Study Of Nonlinear Control Strategy For Unified Power Quality Controller

With the continuous improvement of user’s request for power quality in recent years, many countries in the study of the theory and experimental research of the unified power quality controller (UPQC) have been enhanced. UPQC is composed of series inverter and shunt converter. It has multiple power quality regulation functions like the distortion waveform compensation and the dynamic voltage quick recovery.In this paper, we focus on the study of the synthesis compensating method and the nonlinear control strategy for UPQC. On the basis of the study of the compensating methods respectively to the UPQC series inverter and shunt converter, the voltage and current synthesis compensating method of the UPQC based on dqO coordinates transform is studied. The method does not need to lock the phase of three-phase voltage by phase lock technique, so as to avoid the error caused by phase lock. The simulation results show that this method is good enough to make up for the deficiency of the existing UPQC detection method.UPQC is a multivariable, nonlinear and strong coupling dynamic system in running process. Its control methods include PI control and double loop decoupling control, fuzzy control, etc. But these control methods are often difficult to achieve the precise decoupling control. In this paper, two strategies of nonlinear control are used to design the controller of UPQC. The first method is the exact linearization approach via state feedback combined with the variable structure control. The nonlinear mathematical model in dqO coordinates of UPQC is built and by using the exact linearization approach via state feedback of nonlinear control theory they are decoupled into a pseudo-linear system. Then, the controller of UPQC is designed by use of the variable structure control theory. The second method is Passivity-based control theory. The nonlinear mathematical model in dqO coordinates of UPQC is transform into Euler-Lagrange equation and the system passivity is verified. Then, the controller of the series and shunt converter of UPQC is designed by use of the passivity-based control theory.Combined with the synthesis compensating method of the UPQC, the variable structure control model and the Passivity-based control model are built respectively by using MATLAB software to verify the effectiveness of the designed controller. The simulation results show that the designed two kinds of controller can solve power quality problems such as the voltage and current harmonic, voltage and load disturbances etc. well. They are not sensitive to the changes of voltage and current harmonic and external disturbances. In other words, they have strong robustness.