Research On Parallel Three-phase PWM Rectifier Under Unbalanced Power Grid Conditions

Three-phase PWM rectifier has many excellent properties such as the DC voltage can be controlled, low current harmonics of grid side and unity power factor operation. It’s one of the hotspot in research of power electronic field. Rectifier in parallel operation conditions can not only increase the redundancy of system but olso can improve the power output, this feature is very suitable for high power occasion. However the parallel rectifier system will produce circular current and it’s ususlly appear unbalanced conditions in the actual power grid. These problems will lead to the unbalanced conditions of the AC current and reduce the efficiency of loads. This thesis mainly studies the problem about the AC current cannot achieve stability and circular current under rectifier parallel operation and unbalanced power grid conditions.Summarized the development of PWM rectifier and the parallel system by reading lots of literature at home and abroad, and analyzed the working principle of parallel runtime and circulation characteristics for three-phase PWM rectifier under unbalanced conditions, respectively. The parallel rectifier system mathematical model of circulation and the rectifier model in the three-phase static coordinate system and rotating coordinate system are established, based on which the rectifier double closed loop control system is established.To slove the problem that the AC current distortion of three-phase PWM rectifier under unbalanced conditions, parallel control strategy was put forward in this thesis. This design uses two rectifiers, one of them generates positive sequence current and the other one negative sequence current. The three-phase PWM rectifier achieved capacity expansion and harmonic suppression effect y by the parallel control.When the three-phase PWM rectifier parallel operation, there will many loops of circular current circulation appear. In order to eliminate the effects of the current circulation this thesis proposed coordinated control strategy, which can restrain the negative sequence and zero sequence circular current between the rectifiers, by controlling the positive sequence current achieve load sharing. Building the simulation three-phase PWM rectifier under unbalanced power grid voltages conditions by using Matlab. And buile the model of parallel operation condition in the same way. The results proved that each control strategy in this thesis is correct.Finally, the hardware parameters are determined according to the theoretical analysis and simulation. Each driver circuit and conditioning circuit are designed. Building the experimental platform by taking the TMS320F2812DSP as core. Using the C language design software program by the compiler software CCS. Through the simulation analysis and experimental debugging, the control effect of the AC current and DC voltage in PWM rectifier under unbalanced power grid conditions is better realized.