Research Of Testing Methods Of Capacitive Current For Medium Voltage Grid Based On Signal Injection Method

The choice of power system with neutral grounding method is a comprehensive problem, it is closely linked to the operation reliability of power system, insulation coordination, relay protection, communication interference, electrical safety and the choice of grounding devices. The non-effective grounding operation mode is generally used in neutral point of the medium voltage distribution network. When the single-phase grounding fault occurs in the distribution network, it may produce arc and lead to an overvoltage, which is a threat to safety operation of the system if the capacitive current of fault point is out of limit. Therefore, when the capacitive current of fault point is greater than the specified value, we need to install arc suppression coil in neutral point, which produced inductive current to compensate the capacitive current of fault point so as to achieve the extinction of arcing. So, the accurate measurement of grounding capacitive current in medium voltage distribution network is significant to select the operation mode of the neutral point and as well as determine the installed capacity of arc suppression coil.First of all, in order to measure the capacitive current in power distribution network more accurately, this paper analysed the measuring principles deeply, such as Three Frenquency Method, Frequency Swept Method, Phasor Method and Improved Signal Injection Method etc. And then concluded the measuring accuracy and the applicable scope of each method. Secondly, due to the current measuring methods of capacitive current does not give the theoretical analysis about choices of injection signal in frequency and intensity, and also ignore the influence of feedback voltage signal which contained the noise in power frequency, therefore, this paper has carried on the theoretical analyses and simulation verification of the injection current signal intensity and frequency, and the PT's characteristics of transimission and change. According to the results of simulation, we concluded the appropriate injection signal strength and frequency. Finally, considering the actual distribution network in the operative mode, there were all kinds of power frequency, harmonic, harmonic and noise interferences existed, and the feedback of PT secondary voltage signal amplitude which was easy to be affected was small, therefore, made use of phase sensitive detector to obtain the interference voltage amplitude and phase from the feedback signal. The simulation results showed that the phase sensitive detector could be relatively accurate extraction of signal, which made the measurement become more accurate.