Abstract: This paper introduces the mathematical model and working principle of vector control for a three-phase voltage-source PWM rectifier based on SVPWM. Based on this, two control strategies—one with and one without feedforward decoupling—are compared in terms of power factor, harmonic content, and DC voltage stability. The results show that while the model of the control strategy without feedforward decoupling is simpler, it suffers from drawbacks such as low power factor and high harmonic content. Under the same conditions, simulation results demonstrate that the strategy with feedforward decoupling exhibits superior control performance.
Keywords: rectifier; feedforward decoupling; vector control
Introduction
Research on PWM rectification technology began abroad as early as the 1970s. From the late 1980s, with the advent of fully controlled devices, research on PWM high-frequency rectification using fully controlled devices reached its peak. PWM rectification technology has strong advantages in suppressing harmonics and reactive power compensation, and has advantages such as near-sinusoidal grid-side current input, controllable grid-side power factor, bidirectional energy transmission, and fast dynamic response speed.
Currently, voltage-oriented PWM rectifiers are widely used, primarily employing voltage-oriented vector control. A voltage-oriented PWM rectifier controls two variables: the rectifier's output voltage and its input current. Vector control based on d-q coordinate transformation controls the input current by regulating the active and reactive currents of the PWM rectifier. This control strategy not only possesses the advantages of direct current control, such as fast dynamic response, good steady-state performance, and inherent current-limiting protection, but also eliminates current steady-state errors, resulting in better dynamic and static performance of the system.
Most articles on this topic only simulate and analyze the feedforward decoupling control strategy, but fail to provide a simulation comparison between the two strategies. This paper, based on the feedforward decoupling control strategy, establishes voltage-source PWM rectifier systems with and without feedforward decoupling control. Simulation results are used to compare the two control strategies, demonstrating the superiority of the feedforward decoupling control strategy.
1. Mathematical Model and Working Principle of PWM Rectifier
1.1 Working Principle
The circuit structure of the three-phase PWM rectifier is shown in Figure 1. The AC input inductor L of the rectifier serves as a filter and also increases the DC voltage. The AC current of the rectifier can be considered as a three-phase sinusoidal current. The DC output capacitor C acts as a voltage regulator and a DC energy storage element, and the output exhibits voltage source characteristics. The DC voltage remains constant in steady state.
Figure 1. PWM rectifier circuit structure
Fig.1 Circuit schematic of three-phase PWM rectifier
Figure 2 shows the vector diagram between the input voltage us , the input current, and the AC control voltage uf of the rectifier. In Figure 2a, the rectifier operates in rectification mode, with the current vector and voltage vector us parallel and in the same direction. At this time, the rectifier grid side exhibits positive resistance characteristics, achieving unity power factor rectification control, and the load absorbs active power from the grid. In Figure 2b, the rectifier operates in inverter mode, with is parallel and in opposite directions to us . At this time, the rectifier grid side exhibits negative resistance characteristics, achieving unity power factor inverter control, and the load releases active power to the grid.
Figure 2(a) Rectification state (b) Inverter state
Fig2.(a)The rectifying state (b)The regenerative inverter state
As shown in the diagram above, the key to achieving unity power factor control of the rectifier lies in controlling the grid-side current Is , ensuring it is in phase or out of phase with the grid voltage Us . There are two methods for this control: one is to directly control the grid-side current of the PWM rectifier through closed-loop control; the other is to indirectly control the grid-side current by controlling the AC side voltage U of the rectifier. In other words, unity power factor control is achieved by controlling the phase of the input current Is , and by controlling the magnitude of Is , Udc is controlled.
1.2 Mathematical Model of PWM Rectifier
Mathematical Model in Three-Phase Stationary Coordinate System
Sk—The switching function of the three-phase bridge arm, upper bridge arm on, lower bridge arm off, lower bridge arm on, upper bridge arm off. The above mathematical model has a clear and intuitive physical meaning, but since the AC side of the VSR consists of time-varying AC quantities, it is not conducive to the design of the control system. Therefore, the three-phase symmetrical stationary abc coordinate system is transformed into a dq coordinate system that rotates synchronously with the fundamental frequency of the power grid through coordinate transformation, and the rectifier in the two-phase synchronous speed rotating dq coordinate system is obtained.
The mathematical model is as follows:
2. Comparison of Control Strategies
Voltage-oriented vector control is a control scheme based on synchronous rotating coordinates of the dq axis. The d-axis of the two-phase rotating dq coordinate system is oriented to be coaxial with the grid voltage vector, and the control of the grid phase current is transformed into the control of the DC component of the current in the d and q axes, thereby simplifying the design of the PWM rectifier system controller. This paper adopts a dual closed-loop control system structure for a three-phase bridge rectifier with voltage-oriented vector control.
Figure 3(a) Control system diagram without feedforward decoupling
Fig(a)Control system diagram of without feedforword
From the mathematical model in equation (2), it can be seen that the d-axis and q-axis variables are coupled to each other, which brings certain difficulties to the design of the controller. Therefore, a feedforward decoupling control strategy can be adopted. Furthermore, using a PI regulator as the current loop controller allows for independent control of the two currents by upd and upq respectively. At this point, we have:
3 System Simulation
The figure shows a simulation model of a three-phase voltage-source PWM rectifier system. The simulation parameters are: AC input voltage RMS value of 110V, given DC bus voltage udc=300V, L=4Mh, C=1.1μF, inductor internal resistance R=1.35Ω, variable compensation ode23tb, and simulation time of 1s. At 0.6 seconds, the load current abruptly changes from 10A to 20A.
Figure 3(b) shows the control system with feedforward decoupling.
Fig(b)Control system diagram of feedforword
Figure 4(a) DC output voltage without feedforward decoupling
Fig4.(a) Output voltage of without feedforword
Figure 4(b) DC output voltage with feedforward decoupling
Fig4.(b) Output voltage of feedforword
Figure 5(a) Power factor without feedforward decoupling
Fig5.(a) Power factor of without feedforword
Figure 5(b) Power factor with feedforward decoupling
Fig(b)Power factor of feedforword
Figure 6(a) Voltage and current waveforms of phase a AC side without feedforward decoupling
Fig6.(a)AC-side phase current for phase a, phase voltage of power grid of without feedforword
Figure 6(b) shows the voltage and current waveforms of phase a AC side with feedforward decoupling.
Fig6.(b)AC-side phase current for phase a, phase voltage of power grid of feedforword
Simulation results show that the PWM rectifier with feedforward decoupling control has a more stable output DC voltage. The overshoot is only 0.867%, and the DC output voltage fluctuation is very small under sudden load conditions. The grid-side current dynamic response is fast, and the power factor reaches 1. This is mainly due to the system's adoption of a dual closed-loop control structure for voltage and current, based on feedforward decoupling control. The outer voltage loop ensures stable DC output, while the inner current loop improves the dynamic response speed of the current. Furthermore, this control method keeps the phase of the current consistent with the phase of the input voltage.
5. Conclusion
Simulation waveform analysis shows that the voltage-source control strategy based on SVPWM with feedforward decoupling can achieve an input power factor of 1 for the rectifier, exhibit fast dynamic response of the grid-side current, and more stable output DC voltage. This demonstrates the correctness and superiority of this control performance.
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