Develop Of Three Phase Four Wire Dynamic Voltage Regulator

In three phase four wire low-voltage distribution network,due to the rapid increase of power load,especially the increasing capacity of impulse and nonlinear load,make more and more power quality problems in power grid,in which the voltage sag is one of the problems with the highest frequency.At present,the main solutions include solid-state switch(SSTS),uninterruptible power supply(UPS)and serial type dynamic voltage regulator(DVR).But there is slow switching speed,high maintenance costs and small compensation capacity problems.this paper propose a scheme which is based on super capacitor energy storage type three phase four wire system parallel dynamic voltage restorer(DVR).This scheme solves the problems of slow compensation speed,deep voltage drop or short-time interruption of power supply.Dynamic voltage regulator uses three-level DC/DC converter to control the charge and discharge of super capacitor.When the converter works in the Buck mode,the single current loop is used to charge the super capacitor.When the converter works in Boost mode,it adopts double closed-loop control of high voltage side voltage and inductance current.In order to solve the problem of over-current in zero voltage charging of super capacitor and the problem of maximum power limitation of DC/AC converter,this paper puts forward the method of phased charge control,which mainly includes four methods: hysteresis current limiting charge,constant current charge,constant power charge and small current make-up.In the discharge control of supercapacitor,in order to realize the stable control when the load power changes suddenly,increase the duty ratio feedforward,improve the dynamic response of the control.In order to solve the problem of neutral point potential imbalance in three-level converter,an algorithm based on the balance of capacitance and ampere second and the invariable output voltage is added.By calculating the duty cycle distribution factors of upper and lower half bridge pulses,to achieve the effect of balance control.The dynamic voltage regulator uses tnpc three-level converter to form a three-phase four wire system circuit to supply power for the load.In the grid connected mode,the DC side voltage and inductance current double closed-loop decoupling control is adopted to realize the unit power factor control.In the off grid mode,the output AC voltage and inductor current double closed-loop PI decoupling control is adopted.In order to ensure the output voltage stability,the load current feed-forward control is added to improve the anti disturbance ability of the system.In order to solve the problem that the output voltage contains negative sequence component caused by unbalanced load,the separation method based on the reduced order generalized integrator is used to separate the positive and negative sequence in DQ rotating coordinate system.To counter the problem of nonlinear load lead to the output voltage distortion,using current loop resonance PR control method,through the current loop to compensate the harmonic current,to ensure that the output voltage distortion within the scope of the permit.Based on the above research,the whole control process of DVR is simulated and analyzed to verify the correctness of circuit parameter calculation and the feasibility of control strategy.A three-phase four wire DVR prototype is developed.The process of parameter calculation,selection of main circuit components and design method of control system are described in detail.The control software of the system is developed based on the dual core DSP chip,and the software architecture of multiprocessor parallel computing is designed by making full use of the hardware resources on the chip.Through the DVR prototype experiment,all the indexes are tested to verify the effectiveness of the above control.