Capacitive Current Detecting Method For Eliminating Influence Of Excitation Branch Of Arc Suppression Coil

In recent years,with the large increase in cable outlets in the distribution network,the capacitor current of the medium voltage distribution network has also increased.In order to improve the stability of the medium-voltage distribution network in the occurrence of single-phase ground fault operation,China generally adopts the neutral point through the arc-suppression coil grounding measures to compensate the capacitor current generated during the fault.Therefore,accurately detecting the capacitance current to the ground becomes a prerequisite for rationally configuring the capacity of the arc suppression coil.At present,the current measurement method of the capacitance current of the distribution network mainly adopts the signal injection method,but whether it is injected from the open triangle side of the voltage transformer or the secondary side of the arc suppression coil,the excitation branch is ignored in the measurement process.The influence of the shunt is large,so the measurement error is large in the field application.In order to eliminate the shunting effect caused by the excitation branch of the arc suppression coil,this paper proposes an improved method for detecting the capacitance current of the medium voltage distribution network.The basic idea is to inject two specific frequency current signals from the secondary side of the arc suppression coil.The current transformer installed on the low-voltage side of the grounding transformer directly obtains the injected current phasor flowing through the primary side of the arc-suppression coil,so that the shunt generated on the excitation branch after the signal is injected from the secondary side of the arc-suppression coil can be directly avoided.At the same time,the feedback voltage value of the specific frequency is obtained from the open triangle side of the voltage transformer,and finally the capacitance value is obtained by substituting the measured data into the formula according to the basic principle of the dual frequency method.In view of the key points of the implementation of this method,the following aspects are studied.Firstly,the parameter calculations in the Simulink simulation model of current transformer and voltage transformer are deduced in detail,and the measurement error of current transformer and voltage transformer is obtained through the simulation platform.Secondly,under the condition that the current transformer and the voltage transformer have good transmission characteristics,the suitable range of the injected signal frequency is determined to be 10 Hz~120 Hz,and the injected current frequency is grouped in the range for quantitative analysis.The frequency is selected to influence the accuracy of the capacitance value measurement,so that the injection frequency is selected to be 20 Hz and 120 Hz.Finally,a dual-frequency weak signal detection model is designed based on the principle of orthogonal vector lock-in amplification.The simulation results show that the model can accurately extract the weak signals submerged in power frequency interference.The overall simulation results of the improved capacitance current measurement method show that the measurement error of the method is less than 1% for the distribution network with small capacitance to ground.The accuracy and feasibility of the method are fully proved,which is of great significance for improving the capacitive current detection technology of the distribution network.