Grid Voltage Is Unbalanced Three-phase Pwm Rectifier Control Strategy

Under the operating conditions of unbalanced input voltage, there exists characteristic harmonics such as 6,12,18 etc and uncharacteristic harmonics such as 2,4,8,10 etc in output dc voltage of 3-phase PWM rectifier, which will creat odd harmonics in ac current through the switching function. The second-order dc voltage and three-order ac current can cause perfomance deterioration of PWM rectifier due to its low frequency and relative large amplitude. Because of this the second-order hamonics in dc voltage caused by the unbalanced input voltage should be eliminated. In this paper dual current control scheme for PWM rectifier under unbalanced conditions is analyzed theoretically and simulation is conducted. Based on the control scheme the paper proposes a virtual-flux-based ac voltage-sensorless control scheme for PWM rectifier under unbalanced input voltage.Firstly, the simulation model of dual current control system is established using the MATLAB simulation tools. Simulations are conducted in order to analyse and research the control scheme. Secondly, the situation of reactive power is very complex under unbalanced conditions in 3-phase network and different definition of reactive power is suitable for different control area. Due to this, the application of two different reactive power definitions in PWM rectifier unbalanced control scheme is discussed. Finally, the design principle of ac input inductance under unbalanced control scheme is discussed, which is compared with the design principle under balanced control scheme. The results show that the design principle of ac input inductance under unbalanced and balanced control scheme is very different. Under balanced control scheme second-order harmonics in output dc voltage is eliminated through restraining the negative sequence current. The ac input inductance is less, second-order harmonics in dc voltage is larger. Therefor, in order to eliminate second-order harmonics of dc voltage, the value of ac input inductance should be large as possible as on the premise that transient current tracking fulfill the requirement. While under unbalanced control scheme second-order harmonics of dcvoltage is eliminated through adding the negative sequence current. Because of the additional negative sequence current, the 3-phase ac inductance will produce instantaneous power with second-order ripple, which will lead output instantaneous power to contain second-order ripple under the constant of input instantanous power. The ac input inductance is larger, second-order harmonics of dc voltage is larger. So, in order to eliminate second-order harmonics of dc voltage, the value of ac input inductance should be small as possible as on the premise that the harmonic distortion of ac current can fulfill the requirement. Simulation is conducted for confirmation.In addition, the paper propose two new ac voltage-sensorless control schemes based on virtual flux. One is virtual-flux-based instantaneous power at the input of the rectifier control scheme, which is on the basis of dual current control scheme and estimate the unbalanced input voltage using the concept virtual flux. Simulations confirm that sensorless scheme can estimate source voltage very well and the same control performance can be achieved as the dual current control scheme. But under the control scheme the output dc voltage will be affected by the second ripple of instaneous power on the 3-phase input inductance. So the paper proposes another ac voltage-sensorless control scheme, that is, virtual-flux-based instantaneous power at the poles of the rectifier control scheme. It can be conveniently to predict the instantaneous power at the poles of the rectifier using the concept of virtual flux. Thus the control of instantaneous power at the poles of the rectifier can be realized. The output dc voltage will not be affected by the second-order ripple of instaneous power on the 3-phase input inductance. The control scheme still can eliminate second-harmonics of dc voltage effectively even under the extreme unbalanced input voltage. Simulation results confirm the feasibility of the control method.At last a experiment system of 3-phase PWM rectifier is designed based on DSP. The design of hardware circuit and SVPWM software are finished.