Injection Current Based Suppressing Method And Realization Of Three-phase Unbalanced Overvoltage For Distribution Networks

Distribution network develops very fast in recent years,and power distribution system goes deep into user terminals.These changes take the more and more high request to the safe and stable operation of distribution networks.Unbalanced three-phase parameters to ground will cause three-phase unbalanced voltage in distribution networks with ineffective earthed.If arc suppression coil runs near resonance point and resonant with capacitance to ground,three-phase unbalanced overvoltage will be caused and greatly affect the normal operation of distribution network.Thus,suppression of unbalanced overvoltage means significantly to power supply reliability.Firstly,the traditional three-phase unbalanced overvoltage suppression methods in distribution networks with ineffective earthed was analyzed and the shortages of these methods was contrasted.The uniqueness of optimal injection current was demonstrated by the inhibition mechanism of three-phase unbalanced overvoltage.Relationship between injection current and zero-sequence voltage was derived.The suppression method was proposed which optimal phase of injection current was searched first and optimal amplitude was searched next.The method was proved by simulation analysis.Then,control method which combining iterative method with proportional resonant control was brought out by this paper.First,iterative method is used to seek the optimal phase and the optimal amplitude of injection current.Then the difference between optimal injection current and actual current was given into the current closed-loop.The actual current is used to trace the given reference current and output the optimal injection current ultimately.According to the simulation analysis of unbalanced overvoltage suppression under three different working conditions,control strategy is proved to be feasible.At last,the hardware circuit and software program of the experimental prototype was briefly introduced in this paper.The accuracy of the theory,scientificity of simulation and rationality of the prototype designing was proved by experimental results of flexible grounding prototype under two working conditions.The simulation and prototype experiment showed that,injection current basedthree-phase unbalanced overvoltage suppression method can suppress the neutral point displacement voltage to zero.This method doesn’t need measuring the parameters to ground,and cover the shortages of the traditional methods.The proposed control strategy which combines iterative method with proportional resonant control can track the given reference current steadily.Flexible grounding prototype can suppress unbalanced overvoltage quickly and effectively under a variety of working conditions,and even suppress the neutral point displacement voltage to zero with further parameters optimization.