Research On The Control Strategy Of A Unified Power Quality Controller Based On Modular Multilevel Converters

Unified power quality controller(UPQC),as a comprehensive device which can compensate both voltage and current,is mainly used in low voltage distribution network.In order to improve the voltage and power level of UPQC,the modular multilevel converter(MMC)is introduced into UPQC,which makes up for the blank of UPQC in high voltage and high power applications.When the topology of MMC-UPQC is determined,the compensation effect mainly depends on the control strategy,so it is necessary to study the control strategy.Passivity based control(PBC)is a nonlinear control from the view of energy.By redistributing the system energy,the error storage function of system approaches to zero,so the actual value of the controlled quantity follows the desired one.PBC has the advantages of less adjustable parameters,strong robustness and ensuring global stability of the system.The PBC for UPQC is deeply studied in this paper.In chapter 1,the research background and significance of the subject are described,the main power quality control technology and the development of UPQC are introduced,and the advantages of MMC used in UPQC are analyzed;in chapter 2,the working principle of MMC-UPQC is analyzed in detail,the mathematical model under the dq0 frame of reference is given,and CPS-SPWM and NLM applied for MMC are introduced;in chapter 3,a compensation detection method without phase lock loop based on park transform is presented,and a qualization controller for the sub-module capacitor voltage is designed;in chapter 4,in inner loop PBC is applied to the controller design for the series and shunt links of UPQC,in outer loop PI control is adopted to provide the desired current,in order to solve the problem that the precise mathematical model of the system can hardly be obtained,in outer loop PI control is replaced by active disturbance rejection control(ADRC)with more robustness,finally the compensation effect of the two kinds of outer loop control is compared;in chapter 5,the control of UPQC with photovoltaic energy storage is studied,which makes UPQC have the functions of photovoltaic grid-connected generation and interruption compensation,so the economy and practicability of the device are increased;in chapter 6,the full paper is summarized and prospected.