Research On Measurement Of Capacitance Current Of Medium Voltage Distribution Network By Signal Injection Method

Effectively grasping the basic data of the capacitance current of the distribution network to ground,and regularly conducting the check of the arc suppression coil device are important tasks to ensure the safe and stable operation of the distribution network.According to the relevant regulations of the power department,for a 6～20kV system that is not directly connected to the generator,when a single-phase ground fault occurs,if the power grid to ground capacitance current is less than or equal to 10A,the neutral point ungrounded mode can be used,if the power grid to ground capacitance current is greater than 10A,the neutral point should be grounded via the arc suppression coil.Therefore,accurate measurement of the capacitance current to ground of the medium voltage distribution network is of great significance for the reasonable selection of the neutral point grounding method and the correct selection of the arc suppression coil capacity.This paper mainly researches the related techniques of measuring the capacitance current to ground of the medium voltage distribution network by the signal injection method.Firstly,the measurement principles of the main existing signal inj ection methods,such as the frequency sweep method,tri-frequency method,phasor method,comprehensive method,and high frequency method are explained in detail,and their scope of application,advantages,and limitations comprehensive comparison.Then,the further study of the comprehensive method with better comprehensive evaluation is carried out.Aiming at the rationality of the existence of the equivalent model of the medium voltage distribution network in the comprehensive method,the difficulty of determining the resonance frequency,the search time of the resonance frequency being too long,and the low frequency,the insufficiency of the voltage transformer excitation branch and other deficiencies were reasonably improved,that is,the equivalent model of the medium voltage distribution network was improved based on the frequency characteristics of the medium-voltage distribution network.Based on the characteristic information of series resonance,a variable step size was proposed In-phase search method.By connecting a suitable capacitor in series with the triangular side of the opening of the voltage transformer,the resonance frequency of the system is controlled in a suitable range to reduce the influence of the excitation branch of the voltage transformer.Then,based on the principles of accuracy and safety,the key parameters of the improved comprehensive method are reasonably selected.Finally,considering the influence of the power frequency signal and the zero-sequence harmonic signals on the measurement results in the actual distribution network,combined with the characteristics of the frequency of the injected signal,a digital cross-correlator is designed to eliminate the impact on interference signal to the measurement.Through simulation verification,the improved comprehensive method has the advantages of convenient operation,good safety,wide application range,fast measurement speed and accurate measurement.