Research On Control Strategy Of Cascaded Statcom Under Unbalanced Grid Voltage

With the rapid development of modern industry, more and more nonlinear and impact loads with low power factor are put into power grid. Now imbalance of grid voltage is widespread and will cause a lot of harmful effects on STATCOM if we only use the control strategy assuming an ideal circumstance. This thesis focuses on the control strategy under unbalanced grid voltage based on cascaded STATCOM. Simulation results show the accuracy and effectiveness of the control strategy proposed in this thesis.Firstly, the principle of reactive power compensation of STATCOM has been analyzed. The mathematical model under balanced grid voltage is built, and then the control strategy is proposed. The thesis presents several methods of reference current detection and the principle of CPS-SPWM method applied to cascaded STATCOM. Simulation results prove this modulation method can effectively reduce harmonics.The application of the symmetrical component method and a new PLL under unbalanced system is investigated, and the sequence separation method is employed in the analysis on positive and negative sequence components of the unbalanced grid voltage or current. Simulation verifies using a second harmonic filter can get good results. On the basis of sequence separation, a negative-sequence voltage feed-forward control method is proposed to prevent over-current. Theoretical analysis and simulation results demonstrate this control strategy can effectively improve the degree of symmetry and reduce the amplitude of the output current, so it can prevent device from quitting running under unbalanced system.In order to realize the compensation of reactive power and negative sequence component, this thesis presents a dual current control method. The positive and negative sequence components of load current are detected. Then we can get the reference values of output voltage in independent dual sequence current loop using a PI regulator and the decoupling control method. Finally, it can achieve the goal of compensation and the system provides only active power and maintains unity power factor. The theoretical analysis and simulation results verify the accuracy of the proposed method.