Application Research Of Harmonic Impedance Measurement Based On Power Electronic Disturbance Technique For Ground Capacitance Detection

The neutral non-effective grounding systems are widely applied in MV distribution networks in China. Recent years, with the expansion of the distribution networks, the increasing number of distribution feeder, as well as the enlargement of line-to-earth capacitance, the arc caused by a single-phase grounding fault can not self extinguish. Besides, higher voltage will also come into being, which may do harm to the sound phases and make it being two phase short circuit. Accordingly, so as to ensure reliability of the power system, the devices of arc-suppression coil which have auto tuning function are extensively used to make compensation for fault current. In the resonance grounding net, the arc-suppression coil makes it up to the capacitive current that moves through earth points. On the other hand, the safety and reliability of power system itself are ensured as well. In order to make the most efficient compensation effects, inevitably, System Grounding Capacitance should be accurately detected.Based on generalizations of different calculation methods, herein introduces a new way to measure System Grounding Capacitance with disturbance technique in power electronic equipment. As for the preset Petersen-coil, by switching the power electronic switch of its damping resistance, the passage intends to present a short-time control on its conducting state, so as to generate the needed disturbance voltage and current that contains affluent frequency components. With the signal of these voltage and current as well as the measurement of harmonic impedance, System Grounding Capacitance can be measured to make for the preset Petersen-coil’s self-tuning. During the normal working hours, disturbance voltage and current that needed would be obtained at two sides of the thyristor, whose working conditions will be temporarily changed by switching the damping resistance, and that means the damping resistance can be deactivated in a short time. Due to linear approximations of the power system, there is a liner relationship between disturbance voltage and disturbance current at different frequency. Thus System Grounding Capacitance can be worked out based on the analysis of measurement of harmonic impedance and equivalent fault circuits in the system itself.The passage first introduces the scheme’s working principles. By equivalent simplifying the power network model which based on fault analysis and Circuit theory, and by optimizing the model which based on disturbance technique in power electronic, two different expressions of System Grounding Capacitance and signal duration are presented at the begining. In view of the errors of detected results that exist at specific frequencies, Least Square Method will then be applied to optimize the calculation result, so as to ensure its accuracy. Then, simulation experiments could be used to verify the feasibility and effectiveness of this measurement. By setting simulative parameters, we can promote simulation analysis on resonant earthed system in different situation, such as different harmonic frequency, varied thyristor triggering angles, district out-of-resonance degrees and unbalanced degrees. System Grounding Capacitance can be worked out then. Meanwhile, research on influences of neutral point displacement voltage can be defined due to the removal of damping resistance. Finally, further validation of it is done by Lab Tests. Some practical instructions on the thyristor’s triggering angle and minimum displacement voltage are listed according to results of simulation experiments.A feasible scheme to work out System Grounding Capacitance is provided in this passage, which has an active significance for the Peterson-coil compensation system. Moreover, application of the disturbance technique in power electronic can be enlarged in power system correspondingly.