Voltage-source PWM Rectifier Based on DSP Control

According to the working principle of PWM rectifier circuit, the PWM rectifier control mode is analyzed, and the hysteresis current control mode is adopted. The voltage type PWM rectifier double closed loop automatic control system is established with TMS320F2812 as the controller. The experimental results show that the system has good static and dynamic performance, which provides a certain theoretical basis for the design of PWM rectifier.

Keywords: PWM rectification; hysteresis current control; DSP

Most of the converters in industry need a rectifier link to obtain DC voltage. Conventional rectifier links generally use diode-controlled rectification or thyristor phase-controlled rectification. Although the circuit is simple, economical and reliable, there are the following problems: the grid-side current waveform is seriously distorted, resulting in a low power factor, with the highest power factor being about 0.8; the consumption of a large amount of reactive power will bring additional burden to the power grid, which not only increases the loss of the transmission line, but also seriously affects the power supply quality; the input current contains a lot of low-order harmonic currents, which brings grid pollution. However, the application of fully controlled devices such as MOSFET and IGBT and the use of PWM rectification technology can make the grid-side current sinusoidal, the converter can operate at a unity power factor, and the energy can flow in both directions, truly realizing green power conversion. Therefore, research on it will promote the rapid development of new rectification technology. This paper takes the three-phase voltage-type PWM rectifier as the research object, uses TMS320F2812 as the controller, and establishes a PWM rectifier double closed-loop automatic control system to analyze and study it.

1 Analysis of control methods

1.1 PWM rectification control method

There are many control methods for the evolving PWM rectifiers. As far as the control methods of voltage-type PWM rectifiers are concerned, they are mainly divided into indirect current control and direct current control. Indirect current control refers to controlling the voltage at the input end of the rectifier so that it maintains a certain amplitude and phase relationship with the power supply voltage, thereby controlling the input current on the AC side to be a sinusoidal waveform and making the device operate at a unity power factor state. Direct current control usually introduces the feedback signal of the actual AC input current into the control system, compares it with the given signal, and controls the on and off of the device by adjusting its error, so that within a certain error range, the actual current is guaranteed to be consistent with the given signal, forming a double closed-loop structure of the voltage outer loop and the current inner loop.

Since the amplitude-phase control strategy has poor dynamic performance, weak current adjustment capability, and the calculation module relies on parameter accuracy, the steady-state performance has deviations. The direct current control system has good steady-state and dynamic performance, so it has been widely used. It is mainly divided into hysteresis current control, predictive current control, and fixed-frequency PWM control. Among them, hysteresis PWM current control can accelerate the response of current and transform the object controlled by the voltage outer loop, thereby improving the performance of the voltage outer loop.

1.2 PWM hysteresis current control method

The hysteresis current control is to compare the feedback current if with the given current ir to limit the deviation between the two within the set range. When the feedback current if < ir-0.5ih, ih is the hysteresis width, and the output of the modulation circuit increases the system input side current is; when if> ir-0.5ih, the output of the modulation circuit reduces the system input side current is. In this way, the hysteresis comparison adjustment is continuously performed to make is always track the given current ir, make a sawtooth change around the given current waveform, and limit the error within the hysteresis width range. If the given current waveform is sinusoidal and the hysteresis width ih is constant, the waveform of is will be close to sinusoidal.

The hysteresis PWM current control structure does not have a traditional current regulator, but a nonlinear hysteresis link. When the current deviation exceeds the hysteresis width, the main circuit power switch tube switches, forcing the current deviation to decrease, which is a typical nonlinear control.

2 Three-phase voltage-type PWM rectifier system

2.1 System structure diagram

According to the working principle of three-phase voltage-type PWM rectifier, a double closed-loop automatic control system is established as shown in Figure 1. The controller uses TMS320F2812.

2.2 System main circuit

The main circuit of the system is shown in Figure 2. The six fully controlled devices are composed of three modules of model SMK 100 GB 121 D, with inductance Is = 5mH, C = 2 200 μF, R = 25 Ω, and a three-phase AC voltage of 70V. The drive circuit uses the drive module M57962L produced by Mitsubishi, which has desaturation detection and protection links inside.

2.3 Current and voltage detection

The control loop and the main loop are isolated to ensure the stability of the system. When sampling the DC voltage feedback, the linear optocoupler LOC110 is used for isolation. The DC voltage detection and conditioning circuit is shown in Figure 3. The digital control part can only recognize positive voltage signals, while the sampled AC voltage and AC current signals are AC, with both positive and negative half cycles. The sampled signals must be converted into signals that can be fully recognized by the DSP through the conditioning circuit. The input level range of the A/D sampling on the TMS320F2812 chip is 0 to 3 V, so it is necessary to use the analog sampling and conditioning module to appropriately transform the collected signal to obtain a signal within 0 to 3 V that can be processed by the DSP. The AC sampling and conditioning circuit is shown in Figure 4. The output voltage of the sensor is isolated and sampled, and the sampled voltage signal is converted to a voltage range of -1.5 to +1.5 V through the operational amplifier. Finally, a voltage offset of 1.5 V is added to form an AC voltage of 0 to 3 V and sent to the DSP. 2.4 Software Flowchart

The PWM rectifier uses hysteresis current control, and the voltage regulator uses PI regulation. The software flow is shown in Figure 5.

3 Experimental results analysis

In order to verify the correctness of the control method and control strategy, a three-phase voltage-type PWM rectifier control system was established with TI's TMS320F2812 EVM board as the controller. The experimental system block diagram is shown in Figure 1. The experimental parameters are IS=5 mH, C=2 200 μF, R=25 Ω, the three-phase AC voltage is 70 V, and the voltage feedback coefficient is α=0.01. The rated voltage of the rectifier output is 160 V, and the resistive load power is 1 kw. The waveforms of the power supply voltage and current in steady state are shown in Figure 6, and the experimental waveforms of the system step response and output voltage during load jump are shown in Figure 7. It can be seen from the experimental waveforms that the input current is close to sinusoidal and is almost in phase with the input voltage. The system has good static and dynamic performance.

4 Conclusion

In this paper, a 1 kW PWM rectifier power supply is built by using PWM hysteresis current control. The experimental results show that the system has a high power factor, the input current is close to sine, the harmonic pollution to the power grid is small, and the system has good steady-state and dynamic performance.