Study Of Three-phase PWM Rectifier Under Unbalanced Input Voltage

In this paper, it is studied that the control strategy of PWM rectifier under the unbalancedgrid voltage, on the basis of balanced grid voltage.Unbalanced voltage consists of positive sequence voltage, negative sequence voltage andzero-sequence voltage. Under the unbalanced voltage, for the presence of negative sequencevoltage, it will be difficult to get the angle of positive sequence voltage, then the network sidecurrent and DC bus voltage will be accompanied by harmonic components. So that thenegative sequence current must be controlled, as well as the positive sequence current. For theseparation of positive and negative sequence, three methods are provided, such as notch filter,low-pass filter and delay method.Under the unbalanced voltage, the negative sequence voltage generates negativesequence current. DC bus voltage will be accompanied by harmonic components, by theproduct of negative current and positive switch function. There are two control strategy underunbalanced voltage, the first one achieves symmetrical and sinusoidal network side current,and the second one eliminates DC voltage harmonic components.The control strategy of symmetrical current is achieved under positive synchronousreference frame, through the compensation of negative sequence voltage by the AC side ofrectifier bridge, the negative sequence current becomes zero, so the target of symmetricalcurrent comes true.If the DC bus voltage is stable under constant load, input power is stable, the presence ofnegative current is required. The internal model controller method under two-phase referenceframe and the four closed-loop control method under synchronous reference frame isproposed. Four closed-loop control method is based on the double-loop, increasing thenegative component of double-loop, have good effect. The internal model controller methodunder two-phase reference frame can make positive and negative current control under onechannel, relatively simple than the four-loop control structure. With contact voltage regulator, building a simple three-phase unbalanced voltage,transforming the existing experimental platform, correctness of symmetrical current controlstrategy is verified.