Three-stage motor driving system based on WPT and control method

Abstract

The invention discloses a WPT-based three-stage motor driving system and a control method, according to the classification of electric energy conversion, the three-stage converter motor driving system can be divided into three parts, corresponding to three converters, namely an FSBB converter, a CLLC resonant converter and a motor system, two adjacent stages of converters are connected through a bus capacitor, and the motor system is connected with the FSBB converter, the CLLC resonant converter and the motor system. The FSBB converter and the CLLC resonant converter in the first two stages convert direct current into direct current, the third-stage converter converts the direct current into alternating current and supplies the alternating current to the permanent magnet synchronous motor, in the three-stage converter topology adopted in the invention, the FSBB converter plays a role in voltage regulation, and the CLLC converter adopts a loosely coupled high-frequency rotary transformer and has an electrical isolation function; according to the three-phase inverter power supply system, the circuit working safety is improved, meanwhile, the wireless electric energy transmission function can be achieved, the practical application range of the circuit is expanded, and the final target of the whole system is to provide appropriate and stable voltage for a three-phase inverter. A bus 2 at the input end of a three-phase inverter on the motor side is controlled by variable bus voltage, and if the load of the CLLC is changed, the resonant cavity efficiency of the CLLC converter is also changed according to the change of the load, so that the voltage of the bus 2 can always correspond to the optimal load.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronic converters, and in particular to a three-level motor drive system and control method based on WPT. Background Art

[0002] Electric vehicles have rapidly become popular in the context of the "dual carbon" in recent years due to their zero tail gas emissions and low noise. The motor drive system is an important part of electric vehicles. As people's requirements for motor drive systems become higher and higher, traditional single-stage converters are gradually unable to meet the drive requirements, and multi-stage converters are often required to improve system performance.

[0003] Wireless Power Transmission (WPT) technology is mainly carried out through magnetic induction between coils. The research on magnetic induction coupling WPT is relatively mature, and its magnetic induction loss is low. It can currently achieve high transmission efficiency. Magnetic induction coupling WPT is often used in wireless short-distance transmission scenarios. During the driving process of the car, the motor cable and signal line are often exposed to the external environment and may be disconnected due to bad road conditions. Therefore, WPT technology has gradually been applied to the field of motor drive. WPT technology effectively reduces the connection between lines and improves the safety of electricity use.

[0004] There are many types of multi-level converters, and the control strategies are also varied. In different application scenarios, it is necessary to adopt appropriate topological structures and design appropriate control strategies for different control objectives. For multi-level converters, independent closed-loop control strategies are mostly adopted, that is, by collecting the electrical parameters of their own converters (such as voltage, current, etc.) as feedback quantities to control the on-off status of the switches of each converter. However, for different topological structures of multi-level converters, the independent closed-loop control strategy sometimes cannot achieve the control objectives of the cascade system, and sometimes a joint closed-loop control strategy is needed to achieve the coordinated cooperation between the sub-converters of the multi-level converter system. Summary of the invention

[0005] The technical problem to be solved by the present invention is to address the deficiencies of the above-mentioned prior art and to provide a variable bus voltage control strategy. Under the voltage of the same bus 2, the efficiency of the CLLC converter resonant cavity varies with the load. For different loads, the bus 2 voltage, that is, the CLLC converter output voltage, is adjusted, so that the CLLC converter resonant cavity can continue to maintain a high efficiency without changing with the load.

[0006] In order to achieve the above purpose, the technical solution of the present invention is as follows:

[0007] The overall structure of the three-level series permanent magnet synchronous motor (PMSM) drive system for vehicles based on bidirectional WPT includes a four-switch Buck-Boost converter (FSBB), a CLLC series resonant converter with LCHFT and a three-phase PWM inverter. The adjacent two levels of the three-level converter are connected by a voltage-stabilizing capacitor, which acts as a DC bus. Vin and iin are the input voltage and input current respectively. The bus connecting the first-level FSBB converter and the second-level CLLC resonant converter is called bus 1, and the voltage across bus 1 is V1. The bus connecting the second-level CLLC resonant converter and the third-level three-phase inverter is called bus 2, and the voltage across bus 2 is V2. C1 and C2 are filter capacitors on buses I and II respectively. L1 is the inductor of the FSBB converter, iL1 is the current flowing through the inductor L1, and Lp and Ls are the primary and secondary resonant inductors of the CLLC converter, which can act as the primary resonant inductor and secondary resonant inductor of the CLLC converter respectively. During forward operation, Lm is the excitation inductance equivalent to the primary side of the magnetic coupling transformer, Cp, Cs, Lp, Ls and Lm constitute the resonant cavity of the CLLC converter, the resonant capacitor and the resonant inductor are connected in series to form a series resonant circuit, Cp and Cs are the primary and secondary resonant capacitors of the CLLC converter, Lm is the transformer excitation inductance, irp and irs are the primary and secondary resonant currents of the CLLC converter. In order to improve the efficiency of transformer energy transmission, the leakage inductance Lp and Ls of LCHFT are used as the primary and secondary resonant inductors, respectively, and Si (i = 1, 2, 3,,, 18) is the switch tube.

[0008] The FSBB converter constitutes the first stage of the entire system, which is essentially a Buck-Boost converter that has the function of regulating the output voltage of the FSBB converter;

[0009] The second-stage intermediate converter is a bidirectional CLLC resonant converter. Due to the natural soft switching characteristics of the CLLC resonant converter in a wide frequency range, the power transmission efficiency is high. It is widely used in WPT and switching power supplies. V1 is the input voltage of the CLLC resonant converter, C1 is the input capacitor, C2 is the output capacitor, R2 is the load resistor, and the switch tubes S5-S8 are the primary side full bridge of the CLLC, and S9-S12 are the secondary side full bridge of the CLLC. Cp and Cs are the primary side resonant capacitor and the secondary side resonant capacitor, respectively. Lp and Ls are the primary leakage inductance and the secondary leakage inductance of the magnetic coupling transformer, respectively, which can act as the primary side resonant inductor and the secondary side resonant inductor of the CLLC. In forward operation, Lm is the excitation inductance of the magnetic coupling transformer equivalent to the primary side. Cp, Cs, Lp, Ls and Lm constitute the resonant cavity of the CLLC converter. The resonant capacitor and the resonant inductor are connected in series to form a series resonant circuit.

[0010] The third stage is a three-phase inverter and PMSM, which realizes the conversion of DC to AC. The input end of the three-phase inverter is connected to the output end of the CLLC resonant converter through a voltage stabilizing capacitor.

[0011] Combined with the application scenario of the PMSM drive system composed of the three-level converter in this article, the design of the permanent magnet synchronous motor three-level converter motor drive system should meet the following basic requirements:

[0012] 1. The converters at all levels of the three-level converter motor drive system should be able to achieve bidirectional transmission of electric energy. When the electric energy is converted from DC to AC, the system drives the PMSM to work. This process is called forward transmission of electric energy. Conversely, the PMSM converts AC to DC to achieve electric energy regeneration. This process is called reverse transmission of electric energy.

[0013] 2. The system should be able to achieve fast voltage conversion over a wide range of input voltages and be able to quickly and accurately control the input voltage of the three-phase inverter. This process is mainly completed by the FSBB converter;

[0014] 3. Under different motor loads, the parameters of the components in the circuit should be able to keep the bus voltages at all levels stable, otherwise it may affect the operation of the motor;

[0015] 4. While ensuring that the three-level converter motor drive system can drive the motor to work normally, try to improve the energy conversion efficiency of the system.

[0016] Considering the resistance load of the rectifier bridge and CLLC as an integral load, when CLLC works in the resonant state, the resonant voltage v flowing out of the resonant cavity CD and the resonant current i 2_rec In phase, the overall load can be regarded as a resistor, and its equivalent resistance is:

[0017]

[0018] For the resonant cavity, there are two resonant frequencies:

[0019]

[0020]

[0021] R eq =n 2 R 2_eq , C s ′=C s / n 2 , L s ′=n 2 L s , n is the transformer ratio, R sis the stator winding resistance, fs is the switching frequency, then according to

[0022] The CLLC converter works in the resonant state, and the transmission efficiency of the resonant cavity can be expressed as:

[0023]

[0024] According to formula (4), the efficiency of the CLLC resonant cavity is related to the equivalent resistance load R eq In order to improve the system efficiency when the load of CLLC changes, the partial derivative of equation (4) is taken, and the equivalent resistance (optimal resistance) at the maximum efficiency point can be expressed as:

[0025]

[0026] In the formula, the values ​​of Rp and Rs are very small and their product can be ignored.

[0027] When the motor load changes, that is, when the CLLC load changes, in order to maintain the optimal efficiency of the system, the output voltage V of bus 2 can be controlled 2ref , so that it satisfies:

[0028]

[0029] Where P M is the power consumed by the motor, V 2min is the minimum acceptable voltage value of the three-phase inverter, V 2ref It is the voltage reference value of bus 2.

[0030] For the bus 2 at the input end of the three-phase inverter on the motor side, fixed bus voltage control is generally adopted. However, when the bus 2 adopts fixed bus voltage control, if the load of the CLLC changes, it can be seen from equations (4), (5) and (6) that the resonant cavity efficiency of the CLLC converter will also change according to the load change.

[0031] In order to ensure that the CLLC converter always has a high transmission efficiency when switching loads, the VBVC strategy is used to control the bus 2 voltage V2. The control block diagram is as follows: Figure 2 As shown. The control object is bus 2 voltage V2, so in the actual control process, firstly, bus 2 voltage V2 is sampled to calculate the motor power PM at this time, and then bus 2 voltage reference value V2ref under the load condition is calculated, and then the voltage value is compared with the reference value V2ref, and the difference enters the PI regulator, and then the appropriate FSBB working mode is selected to control the on-off state of the FSBB switch tube for voltage regulation, so that bus 2 voltage V2 follows the reference value V2ref.

[0032] The present invention has the following beneficial effects:

[0033] According to different loads, the bus 2 voltage, that is, the CLLC converter output voltage, is adjusted, so that the CLLC converter resonant cavity can continuously maintain a high efficiency without changing with the load. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 A general diagram of the topology of a three-level motor drive system based on WPT

[0035] Figure 2 Block diagram of the joint control strategy of the three-level converter of the three-level motor drive system based on WPT

[0036] Figure 3 The block diagram of the bus voltage control for the three-level motor drive system based on WPT

[0037] Figure 4 Comparison of the resonant cavity efficiency of the CLLC converter under the variable bus voltage control strategy and the constant bus voltage control strategy.

Claims

1. A three-level motor drive system based on WPT, characterized by: The overall structure of the automotive three-level series permanent magnet synchronous motor (PMSM) drive system based on bidirectional WPT, including a four-switch Buck-Boost converter (FSBB), a CLLC series resonant converter with LCHFT, and a three-phase PWM inverter; The adjacent two stages of the three-stage converter are connected by a voltage-stabilizing capacitor, which acts as a DC bus. For convenience, the bus connecting the first-stage FSBB converter and the second-stage CLLC resonant converter is called bus 1, and the voltage across bus 1 is V1. The bus connecting the second-stage CLLC resonant converter and the third-stage three-phase inverter is called bus 2, and the voltage across bus 2 is V2. Vin and iin are the input voltage and input current respectively. The bus connecting the first-stage FSBB converter and the second-stage CLLC resonant converter is called bus 1, and the voltage across bus 1 is V1. The bus connecting the second-stage CLLC resonant converter and the third-stage three-phase inverter is called bus 2, and the voltage across bus 2 is V2. C1 and C2 are the filter capacitors on bus I and II respectively. L1 is the inductor of the FSBB converter, iL1 is the current flowing through the inductor L1, and Lp and Ls are the primary and secondary resonant inductors of the CLLC converter respectively, which can act as the primary and secondary resonant inductors of the CLLC. During forward operation, Lm is the excitation inductance equivalent to the primary side of the magnetic coupling transformer, Cp, Cs, Lp, Ls and Lm constitute the resonant cavity of the CLLC converter, the resonant capacitor and the resonant inductor are connected in series to form a series resonant circuit, Cp and Cs are the primary and secondary resonant capacitors of the CLLC converter, Lm is the transformer excitation inductance, irp and irs are the primary and secondary resonant currents of the CLLC converter. In order to improve the efficiency of transformer energy transmission, the leakage inductance Lp and Ls of LCHFT are used as the primary and secondary resonant inductors, respectively, and Si (i = 1, 2, 3,,, 18) is the switch tube.

2. The three-level motor drive system based on WPT according to claim 1 is characterized in that: According to the classification of electric energy conversion, the three-level converter motor drive system can be divided into three parts, corresponding to three converters, namely FSBB converter, CLLC resonant converter and motor system. The adjacent two-level converters are connected through bus capacitors; FSBB converter is suitable for many occasions with a wide input voltage range. At the same time, the input voltage and output voltage of FSBB converter have the same polarity, which is suitable for bidirectional transmission of electric energy. In addition, since FSBB converter has four switch tubes, the on-off sequence and on-off time of the four switch tubes of FSBB can be flexibly controlled. Therefore, FSBB converter also has a variety of control methods and voltage regulation modes, which can be in boost mode, buck mode and buck-boost mode. The CLLC resonant converter has a completely symmetrical structure and can realize bidirectional transmission of electric energy. When running in the forward direction, the switch tubes S5-S8 are turned on in order, S5 and S6 are turned on complementary, S7 and S8 are turned on complementary, and the duty cycle of S5 and S8 is equal, which is equivalent to an inverter full bridge. 12 Always off, using its own anti-parallel diode to form a rectifier full bridge; Permanent magnet synchronous motors (PMSMs) are lightweight, small, and highly efficient, and are widely used in motor drive systems; The first two stages of FSBB converter and CLLC resonant converter convert DC power into DC power, and the third stage converter converts DC power into AC power to supply the permanent magnet synchronous motor.

3. The three-level motor drive system based on WPT according to claim 1 is characterized in that: When the power flows in the forward direction, the battery pack serves as the input side to provide DC power, and voltage conversion is achieved through the FSBB converter; the FSBB output voltage is provided to the CLLC converter, and wireless power transmission is achieved through the CLLC resonant converter, in which the full bridge H1 close to the FSBB in the CLLC converter serves as the inverter bridge, and the full bridge H2 close to the three-phase PWM converter in the CLLC converter serves as the rectifier bridge; the three-phase PWM converter serves as an inverter to convert the DC power output by the CLLC into three-phase symmetrical AC power and provide it to the load PMSM to drive the PMSM to rotate.

4. A joint control method for a three-level motor drive system based on WPT, characterized in that: In the three-level converter topology adopted in this paper, the FSBB converter takes on the role of voltage regulation, and the ultimate goal of the entire system is to provide a suitable and stable voltage for the three-phase inverter; Therefore, this paper adopts a joint control strategy to control the bus 2 voltage, collects the bus 2 voltage V2, and sends the difference between the voltage value V2 and the reference value V2ref to the PI controller for PWM modulation, and then outputs a square wave with a corresponding duty cycle to control the on and off of the relevant switching tubes of the FSBB converter, thereby adjusting the bus 2 voltage to provide a stable voltage for the three-phase inverter.

5. A method for controlling the variable bus voltage of a three-level motor drive system based on WPT, characterized in that: The CLLC converter is most efficient when operating in a resonant state. Although the voltage regulation effect can be achieved by adjusting the CLLC switching frequency, the efficiency of the CLLC converter will be greatly reduced because it does not operate at the resonant point. This can be avoided. Therefore, the CLLC converter adopts fixed-frequency control and ensures that the switching frequency is close to the resonant frequency fr1. Due to the flexible control method and good voltage regulation performance of the FSBB converter, the task of controlling the bus 2 voltage V2 in this article is completed by the FSBB converter.

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