Analysis And Control Of Unified Power Quality Conditioner(UPQC)

DFACTS/CUSPOW, based on the power electronics technology for power distribution system, has drawn more attention since 1988. Unified Power Quality Conditioner (UPQC), composed of a parallel and series conversion system, is the most promising among all the DFACTS devices. It cannot only improve the network current quality but also the load voltage quality. This dissertation is dedicated to investigate the application of three-phase four-wire UPQC system in distribution network.The operation principle of three-phase four-wire UPQC system is analyzed. It shows that the combined power quality regulation of UPQC can be achieved by controlling the series converter as a sinusoidal current source and the parallel converter as a sinusoidal voltage source. The static operation principle is deeply analyzed based on the ideal control and the equivalent circuit model. The equations of system power flow, compensated voltage and current are established. The active and reactive power flow among two converters, power network and load are investigated under different operated conditions. Furthermore, the factors influencing system VA capability are also discussed. Two difference topologies are compared, including their application, merits and drawbacks. Moreover, the different connection types for secondary side of series transformer are analyzed in the three-phase three-wire or three-phase four-wire system. The analysis indicates that the Y-type connection with neutral line for series transformer is suitable for the three-phase four-wire UPQC system. A new simulation strategy based on Matlab Simulink environment is also introduced.Mathematical models of the three-phase four-wire series converter and parallel converter are established in ABC frame and dq0 frame respectively. The models show that each phase of the series/parallel converter is individual in ABC frame and 0 axis is independent of dq axis in dq0 frame. The current control strategies for series converter of UPQC are studied under ideal and non-ideal input voltage conditions. The influences of non-ideal input voltage are analyzed. The output dc voltage has the second harmonic and input current has the third harmonic due to the unbalanced supply. Nth input voltage harmonic results in the (N+1)th and (N-1)th harmonics in the dc voltage and Nth harmonic in the input current. In order to suppress the influence of non-ideal input voltage, two effective control methods are introduced. One is three-phase ABC individual control and the other is dq0 axis control. Special treatments and considerations for current controller are also indicated. The voltage unbalance between the two group capacitors is analyzed and a simple and valid method is provided. Simulation and experimental results verified the validity of the two control methods.The voltage control strategies for parallel converter of UPQC are investigated. The non-ideal load characteristics are discussed and their influences on output voltage are also indicated. Furthermore, a three-dimensional space vector modulation technique is introduced and its validity in three-leg four-wire converter is verified by simulation results. Several control methods are compared in order to suppress the influences of non-ideal load. These methods include three-phase ABC individual control, dq0 axis voltage and current dual loop control, dq axis negative sequence control, dq axis harmonics control, combined repetitive algorithms ABC control. Simulation and experimental results verified the effectiveness of these control methods.A 10kVA prototype of the three-phase four-wire UPQC is designed and built. The parameters of power circuit are designed and the coordinated control systems based on two digital signal processors (DSP) for series and parallel converter are introduced. Program flows are also provided. The instantaneous circuit simulation model of the whole system is built with Matlab Simulink. The basic characteristics of the UPQC are well verified by a large number of simulation and experimental results under fluctuating utility voltages and various linear and non-linear loads.