Research And Realization On Direct Power Control Of Three-phase Voltage-source PWM Rectifier

Three-phase PWM rectifiers, which have more and more widespread applications, is in a hot research area, because their ability of two-way energy flowing, as well as arbitrary AC current wave and power factor. Direct power control is a kind of method, which directly control converters' instantaneous power with closed-loop. The main content of this paper is to conduct a comprehensive in-depth study about direct power control in PWM rectifiers.According to the instantaneous power theory, the three-phase PWM rectifier's mathematical power model is established. By analyzing the energe flow of PWM rectifier's active power and reactive power, its working principle is powered on. Based on the mathematical model to examine the different effects for instantaneous power of each switch vector in the vector space, a non-fixed 18 sectors dividing method with a novel switching vector table is proposed. They not only can overcome the defects of the traditional one in reactive power control, resulting better steady-state and dynamic effects, but also can give a certain explanation to the lack of other tables.Under the features of direct power control, main circut components design methods are proposed, including: value of the AC side inductor, DC capacitor and DC side voltage. The inter-relationship between the system parameters is researched to guid system design. A concept of real-time gene is introduced to digital direct power control system, to scale the influence of the digitalized system. At the same time, the greatest tolerance value of the entire digital control system delay is given. Another concept: real hysteresis-band is also introduced to measure effect of digital control system. Dead-time is one of the most impacts on the waves' qualities. Through analysis of impact on the AC side voltage of the converter and grid current respectively, dead-time effect can be summed up as voltage-effect and current-effect to the PWM rectifier. The expressions of these two effects are dedused.The switch frequency of the direct power control system using hysteresis comparator is not fixed, which is a technical issue in practical needing be solved. By comparing to direct current control method using PI regulator, the contradictory relationship between the hysteresis-band and the switching frequency is analysed. Then a method of switching frequency limitation is proposed, which base on the novel 18 sectors space division method. By controlling the hysteresis-band of reactive power, the switching frequency can be regulated within a safty range, at the same time achiveing an improved wave quality. In order to achieve a direct power control strategy with constant switching frequency, a combining control method named DPC-SVM is proposed, which maintains the characters of simple control structure, no impact of coordinate transformation.In the paper, the stator flux concept is introducted from induction motor to three-phase PWM rectifier, which is defined as virtual flux: VF. And the instantaneous power can be estimated using of virtual flux. With virtual flux orientation, a new method of direct power control is gotten, called VF-DPC. This novle method has attractive merits, such as: saving voltage sensor and the low-pass filter behave of the integral. In the realization process, a second-order filter is used in place of pure integrator to solve the initial value problem. At the same time, a better filtering property is achieved. By combining space vector modulation method with VF-DPC, a constant frequency control strategy called VF-DPC-SVM is discussed in the paper, which is an ideal control method with great research value.