The Adaptive Sliding Mode Control Strategy Of Three-phase PWM Rectifier Under Unbalanced Grid Voltages

As a new generation of green converter devices, three-phase PWM rectifier has re-ceived more and more attentions and has been widely used in power converting area dueto its advantages such as bidirectional power flow, low harmonic distortion of line current,regulation of input power factor to unity and so on. However, the three-phase voltage un-balance is widely existed because of the use of unbalanced loads and nonlinear loads in areal grid. The unbalanced voltages will seriously influence the performance of the three-phase PWM rectifier. As a consequence, to find a way to deal with the control problem ofthe three PWM rectifier under unbalanced gird voltage is really important.This paper firstly derives the instantaneous power model under unbalanced voltagebased on Kirchhof’s basic law and instantaneous power theory. By using the model,this paper analyses the cause of the100Hz ripples in the active and reactive power aswell as discusses the best control performance which can be achieved.Then, a modifiedleast squares method is proposed in order to realize online monitoring. The modified leastsquares method reduces the amount of the computation and the sensitivity to the round-oferror by the methods of separating the time-varying part from constant part as well as de-composing matrix inversion. Based on the above, a positive and negative sequence com-ponent separation method is derived combined with symmetrical components method.This method enhances the ability of harmonic suppression as well as guaranteeing thereal-time separation speed. On the basis of direct power control, this paper modifies thecontrol strategy of three-phase PWM rectifier under grid unbalance. This strategy takesthe100Hz ripples in the active and reactive power into account and uses a power compen-sating part to eliminate the influence. The control strategy includes two parts structurallywhich are basic controller and the anti-disturbance controller. The basic controller aims tokeep the basic performance of the control strategy, while the anti-disturbance controller’s goal is to increase the robustness of the control strategy. The basic controller is built af-ter decoupling and linearizing by inverse system method, and the anti-disturbance adoptsadaptive sliding mode method.The proposed strategy can solve the problems of the three-phase PWM rectifier under unbalanced grid voltages including current distortion, powerripples and decrease of the power factor efectively. The experiments based on Simulinkand the dSPACE prove the efectiveness of the proposed control strategy. The strategy can eliminate the influence caused by the unbalanced grid voltage efectively, achieve the con-trol goal accurately. Besides, the strategy also has advantages of rapid dynamic responseand high degree of robustness.