Research On The Control Strategy For Cascade Multilevel STATCOM Under Unbalanced Condition

In this paper, a brief introduction about the operation principle of cascade multilevel static synchronous compensator (STATCOM) is given firstly. Then the modeling is analyzed and parameters are designed for the main circuit of a±2.8kVar delta-connected cascade multilevel STATCOM. Furthermore, the key technologies of DC capacitor voltage balance control, control strategy under unbalanced condition, instantaneous current tracking control and single phase SVPWM are all researched.DC capacitor voltage balance control is always one of the key problems for cascade multilevel STATCOM. Its mechanism is analyzed in view of the energy conservation, which can draw a conclusion that the imbalance is caused by the inconsistency of energy absorbed and consumed by H-bridge. Based on this conclusion, changing the energy absorbed by H-bridge might be an effective way to balance the DC voltage. By analyzing the influence of the current and output voltage of H-bridge on the capacitor state (charging or discharging), a balance control strategy based on active voltage vector superposition is proposed, in which the voltage vector is determined by both the capacitor state and instantaneous DC voltage error. Matlab simulation has been done to verify this strategy.The control strategies for cascade multilevel STATCOM under unbalanced power system is another key problem. The influence of this imbalance is analyzed, from which it can be known that STATCOM outputs positive sequence, negative sequence voltage as well as third harmonic components when the system voltage is unbalanced and the individual phase instantaneous current tracking control strategy is introduced. A kind of phase-locked-loop is presented, which can capture the information about phase and amplitude of unbalanced system voltage quickly and accurately. Three compensation modes of cascade multilevel STATCOM are studied in this paper: reactive power control mode, voltage control mode and unbalanced load current compensation mode. Hybrid controller composed by proportional controller and repetitive controller is adapted to track the instantaneous current without steady-state error. And the detailed designation of the controller is given. Simulation results show that the proposed control strategy can maintain cascade multilevel STATCOM running normally and effectively under unbalanced condition. The comparison of simulation between hybrid controller and PI controller proves the former has a better performance on current tracking and harmonic suppression.A new single-phase space-vector PWM algorithm is proposed for cascade H-bridge converters, which is simple and suitable for any level cascaded converters as well as unbalanced condition. What's more, the existence of redundant state vectors provides possibilities for control performance optimizing. Two rules for redundant state vectors selecting are given out, while one is for reducing switching loss as well as balancing switching loads and the other is for balancing the DC capacitor voltage. Matlab simulations are been done to verify this algorithm and the selecting rules.