1 Overview
The three-phase voltage type PWM rectifier has the characteristics of energy bidirectional flow, grid side current sinusoidal, low harmonic input current, constant DC voltage control, small capacity filter and high power factor (approximate unit power factor), effectively eliminating The traditional rectifier has large input current harmonic content and low power factor. It is widely used in four-quadrant AC drive, active power filter, superconducting energy storage, new energy power generation and other industrial fields.
There are many control strategies for PWM rectifiers. Direct current and indirect current control are the main control strategies. These two closed-loop control strategies require complex algorithms and modulation modules. The three-phase voltage type PWM rectifier direct power control (DPC) has been widely studied in recent years due to its simple control method, strong anti-interference ability, good dynamic performance, and decoupling control of active and reactive power. There are also endless streams [1-2].
This paper introduces the topology of the main circuit of the three-phase voltage-type PWM rectifier and the control strategy based on DPC, and compares and analyzes it. Based on this, the control strategy of the PWM rectifier is prospected.
2 circuit topology
In recent years, the research on the topology of three-phase voltage-type PWM rectifiers has focused on reducing power switches [3] and improving DC output performance in low-power applications. For high-power applications, it is mainly concentrated in multi-level [4], converters. Combination and soft switching technology [5]. Currently more mature topologies have two-level and three-level PWM rectifier structures.
The three-phase voltage type two-level PWM rectifier is the most basic PWM rectifier circuit. Because of its simple structure and relatively mature control algorithm, it has been widely used. Compared with the three-level PWM rectifier, each bridge arm has two more switching tubes and two clamping diodes. The circuit structure is complex, there is a midpoint potential balance problem, and the control algorithm is cumbersome, but the circuit has a larger conversion power. The lower input current distortion rate and other advantages have also been widely studied and applied.
3 direct power control method
The direct power control (DPC) system structure is a double closed loop system in which the DC side voltage is the outer loop and the instantaneous power is controlled as the inner loop.
From the point of view of power conservation, the direct power control PWM rectifier achieves the purpose of controlling 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 the preset power factor. And power flow direction.
3.1 Voltage-based direct power control (V-DPC)
Compared with the previous PWM rectifier control strategies, the outstanding advantages of this control strategy are:
(1) The PWM modulation module is not needed, the current closed-loop adjustment is not required, and the active power and the reactive power are directly controlled by means 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 uses a predictive model of the voltage-free sensor, 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 for the selection of the AC side inductance;
(2) High dependence on sensor conversion accuracy and system sampling frequency.
3.2 Direct flux control based on virtual flux linkage (VF-DPC)
In addition to the advantages of V-DPC, the direct flux control strategy based on virtual flux linkage has [10]:
(1) a lower sampling frequency;
(2) There is a lower total current harmonic content (THD) in the case where the input three-phase grid voltage is not ideal.
Similarly, VF-DPC does not solve the problem that the switching frequency is not fixed.
3.3 Direct power control based on instantaneous power theory
The active power and reactive power in the traditional theory are defined on the basis of the average value. It is only applicable to the case where the voltage and current are sinusoidal. The concept of instantaneous power theory is based on the instantaneous value, on the sine, Non-sinusoidal voltage and current conditions apply [12]. Figure 5 shows a block diagram of a direct power control system based on instantaneous power theory [13]. The control principle is similar to that of V-DPC. The calculated active power P, reactive power Q and power are given to the difference. The result is determined by the power hysteresis loop and the sector where the voltage vector is located.
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 switching state, which greatly simplifies the algorithm and provides more accurate active and reactive power transients. the amount. At the same time, the control strategy also has the advantages of fast dynamic response and low current distortion on the input side. weakness is:
(1) The switching frequency is not fixed;
(2) A higher sampling frequency is required.
3.4 Space vector based direct power control (SVM-DPC)
Spatial vector-based direct power control (SVM-DPC) replaces the switch vector table and power hysteresis in traditional DPC systems with space vector PWM modulation modules and PI links [14-16].
Advantages of space vector modulation direct power control strategy:
(1) Do not use a nonlinear controller;
(2) The switching frequency is fixed, thus facilitating the selection of the inductance parameters of the grid side;
(3) Reduce the sampling frequency;
(4) A voltage vector in any direction can be obtained, and there is no reactive offset region;
(5) has a lower input current distortion rate.
Disadvantages:
(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 obtain more accurate instantaneous power, some scholars have proposed a control scheme to increase the voltage sensor on the grid side, and calculate instantaneous active and reactive power according to the instantaneous power theory. This method is not ideal in the case of three-phase input voltage asymmetry. A better control effect is obtained.
3.5 Direct Power Control Based on Power Prediction (P-DPC)
Power prediction based DPC systems [17-19] can be divided into fixed frequency and indefinite frequency. The literature [18] introduced the control algorithms of the two PDPCs in detail and made a simulation study. From the simulation results of the two, the effect of the fixed frequency control is better.
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 optimal voltage vector sequence and its corresponding action time according to the given power to control the PWM rectifier. Operation at a constant switching frequency. The power prediction is calculated by the formula (15) and the formula (16).
Direct power control based on fixed-frequency power prediction maintains the advantages of traditional DPC, such as fast dynamic response, while achieving a fixed switching frequency in a novel way, simplifying the parametric system parameter design. The shortcomings of this control strategy are mainly reflected in the relatively complicated power algorithm.
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 the reactive power are coupled to 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, and obtain independent expressions to provide a more accurate control model for the system [20-22].
Figure 8 is a block diagram of a direct power control architecture using passive control for power decoupling [22]. The active power reference can be calculated by equation (17), and the specific passive power control law is given by equations (18) and (19). Substituting Sd and Sq into the rectifier mathematical model [22] to obtain equations (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 to current power control, power decoupling control gives rectifiers the following advantages:
(1) Faster power and DC voltage tracking capability;
(2) better static performance;
(3) Strong anti-load disturbance ability.
Disadvantages:
(1) The algorithm is complex;
(2) The control effect depends on the accuracy of the estimated parameter values ​​Ra1, Ra2.
3.7 Direct power control based on dual switch tables
The traditional switch meter is based on the simultaneous action of active power and reactive power, that is, the same voltage vector must simultaneously adjust the active power and reactive power, but this combination is actually difficult to achieve perfectly, more The case is that the selected voltage vector has a strong adjustment capability for one side and a weak adjustment capability for the other, resulting in a slow tracking speed of the overall system.
The dual switch meter is a switch vector table for independent adjustment control of active power and reactive power. In a certain sense, the use of the dual switch meter reduces the coupling between active power and reactive power. The control idea is that in a control cycle, if the adjustment ability of the 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 two-switch table.
Based on the dual-switch table DPC control strategy, the traditional DPC single logic switch table is used to adjust the DC voltage and power during the start-up transient process. The steady-state load disturbance causes large DC-side voltage fluctuation and slow power tracking. And other issues, with 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, according to the instantaneous active and reactive power derivatives, select the rectifier input voltage vector in time to control the instantaneous active power and The increase or decrease of reactive power completes the power pre-control to achieve the purpose of system unit power factor operation and balancing DC voltage [23-24]. Compared with the traditional DPC strategy, it has the advantage of improving the dynamic performance of the system and achieving good results under the condition of unbalanced input voltage, at the cost of greatly increasing the complexity of the algorithm.
3.9 Other improved direct power control strategies
Literature [25] proposed a direct power control based on fuzzy control. The main idea is to replace the PI link in the traditional DPC with fuzzy control to obtain the system active power reference.
Because the traditional DPC has weak ability to adjust the active power, the literature [26] adopts the method of changing the reactive power to increase the ability to adjust the active power and improve the power response speed.
The power control strategy of power inner loop and voltage square outer loop further improves DC voltage tracking and power tracking capability.
In order to reduce the influence of fan-shaped boundary on power control and DC voltage, the literature [28] proposed a DPC control strategy for setting the dead zone of the sector boundary.
In order to obtain the phase angle of the voltage vector more accurately, some scholars have introduced a phase-locked loop (PLL) into the PWM rectifier DPC control, and realize the positioning of the voltage vector by detecting the phase of the AC side input voltage.
4 Prospects of Direct Power Control Strategy for Three-Phase Rectifiers With the development of power electronics technology and control theory, the research on the control strategy of three-phase PWM rectifiers will continue to deepen. According to the performance requirements of the rectifier itself, like the smaller current distortion rate, The DC ripple coefficient is reduced, and the power factor is further improved. The corresponding control strategy is mainly developed in the following aspects.
1) For voltage-type PWM rectifier models with nonlinear multivariable coupling characteristics, the conventional control strategy and its controller design are insufficiency in that the stability of the control system over a wide range of disturbances cannot be guaranteed. To this end, scholars have proposed a DPC control strategy based on stability theory to change the robustness of the system.
2) In the case of unbalanced three-phase power grid, the DC side voltage of the rectifier and the low-order harmonic amplitude of the AC side current increase, and at the same time, the current imbalance of the grid side is generated, and the rectifier device may be damaged in severe cases. Some scholars have also done some work on the rectifier DPC control strategy under grid imbalance conditions [29].
3) Because multi-level three-phase PWM rectifiers have outstanding advantages in controlling current harmonics, stabilizing DC voltage, and higher conversion capacity, some scholars have also studied multi-level DPC control strategies [30]. .
4) Since the traditional rectifier control system is given on the basis of the grid balance and the power switch device is an ideal model, the system is less robust. For these problems, some scholars try to control the intelligent control, such as the neural network controller. Fuzzy logic controllers and the like are applied to the rectifier DPC control strategy to solve.
5 Conclusion This paper first introduces the application advantages of direct power control in three-phase voltage-type PWM rectifiers and explains its control ideas. It focuses on the two-level, three-level circuit topology of three-phase voltage rectifiers, and the current direct The main method and implementation principle of power control, and finally the development direction of direct power control technology of three-phase PWM rectifier is prospected.
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