal measurement, the harmonicmeasurement and transient performances. The results of simulation analysis haveproved the correctness and effectiveness of the adaptive integral circuit.(4) The error compensation methods to improve the operation stability of thecurrent-based EVT are investigated in this dissertation. The multi-sensor informationfusion method is employed for error compensation of the capacitive sensor, to dealwith the problem that temperature and SF6gas pressure influence the measurementaccuracy and stability of the capacitive sensor. Firstly, the information fusion modelbased on the ternary regression analysis is established, which include temperature, gaspressure and ideal capacitance of the capacitive sensor. Secondly, the least squaremethod is used to get the optimal parameters in the regression model. Thirdly, theestimated capacitance value of the capacitive sensor is calculated. Finally, theestimated value is used to modify the output voltage of the current-based EVT,realizing the error compensation of the capacitive sensor. The simulation results haveproved the effectiveness of the information fusion method. To deal with thetemperature drift of the analog signal processing circuit, this dissertation establishesthe temperature error model of the analog signal processing circuit, and proposes atemperature compensation method based on the platinum resistor. The temperatureerror experimental results have demonstrated that this method can effectively improve the temperature stability of the analog signal processing circuit.(5) This dissertation designs the high-voltage capacitive sensor and the signalprocessing unit of the current-based EVT. The electromagnetic interference path tothe current-based EVT is analyzed, and the strategies to improve the EVT’selectromagnetic compatibility are studied. The effectiveness of the strategies isdemonstrated by the electromagnetic compatibility test results. Several experimentson the110kV current-based EVT prototype are conducted. The experimental resultshave shown that the EVT prototype assures0.2measurement precision class and3Pprotective class, as well as complies with other relevant performance requirements ofEVT standards.The proposed current-based EVT is based on the sensing theories and methodsfor measuring high voltage by directly detecting capacitive current. It has manyadvantages, among these are strong anti-interference performance, good temperaturestability, and good transient performance, and satisfactory bandwidth meeting therequirements of harmonic measurement. The research in this dissertation provides anew design idea and solution for realizing EVT. In addition, the researched andmanufactured current-based EVT has achieved the prospective design goals and canmeet the requirements of engineering applications, therefore, it has very promisingapplications in this field.