Research On Three-phase Unbalanced Compensation Technology For Low Voltage Distribution Network

Low-voltage distribution systems in our country are mostly used in agricultural load and household load.The size and running time of these loads are uncontrollable,which leads to the three-phase imbalance in low-voltage distribution system,which seriously affects the quality of power supply.At present,the means to deal with the three-phase imbalance are relatively lacking,and the technology is not mature enough.In view of this background,the three-phase current imbalance compensation technology in low-voltage distribution network is studied in this paper.Firstly,according to the characteristics of zero sequence current passing through the neutral line when the load imbalance occurs in three-phase four-wire system,the three-level neutral-point clamped(NPC)inverter is selected as the main circuit topology of the compensation device designed in this paper.The operation principle is analyzed and the mathematical model is established in ??0 coordinate system.Secondly,in view of the shortcomings of the traditional current detection method because of the use of phase locked loop(PLL),the positive and negative sequence current extraction method based on cascaded filter is designed to extract the positive and negative sequence current in the case of voltage distortion.The simulation results show that the cascaded filter can quickly extract the positive and negative sequence components of the fundamental wave in the unbalanced current.The control strategy of the compensation device determines the compensation performance of the device.The mathematical model of the three-level compensation device in??0 the coordinate system is analyzed.It is found that there is no coupling relationship among the three bridge arms of the three-level inverter and can be controlled separately.According to this characteristic,the reference signal is tracked and the instruction signal is generated by the non-difference beat control of the embedded repetitive controller.The space vector pulse width modulation strategy is used to control the state of each bridge arm switch,and the positive and negative small vectors are used to balance the midpoint voltage.The output of the phase voltage with three levels is realized,and the utilization ratio of DC side voltage is improved.Reduce the harmonic content of the output waveform.The simulation model of the device is built in the environment of MATLAB.The simulation results verify the feasibility and effectiveness of the control strategy and modulation strategy.Finally,on the basis of sufficient theoretical research and simulation model,the physical experiment platform is built,and the load compensation experiment of resistance-induced unbalanced load is carried out.The experimental results show that the compensation devicebased on three-level midpoint clamp inverter can effectively solve the problem of three-phase current imbalance in low-voltage distribution network,and can quickly compensate the new unbalanced state when the load changes suddenly.