Study On Control Method Of VIENNA Rectifier Under Unbalanced Power Grid

With the development of industry,in order to meet the increasing power demand and adapt to the complex and changing power environment,a large number of power electronic devices have been widely used in power grids.Although traditional phase-controlled rectifiers and diode rectifiers are widely used in high-power applications,a large number of low-order harmonics will be generated on the grid-side input current due to those devices,which will cause power quality degradation and it is one of the main sources of grid harmonics.Compared with the traditional rectification topology with power factor correction,the three-phase three-level VIENNA rectifier has the following advantages: 1)the voltage stress on the power device is half of the output DC voltage;2)the total harmonic distortion of the input current is comparatively low;3)the power and control circuit are relatively simple.Therefore,VIENNA rectifier is widely used in active filters,aviation power,wind power generation and other occasions.Due to the influence of many non-ideal factors such as external interference,parameter asymmetry and three-phase load fluctuations,the three-phase voltage unbalance of the AC grid will be caused.Therefore,realizing the stable operation of the VIENNA rectifier under the imbalanced power grid condition and improving the power quality at the same time have become the key problem to the control of rectifer.In this paper,the in-depth research on the control strategy of three-phase three-level VIENNA rectifiers under unbalanced grid voltage conditions is carried out.This paper will start from the following aspects:Firstly,the topology and working principle of VIENNA rectifier are analyzed.Based on instantaneous power calculation theory,mathematical model in dq coordinate system and ?? coordinate system are established under balanced grid voltage condition and unbalanced grid voltage condition.Secondly,based on the mathematical model of VIENNA rectifier,this paper designs the double closed-loop PI control strategy under balanced grid voltage condiition,and discusses the parameter design of the PI controller.Then,based on the voltage and current double-loop structure,a control strategy of sliding mode variable structure is proposed.A voltage and current controller was designed by combining auto-disturbance rejection technology and sliding mode approach law.Finally,Finally,the two control strategies are simulated and verified.Then,considering the unbalanced grid voltage condition,this paper studies and comparatively analyzes the three positive and negative sequence component separation methods,and adopts a cross-decoupled dual complex coefficient filter phase-locked loop with simple design structure and strong anti-interference performance.At the same time,by analyzing the grid voltage complex vector and the instantaneous power flow,the traditional dual-loop PI unbalance control strategy is desgined However,considering the complex parameter design of the method,an unbalanced control strategy based on sliding mode variable structure is proposed.This strategy takes into account the impact of the secondary ripple of active and reactive power.The sequence double-loop independent structure and the hyperbolic tangent function sliding mode control algorithm are introduced into the current loop,which improves the system's robustness and anti-interference performance.Through MATLAB / Simulink simulation,it is compared with the traditional unbalanced control strategy to verify the effectiveness and feasibility of the control strategy based on sliding mode variable structure under the condition of unbalanced power grid.Finally,based on the theory in this paper,a prototype of the VIENNA rectifier experiment was designed.The software control part and hardware circuit design of the prototype were introduced to verify the correctness of the control algorithm.