NPC three-level neutral-point voltage balancing method and device under condition of low carrier wave ratio or ultralow carrier wave ratio

Abstract

The invention belongs to the technical field of motor driving control, and particularly relates to an NPC three-level neutral-point voltage balancing method and device under the condition of a low carrier wave ratio or an ultra-low carrier wave ratio. Three-level bridge arms of the same type are additionally arranged on an NPC three-level main circuit to form an auxiliary single-phase, three-phase or hybrid three-level circuit; the auxiliary three-level circuit selects a power device with a small rated current parameter and is set as a high carrier wave ratio, capacitor neutral-point voltage balance control can be simply and conveniently realized when the NPC three-level main circuit is in a low carrier wave ratio or an ultra-low carrier wave ratio by using a space vector modulation method or a zero-sequence injection carrier wave modulation method, the internal key problem of the three-level topology is solved, and the reliability of the three-level topology is improved. The application of the three-level technology in the fields of high-power electric propulsion, high-speed motor driving and the like is greatly promoted, and the high practical value is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of motor drive control, and in particular relates to a method and a device for balancing the midpoint voltage of an NPC three-level under low carrier ratio or ultra-low carrier ratio conditions. Background Art

[0002] Medium voltage, high power, high power density and low carrier ratio are the outstanding features of electric propulsion and high-speed motor drive systems. In order to make the drive meet these requirements, the neutral-point-clamped (NPC) three-level circuit has better output voltage waveform and lower voltage change rate, and can use low-rated voltage power devices to show lower economic cost and better performance. However, to fully utilize the excellent characteristics of the NPC three-level circuit topology, the balance of the DC bus capacitor midpoint voltage must be ensured.

[0003] The current three-level midpoint voltage balance control strategy is mainly divided into: carrier modulation method and space vector modulation method. The basic principle of the carrier modulation method is to control the midpoint voltage by superimposing the same zero-sequence component in the modulation signal; the space vector modulation method uses the complementary characteristics of the midpoint current corresponding to the positive and negative small vectors to adjust the relative action time of the positive and negative vectors to compensate the midpoint voltage. Both methods establish an average midpoint current mathematical model, and both assume that the output load current remains unchanged during the sampling period, that is, the carrier frequency is required to be much larger than the fundamental frequency, that is, a high carrier ratio. When the carrier ratio is low or ultra-low, the established average midpoint current model does not hold, so the theoretical basis of the NPC three-level circuit topology voltage balancing method based on this premise does not hold. These two methods will not be able to effectively control the midpoint voltage, and the midpoint voltage will eventually be unbalanced, and the NPC three-level circuit will not work properly.

[0004] In view of the above analysis, the technical problems that need to be solved urgently in the prior art are:

[0005] (1) The existing NPC three-level midpoint voltage balance control method is only applicable to high-carrier ratio NPC three-level circuits. When the carrier ratio is low or ultra-low, the average midpoint current model established based on the high carrier ratio condition does not hold, and thus the proposed midpoint voltage balance control method fails.

[0006] (2) Under low carrier ratio or ultra-low carrier ratio conditions, NPC three-level high-power electric propulsion and high-speed motor drive are subject to the midpoint voltage. How to effectively control the midpoint voltage has become a difficult problem that needs to be solved urgently in the development of NPC multi-level conversion technology. Summary of the invention

[0007] In view of the problems existing in the prior art, the present invention provides a method and device for balancing the midpoint voltage of an NPC three-level circuit under low carrier ratio or ultra-low carrier ratio conditions.

[0008] The present invention is implemented as follows: a method for balancing the midpoint voltage of an NPC three-level circuit under low carrier ratio or ultra-low carrier ratio conditions is provided. A three-level bridge arm of the same type is added to the NPC three-level main circuit to form an auxiliary single-phase, three-phase, or mixed three-level circuit. The auxiliary three-level circuit selects power devices with small rated current parameters and is set to a high carrier ratio to be responsible for the midpoint voltage balancing work of the main power circuit.

[0009] Furthermore, a three-level bridge arm of the same type is added to the NPC three-level main circuit to form an auxiliary single-phase three-level circuit. The auxiliary single-phase three-level circuit selects power devices with small rated current parameters and is set to a high carrier ratio. When the NPC three-level main circuit operates at a low carrier ratio or an ultra-low carrier ratio, the auxiliary three-level circuit balances the midpoint voltage using a single-phase space vector modulation method or a zero-sequence injection carrier modulation method.

[0010] Furthermore, a three-level bridge arm of the same type is added to the NPC three-level main circuit to form two sets of parallel auxiliary single-phase three-level circuits, and the two sets of parallel auxiliary single-phase three-level circuits select power devices with small rated current parameters, and are all set to high carrier ratio. When the NPC three-level main circuit operates at a low carrier ratio or an ultra-low carrier ratio, the two sets of parallel auxiliary three-level circuits use a single-phase space vector modulation method or a zero-sequence injection carrier modulation method to balance the midpoint voltage together.

[0011] Furthermore, a three-level bridge arm of the same type is added to the NPC three-level main circuit to form an auxiliary three-phase three-level circuit. The auxiliary three-phase three-level circuit selects power devices with small rated current parameters and is set to a high carrier ratio. When the NPC three-level main circuit operates at a low carrier ratio or an ultra-low carrier ratio, the auxiliary three-level circuit balances the midpoint voltage using a three-phase space vector modulation method or a zero-sequence injection carrier modulation method.

[0012] Furthermore, three-level bridge arms of the same type are added to the NPC three-level main circuit to form two sets of parallel auxiliary three-phase three-level circuits. The two sets of parallel auxiliary three-phase three-level circuits select power devices with small rated current parameters and are all set to high carrier ratios. When the NPC three-level main circuit operates at a low carrier ratio or an ultra-low carrier ratio, the two sets of parallel auxiliary three-level circuits use a three-phase space vector modulation method or a zero-sequence injection carrier modulation method to balance the midpoint voltage.

[0013] Furthermore, a three-level bridge arm of the same type is added to the NPC three-level main circuit to form an auxiliary single-phase and three-phase hybrid three-level circuit. The auxiliary single-phase and three-phase hybrid three-level circuit selects power devices with small rated current parameters, and all are set to high carrier ratios. When the NPC three-level main circuit operates at a low carrier ratio or an ultra-low carrier ratio, the auxiliary single-phase and three-phase hybrid three-level circuit uses a space vector modulation method or a zero-sequence injection carrier modulation method to balance the midpoint voltage.

[0014] The present invention also provides an NPC three-level midpoint voltage balancing device under low carrier ratio or ultra-low carrier ratio conditions, comprising:

[0015] The main circuit module is an NPC three-level main circuit;

[0016] The auxiliary circuit module is arranged in the main circuit module, and includes a single-phase, three-phase or single-phase and three-phase mixed three-level circuit composed of three-level bridge arms of the same type. The auxiliary circuit selects power devices with smaller rated current parameters and is set to a high carrier ratio working mode;

[0017] The modulation control module is used to control the auxiliary circuit module to adjust the midpoint voltage of the main circuit module by using the space vector modulation method or the zero-sequence injection carrier modulation method.

[0018] Furthermore, the auxiliary circuit module is a single-phase three-level circuit, and the modulation control module adjusts the midpoint voltage of the main circuit module by a single-phase space vector modulation method or a zero-sequence injection carrier modulation method.

[0019] Furthermore, the auxiliary circuit module is a three-phase three-level circuit, and the modulation control module adjusts the midpoint voltage of the main circuit module by a three-phase space vector modulation method or a zero-sequence injection carrier modulation method.

[0020] Furthermore, the auxiliary circuit module is composed of two groups of single-phase or three-phase three-level circuits connected in parallel, and the modulation control module jointly adjusts the midpoint voltage of the main circuit module through a space vector modulation method or a zero-sequence injection carrier modulation method.

[0021] Furthermore, the auxiliary circuit module is composed of a single-phase and three-phase hybrid three-level circuit, and the modulation control module jointly adjusts the midpoint voltage of the main circuit module through a space vector modulation method or a zero-sequence injection carrier modulation method.

[0022] In combination with the above technical solutions and the technical problems solved, the advantages and positive effects of the technical solutions to be protected by the present invention are as follows:

[0023] First, a three-level bridge arm of the same type is added to the NPC three-level main circuit, and the newly added three-level bridge arm constitutes one (or more) auxiliary single-phase, three-phase, or hybrid three-level circuit. The auxiliary three-level circuit is set to a high carrier ratio, and the space vector modulation method or the zero-sequence injection carrier modulation method can be used to easily realize the capacitor midpoint voltage balance control when the NPC three-level main circuit has a low carrier ratio or an ultra-low carrier ratio, which solves the key problems inherent in the three-level topology, greatly promotes the application of three-level technology in high-power electric propulsion and high-speed motor drive and other fields, and has high practical value.

[0024] Secondly, the present invention addresses the problem in the prior art that the midpoint voltage of the NPC three-level circuit is difficult to balance under low carrier ratio or ultra-low carrier ratio conditions. By adding an auxiliary three-level circuit and combining a space vector modulation method or a zero-sequence injection carrier modulation method, the circuit instability problem caused by midpoint voltage drift is solved, thereby significantly improving the safety and reliability of the NPC three-level circuit.

[0025] Compared with the traditional technical solutions that rely on complex control algorithms or high-power devices, the present invention not only reduces the dependence on the main circuit hardware, but also reduces the design cost of the system by introducing an auxiliary three-level circuit with a smaller rated current parameter. At the same time, the auxiliary circuit with a high carrier ratio has a higher response speed when adjusting the midpoint voltage, thereby meeting the demand for fast voltage balance in actual industrial scenarios.

[0026] The auxiliary single-phase, three-phase and hybrid three-level circuit structures proposed in the present invention are highly flexible and can select appropriate structural forms according to different application scenarios to adapt to a variety of complex load conditions. In addition, by connecting multiple groups of auxiliary circuits in parallel, the balance accuracy of the midpoint voltage is further improved, providing a reliable solution for large-scale power electronic systems.

[0027] The technical solution of the present invention has also demonstrated significant technological progress in industrial applications, including effectively reducing power consumption during midpoint voltage regulation, improving circuit operating efficiency, and enhancing the system's ability to adapt to low carrier ratio and ultra-low carrier ratio operating environments. These technological advances not only meet the high-performance requirements in actual production, but also provide technical support for the promotion and application of NPC three-level circuits in energy, industrial control and other fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 An NPC three-level main circuit and a method and circuit diagram for assisting a single-phase three-level circuit in balancing a midpoint voltage under a low carrier ratio or ultra-low carrier ratio provided by an embodiment of the present invention;

[0029] Figure 2A method and circuit diagram for balancing the midpoint voltage of an NPC three-level main circuit and two sets of parallel auxiliary single-phase three-level circuits under a low carrier ratio or ultra-low carrier ratio provided by an embodiment of the present invention;

[0030] Figure 3 An NPC three-level main circuit and a method and circuit diagram for assisting a three-phase three-level circuit to balance the midpoint voltage under a low carrier ratio or ultra-low carrier ratio provided by an embodiment of the present invention;

[0031] Figure 4 A method and circuit diagram for balancing the midpoint voltage of an NPC three-level main circuit and two sets of parallel auxiliary three-phase three-level circuits under a low carrier ratio or ultra-low carrier ratio provided by an embodiment of the present invention;

[0032] Figure 5 An NPC three-level main circuit and a method and circuit diagram for assisting a single-phase and three-phase hybrid three-level circuit to balance the midpoint voltage under a low carrier ratio or ultra-low carrier ratio provided by an embodiment of the present invention;

[0033] Figure 6 This is the effect waveform of assisting a single-phase three-level circuit to balance the midpoint voltage at an ultra-low carrier ratio provided by an embodiment of the present invention;

[0034] Figure 7 This is the effect waveform of the balanced midpoint voltage of the auxiliary three-phase three-level circuit at the ultra-low carrier ratio provided by the embodiment of the present invention;

[0035] Figure 8 This is the effect waveform of assisting a single-phase and three-phase hybrid three-level circuit to balance the midpoint voltage at an ultra-low carrier ratio provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0037] The present invention proposes a NPC three-level midpoint voltage balance control method under low carrier ratio or ultra-low carrier ratio conditions. A three-level bridge arm of the same type is added to the NPC three-level main circuit to form an auxiliary single-phase, three-phase, or hybrid three-level circuit. The auxiliary three-level circuit selects power devices with small rated current parameters and is set to a high carrier ratio to be responsible for the midpoint voltage balance of the main power circuit. The specific implementation is as follows:

[0038] The same type of three-level bridge arm is added to the NPC three-level main circuit to form an auxiliary single-phase three-level circuit, as shown in the attached Figure 1As shown, the auxiliary single-phase three-level circuit selects power devices with small rated current parameters and sets them to high carrier ratio. When the NPC three-level main circuit operates at low carrier ratio or ultra-low carrier ratio, the auxiliary three-level circuit uses single-phase space vector modulation method or zero-sequence injection carrier modulation method to balance the midpoint voltage.

[0039] The same type of three-level bridge arms are added to the NPC three-level main circuit to form two sets of parallel auxiliary single-phase three-level circuits, as shown in the attached figure. Figure 2 As shown, the two sets of parallel auxiliary single-phase three-level circuits select power devices with small rated current parameters and are all set to high carrier ratio. When the NPC three-level main circuit operates at low carrier ratio or ultra-low carrier ratio, the two sets of parallel auxiliary three-level circuits use single-phase space vector modulation method or zero-sequence injection carrier modulation method to balance the midpoint voltage.

[0040] The same type of three-level bridge arm is added to the NPC three-level main circuit to form an auxiliary three-phase three-level circuit, as shown in the attached figure. Figure 3 As shown, the auxiliary three-phase three-level circuit selects power devices with small rated current parameters and sets them to high carrier ratio. When the NPC three-level main circuit operates at low carrier ratio or ultra-low carrier ratio, the auxiliary three-level circuit balances the midpoint voltage using a three-phase space vector modulation method or a zero-sequence injection carrier modulation method.

[0041] The same type of three-level bridge arms are added to the NPC three-level main circuit to form two sets of parallel auxiliary three-phase three-level circuits, as shown in the attached figure. Figure 4 As shown, the two sets of parallel auxiliary three-phase three-level circuits select power devices with small rated current parameters, and are all set to high carrier ratio. When the NPC three-level main circuit operates at low carrier ratio or ultra-low carrier ratio, the two sets of parallel auxiliary three-level circuits use the three-phase space vector modulation method or the zero-sequence injection carrier modulation method to balance the midpoint voltage.

[0042] The same type of three-level bridge arm is added to the NPC three-level main circuit to form an auxiliary single-phase and three-phase mixed three-level circuit, as shown in the attached Figure 5 As shown, the auxiliary single-phase and three-phase hybrid three-level circuit selects power devices with small rated current parameters, all of which are set to high carrier ratio. When the NPC three-level main circuit operates at low carrier ratio or ultra-low carrier ratio, the auxiliary single-phase and three-phase hybrid three-level circuit uses space vector modulation method or zero-sequence injection carrier modulation method to balance the midpoint voltage.

[0043] Example 1

[0044] Embodiment 1 of the present invention provides a method and circuit diagram for assisting single-phase three-level balanced midpoint voltage under ultra-low carrier ratio conditions, corresponding to the attached Figure 1Among them, the ultra-low carrier ratio of the NPC three-level main circuit is 4, and the high carrier ratio of the auxiliary single-phase three-level circuit is 100. During operation, the auxiliary single-phase three-level circuit uses a single-phase space vector modulation method to balance the capacitor midpoint voltage, and the NPC three-level main circuit is only responsible for driving the working motor. Figure 6 The typical step-shaped three-level line voltage waveform U is given. ab , and the upper and lower half capacitor voltage waveforms U dc1 with U dc2 Obviously, the auxiliary single-phase three-level circuit added at ultra-low carrier ratio can balance the midpoint voltage well.

[0045] Example 2

[0046] Embodiment 2 of the present invention provides a method and circuit diagram for assisting three-phase three-level balanced midpoint voltage under ultra-low carrier ratio conditions, corresponding to the attached Figure 3 ; Among them, the ultra-low carrier ratio of the NPC three-level main circuit is 4, and the high carrier ratio of the auxiliary three-phase three-level circuit is 142.8. During operation, the auxiliary three-phase three-level circuit uses a three-phase space vector modulation method to balance the capacitor midpoint voltage, and the NPC three-level main circuit is only responsible for driving the working motor. Figure 7 The typical step-shaped three-level line voltage waveform U is given. ab , and the upper and lower half capacitor voltage waveforms U dc1 with U dc2 Obviously, the auxiliary three-phase three-level circuit added at the ultra-low carrier ratio can balance the midpoint voltage very well.

[0047] Example 3

[0048] Embodiment 3 of the present invention provides a method and circuit diagram for assisting a single-phase and three-phase hybrid three-level balanced midpoint voltage under ultra-low carrier ratio conditions, corresponding to the attached Figure 5 Among them, the ultra-low carrier ratio of the NPC three-level main circuit is 4, and the high carrier ratios of the auxiliary single-phase and three-phase three-level circuits are 385 and 142.8 respectively. During operation, the auxiliary single-phase and three-phase three-level circuits use the space vector modulation method to coordinately balance the capacitor midpoint voltage, and the NPC three-level main circuit is only responsible for driving the working motor. Figure 8 The typical step-shaped three-level line voltage waveform U is given. ab , and the upper and lower half capacitor voltage waveforms U dc1 with U dc2 Obviously, the auxiliary single-phase and three-phase hybrid three-level circuit added at ultra-low carrier ratio can balance the medium voltage well.

[0049] The present invention discloses a method and device for balancing the midpoint voltage of an NPC three-level converter under low carrier ratio or ultra-low carrier ratio conditions. An auxiliary three-level bridge arm is added to the NPC three-level main circuit, and the newly added three-level bridge arm constitutes one (or more) auxiliary single-phase, three-phase, or hybrid three-level circuits. The auxiliary three-level circuit is set to a high carrier ratio and is responsible for balancing the midpoint voltage of the capacitor, thereby solving the key problems inherent in the three-level topology, thereby ensuring the reliable, safe and stable operation of the three-level converter system. The present invention is not only applicable to three-phase and multi-phase NPC three-level circuits; at the same time, in the motor drive system, it can be applied to both sinusoidal current motors in 180° working mode and brushless DC motors in 120° working mode.

[0050] The present invention provides an NPC three-level midpoint voltage balancing method under low carrier ratio or ultra-low carrier ratio conditions, the core of which is to achieve dynamic balance of midpoint voltage by adding an auxiliary three-level circuit. The auxiliary circuit is composed of three-level bridge arms of the same type, and can adopt a single-phase, three-phase or single-phase and three-phase mixed structure. The auxiliary circuit selects power devices with smaller rated current parameters, and adjusts the midpoint voltage of the main circuit through a high carrier ratio working mode, thereby ensuring the stable operation of the entire system under low carrier ratio or ultra-low carrier ratio conditions.

[0051] In the NPC three-level main circuit, an auxiliary single-phase three-level circuit is added, power devices with smaller rated current are selected, and the circuit is set to high carrier ratio working mode. When the main circuit operates at low carrier ratio or ultra-low carrier ratio, the auxiliary circuit uses single-phase space vector modulation method or zero-sequence injection carrier modulation method to balance and adjust the midpoint voltage. The addition of the auxiliary single-phase circuit can effectively suppress the midpoint voltage drift and improve the working stability of the main circuit.

[0052] In order to enhance the neutral point voltage balancing effect, two sets of parallel auxiliary single-phase three-level circuits can be introduced into the NPC three-level main circuit. Through the coordinated control of these two sets of circuits, the single-phase space vector modulation method or the zero-sequence injection carrier modulation method is adopted to jointly balance the neutral point voltage. Compared with a single auxiliary circuit, the parallel circuit can provide higher redundancy and control flexibility, thereby significantly improving the stability and voltage balancing ability of the system.

[0053] In the NPC three-level main circuit, the balance of the midpoint voltage can also be achieved by adding an auxiliary three-phase three-level circuit. The auxiliary circuit also uses power devices with smaller rated current parameters and is set to a high carrier ratio. When the main circuit operates at a low carrier ratio or an ultra-low carrier ratio, the auxiliary three-phase circuit adjusts the midpoint voltage in real time through a three-phase space vector modulation method or a zero-sequence injection carrier modulation method. Compared with the auxiliary single-phase circuit, the auxiliary three-phase circuit can cover a wider modulation range and is suitable for more complex load conditions.

[0054] In the NPC three-level main circuit, two sets of parallel auxiliary three-phase three-level circuits can also be added. Both sets of auxiliary circuits work together with a high carrier ratio, using a three-phase space vector modulation method or a zero-sequence injection carrier modulation method to collaboratively complete the task of balancing and regulating the midpoint voltage. The parallel design further improves the redundancy and dynamic response capability of the circuit, and can ensure the stability and efficient operation of the system under more complex operating conditions.

[0055] In order to take into account the advantages of single-phase and three-phase circuits, an auxiliary single-phase and three-phase hybrid three-level circuit can also be designed. In this structure, both the auxiliary single-phase and three-phase circuits use a high carrier ratio, and use the space vector modulation method or the zero-sequence injection carrier modulation method to coordinately adjust the midpoint voltage. This hybrid structure not only improves the adaptability of the circuit, but also can flexibly switch the regulation strategy under different load modes to ensure that the midpoint voltage of the NPC three-level main circuit is always balanced.

[0056] Example 4

[0057] In a certain industrial control scenario, the NPC three-level main circuit operates under low carrier ratio conditions, with an output frequency of 50Hz and a carrier ratio of 15. To solve the problem of midpoint voltage imbalance, an auxiliary single-phase three-level circuit is added. The auxiliary circuit selects power devices with a rated current of 0.1 to 1 times the rated current of the main circuit, and sets the carrier ratio to 200. Through the single-phase space vector modulation method, the auxiliary circuit adjusts the midpoint voltage of the main circuit in real time, ensuring the symmetry and stability of the output voltage of the main circuit.

[0058] Example 5

[0059] In a certain flywheel energy storage system, the NPC three-level main circuit operates under ultra-low carrier ratio conditions, with an output frequency of 300Hz and a carrier ratio of only 5. In order to solve the problem of long-term drift of the midpoint voltage, two sets of parallel auxiliary three-phase three-level circuits are added. The auxiliary circuits use power devices with a rated current of 0.1 to 0.5 times the rated current of the main circuit, and the carrier ratios are set to 120. The two sets of auxiliary circuits work together through the three-phase space vector modulation method to adjust the midpoint voltage of the main circuit with high precision, ensuring the dynamic balance of the midpoint voltage under complex load fluctuation conditions.

[0060] Through the above two specific embodiments, the technical solution of the present invention demonstrates the effectiveness of midpoint voltage balance in different application scenarios. Whether it is the rapid response of the auxiliary single-phase circuit or the high-precision regulation of the parallel auxiliary three-phase circuit, it reflects the applicability and flexibility of the technology of the present invention, and provides a reliable solution for solving technical problems under low carrier ratio or ultra-low carrier ratio operating conditions.

[0061] The embodiments of the present invention also show that the auxiliary circuit uses power devices with a small rated current and operates at a high carrier ratio, which not only reduces the system cost, but also significantly improves the rapidity and stability of regulation. These advantages make the technical solution of the present invention have a wide range of application potentials in industrial control, new energy generation and power electronic systems.

[0062] In the description of the present invention, unless otherwise specified, "plurality" means two or more than two; the orientations or positional relationships indicated by the terms "upper", "lower", "left", "right", "inner", "outer", "front end", "rear end", "head", "tail", etc. are based on the orientations or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", "third", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0063] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principle of the present invention should be covered by the protection scope of the present invention.

Claims

1. A NPC three-level midpoint voltage balancing method under low carrier ratio or ultra-low carrier ratio conditions, characterized in that: The following steps are involved: A three-level bridge arm of the same type is added to the NPC three-level main circuit to form an auxiliary circuit, wherein the auxiliary circuit includes a single-phase, three-phase or single-phase and three-phase mixed three-level circuit; The auxiliary circuit selects a power device with a smaller rated current parameter and sets it to a high carrier ratio working mode; When the main circuit is running, the midpoint voltage is adjusted by the auxiliary circuit using a space vector modulation method or a zero-sequence injection carrier modulation method.

2. The method according to claim 1, characterized in that The auxiliary circuit is a single-phase three-level circuit, and adopts a single-phase space vector modulation method or a zero-sequence injection carrier modulation method to adjust the midpoint voltage.

3. The method according to claim 1, characterized in that: The auxiliary circuit is a three-phase three-level circuit, which uses a three-phase space vector modulation method or a zero-sequence injection carrier modulation method to adjust the midpoint voltage.

4. The method according to claim 1, characterized in that: The auxiliary circuit is two sets of parallel single-phase three-level circuits, which use a single-phase space vector modulation method or a zero-sequence injection carrier modulation method to jointly adjust the midpoint voltage.

5. The method according to claim 1, characterized in that The auxiliary circuit is two sets of three-phase three-level circuits connected in parallel, and the midpoint voltage is jointly adjusted by using a three-phase space vector modulation method or a zero-sequence injection carrier modulation method.

6. The method according to claim 1, characterized in that The auxiliary circuit is a single-phase and three-phase hybrid three-level circuit, and adopts a space vector modulation method or a zero-sequence injection carrier modulation method to adjust the midpoint voltage.

7. An NPC three-level midpoint voltage balancing device under low carrier ratio or ultra-low carrier ratio conditions, characterized in that: include: The main circuit module is an NPC three-level main circuit; The auxiliary circuit module is arranged in the main circuit module, and includes a single-phase, three-phase or single-phase and three-phase mixed three-level circuit composed of three-level bridge arms of the same type. The auxiliary circuit selects power devices with smaller rated current parameters and is set to a high carrier ratio working mode; The modulation control module is used to control the auxiliary circuit module to adjust the midpoint voltage of the main circuit module by using the space vector modulation method or the zero-sequence injection carrier modulation method.

8. The device according to claim 7, characterized in that The auxiliary circuit module is a single-phase three-level circuit, and the modulation control module adjusts the midpoint voltage of the main circuit module by a single-phase space vector modulation method or a zero-sequence injection carrier modulation method.

9. The device according to claim 7, characterized in that The auxiliary circuit module is a three-phase three-level circuit, and the modulation control module adjusts the midpoint voltage of the main circuit module by a three-phase space vector modulation method or a zero-sequence injection carrier modulation method.

10. The device according to claim 7, characterized in that The auxiliary circuit module is composed of two sets of single-phase or three-phase three-level circuits connected in parallel, and the modulation control module jointly adjusts the midpoint voltage of the main circuit module through a space vector modulation method or a zero-sequence injection carrier modulation method; The auxiliary circuit module is composed of a single-phase and three-phase hybrid three-level circuit, and the modulation control module jointly adjusts the midpoint voltage of the main circuit module through a space vector modulation method or a zero-sequence injection carrier modulation method.

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