Carrier modulation method and system for realizing accurate control of midpoint potential of three-level inverter

Abstract

The invention belongs to the field of power electronic converter control and design, and discloses a carrier modulation method and system for achieving accurate control over the neutral-point potential of a three-level inverter. The neutral-point voltage is kept in a relatively balanced state by detecting and correcting the voltage difference of an upper capacitor and a lower capacitor on the direct current side in time; and the distortion of the output voltage at the alternating current side caused by large fluctuation of the neutral-point potential is reduced or avoided. A midpoint potential correction link is introduced while the advantages of a conventional SVM or carrier modulation are not damaged; and when the neutral-point potential deviates, the zero-sequence voltage is automatically injected or a proper switching vector sequence is selected to compensate, suppress harmonic waves and reduce the distortion degree, so that the waveform quality of the output voltage is improved.

Description

Technical Field

[0001] The present invention belongs to the field of power electronic converter control and design, and particularly relates to a carrier modulation method and system for realizing precise control of the neutral point potential of a three-level inverter. Background Art

[0002] The use of multilevel inverters makes it possible to consume renewable energy on a large scale. At the same time, it also provides the possibility to increase the power level, providing strong support for promoting the green and low-carbon transformation of energy. Compared with traditional two-level inverters, three-level inverters have been widely used in medium and high-voltage scenarios due to their advantages such as lower output harmonic content and higher system efficiency. However, due to the interference of external factors such as the topological structure and system parasitic parameters, the neutral point potential of the three-level inverter will fluctuate or even become unbalanced, resulting in a decline in the quality of the AC-side output voltage waveform, and in severe cases, even causing the inverter to stop working.

[0003] The method of adjusting the action time of positive and negative small vectors to offset the influence of medium vectors and balance the neutral point potential is the most widely used method. However, this method is only applicable to the working conditions with a small modulation degree and a large power factor of the system, and it is difficult to completely eliminate the neutral point potential deviation under other working conditions. Moreover, in the process of adjusting the action time of small vectors, it often brings new challenges to other control objectives such as system efficiency improvement and leakage current suppression. Therefore, precise control of the neutral point potential under all working conditions cannot be achieved only by adjusting the action time of small vectors, and the problem of coupling between improving the neutral point potential control ability and other control objectives is also a difficult problem faced in practical engineering applications. Summary of the Invention

[0004] The purpose of the present invention is to overcome the above deficiencies such as the decline in the quality of the AC-side output voltage waveform and even the inability of the inverter to work, and provide a carrier modulation method and system for realizing precise control of the neutral point potential of a three-level inverter.

[0005] To achieve the above purpose, the present invention adopts the following technical solutions: In the first aspect, the present invention provides a carrier modulation method for realizing precise control of the neutral point potential of a three-level inverter, including the following steps: Obtain the voltage difference between the upper and lower capacitors on the DC side of the three-level inverter, preprocess the capacitor voltage difference to obtain the modulation signal output by the neutral point potential control link, compare the modulation signal output by the neutral point potential control link with the sawtooth wave, and judge whether to add neutral point potential control according to the comparison result; When adding neutral point potential control, obtain the three-phase reference voltages of the three-level inverter, calculate the large sectors and small sectors according to the three-phase reference voltages; sort the three-phase reference voltages and obtain the corresponding three-phase inductor currents; According to the voltage difference between the upper and lower capacitors on the DC side, the small sector, and the three-phase inductor current, select a switching vector sequence that is beneficial to the restoration of the balance of the neutral point potential, and determine the corresponding carrier modulation mode according to the switching vector sequence; Based on the determined carrier modulation mode, inject a zero-sequence voltage component on the basis of the three-phase reference voltage, and inject a second zero-sequence voltage component into the middle phase of the reference voltage to obtain the upper modulation wave and the lower modulation wave corresponding to the three-phase reference voltage; Compare the upper modulation wave and the lower modulation wave corresponding to the three-phase reference voltage with the corresponding upper carrier wave and lower carrier wave in the carrier modulation mode to obtain the switching pulse signal, and distribute the switching pulse signal to the three-phase bridge arms in combination with the large sector to obtain the drive signals of each switching tube.

[0006] A further improvement of the present invention lies in the specific method of preprocessing the voltage difference between the capacitors to obtain the modulation signal output by the neutral point potential control link as follows: Obtain the voltage difference between the upper and lower capacitors on the DC side in the three-level inverter; Obtain the absolute value of the voltage difference between the upper and lower capacitors on the DC side; Perform a proportional link processing on the obtained absolute value; Perform a clipping link processing on the value after the proportional link processing to obtain the modulation signal output by the neutral point potential control link.

[0007] A further improvement of the present invention lies in comparing the modulation signal output by the neutral point potential control link with a sawtooth wave, and according to the comparison result, judging whether to add the neutral point potential control. The specific method is as follows: Compare the modulation signal output by the neutral point potential control link with a sawtooth wave; If the modulation signal output by the neutral point potential control link is greater than the sawtooth wave, add the neutral point potential control; Otherwise, do not add the neutral point potential control.

[0008] A further improvement of the present invention lies in that when the neutral point potential control is added, obtain the three-phase reference voltage of the three-level inverter, and calculate the large sector and the small sector according to the three-phase reference voltage. The specific method is as follows: Obtain the three-phase reference voltage, and obtain the component of the reference voltage in the α coordinate system and the component of the reference voltage in the β coordinate system by performing an equal-amplitude coordinate transformation on the three-phase reference voltage; According to the component of the reference voltage in the α coordinate system and the component of the reference voltage in the β coordinate system, calculate the amplitude and angle of the reference voltage vector; Calculate the large sector and the vector equivalent angle through the reference voltage vector angle; Calculate the equivalent reference voltage component in the α coordinate system and the equivalent reference voltage component in the β coordinate system through the amplitude of the reference voltage vector and the vector equivalent angle of the reference voltage vector angle; Divide the small sectors of the three-phase reference voltage according to the equivalent reference voltage components in the α coordinate system and the equivalent reference voltage components in the β coordinate system.

[0009] A further improvement of the present invention lies in sorting the three-phase reference voltage and obtaining the corresponding three-phase inductor currents. The specific method is as follows: Sort the three-phase reference voltage, which are the maximum reference voltage, the intermediate reference voltage, and the minimum reference voltage in descending order; The three-phase inductor currents corresponding to the three-phase reference voltage are the maximum inductor current, the intermediate inductor current, and the minimum inductor current.

[0010] A further improvement of the present invention lies in that the carrier modulation mode includes in-phase stacked carrier modulation and reverse stacked carrier modulation.

[0011] A further improvement of the present invention lies in injecting a zero-sequence voltage component based on the determined carrier modulation mode on the basis of the three-phase reference voltage, and injecting a secondary zero-sequence voltage component into the intermediate phase of the reference voltage to obtain the upper modulation wave and the lower modulation wave corresponding to the three-phase reference voltage. The specific method is as follows: Inject the zero-sequence voltage into the three-phase reference voltage to obtain the upper modulation wave and the lower modulation wave corresponding to the maximum-phase reference voltage and the minimum-phase reference voltage; Split the intermediate phase of the reference voltage injected with the zero-sequence voltage component into two components, and inject the secondary zero-sequence voltage component into each of them to obtain the upper modulation wave and the lower modulation wave corresponding to the intermediate phase of the reference voltage.

[0012] A further improvement of the present invention lies in that the three-phase bridge arm includes drive signals of 12 switching tubes.

[0013] In a second aspect, the present invention provides a carrier modulation system for realizing precise control of the neutral point potential of a three-level inverter, including: A neutral point potential control module, configured to obtain the voltage difference between the upper and lower capacitors on the DC side of the three-level inverter, preprocess the voltage difference between the capacitors to obtain the modulation signal output by the neutral point potential control link, compare the modulation signal output by the neutral point potential control link with a sawtooth wave, and determine whether to add neutral point potential control according to the comparison result; A sector current acquisition module, configured to obtain the three-phase reference voltage of the three-level inverter when neutral point potential control is added, calculate the large sector and the small sector according to the three-phase reference voltage; sort the three-phase reference voltage and obtain the corresponding three-phase inductor currents; A carrier modulation mode switching module, configured to select a switching vector sequence that is beneficial to the restoration of the balance of the neutral point potential according to the voltage difference between the upper and lower capacitors on the DC side, the small sector, and the three-phase inductor currents, and determine the corresponding carrier modulation mode according to the switching vector sequence; A modulation wave acquisition module, which is used to inject a zero-sequence voltage component and a second-order zero-sequence voltage component based on the carrier modulation mode on the basis of the three-phase reference voltage to obtain an upper modulation wave and a lower modulation wave corresponding to the three-phase reference voltage; A drive signal acquisition module, which is used to compare the upper modulation wave and the lower modulation wave corresponding to the three-phase reference voltage with the corresponding upper carrier wave and lower carrier wave in the carrier modulation mode to obtain a switching pulse signal, and combine large sectors to distribute the switching pulse signal to the three-phase bridge arms to obtain drive signals for each switching tube.

[0014] Compared with the prior art, the present invention has the following beneficial effects: By timely detecting and correcting the voltage difference between the upper and lower capacitors on the DC side, the present invention keeps the midpoint voltage in a relatively balanced state, reducing or avoiding the distortion of the AC-side output voltage caused by large fluctuations in the midpoint potential. While not compromising the advantages of conventional SVM or carrier modulation, a midpoint potential correction link is introduced; when the midpoint potential deviates, a zero-sequence voltage is automatically injected or a suitable switching vector sequence is selected for compensation to suppress harmonics and reduce the distortion degree, thereby improving the quality of the output voltage waveform. After ensuring the midpoint balance, the three-level inverter of the present invention can better utilize the advantages of a higher voltage level, lower device stress, and a richer vector synthesis scheme, so that it can still maintain high efficiency and high power factor output under wide-range speed regulation or different load conditions. The present invention realizes the rapid correction of the midpoint potential through the comprehensive consideration and real-time judgment of multiple factors such as the voltage difference between the upper and lower capacitors, the reference voltage, and the carrier modulation mode. This method can adapt to various actual working conditions such as load mutation and bus voltage fluctuation, enhancing the robustness and stability of the system. Based on the common three-level carrier modulation architecture, the present invention only needs to add a midpoint potential adjustment logic to the control algorithm without excessive modification of the hardware or system framework, which is convenient to be directly implemented on the existing control platform and has high practical value. In summary, through precise control of the midpoint potential and targeted carrier modulation strategies, the present invention realizes the balanced control of the midpoint potential while ensuring or improving the output voltage quality of the three-level inverter, avoiding output waveform distortion and potential device overstress caused by midpoint drift, and contributing to improving the safety, reliability, and power quality of the system. Description of the Drawings

[0015] Figure 1 is the topological structure of a T-NPC three-level inverter; Figure 2 is the switching vector distribution diagram of the three-level inverter; Figure 3 is the block diagram of actively indirectly adding midpoint potential control; Figure 4 is the schematic diagram of midpoint potential control; Figure 5 is the flowchart of small sector division; Figure 6 It is a diagram for small sector division; Figure 7(a) is a diagram corresponding to the carrier type of PDPWM; Figure 7(b) is a diagram corresponding to the carrier type of PODPWM; Figure 8 When the neutral point potential control is added u z Selection flowchart; Figure 9 When the neutral point potential control is added u z2 Selection flowchart; Figure 10 It is an overall modulation flowchart; Figure 11 It is a schematic diagram of a three-level inverter simulation platform; Figure 12(a) is m =0.4, φ Neutral point potential control waveform diagram when =0°; Figure 12(b) is m =0.7, φ Neutral point potential control waveform diagram when =60°; Figure 12(c) is m =1.0, φ Neutral point potential control waveform diagram when = -90°; Specific implementation manner

[0016] To further understand the content of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the embodiments are only for explaining the present invention and not for limiting it.

[0017] Embodiment 1: A carrier modulation method for achieving precise control of the neutral point potential of a three-level inverter includes the following steps: S1. Obtain the voltage difference between the upper and lower capacitors on the DC side of the three-level inverter, preprocess the voltage difference between the capacitors to obtain the modulation signal output by the neutral point potential control link, compare the modulation signal output by the neutral point potential control link with the sawtooth wave, and judge whether to add the neutral point potential control according to the comparison result.

[0018] S2. When adding the neutral point potential control, obtain the three-phase reference voltages of the three-level inverter, calculate the large sectors and small sectors according to the three-phase reference voltages; sort the three-phase reference voltages and obtain the corresponding three-phase inductor currents.

[0019] S3. According to the voltage difference between the upper and lower capacitors on the DC side, the small sectors and the three-phase inductor currents, select a large sector switch vector sequence that is beneficial to the restoration of the balance of the neutral point potential, and determine the corresponding carrier modulation mode according to the large sector switch vector sequence.

[0020] S4. Based on the determined carrier modulation mode, inject a zero-sequence voltage component on the basis of the three-phase reference voltage, and inject a second zero-sequence voltage component into the middle phase of the reference voltage to obtain the upper modulation wave and the lower modulation wave corresponding to the three-phase reference voltage.

[0021] S5. Compare the upper modulation wave and the lower modulation wave corresponding to the three-phase reference voltage with the corresponding upper carrier wave and lower carrier wave in the carrier modulation mode to obtain the switching pulse signal, and distribute the switching pulse signal to the three-phase bridge arms in combination with the large sector to obtain the driving signals of each switching tube.

[0022] Embodiment 2: The neutral-point potential control module is used to obtain the voltage difference between the upper and lower capacitors on the DC side of the three-level inverter, preprocess the voltage difference between the capacitors to obtain the modulation signal output by the neutral-point potential control link, compare the modulation signal output by the neutral-point potential control link with the sawtooth wave, and judge whether to add the neutral-point potential control according to the comparison result; The sector current acquisition module is used to obtain the three-phase reference voltage of the three-level inverter when the neutral-point potential control is added, calculate the large sector and the small sector according to the three-phase reference voltage; sort the three-phase reference voltage and obtain the corresponding three-phase inductor currents; The carrier modulation mode switching module is used to select the switching vector sequence that is beneficial to the restoration of the balance of the neutral-point potential according to the voltage difference between the upper and lower capacitors on the DC side, the small sector and the three-phase inductor currents, and determine the corresponding carrier modulation mode according to the switching vector sequence; The modulation wave acquisition module is used to inject a zero-sequence voltage component and a second zero-sequence voltage component on the basis of the three-phase reference voltage according to the carrier modulation mode to obtain the upper modulation wave and the lower modulation wave corresponding to the three-phase reference voltage; The driving signal acquisition module is used to compare the upper modulation wave and the lower modulation wave corresponding to the three-phase reference voltage with the corresponding upper carrier wave and lower carrier wave in the carrier modulation mode to obtain the switching pulse signal, and distribute the switching pulse signal to the three-phase bridge arms in combination with the large sector to obtain the driving signals of each switching tube.

[0023] Embodiment 3: Taking the T-type neutral-point clamped (T-NPC) three-level inverter as an example, its topological structure is as Figure 1 shown, which consists of a DC power supply, DC-side capacitors, three-phase bridge arms and an AC-side LC filter. Each phase bridge arm has three switching states, and its switching function is shown in Equation (1), where S x represents the switching combination state of the corresponding phase, x=(a, b, c). There are 27 switching state combinations in three phases. After coordinate transformation of the three-phase output voltages corresponding to these switching state combinations, they correspond to 19 switching vectors in the αβ coordinate system. When the neutral point potential is balanced, the switching vector distribution of the three-level inverter is as Figure 2 shown.

[0024] (1) The optimization objective of the present invention is to decouple the neutral point potential control and other target controls by optimizing the modulation strategy, and at the same time achieve precise control of the neutral point potential under all working conditions.

[0025] Step 1: The DC side upper and lower capacitor voltage difference Δ u C After passing through the absolute value link, the proportional link and the limiting link, the modulation signal output by the neutral point potential control link is obtained u NPVL , and compared with the sawtooth wave u CNPV through a comparator to obtain the control quantity k B . Its control architecture is as Figure 3 shown, u C1 and u C2 are the DC side upper capacitor voltage and lower capacitor voltage respectively. When u NPVL ≤ u CNPV , k B =0, the neutral point potential control is not added, and other control objectives are preferentially guaranteed; when u NPVL > u CNPV , k B =1, at this time the neutral point potential control is added, and the neutral point potential balance is preferentially guaranteed.

[0026] Figure 4 The implementation schematic diagram of the neutral point potential control is given. The time when the neutral point potential control is added can be adjusted according to the degree of neutral point potential deviation. Such treatment not only decouples the neutral point potential control and other target inhibitions, but also does not need to passively set the neutral point potential boundary to switch the control method. The neutral point potential control can be automatically added according to the degree of neutral point potential deviation and the sawtooth wave period.

[0027] Step 2: Based on Step 1, when k B =1, at this time it is necessary to achieve precise control of the neutral point potential under all working conditions. For the three-phase reference voltage u refa, u refb and u refc further processed. Specifically, it includes: In the first step, the large sector L S and small sector S S are calculated through the three-phase reference voltage. The specific process is as follows: 1) The three-phase reference voltage is transformed through an equal-amplitude αβ coordinate transformation to obtain the component of the reference voltage in the α coordinate system u refα and the component of the reference voltage in the β coordinate system u refβ , as shown in Equation (2); 2) The magnitude u refα and angle u refβ of the reference voltage vector V ref are calculated through the component of the reference voltage in the α coordinate system A ref and the component of the reference voltage in the β coordinate system θ ref , as shown in Equation (3) and Equation (4) respectively; 3) The θ ref and the vector equivalent angle L S are calculated through the angle θ' ref of the reference voltage vector, as shown in Equation (5) and Equation (6) respectively, where [] is the rounding function; 4) The equivalent reference voltage component in the α coordinate system A ref and the equivalent reference voltage component in the β coordinate system θ' ref are calculated through the magnitude u' refα of the reference voltage vector and the vector equivalent angle u' refβ of the reference voltage vector angle, as shown in Equation (7); 5) The small sector u' refα is divided through the equivalent reference voltage component in the α coordinate system u' refβ and the equivalent reference voltage component in the β coordinate system S S . The small sector division and the division flow chart are shown in Figure 5 and Figure 6 respectively.

[0028] (2) (3) (4) (5) (6) (7) Step 2: Sort the magnitudes of the three-phase reference voltages, from largest to smallest as the maximum reference voltage u max , the intermediate reference voltage u mid and the minimum reference voltage u min . The corresponding three-phase inductor currents are the maximum inductor current i umax , the intermediate inductor current i umid and the minimum inductor current i umin .

[0029] Step 3: Based on Step 2, by determining the DC-side upper and lower capacitor voltage difference Δ u C , the small sector S S and the three-phase inductor currents, and avoiding excessive additional switching losses caused by switching vector switching, select a switching vector sequence that is beneficial to the restoration of the neutral point potential balance, and then clarify the carrier modulation mode through the switching vector sequence. The carrier modulation modes are divided into in-phase stacked carrier modulation (phase disposition pulse width modulation, PDPWM) and anti-phase stacked carrier modulation (phase opposition disposition pulse width modulation, PODPWM), as shown in Figures 7(a) and 7(b) respectively. Taking the reference voltage vector located in the first largest sector as an example, Table 1 gives the required switching vectors and corresponding carrier modulation modes under all working conditions.

[0030] Table 1 Carrier modulation mode and switching vector selection in the first sector under different working conditions when adding neutral point potential control

[0031] Step 4: As can be seen from Step 3, in order to achieve the corresponding switching vector sequence through carrier modulation, a zero-sequence voltage component u z needs to be injected on the basis of the original three-phase reference voltage.Figure 8 gives the zero-sequence voltage component u z The selection flowchart under different working conditions. However, in order to achieve precise control of the midpoint potential under all working conditions, it can be noted that there are multiple state transitions and working conditions of P and N state transitions in the middle value of the reference voltage u mid . Therefore, for the middle value of the reference voltage u mid , on the basis of injecting the original zero-sequence voltage u z , it needs to be split and the secondary zero-sequence voltage u z2 and - u z2 are respectively injected, as shown in Equation (8), and then the upper modulation wave u mid1 and the lower modulation wave u mid2 at the middle value are obtained. When generating the switching pulses, these two modulation waves are respectively compared with the upper and lower carrier waves to generate the driving signals for the switching tubes. Figure 9 gives the selection flowchart of the zero-sequence voltage u z2 under different working conditions.

[0032] (8) Step 5: First, inject the u z obtained from Step 4 into the three-phase reference voltage, and then inject the zero-sequence voltage component u z2 to obtain the upper modulation wave u mid1 and the lower modulation wave u mid2 at the middle value. Compare the maximum value of the reference voltage u max , the upper modulation wave u mid1 and the lower modulation wave u mid2 and the minimum value of the reference voltage u min with the upper and lower carrier waves determined in Step 3 to obtain the switching pulse signals. Finally, combine L S to distribute the switching pulse signals to the three-phase bridge arms to obtain the driving signals for each switching tube.

[0033] In summary, the overall modulation flowchart is as shown in Figure 10 . In the figure, u cb_up and u cb_downThe upper and lower carrier waves respectively S xn represent the drive signals of 12 switching tubes x = (a, b, c), n = (1, 2, 3, 4).

[0034] The present invention proposes a carrier modulation method for precisely controlling the neutral point potential of a three-level inverter under all operating conditions. This method fully considers the influence of the modulation degree change and the power factor change on the neutral point potential control, realizes the precise control of the neutral point potential under all operating conditions, and adopts an active indirect method of adding neutral point potential control to decouple the neutral point potential control and other target controls. Moreover, by using the carrier modulation method, the proposed method is realized only by adjusting the carrier type and the zero-sequence voltage component, which greatly simplifies the modulation process and is convenient for engineering applications.

[0035] To test the present invention, a simulation platform of a three-level inverter was built in the PSIM software, as Figure 11 shown. The given operating conditions and related parameters are shown in Table 2.

[0036] Table 2 Given operating conditions and related parameters

[0037] To better verify the effectiveness of the proposed method, switching switches NPVS1 and NPVS2 for connecting the DC side capacitors are set. Before 0.4 s, NPVS1 is closed and NPVS2 is open, and the neutral point potential is ensured to be stable through two DC sources; after 0.4 s, NPVS1 is open and NPVS2 is closed, and the connection is switched to the DC side capacitor. And the initial Δ u C of the DC side capacitor is set to 100 V, so as to better verify the effectiveness of the proposed method.

[0038] The simulation results are shown in Figure 12, where u ' refa 、 u ' refb and u ' refc are the three-phase modulation waves after injecting u z and u z2 respectively. The following conclusions are obtained from Figure 12:[[]] Under different system modulation degrees m and power factor angles φ , the modulation method proposed by the present invention can quickly and effectively restore the unbalanced neutral point potential to the balanced state; By actively and indirectly adding neutral point potential control, the contradiction between the neutral point potential control and other control targets can be effectively decoupled, that is, it can be atk B During the stage of =0, other control objectives are pursued for implementation; The overall architecture is based on carrier modulation, without the need for a large number of computational processes. Only u z and u z2 injecting three-phase modulation waves and comparing them with the carrier wave are required. Compared with space vector modulation, the modulation complexity and computational amount are reduced, which is convenient for engineering implementation.

[0039] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: modifications or equivalent replacements can still be made to the specific implementation manners of the present invention. Any modification or equivalent replacement without departing from the spirit and scope of the present invention shall be covered by the protection scope of the claims of the present invention.

Claims

1. A carrier modulation method for precisely controlling the neutral point potential of a three-level inverter, characterized in that, It includes the following steps: Obtain the voltage difference between the upper and lower capacitors on the DC side of the three-level inverter, preprocess the voltage difference between the capacitors to obtain the modulation signal output by the neutral point potential control link, compare the modulation signal output by the neutral point potential control link with the sawtooth wave, and judge whether to add neutral point potential control according to the comparison result; When adding neutral point potential control, obtain the three-phase reference voltages of the three-level inverter, calculate the large sector and small sector according to the three-phase reference voltages; sort the three-phase reference voltages and obtain the corresponding three-phase inductor currents; According to the voltage difference between the upper and lower capacitors on the DC side, the small sector and the three-phase inductor currents, select a switching vector sequence that is beneficial to the restoration of the balance of the neutral point potential, and determine the corresponding carrier modulation mode according to the switching vector sequence; Based on the determined carrier modulation mode, inject a zero-sequence voltage component on the basis of the three-phase reference voltages, and inject a second zero-sequence voltage component into the middle phase of the reference voltages to obtain the upper modulation wave and the lower modulation wave corresponding to the three-phase reference voltages; Compare the upper modulation wave and the lower modulation wave corresponding to the three-phase reference voltages with the upper carrier wave and the lower carrier wave in the carrier modulation mode to obtain the switching pulse signals, and distribute the switching pulse signals to the three-phase bridge arms in combination with the large sector to obtain the drive signals of each switching tube.

2. The carrier modulation method for precisely controlling the neutral point potential of a three-level inverter according to claim 1, wherein The specific method for preprocessing the voltage difference between the capacitors to obtain the modulation signal output by the neutral point potential control link is as follows: Obtain the voltage difference between the upper and lower capacitors on the DC side of the three-level inverter; Obtain the absolute value of the voltage difference between the upper and lower capacitors on the DC side; Perform proportional link processing on the obtained absolute value; Perform clipping link processing on the value after proportional link processing to obtain the modulation signal output by the neutral point potential control link.

3. The carrier modulation method for precisely controlling the neutral point potential of a three-level inverter according to claim 1, characterized in that, The specific method for comparing the modulation signal output by the neutral point potential control link with the sawtooth wave and judging whether to add neutral point potential control according to the comparison result is as follows: Compare the modulation signal output by the neutral point potential control link with the sawtooth wave; If the modulation signal output by the neutral point potential control link is greater than the sawtooth wave, add neutral point potential control; Otherwise, do not add neutral point potential control.

4. The carrier modulation method for precisely controlling the neutral point potential of a three-level inverter according to claim 1, characterized in that, When adding neutral point potential control, the specific method for obtaining the three-phase reference voltages of the three-level inverter and calculating the large sector and small sector according to the three-phase reference voltages is as follows: Obtain the three-phase reference voltages, and obtain the components of the reference voltages in the α coordinate system and the components of the reference voltages in the β coordinate system through equal-amplitude coordinate transformation of the three-phase reference voltages; Calculate the amplitude and angle of the reference voltage vector according to the components of the reference voltage in the α coordinate system and the components of the reference voltage in the β coordinate system; Calculate the large sector and the vector equivalent angle through the reference voltage vector angle; Calculate the equivalent reference voltage component in the α coordinate system and the equivalent reference voltage component in the β coordinate system through the amplitude of the reference voltage vector and the vector equivalent angle of the reference voltage vector angle; Divide the small sector of the three-phase reference voltages according to the equivalent reference voltage component in the α coordinate system and the equivalent reference voltage component in the β coordinate system.

5. The carrier modulation method for precisely controlling the neutral point potential of a three-level inverter according to claim 1, characterized in that The specific method for sorting the three-phase reference voltages and obtaining the corresponding three-phase inductor currents is as follows: Sort the three-phase reference voltages, from largest to smallest are the reference voltage maximum value, the reference voltage intermediate value and the reference voltage minimum value; The three-phase inductor currents corresponding to the three-phase reference voltages are the maximum inductor current, the intermediate inductor current, and the minimum inductor current.

6. The carrier modulation method for precisely controlling the neutral point potential of a three-level inverter according to claim 1, characterized in that, The carrier modulation modes include in-phase stacked carrier modulation and anti-phase stacked carrier modulation.

7. The carrier modulation method for precisely controlling the neutral point potential of a three-level inverter according to claim 1, characterized in that, Based on the determined carrier modulation mode, a zero-sequence voltage component is injected on the basis of the three-phase reference voltages, and a second-order zero-sequence voltage component is injected into the intermediate phase of the reference voltages. The specific method for obtaining the upper modulation wave and the lower modulation wave corresponding to the three-phase reference voltages is as follows: Inject the zero-sequence voltage component into the three-phase reference voltages to obtain the upper modulation wave and the lower modulation wave corresponding to the maximum-phase reference voltage and the minimum-phase reference voltage; Split the intermediate phase of the reference voltages injected with the zero-sequence voltage component into two components, and inject the second-order zero-sequence voltage component into each of them respectively to obtain the upper modulation wave and the lower modulation wave corresponding to the intermediate phase of the reference voltages.

8. The carrier modulation method for precisely controlling the neutral point potential of a three-level inverter according to claim 1, wherein The three-phase bridge arms include the drive signals of 12 switching tubes.

9. A carrier modulation system for precisely controlling the neutral point potential of a three-level inverter, characterized in that, It includes: A neutral-point potential control module, which is used to obtain the voltage difference between the upper and lower capacitors on the DC side in the three-level inverter, preprocess the voltage difference between the capacitors to obtain the modulation signal output by the neutral-point potential control link, compare the modulation signal output by the neutral-point potential control link with the sawtooth wave, and judge whether to add neutral-point potential control according to the comparison result; A sector current acquisition module, which is used to obtain the three-phase reference voltages of the three-level inverter when neutral-point potential control is added, calculate the large sector and the small sector according to the three-phase reference voltages; sort the three-phase reference voltages, and obtain the corresponding three-phase inductor currents; A carrier modulation mode switching module, which is used to select a switching vector sequence that is beneficial to the restoration of the balance of the neutral-point potential according to the voltage difference between the upper and lower capacitors on the DC side, the small sector, and the three-phase inductor currents, and determine the corresponding carrier modulation mode according to the switching vector sequence; A modulation wave acquisition module, which is used to inject a zero-sequence voltage component and a second-order zero-sequence voltage component on the basis of the three-phase reference voltages according to the carrier modulation mode to obtain the upper modulation wave and the lower modulation wave corresponding to the three-phase reference voltages; A drive signal acquisition module, which is used to compare the upper modulation wave and the lower modulation wave corresponding to the three-phase reference voltages with the corresponding upper carrier wave and lower carrier wave in the carrier modulation mode to obtain a switching pulse signal, and distribute the switching pulse signal to the three-phase bridge arms in combination with the large sector to obtain the drive signals of each switching tube.

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