Research On Control Strategy Of Parallel Dynamic Voltage Regulator

As an effective solution to the problem of voltage drop,Dynamic Voltage Regulator(DVR)has a wide range of application scenarios in the distribution network,the dynamic response time and power supply reliability of the system will directly affect the power quality of the load,which is one of the most important performance indicators of the prototype.In this paper,the dynamic performance of DVR is improved from the aspects of grid voltage drop detection,seamless switching control and off grid control strategy;from the analysis of parallel expansion,the reliability of power supply is enhanced.The main research contents are as follows:(1)The DVR main circuit adopts a two-stage parallel structure based on super capacitor energy storage.By analyzing the circuit modeling of the front three-level bidirectional DC converter and the rear TNPC converter under the grid connected and off grid conditions,the corresponding control strategy of DVR device is proposed,simulation and prototype power experiments prove that DVR device can achieve normal function.(2)Fast and accurate detection of grid voltage drop is an important premise for the normal operation of DVR,traditional detection methods are not suitable for single-phase voltage drop conditions and cannot meet the response time requirements of DVR.In order to solve the above problems,the software phase-locked loop based on SOGI integrator is used to accurately obtain the grid voltage phase;on the basis of the delay small-angle detection method,a method of voltage drop judgment based on the amplitude fluctuation characteristics of grid voltage drop is adopted.when the grid voltage drops,this method can quickly judge the voltage sag by predicting the amplitude change trend in advance,it can not only reduce the detection delay,but also avoid the impact of small disturbances in the grid-side voltage on the detection.(3)After the DVR system detects the grid voltage drop,the working mode of the whole machine needs to be quickly switched to the off-grid compensation state.In this paper,a hysteresis PWM current control strategy suitable for TNPC topology.is analyzed,by controlling the converter output to switch between positive,negative,and zero levels,the grid-side current can be quickly tracked,the simulation found that this scheme is greatly affected by the sampling period and is not suitable for engineering design.In order to solve this problem,a back-pressure forced shutdown strategy is adopted,which indirectly realizes current transfer by controlling the voltage across the inductor.After the grid-side voltage is restored,in order to avoid too long pre-synchronization time,an open-loop phase synchronization scheme is adopted to achieve the pre-synchronization for the voltage on the machine side and the grid side,finally,the switch from off grid to on grid mode is completed.(4)The traditional PI double closed-loop control strategy cannot achieve fast voltage tracking output.In order to solve this problem,the control system of the rear TNPC converter is modeled accurately under off grid condition,then according to the requirement of regulation time,the PI-P double loop control system is designed based on the principle of pole zero cancellation and dominant pole assignment.The simulation and experimental results prove that the improved prototype has excellent dynamic response performance and meets the design requirements.(5)In order to improve the reliability of the power supply of the DVR,a parallel expansion strategy is adopted in this paper.Based on the principle of master-slave control,a simplified parallel circuit and control architecture are designed,the CAN communication and interconnection lines are used for information exchange;at the same time,the stability of the parallel system is analyzed through complex vector modeling,the corresponding digital delay size under the critical stable state is deduced,the above control strategy and theoretical analysis are verified by simulation and experiment.