Study On Key Techniques Of Measuring Capactive Current In Distribution Network By High-frequency Method

Distribution network’s capacity are gradually increasing,power cables in the distribution network are also used more than ever.the capacitive current is greatly rose when single-phase-to-ground fault occurs.It cause arc grounding over-voltage,which harms equipment and personal safety.To reduce current at the fault point and extinct ground arc reliably,the arc suppression coil at the neutral point of the large number of low-voltage network has been installed.The accurate measurement of capacitive current of the distribution network is the prerequisite for effective operation of the automatic tuning device.High-frequency method which apply high frequency to estimate the leakage inductance of voltage transformer,which eliminate the transmission error caused by the unknown leakage inductance in the process of calculating capacitance current of the distribution network.At the same time,high-frequency method combine dual-frequency method with sweep frequency method to expand detection range and solve the problem of selecting frequency difficultly.However,high-frequency method is seriously interfered by working frequency signal of distribution network,and it not only needs to measure the phase difference of the open triangular side voltage and current of the voltage transformer to calculate,but also put forward high request for precision of the research of resonant frequency.For the above three problems,the existing literature has not put forward effective solutions.Based on the principle of high-frequency method,several key techniques in the application of high-frequency method are deeply discussed in this paper.Firstly,in order to solve the problem of interference of working frequency in high-frequency method,double T working frequency band-stop filter is used when high frequency signal is injected;FIR working frequency band-stop filter is used when low frequency signal is injected.Two kinds of filters are analyzed and designed respectively.Secondly,in order to solve the problem of the phase difference measurement in the high-frequency method,a phase comparator is designed.The phase comparator consists of a phase delay elimination unit and a phase comparison unit.This article has carried on the detailed elaboration to each unit and the realization process.It match with the FIR working frequency band-stop filter well through the verification of simulation,which can measure the phase difference between voltage and current accurately.thirdly,in order to solve the problem of resonant frequency searching in the high-frequency method.In this paper,a method of second-frequency sweep with in phase is presented.The first rough sweep can save time and determine the resonant frequency at the same time.The second precise sweep can measure the resonant frequency accurately.Finally,considering interference of working frequency in the process of actual measurement of high-frequency method,this paper combines the above three research results of key technology with high-frequency method itself to conduct comprehensive simulation verification and compares the measurement results of high-frequency method in ideal situation.It turned out that the results of key technology research can better meet the actual measurement requirements of the high-frequency method.The measurement time is greatly reduced,and the relative error of measurement is no more than 4.4%,decreased by 5.6%,which ensures the rapidity and measurement accuracy of high-frequency method.