| The distribution network goes deep into the user terminal,and the structure is changeable and the shape is complex.The line asymmetric transposition and instantaneous single-phase grounding faults frequently occur,which easily lead to displacement overvoltage and arc grounding overvoltage.The distribution network goes deep into the user terminal,and the structure is changeable and the shape is complex.The line asymmetric transposition and instantaneous single-phase grounding faults frequently occur,which easily lead to displacement overvoltage and arc grounding overvoltage.The flexible grounding device can adjust the neutral point voltage of the distribution network by injecting current into the neutral point,so as to suppress overvoltage and ensure the reliable operation of the distribution network.Due to the randomness of the occurrence of grounding faults,the flexible grounding device operating in the displacement overvoltage suppression state is prone to overcurrent during single-phase grounding fault detection,resulting in overcurrent shutdown of the device and reducing the power supply reliability of the distribution network.In order to solve the problem of smooth switching between the two operating modes of flexible grounding device displacement overvoltage suppression and ground fault arc suppression,this paper studies a smooth switching control method for flexible grounding device operating modes,so that the flexible grounding device does not stop,smooth switching of the device between the two operating modes.Firstly,starting from the zero-sequence equivalent model of the distribution network and the control system model of the flexible grounding device,the paper deduces the expression of the transfer function between the output current of the flexible grounding device and the fault phase electromotive force.The switching process is quantitatively analyzed from the two aspects of transient state and steady state,and the overcurrent generation mechanism during mode switching is revealed.Then,the paper proposes a smooth switching control method of flexible grounding device mode combining software and hardware.This method adopts the overcurrent suppression control strategy based on virtual resistance in software,and limits instantaneous overcurrent by introducing virtual resistance in the voltage outer loop.On the hardware,an active blocking strategy based on the zero-sequence voltage amplitude exceeding the limit is adopted.When the zero-sequence voltage amplitude difference exceeds the limit,the instantaneous overcurrent is suppressed by actively blocking the control pulse.Finally,a 10 kV distribution network flexible grounding system model is built in the PSIM simulation software,and a detailed simulation verification is carried out on the smooth switching control method of the flexible grounding device operation mode proposed in the paper.The simulation results show that the smooth switching control method of the operation mode of the flexible grounding device proposed in this paper can effectively suppress the neutral point displacement overvoltage,the ground fault arc grounding overvoltage and the overcurrent generated at the moment of switching between the two operation modes.When the point displacement overvoltage is suppressed,the ground fault suddenly occurs and the flexible grounding device is shut down.The smooth switching of the two operation modes is realized,and the reliability of the flexible grounding device and the distribution network is effectively improved. |
