Three-level inverter direct-current neutral-point balancing method and device in open-loop state

Abstract

The invention discloses a three-level inverter direct-current neutral-point balancing method and device in an open-loop state, and relates to the field of power electronic control. The method comprises the following steps: converting three power grid voltages of a three-level inverter in an open-loop state into power grid voltages ealpha and ebeta in an alpha coordinate system; generating a corresponding positive modulation wave mabc + and a corresponding negative modulation wave mabc-based on the power grid voltage e [alpha] and e [beta]; adjusting the positive modulation wave mabc + and the negative modulation wave mabc-through a PI controller to generate a new positive modulation wave m'abc + and a new negative modulation wave m'abc-, and comparing the new positive modulation wave m'abc + and the new negative modulation wave m'abc-with a carrier signal to generate a PWM signal; a PWM signal is used to drive a power switch of a three-level inverter so as to adjust the balance of the DC neutral-point voltage. According to the scheme, the voltage balance of the DC midpoint of the inverter is realized, the regulated modulation wave ensures the stability of the DC midpoint voltage under the open-loop control condition, and the stability and reliability of the system are improved.

Description

Technical Field

[0001] The present invention relates to the field of power electronic control, and in particular to a method and a device for balancing a DC midpoint of a three-level inverter in an open-loop state. Background Art

[0002] Three-level inverters are widely used in the field of power electronics because of their lower output harmonics and higher conversion efficiency. However, due to differences in the parameters of the internal components of the three-level inverter, such as the inconsistent capacitance of the upper and lower capacitors on the DC side, the differences in the switching characteristics of the power switching devices, and the errors introduced in the digital control process (such as the inherent dead time insertion mechanism of the CPU), the upper and lower capacitor voltages of the DC bus are prone to imbalance. This voltage imbalance will not only increase the harmonic content of the output current and reduce the conversion efficiency of the system, but may also cause overstress of the inverter devices and damage to the equipment. Therefore, in order to maintain the reliable operation of the three-level inverter, an appropriate DC midpoint balancing strategy must be adopted.

[0003] The DC midpoint balancing method of the three-level inverter in the prior art is usually based on the sector where the current vector is located and the current direction, and controls the current flowing into and out of the midpoint by adjusting the action time of the redundant small vector, so that the voltage of the positive and negative bus capacitors tends to be consistent. However, in the open-loop state, due to the small current amplitude, it only appears as a ripple, and there may be errors in the sampling process, which may cause the sampled current direction to be inconsistent with the actual value. Therefore, in this case, the existing DC midpoint balancing method is difficult to work effectively, and other more effective balancing strategies need to be adopted.

[0004] Therefore, in the open-loop operation state, a midpoint balancing method that does not depend on the current direction and vector sector information is urgently needed to effectively solve the voltage balance problem of the DC midpoint of the three-level inverter. Summary of the invention

[0005] The object of the present invention is to provide a method and device for balancing the DC midpoint of a three-level inverter in an open-loop state, which can achieve effective balance of the DC midpoint voltage by calculating the voltage deviation and dynamically adjusting the modulation wave, thereby effectively improving the stability of the system.

[0006] To achieve the above object, the present invention discloses the following technical solution:

[0007] A first aspect of the present invention provides a method for balancing a DC midpoint of a three-level inverter in an open-loop state, the method comprising the following steps:

[0008] Get the three grid voltages e of the three-level inverter in the open-loop state a 、e b and e c ;

[0009] Through abc / αβ coordinate transformation, the grid voltage e a 、e b and e c Transformed into the grid voltage e in the αβ coordinate system α and e β ;

[0010] Based on the grid voltage e in the αβ coordinate system α and e β , generating the corresponding positive modulation wave m abc+ and negative modulation wave m abc- ;

[0011] According to the positive bus voltage u p and negative bus voltage u N , the positive modulation wave m is controlled by a PI controller abc+ and the negative modulation wave m abc- Adjust to generate a new positive modulation wave m′ abc+ and the new negative modulation wave m′ abc- ;

[0012] The new positive modulation wave m abc+ and the new negative modulation wave m abc- Compare with the carrier signal to generate a PWM signal for driving the power switch of the three-level inverter;

[0013] The PWM signal is used to drive the power switch of the three-level inverter to adjust the balance of the DC midpoint voltage.

[0014] Optionally, in the above three-level inverter DC midpoint balancing method, the positive bus voltage u p and negative bus voltage u N , the positive modulation wave m is controlled by a PI controller abc+ and the negative modulation wave m abc- Make adjustments, including:

[0015] Get the positive bus voltage u p and negative bus voltage u N ;

[0016] Calculate the positive bus voltage u p With the negative bus voltage u N The voltage deviation between dif ;

[0017] The reference voltage u ref The voltage deviation u dif By comparison, the voltage deviation error u is obtained. E ;

[0018] The voltage deviation error u E Input to the PI controller to generate and output the control signal u PI ;

[0019] According to the control signal u PI Adjust the positive modulation wave m abc+ and the negative modulation wave m abc- The magnitude of , where:

[0020] When the control signal u PI ≥0, reduce the positive modulation wave m abc+ The amplitude of

[0021] When the control signal u PI <0, reduce the negative modulation wave m abc- The amplitude of .

[0022] Preferably, in the above three-level inverter DC midpoint balancing method, the control signal u PI The absolute value of the positive modulation wave m abc+ The amplitude and the negative modulation wave m abc- are inversely proportional to the amplitude.

[0023] Furthermore, in the above three-level inverter DC midpoint balancing method, when the control signal u PI ≥0, the new positive modulation wave m′ abc+ Equal to the original positive modulation wave m abc+ Divided by the control signal u PI , the new negative modulation wave m′ abc- Equal to the original negative modulation wave m abc- ,Right now:

[0024]

[0025] When the control signal u PI <0, the new positive modulation wave m′ abc+ Equal to the original positive modulation wave m abc+ , the new negative modulation wave m′ abc- Equal to the original negative modulation wave m abc- Divided by the control signal u PI The absolute value of is:

[0026]

[0027] Furthermore, in the above three-level inverter DC midpoint balancing method, the positive bus voltage u p With the negative bus voltage u N The voltage deviation between dif Calculated by the following formula:

[0028] u dif =u p -u N (3).

[0029] Furthermore, in the above three-level inverter DC midpoint balancing method, the reference voltage u ref Set to 0V, the voltage deviation error u E Calculated by the following formula:

[0030] u E =0-u dif (4).

[0031] A second aspect of the present invention provides a three-level inverter DC midpoint balancing device in an open-loop state, the device comprising:

[0032] Coordinate transformation module, used to obtain the three grid voltages e of the three-level inverter in the open-loop state a 、e b and e c , and through abc / αβ coordinate transformation, the grid voltage e a 、e b and e c Transformed into the grid voltage e in the αβ coordinate system α and e β ;

[0033] A modulation module is used for the grid voltage e based on the αβ coordinate system. α and e β , generating the corresponding positive modulation wave m abc+ and negative modulation wave m abc- ;

[0034] The voltage balancing module is used to p and negative bus voltage u N , the positive modulation wave m is controlled by a PI controller abc+ and the negative modulation wave m abc- Adjust to generate a new positive modulation wave m′ abc+ and the new negative modulation wave m′ abc- ;

[0035] PWM generation module, used to convert the new positive modulation wave m abc+ and the new negative modulation wave m abc- Compare with the carrier signal to generate a PWM signal for driving the power switch of the three-level inverter;

[0036] The driving module is used to drive the power switch of the three-level inverter using the PWM signal to adjust the balance of the DC midpoint voltage.

[0037] Optionally, in the above-mentioned three-level inverter DC midpoint balancing device, the voltage balancing module includes:

[0038] Voltage deviation calculation unit, used to obtain the positive bus voltage u p and negative bus voltage u N , and calculate the positive bus voltage u p With the negative bus voltage u N The voltage deviation between dif ;

[0039] Deviation error calculation unit, used to convert the reference voltage u ref The voltage deviation u dif By comparison, the voltage deviation error u is obtained. E ;

[0040] PI control unit, used to convert the voltage deviation error u E Input to the PI controller to generate and output the control signal u PI ;

[0041] The modulation wave modulation unit is used to modulate the PI Adjust the positive modulation wave m abc+ and the negative modulation wave m abc- The amplitude of the positive modulation wave m′ is generated abc+ and the new negative modulation wave m′ abc- .

[0042] Furthermore, in the above-mentioned three-level inverter DC midpoint balancing device, the modulation wave modulation unit comprises:

[0043] A comparison subunit, used to compare the control signal u PI Compare the value with zero and output the comparison result;

[0044] A modulation subunit, used for adjusting the modulation wave according to the output result of the comparison subunit;

[0045] in,

[0046] When the control signal u PI ≥0, reduce the positive modulation wave m abc+ The amplitude of

[0047] When the control signal u PI <0, reduce the negative modulation wave m abc- The amplitude of

[0048] The control signal u PI The absolute value of the positive modulation wave m abc+The amplitude and the negative modulation wave m abc- are inversely proportional to the amplitude.

[0049] The effects provided in the content of the invention are only the effects of the embodiments, not all the effects of the invention. One of the above technical solutions has the following advantages or beneficial effects:

[0050] The present application provides a method and device for balancing the DC midpoint of a three-level inverter in an open-loop state. By performing αβ coordinate transformation on the grid voltage, generating positive and negative modulation waves based on the transformed grid voltage, and then adjusting the modulation wave amplitude through a PI controller, the voltage balance of the DC midpoint of the inverter is achieved, and the adjusted modulation wave ensures the stability of the DC midpoint voltage under open-loop control conditions. At the same time, by accurately generating and adjusting the modulation wave, and generating a corresponding PWM signal to drive the power switch, the stability of the inverter system is significantly improved, the voltage fluctuation is reduced, and the overall performance and reliability of the system are improved. In addition, the present application does not require a complex feedback control loop, and can achieve the balance of the DC midpoint voltage only by dynamically adjusting the modulation wave, which simplifies the system structure and reduces the difficulty and cost of implementation. In addition, compared with the existing method of adjusting through redundant small vectors, this solution has a better effect in suppressing the midpoint imbalance under open-loop conditions and has a stronger control capability. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0052] Figure 1 It is a schematic diagram of the structure of a T-type inverter in the prior art;

[0053] Figure 2 This is a schematic flow chart of a method for balancing the DC midpoint of a three-level inverter in an open-loop state according to an embodiment of the present application;

[0054] Figure 3 for Figure 2 A flowchart of a method for implementing step S140;

[0055] Figure 4 A curve diagram showing the variation of the DC midpoint voltage difference of a three-level inverter over time according to an embodiment of the present application;

[0056] Figure 5 This is a schematic structural diagram of a DC midpoint balancing device for a three-level inverter in an open-loop state according to an embodiment of the present application;

[0057] Figure 6 for Figure 5 A structural diagram of a medium voltage equalizing module. DETAILED DESCRIPTION

[0058] 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 accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0059] It should be noted that references to "one embodiment", "embodiment", "example embodiment", etc. in this specification refer to the embodiment being described which may include specific features, structures or characteristics, but not every embodiment must include these specific features, structures or characteristics. In addition, such expressions do not refer to the same embodiment. Furthermore, when describing specific features, structures or characteristics in conjunction with an embodiment, whether or not there is an explicit description, it is indicated that incorporating such features, structures or characteristics into other embodiments is within the knowledge of those skilled in the art.

[0060] The DC midpoint balancing method of a three-level inverter in the prior art mainly controls the midpoint current according to the current direction and the sector where the vector is located by adjusting the action time of the redundant small vector, so as to achieve the balance of the positive and negative bus voltages. However, under open-loop control, since the current amplitude is small and appears in the form of ripples, and the sampling error may lead to inaccurate judgment of the current direction, the conventional midpoint balancing method is difficult to play an effective role. Therefore, the DC midpoint balancing problem under open-loop state needs to be solved by a more effective strategy. Based on this, the present application proposes a DC midpoint balancing method and device for a three-level inverter under open-loop state, which effectively solves the midpoint imbalance problem caused by small current amplitude and sampling error under open-loop state by calculating the voltage deviation and dynamically adjusting the amplitude of the modulation wave. This scheme can ensure the stability of the DC midpoint voltage under open-loop control conditions. Next, with Figure 1 Taking the T-type inverter shown as an example, the solution of the present application is described in detail.

[0061] refer to Figure 2 , Figure 2 A schematic flow chart of a method for balancing the DC midpoint of a three-level inverter in an open-loop state according to an embodiment of the present application is shown. The method comprises the following steps:

[0062] S110, obtaining three grid voltages e of the three-level inverter in the open-loop state a 、e b and e c ;

[0063] S120, through abc / αβ coordinate transformation, the grid voltage e a 、e b and e c Transformed into the grid voltage e in the αβ coordinate system α and eβ ;

[0064] S130, grid voltage e based on αβ coordinate system α and e β , generating the corresponding positive modulation wave m abc+ and negative modulation wave m abc- ;

[0065] S140, according to the positive bus voltage u p and negative bus voltage u N , the modulation wave m is adjusted by the PI controller abc+ and negative modulation wave m abc- Adjust to generate a new positive modulation wave m′ abc+ and the new negative modulation wave m′ abc- ;

[0066] S150, the new positive modulation wave m abc+ and the new negative modulation wave m abc- Compare with the carrier signal to generate a PWM signal for driving the power switch of the three-level inverter;

[0067] S160: Use the PWM signal to drive the power switch of the three-level inverter to adjust the balance of the DC midpoint voltage.

[0068] The solution of this embodiment realizes the voltage balance of the DC midpoint of the inverter by performing αβ coordinate transformation on the grid voltage, generating positive and negative modulation waves based on the transformed grid voltage, and then adjusting the amplitude of the modulation wave through a PI controller. The adjusted modulation wave ensures the stability of the DC midpoint voltage under open-loop control conditions, so that the system can maintain a balanced state under open-loop control conditions, thereby improving the stability and reliability of the system.

[0069] Specifically, in the above step S110, in the open-loop state, the three-phase voltage signal e is collected from the grid input terminal of the three-level inverter. a 、e b and e c , these signals usually come from grid voltage sensors or measuring devices. The grid voltage signal e a 、e b and e c It can accurately reflect the actual grid voltage in the open-loop state and provide a reliable data basis for the subsequent midpoint voltage balancing method.

[0070] In some embodiments, in the above step S120, the grid voltage e is converted into a 、e b and e c Transformed into the grid voltage e in the αβ coordinate system α and eβ The process is realized by the following formula:

[0071]

[0072] In some embodiments, in the above step S130, based on the grid voltage e in the αβ coordinate system α and e β , generating the corresponding positive modulation wave m abc+ and negative modulation wave m abc- , which can be achieved through space vector modulation (SVM). Specifically, after the grid voltage is converted to the αβ coordinate system, the voltage vector generated by the SVM technology can be used to generate the required positive modulation wave m abc+ and negative modulation wave m abc- , used to control the inverter power switch.

[0073] refer to Figure 3 In some embodiments, in step S140, according to the positive bus voltage u p and negative bus voltage u N , the modulation wave m is adjusted by the PI controller abc+ and negative modulation wave m abc- Adjustment includes the following steps:

[0074] S141, obtain positive bus voltage u p and negative bus voltage u N ;

[0075] By obtaining the positive bus voltage u p and negative bus voltage u N These two voltage signals are used to understand the changes in the positive and negative bus voltages in real time, and provide necessary input data for subsequent voltage deviation calculation and modulation wave adjustment.

[0076] S142, calculate the positive bus voltage u p With negative bus voltage u N The voltage deviation between dif ;

[0077] Specifically, the positive bus voltage u p With negative bus voltage u N The voltage deviation between dif Calculated by the following formula:

[0078] u dif =u p -u N (2).

[0079] S143, the reference voltage u refWith voltage deviation u dif By comparison, the voltage deviation error u is obtained. E ;

[0080] In some embodiments, the reference voltage u ref Set to 0V, voltage deviation error u E Calculated by the following formula:

[0081] u E =0-u dif (3).

[0082] S144, the voltage deviation error u E Input to the PI controller to generate and output the control signal u PI ;

[0083] Specifically, the PI controller is based on the input voltage deviation error u E Perform proportional P and integral I regulation to generate control signal u for regulating the system PI , to dynamically adjust the modulation wave.

[0084] S145, according to the control signal u PI Adjust the positive modulation wave m abc+ and negative modulation wave m abc- The specific adjustment principle of this step is:

[0085] When the positive bus voltage u p Greater than negative bus voltage u N When the positive bus voltage u p With negative bus voltage u N The voltage deviation between dif If it is greater than 0, it means that the positive bus voltage is too high. At this time, the voltage deviation error u E Less than 0, the control signal u output by the PI controller PI is also less than 0. To correct this imbalance, the negative modulation wave m abc- The magnitude of the N state (the negative bus related state) is reduced, the current average value increases, the current flowing into the inverter DC midpoint increases, the charging of the negative bus capacitor increases, and the negative bus voltage u is increased. N . With the negative bus voltage u N As the voltage rises, the positive and negative bus voltages tend to be consistent, thus balancing the DC midpoint voltage.

[0086] When the positive bus voltage u p Less than negative bus voltage u N When the positive bus voltage u p With negative bus voltage u N The voltage deviation between difLess than 0, indicating that the negative bus voltage is too high. At this time, the voltage deviation error u E Greater than 0, the control signal u output by the PI controller PI is also greater than 0. To correct this imbalance, the positive modulation wave m abc+ The magnitude of , then the duty cycle of the P state (positive bus related state) decreases. As the duty cycle of the P state decreases, the current flowing out of the inverter DC midpoint increases, the charging of the positive bus capacitor increases, and the positive bus voltage u p . As the positive bus voltage u p The positive and negative bus voltages tend to be consistent, thus balancing the DC midpoint voltage.

[0087] Compared with the conventional method of adjusting the redundant small vector, the method of directly changing the amplitude of the modulation wave in this embodiment is more effective and can effectively suppress the serious midpoint imbalance in the open-loop state.

[0088] Based on the above regulation principle, the regulation method of this step is: when the control signal u PI ≥0, reduce the positive modulation wave m abc+ The amplitude of the control signal u PI When <0, reduce the negative modulation wave m abc- And, the control signal u PI The absolute value of is inversely proportional to the amplitude of the positive and negative modulation waves. Preferably:

[0089] When the control signal u PI ≥0, the new positive modulation wave m′ abc+ Equal to the original positive modulation wave m abc+ Divided by the control signal u PI , the new negative modulation wave m′ abc- Equal to the original negative modulation wave m abc- ,Right now:

[0090]

[0091] When the control signal u PI <0, the new positive modulation wave m′ abc+ Equal to the original positive modulation wave m abc+ , the new negative modulation wave m′ abc- Equal to the original negative modulation wave m abc- Divided by the control signal u PI The absolute value of is:

[0092]

[0093] In this step, the control signal u PIDirectly associated with the modulation wave amplitude, it avoids the complex redundant vector adjustment process and enables the system to correct voltage deviation more quickly. Since the modulation wave amplitude changes in proportion to the voltage deviation, the larger the voltage deviation, the larger the adjustment amplitude of the modulation wave, so that the bus voltage can quickly restore balance. Through this fine-grained control, the voltage adjustment process is smoother and more continuous, the system stability is enhanced, and the voltage fluctuation is significantly reduced.

[0094] Next, through the above step S150, the new positive modulation wave m abc+ and the new negative modulation wave m abc- The PWM signal is compared with the carrier signal to generate a PWM signal for driving the power switch of the three-level inverter. Further through the above step S160, the PWM signal is used to drive the power switch of the three-level inverter, thereby controlling the switching time and duty cycle of the power switch to ensure accurate regulation of the inverter output voltage and balance of the DC midpoint voltage.

[0095] refer to Figure 4 , Figure 4 The curve diagram of the DC midpoint voltage difference of the three-level inverter according to an embodiment of the present application over time is shown in FIG. In this embodiment, the DC bus voltage is 660V, the effective value of the grid voltage is 230V, the proportional coefficient of the PI controller is 0.2, and the integral coefficient is 0. After adopting the above-mentioned three-level inverter DC midpoint balancing method, the DC midpoint voltage difference is rapidly reduced from 100V to 0V.

[0096] In summary, the DC midpoint balancing method of the three-level inverter of this embodiment can significantly reduce the difference of the DC midpoint voltage and ensure the stable operation of the system in an open-loop state by performing coordinate transformation on the grid voltage, generating positive and negative modulation waves based on the transformed grid voltage, and then using the PI controller to dynamically adjust the amplitude of the modulation wave. Furthermore, the PI controller can accurately adjust the amplitude of the positive and negative modulation waves by calculating the voltage deviation and generating a control signal, thereby ensuring the accuracy of the voltage balance. Furthermore, the regulation method in which the absolute value of the control signal is inversely proportional to the amplitude of the modulation wave makes the voltage adjustment more effective. The present application scheme simplifies the adjustment process, reduces the difficulty and cost of implementation, and is particularly suitable for the DC midpoint voltage balance control of the three-level inverter in an open-loop state.

[0097] refer to Figure 5 The present application also provides a three-level inverter DC midpoint balancing device in an open-loop state. The device includes:

[0098] The coordinate transformation module 100 is used to obtain the three grid voltages e of the three-level inverter in the open loop state. a 、e b and e c , and through abc / αβ coordinate transformation, the grid voltage ea 、e b and e c Transformed into the grid voltage e in the αβ coordinate system α and e β ;

[0099] Modulation module 200, for grid voltage e based on αβ coordinate system α and e β , generating the corresponding positive modulation wave m abc+ and negative modulation wave m abc- ;

[0100] The voltage balancing module 300 is used to p and negative bus voltage u N , the modulation wave m is adjusted by the PI controller abc+ and negative modulation wave m abc- Adjust to generate a new positive modulation wave m′ abc+ and the new negative modulation wave m′ abc- ;

[0101] PWM generation module 400, used to generate a new positive modulation wave m abc+ and the new negative modulation wave m abc- Compare with the carrier signal to generate a PWM signal for driving the power switch of the three-level inverter;

[0102] The driving module 500 is used to drive the power switch of the three-level inverter using a PWM signal to adjust the balance of the DC midpoint voltage.

[0103] refer to Figure 6 In one embodiment, the pressure equalization module 300 includes:

[0104] The voltage deviation calculation unit 310 is used to obtain the positive bus voltage u p and negative bus voltage u N , and calculate the positive bus voltage u p With negative bus voltage u N The voltage deviation between dif ;

[0105] The deviation error calculation unit 320 is used to convert the reference voltage u ref With voltage deviation u dif By comparison, the voltage deviation error u is obtained. E ;

[0106] PI control unit 330, used to convert the voltage deviation error u E Input to the PI controller to generate and output the control signal u PI ;

[0107] The modulation wave modulation unit 340 is used to modulate the PI Adjust the positive modulation wave m abc+ and negative modulation wave m abc- The amplitude of the positive modulation wave m′ is generated abc+ and the new negative modulation wave m′ abc- .

[0108] In one embodiment, the modulation wave modulation unit 340 includes:

[0109] Comparison subunit 341, used to compare control signal u PI Compare the value with zero and output the comparison result;

[0110] The modulation subunit 342 is used to adjust the modulation wave according to the output result of the comparison subunit 341; wherein,

[0111] When the control signal u PI ≥0, reduce the positive modulation wave m abc+ The amplitude of

[0112] When the control signal u PI When <0, reduce the negative modulation wave m abc- The amplitude of

[0113] And, the control signal u PI The absolute value of the positive modulation wave m abc+ The amplitude and negative modulation wave m abc- are inversely proportional to the amplitude.

[0114] Regarding the working principle and working process of the three-level inverter DC midpoint balancing device in the open-loop state in this embodiment that are not described in detail, reference can be made to the relevant parts of the embodiment of the three-level inverter DC midpoint balancing method in the open-loop state, which will not be repeated here.

[0115] The above description is only a preferred embodiment of the present invention and does not limit the present invention in other forms. Any technician familiar with the profession may use the technical content disclosed above to change or modify it into an equivalent embodiment with equivalent changes and apply it to other fields. However, any simple modification, equivalent change and modification made to the above embodiment based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A DC midpoint balancing method for a three-level inverter in an open-loop state, characterized in that: The following steps are involved: Get the three grid voltages e of the three-level inverter in the open-loop state a 、e b and e c ; Through abc / αβ coordinate transformation, the grid voltage e a 、e b and e c Transformed into the grid voltage e in the αβ coordinate system α and e β ; Based on the grid voltage e in the αβ coordinate system α and e β , generating the corresponding positive modulation wave m abc+ and negative modulation wave m abc- ; According to the positive bus voltage u p and negative bus voltage u N , the positive modulation wave m is controlled by a PI controller abc+ and the negative modulation wave m abc- Adjust to generate a new positive modulation wave m′ abc+ and the new negative modulation wave m′ abc- ; The new positive modulation wave m abc+ and the new negative modulation wave m abc- Compare with the carrier signal to generate a PWM signal for driving the power switch of the three-level inverter; The PWM signal is used to drive the power switch of the three-level inverter to adjust the balance of the DC midpoint voltage.

2. The three-level inverter DC midpoint balancing method according to claim 1, characterized in that: According to the positive bus voltage u p and negative bus voltage u N , the positive modulation wave m is controlled by a PI controller abc+ and the negative modulation wave m abc- Make adjustments, including: Get the positive bus voltage u p and negative bus voltage u N ; Calculate the positive bus voltage u p With the negative bus voltage u N The voltage deviation between dif ; The reference voltage u ref The voltage deviation u dif By comparison, the voltage deviation error u is obtained. E ; The voltage deviation error u E Input to the PI controller to generate and output the control signal u PI ; According to the control signal u PI Adjust the positive modulation wave m abc+ and the negative modulation wave m abc- The magnitude of , where: When the control signal u PI ≥0, reduce the positive modulation wave m abc+ The amplitude of When the control signal u PI <0, reduce the negative modulation wave m abc- The amplitude of .

3. The three-level inverter DC midpoint balancing method according to claim 2, characterized in that: The control signal u PI The absolute value of the positive modulation wave m abc+ The amplitude and the negative modulation wave m abc- are inversely proportional to the amplitude.

4. The three-level inverter DC midpoint balancing method according to claim 3, characterized in that: When the control signal u PI ≥0, the new positive modulation wave m′ abc+ Equal to the original positive modulation wave m abc+ Divided by the control signal u PI , the new negative modulation wave m′a bc - equal to the original negative modulation wave m abc- ,Right now: m′ abc- =m abc- (1)。 5. The three-level inverter DC midpoint balancing method according to claim 4, characterized in that: When the control signal u PI <0, the new positive modulation wave m′ abc+ Equal to the original positive modulation wave m abc+ , the new negative modulation wave m′ abc- Equal to the original negative modulation wave m abc- Divided by the control signal u PI The absolute value of is: m′ abc+ =m abc+ 6. The three-level inverter DC midpoint balancing method according to any one of claims 1 to 5, characterized in that: The positive bus voltage u p With the negative bus voltage u N The voltage deviation between dif Calculated by the following formula: in dif =in p -in N (3).

7. The three-level inverter DC midpoint balancing method according to claim 6, characterized in that: The reference voltage u ref Set to 0V, the voltage deviation error u E Calculated by the following formula: in E =0-in dif (4).

8. A three-level inverter DC midpoint balancing device in an open-loop state, characterized in that: include: Coordinate transformation module, used to obtain the three grid voltages e of the three-level inverter in the open-loop state a 、e b and e c , and through abc / αβ coordinate transformation, the grid voltage e a 、e b and e c Transformed into the grid voltage e in the αβ coordinate system α and e β ; A modulation module is used for the grid voltage e based on the αβ coordinate system. α and e β , generating the corresponding positive modulation wave m abc+ and negative modulation wave m abc -; The voltage balancing module is used to p and negative bus voltage u N , the positive modulation wave m is controlled by a PI controller abc+ and the negative modulation wave m abc - Adjust to generate a new positive modulation wave m' abc+ and the new negative modulation wave m′ abc- ; PWM generation module, used to convert the new positive modulation wave m abc+ and the new negative modulation wave m abc -Compare with the carrier signal to generate a PWM signal for driving the power switch of the three-level inverter; The driving module is used to drive the power switch of the three-level inverter using the PWM signal to adjust the balance of the DC midpoint voltage.

9. The three-level inverter DC midpoint balancing device according to claim 8, characterized in that: The pressure equalizing module comprises: Voltage deviation calculation unit, used to obtain the positive bus voltage u p and negative bus voltage u N , and calculate the positive bus voltage u p With the negative bus voltage u N The voltage deviation between dif ; Deviation error calculation unit, used to convert the reference voltage u ref The voltage deviation u dif By comparison, the voltage deviation error u is obtained. E ; PI control unit, used to convert the voltage deviation error u E Input to the PI controller to generate and output the control signal u PI ; The modulation wave modulation unit is used to modulate the PI Adjust the positive modulation wave m abc+ and the negative modulation wave m abc- The amplitude of the positive modulation wave m′ is generated abc+ and the new negative modulation wave m′ abc- .

10. The three-level inverter DC midpoint balancing device according to claim 9, characterized in that: The modulation wave modulation unit comprises: A comparison subunit, used to compare the control signal u PI Compare the value with zero and output the comparison result; A modulation subunit, used for adjusting the modulation wave according to the output result of the comparison subunit; in, When the control signal u PI ≥0, reduce the positive modulation wave m abc+ The amplitude of When the control signal u PI <0, reduce the negative modulation wave m abc- The amplitude of The control signal u PI The absolute value of the positive modulation wave m abc+ The amplitude and the negative modulation wave m abc- are inversely proportional to the amplitude.

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