Low voltage auxiliary inverter based multi-set three-phase motor neutral point current control system and method

Abstract

The present application relates to the technical field of motor control, and particularly relates to a neutral point current control system and method for multiple three-phase motors based on a low-voltage auxiliary converter; an independent isolated low-voltage power supply is used to supply power to the low-voltage auxiliary converter, so that the DC bus voltage is lower than that of the main power converter, effectively reducing the requirements for device withstand voltage, insulation and switching loss; at the same time, the auxiliary converter only adjusts the neutral point current through the differential mode voltage, does not bear the fundamental power output of the main phase terminal, realizes the decoupling of the main and auxiliary functions, so that the main power converter can focus on the fundamental power output; the controller can collect the electric record to realize accurate current closed-loop control, and only adjusts the differential mode voltage, which is naturally decoupled with the external common mode voltage control, avoiding common mode interference; at the same time, due to the use of a low-voltage auxiliary power supply, the switching frequency of the auxiliary converter can be significantly higher than that of the main power converter, which can quickly track and accurately compensate the high-frequency harmonic current.

Description

Technical Field

[0001] This invention relates to the field of motor control technology, and in particular to a neutral point current control system and method for multiple three-phase motors based on a low-voltage auxiliary converter. Background Technology

[0002] In high-power motor drive systems, the main power converter has a high DC bus voltage and large rated current. Due to limitations in switching losses, heat dissipation capacity, electromagnetic compatibility requirements, and device safety margins, the switching frequency of the main power converter is generally difficult to increase significantly. Especially in high-speed, high-power multiphase motor applications, a lower switching frequency increases the difficulty of controlling current ripple, low-order voltage harmonics, air gap harmonic magnetic fields, and torque pulsation.

[0003] Multi-phase three-phase motors consist of two or more sets of three-phase windings, each typically connected in a star configuration and forming an independent neutral point. The presence of multiple neutral points provides additional degrees of control freedom for zero-sequence current regulation, winding magnetomotive force reconstruction, and local harmonic compensation. Theoretically, by injecting a controlled neutral current into the motor through the neutral point branch, effective compensation for specific harmonic currents, zero-sequence components, and spatial harmonic magnetomotive forces can be achieved without significantly increasing the capacity of the main power circuit.

[0004] However, existing technologies that directly increase the switching frequency by relying on the main power converter are difficult to implement in practical engineering due to the switching losses and heat dissipation conditions of high-voltage devices. If a fourth bridge arm with the same voltage level as the main DC bus is added to the main power converter to regulate the neutral point current, additional high-voltage power devices and corresponding insulation, drive, heat dissipation and protection designs are required, which will significantly increase the system cost and switching losses. If multiple neutral points are connected passively (such as by direct shorting), although the structure is simple, it is impossible to accurately regulate the amplitude, phase and frequency of the neutral current in a closed loop. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a control system and method for neutral point current of multiple three-phase motors based on a low-voltage auxiliary converter. This solves the technical problems of existing technologies that rely on the main power converter, require additional high-voltage power devices and supporting components, or have poor adjustment accuracy when regulating neutral point current.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a multi-group three-phase motor neutral point current control system based on a low-voltage auxiliary converter, comprising at least two groups of three-phase star-connected windings driven by a main power converter, wherein each group of three-phase star-connected windings has an independent neutral point at its end serving as a common junction point, and further comprising: A low-voltage auxiliary converter has multiple bridge arms corresponding to the number of neutral points, and each bridge arm is connected to a neutral point through a neutral inductor. An isolated low-voltage power supply connected to a low-voltage auxiliary converter is used to provide each bridge arm with a low-voltage DC bus voltage lower than that of the main power converter DC bus. The controller is used to collect electrical data of multiple sets of three-phase star-connected windings and calculate the neutral current command. Based on the neutral current command, it controls the low-voltage auxiliary converter to adjust the differential mode voltage between each neutral point, so as to drive the branch where the neutral inductor is located to generate a controlled neutral current.

[0007] Preferably, the electrical data collected by the controller includes the phase current of the multiple sets of three-phase star-connected windings and the neutral current of each neutral point branch; the controller performs a closed-loop comparison between the calculated neutral current command and the collected neutral current sample value to generate a differential mode voltage command, thereby realizing closed-loop control of the neutral current, and decoupling from the external common mode voltage control by adjusting the differential mode voltage between each neutral point.

[0008] Preferably, the switching frequency of the power switching devices in the low-voltage auxiliary converter is higher than the switching frequency of the power switching devices in the main power converter.

[0009] Preferably, the controlled neutral current is used to suppress or compensate for at least one of zero-sequence current, specified harmonic current, air gap harmonic magnetic field, torque pulsation, and rotor additional losses.

[0010] Preferably, the bridge arm includes two switching devices, which are connected in parallel and then in series with the neutral inductor. The two switching devices are driven by complementary means and have a dead zone.

[0011] Preferably, the three-phase star-connected windings are in two sets, and the low-voltage auxiliary converter is a low-voltage H-bridge structure. The first and second arms of the H-bridge are respectively driven by complementary forces and have dead zones. When the upper switch of the first arm and the lower switch of the second arm are simultaneously turned on, the low-voltage H-bridge outputs a positive differential mode voltage. When the lower switch of the first arm and the upper switch of the second arm are simultaneously turned on, the low-voltage H-bridge outputs a negative differential mode voltage. When the first and second arms are simultaneously connected to the same side of the low-voltage DC bus, the low-voltage H-bridge outputs a zero differential mode voltage.

[0012] This invention also provides a method for controlling the neutral point current of multiple three-phase motors based on a low-voltage auxiliary converter, applicable to a multi-group three-phase motor neutral point current control system, comprising the following steps: S1. Collect electrical data including phase currents of multiple sets of three-phase star-connected windings, neutral currents of each neutral point branch, and operating status of each motor. S2. Based on the collected electrical data, calculate the compensation targets for the zero-sequence current, specified harmonic current, or air gap harmonic magnetic field components that need to be compensated, and generate reference values ​​for the neutral current of each neutral point branch. S3. Compare the sampled value of the neutral current of each neutral point branch with the corresponding reference value of the neutral current, and use the closed-loop control algorithm to generate the initial voltage command of each neutral point. S4. Based on the DC bus voltage of the low-voltage auxiliary converter and the voltage constraints of each neutral point, the initial voltage command is allocated to generate the differential mode voltage control command for each bridge arm. S5. According to the differential mode voltage control command, the switching modulation of each bridge arm of the low-voltage auxiliary converter is performed to output the differential mode voltage and drive the neutral line inductor to generate a controlled neutral line current.

[0013] Preferably, the compensation objective includes at least one of suppressing third harmonic zero-sequence harmonics, reducing torque ripple, improving phase current waveform, and reducing rotor additional losses.

[0014] By employing the above technical solution, the present invention provides a control system and method for neutral point current of multiple three-phase motors based on a low-voltage auxiliary converter, which has at least the following beneficial effects: 1. This invention employs an independent, isolated low-voltage power supply to power the low-voltage auxiliary converter, ensuring its DC bus voltage is lower than that of the main power converter. The auxiliary bridge arm does not need to withstand the high voltage of the main bus, effectively reducing the requirements for device withstand voltage, insulation, and switching losses. Simultaneously, the auxiliary converter only regulates the neutral point current through differential mode voltage and does not bear the fundamental power output of the main phase, achieving decoupling of main and auxiliary functions. This allows the main power converter to focus on fundamental power output, greatly reducing system control complexity.

[0015] 2. The controller of this invention can collect phase current and neutral current to achieve precise current closed-loop control, and only adjusts the differential mode voltage, achieving natural decoupling from external common mode voltage control and avoiding common mode interference. Simultaneously, due to the use of a low-voltage auxiliary power supply, the switching frequency of the auxiliary converter can be significantly higher than that of the main power converter, greatly improving the current loop control bandwidth. This enables rapid tracking and precise compensation of high-frequency harmonic currents, significantly improving the harmonic suppression response speed.

[0016] 3. This invention, through controlled neutral current injection, can specifically suppress various targets such as zero-sequence current, air gap harmonic magnetic field, and torque pulsation, effectively reducing rotor eddy current losses and electromagnetic noise. Simultaneously, the number of bridge arms corresponds one-to-one with the number of neutral points in the three-phase star-connected windings, supporting modular expansion. Simply adding or removing bridge arm modules can adapt to dual three-phase, three-phase, or even higher-phase multi-phase motors, greatly expanding the system's applicability and engineering compatibility. Attached Figure Description

[0017] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings: Figure 1 This is a circuit diagram of the neutral point current control system according to Embodiment 1 of the present invention; Figure 2 This is a circuit diagram of the neutral point current control system according to Embodiment 2 of the present invention. Detailed Implementation

[0018] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. This will allow for a full understanding of how the present application uses technical means to solve technical problems and achieve technical effects, and to facilitate its implementation.

[0019] To address the technical problems of existing technologies that rely on the main power converter, require additional high-voltage power devices and supporting components, or have poor adjustment accuracy when regulating neutral point current, this embodiment provides a multi-group three-phase motor neutral point current control system based on a low-voltage auxiliary converter. This invention is applicable to dual three-phase motors, three-group three-phase motors, and n-group three-phase winding motors. It is also applicable to permanent magnet synchronous motors, induction motors, synchronous reluctance motors, and other AC motors with multiple independent star neutral points that require adjustment of zero-sequence current or specified harmonic current through neutral point branches. Specific embodiments are described below: Example 1 Please refer to Figure 1 In this embodiment, the control system includes at least two sets of three-phase star-connected windings driven by a main power converter. Each set of three-phase star-connected windings has an independent neutral point at its end, serving as a common junction point. Specifically, the two sets of three-phase windings form neutral points N1 and N2, respectively. The main power converter connects to each phase terminal and outputs fundamental power.

[0020] The system also includes a low-voltage auxiliary converter, which has multiple bridge arms corresponding to the number of neutral points. Each bridge arm is connected to a neutral point via a neutral inductor. In this embodiment, a neutral inductor is connected in series between each neutral point and the low-voltage auxiliary converter. The neutral inductor serves two purposes: firstly, it constitutes a controllable first-order current object; secondly, it limits the current ripple during switching transients, preventing the high-frequency switching voltage of the low-voltage bridge arm from directly acting on the motor neutral point. The neutral inductor can be set independently or can be a combination of coupled inductors, integrated inductors, or distributed inductors from the motor leads with external inductors. The sampling unit includes neutral current sampling, neutral point voltage sampling, main power converter phase current sampling, and DC bus voltage sampling, used to provide the controller with the physical quantities required for closed-loop control. The low-voltage auxiliary converter can adopt a low-voltage H-bridge or an equivalent double-bridge arm structure, with the midpoints of the two arms connected to the neutral inductor. and They are connected to neutral points N1 and N2 respectively. The DC side of the H-bridge is powered by an isolated low-voltage power supply connected to the low-voltage auxiliary converter. This isolated low-voltage power supply provides a low-voltage DC bus for each bridge arm with a voltage lower than that of the main power converter DC bus. In other words, there is no direct electrical connection between the DC side of the low-voltage H-bridge and the main power converter DC bus.

[0021] Regarding the switching states of the bridge arms, in this embodiment, the first and second bridge arms of the H-bridge employ complementary drives and are configured with dead zones. Let the switching functions of the first and second bridge arms be respectively... , The value is 1 when the upper tube is on and 0 when the lower tube is on; the low-voltage DC bus voltage is , with neutral point and The midpoint voltages of the corresponding bridge arms are respectively and Then the differential mode voltage applied between the two neutral ports of the H-bridge satisfy: Specifically, the first and second arms of the H-bridge each include two switching devices. When the switching transistor on the first arm (such as...) ) and the second bridge arm lower switch (such as When both are turned on, the low-voltage H-bridge outputs a positive differential mode voltage; when the lower switch of the first bridge arm (such as...) is turned on... ) and the switching transistor on the second bridge arm (such as When both arms are simultaneously connected, the low-voltage H-bridge outputs a negative differential-mode voltage; when the first and second arms are simultaneously connected to the same side of the low-voltage DC bus, the low-voltage H-bridge outputs a zero differential-mode voltage. By switching between the above switching states using PWM, hysteresis control, or model predictive control, a continuously adjustable average differential-mode voltage can be synthesized within one switching cycle, making the auxiliary module a bidirectional controllable voltage source connecting N1 and N2.

[0022] This system also includes a controller for acquiring electrical data from multiple sets of three-phase star-connected windings and calculating the neutral current command. Based on the neutral current command, the controller controls the low-voltage auxiliary converter to adjust the differential-mode voltage between each neutral point, thereby driving a controlled neutral current in the branch containing the neutral inductor. Specifically, the electrical data acquired by the controller includes the phase currents of multiple sets of three-phase star-connected windings and the neutral current in each neutral point branch. The controller performs a closed-loop comparison between the calculated neutral current command and the acquired neutral current sample values ​​to generate a differential-mode voltage command, achieving closed-loop control of the neutral current. Decoupling from external common-mode voltage control is achieved by adjusting the differential-mode voltage between each neutral point. This closed-loop control can employ PI control, PR control, resonant controller, repetitive controller, model predictive controller, or a combination thereof.

[0023] When the controller drives the low-voltage auxiliary converter to generate a controlled neutral current, the formation of the neutral current strictly follows Kirchhoff's current law. Specifically, the neutral circulating current is defined as follows: The positive direction is from port a of bridge H via N1 enters the first group of three-phase windings and returns to N2 via the zero-sequence circuit of the main power converter and the second group of three-phase windings. And port b. Applying Kirchhoff's current law to the neutral point of each group of three-phase windings, the relationship between the neutral current and the sum of the three-phase currents in that group can be obtained as follows: In the above formula, , and Neutral point The corresponding three-phase current of the three-phase winding. , and Neutral point The three-phase currents of the corresponding three-phase windings, neutral point and The currents at are respectively and , and These represent the zero-sequence currents of the first and second three-phase windings, respectively. Therefore, the low-voltage H-bridge injects zero-sequence currents of equal amplitude and opposite direction into the two sets of three-phase windings. This current does not directly change the positive-sequence fundamental current (dq) of the two sets of three-phase windings, so the auxiliary module does not need to bear the main torque current and the main fundamental power; its main function is to reconstruct the phase current harmonics and air gap harmonic magnetomotive force using the additional zero-sequence degrees of freedom. In actual systems, the zero-sequence current may still exchange a small amount of active power with the motor harmonic back EMF, so the low-voltage power supply and its DC / DC module can be designed as energy feedback type as needed.

[0024] Let the equivalent series resistance of the centerline be denoted as... , and , And the voltage difference between the two neutral points is denoted as Then the equivalent dynamic equation of the midline circulation can be written as: This equation shows that the rate of change of the neutral current is directly determined by the difference between the H-bridge differential voltage and the inherent voltage difference between the two neutral points of the motor. The controller responds according to the neutral current command. With the sampling value of the neutral current A closed-loop comparison is performed, and a differential-mode voltage command is generated based on the error between the two comparisons. To achieve closed-loop control of the neutral current, when using neutral point voltage difference, resistor voltage drop, and reference current rate of change as feedforwards, the general form of the following equation can be expressed: in It can be a PI controller, PR controller, repetitive controller, resonant controller, model predictive controller, or a combination thereof. The neutral inductor serves two purposes: firstly, it constitutes a controllable first-order current object; secondly, it limits the transient current flow during switching. This reduces current ripple and prevents the high-frequency switching voltage of the low-voltage H-bridge from directly acting on the motor neutral point.

[0025] This embodiment achieves low voltage, high bandwidth, and excellent harmonic suppression performance, mainly based on the following physical mechanism: First, the low-voltage auxiliary converter only needs to establish the differential-mode voltage required for the neutral circulating current; it does not need to synthesize the fundamental back EMF at the motor phase terminals and the main output voltage. Its DC bus voltage is mainly determined by the voltage difference between the two neutral point harmonics, the voltage drop across the neutral resistance, the dynamic voltage drop across the neutral inductor, and the modulation margin. For an angular frequency of... Amplitude To determine the target harmonics, the selection of the low-voltage busbar can be checked using the following formula: In the above formula, The harmonic voltage difference amplitude between the two neutral points. and The equivalent total resistance and total inductance of the neutral line are given. This provides the voltage margin required for modulation and dynamic response. Since the aforementioned compensation voltage is significantly lower than the fundamental voltage at the main phase terminals, devices with lower withstand voltage ratings and lower switching losses can be used, and each bridge arm is provided with a low-voltage DC bus with a voltage lower than the DC bus voltage of the main power converter. This low-voltage designation is not only determined by the "auxiliary" designation but is also jointly verified by the most unfavorable speed, maximum harmonic current, maximum neutral point voltage difference, and device voltage drop.

[0026] Second, the auxiliary module only carries the neutral current that needs compensation, and its capacity can be determined based on the differential mode port voltage and the neutral current, approximately satisfying... This capacity is typically smaller than the fundamental output capacity of the main power converter, and the average active power is mainly used to compensate for branch copper losses, switching losses, and energy exchanged with harmonic back EMF. Therefore, additional high-frequency control degrees of freedom can be obtained without significantly increasing the capacity of the main power devices. More importantly, the main power converter and the low-voltage auxiliary converter are separated in terms of hardware and control functions. When the auxiliary module experiences overcurrent, overtemperature, or drive failure, the switching devices can be blocked and the module can be shut down through bypass or circuit breaker devices, while the main power converter can still maintain the basic operation of the motor; the neutral inductance also limits the rate of rise of the fault current. This allows for precise harmonic control while improving modular maintenance and fault derating capabilities.

[0027] Third, the controller can extract the target component to be suppressed from the sum of the three-phase currents of each group, the harmonic observer, the air gap magnetic field observation, or the torque ripple signal, and convert it into... , Since the neutral current is directly equal to the negative sum of the three-phase currents in each group, the auxiliary module can directly perform closed-loop regulation of the zero-sequence current and specified harmonics related to the zero-sequence subspace. Because the auxiliary converter uses a low-voltage bus, the switching frequency of the power switching devices in the low-voltage auxiliary converter is higher than that in the main power converter, thus achieving rapid tracking of high-frequency harmonics (such as the fifth and seventh harmonics). For double three-phase windings with electrically offset phases, the controller can also map the two sets of equal-amplitude reverse zero-sequence currents into the spatial harmonic magnetomotive force to be canceled based on the winding displacement angle and winding coefficient. The specific harmonic order that can be suppressed depends on the winding phase shift, slot-pole matching, winding distribution, and the spatial properties of the target harmonic.

[0028] Fourth, when the specified harmonic current and its corresponding air-gap harmonic magnetic field decrease, the interaction between the harmonic magnetic field and the permanent magnet, rotor conductor, or rotor salient pole weakens, thereby reducing torque pulsation, rotor eddy current loss, stator additional copper loss, and electromagnetic noise at the corresponding frequency. This technical effect stems from a complete control chain of "harmonic target extraction—neutral current closed loop—differential mode voltage drive," and cannot be obtained simply by shorting the neutral point.

[0029] Fifth, the main power converter and the low-voltage auxiliary converter are separated in terms of hardware and control functions. When the auxiliary module experiences overcurrent, overtemperature, or drive failure, the four switching devices can be blocked and the circuit can be shut down through a bypass or circuit breaker, while the main power converter can still maintain the basic operation of the motor. , It also limits the rate of rise of fault current. This allows for precise harmonic control while improving modular maintenance and fault derating capabilities.

[0030] It should be noted that in this embodiment, the low-voltage auxiliary converter is only responsible for neutral current regulation and does not need to synthesize the fundamental back EMF and main output voltage at the motor phase terminals. The voltage level of this isolated low-voltage power supply is mainly determined by the voltage difference between the two neutral point harmonics, the voltage drop of the neutral resistance, the dynamic voltage drop of the neutral inductance, and the modulation margin. Furthermore, the switching frequency of the power switching devices in the low-voltage auxiliary converter is higher than that in the main power converter, which allows the auxiliary module to undertake high-frequency or specified harmonic current regulation tasks, improving the system's harmonic control capability. Simultaneously, the controlled neutral current is used to suppress or compensate for at least one of the following: zero-sequence current, specified harmonic current, air-gap harmonic magnetic field, torque pulsation, and rotor additional losses. When the specified harmonic current and its corresponding air-gap harmonic magnetic field decrease, the interaction between the harmonic magnetic field and the permanent magnet, rotor conductor, or rotor salient pole weakens, thereby reducing torque pulsation, rotor eddy current losses, stator additional copper losses, and electromagnetic noise.

[0031] In addition, the isolated low-voltage power supply provides a low-voltage DC bus to the low-voltage auxiliary converter. This low-voltage DC bus can be a fixed voltage or dynamically adjusted by the DC / DC module according to the neutral current amplitude, harmonic frequency, speed range, or main power converter operating conditions. When the harmonic voltage to be compensated is significantly lower than the fundamental voltage at the main phase terminals, the auxiliary bridge arm devices can use devices with lower withstand voltage ratings and lower switching losses, and the switching frequency can be increased under the same heat dissipation conditions. After adopting the low-voltage auxiliary power supply, the voltage rating of the auxiliary bridge arm devices is reduced, and power devices with higher switching speeds, lower losses, and simpler drives can be selected. Compared with directly adding a high-voltage fourth bridge arm to the main power converter, this solution is more suitable for fine control of the neutral current in high-power, low-switching-frequency main converter scenarios.

[0032] Example 2 Please refer to Figure 2 For three or more three-phase motors, the low-voltage auxiliary converter has multiple bridge arms corresponding to the number of neutral points. Each bridge arm can share the same isolated low-voltage DC bus or use multiple isolated low-voltage power supply modules.

[0033] In this embodiment, each bridge arm includes two switching devices connected in parallel and then in series with the neutral inductor. The two switching devices employ complementary driving and a dead time is set. Specifically, each bridge arm consists of upper and lower power switching transistors connected in series to form a half-bridge structure, with its switching node connected in series with the neutral inductor. The complementary driving and dead time setting prevents a shoot-through short circuit on the DC bus caused by simultaneous conduction of the upper and lower bridge arms. The control logic of each bridge arm is independently modulated, and the controller coordinates and distributes neutral point voltage commands to meet the differential mode control objective.

[0034] Each neutral branch can be sampled and current-limited independently. The modular structure allows for rapid addition or removal of neutral control arms based on the number of motor windings, making it suitable for serialized designs of dual three-phase, three-phase, and higher phase-number motors. Each neutral branch can also be equipped with independent current-limiting protection, bypass switches, or circuit breakers. When an auxiliary module experiences overcurrent, overtemperature, or drive failure, the corresponding arm's switching devices can be blocked, and the circuit can be bypassed or disconnected, while the main power converter can still maintain basic motor operation. The neutral inductor also limits the rate of rise of fault current, thereby improving modular maintenance and fault derating capabilities while achieving fine harmonic control.

[0035] Example 3 This embodiment provides a method for controlling the neutral point current of multiple three-phase motors based on a low-voltage auxiliary converter. The method includes the following steps: S1. Collect electrical data including phase currents of multiple sets of three-phase star-connected windings, neutral currents of each neutral point branch, and operating status of each motor. Specifically, the controller provides a data basis for calculating subsequent control objectives based on information such as motor operating status, phase current sampling values, neutral current sampling values, speed, electrical angle, bus voltage, and temperature.

[0036] S2. Based on the collected electrical data, calculate the compensation targets for the zero-sequence current, specified harmonic current, or air-gap harmonic magnetic field component that need to be compensated, and generate reference values ​​for the neutral current of each neutral point branch. Specifically, the controller generates reference values ​​for the neutral current based on the sum of phase currents, the zero-sequence component, the specified harmonic component, or the observed air-gap magnetic field. Control targets may include suppressing third harmonics or other zero-sequence harmonics, reducing torque ripple, improving phase current waveforms, and reducing rotor additional losses. That is, the compensation targets include at least one of suppressing third harmonics, reducing torque ripple, improving phase current waveforms, and reducing rotor additional losses. For double three-phase windings with electrically offset phase angles, the controller can also map the two sets of equal-amplitude reverse zero-sequence currents into spatial harmonic magnetomotive forces that need to be canceled based on the winding displacement angle and winding coefficients.

[0037] S3. Compare the sampled neutral current values ​​of each neutral point branch with the corresponding neutral current reference values, and generate the initial voltage command for each neutral point using a closed-loop control algorithm. In the neutral current closed-loop control, the controller compares each sampled neutral current value with the reference value and generates the initial neutral point voltage command using PI control, PR control, repetitive control, model predictive control, or a combination thereof. For fixed-frequency harmonic compensation, a resonant controller can be used to improve the tracking accuracy of the current at the specified frequency; for multi-frequency harmonic compensation, multi-resonant control or model predictive control can be used.

[0038] S4. Based on the DC bus voltage of the low-voltage auxiliary converter and the voltage constraints of each neutral point, the initial voltage command is allocated to generate differential mode voltage control commands for each bridge arm. The DC bus voltage of the low-voltage auxiliary converter is mainly determined by the harmonic voltage difference of each neutral point, the voltage drop across the neutral line resistance, the dynamic voltage drop across the neutral line inductance, and the modulation margin. According to this voltage constraint, the controller allocates the initial voltage command to each bridge arm, generating the corresponding differential mode voltage control signal. This control process can be implemented in the main motor controller or in the independent neutral current controller, and exchanges speed, electrical angle, phase current, power command, and fault status with the main power converter controller through a communication interface.

[0039] S5. Based on the differential mode voltage control command, the switching modulation of each bridge arm of the low-voltage auxiliary converter is performed to output the differential mode voltage, driving the neutral inductor to generate a controlled neutral current. Since the auxiliary converter uses a low-voltage bus, its switching frequency can be higher than that of the main power converter, thereby improving the neutral current bandwidth and control resolution. The auxiliary module only flows through the neutral current that needs compensation; its capacity can be determined based on the differential mode port voltage and the neutral current. This capacity is typically smaller than the fundamental output capacity of the main power converter, allowing for additional high-frequency control freedom without significantly increasing the capacity of the main power devices.

[0040] The main power converter and the low-voltage auxiliary converter are separated in terms of hardware and control functions. When the auxiliary module experiences overcurrent, overtemperature, or drive failure, the four switching devices can be blocked and the circuit can be shut down via bypass or circuit breaker, while the main power converter can still maintain the basic operation of the motor. Therefore, this invention allows the low-voltage auxiliary converter to handle high-frequency or specified harmonic current regulation while the main power converter maintains a low switching frequency, significantly improving the harmonic control capability, electromagnetic compatibility performance, and engineering adaptability of multiphase motors.

[0041] Those skilled in the art will understand that all or part of the steps in the methods of the above embodiments can be implemented by a program instructing related hardware. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Moreover, this application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0042] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. Since the above embodiments are substantially similar to the method embodiments, their descriptions are relatively simple; relevant parts can be referred to the descriptions of the method embodiments.

[0043] The above embodiments provide a detailed description of the present invention. Specific examples have been used to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A multi-group three-phase motor neutral point current control system based on a low-voltage auxiliary converter, comprising at least two groups of three-phase star-connected windings driven by a main power converter, wherein each group of three-phase star-connected windings has an independent neutral point at its end serving as a common junction point, characterized in that, Also includes: A low-voltage auxiliary converter has multiple bridge arms corresponding to the number of neutral points, and each bridge arm is connected to a neutral point through a neutral inductor. An isolated low-voltage power supply connected to a low-voltage auxiliary converter is used to provide each bridge arm with a low-voltage DC bus voltage lower than that of the main power converter DC bus. The controller is used to collect electrical data of multiple sets of three-phase star-connected windings and calculate the neutral current command. Based on the neutral current command, it controls the low-voltage auxiliary converter to adjust the differential mode voltage between each neutral point, so as to drive the branch where the neutral inductor is located to generate a controlled neutral current.

2. The multi-group three-phase motor neutral point current control system according to claim 1, characterized in that, The electrical data collected by the controller includes the phase current of the multiple sets of three-phase star-connected windings and the neutral current of each neutral point branch. The controller performs a closed-loop comparison between the calculated neutral current command and the collected neutral current sample value to generate a differential mode voltage command, thereby realizing closed-loop control of the neutral current. It also decouples from the external common mode voltage control by adjusting the differential mode voltage between each neutral point.

3. The multi-group three-phase motor neutral point current control system according to claim 1, characterized in that, The switching frequency of the power switching devices in the low-voltage auxiliary converter is higher than that of the power switching devices in the main power converter.

4. The multi-group three-phase motor neutral point current control system according to claim 1, characterized in that, The controlled neutral current is used to suppress or compensate for at least one of zero-sequence current, specified harmonic current, air gap harmonic magnetic field, torque pulsation, and rotor additional losses.

5. The multi-group three-phase motor neutral point current control system according to claim 1, characterized in that, The bridge arm includes two switching devices, which are connected in parallel and then in series with the neutral inductor. The two switching devices are driven by complementary means and have a dead zone.

6. The multi-group three-phase motor neutral point current control system according to claim 1, characterized in that, The three-phase star-connected winding consists of two sets. The low-voltage auxiliary converter is a low-voltage H-bridge structure. The first and second arms of the H-bridge are driven by complementary forces and have dead zones. When the upper switch of the first arm and the lower switch of the second arm are simultaneously turned on, the low-voltage H-bridge outputs a positive differential mode voltage. When the lower switch of the first arm and the upper switch of the second arm are simultaneously turned on, the low-voltage H-bridge outputs a negative differential mode voltage. When the first and second arms are simultaneously connected to the same side of the low-voltage DC bus, the low-voltage H-bridge outputs a zero differential mode voltage.

7. A method for controlling the neutral point current of multiple three-phase motors based on a low-voltage auxiliary converter, applied to the neutral point current control system of multiple three-phase motors as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. Collect electrical data including phase currents of multiple sets of three-phase star-connected windings, neutral currents of each neutral point branch, and operating status of each motor. S2. Based on the collected electrical data, calculate the compensation targets for the zero-sequence current, specified harmonic current, or air gap harmonic magnetic field components that need to be compensated, and generate reference values ​​for the neutral current of each neutral point branch. S3. Compare the sampled value of the neutral current of each neutral point branch with the corresponding reference value of the neutral current, and use the closed-loop control algorithm to generate the initial voltage command of each neutral point. S4. Based on the DC bus voltage of the low-voltage auxiliary converter and the voltage constraints of each neutral point, the initial voltage command is allocated to generate the differential mode voltage control command for each bridge arm. S5. According to the differential mode voltage control command, the switching modulation of each bridge arm of the low-voltage auxiliary converter is performed to output the differential mode voltage and drive the neutral line inductor to generate a controlled neutral line current.

8. The method for controlling the neutral point current of multiple three-phase motors according to claim 7, characterized in that, The compensation objectives include at least one of the following: suppressing third harmonic zero-sequence harmonics, reducing torque ripple, improving phase current waveforms, and reducing rotor additional losses.

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