Study On The Key Techniques Of Measuring Capacitive Current By Signal Injection Method

The capacitive current of medium voltage distribution network is increasing with the large-scale renovation of distribution network and a significant investment of cable lines.In order to reduce the harm of the capacitive current and improve the operation reliability of the medium voltage distribution network,it is widely used to install the arc suppression coil at the neutral point of the medium voltage distribution network.In order to configure the capacity of arc suppression coil properly,the capacitive current of medium voltage distribution network must be measured accurately.There are many methods to measure the capacitive current now,but the measurement accuracy of the traditional methods for measuring capacitive current based on the asymmetry in primary side is reduced or even the capacitive current cannot be measured with the aggrandizement of the proportion of cable lines and the increase of the symmetry in the grid network.In addition,the traditional methods have the disadvantages of complicated operation and insecurity.Due to the above reasons,the signal injection method,which is operated in the secondary side and does not need to rely on the asymmetry to maintain the accuracy of measurement,can be adopted to measure the capacitive current in medium voltage distribution network.However,the measurement error of the method is prominent in field application,the cause of the measurement error is that some key technologies of measuring capacitive current by signal injection method are still insufficient,such as inaccurate equivalent model of the zero sequence circuit in the medium voltage distribution network,the inappropriate frequency range and improper capacity of the injected signal,the inaccurate detection algorithm,etc.In order to measure the capacitive current in medium voltage distribution network more accurately,several key technologies of signal injection method were studied in-depth in this paper based on the principle of the signal injection method to measure the capacitive current.Firstly,the measuring principles and errors of triple frequency method,Frequency division method and frequency sweeping method in the signal injection method were thoroughly analyzed and verified by simulation in this paper,and then made a comprehensive evaluation of the characteristics and application scope of each method.Secondly,Analyzed frequency characteristics of zero sequence impedance in medium voltage distribution network based on the frequency characteristics of zero sequence parameters of three-core cable,and improved equivalent model of zero sequence circuit in medium voltage distribution network by the series equivalent method for one port network.After combining the equivalent model and PT model,the precise measurement model of signal injection method was obtained.Compared with the precise measurement model,the accuracy of the traditional measurement model was verified,it also showed that there was no method error in signal injection method.Thirdly,the PT excitation characteristic test and PT transmission characteristic test were carried out and the test data were analyzed to determine the frequency range of the injected signal.The most suitable capacity of the injected signal was identified by taking into account the attenuation of injection signal and the limitations of PT capacity limits.Finally,the digital phase sensitive detection(DPSD)technique was used to detect the low signal to noise ratio(SNR)reflected voltage signal in medium voltage distribution network.The detection algorithm was designed and verified by simulation,the simulation results showed that the DPSD algorithm could suppress the harmonic signal and the random noise of the power grid effectively,and extract the return voltage signal accurately.Accordingly,the study results of the techniques of measuring capacitive current by signal injection method have guiding significance for the site measurement and the development of the measuring instrument.