Research On The Control Scheme Of PWM Rectifier Under Unbalanced Voltage

The three-phase voltage source PWM rectifier designed based on the condition of balanced grid voltage will appear abnormal operaeing condition under unbalanced grid voltage.The problems such as negative sequence current, voltage and current harmonic will come out. The negative sequence current results in current asymmetry, non-characteristic harmonic component and the total harmonic distortion increase.The dc voltage will also appear the non-characteristic harmonic component, which makes dc voltage waveform distortion happens. Therefore, it is necessary to study the control strategy of the-phase voltage source PWM rectifier under unbalanced grid voltage.The main contents of research of this paper are as follows.Some basic concepts and analysis methods of the unbalanced three-phase power grid and the mathmetical model and modeling analysis are discussed, based on the mathematical modeling, two unbalanced control strategies are focused on.One is the traditional double current power balance control strategy, and the other is the improved power balance control strategy.Meanwhile, two methods for the extraction of the voltage positive and negative sequence components.At last, the system of the three-phase voltage source PWM rectifier under unbalanced grid voltage including the inner current loop,the outer voltage loop and the parameters of the main circuit.In the above theory, the simulation of the control strategies under the balanced and unbalanced grid voltage are comparatively detailed in this paper, including the balanced control strategy, the traditional double current power balance control strategy,the improved power balance control strategy.The simulation results show that the improved power balanced control strategy can replace the conventional control strategy under balanced voltage, achieve the similar control effect with the double current power balance control strategy under unbalanced voltage, and realize the inhibition of the dc voltage and ac current harmonic components, the energy feedback and good active and static characteristic.