Feedforward decoupling control strategy for three-phase voltage-source PWM rectifier

1. Introduction

Based on the analysis of the working principle of the three-phase voltage-type PWM rectifier, this paper establishes the mathematical model of the three-phase voltage-type PWM rectifier in the dq synchronous rotating coordinate system, and designs the current inner loop controller and voltage outer loop controller based on feedforward decoupling control. Finally, the correctness and effectiveness of the control strategy are verified through simulation and experiments.

Compared with the traditional uncontrolled rectification or phase-controlled rectification, PWM rectifier has the advantages of less grid-side current harmonic content, high power factor, bidirectional energy flow, and fast dynamic response, and has become a hot topic in the field of power electronics. Therefore, it is of great practical significance to study the control strategy of high-performance PWM rectifier.

2. Mathematical model of three-phase voltage-type PWM rectifier

The schematic diagram of the three-phase voltage-type PWM rectifier is shown in Figure 1. The physical quantities in Figure 1 are defined as follows: ea, eb, ec are the grid voltages, ia, ib, ic are the phase currents on the AC side, Udc represents the DC side voltage, and ua, ub, uc are the AC side input voltages of the PWM rectifier. Assuming that the three-phase grid voltage is balanced, we can get:

Transform the mathematical model of the PWM rectifier in the above abc three-phase stationary coordinate system into the mathematical model in the dq synchronous rotating coordinate system:

3. Three-phase voltage-source PWM rectifier dual closed-loop control strategy

3.1 Three-phase voltage-source PWM rectifier current inner loop controller

By rearranging formula (3), we can get:

From the above dq mathematical model, we can see that the d and q axis variables are mutually coupled, which makes it difficult to design the controller. To this end, the feedforward decoupling control strategy can be adopted, and the PI regulator introduced can be used to achieve closed-loop stable control of the system. The control equation is:

It can be seen from equation (6) that the introduction of the current state feedback decoupling control strategy in the current inner loop controller can realize the decoupling control of active current and reactive current. The block diagram of the current inner loop controller is shown in Figure 2.

3.2 Three-phase voltage-type PWM rectifier voltage outer loop controller

According to the instantaneous power theory, the active power P and reactive power Q in the dq synchronous rotating coordinate system can be expressed as:

From equation (8), we can see that id and iq are linearly proportional to active power P and reactive power Q respectively. By adjusting id and iq, the active power and reactive power of the PWM rectifier can be independently controlled, thus realizing the decoupling control of active power and reactive power. In order to stabilize the DC side voltage of the PWM rectifier, the DC voltage outer loop adopts PI control, and its simplified control structure is shown in Figure 3 [6]. According to the typical type II system, the PI regulator parameters can be obtained as follows:

4. Simulation study

A three-phase voltage-type PWM rectifier simulation model was built in the Matlab/Simulink simulation platform. The simulation parameters are as follows: the effective value of the grid phase voltage is 110V, the grid voltage frequency is 50Hz, the transformer is a step-up isolation transformer, the three-phase filter inductance is 3mH, the DC side load is 24Ω, and the DC side given voltage is 300V. In order to test the anti-interference performance of the system, at 3s, the load suddenly increased and the resistance suddenly changed from 24Ω to 12Ω. At this time, the DC bus voltage dropped slightly and can be restored to stability in a short time, as shown in Figure 6. Figure 6 shows the simulation waveforms of the voltage and current of phase A when the load suddenly changes. It can be seen from the figure that when the load suddenly changes, the input current on the AC side increases rapidly to ensure the balance of the input and output power of the rectifier, while the system still runs at a unity power factor, reflecting the good dynamic response of the system.

5. Experimental studies

A three-phase voltage-type PWM rectifier prototype was built for experimental verification. The experimental parameters are the same as the simulation parameters. The switching frequency is 10kHz and the dead time is 2us. The main control chip of the rectifier is TMS320F28335 from TI.

Figure 7 is the experimental waveform of the DC bus voltage of the three-phase voltage-type PWM rectifier recorded by Fluke435 power quality tester. Figure 8 is the experimental waveform of the voltage and current of phase A recorded by Fluke (the waveform with larger amplitude is the voltage waveform). Figure 9 is the total harmonic content of current measured by Fluke, and the current harmonic content (THD) is 2.7%. The experimental results show that the control strategy proposed in this paper can effectively suppress harmonic current and achieve high power factor operation. The DC bus voltage has a small fluctuation (fluctuation within 5V), basically stable at 300V, and the system has good stability.

6. Conclusion

Based on the analysis of the mathematical model of the three-phase voltage-type PWM rectifier, this paper designs the current inner loop controller and the voltage outer loop controller respectively. The simulation and experimental results show that the PWM rectifier basically operates at the unit power factor on the grid side, effectively suppressing the grid-side input harmonic current. The system has good steady-state characteristics and dynamic characteristics. Therefore, this control strategy is an effective control strategy with certain practical value.