A Review of Direct Power Control Methods for Three-Phase Voltage-Source PWM Rectifiers

1 Overview

The three-phase voltage-type PWM rectifier has the characteristics of bidirectional energy flow, grid-side current sinusoidalization, low harmonic input current, constant DC voltage control, smaller capacity filter and high power factor (approximately unity power factor). It effectively eliminates the problems of large harmonic content and low power factor of traditional rectifier input current. It is widely used in four-quadrant AC transmission, active power filtering, superconducting energy storage, new energy power generation and other industrial fields.

There are many control strategies for PWM rectifiers. The current control strategies are mainly direct current control and indirect current control. These two closed-loop control strategies require complex algorithms and modulation modules. The direct power control (DPC) of three-phase voltage-type PWM rectifiers has been widely studied in recent years because of its many advantages such as simple control method, strong anti-interference ability, good dynamic performance, and the ability to achieve active and reactive power decoupling control. Control methods are also emerging in an endless stream [1-2].

This paper will introduce the topology of the main circuit of the three-phase voltage-type PWM rectifier and the control strategy based on DPC, and conduct a comparative analysis. On this basis, the control strategy of the PWM rectifier is prospected.

2 Circuit Topology

In recent years, the research on three-phase voltage-source PWM rectifier topology has mainly focused on reducing power switches[3] and improving DC output performance in low-power applications; and on multi-level[4], converter combination and soft switching technology[5] in high-power applications. Currently, the more mature topologies include two-level and three-level PWM rectifier structures.

The three-phase voltage-type two-level PWM rectifier is the most basic PWM rectifier circuit. It has been widely used because of its simple structure and relatively mature control algorithm. Compared with it, the three-level PWM rectifier has two more switches and two clamping diodes in each bridge arm, and the circuit structure is complex, there is a problem of midpoint potential balance, and the control algorithm is cumbersome. However, this circuit has the advantages of greater conversion power and lower input current distortion rate, and has also been widely studied and applied.

3 Direct Power Control Method

The direct power control (DPC) system structure is a double closed-loop system with the DC side voltage as the outer loop and the instantaneous power control as the inner loop.

From the perspective of power conservation, the direct power control PWM rectifier controls the instantaneous input current by controlling the instantaneous active power and reactive power flowing into the rectifier when the voltage on the AC side is constant, thereby obtaining a preset power factor and power flow direction.

3.1 Voltage-based Direct Power Control (V-DPC)

Compared with various previous PWM rectifier control strategies, the outstanding advantages of this control strategy are:

(1) No PWM modulation module is required, no current closed-loop regulation is required, active power and reactive power are directly controlled with the help of the switch vector table, and the control algorithm is simple;

(2) The system has a faster dynamic response speed;

(3) The input current has a lower distortion rate;

(4) The acquisition of instantaneous power adopts a prediction model without voltage sensors, which saves hardware costs to a certain extent.

At the same time, it also has the following shortcomings:

(1) The switching frequency is not fixed, which increases the difficulty of selecting the AC side inductor;

(2) Highly dependent on sensor conversion accuracy and system sampling frequency.

3.2 Virtual Flux-Based Direct Power Control (VF-DPC)

In addition to the advantages of V-DPC, the direct power control strategy based on virtual flux linkage also has the following advantages[10]:

(1) Lower sampling frequency;

(2) When the input three-phase grid voltage is not ideal, the total harmonic content (THD) of the current is lower.

Likewise, VF-DPC does not solve the problem of non-fixed switching frequency.

3.3 Direct Power Control Based on Instantaneous Power Theory

Active power and reactive power in traditional theory are defined on the basis of average values, which is only applicable to the case where voltage and current are sinusoidal waves; while the concept of instantaneous power theory is based on instantaneous values, which is applicable to both sinusoidal and non-sinusoidal voltage and current [12]. Figure 5 shows the block diagram of direct power control system based on instantaneous power theory [13]. The control principle is similar to that of V-DPC, where the calculated active power P and reactive power Q are used to make a difference with the power reference, and the result is compared with the sector where the voltage vector is located after the power hysteresis loop to determine the switching state of the system.

Compared with V-DPC and VF-DPC, although the system uses an additional voltage sensor, the calculation of instantaneous power does not depend on the system switch state, which greatly simplifies the algorithm and provides more accurate instantaneous active and reactive power. At the same time, this control strategy also has the advantages of fast dynamic response and low input side current distortion rate. Disadvantages are:

(1) The switching frequency is not fixed;

(2) Requires a higher sampling frequency.

3.4 Space Vector Based Direct Power Control (SVM-DPC)

Space vector based direct power control (SVM-DPC) replaces the switching vector table and power hysteresis loop in the traditional DPC system with a space vector PWM modulation module and PI link [14-16].

Advantages of space vector modulation direct power control strategy:

(1) No nonlinear controller is used;

(2) The switching frequency is fixed, which facilitates the selection of grid-side inductance parameters;

(3) Reduced sampling frequency;

(4) Voltage vectors in any direction can be obtained, and there is no reactive imbalance zone;

(5) Has lower input current distortion rate.

shortcoming:

(1) The control algorithm is complicated, and the estimation of instantaneous power depends on the current switching state of the system;

(2) Multiple PI links increase the complexity of system debugging.

In addition, in order to further obtain more accurate instantaneous power, some scholars have proposed a control scheme of adding voltage sensors on the grid side, and calculating the instantaneous active and reactive power according to the instantaneous power theory. This method has achieved better control effects under non-ideal conditions such as asymmetric three-phase input voltage.

3.5 Power Prediction-Based Direct Power Control (P-DPC)

The DPC system based on power prediction [17-19] can be divided into two types: fixed frequency and variable frequency. Reference [18] introduced the control algorithms of the two PDPC systems in detail and conducted simulation research. The simulation results of the two showed that the fixed frequency control system has better performance.

Figure 7 shows the block diagram of the fixed-frequency direct power control system based on power prediction. The system obtains the current instantaneous power through the power prediction model, and selects the best voltage vector sequence and its corresponding action time based on the given power to control the operation of the PWM rectifier at a constant switching frequency. The power prediction is calculated by formula (15) and formula (16).

Direct power control based on fixed frequency power prediction maintains the advantages of traditional DPC, such as fast dynamic response, and achieves the purpose of fixed switching frequency in a novel way, which simplifies the design of rectifier system parameters. The disadvantage of this control strategy is that the power algorithm is relatively complex.

3.6 Direct Power Control Based on Power Decoupling

Since the three-phase voltage-type PWM rectifier is a hybrid nonlinear system, the active power and reactive power are coupled with each other, which affects the control performance of the system. The idea of ​​power decoupling control is to separate the active power and reactive power from the complex relationship of mutual coupling, obtain independent expressions, and provide a more accurate control model for the system [20-22].

Figure 8 is a block diagram of the direct power control structure using passive control to achieve power decoupling [22]. The active power given can be calculated by formula (17), and formulas (18) and (19) give the specific passive power control law. Substituting Sd and Sq into the rectifier mathematical model [22], we get formulas (20) and (21). It can be seen that the expressions of P and Q no longer contain the coupling terms in the power expression of the traditional DPC control strategy.

Compared with the current power control, power decoupling control gives the rectifier the following advantages:

(1) Faster power and DC voltage tracking capabilities;

(2) Better static performance;

(3) Strong ability to resist load disturbance.

shortcoming:

(1) Complex algorithm;

(2) The control effect depends on the accuracy of the estimated parameter values ​​Ra1 and Ra2.

3.7 Direct Power Control Based on Dual Switch Meters

Traditional switch meters are based on the simultaneous action on active power and reactive power, that is, the same voltage vector must take into account the regulation of both active power and reactive power. However, this balance is actually difficult to achieve perfectly. In most cases, the selected voltage vector has a strong regulation capability for one side and a weak regulation capability for the other, resulting in a slow overall tracking speed of the system.

The dual switch table is a switch vector table for independent regulation and control of active power and reactive power [2]. In a sense, the use of the dual switch table reduces the coupling degree of active power and reactive power. The control idea is that in a control cycle, if the regulation capability of active power is to be enhanced, the action time of the active switch table is increased and the action time of the reactive switch table is reduced, and vice versa. Figure 9 is a block diagram of a direct power control system based on a dual switch table.

The dual-switch-meter DPC control strategy solves the problems of large fluctuations in DC voltage and power during startup transients caused by power regulation using a single logic switch table of the traditional DPC, large DC side voltage fluctuations caused by steady-state load disturbances, and slow power tracking, and has better dynamic and static performance.

3.8 Direct Power Control Based on Output Regulation Subspace

The control idea of ​​the PWM rectifier DPC strategy based on the output regulation subspace (ORS) is: take the instantaneous active and reactive power as the output, and select the rectifier input voltage vector in time to control the increase and decrease of the instantaneous active and reactive power according to the instantaneous active and reactive power derivatives, and complete the power pre-control to achieve the purpose of unit power factor operation and balanced DC voltage of the system [23-24]. Compared with the traditional DPC strategy, its advantage is that it improves the dynamic performance of the system and achieves good results under the condition of unbalanced input voltage, but the cost is that the algorithm complexity is greatly increased.

3.9 Other Improved Direct Power Control Strategies

Reference [25] proposed a direct power control based on fuzzy control. The main idea is to use fuzzy control to replace the PI link in traditional DPC to obtain the system active power setting.

Since the traditional DPC has weak active power regulation capability, reference [26] adopted a variable reactive power setting method to increase the active power regulation capability and improve the power response speed.

The literature adopts a power control strategy of power inner loop and voltage square outer loop to further improve the DC voltage tracking and power tracking capabilities.

In order to reduce the impact of sector boundaries on power control and DC voltage, reference [28] proposed a DPC control strategy with a sector boundary dead zone.

In order to obtain the phase angle of the voltage vector more accurately, some scholars introduced the phase-locked loop (PLL) into the PWM rectifier DPC control, and realized the positioning of the voltage vector by detecting the input voltage phase on the AC side.

4 Prospects of direct power control strategies for three-phase rectifiers

With the development of power electronics technology and control theory, the research on control strategies for three-phase PWM rectifiers will continue to deepen. According to the performance requirements of the rectifier itself, such as smaller current distortion rate, reduced DC side ripple coefficient, and further improvement of power factor, the corresponding control strategies will mainly develop in the following aspects [1].

1) For the voltage-type PWM rectifier model with nonlinear multivariable coupling characteristics, the disadvantage of conventional control strategies and their controller designs is that they cannot guarantee the stability of the control system under large-scale disturbances. To this end, scholars have proposed a DPC control strategy based on stability theory to change the robustness of the system.

2) When the three-phase power grid is unbalanced, the amplitude of the low-order harmonics of the DC voltage and AC current of the rectifier increases, and the grid current is unbalanced, which may damage the rectifier in severe cases. Some scholars have also done some work on the DPC control strategy of rectifiers under unbalanced power grid conditions [29].

3) Since multi-level three-phase PWM rectifiers have outstanding advantages in controlling current harmonics, stabilizing DC voltage, and having higher conversion capacity, some scholars have also studied multi-level DPC control strategies [30].

4) Since traditional rectifier control systems are given on the basis of ideal models of grid balance and power switching devices, the system robustness is poor. To address these problems, some scholars have tried to apply intelligent control, such as neural network controllers and fuzzy logic controllers, to rectifier DPC control strategies to solve them.

5 Conclusion

This paper first introduces the application advantages of direct power control in three-phase voltage-source PWM rectifiers and explains its control ideas. It focuses on the two-level and three-level circuit topologies of three-phase voltage-source rectifiers, as well as the main methods and implementation principles of current direct power control. Finally, it looks forward to the development direction of direct power control technology for three-phase PWM rectifiers.