Research On Control Strategy Of Three-level PWM Rectifier Based On DSP

Rectifiers exist massively in power system and are commonly used as pre-converters in high-power electricity equipment. Due to switch characteristics of traditional rectifiers, the current injected into the grid is no longer standard sine waves. As a result, power factors of grid would be worsened and total harmonic current injected into the grid would upsurge. These factors would bring serious pollution to power environment,especially to high-power electricity equipment. In the meantime, Organizations domestic and abroad formulate strict standards about harmonic current allowed into the grid. With the development of control theories and the increasing of switching frequency, PWM rectifier technology come into the world. PWM rectifiers not only make grid-side power factor into 1, but also significantly reduce the harmonic current that flows into the grid. Based on the topology improvement of parallel connection of dual Boost single phase PFC,A new three-phase four-wire PWM rectifier topology is proposed,which uses less inductance so that cost is reduced and unidirectional magnetization of the inductance is avoided. The control strategy of three phase PWM rectifier can be used in single-phase situation,which is suitable for low power equipment.First, several kinds of PWM rectifier topology and a variety of control strategies are discussed and analyzed, then, The significance of the research and application prospect of three-phase four-wire PWM rectifier is elaborated.Through analysis of single-phase PWM rectifier topology, the relationship between inductance current ripple as well as output voltage ripple and the system can be figured out. By derivation of current ripple, lower value of the inductance is derived, subsequently, upper value of the inductance and lower limit value of the capacitor can also be figured out to meet the demands of the rectifiers. After that, design methodology for key components in PWM rectifiers is completed.Dual-loop control strategy for CCM and single-loop control strategy for DCM are adopted. Under the condition of CCM, small signal model is established. From the perspective of crossover frequency and phase margin, PI parameters of dual-loop are designed. Based on the relationship between inductance’s current and circuit parameters, critical power between DCM mode and CCM mode is found out. Simulation results show that dual-loop control strategy for CCM mode and single-loop strategy control for DCM is steady and credible.In addition, by comparing THD effect of LCL filter and L filter, idea of step by step is adopted to design LCL filtering PWM rectifier. By drawing bode pictures of LCL filter and L filter, LCL filter’s dual-loop control strategy invariance is approved under CCM mode. The theory is verified by simulation experiment under the environment of MATLAB/SIMULINK.