Research On Compensation Technology Of Three Phase Unbalanced Load In Low Voltage Distribution Network

In the low voltage distribution network,most of the user side is single phase load.Due to the uneven distribution of the load and the randomness of the user's electricity,the unbalanced three-phase load of the power supply system is caused,which seriously affects the power quality of the power grid.At present,most of the active compensation systems for the unbalanced three-phase load control adopt the four bridge arm topology,and the four bridge arm topology can only be used in the three-phase four wire system and is not conducive to the current sequence control.Aiming at this background,this paper adopts the three bridge arm circuit topology which is suitable for three phase three wire system and three phase four wire system,and designs the control algorithms for the two systems respectively.The key technologies such as the current sequence algorithm,the influence of the bus voltage fluctuation and the current tracking and control algorithm are deeply researched.The topology and working principle of the three bridge arm active compensation system are studied,and the mathematical model and compensation device,the energy flow model between the power grid and the load are set up in the synchronous rotating coordinate system.The current partial sequence method is studied.According to the different changing rules and phase sequence of each sequence current in coordinate transformation,the current partial order method based on synchronous rotating coordinate transformation is introduced.In view of the problem that low-pass filter can not effectively filter low frequency harmonics in the synchronous rotating coordinate transformation method,a hybrid low pass filter is proposed.The validity of the hybrid filter is verified by simulation.The control strategy for three-phase three wire system and three-phase four wire system is designed for three leg active compensation system.The DC bus voltage of the three bridge arm active compensation system has a fluctuation of two frequency doubling,which is finally reflected on the current loop,thus affecting the output and control of the current ring.This paper analyzes the cause of the two frequency doubling of the bus voltage,and puts forward a Proportional Integral,PI doubler.A voltage controller.At the same time,the compensation device in the three-phase four wire system still has the problem of the DC bus capacitor voltage equalizing.In order to avoid the imbalance of the capacitor voltage of the bus capacitor,the pressure sharing ring is designed to deal with it.In this paper,the quasi proportional resonance(Proportional Resonance,PR)controller is designed to compensate the zero sequence current,and the parameters that affect the performance of the quasi PR controller are studied.On the Simulink simulation platform,the three bridge arm active compensation system is simulated and analyzed,and the experimental platform is built to verify the three-phase three wire three-phase load unbalance compensation system.The simulation results and experimental results show that the compensation system greatly reduces the imbalance of three-phase current,and the three-phase load imbalance is effectively compensated.