Research On Active Compensation Equipment Of Three Phase Unbalanced Load

In the low voltage distribution network,most of the user side of the system are single-phase load,and the power consumption is not synchronous.Therefore,the distribution system is prone to appearing three-phase imbalance,which has a serious impact on power quality.At present,most of the active devices for unbalanced compensation are three-phase three wire system,which can only eliminate the negative sequence current,but not the zero sequence current.Therefore,the research on the three-phase four wire load unbanlance compensation device is of great practical significance to improve the power quality and the safe and stable operation of the distribution network.In this paper,the key problems of unbalanced three-phase four wire system are studied,such as unbalanced current detection,power grid synchronization algorithm,modulation algorithm,current tracking control algorithm and so on.According to the characteristics of three-phase four wire load imbalance,the negative sequence current can be compensated and the zero sequence current of the neutral line can be compensated.The converter of the four bridge arm is selected as the topology of the device.The filter uses the LCL filter with four branches.In order to solve the unbalanced load current decoupling problem,this paper analyzes and compares the notch filter,time delay signal and decoupling method based on second order generalized integrator from the point of view of the anti-jamming capability.It is proved that the decoupling method based on second order generalized integrator has strong anti-jamming capability and good stability and suitable for compensation device.The simulation results show that the proposed method can realize the fast zero error decoupling of the fundamental positive sequence,negative sequence and zero sequence components.Aiming at the poor performance of the traditional PLL under unbalanced grid conditions.In this paper,from the view of the phase locked precision and the dynamic response speed,the power grid synchronization algorithm based on dual synchronous coordinate system decoupling and the power grid synchronization algorithm based on second order generalized integrator are studied.In view of the shortcoming of the system slow dynamic response,this paper makes an improvement,which is the SOGI-αβ hybrid grid synchronization algorithm.Four-leg converter has so many switch states and the modulation voltage is also complex.This paper designs a three-dimensional space vector pulse width modulation algorithm.The simulation results show that the modulation output voltage is consistent with the reference voltage.The control strategy of compensation device is the focus of this research.In order to improve the performance of control system,under the premise of satisfiy the system stability,an improved scheme is proposed to overcome the slow dynamic response of the traditional repetitive controller.The improved repetitive controller has a good ability to track the command current,and can accurately track the fundamental signal in the harmonic signal.In addition,the parameters of the voltage loop controller,the parameters of the DC bus capacitor and the hardware of the system are designed.Finally,the correctness and effectiveness of the system is verified by simulation and experiment.