Research On Electronic Voltage Transformer Based On Resistor Divider

The voltage transformer is the important instrument for energy measurement and relay protection in the electric power system, its measuring accuracy and the reliability are important to the electrical power system security, stability and economic operation. At present, electromagnetic PT and capacitance-voltage transformers to achieve voltage measurement have been used in power system. Traditional voltage transformer has disadvantages of larger volume, high ferroresonance, insulation complex structure, narrow band of frequency, smaller linear scope. Traditional voltage transformer's quadratic side can not be short-circuit. With the development of the relay protection technology and modern microelectronics technology, relay protection and the secondary measuring devices are no longer required high-power-driven. The secondary output of traditional voltage transformer is 100V or 100/(?)3V which can not directly connected with computer, it is difficult to adapt to the trend of power system automation, digital and intelligent development. Therefore, the research and development of the novel voltage transformer replacing traditional voltage transformer become the requirement of further development of the power system.This paper has analyzed the error of the resistance voltage transformer theoretically, and has established equivalent circuit to research the main factors which impact the accuracy of the resistive voltage divider. The resistance voltage divider structure of preliminary design is obtained by the theoretical analysis. Based on this design, in order to determine the structure of mask and the position of installment, using finite element software package Maxwell 2D of Ansoft Corporation establishes the finite element model of resistance voltage transformer, and calculates its electric-field intensity and the distributed capacity. According to the results, the preliminary design plan of the voltage divider has been obtained by comparing with several kind of structures plans' maximum of electric field; optimum plan of voltage divider has been obtained by the stray capacitance between mask and resistance. Under ideal conditions, the phase error of primary voltage and secondary voltage of resistance voltage divider is the zero; however, the existence of stray capacitance caused the voltage divider's phase error. In order to eliminate the phase error, it is necessary to increase corresponding masking measures, but it is still unable to completely eliminate the phase error. Against with the above problem, secondary electronic circuit should be used to adjust amplitude and phase of the transformer. The first stage of the secondary circuit is follower, high input impedance and paralleling with low voltage arm resistance, the impact of divider is nearly zero. The amplitude of voltage signal has been adjusted by the simple amplification circuit. The output is the standard value 3.25/(?)3V. Basic on the analysis in the theory, a model machine of 10kV electric voltage transformer whose rated output voltage is 3.25/(?)3V has been produced, the measurement accuracy of the developed transformer can meet the requirements of both metering PT at 0.2 class and protection at 3P class. During the process of research, in order to inspect insulating ability, the accuracy and the stability of electronic voltage transformer, we have carried on a series of experiments in laboratory, Sichuan Electrical apparatus co., LTD. and Chengdu Xuguang Electronic Corporation. Among them, voltage transformer carries on a series of experiments, including accuracy testing and insulating testing. The testing device samples the secondary output of the standard voltage transformer by 0.01% precision resistor, acquisition card which is controlled by Labwindows synchronized sample to standard voltage transformer and tested electronic voltage transformer, and save the results. The test results showed that the transformer could satisfy the requirement of IEC 60044-7 and the measurement accuracy could reach class 0.2, the protection requirement could reach class 3P. During the process of producing model transformer, the anti-interference of the electronic circuit has been used in the EVT.