Control method of three-phase VIENNA rectifier

Abstract

The invention discloses a control method of a three-phase VIENNA rectifier, and relates to the technical field of power supply control. According to the technical key points, backstepping control is adopted on the basis of a voltage outer loop, an improved variable exponential reaching law sliding mode controller is adopted on a current inner loop, a nonlinear part in a system can be reserved through backstepping control, and the control performance of the system is improved. The improved variable exponential approach rate can change along with time and the output condition of the system, the shaking condition of the system on a sliding mode surface can be effectively reduced, the control effect in a VIENNA rectifier system is improved, the change can be made according to the change of the system structure by using sliding mode control, and the control efficiency is improved. The motion state of the system finally returns to the constraint point, and the stability of sliding mode control is better than that of PI control.

Description

Technical Field

[0001] The invention relates to the technical field of power supply control, and in particular to a control method for a three-phase VIENNA rectifier. Background Art

[0002] In recent years, VIENNA rectifiers have been rapidly and widely used in medium and high voltage high power occasions due to their advantages of high power density, high power factor, simple control and high reliability. Three-phase VIENNA rectifiers have been widely used in high power occasions such as charging piles, and there are countless theoretical studies on their control strategies.

[0003] Traditional PI control has a long startup response time and a high overshoot, while sliding mode control has excellent control performance for nonlinear systems. Sliding mode variable structure control is often used in nonlinear control systems. Its unique variable structure control is mainly reflected in the discontinuity of the control process. It has the advantages of fast response speed and simple implementation. Backstepping control is a recursive design method for uncertain parameters. Its advantage is that it can retain the nonlinear part of the system, thereby improving the control performance of the system. The traditional PI dual-loop control strategy has achieved relatively good results in the VIENNA rectifier, but it still lacks in response speed and stability. Summary of the invention

[0004] In view of the deficiencies in the prior art, the object of the present invention is to provide a control method for a three-phase VIENNA rectifier, which reduces the time required for the DC side voltage to reach the target steady state, while reducing the voltage overshoot and improving the steady-state performance and safety performance of the system.

[0005] The above object of the present invention is achieved through the following technical solutions:

[0006] A control method for a three-phase VIENNA rectifier is disclosed. The input of each phase of the VIENNA rectifier is regarded as being connected to the positive electrode, the midpoint or the negative electrode of the output capacitor by a three-way switch, and a function description is performed on the switch. According to Park transformation and Clarke transformation, a mathematical model of the VIENNA rectifier in a two-phase stationary coordinate system and a two-phase rotating coordinate system can be obtained, and the transformation matrices T1 and T2 of the two are obtained respectively: The mathematical models of the αβ coordinate system and the dq coordinate system are obtained at the same time. The sliding mode control is selected and the improved variable exponential reaching law is used to control the inner current loop and the backstepping control is used to control the outer voltage loop.

[0007] In a preferred example, the present invention can be further configured as follows: the function of the switch state is described as: Where P, n, and O are the positive electrode, midpoint, and negative electrode of the output side; and the switching function satisfies: Where Q xP , Q xO , Q xn They are the switching states when the x phase (x=a, b, c) is connected to the positive pole, midpoint, and negative pole of the DC output.

[0008] By adopting the above technical solution,

[0009] In a preferred example, the present invention can be further configured as follows: the mathematical models of the αβ coordinate system and the dq coordinate system are respectively

[0010]

[0011] Where: i α 、i β is the component of the current on the α and β axis grid sides; u α 、u β is the component of the three-phase AC voltage on the α and β axes; Q αi , Q βi (i=p, n) is the switching function represented by the α and β axes; ω is the fundamental angular frequency; i d 、i q is the grid-side current component of the d and q axes; u d 、u q is the d and q axis three-phase AC voltage component; Q di , Q qi (i=p, n) is the switching function represented by the d and q axes.

[0012] In a preferred example, the present invention can be further configured as follows: the sliding mode controller in the above-mentioned embodiment: select the control variable

[0013]

[0014] Where: is the VIENNA rectifier current command in the dq coordinate system.

[0015] In a preferred example, the present invention can be further configured as follows: for the above control variables, an exponential approach rate is selected:

[0016]

[0017] ε i 、n i , α i ,β1,k i are the sliding mode parameters of the current loop sliding mode controller respectively. are the control voltages in the dq coordinate system respectively.

[0018] In summary, the present invention includes at least one of the following beneficial technical effects:

[0019] 1. By improving the variable exponential approach rate, which changes with time and the output of the system, the jitter of the system on the sliding surface can be effectively reduced, and the control effect in the VIENNA rectifier system is improved;

[0020] 2. By using sliding mode control, changes can be made according to changes in the system structure. The motion state of the system will eventually return to the constraint point, and the stability of sliding mode control is better than PI control. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the topological structure of the VIENNA rectifier in this technical solution;

[0022] Figure 2 It is a control block diagram of the current inner loop SMC in this technical solution;

[0023] Figure 3 It is the simulated output waveform of the DC voltage controlled by PI in this technical solution;

[0024] Figure 4 It is the DC voltage simulation output waveform of the backstepping sliding mode control in this technical solution;

[0025] Figure 5 It is the current and voltage waveform diagram of the AC side of the sliding mode control in this technical solution. DETAILED DESCRIPTION

[0026] The present invention is further described in detail below in conjunction with the accompanying drawings.

[0027] like Figure 1-4 As shown in FIG. 1 , a control method for a three-phase VIENNA rectifier disclosed in the technical solution is used. For the traditional dual-loop control strategy, according to the rectifier's own model parameters, such as Figure 1 As shown:

[0028] From the working state analysis of its circuit structure, it can be seen that the input of each phase of the VIENNA rectifier can be regarded as being connected to the positive electrode, midpoint or negative electrode of the output capacitor through a three-way switch after passing through the filter inductor.

[0029] Its switching state can be described by a function, as shown in formula (1).

[0030]

[0031] P, n, and O in the formula are the positive electrode, midpoint, and negative electrode of the output side, and the switching function satisfies formula (2).

[0032]

[0033] Where Q xP , Q xO , Q xn They are the switching states when the x phase (x=a, b, c) is connected to the positive pole, midpoint, and negative pole of the DC output.

[0034] The three-phase voltage equations of a, b, and c can be written using KVL:

[0035]

[0036] Where: u a 、u b 、u c is the three-phase AC voltage, i a 、i b 、i c are the three-phase corresponding AC currents; L is the filter inductance; r is the filter inductance equivalent resistance.

[0037] From the relationship between the switch state and the potential, we can get:

[0038]

[0039] According to the characteristics of the three-phase power grid: a +u b +u c =0,i a +i b +i c =0 into formula (3) and add them, then substitute formula (4) to get:

[0040]

[0041] Substituting formula (5) into formula (3), we can get:

[0042]

[0043] According to the KCL equation at point P, we can get:

[0044]

[0045] Similarly, we can get the n-point KCL equation:

[0046]

[0047] Where: i P and i n is the node current; is the load current.

[0048] According to Park transformation and Clarke transformation, the mathematical model of VIENNA rectifier in two-phase stationary coordinate system and two-phase rotating coordinate system can be obtained. The transformation matrices T1 and T2 of the two are:

[0049]

[0050] Then the mathematical models in the αβ coordinate system and the dq coordinate system are obtained as follows:

[0051]

[0052] Where: i α 、i β is the component of the current on the α and β axis grid sides; u α 、u β is the component of the three-phase AC voltage on the α and β axes; Q αi , Q βi (i=p, n) is the switching function represented by the α and β axes; ω is the fundamental angular frequency; i d 、i q is the grid-side current component of the d and q axes; u d 、u q is the d and q axis three-phase AC voltage component; Q di , Q qi (i=p, n) is the switching function represented by the d and q axes.

[0053] In view of the instability and susceptibility to external interference of traditional PI control, this technical solution adopts sliding mode control and uses an improved variable exponential reaching rate to control the inner current loop. Sliding mode control can make changes according to changes in the system structure, and the motion state of the system will eventually return to the constraint point. Its control stability is better than PI control.

[0054] The sliding mode controller is designed as follows:

[0055] Selecting control variables

[0056]

[0057] Where: is the VIENNA rectifier current command in the dq coordinate system.

[0058] The exponential reaching law sliding mode control function is selected for the control variable.

[0059] In order to achieve better control effect, this paper adopts the improved variable exponential approach rate:

[0060]

[0061] According to the first two terms of formula (11), formula (12) and formula (13), the SMC control model is obtained:

[0062]

[0063] ε i 、n i , α i ,β1,k i are the sliding mode parameters of the current loop sliding mode controller respectively. are the control voltages in the dq coordinate system respectively. At this point, the design of the current inner loop sliding mode controller of the three-phase VIENNA rectifier is completed. The current inner loop control block diagram is shown in Figure 2 As shown in Equation 15, the controller output is a current reference value, which is mainly related to the DC side voltage and load current. It is easy to control. The adjustment value can adjust the control effect of the sliding mode controller. The smaller the value, the faster the dynamic response of the system. However, if the value is too small, it will cause oscillation of the system and cause instability. Therefore, the value should be reasonably adjusted according to the actual situation to achieve the most ideal output effect.

[0064] In the voltage outer loop control, since the PI controller cannot accurately track the target voltage and is easily affected by external environmental interference and other factors, PI control is not the best choice for the voltage outer loop. The backstepping control strategy can retain the nonlinear control part of the system, can track the target voltage more accurately and have a faster dynamic response. Its control effect is better than PI control. Therefore, this paper adopts the voltage inner loop backstepping controller.

[0065] According to the law of conservation of power, the power on the DC side and the AC side of the rectifier is conserved. Combining the last two terms of equation (11) in the dq coordinate system, the DC side power Pdc can be obtained:

[0066]

[0067] The combined equations can be expressed as:

[0068]

[0069] After the combined equation (11), we can get:

[0070]

[0071] According to the quasi-linearization method, we can set:

[0072] ωLi q =V d ,ωLi d =V q #(19)#

[0073] According to the analysis of the system after dq coordinate transformation, it is known that the voltage vector u d =U n ,u q =0.

[0074] Combining the first term of equation (11) with equation (18) and substituting it into equation (19), we can get the voltage outer loop power model:

[0075]

[0076] Set the voltage outer loop system input state variable to the output reference voltage U dcref The actual output voltage U dc The error of . We can get:

[0077]

[0078] Take the subsystem Lyapunov function V1 as:

[0079]

[0080] Where c2 is the integral weight coefficient, and its value must be a positive real number to ensure that V1≥0 is positive definite.

[0081] From formula (21), we can get:

[0082]

[0083] Since the voltage outer loop input is E r ,i d In the backstepping method, it is called a virtual control model, which can be designed as:

[0084]

[0085] Substituting formula (24) and formula (23) into formula (21), we can get:

[0086]

[0087] Let σ2=i d +σ1, and taking the derivative of V1, we get:

[0088]

[0089] Taking the derivative of σ2, we get:

[0090]

[0091] The second-order virtual control model σ3 is defined as follows:

[0092]

[0093] Let the intermediate variable u Ld =σ3, take the overall Lyapunov function V2 of the system and its derivative:

[0094]

[0095] In the formula, c3 is the integral weight coefficient. Only when its value is a positive real number can V2≥0 be positive definite, and its derivative is negative definite. According to the Lyapunov stability criterion, the overall system is stable.

[0096] In order to verify the effect of the improved sliding mode control based on the current inner loop and the voltage outer loop backstepping controller control, the simulation model of the traditional dual-loop PI control method, the backstepping control method and the improved approaching rate sliding mode control method is first built in Matlab. The system input is the three-phase balanced grid voltage u = 220V, the DC side output reference voltage Vdc = 750V, the DC side capacitor C1 = C2 = 2200μF, the inductor equivalent resistance r = 0.2Ω, the inductor L = 1.5mH, the system switching frequency is 20kHz, and the DC side output capacitor Rs = 50Ω.

[0097] like Figure 3 As shown in the figure, the DC voltage output obtained by using PI control for both the traditional voltage outer loop and the current inner loop, it can be observed from the figure that the traditional PI control output DC voltage has a large overshoot, the overshoot is 884V, and then the system reaches the target steady state in 0.18s, which takes a long time, and the large overshoot voltage is an unstable factor. Figure 4 As shown in the figure, the DC voltage output is simulated by the voltage outer loop backstepping controller control and the current inner loop sliding mode control. The DC voltage overshoot of the sliding mode control output is 753.4V, and the system reaches steady state in 0.006s. The control strategy used in this paper reduces the time required for the DC side voltage to reach the target steady state, and the voltage overshoot is smaller, which improves the steady-state performance and safety performance of the system. The backstepping controller voltage outer loop control and sliding mode inner loop control strategy used in this paper reduces the time required for the DC side voltage to reach the target steady state, and the voltage overshoot is smaller, which improves the steady-state performance and safety performance of the system.

[0098] like Figure 5 As shown in the figure, the A-phase current and voltage waveforms of the AC measurement when the sliding mode control strategy is used. From the figure, we can see that the input AC voltage u a With input current i a It can quickly track and be in the same phase, and the AC current can quickly reach a steady state in a shorter time.

[0099] In summary, the control strategy based on the backstepping controller in the voltage outer loop and the sliding mode controller with improved variable exponential reaching law in the current inner loop can balance the system in a very short time and maintain the bus voltage balance at the same time, which meets the operation requirements of the VIENNA rectifier system.

[0100] The embodiments of this specific implementation method are all preferred embodiments of the present invention, and are not intended to limit the protection scope of the present invention. Therefore, any equivalent changes made based on the structure, shape, and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A control method for a three-phase VIENNA rectifier, characterized in that: The input of each phase of the VIENNA rectifier is regarded as a three-way switch connected to the positive pole, midpoint or negative pole of the output capacitor, and its switch is described by function; according to Park transformation and Clarke transformation, the mathematical model of the VIENNA rectifier in the two-phase stationary coordinate system and the two-phase rotating coordinate system can be obtained, and the transformation matrices T1 and T2 of the two are obtained respectively: The mathematical models of the αβ coordinate system and the dq coordinate system are obtained at the same time. The sliding mode control is selected and the improved variable exponential reaching law is used to control the inner current loop and the backstepping control is used to control the outer voltage loop.

2. The control method of a three-phase VIENNA rectifier according to claim 1, characterized in that: The function description of the switch state is: Where P, n, and O are the positive electrode, midpoint, and negative electrode of the output side; and the switching function satisfies: Where Q xP , Q xO , Q xn They are the switching states when the x phase (x=a, b, c) is connected to the positive pole, midpoint, and negative pole of the DC output.

3. The control method of a three-phase VIENNA rectifier according to claim 1, characterized in that: The mathematical models of the above αβ coordinate system and dq coordinate system are respectively Where: i α 、i β is the component of the current on the α and β axis grid sides; u α 、u β is the component of the three-phase AC voltage on the α and β axes; Q α i.Q βi (i=p, n) is the switching function represented by the α and β axes; ω is the fundamental angular frequency; i d 、i q is the grid-side current component of the d and q axes; u d 、u q is the d and q axis three-phase AC voltage component; Q di , Q qi (i=p, n) is the switching function represented by the d and q axes.

4. The control method of a three-phase VIENNA rectifier according to claim 1, characterized in that: Sliding mode controller in the above: Select the control variable Where: is the VIENNA rectifier current command in the dq coordinate system.

5. The control method of a three-phase VIENNA rectifier according to claim 4, characterized in that: For the above control variables, select the exponential convergence rate: ε i 、n i , α i ,β1,k i are the sliding mode parameters of the current loop sliding mode controller respectively. are the control voltages in the dq coordinate system respectively.

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