Method and system for controlling three-phase six-switch VIENNA rectifier

Abstract

The invention discloses a control method for a three-phase six-switch VIENNA rectifier, and the method comprises the steps: building a double-closed-loop PI control model of the three-phase six-switch VIENNA rectifier, and enabling the double-closed-loop PI control model to comprise current inner-loop control and voltage outer-loop control; current inner loop control in the double-closed-loop PI control model is improved by adopting a fractional order PI algorithm; and adopting a linear active disturbance rejection controller (LADRC) to improve voltage outer loop control in the double closed loop PI control model. Based on the method, the invention further provides a control system of the three-phase six-switch VIENNA rectifier, and the control system comprises a fractional order PI current inner loop control module, an LADRC voltage outer loop control module, an SVPWM modulation module and a neutral-point potential balance module. According to the invention, the output accuracy of the rectifier in a steady state is improved, the purposes of high steady-state current accuracy and low harmonic distortion rate are achieved, the overshoot of the system is reduced, and the dynamic response speed is high and the robustness is strong.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of rectifier control, more particularly, to a three-phase six-switch VIENNA rectifier control method and system. BACKGROUND

[0002] VIENNA rectifier, also known as Vienna rectifier, is a three-phase three-level high power factor pulse width modulation rectifier with unidirectional energy flow, among which the VIENNA rectifier with three-phase six-switch topology is widely used due to its simple circuit structure, high stability and better adaptability to engineering practical needs. The control strategy of VIENNA rectifier is the premise of realizing efficient power conversion, and the control effect directly relates to the operation stability and output power quality of VIENNA rectifier. The control of VIENNA rectifier needs to meet the following three objectives: 1. Keep the phases of three-phase voltage and current consistent, and make the harmonic content of input current as low as possible; 2. The output voltage on the DC side is smoothly tracked to the given value, and has good dynamic performance; 3. The midpoint potential on the DC side is stable, and the upper and lower capacitor voltages are balanced.

[0003] At present, the control strategy of three-phase six-switch VIENNA rectifier adopts double closed-loop control, which is a control framework designed based on its three-phase six-switch topology structure. The essence is to solve the problems of DC side voltage stability and AC side current sinusoidalization through the hierarchical control logic of "outer ring stable voltage and inner ring control current", plus midpoint potential balance control, to meet the above three core objectives and realize the high performance operation of the rectifier. The double closed-loop control system includes a voltage outer ring controller, a current inner ring controller, a midpoint potential balance module and a modulation module. The input of the voltage outer ring controller is the difference between the actual output voltage and the target voltage, and the output is the amplitude instruction of the AC side current, which is used as the control target of the current inner ring; the input of the current inner ring controller is the difference between the actual current and the target current, and the target current comes from the voltage outer ring, and the output is the voltage modulation instruction of the three-phase bridge arm of the rectifier after the midpoint potential balance; the modulation module receives the voltage modulation instruction and converts it into the switching signal of the IGBT (Insulated Gate Bipolar Transistor) on the three bridge arms.

[0004] In the design of voltage outer ring controller and current inner ring controller, PI (Proportional-integral Control) is the most common industrial paradigm. PI controller has the advantages of simple design and implementation, and is suitable for engineering practice, but has defects such as insufficient dynamic performance and poor disturbance rejection. Therefore, many researches are devoted to improving the PI controller to achieve better control effect.

[0005] In the Chinese patent document with application number ZL 202311509544.6 and invention name "Electric vehicle charging pile control method and device", an electric vehicle charging pile control method and device are disclosed, which includes: establishing a traditional double closed loop PI control model of T-type VIENNA rectifier, the traditional double closed loop PI control model including current inner loop control and voltage outer loop control; improving the current inner loop control in the traditional double closed loop PI control model by using a weak chattering super-helix second-order sliding mode algorithm; improving the voltage outer loop control in the traditional double closed loop PI control model by using a linear active disturbance rejection controller LADRC; and controlling the electric vehicle charging pile according to the improved traditional double closed loop PI control model. The above scheme can improve the dynamic performance of the charging pile device and reduce the distortion rate of the grid-side current harmonics. However, the sliding mode control is essentially a "quasi-sliding mode" (which cannot completely move along the sliding surface in the actual system), and even with weak chattering, there is still a bounded steady-state error, and the current has a small steady-state ripple, which is not suitable for scenarios with very high steady-state accuracy requirements, such as precision instruments and medical devices. SUMMARY

[0006] In order to further improve the steady-state accuracy of VIENNA rectifier current control, the present application proposes a control method and system for a three-phase six-switch VIENNA rectifier, which improves the current inner loop control in the traditional double closed loop PI control model based on a fractional order PI algorithm, thereby improving the dynamic response speed and disturbance rejection of the rectifier while achieving no chattering, low harmonics, and high steady-state accuracy.

[0007] In a first aspect, the present application provides a control method for a three-phase six-switch VIENNA rectifier, which includes: establishing a double closed loop PI control model for a three-phase six-switch VIENNA rectifier, the double closed loop PI control model including current inner loop control and voltage outer loop control, characterized by improving the current inner loop control in the double closed loop PI control model using a fractional order PI algorithm.

[0008] Further, improving the current inner loop control in the double closed loop PI control model using a fractional order PI algorithm specifically includes: establishing a mathematical model for a three-phase six-switch VIENNA rectifier in d-q coordinate system; eliminating the influence of coupling in the model using a feedforward decoupling method; establishing a control object expression based on integer order PI current inner loop using the decoupled model; adjusting the integral order of the integer order PI controller transfer function from integer 1 to , establishing a control object expression for fractional order PI current inner loop; tuning the parameters of the fractional order PI current inner loop controller.

[0009] Further, the transfer function of the fractional order PI current inner loop controller is:

[0010] The control object expression based on the fractional order PI current inner loop is:

[0011] wherein R is the DC side output load, Ls is the expression of inductance in the complex domain, Ts is the sampling period, is the inertia link considering the comprehensive delay, K P is the proportional coefficient, K I is the integral coefficient, is the fractional order integral operator in the complex domain, is the integral order, .

[0012] Further, the tuning parameters of the fractional order PI current inner loop controller adopts a phase amplitude margin combination method, specifically including: At the cut-off frequency , the control system open loop transfer function has the following amplitude characteristics:

[0013] The control system has the following phase angle characteristics at :

[0014] The system gain satisfies the following conditions:

[0015] Let the amplitude margin be , and the control system at the crossover frequency needs to satisfy: .

[0016] Further, the tuning parameters of the fractional order PI current inner loop controller also include using a fractional calculus operator approximation method based on joint optimal rational approximation to rationally approximate the fractional integral operator.

[0017] Further, the method also includes using a linear active disturbance rejection control (LADRC) algorithm to improve the voltage outer loop control in the double closed loop PI control model.

[0018] Further, a particle swarm optimization algorithm is used to determine the parameters of the LADRC voltage outer loop controller.

[0019] In a second aspect, the present application provides a control system of a three-phase six-switch VIENNA rectifier, comprising a current inner loop control module, a voltage outer loop control module, a three-level modulation module and a neutral point potential balancing module, characterized in that the current inner loop control module is a fractional order PI control module.

[0020] Further, the voltage outer loop control module is a linear active disturbance rejection control (LADRC) module.

[0021] The present application has the beneficial effect that the fractional order PI current inner loop controller, on the basis of the integer order PI current inner loop controller, generalizes the integral order from integer 1 to any number, improving the degree of freedom of parameter adjustment. By adjusting the order value, the control structure of the fractional order PI current inner loop controller can be designed to adapt to the control object to achieve better control performance. The continuous adjustability of the fractional order integral order can more subtly compensate for current tracking errors. Compared with the traditional integer order PI current inner loop controller, it has stronger suppression ability for low-order harmonics (such as 3 times and 5 times) other than the fundamental of the power grid, can improve the accuracy of the system output in the steady state, realize high steady-state current accuracy and low harmonic distortion rate, and at the same time reduce the overshoot of the system, showing faster dynamic response speed and stronger robustness. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 The three-phase six-switch VIENNA rectifier control method flowchart of the first embodiment of the present application; Figure 2 、 Three Topological structure diagram of the three-phase six-switch VIENNA rectifier; Figure 3 、 Three Equivalent circuit diagram of the three-phase six-switch VIENNA rectifier in d-q coordinate system; Figure 4 Flowchart of improving PI current inner loop control by using fractional order PI algorithm; Figure 5 、 Three Control block diagram of the fractional order PI current inner loop of the three-phase six-switch VIENNA rectifier; Figure 6 Input voltage and current waveform diagram of phase a during simulation running of the first embodiment of the present application; Figure 7 Three-phase input current waveform diagram during simulation running of the first embodiment of the present application; Figure 8 Three-phase input current waveform diagram of the simulation model of the PI voltage outer loop and PI current inner loop during half-load power start; Figure 9, the simulation model of the PI voltage outer loop and the fractional order PI current inner loop, three-phase input current waveform diagram during half load power starting; Figure 10 , the closed-loop control structure block diagram of the first-order linear active disturbance rejection control (LADRC) voltage outer loop in the second embodiment of the present application; Figure 11 , the DC side voltage and a-phase voltage and current waveform diagram during simulation running of the second embodiment of the present application; Figure 12 , the simulation model of the PI voltage outer loop and the fractional order PI current inner loop, and the simulation model of the LADRC voltage outer loop and the fractional order PI current inner loop, comparison diagram during half load power starting process; Figure 13 , the structure schematic diagram of the three-phase six-switch VIENNA rectifier control system in the third embodiment of the present application. DETAILED DESCRIPTION

[0023] In order to make the above objectives, features and advantages of the present application more apparent, specific embodiments of the present application are described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work should belong to the protection scope of the present application.

[0024] In the following description, many specific details are set forth in order to provide a thorough understanding of the present application. However, the present application can be practiced without the specific details, which are not described in the present application, and it can be apparent to those skilled in the art that the present application can be implemented in other different ways. Therefore, the present application is not limited to the specific embodiments disclosed below.

[0025] Secondly, the "one embodiment" or "embodiment" referred to herein can include specific features, structures or characteristics contained in at least one implementation of the present application. "In one embodiment" appearing in different places in the specification does not mean the same embodiment, nor is it an independent or alternative embodiment that excludes other embodiments.

[0026] The present application is described in detail in conjunction with the schematic diagram. In the detailed description of the embodiments of the present application, the cross-sectional view of the device structure is locally enlarged without the general proportion for the convenience of description, and the schematic diagram is only an example, which should not limit the protection scope of the present application herein. In addition, the three-dimensional spatial dimensions of length, width and depth should be included in the actual manufacture.

[0027] Meanwhile, in the description of the present application, it should be noted that the terms "upper, lower, inner and outer" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application. In addition, the terms "first, second or third" are only for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0028] Unless otherwise explicitly specified and limited, the terms "mounting, connection, connection" in the present application should be understood broadly, for example: it can be fixed connection, detachable connection or integral connection; it can also be mechanical connection, electrical connection or direct connection, it can also be indirectly connected through an intermediate medium, or it can be the communication between two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0029] Embodiment one As Figure 1 shown, an embodiment of the present application provides a control method of a three-phase six-switch VIENNA rectifier, comprising: A conventional double-loop PI control model of the three-phase six-switch VIENNA rectifier is established, and the conventional double-loop PI control model comprises PI current inner loop control and PI voltage outer loop control. The conventional double-loop PI control model of the three-phase six-switch VIENNA rectifier can be established by using Kirchhoff's law, Laplace transform and the like. It should be noted that the establishment of the conventional double-loop PI control model of the three-phase six-switch VIENNA rectifier belongs to the prior art, and the present application will not be described in detail here.

[0030] The current inner loop control in the conventional double-loop PI control model is improved by using a fractional order PI algorithm, as Figure 4 shown, the specific steps are as follows: Step one, according to the topology structure of the three-phase six-switch VIENNA rectifier, a mathematical model thereof in dq coordinates is established.

[0031] 1.1 The topology structure of the three-phase six-switch VIENNA rectifier is as Figure 2 shown, the KVL equation and the KCL equation in the three-phase coordinate system are established according to Kirchhoff's law.

[0032] 1.2 The loop equation in the three-phase coordinate system is transformed into the d-q coordinate system, and the mathematical model is as follows: (Formula 1) Wherein, L is the AC side boost inductance, C1, C2 are DC side capacitors, up, un are the voltage on the DC side capacitors C1, C2, id, iq are the d-axis current and q-axis current respectively, ud, uq are the d-axis and q-axis AC input voltage, R is the DC side resistance, iR is the current flowing through the resistance R, Sxy (x=d, q, y=p, n) is the different state of d-axis and q-axis switch.

[0033] The equivalent circuit of three-phase six-switch VIENNA rectifier in d-q coordinate system is shown in Figure 3

[0034] Step two, eliminate the influence of coupling in the model by using feedforward decoupling method. One of the goals of VIENNA rectifier current inner loop control is to maintain good sinusoidal degree of three-phase input current, but from the mathematical model of three-phase six-switch VIENNA rectifier in d-q coordinate system in (formula 1), there is still an AC coupling part between the component in d-axis and the component in q-axis. Therefore, based on d-q coordinate system, the influence of coupling is eliminated by using feedforward decoupling method. Specifically as follows: 2.1 Define variables As follows: (Formula 2) According to (formula 1), we can get: (Formula 3) 2.2 The general transfer function of integer order PI controller is: (Formula 4) Based on the design principle of PI controller, in the complex domain, the current inner loop control should satisfy the following formula: (Formula 5) Wherein, is the proportional coefficient, is the integral coefficient, and are the d-axis and q-axis current inner loop given values from the voltage outer loop control, 1 / s is the integral operator in the complex domain, After converting (formula 3) in the complex domain, combined with (formula 5), we can get: (Formula 6)

[0035] Step three: establish the control object expression based on integer order PI current inner loop. Specifically as follows: ​After the above decoupling control, the d-axis and q-axis of the current inner loop are separated and can be independently controlled. When the system works at unity power factor, the q-axis component of the current should be 0, and the d-axis current is given as the output of the voltage outer loop. The d-axis current control is taken as an example due to the symmetry of the dq two-phase system, and the current formula is expressed as: (7) Considering the combined effect of current sampling delay and output delay of the rectifier, the d-axis current control object expression is: (8) where Ts is the sampling frequency, is the inertia element considering the comprehensive delay; Combining (4) and (8), the control object expression of the integer-order PI current inner loop is: (9).

[0036] Step four: adjust the integral order from integer 1 to , and establish the current inner loop control expression based on the fractional-order PI controller. Specifically as follows: 4.1 The transfer function time domain expression of the fractional-order PI controller is: (10) where e(t) is the error signal, is the proportional coefficient, is the integral coefficient, represents the integral operator of order; after Laplace transform to S domain, the expression is: (11); 4.2 According to (9) and (11), the current inner loop control expression of the fractional-order PI is: (12) where R is the output load on the DC side, Ls is the expression of inductance in the complex domain, Ts is the sampling period, is the inertia element considering the comprehensive delay, KP is the proportional coefficient, KI is the integral coefficient, is the fractional-order integral operator in the complex domain, is the integral order, .

[0037] Step five: adjust the parameters of the fractional-order PI controller.

[0038] In the design of fractional-order PI controllers, the control effect obtained by the controlled object varies with the selected integral order λ. Increasing the value of λ can improve the accuracy of the system's output in steady state, thereby reducing the system's overshoot, but this must be within a reasonable range. If the value of λ is adjusted too large, the system's settling time and overshoot will increase, resulting in severe oscillations and reducing the accuracy of the steady-state output. Compared with traditional PI controllers, the added integral order parameter λ of fractional-order PI controllers gives the controller better control performance, but it also increases the control complexity. Therefore, selecting an appropriate parameter tuning method and performing reasonable parameter tuning based on the known information of the system is particularly important for achieving the controller design objectives.

[0039] This embodiment uses the phase margin combined method to tune the parameters of the fractional-order PI controller. The tuning steps of the phase margin combined method include: 5.1 At the cutoff frequency At this point, the open-loop transfer function of the control system has the following amplitude characteristics: (Equation 13); 5.2 Control System It has the following phase angle characteristics: (Equation 14); 5.3 The system gain satisfies the following condition: (Equation 15); 5.4 Let the gain margin be The control system crosses the frequency The following conditions must be met: (Equation 16); 5.5 The equations are solved using the fmincon function in the MATLAB optimization toolkit, at the switching frequency. In a VIENNA rectifier set to 30kHz, the current loop cutoff frequency is selected. Switching frequency 1 / 10, =3×10³Hz, phase margin Input current sampling period The fractional-order PI controller parameters are obtained as follows: .

[0040] The transfer function of the VIENNA rectifier current inner loop fractional-order PI controller established based on this parameter is as follows: (Equation 17).

[0041] Step six: the integral operator is approximated by the joint optimal rational approximation based fractional calculus operator approximation method .

[0042] Since the fractional calculus operator is irrational, it is difficult to realize fractional order control using a circuit, and in practical applications, it needs to be discretized to adapt to the form of a digital controller. The joint optimal rational approximation based fractional calculus operator approximation method is the prior art, and this paper will not expand on it. Using the above method, the rational approximation expression A(s) / B(s) of the integral operator is: (Formula 18).

[0043] After the above steps, the current inner loop control method in the traditional double closed loop PI control model has been improved to an implementable fractional order PI current inner loop control method. To verify the performance of the fractional order PI current inner loop controller, a Simulink simulation model of the fractional order PI current inner loop controller is built based on the above method, and the PI controller of the voltage outer loop uses the original traditional PI controller. The simulation parameters of the three-phase six-switch VIENNA rectifier circuit are as follows:

[0044] Running the simulation model, the a-phase input voltage and current waveforms are as shown in Figure 6 and the three-phase current waveforms are as shown in Figure 7 The three-phase input voltage and current are in phase and maintain sinusoidal, which preliminarily verifies the stability of the fractional order PI controller.

[0045] To further verify the performance of the controller, the traditional integer order PI control and the improved fractional order PI control are used in the current inner loop respectively under the same PI voltage outer loop, and the simulation model is set to start at half load 15kW power. The three-phase input current waveforms of the improved three-phase six-switch VIENNA rectifier are as shown in Figure 8 and the three-phase input current waveforms of the improved three-phase six-switch VIENNA rectifier are as shown in Figure 9 As can be seen from the figure, after the system starts, the three-phase current waveforms under the traditional integer order PI current inner loop control reach steady state after about 15ms, and the current peak value reaches 105A and the valley value reaches -106A during this process. While the three-phase current waveforms under the improved fractional order PI current inner loop control reach steady state after 10ms, and the current peak value and valley value are 92A and -95A respectively. Compared with the original control method, the improved fractional order PI current inner loop control reaches steady state faster and has smaller current fluctuation, showing better dynamic performance.

[0046] Example two: On the basis of embodiment one, in order to meet the demand of higher dynamic response, this embodiment introduces the linear adjust disturbance rejective control (LADRC) method with the ability of disturbance estimation and elimination to improve the original voltage outer loop PI controller.

[0047] Step one: establish the basic framework of LADRC.

[0048] LADRC consists of the following three parts: (1) Tracking Differentiator (TD), its function is to differentiate the input signal, extract the discrete signal as a continuous signal, and soften the starting link. The input of the actual voltage outer loop controller is a fixed DC voltage reference value, so there is no need to configure an additional tracking differentiator link.

[0049] (2) Linear Extended State Observer (LESO), as the most important part of LADRC, this module is inspired by the state observer, which combines internal and external disturbances in the system, observes and compensates the state variables, and converts the observed variables into integral series standard form to eliminate the disturbance effect. It has the advantages of obtaining estimated values without direct measurement and almost no dependence on the model that produces disturbance. At the same time, the concept of observer bandwidth is introduced, which connects the physical meaning and mathematical expression of parameters, effectively simplifying the parameter tuning process. The standard form is: (Equation 19) Where, , : the time derivative of the state of the observer; , : the observed value of the system output (or the observed state); : the observed value of the total disturbance of the system (including internal and external disturbances); v: the input signal of the system (such as the output of the controller); y: the actual output of the system (used for feedback correction of the observer); , : the gain parameter of the observer, used to adjust the convergence speed and observation accuracy of the observer, which needs to be adjusted according to the system characteristics (such as bandwidth, stability requirements); : the known control gain parameter of the system, which is related to the inherent characteristics of the system's actuator or controlled object.

[0050] ​(3) Linear State Error Feedback (LSEF) control law, which is designed using linear PID combination. The expression is: (20) wherein, is the adjustable combination coefficient of the system.

[0051] Step two: design the voltage outer loop controller of three-phase six-switch VIENNA rectifier based on LADRC.

[0052] In the voltage outer loop of three-phase six-switch VIENNA, in order to realize real-time observation of the system DC side voltage and disturbance, the input quantity v of the controller and the output y of the controller are respectively the DC voltage reference value and the d-axis current reference value , combined with (19) and (20), the closed-loop control structure block diagram of the first-order linear active disturbance rejection control voltage outer loop is shown in Figure 10 .

[0053] Step three: use bandwidth tuning method to tune the parameters of LADRC voltage outer loop controller.

[0054] The poles of LESO control equation are configured at , and the poles of LSEF control equation are configured at , wherein, is the bandwidth of the observer, is the bandwidth of the controller, and at this time the parameters of the LADRC controller are: (21) At this time, the solution of the controller parameters is essentially the configuration of the observer bandwidth and the controller bandwidth , by adjusting the observer bandwidth , the controller bandwidth and the gain coefficient , the dynamic performance of the controller can meet the requirements. Among the parameters, determines the speed of LESO disturbance estimation, and reasonable increase of can reduce the state estimation error and improve the tracking performance of the observer, but too large may cause the observer to oscillate due to noise and other factors, thereby affecting the output of the system; within a certain adjustment range, the response speed of the system is proportional to the value of , but too large will cause system oscillation, so it should be determined according to the requirements of the system on transient response; b0 represents the known part of the system gain, which can be obtained by modeling the characteristics of the controlled object.

[0055] Preferably, the LADRC controller parameters are determined based on a particle swarm optimization algorithm.

[0056] The parameter tuning method based on the bandwidth configuration method is widely used in the application process of the LADRC, but in actual engineering problems, the bandwidth parameter still needs to be debugged for a long time to obtain an ideal control effect. The swarm intelligence optimization algorithm (SIA) using computer technology to seek an optimal solution has a good effect on solving multi-objective optimization problems. As one of the SIA, the particle swarm optimization algorithm has the advantages of fast iteration speed and small difficulty in obtaining an optimal solution, and is widely used in the field of function optimization of industrial control. The principle and application mode of the particle swarm optimization algorithm are prior art, and will not be described in detail herein.

[0057] The optimization purpose of the embodiment is to determine suitable active disturbance rejection controller parameters 、 and , so that the VIENNA rectifier voltage outer loop obtains good dynamic performance, and therefore the integral of time multiplied by absolute error (ITAE) of the absolute value of the difference between the output and the given value and time is used as a performance index. A larger ITAE value indicates that the value is farther away from the expected value, and a smaller ITAE indicates that the value is closer to the expected value, and the convergence speed is faster. In the embodiment, the number of population particles is set to 70, and the number of iterations is 70, that is, 70 parameters participate in optimization each time, and after 4900 times of optimization, the optimal voltage outer loop controller parameters under the ITAE performance index with a penalty function are obtained.

[0058] The optimized voltage outer loop controller parameters obtained after the algorithm runs optimization in the simulation model in Simulink are as follows:

[0059] After the above steps, the voltage outer loop control method has been improved to the linear active disturbance rejection controller LADRC. To verify the method of the embodiment, a simulation model is built on the MATLAB / Simulink platform based on the above derivation. The simulation parameters of the three-phase six-switch VIENNA rectifier are as follows:

[0060] First, the simulation verification under steady state is performed. Under the improved fractional order PI current inner loop control and the linear active disturbance rejection voltage outer loop controller LADRC control, the DC side output voltage and the AC side a-phase input voltage and current waveforms when the system is in steady state are as follows:Figure 11 As shown in the figure, it can be observed that the DC side output voltage is stably tracked at the given value of 750 V, the input voltage and current waveforms are in phase, and the stability of the controller model is preliminarily verified.

[0061] To verify the performance of the LADRC voltage outer loop controller, the VIENNA rectifier under the PI voltage outer loop control and the LADRC voltage outer loop control is compared respectively, and the dynamic response process of the system is observed, while the inner loop controller simultaneously adopts the fractional order PI current inner loop controller designed in embodiment one. Figure 12 As shown in the figure, the upper and lower capacitor voltages are set to 200 V in advance to simulate the pre-charging condition of the capacitor in the actual start-up. As can be observed from the figure, when the PI control is used in the voltage outer loop, the system has an overshoot of about 33 V at the start-up, and it takes about 35 ms to reach the steady state; when the LADRC voltage outer loop control is used, the system has almost no overshoot, and it only takes 14 ms to reach the steady state.

[0062] Embodiment three: On the basis of the method described in embodiment one and embodiment two, a control system of a three-phase six-switch VIENNA rectifier is provided, as shown in Figure 13 The control system includes a fractional order PI current inner loop control module, a linear active disturbance rejection LADRC voltage outer loop control module, an SVPWM modulation module, and a midpoint potential balancing module.

[0063] SVPWM modulation is the most commonly used modulation strategy for a three-phase six-switch VIENNA rectifier, and SVPWM modulation and midpoint voltage balancing control based on SVPWM modulation belong to the prior art, which will not be described in detail here.

Claims

1. A control method for a three-phase six-switch VIENNA rectifier, comprising establishing a dual-closed-loop PI control model for the three-phase six-switch VIENNA rectifier, wherein the dual-closed-loop PI control model includes an inner current loop control and an outer voltage loop control, characterized in that... The current inner loop control in the dual closed-loop PI control model is improved by adopting a fractional-order PI algorithm.

2. The method according to claim 1, characterized in that... Improving the inner current loop control in the dual-closed-loop PI control model using a fractional-order PI algorithm specifically includes: A mathematical model of the three-phase six-switch VIENNA rectifier in the dq coordinate system is established. The influence of coupling in the model is eliminated by using feedforward decoupling. The decoupled model is used to establish the expression for the control object based on the integer-order PI current inner loop. Adjust the integral order of the transfer function of the integer-order PI controller from integer 1 to... , Establish the expression for the control object based on the fractional-order PI current inner loop; Set the parameters of the fractional-order PI current inner loop controller.

3. The method as described in claim 2, characterized in that... The transfer function of the fractional-order PI current inner loop controller is: The expression for the control object based on the fractional-order PI current inner loop is: Where R is the DC-side output load, Ls is the inductance in the complex domain, and Ts is the sampling period. It is an inertial element that takes into account the overall delay, K P It is the proportionality coefficient, K I It is the integral coefficient. It is a fractional integral operator in the complex field. For the order of integration, .

4. The method as described in claim 2, characterized in that... The parameters of the fractional-order PI current inner loop controller are set using a combination of phase and magnitude margins, specifically including: at cutoff frequency At this point, the open-loop transfer function of the control system has the following amplitude characteristics: control system It has the following phase angle characteristics: The system gain must satisfy the following condition: Let the magnitude margin be The control system crosses the frequency The following conditions must be met: 。 5. The method as described in claim 4, characterized in that... The parameters for tuning the fractional-order PI current inner loop controller also include a rational approximation of the fractional-order integral operator using a fractional-order calculus operator approximation method based on joint optimal rational approximation.

6. The method as described in claim 5, characterized in that... At switching frequency In a three-phase six-switch VIENNA rectifier set to 30kHz, the current loop cutoff frequency is selected. Switching frequency 1 / 10, =3×10³Hz, phase margin Input current sampling period The fractional-order PI controller parameters are obtained as follows: Based on these parameters, the transfer function of the fractional-order PI controller for the inner current loop of the three-phase six-switch VIENNA rectifier is as follows: The rational approximation expression A(s) / B(s) is: 。 7. The method according to claim 1, characterized in that... The method also includes improving the voltage outer loop control in the dual-closed-loop PI control model by using the Linear Active Disturbance Rejection Control (LADRC) algorithm.

8. The method according to claim 7, characterized in that The parameters of the LADRC voltage outer loop controller were determined using the particle swarm optimization algorithm.

9. A control system for a three-phase six-switch VIENNA rectifier, comprising an inner current control module, an outer voltage control module, a three-level modulation module, and a neutral point potential balancing module, characterized in that... The current inner loop control module is a fractional-order PI control module.

10. The system as described in claim 9, characterized in that... The voltage outer loop control module is a linear active disturbance rejection (LADRC) control module.

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