Research on Direct Current Control Strategy of Single-phase PWM Rectifier

1. Introduction

With the development of power electronics technology, power electronic devices are being used more and more widely, resulting in a large number of nonlinear loads pouring into the power grid, causing increasingly serious harmonic pollution to the voltage and current of the power system. However, PWM rectifiers improve the power factor of the system and reduce the harmonic pollution to the power grid, which has attracted people's attention.

According to the input inductor current state, PWM rectifiers can be divided into discontinuous current mode (DCM) and continuous current mode (CCM). CCM mode has the advantages of small input and output current ripple, easy filtering, small device conduction loss, and suitability for high-power occasions, and has received more attention. In CCM mode, according to whether the transient inductor current is directly selected as the feedback quantity, it can be divided into direct current control and indirect current control. Indirect current control has a simple structure and does not require a current sensor, but its biggest disadvantage is that the current dynamic response is slow, and even the AC side current contains a DC component, and it is sensitive to system parameter fluctuations. Compared with indirect current control, direct current control uses the input current of the rectifier as feedback and controlled quantity to form a current closed-loop control, which improves the dynamic and static performance of the current, and also makes the grid-side current control insensitive to system parameters, thereby enhancing the robustness of the current control system. Therefore, direct current control technology has a very broad application prospect and use value.

2. Single-phase voltage PWM rectifier principle block diagram

The topology of the single-phase voltage-type PWM rectifier is shown in Figure 1. It mainly consists of three parts: AC circuit, power switch bridge, and DC circuit. The AC circuit includes AC electromotive force UN, grid-side resistance RN, and grid-side inductance LN, etc. The DC circuit includes a series resonant circuit composed of inductor L2 and capacitor C2 to filter out the second harmonic components of the grid, filter capacitor Cd, and load RL, etc. The power switch bridge is composed of four IGBTs with anti-parallel diodes.

The control idea of ​​the single-phase PWM inverter is to keep the current and voltage on the AC side in phase as much as possible while ensuring the stability of the DC side voltage, so that the power factor on the AC side is 1.

3. Analysis of direct current control technology of single-phase PWM rectifier

Direct current control can be divided into hysteresis current control, peak current control, predictive current control, average current control, state feedback control, single cycle control, etc. according to different control methods.

3.1 Peak current control

The principle of peak current control is to compare the instantaneous value of actual current and command current in real time. The command current value is the upper limit of actual current. Once the actual current reaches this upper limit, it immediately turns to decay downward. The size of inductance, line impedance and pulse width modulation switching frequency affect the final value of this decay. The control principle block diagram is shown in Figure 2 below.

Advantages of peak current: ① Fast transient closed-loop response, fast transient response to changes in input voltage and output load; ② Easy to design control loop; ③ Input voltage adjustment can be matched with input voltage feedforward technology of voltage mode control; ④ Simple and automatic flux balance function; ⑤ Instantaneous peak current limiting function, that is, inherent pulse-by-pulse current limiting function; ⑥ Automatic current-sharing parallel function. Disadvantages: ① Open-loop instability when duty cycle is greater than 5%, the error between peak current and average current is difficult to correct; ② Closed-loop response is not as ideal as average current mode control; ③ Subharmonic oscillation occurs in the system when duty cycle is greater than 0.5; ④ Sensitive to noise, poor noise resistance; ⑤ Circuit topology is limited; ⑥ Poor interactive regulation performance for multiple output power supplies.

3.2 Hysteresis Current Control

As an improvement of the peak current control method, the hysteresis current control method only adds a lower limit to limit the current decay. The principle is still to compare the real-time values ​​of the command current and the actual current. When the actual current reaches the upper limit command current, it will immediately decay, decay to the lower limit command current, and then start to rise again. This will be repeated, and the actual current will be a sawtooth wave jumping between the upper and lower limit command currents.

The control principle diagram is shown in Figure 3. In the figure, the command current i* and the actual current i are compared, and the deviation △i between the two is used as the input of the hysteresis comparator. The hysteresis comparator generates a PWM signal for controlling the on and off of the switch in the main circuit of the control circuit. The PWM signal controls the on and off of the power device through the drive circuit, thereby controlling the change of the current i.

The advantages of hysteresis current control are simple structure, easy implementation, strong robustness and fast dynamic response. The disadvantages are that the switching frequency is not fixed, the filter design is difficult, and the detection and control of the full cycle of the inductor current is required.

3.3 Average Current Control

The working principle of average current control is to add the inductor current signal to the sawtooth wave signal. When the sum of the two signals exceeds the reference current, the switch tube is turned off. When the sum is less than the reference current, the switch tube is turned on. The sampling current comes from the actual input current rather than the switch current. The control principle diagram is shown in Figure 4.

The advantages of average current control are: ① The average inductor current can track the current programming signal with high accuracy; ② The circuit with good debugging has excellent anti-noise performance; ③ It is suitable for any circuit topology to control the input or output current; ④ It is easy to achieve current sharing. The disadvantages are: ① The gain of the current amplifier at the switching frequency has a maximum limit; ② The design and debugging of the matching parameters such as bandwidth and gain of the dual closed-loop amplifier are complicated.

3.4 Predictive Circuit Control

The principle of predictive current control is to sample the input, output voltage and input current at the beginning of each regulation cycle, and select the optimized voltage vector to act on the next cycle according to the error between the actual current and the reference current, so that the actual current can track the reference current within one cycle and achieve steady-state error-free. The block diagram of the predictive current control principle is shown in Figure 5, where Uref is the given voltage and Udc is the DC side feedback voltage.

Advantages of this control: fixed switching frequency, good dynamic performance, small current harmonics, small device switching stress, and simple digital implementation. Disadvantages: requires higher sampling frequency and switching frequency, and at low sampling frequency, periodic current errors will occur.

3.5 Deadbeat Control

Deadbeat control is a fully digital control technology developed on the basis of current hysteresis comparison control technology. It uses the current reference value of the previous moment and the current output value of the converter under various switching states, and calculates the switching mode that the rectifier should meet at the next moment according to the space vector theory. This switching mode is selected as the switching state at the next moment, so as to achieve the goal of current error equal to zero. The advantages of using deadbeat control are rigorous mathematical derivation, no overshoot in tracking, good dynamic performance, easy computer execution, can eliminate steady-state errors, and end the transition process in the shortest time. However, it also has disadvantages such as poor robustness, large transient response overshoot, strong real-time calculation, and high hardware requirements. With the increasing popularity of digital signal processor applications, this is a very promising control method.

3.6 State Feedback Control

State feedback control is a control method to eliminate the oscillation that is prone to occur in the input filter of the current-type reversible rectifier. The control principle diagram is shown in Figure 6.

4. Conclusion

Direct current control is the main control method of single-phase high power factor rectifier, and it is also the focus of our future research. Taking advantage of the advantages and disadvantages of various control strategies, combining multiple control strategies to form a complementary relationship and achieve the desired effect is also a direction for the development of control technology. The research of new direct current control strategies will be another direction. With the widespread application of digital signal processors and the gradual maturity of artificial intelligence technology. Digital control and intelligent control will also be the goal of our research, which will be the mainstream of the development of direct current control technology.