CN111654200A - A Novel Asymmetric Three-Level Midpoint Clamped Power Converter - Google Patents

Abstract

The invention discloses a novel asymmetric three-level midpoint clamp type power converter, which belongs to the field of novel power converter design. Compared with the asymmetric half-bridge power converter, the power converter adds two power transistors and two clamping diodes per phase, which can realize the multi-level control of the switched reluctance motor system. The maximum withstand voltage of the power transistor. In addition, for a new switched reluctance motor system based on asymmetric three-level midpoint-clamped power converters, a double closed-loop pulse width modulation (PWM) strategy for speed and current is proposed. Compared with the traditional PWM control strategy, this control strategy The strategy increases the current closed-loop control, which has better current control effect; the five-stage working mode distribution method can effectively suppress the mid-point potential offset problem caused by the unbalanced charging and discharging of the capacitor; rational use of a variety of effective working modes can greatly The advantages of the power converter are maximized to optimize the control performance of the system.

Description

A Novel Asymmetric Three-Level Midpoint Clamped Power Converter

technical field

The invention relates to the design field of a novel power converter, in particular to a power converter of a switched reluctance motor system.

Background technique

The switched reluctance motor is a double salient structure, the rotor does not contain windings and permanent magnets, the stator winding is composed of several concentrated coils, each phase winding is independent of each other, and the electromagnetic torque has nothing to do with the direction of the winding current, so it has a simple structure and low cost. , large starting torque, high reliability and strong fault tolerance. At the same time, the motor also has certain limitations, such as large instantaneous torque ripple, vibration and noise problems. The switched reluctance motor system integrates the switched reluctance motor with power electronic technology and control technology, and has the advantages of traditional AC and DC speed control systems, and has a very broad application prospect. At present, switched reluctance motors have been widely used in textiles, home appliances, aerospace and electric vehicles and many other fields. However, since permanent magnet synchronous motors, asynchronous motors and brushless DC motors have been researched earlier and occupy a large market share, there are still many problems to be solved in order to further expand the application range of switched reluctance motors. With the expansion of the application field of switched reluctance motors and the advancement of modern industrial technology, various applications have also put forward different requirements for switched reluctance motor systems. As the core part of the energy conversion of the switched reluctance motor system, the power converter has a great impact on the system performance. The traditional asymmetric half-bridge power converter can ensure the independent control of each phase of the motor, effectively improve the fault tolerance of the system, and has no requirements and restrictions on the number of motor phases. However, the converter has few working modes, which is not conducive to the flexible control of the system, and the maximum withstand voltage of each power transistor is the power supply voltage, which limits the application of the system in high-voltage and high-power applications.

SUMMARY OF THE INVENTION

In view of the problems existing in the above-mentioned technologies, the present invention proposes an asymmetric three-level midpoint clamp type power converter that provides an increased number of levels and is flexible and convenient to control.

In order to realize the above-mentioned technical purpose, the present invention adopts the following technical scheme to realize:

A new type of asymmetric three-level midpoint clamped power converter. Compared with the asymmetrical half-bridge power converter, two power transistors and two clamp diodes are added to each phase. Corresponding winding voltage and current paths; study the potential offset of the midpoint of the DC side capacitor under different working modes, and then analyze the impact on the system performance; propose a double closed-loop PWM control strategy for speed and current, and give a five-stage working mode distribution method.

The present invention lies in that, while the power converter maintains the independent control of each phase of the asymmetric half-bridge power converter, through a reasonable combination of the switching states of the power triodes, nine effective working modes can be generated, five kinds of winding voltages can be provided, and the control is more flexible .

The invention lies in that the power tube in the power converter bears only half of the power supply voltage, which greatly reduces the damage and breakdown probability of the power tube, is beneficial to improve the competitiveness of the system in high-voltage and high-power occasions, and expands the application range of the system .

The present invention lies in that, for a switched reluctance motor system based on a novel asymmetric three-level midpoint clamp type power converter, the double closed-loop PWM control strategy of the rotational speed and current can effectively suppress the midpoint potential offset problem, and rationally utilize a variety of effective working mode to give full play to the advantages of multi-level power converters.

The invention lies in that the novel asymmetric three-level midpoint clamp type power converter combined with the speed and current double closed-loop PWM control strategy can effectively reduce the system current fluctuation and optimize the control performance of the system.

Beneficial effects:

_A new type of asymmetric _{three -} level midpoint clamp type power converter described in the present invention takes the _A phase as an example. It is composed of two clamping diodes (D ₁ , D ₂ ) and two freewheeling diodes (D ₃ , D ₄ ), five levels can be generated through the combination of the switching states of the power tubes, and there are nine effective working modes in total, which is beneficial to the system flexible control. Through the analysis of nine working modes, it can be seen that the maximum voltage that the power tube can withstand is only half of the power supply voltage, which is conducive to improving the competitiveness of the system in high-voltage and high-power occasions and further expanding the application scope of the system. In order to solve the problems that the traditional PWM control strategy has large current fluctuation and cannot suppress the midpoint potential offset, a double closed-loop PWM control strategy for speed and current is proposed, which not only helps to suppress the midpoint potential offset of the DC side capacitor, The effective working mode of the power converter is rationally utilized, and the advantages of the asymmetric three-level midpoint clamped power converter are brought into play to a great extent. The asymmetric three-level mid-point clamped power converter combined with the speed and current double closed-loop PWM control strategy effectively reduces the system current fluctuation and improves the system performance.

Description of drawings

FIG. 1 is a topological structure diagram of a three-phase asymmetric three-level midpoint clamp type power converter of the present invention.

FIG. 2 shows the current paths of nine effective operating modes of the three-phase asymmetric three-level midpoint-clamped power converter of the present invention.

FIG. 3 is a control block diagram of a novel switched reluctance motor system based on the speed and current double closed-loop PWM control strategy of the present invention.

FIG. 4 shows the action sequence and action time distribution of the working mode in the synthesis interval 3 of the speed-current double closed-loop PWM control strategy of the present invention.

FIG. 5 shows the traditional switched reluctance motor system based on the traditional PWM control strategy, the traditional switched reluctance motor system based on the speed and current double closed-loop PWM control strategy, and the new switched reluctance motor system based on the speed and current double closed-loop PWM control strategy of the present invention. The current fluctuation diagram when the torque is 0.1N·m.

FIG. 6 shows the traditional switched reluctance motor system based on the traditional PWM control strategy, the traditional switched reluctance motor system based on the speed current double closed-loop PWM control strategy and the new switched reluctance motor system based on the speed current double closed loop PWM control strategy of the present invention Current fluctuation diagram at a torque of 0.5N·m.

Detailed ways

Below in conjunction with accompanying drawing, one embodiment of the present invention is further described:

Figure 1 shows the topology diagram of the asymmetric three-level mid-point clamped power converter. The DC side of the converter contains two equal-sized voltage divider capacitors C ₁ , C ₂ , A, B and C respectively represent Three-phase windings of switched reluctance motors. Taking phase A as an example, the converter contains two clamp diodes (D ₁ , D ₂ ), two freewheeling diodes (D ₃ , D ₄ ) and four power transistors (S ₁ , S ₂ , S ) in each phase ₃ , S4 ₎ .

Through the combination of the switching states of the four power tubes, the converter has nine effective working modes for each phase, and there are situations in which different working modes correspond to the same winding working state. All the working modes of the A-phase bridge arm are shown in Figure 2. It is defined that the state of the power tube is 1 when it is turned on and 0 when it is turned off. The working modes of the power converter are analyzed one by one below. Table 1 shows all valid operating modes, their effects on the midpoint potential, and the corresponding winding voltages and operating states.

Working mode 1: As shown in Figure 2(a), all power tubes are turned on, the current in the A-phase bridge arm flows sequentially through S ₁ , S ₂ , windings A, S ₃ and S ₄ , and the phase voltage of the winding is U, at this time Phase A works in a positive full-voltage excitation state. Capacitors _C1 and _C2 discharge the windings at the same time, and the midpoint potential of the capacitors will not shift.

Working mode 2: As shown in Figure 2(b), the power tubes S ₁ , S ₂ and S ₃ are turned on, S ₄ is turned off, and the current in the A-phase bridge arm flows through S ₁ , S ₂ , windings, S ₃ and D ₂ , the winding voltage is U/2. At this time, the A phase works in the positive half-voltage excitation state, only the capacitor C ₁ discharges the winding, and the voltage of C ₁ decreases, causing the mid-point potential to rise.

Working mode 3: The power tubes S ₂ , S ₃ and S ₄ are turned on, S ₁ is turned off, and the current in the A-phase bridge arm flows through D ₁ , S ₂ , windings, S ₃ and S ₄ in turn, and the current path is shown in Figure 2 ( As shown in c), the winding voltage is U/2. At this time, the A-phase works in the positive half-voltage excitation state, and only the capacitor _C2 discharges the winding, and the voltage of _C2 decreases, resulting in a decrease in the midpoint potential.

Working mode 4: As shown in Figure 2(d), the power tubes S ₁ and S ₂ are turned on, S ₃ and S ₄ are turned off, and the current in the A-phase bridge arm flows sequentially through S ₁ , S ₂ , the winding and D ₄ , The winding voltage is 0, and the A-phase winding works in a zero-voltage freewheeling state. At this time, there is no capacitor discharge, and the midpoint potential does not shift.

Working mode 5: As shown in Figure 2(e), the power tubes S ₂ and S ₃ are turned on, S ₁ and S ₄ are turned off, and the current in the A-phase bridge arm flows through D ₁ , S ₂ , windings, S ₃ and D ₂ , the winding voltage is 0, and the A-phase winding works in a zero-voltage freewheeling state. At this time, there is no capacitor discharge, and the midpoint potential does not shift.

Working mode 6: As shown in Fig. 2(f), the power tubes S ₃ and S ₄ are turned on, S ₁ and S ₂ are turned off, and the current flows through D ₃ , winding, S ₃ and S ₄ in sequence, and the winding voltage is 0. The A-phase windings work in a zero-voltage freewheeling state. At this time, there is no capacitor discharge, and the midpoint potential does not shift.

Working mode 7: As shown in Figure 2(g), the power tubes S ₁ , S ₂ , and S ₄ are turned off, only S ₃ is turned on, and the current in the A-phase bridge arm flows through D ₃ , windings, S ₃ and D ₂ in turn , the winding voltage is -U/2, and the A-phase winding works in a negative half-voltage freewheeling state. At this time, the capacitor _C1 does not work, while the capacitor _C2 works in a charged state, and the midpoint potential increases.

Working mode 8: As shown in Figure 2(h), the power tubes S ₁ , S ₃ , and S ₄ are turned off, only S ₂ is turned on, and the current in the A-phase bridge arm flows through D ₁ , S ₂ , windings and D ₄ in turn , the winding voltage is -U/2, and the A-phase winding works in a negative half-voltage freewheeling state. At this time, the capacitor C ₂ does not work, while the capacitor C ₁ works in a charged state, and the midpoint potential decreases.

Table 1 Power converter working mode and winding working state

Working mode 9: As shown in Figure 2(i), all power tubes are turned off, the current in the A-phase bridge arm flows through D ₃ , winding and D ₄ in turn, the winding voltage is -U, and the A-phase winding works at negative full voltage The freewheeling state can speed up the current reduction process. At this time, the capacitors are all working in a charged state, and the midpoint potential will not shift.

From the analysis of the above working modes and Table 1, the relationship between the switching state of the power tube and the phase voltage of the winding can be obtained

Wherein, S ₁ , S ₂ , S ₃ and S ₄ respectively represent the switching states of the power transistors S ₁ , S ₂ , S ₃ and S ₄ .

Table 2 The withstand voltage of A-phase power transistors in different working modes

Theoretically, the mid-point n voltage U _n should be half of the DC voltage, but in practice, due to the error of the capacitance value of the DC side capacitor, different characteristics of the power tube, asymmetry of the three-phase winding of the motor, and unbalanced charging and discharging of the capacitor in different working modes, etc. , there will be deviations in the voltages of the two capacitors on the DC side, resulting in an unbalanced potential at the midpoint of the DC side. The unbalance of the mid-point potential will cause the output voltage of the power converter to shift, and when the mid-point potential shift is large, the output voltage will even degenerate into two levels.

From the above analysis of the working mode of the power converter, it can be seen that if the working time of working mode 2 or 7 is too long, that is, the discharge time of capacitor _C1 or the charging time of _C2 is too long, the midpoint potential will be greater than U/2; otherwise, if If the working time of working mode 3 or 8 is too long, that is, the charging time of capacitor _C1 or the discharging time of _C2 is too long, the midpoint potential will be less than U/2. It can be seen from Table 2 that if the mid-point potential Un _is offset and is greater than U/2, the withstand voltage of the power tube S ₄ will be greater than U/2 when the working modes 2, 5 and 7 act; on the contrary, if the mid-point potential U _n When the offset occurs and is less than U/2, the withstand voltage of the power tube S ₁ will be greater than U/2 when operating modes 3, 5 and 8 act. Moreover, the greater the offset of the mid-point potential, the greater the voltage stress the power tube is subjected to. If the power converter degenerates into two-level, there is a power tube that bears all the power supply voltage in a certain working mode, which will seriously affect the power The service life of the tube even directly breaks down the power tube, which affects the safe and stable operation of the system. Therefore, in order to ensure the reliability of the system operation, this paper takes measures in the control strategy of the system to solve the problem of the mid-point potential offset of the power converter.

According to the characteristics of asymmetric half-bridge power converters, there are three basic control strategies. Among them, the current chopping control strategy adopts hysteresis control. Hysteresis control is a kind of tolerance control, which will inevitably bring potential ripple; voltage; The PWM and angular position control strategies only perform closed-loop speed, which will also cause large current fluctuations. In order to reduce the system current ripple and make reasonable use of the various effective working modes of the asymmetric three-level midpoint clamped power converter, a double closed-loop PWM control strategy for speed and current is proposed in this paper. Compared with the traditional PWM control strategy, this control strategy adds a current controller, which can effectively reduce the current fluctuation of the system, and its working mode distribution principle can effectively suppress the midpoint potential offset of the DC side capacitor. Figure 3 shows a block diagram of the control strategy of the switched reluctance motor system. The following is a detailed analysis of the implementation process of the control strategy.

In order to realize the double closed-loop control of the speed and current, the speed and current controllers must be established first. In this paper, the most widely used PI controller is used as the speed and current controller. As shown in Figure 3, the input of the speed controller is the difference between the given speed and the actual speed, and the reference current value is output through the PI control link; the current controller input is the difference between the reference current value and the actual current value, and the output is the reference voltage value U _ref . The key to the realization of PI algorithm is the setting of proportional and integral coefficients, which will affect the control performance of the system. However, the switched reluctance motor has nonlinear characteristics, and an accurate mathematical model cannot be established. Therefore, the optimal parameters of the two controllers under different working conditions are determined through debugging. The input of the PWM controller is the reference voltage, and the voltage duty ratio is adjusted by adjusting the drive signal of the power tube, so that the winding voltage is equivalent to the reference voltage. When there is a deviation between the motor speed and the given speed, the reference current value first increases, and then the reference voltage value increases. Therefore, the PWM controller controls the winding voltage by adjusting the drive signal to reduce the speed difference. When the winding current and speed control The adjustment process is similar when the output reference current of the controller deviates. It can be seen that the closed-loop control of the system can be realized through the speed and current double closed-loop controller.

After the reference voltage value is obtained, a PWM controller needs to be established to generate the drive signal. The first is the selection of the synthesis voltage and the synthesis interval. The asymmetric three-level midpoint clamped power converter has nine operating modes, which can generate five winding voltages and form a synthesis interval of six reference voltages. They are numbered from 1 to 6 in descending order. When the reference voltage is greater than or equal to U or less than or equal to -U, the combined voltage is taken as U and -U, respectively, and two adjacent winding voltages are selected to synthesize the reference voltage in the rest of the interval. For example, when the reference voltage is less than U and greater than 0.5U, then interval 2 is the combined interval of the reference voltage, and the combined voltage is U and 0.5U.

The second step is to calculate the respective action times of the two synthesized voltages in one PWM cycle. When the reference voltage value is U _ref (k), in order to achieve equivalence with the reference voltage, the action time of the two combined voltages is calculated based on the principle of "volt-second balance", that is, the reference voltage U _ref (k) and the PWM period T The product of _p is equal to the sum of the products of the two resultant voltages and the corresponding action times, and the expression is as follows

Among them, U ₁ and U ₂ are the two synthesized voltages of the kth sampling period, and T ₁ and T ₂ are the corresponding action times, respectively. Therefore, the values of T ₁ and T ₂ in any period can be calculated from the reference voltage U _ref and the PWM period T _p according to Expression (2-5). Due to the different synthesis voltages in different synthesis intervals, the action times T ₁ and T ₂ calculated in different synthesis intervals are also different, as shown in Table 3.

Table 3 Synthesis interval, synthetic voltage and action time distribution

When the combined voltage and the corresponding action time are known, the third step is the assignment of the action sequence and action time of the working mode. When the combined voltage is the power supply voltage U and the negative power supply voltage -U, the corresponding operating modes are 1 and 9, respectively. However, the combined voltages of 0.5U, -0.5U and 0 all correspond to more than one operating mode. When the resultant voltage is zero, the corresponding working modes are modes 4, 5 and 6, and none of these three working modes will cause the midpoint potential shift. Since only one power transistor needs to be switched when switching between operating mode 5 and the operating modes corresponding to the adjacent combined voltages of 0.5U and -0.5U, in order to reduce switching loss, operating mode 5 is selected when the combined voltage is zero. The combined voltage corresponding to operating modes 2 and 3 is 0.5U, and the combined voltage corresponding to operating modes 7 and 8 is -0.5U. The aforementioned operating modes will cause the midpoint potential to increase or decrease. In order to maintain the balance of the midpoint potential, it should be evenly distributed. Corresponding to the action time of the two working modes of the same voltage.

Assuming that the reference voltage is located in the synthesis interval 3 in the kth sampling period, it can be obtained from equation (2)

Among them, T ₁ and T ₂ are the action times corresponding to 0.5U and 0, respectively. Therefore, the action time of working modes 2 and 3 should both be 0.5T ₁ , and the action time of mode 5 should be T ₂ . In this paper, the triangular wave is used as the carrier signal, and the frequency of the triangular wave is the PWM frequency. Each PWM cycle adopts a five-segment method to realize the distribution of the working mode and the respective action time. As shown in Figure 4, the triangular wave divides a PWM cycle into five segments, the action sequence of the working mode can be selected as 2→5→3→5→2, and the corresponding action times are 0.25T ₁ , 0.5T ₂ , and 0.5T ₁ respectively , 0.5T ₂ and 0.25T ₁ . When selecting the action sequence of the combined voltage, the same starting and ending voltages should be selected as far as possible for adjacent intervals. When the reference voltage changes continuously, the switching loss can be minimized.

According to the above analysis, Table 4 lists the action sequence and action time distribution results of all working modes in the interval. This distribution principle has the following advantages:

(1) When the working mode changes in sequence in the same interval, only one switch needs to be switched.

(2) The same working mode is used at the beginning and end of the same interval. If the reference voltages of adjacent PWM cycles are in the same synthesis interval, the switch state does not need to be switched when the PWM cycle changes.

(3) The first and last working modes of adjacent intervals differ by one switch state. Since the input reference voltage of the PWM controller is a continuous value, when the reference voltages of adjacent PWM cycles are in different synthesis intervals, only one switch state needs to be switched to change the PWM cycle. .

(4) The average distribution of the action time of the working mode corresponding to the same voltage is realized, so the midpoint potential balance of the DC side capacitor can be maintained.

Figures 5 and 6 are the traditional switched reluctance motor system based on the traditional PWM control strategy, the traditional switched reluctance motor system based on the speed and current double closed-loop PWM control strategy, and the new switched reluctance motor system based on the speed and current double closed-loop PWM control strategy Current fluctuation diagrams at torques of 0.1 N·m and 0.1 N·m. Fluctuations 1, 2 and 3 represent the current fluctuations of the traditional switched reluctance motor system based on the voltage PWM control strategy, the speed and current double closed-loop PWM control strategy and the new switched reluctance motor system based on the speed and current double closed-loop PWM strategy, respectively. It can be seen from the figure that compared with the voltage PWM control strategy, the speed and current double closed-loop PWM control strategy adds a current loop, which can effectively reduce the current fluctuation of the traditional switched reluctance motor system; The new switched reluctance motor system of the asymmetric three-level midpoint-clamped power converter has a smaller chopping voltage amplitude, so it has smaller current fluctuation and more flexible control.

Table 4 Working mode, action sequence and action time allocation

Claims (5)

1. a novel asymmetric three-level midpoint clamp type power converter, is characterized in that, comprises the steps: A. Design a new type of asymmetric three-level mid-point clamped power converter. Compared with the traditional asymmetrical half-bridge power converter, each phase adds two power transistors and two clamp diodes; B. By analyzing the effective working mode, winding voltage and current path of the power converter, it is found that the power converter has nine effective working modes and can provide five kinds of winding voltages; C. Analyze the withstand voltage of the power tube under the above nine effective working modes, and obtain the power supply voltage that the maximum voltage of the power tube in the power converter is only half; D. Study the change of the midpoint potential of the DC side capacitor under the action of the above nine effective working modes, and then analyze the influence of the midpoint potential offset on the system; E. A double closed-loop PWM control strategy for speed and current is proposed. Compared with the traditional PWM control strategy, a current controller is added to achieve better current control effect, and the five-stage working mode distribution method can effectively suppress the midpoint potential offset. 2. A novel asymmetric three-level midpoint clamp type power converter according to claim 1, characterized in that: taking phase A as an example, each phase includes four power triodes (S ₁ , S ₂ , S ₃ , S ₄ ), two clamping diodes (D ₁ , D ₂ ) and two freewheeling diodes (D ₃ , D ₄ ), five levels can be generated through the combination of the switching states of the power tubes, a total of nine Effective working mode is conducive to the flexible control of the system. 3. A new type of asymmetric three-level midpoint clamp type power converter according to claim 1, characterized in that: in the power converter, the power tube withstands only half of the maximum voltage of the power supply voltage, which greatly reduces the power supply voltage. The probability of damage and breakdown of the power tube is conducive to improving the competitiveness of the system in high-voltage and high-power occasions and expanding the application scope of the system. 4. A new type of asymmetric three-level midpoint clamp type power converter according to claim 1, characterized in that: when different working modes act, the DC side capacitors work in different charging and discharging states, and if they cannot be reasonably allocated In the working mode, the mid-point potential may be shifted due to the imbalance of charge and discharge, which will affect the stable operation of the system. 5. A new type of asymmetric three-level midpoint clamped power converter according to claim 1, characterized in that: for the switched reluctance based on the new asymmetrical three-level midpoint clamped power converter In the motor system, the speed and current double closed-loop PWM control strategy can effectively suppress the midpoint potential offset problem, and rationally use a variety of effective working modes to reduce the system current fluctuation and improve the control performance of the system.

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