Method for neutral point voltage balancing control of a three-level converter

Abstract

Embodiments of the present application provide a method for midpoint voltage balancing control of a three-level converter (10), comprising: generating a reference voltage vector V ref In the case of being in the inner hexagon of the space vector of the three-level converter, doubling the reference voltage vector 2V ref The direction decomposition of the small vector group into a first space sub-vector V1 and a second space sub-vector V2; in the first PWM period, the first space sub-vector V1 is modulated using the first small vector group, and the three-level converter is controlled based on the modulated first small vector group, the action time of the first small vector and the corresponding first redundant small vector being equal; in the second PWM period immediately following the first PWM period, the second sub-space vector V2 is modulated with respect to the second small vector group, and the three-level converter (10) is controlled based on the modulated second small vector group, the action time of the second small vector and the corresponding second redundant small vector being equal.

Description

Technical Field

[0001] This application relates to the field of power converter control for motors, and more particularly to a method for midpoint voltage balance control of a three-level converter, a three-level converter, a permanent magnet synchronous motor including the three-level converter according to this application, and a computer program product for at least auxiliaryly implementing the steps of the method according to this application. Background Technology

[0002] Permanent magnet synchronous motors (PMSMs) based on three-level converters have been widely used due to their advantages such as simple structure, low moment of inertia, high power density, and high torque-to-current ratio, especially as high-speed air compressors in fuel cell systems. However, during the low-speed operation phase of the PMSM, particularly during startup, traditional midpoint voltage balancing methods fail, causing midpoint voltage drift in the three-level converter and resulting in a large voltage drop at the midpoint within a short period. This midpoint voltage drift severely affects the startup performance and operational stability of the PMSM.

[0003] Therefore, how to eliminate the midpoint voltage drift phenomenon of permanent magnet synchronous motors based on three-level converters during low-speed operation has become a technical problem that needs to be solved. Summary of the Invention

[0004] The purpose of this application is to provide a method for midpoint voltage balance control of a three-level converter, a three-level converter, a permanent magnet synchronous motor, and a computer program product to at least partially solve the problems in the prior art.

[0005] According to a first aspect of this application, a method for midpoint voltage balance control of a three-level converter is provided, the method comprising:

[0006] -In the generated reference voltage vector V ref When the voltage is within the inner hexagon of the space vector of the three-level converter, the reference voltage vector will be twice 2V. ref The direction of the small vector group is decomposed into a first spatial sub-vector V1 and a second spatial sub-vector V2, wherein the inner hexagon is formed by the ends of each small vector, and the small vector group includes a small vector and a redundant small vector corresponding to this small vector.

[0007] - In the first PWM cycle, the first spatial sub-vector V1 is modulated using a first small vector group, and the three-level converter is controlled based on the modulated first small vector group, wherein,

[0008] The first small vector and its corresponding first redundant small vector have equal durations; and

[0009] - In the second PWM cycle that immediately follows the first PWM cycle, the second subspace vector V2 is modulated with respect to the second small vector group, and the three-level converter is controlled based on the modulated second small vector group, wherein the second small vector and the corresponding second redundant small vector have the same duration.

[0010] The core concept of this application includes at least: generating a reference voltage vector V located within the inner hexagon of the space vector of the three-level converter. ref The reference voltage vector is twice 2V. ref The direction of the small vector group is decomposed into two spatial sub-vectors. In two consecutive PWM cycles, one of the decomposed spatial sub-vectors is modulated using one small vector group and, if necessary, a zero vector. Since the two small vectors of the same small vector group have equal action time and generate phase currents in opposite directions, the midpoint voltage drift phenomenon during each PWM cycle can be effectively eliminated, thereby effectively improving the starting performance and running stability of the permanent magnet synchronous motor.

[0011] According to a second aspect of this application, a three-level converter is provided, which includes a control unit for performing the method of this application.

[0012] According to a third aspect of this application, a permanent magnet synchronous motor is provided, the permanent magnet synchronous motor including a three-level converter according to this application.

[0013] According to a fourth aspect of this application, a computer program product, such as a computer-readable program carrier, is provided, comprising computer program instructions that, when executed by a processor, at least partially implement the steps of the method described in this application. Attached Figure Description

[0014] The principles, features, and advantages of this application will be better understood below with reference to the accompanying drawings. The drawings include:

[0015] Figure 1 A flowchart illustrating a method for midpoint voltage balance control of a three-level converter according to an exemplary embodiment of this application is shown.

[0016] Figure 2 A topology diagram of a midpoint clamped three-level converter according to an exemplary embodiment of this application is shown;

[0017] Figure 3 A three-level space vector diagram according to the prior art is shown;

[0018] Figure 4 A three-level space vector diagram according to an exemplary embodiment of this application is shown;

[0019] Figure 5 A schematic diagram of a reference voltage vector according to an exemplary embodiment of this application is shown; and

[0020] Figure 6 A schematic diagram of a permanent magnet synchronous motor according to an exemplary embodiment of this application is shown. Detailed Implementation

[0021] To make the technical problems to be solved, the technical solutions, and the beneficial technical effects of this application clearer, the application will be further described in detail below with reference to the accompanying drawings and several exemplary embodiments. It should be understood that the specific embodiments described herein are only for explaining this application and are not intended to limit the scope of protection of this application.

[0022] Figure 1 A flowchart illustrating a method for midpoint voltage balance control of a three-level converter according to an exemplary embodiment of this application is shown. The following exemplary embodiments describe the method according to this application in more detail.

[0023] The method according to this application is applicable to the three-level converter 10 of a permanent magnet synchronous motor 1, especially the three-level converter of a high-speed air compressor. Currently, various modulation algorithms have been proposed for use in three-level converters. Among them, space vector modulation (SVPWM) has been widely used due to its advantages such as high voltage utilization, low harmonic content of the output waveform, and ease of digitization. The three-level converter 10 can be configured, in particular, as a midpoint clamped three-level converter, such as... Figure 2 The illustrated topology diagram has three phases: a, b, and c. Each phase has an upper arm and a lower arm. U dc The bus voltage of the three-level converter is given, and the upper arm of phase a has a controllable switching device S. a1 S a2 The lower bridge arm of phase a has a controllable switching device S. a3 S a4 The upper arm of phase b has a controllable switching device S. b1 S b2 The lower arm of phase b has a controllable switching device S. b3 S b4 The upper arm of phase C has a controllable switching device S. c1 S c2 The lower arm of phase c has a controllable switching device S. c3 S c4 .

[0024] like Figure 3The three-level space vector diagram shown is an exemplary embodiment of this application. The α-axis and β-axis represent the voltage vector directions in a two-phase stationary coordinate system. The space vector of the three-level converter is divided into six sectors, each sector being 60°. Each phase of the three-level converter can have three phase states: P, N, and O. The switching states of the controllable switching devices corresponding to each phase state are shown in Table 1.

[0025]

[0026]

[0027] Table 1: Correspondence between Phase States and Switching States of a Three-Level Converter

[0028] Therefore, the three-level converter has a total of 27 switching states, corresponding to 27 basic voltage vectors, such as... Figure 3 The three-level space vector diagram shown is based on the prior art, and these basic voltage vectors include:

[0029] -12 small vectors with an amplitude of U dc / 3, which is divided into 6 small vector groups. Each small vector group includes a small vector and a redundant small vector corresponding to that small vector, namely PPO and OON, POO and ONN, POP and ONO, OOP and NNO, OPP and NOO.

[0030] And OPO and NON;

[0031] -6 medium vectors, with magnitudes of That is, PON, OPN, NPO, NOP, OPN, and PNO;

[0032] -6 large vectors with an amplitude of 2U dc / 3, namely PNN, PPN, NPN, NPP, NNP and PNP;

[0033] -3 zero vectors with an amplitude of 0, namely PPP, OOO, and NNN.

[0034] The theoretical basis of space vector modulation is the volt-second balance principle, which involves combining basic voltage vectors within a switching cycle and assigning a corresponding duration and sequence to each basic voltage vector so that the average value of the combined basic voltage vectors over their respective durations is equal to the value of a given reference voltage vector. When selecting voltage vectors, starting with the shortest vector, any change in voltage vector can only trigger the switching action of one bridge arm. For this purpose, a zero vector can be used for coordination, and the sum of the durations of all voltage vectors equals the duration of the reference voltage vector.

[0035] During the low-speed operation phase, especially the startup phase, of the permanent magnet synchronous motor 1, a reference voltage vector can be set based on a virtual given angle, with the angle of this reference voltage vector increasing sequentially from 0°. Since the high-speed air compressor employs a sensorless control strategy, the actual rotor position is unknown and random, resulting in a corresponding current vector. When the generated current vector is approximately perpendicular to the reference voltage vector, the traditional midpoint voltage balancing method will fail, causing midpoint voltage drift, and subsequently, a large midpoint voltage drift within a short period. For example... Figure 3 The three-level space vector diagram shown in the prior art produces a reference voltage vector V0 with an angle of -29.9° and a small amplitude. According to the voltage vector selection principle of space vector modulation, voltage vectors POO, ONN, ONO, and OOO can be used to modulate the reference voltage vector V0. Specifically, the first reference voltage component V is obtained by decomposing the direction of the small vector group of reference voltage vector V0 with respect to POO and ONN. 01 And use small vector groups POO and ONN to pair the first reference voltage component V 01 Modulation is performed where the durations of POO and ONN are kept equal, thereby generating a phase current -i through phase state POO. a With the phase current i generated through phase state ONN a The effects of the midpoint voltage cancel each other out. Simultaneously, the reference voltage vector V0 is decomposed about the direction of the ONO small vector to obtain the second reference voltage component V. 02 And use the small vector ONO to pair the first reference voltage component V 01 Modulation is performed by connecting the terminals of phase a and phase c in phase state ONO, thereby allowing the phase current -i to pass through. b This affects the neutral point voltage by absorbing positive charge from the neutral points of the two DC support capacitors C1 and C2 of the three-level converter. A larger phase current -i will be generated when the actual rotor angle is, for example, -150°. b Meanwhile, the reference voltage vector V0 at -29.9° is at the phase current -i b The voltage vector has a large voltage component in the direction of the voltage, which easily leads to voltage drift. Traditional midpoint voltage balancing methods cannot effectively eliminate this voltage drift. Because the voltage vector angle changes slowly during the low-speed operation phase of the permanent magnet synchronous motor 1, especially during the startup phase, it remains within a certain angle range for a long time. This midpoint voltage drift will persist for a long time, leading to a large midpoint voltage drift. This midpoint voltage drift seriously affects the starting performance and operational stability of the permanent magnet synchronous motor. The method according to this application can eliminate this midpoint voltage drift.

[0036] like Figure 1As shown, the method may include steps S1 to S3. In step S1, the generated reference voltage vector V... ref When located within the inner hexagon of the space vector of the three-level converter 10, it can convert twice the reference voltage vector 2V. ref The direction of the small vector group is decomposed into a first spatial sub-vector V1 and a second spatial sub-vector V2, wherein the inner hexagon is formed by the ends of each small vector, and the small vector group includes a small vector and a redundant small vector corresponding to this small vector. Figure 4 The three-level space vector diagram shown according to an exemplary embodiment of this application, with the inner hexagon marked by solid lines, is a hexagon formed by the ends (i.e., endpoints different from the coordinate origin) of the six small vector groups POO / ONN, POO / ONN, POP / ONO, OOP / NNO, OPP / NOO, and OPO / NON. It can be understood that, in the reference voltage vector V... ref When inside the inner hexagon, due to the magnitude of the large vector 2U dc / 3 is the magnitude U of the small vector. dc / 3 times, twice the reference voltage vector 2V ref It must also lie within the outer hexagon of the space vector, allowing the combination of the basic voltage vectors to obtain twice the reference voltage vector 2V. ref Modulation is performed, wherein the outer hexagon marked with a dashed line is a hexagon formed by the ends (i.e., the endpoints different from the origin) of the large vectors PNN, PPN, NPN, NPP, NNP and PNP.

[0037] Considering that the midpoint voltage drift of the three-level converter usually occurs during the low-speed operation of the permanent magnet synchronous motor 1, especially during the startup phase, the speed of the permanent magnet synchronous motor 1 can be monitored. Furthermore, when the speed of the permanent magnet synchronous motor 1 is lower than a predetermined speed threshold, for example, the reference voltage vector V for each PWM cycle can be generated through the current loop of the field-oriented control circuit of the permanent magnet synchronous motor 1. ref (n). Specifically, the three-phase stator current i is detected in real time by a current sensor installed in the stator circuit of the permanent magnet synchronous motor 1. a i b i c The detected three-phase stator current is converted into a two-phase stationary coordinate system current i using Clark transformation. α i β Then, through the Park transformation, it is converted into a direct-axis current i in a synchronous rotating coordinate system. d and cross-axis current i q , where the direct-axis current i d Primarily used to control the magnetic field of a motor, quadrature-axis current i qIt is mainly used to control the torque of the motor. Next, the direct-axis current i will be calculated. d and cross-axis current i q With the given current command value i d * i q * The deviation between the two phases is analyzed by a PI controller, which outputs a corresponding reference voltage vector based on the magnitude and trend of the deviation. This reference voltage vector is then transformed by an inverse Park transformation into the reference voltage vector V in the two-phase stationary coordinate system. ref (n), which can thus participate in the space vector modulation method according to this application.

[0038] Optionally, when the speed of the permanent magnet synchronous motor 1 is lower than a predetermined speed threshold, the reference voltage vector V for each PWM cycle can also be generated through voltage frequency control of the permanent magnet synchronous motor 1. ref (n). For example, a linear voltage-frequency control method can be used to adjust the reference voltage vector based on the motor frequency, so that the magnetic flux of the motor remains basically constant during speed regulation. Considering the differences in the frequency characteristics of the motor, the voltage-to-frequency ratio of the motor can be increased in the low-frequency range to ensure sufficient torque output, and the voltage-to-frequency ratio of the motor can be decreased in the high-frequency range to prevent overvoltage operation, thereby realizing segmented voltage-frequency control.

[0039] For example, in Figure 4 The reference voltage vector V is shown with an angle between 0° and 30° relative to the α axis. ref The reference voltage vector is twice 2V. ref The direction of the small vector group POO / ONN is decomposed into the first spatial sub-vector V1, and the direction of the small vector group PPO / OON is decomposed into the second spatial sub-vector V2.

[0040] In step S2, during the first PWM cycle, the first spatial sub-vector V1 can be modulated using the first small vector group, and the three-level converter 10 is controlled based on the modulated first small vector group, wherein the duration of the first small vector and the corresponding first redundant small vector is equal. Here, the first spatial sub-vector V1 can be modulated using the first small vector POO and the first redundant small vector ONN, and an appropriate modulation ratio is assigned to the first small vector POO and the first redundant small vector ONN so that the duration of the first small vector POO and the first redundant small vector ONN is equal. For the two small vectors POO / ONN of the same small vector group, the switching states of the controllable switching devices corresponding to them are different, but the output voltages generated are the same, and the phase currents generated are in opposite directions. When the duration of these two small vectors is equal, the influence of these two small vectors on the midpoint voltage of the DC support capacitors C1 and C2 during the first PWM cycle can cancel each other out. Since no other basic voltage vectors participate in the modulation during the first PWM cycle, the midpoint voltage drift phenomenon during the first PWM cycle can be effectively eliminated.

[0041] In step S3, during the second PWM cycle immediately following the first PWM cycle, the second spatial sub-vector V2 can be modulated with respect to the second small vector group, and the three-level converter 10 can be controlled based on the modulated second small vector group, wherein the second small vector and the corresponding second redundant small vector have equal durations. Here, the second spatial sub-vector V2 can be modulated using the second small vector PPO and the second redundant small vector OON, and appropriate modulation ratios can be assigned to the second small vector PPO and the second redundant small vector OON so that the durations of the second small vector PPO and the second redundant small vector OON are equal. For the two small vectors PPO / OON of the same small vector group, the switching states of the controllable switching devices corresponding to them are different, but the output voltages generated are the same, and the phase currents generated are in opposite directions. When the durations of these two small vectors are equal, the influence of these two small vectors on the midpoint voltage of the DC support capacitors C1 and C2 during the second PWM cycle can cancel each other out. Since no other basic voltage vectors participate in the modulation during the second PWM cycle, the midpoint voltage drift phenomenon during the second PWM cycle can be effectively eliminated.

[0042] As long as the speed of permanent magnet synchronous motor 1 is lower than the predetermined speed threshold, or as long as the generated reference voltage vector V ref Within the inner hexagon, the method according to this application can be used to continuously adjust the reference voltage vector V over two PWM cycles. ref Modulation is performed until the speed of the permanent magnet synchronous motor 1 exceeds a predetermined speed threshold or the generated reference voltage vector V. refThe method according to this application ceases to be performed only when the object is outside the inner hexagon.

[0043] Furthermore, the reference voltage vector V generated through current loop or voltage frequency control... ref (n) During modulation, the three-level converter 10 generates a large current ripple. To minimize this ripple, the reference voltage vector V generated in the (n-1)th PWM cycle can be used. ref (n-1) and the reference voltage vector V of the nth PWM cycle ref (n) Calculate the equivalent reference voltage vector V ref_equ (n), and in the nth PWM cycle and the (n+1)th PWM cycle, the equivalent reference voltage vector is doubled to 2V. ref_equ (n) is decomposed and modulated. Specifically, the equivalent reference voltage vector V can be calculated as follows: ref_equ (n):

[0044] When n is odd

[0045] When n is even, V ref_equ (n)=V ref_equ (n-1).

[0046] Figure 5 A schematic diagram of a reference voltage vector according to an exemplary embodiment of this application is shown. The voltage vector marked by dashed lines is the reference voltage vector V generated for each PWM cycle by a current loop or voltage frequency control. ref (n). The reference voltage vector V for the first PWM cycle. ref (1) It can be generated by current loop or voltage frequency control, but at this time the reference voltage vector V generated by the second PWM cycle has not yet been received. ref (2) The equivalent reference voltage vector V for the first PWM cycle can be calculated using the above formula. ref_equ (1):

[0047]

[0048] Wherein, the reference voltage vector V ref (0) The initial value can be set based on a virtual given angle. According to the space vector modulation method of this application, the same equivalent reference voltage vector 2V can be used in the first PWM cycle and the second PWM cycle. ref_equ (1) Decompose and modulate, and then use the above formula to calculate the equivalent reference voltage vector V for the second PWM cycle. ref_equ (2):

[0049] V ref_equ (2) = Vref_equ (1).

[0050] The reference voltage vector V in the third PWM cycle ref (3) It can be generated by current loop or voltage frequency control, but at this time the reference voltage vector V generated by the fourth PWM cycle has not yet been received. ref (4) The equivalent reference voltage vector V for the third PWM cycle can be calculated using the above formula. ref_equ (3):

[0051]

[0052] And the equivalent reference voltage vector V used in the fourth PWM cycle ref_equ (4) The calculation is as follows:

[0053] V ref_equ (4) = V ref_equ (3).

[0054] Therefore, the same equivalent reference voltage vector 2V can be used in the third and fourth PWM cycles. ref_equ (3) Decompose and modulate, and so on. The equivalent reference voltage vector calculated in this way can approximate the variation trend of the reference voltage vector of the previous PWM cycle as closely as possible, thereby minimizing the current ripple generated by the three-level converter 10.

[0055] Understandably, according to the vector selection principle of the space vector modulation method, any change in voltage vector can only perform the switching action of one bridge arm. Therefore, a zero vector can be introduced to coordinate the switching of voltage vectors in the same small vector group. This zero vector will not cause the midpoint voltage drift of the three-level converter.

[0056] It should be noted that the method according to this application is not only compatible with the traditional midpoint voltage balancing method of three-level inverters, but also the existing field orientation control and voltage frequency control strategies and implementation methods of permanent magnet synchronous motors can remain unchanged. Therefore, the method according to this application can participate in the regulation of the low-speed operation stage, especially the start-up stage of the permanent magnet synchronous motor, without changing the software and hardware framework of the permanent magnet synchronous motor.

[0057] According to the method of this application, a reference voltage vector V is generated within the inner hexagon of the space vector of the three-level converter. ref The reference voltage vector is twice 2V. refThe direction of the small vector group is decomposed into two spatial sub-vectors. In two consecutive PWM cycles, one of the decomposed spatial sub-vectors is modulated using one small vector group and, if necessary, a zero vector. Since the two small vectors of the same small vector group have equal action time and generate phase currents in opposite directions, the midpoint voltage drift phenomenon during each PWM cycle can be effectively eliminated, thereby effectively improving the starting performance and running stability of the permanent magnet synchronous motor.

[0058] In addition, it should be noted that the step numbers described herein do not necessarily represent the order of steps, but are merely a reference numeral. The order may be changed depending on the specific circumstances, as long as the technical objective of this application can be achieved.

[0059] Figure 6 A schematic diagram of a permanent magnet synchronous motor according to an exemplary embodiment of this application is shown.

[0060] like Figure 6 As shown, the permanent magnet synchronous motor 1 may include a three-level converter 10, which is configured, for example, as a midpoint clamped three-level converter, and its topology is as follows: Figure 2 As shown. The three-level converter 10 may include a control unit 11, which is used to perform the method according to this application.

[0061] It should be understood that in this document, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance, nor should they be construed as implicitly specifying the number of technical features indicated.

[0062] If an embodiment includes an "and / or" association between a first feature and a second feature, it should be interpreted as follows: according to one implementation, the embodiment has not only the first feature but also the second feature; according to another implementation, the embodiment has either only the first feature or only the second feature.

[0063] Although specific embodiments have been described above, these embodiments are not intended to limit the scope of this application, even when only a single embodiment is described with respect to a particular feature. The feature examples provided in this application are intended for illustrative purposes and not for limitation, unless otherwise stated. In practice, multiple features may be combined with each other as needed and where technically feasible. Various substitutions, modifications, and alterations are also conceived without departing from the spirit and scope of this application.

Claims

1. A method for midpoint voltage balance control of a three-level converter (10), the method comprising: The generated reference voltage vector V ref When the reference voltage vector 2V is located within the inner hexagon of the space vector of the three-level converter (10), it will be twice the reference voltage vector 2V. ref The direction of the small vector group is decomposed into a first spatial sub-vector V1 and a second spatial sub-vector V2, wherein the inner hexagon is formed by the ends of each small vector, and the small vector group includes a small vector and a redundant small vector corresponding to this small vector. In the first PWM cycle, the first spatial sub-vector V1 is modulated using a first small vector group, and the three-level converter (10) is controlled based on the modulated first small vector group, wherein the first small vector and the corresponding first redundant small vector have equal durations; and In the second PWM cycle that follows the first PWM cycle, the second subspace vector V2 is modulated about the second small vector group, and the three-level converter (10) is controlled based on the modulated second small vector group, wherein the second small vector and the corresponding second redundant small vector have the same duration.

2. The method according to claim 1, wherein, When the speed of the permanent magnet synchronous motor (1) is lower than the predetermined speed threshold, the reference voltage vector V for each PWM cycle is generated through the current loop of the field orientation control circuit of the permanent magnet synchronous motor (1). ref (n).

3. The method according to claim 1, wherein, When the speed of the permanent magnet synchronous motor (1) is lower than the predetermined speed threshold, the reference voltage vector V for each PWM cycle is generated by the voltage frequency control of the permanent magnet synchronous motor (1). ref (n).

4. The method according to claim 2 or 3, wherein, Using the generated reference voltage vector V of the (n-1)th PWM cycle ref (n-1) and the reference voltage vector V of the nth PWM cycle ref (n) Calculate the equivalent reference voltage vector V ref_equ (n), and in the nth PWM cycle and the (n+1)th PWM cycle, the equivalent reference voltage vector is doubled to 2V. ref_equ (n) is decomposed and modulated.

5. The method according to claim 4, wherein, The equivalent reference voltage vector V is calculated as follows: ref_equ (n): When n is odd When n is even, V ref_equ (n)=V ref_equ (n-1).

6. A three-level converter (10) comprising a control unit (11) for performing the method according to any one of claims 1 to 5.

7. The three-level converter (10) according to claim 6, wherein, The three-level converter (10) includes a midpoint clamped three-level converter.

8. A permanent magnet synchronous motor (1), the permanent magnet synchronous motor (1) comprising a three-level converter (10) according to claim 6 or 7.

9. The permanent magnet synchronous motor (1) according to claim 8, wherein, The permanent magnet synchronous motor (1) includes a high-speed air compressor.

10. A computer program product, such as a computer-readable program carrier, comprising computer program instructions that, when executed by a processor, at least assist in implementing the steps of the method according to any one of claims 1 to 5.

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