Analysis And Control Of Quadruple-Active-Bridge Converter Based Unified Power Quality Conditioner

In the low voltage distribution network,the voltage-and current-related power quality issues have always been the focus of attention in the power system.Due to the excellent electrical characteristics of providing highquality grid current and load voltage simultaneously,the unified power quality conditioner(UPQC)has broad application prospects in the areas of power quality control and fault current limitation.Recently,with the rapid development and wide application of power conversion technology,new requirements for the UPQC have been put forward,including the power density,control performance,functionality and adaptability of the application environment.In view of the above requirements,this paper mainly focuses on the topologies,control strategies and stability for the UPQC system.The main contributions are as follows:(1)A three-phase UPQC topology based on a quadruple-active-bridge(QAB)is presented to improve the power density.It is composed of the series converters made up of three H-bridge converters,a three-phase shunt converter and a QAB converter.In the three-phase UPQC-QAB,the QAB converter connects the series converter with three DC ports and the shunt converter with one DC port,which can completely avoid the use of conventional line frequency transformer.It leads to higher power density.Besides,the QAB converter could function as an energy router to provide multiple DC-link voltage access ports in the UPQC,which increases the flexibility of the UPQC.(2)The small signal models of the UPQC-QAB with different compensation strategies are established and their performance characteristics are revealed,which provides the guidelines for the selection of compensation schemes in specific applications.Based on the three-phase conventional and dual UPQC-QAB,this article establishes the small signal models of the UPQC-QAB with consideration of the effects of the phase-locked-loop and grid impedance.Furthermore,from the aspects of topologies and system models,the related performance of them are analyzed and compared under different conditions.The comparative analysis results show that the dual UPQC-QAB can provide better load voltage and grid current quality under most conditions when compared with the conventional UPQC-QAB.(3)For the harmonic oscillation issues of the UPQC-QAB in weak grid,the stability of the UPQC-QAB based on impedance model is studied and the effects of power grid,load and phase-locked-loop on system stability are revealed.Ignoring the effect of DC-link voltage in the UPQC-QAB system,its performance will be affected by the electrical and control parameters of series and shunt converters,as well as power grid and load.Based on the small signal models with consideration of the phase-locked-loop and load impedance,the article further deduces and establishes the output impedance model for the conventional and dual UPQC-QAB.Applying the Generalized Nyquist Criterion,the stability of the conventional and dual UPQC-QAB are compared and discussed.The impedance analysis results show that the stability margin of the UPQC-QAB will be affected by multiple factors,and the dual UPQCQAB has higher stability margin in most cases compared with the conventional UPQC-QAB.The findings provide the guideline for the stable operation of the UPQC-QAB in weak grid.(4)For the cross-coupling issue among the DC-link voltages in the UPQC-QAB system,an extended-state-observer(ESO)based second-order sliding-mode-control(SOSMC)is proposed.The proposed SOSMC-ESO controller is formed from the input matrix diagonalization,ESO and SOSMC.The input matrix diagonalization can eliminate most of the coupling terms of the QAB and the SOSMC-ESO is used to deal with the remaining coupling terms.Thus,by adopting the proposed SOSMC-ESO decoupling strategies,the multi-input-multi-output(MIMO)system can be transformed into several independent single-input-single-output(SISO)systems.Compared with the conventional PI control,the SOSMC-ESO scheme has the advantages of strong robustness and good decoupling performance.(5)Taking into account the power quality and fault overcurrent issues simultaneously,a dual functional three-phase UPQC-L-QAB topology and related control schemes are proposed.Due to the removal of the series line frequency transformer,the proposed UPQC-L-QAB could eliminate the effect on the series transformers caused by the nonlinear load.In addition,since the series converter is located at the load side,it can not only work as a voltage source to control the load voltage when the load is normal,but also be controlled as a current source to limit the load fault current when the load fault occurs.Furthermore,model predictive control schemes are proposed to control the series and shunt converters,which are convenient for smooth mode switching between the UPQC and fault current limiter operation modes.