A square wave modulation midpoint voltage control method, system and related components

Abstract

The present application discloses a square wave modulation midpoint voltage control method, system and related components, which are applied to a three-level inverter, including: obtaining the value of the current three-phase modulation wave; determining the output voltage vector type according to the value of the current three-phase modulation wave; judging whether the output voltage vector type is a preset output voltage vector type; if so, determining the zero-sequence component according to the vector flag and the value of the current three-phase modulation wave; determining the target three-phase modulation wave according to the zero-sequence component to control the midpoint voltage balance of the three-level inverter. The present application is used to control a three-level inverter, by determining that the output voltage vector type is a preset output voltage vector type, determining the zero-sequence component according to the vector flag and the value of the current three-phase modulation wave, and injecting the zero-sequence component into the current three-phase modulation wave, the midpoint voltage deviation can be eliminated, and the midpoint voltage balance control under square wave modulation can be achieved.

Description

Technical Field

[0001] The present invention relates to the field of inverters, and in particular to a square wave modulation midpoint voltage control method, system and related components. Background Art

[0002] The three-level neutral point clamped (NPC) inverter is the most commonly used three-level inverter. Its main circuit topology is as follows: Figure 1 Compared with the traditional two-level inverter, the three-level NPC inverter can reduce the device voltage stress and improve the output power quality; compared with other three-level inverters such as cascade H-bridge, the three-level NPC inverter has a relatively simple structure and control, and is easy to achieve bidirectional energy flow. Therefore, the three-level NPC inverter has been widely used in motor traction drive, photovoltaic and wind power generation, metallurgy and mining and other fields.

[0003] When the three-level NPC inverter is used in the motor traction drive system, in order to increase the output fundamental frequency as much as possible without exceeding the maximum switching frequency of the power device, the following is usually used when the output fundamental frequency exceeds 120Hz. Figure 2 Square wave modulation is shown in the figure. In square wave modulation, each power device is switched on and off only once in each fundamental wave cycle, and the switching frequency is equal to the output fundamental wave frequency, so the corresponding power device has the lowest switching loss.

[0004] Carrier modulation is a commonly used modulation strategy for three-level NPC inverters, which has the advantages of simple calculation and convenient expansion. The document "Research on Improved Synchronous Carrier Pulse Width Modulation Strategy for Three-Level Neutral-Point Clamped Converter under Low Carrier Ratio" (Gao Zhan. [J]. Transactions of China Electrotechnical Society, 2020, 35(18): 3894-3907.) points out that traditional carrier modulation cannot normally enter square wave modulation, and thus cannot be applied to high output fundamental frequency conditions. To overcome this defect, the patent "A Square Wave Modulation Method for Three-Level Converter Based on Carrier" (Zhao Lu. [P]. Invention Patent, 2020, CN201910658638.7) proposes a square wave modulation method based on carrier implementation that calculates the switching angle online according to the modulation ratio. This method can make the output phase voltage meet three-phase symmetry, half-wave symmetry and quarter-cycle symmetry while entering the square wave condition, which broadens the application range of carrier modulation under high output frequency of three-level NPC inverters.

[0005] The midpoint voltage is the voltage difference between the upper capacitor and the lower capacitor on the DC side of the three-level NPC inverter. The literature "A general carrier modulation equalization strategy for diode-clamped converters based on predictive control" (Cui Dongdong. [J]. New Technology of Electrical Engineering and Energy, 2018, 37(4): 24-34.) points out that when the midpoint voltage loses balance, it will cause output current distortion and increase voltage stress of the switching devices of the three-level NPC inverter. Therefore, in order to improve the reliability of the three-level NPC inverter, it is necessary to design an effective midpoint voltage balance control method for the three-level NPC inverter.

[0006] Define the three-level NPC inverter output from high to low as P, O, and N, then the space vector of the three-level NPC inverter can be summarized as Figure 3 As shown in Table 1, Figure 3 Each space vector in can be classified into zero vector, P-type small vector, N-type small vector, medium vector and large vector. Among them, P-type small vectors and N-type small vectors with the same position are redundant small vectors.

[0007] Table 1 Types of voltage space vectors for three-level converters

[0008]

[0009] Under the action of traditional carrier modulation, there are two redundant small vectors in the space vector sequence output by the three-level NPC inverter in a single sampling period. The literature "Research on the Midpoint Potential Control Strategy of Three-Level ANPC Converter" (Zhang Bo. [J]. New Technology of Electrical Engineering and Power, 2016, 35(8): 1-7.) points out that the two redundant small vectors in the same position have opposite effects on the midpoint voltage, so the midpoint voltage can be controlled to restore balance by redistributing the action time of the two redundant small vectors.

[0010] Depend on Figure 2 It can be seen that under the square wave modulation based on the carrier, the space vector state output by the three-level NPC inverter in a single sampling period may not change, and there is no redundant small vector in the corresponding space vector sequence. Therefore, the traditional midpoint voltage balance control method by redistributing the action time of two redundant small vectors is not suitable for square wave modulation. In order to improve the reliability of the three-level NPC inverter under square wave modulation, it is of great theoretical and practical significance to design a midpoint voltage balance control method suitable for square wave modulation.

[0011] Therefore, how to provide a solution to the above technical problems is a problem that those skilled in the art need to solve. Summary of the invention

[0012] In view of this, the purpose of the present invention is to provide a square wave modulation midpoint voltage control method to solve the square wave modulation midpoint voltage balance control problem. The specific scheme is as follows:

[0013] A square wave modulation midpoint voltage control method is applied to a three-level inverter, comprising:

[0014] Get the value of the current three-phase modulation wave;

[0015] Determining the output voltage vector type according to the value of the current three-phase modulation wave;

[0016] Determining whether the output voltage vector type is a preset output voltage vector type;

[0017] If so, determining the zero-sequence component according to the vector flag and the value of the current three-phase modulation wave;

[0018] The zero-sequence component is injected into the current three-phase modulation wave to control the midpoint voltage balance of the three-level inverter.

[0019] Further, determining the output voltage vector type according to the value of the current three-phase modulation wave includes:

[0020] When the amplitudes of the phases of the current three-phase modulation wave are equal, determining that the output voltage vector type of the three-level inverter is a first vector type;

[0021] When the sum of the amplitudes of the phases of the current three-phase modulation wave is zero, determining that the output voltage vector type of the three-level inverter is a second vector type;

[0022] When the sum of the absolute values ​​of the amplitudes of the phases of the current three-phase modulation wave is 3K, determining that the output voltage vector type of the three-level inverter is a third vector type;

[0023] When the sum of the absolute values ​​of the amplitudes of the phases of the current three-phase modulation wave is K or 2K, determining that the output voltage vector type of the three-level inverter is a fourth vector type, and the fourth vector type is the preset output voltage vector type;

[0024] Wherein, K represents the amplitude of the three-phase modulation wave, and the corresponding value range of the three-phase modulation wave is -K to K.

[0025] Furthermore, before the process of determining the zero-sequence component according to the vector flag and the value of the current three-phase modulation wave, it also includes determining the vector flag according to the values ​​of the phase angle position, the midpoint voltage and the output current.

[0026] Further, determining the vector flag according to the phase angle position, the midpoint voltage and the output current value includes:

[0027] When the phase angle position is in the phase angle region of 0° to 60° or 120° to 180° or 240° to 300°, when the product of the midpoint voltage and the output current is greater than zero, the vector flag is determined to be negative, and when the product of the midpoint voltage and the output current is less than zero, the vector flag is determined to be positive;

[0028] When the phase angle position is in the phase angle region of 60° to 120° or 180° to 240° or 300° to 360°, when the product of the midpoint voltage and the output current is greater than zero, the vector flag is determined to be positive, and when the product of the midpoint voltage and the output current is less than zero, the vector flag is determined to be negative.

[0029] Further, the output current is determined according to the phase angle position and the three-phase current, including:

[0030] When the phase angle position is in a phase angle region of 0° to 60° or 180° to 240°, determining that the output current is a B-phase output current of the three-phase inverter;

[0031] When the phase angle position is in the phase angle region of 60° to 120° or 240° to 300°, determining that the output current is the A-phase output current of the three-phase inverter;

[0032] When the phase angle position is in a phase angle region of 120° to 180° or 300° to 360°, the output current is determined to be a C-phase output current of the three-phase inverter.

[0033] Further, determining the zero-sequence component according to the vector flag and the value of the current three-phase modulation wave includes:

[0034] When the vector flag is positive and the sum of the amplitudes of the current three-phase modulation wave is greater than zero, or when the vector flag is negative and the sum of the amplitudes of the current three-phase modulation wave is less than zero, determine that the value of the zero-sequence component is zero;

[0035] When the vector flag is positive and the sum of the amplitudes of each phase of the current three-phase modulation wave is less than zero, determining that the value of the zero-sequence component is K;

[0036] When the vector flag is negative and the sum of the amplitudes of each phase of the current three-phase modulation wave is greater than zero, the value of the zero-sequence component is determined to be -K, where K represents the amplitude of the three-phase modulation wave.

[0037] Further, injecting the zero-sequence component into the current three-phase modulation wave to control the midpoint voltage balance of the three-level inverter includes:

[0038] The method of injecting the zero-sequence component into the current three-phase modulation wave is:

[0039]

[0040] Among them, U ban_neu is the zero sequence component, U am2 is the target A phase modulation wave after the zero sequence component is injected, U bm2 is the target B phase modulation wave after the zero sequence component is injected, U cm2 is the target C phase modulation wave after injecting zero sequence component, U am is the current A phase modulation wave, U bm is the current B phase modulation wave, U cm is the current C phase modulation wave.

[0041] Correspondingly, the present application also discloses a square wave modulation midpoint voltage control system, which is applied to a three-level inverter, comprising:

[0042] An acquisition module is used to obtain the value of the current three-phase modulation wave;

[0043] A first determination module, configured to determine the output voltage vector type according to the value of the current three-phase modulation wave;

[0044] A judging module, used for judging whether the output voltage vector type is a preset output voltage vector type;

[0045] A second determination module is used to determine the zero-sequence component according to the vector flag and the value of the current three-phase modulation wave;

[0046] An action module is used to inject the zero-sequence component into the current three-phase modulation wave to control the midpoint voltage balance of the three-level inverter.

[0047] Correspondingly, the present application also discloses a square wave modulation midpoint voltage control device, comprising:

[0048] Memory for storing computer programs;

[0049] A processor is used to implement the steps of the square wave modulation midpoint voltage control method as described in any one of the above items when executing the computer program.

[0050] Correspondingly, the present application also discloses a readable storage medium, on which a computer program is stored. When the computer program is executed by a processor, the steps of the square wave modulation midpoint voltage control method as described in any one of the above items are implemented.

[0051] The present invention discloses a square wave modulation midpoint voltage control method, which is applied to a three-level inverter, including: obtaining the value of the current three-phase modulation wave, determining the output voltage vector type according to the value of the current three-phase modulation wave, judging whether the output voltage vector type is a preset output voltage vector type, and if so, determining the zero-sequence component according to the vector flag and the value of the current three-phase modulation wave, and injecting the zero-sequence component into the current three-phase modulation wave to control the midpoint voltage balance of the three-level inverter. The present application can eliminate the midpoint voltage deviation by determining that the output voltage vector type is a preset output voltage vector type, determining the zero-sequence component according to the vector flag and the value of the current three-phase modulation wave, and injecting the zero-sequence component into the current three-phase modulation wave, thereby improving the reliability of the three-level inverter during square wave modulation and achieving midpoint voltage balance. In addition, the present invention does not require the use of a PI controller and has the advantages of simple calculation and easy implementation. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0053] Figure 1 It is a main circuit topology diagram of a three-level inverter in the prior art;

[0054] Figure 2 is a phase voltage waveform diagram of a three-level inverter under square wave modulation in the prior art;

[0055] Figure 3 is a spatial vector diagram of a three-level inverter in the prior art;

[0056] Figure 4 This is a flow chart of the steps of a square wave modulation midpoint voltage control method in an embodiment of the present application;

[0057] Figure 5-Figure 9 They are schematic diagrams of simulation experimental results of square wave modulation midpoint voltage control method;

[0058] Fig.10 This is a structural distribution diagram of a square wave modulated midpoint voltage control system in an embodiment of the present application. DETAILED DESCRIPTION

[0059] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0060] In the prior art, under the square wave modulation based on the carrier, the space vector state output by the three-level inverter in a single sampling period may not change, and there is no redundant small vector in the corresponding space vector sequence. The traditional midpoint voltage balance control method by redistributing the action time of two redundant small vectors is not suitable for square wave modulation.

[0061] The present application discloses a square wave modulation midpoint voltage control method, which is applied to a three-level inverter. Figure 4 As shown, including:

[0062] S1: Get the value of the current three-phase modulation wave;

[0063] S2: Determine the output voltage vector type according to the value of the current three-phase modulation wave;

[0064] S3: Determine whether the output voltage vector type is a preset output voltage vector type;

[0065] S4: If yes, determine the zero-sequence component according to the vector flag and the value of the current three-phase modulation wave;

[0066] S5: injecting a zero-sequence component into the current three-phase modulation wave to control the midpoint voltage balance of the three-level inverter;

[0067] It is understandable that if the output voltage vector type is not the preset output voltage vector type, the midpoint voltage is balanced at this time, and there is no need to perform step S4 or inject zero-sequence components, and the three-level inverter can be controlled with the current three-phase modulation wave.

[0068] Further, step S5 specifically includes: injecting a zero-sequence component into the current three-phase modulation wave to control the midpoint voltage balance of the three-level inverter. It can be understood that when the output voltage vector type is the preset output voltage vector type at this time, that is, the midpoint voltage is unbalanced, it is necessary to inject a zero-sequence component to obtain a new three-phase modulation wave, thereby realizing midpoint voltage control.

[0069] Among them, the method of injecting the zero-sequence component into the current three-phase modulation wave is as follows:

[0070]

[0071] Among them, U ban_neu is the zero sequence component, Uam2 is the target A phase modulation wave after the zero sequence component is injected, U bm2 is the target B phase modulation wave after the zero sequence component is injected, U cm2 is the target C phase modulation wave after injecting zero sequence component, U am is the current A phase modulation wave, U bm is the current B phase modulation wave, U cm is the current C phase modulation wave.

[0072] This embodiment determines the output voltage vector type of the three-level inverter by the value of the current three-phase modulation wave. When the output voltage vector type is the preset output voltage vector type, the zero-sequence component is determined according to the vector flag and the value of the current three-phase modulation wave, and the zero-sequence component is injected into the current three-phase modulation wave, thereby controlling the midpoint voltage to restore balance, improving the reliability of the three-level inverter during square wave modulation, and achieving midpoint voltage balance. In addition, this embodiment does not need to use a PI controller, and it also has the advantages of simple calculation and easy implementation.

[0073] The embodiment of the present application discloses a specific square wave modulation midpoint voltage control method. Compared with the previous embodiment, this embodiment further explains and optimizes the technical solution.

[0074] Specifically, the process of determining the output voltage vector type according to the value of the current three-phase modulation wave includes:

[0075] When the amplitudes of the phases of the current three-phase modulation waves are equal, determining that the output voltage vector type of the three-level inverter is a first vector type;

[0076] When the sum of the amplitudes of the phases of the current three-phase modulation wave is zero, determining that the output voltage vector type of the three-level inverter is a second vector type;

[0077] When the sum of the absolute values ​​of the amplitudes of the phases of the current three-phase modulation wave is 3K, determining that the output voltage vector type of the three-level inverter is the third vector type;

[0078] When the sum of the absolute values ​​of the amplitudes of the phases of the current three-phase modulation wave is K or 2K, the output voltage vector type of the three-level inverter is determined to be a fourth vector type, and the fourth vector type is a preset output voltage vector type.

[0079] Where K represents the amplitude of the three-phase modulation wave, and the corresponding value range of the three-phase modulation wave is -K to K, which can be determined according to the actual working conditions. For example, in a specific embodiment, the first vector type, the second vector type, and the third vector type are zero vector, medium vector, and large vector, respectively, and the fourth vector type is a small vector. The amplitude of the three-phase modulation wave is taken as 2, and the method for judging the output voltage vector type of the three-level inverter according to the value of the three-phase modulation wave is as follows:

[0080] 1) When U am =U bm =U cm When , the output voltage vector type of the three-level inverter is zero vector;

[0081] 2) When U am +U bm +U cm =0, the output voltage vector type of the three-level inverter is a medium vector;

[0082] 3) When abs(U am )+abs(U bm )+abs(U cm )=2 or abs(U am )+abs(U bm )+abs(U cm )=4, the output voltage vector type of the three-level inverter is a small vector;

[0083] 4) When abs(U am )+abs(U bm )+abs(U cm )=6, the output voltage vector type of the three-level inverter is a large vector;

[0084] In the above judgment method, U am is the A phase modulation wave, abs(U am ) is the absolute value of the A phase modulation wave, U bm is the B phase modulation wave, abs(U bm ) is the absolute value of the B-phase modulation wave, U cm is the C phase modulation wave, abs(U cm ) is the absolute value of the C-phase modulation wave.

[0085] When the vector type is zero vector, medium vector and large vector, there is no need to inject zero-sequence components, and the three-level inverter can be controlled with the current three-phase modulation wave.

[0086] When the output voltage vector type is a small vector, the midpoint voltage is unbalanced. The vector flag is judged according to the phase angle position, the midpoint voltage and the output current. The zero-sequence component is determined by the vector flag and the value of the three-phase modulation wave. The zero-sequence component is injected into the current three-phase modulation wave to control the midpoint voltage to restore balance, thereby improving the reliability of the three-level inverter during square wave modulation and achieving midpoint voltage balance.

[0087] The embodiment of the present application discloses a specific square wave modulation midpoint voltage control method. Compared with the previous embodiment, this embodiment further explains and optimizes the technical solution.

[0088] Specifically, before determining the zero-sequence component according to the vector flag and the value of the current three-phase modulation wave, the vector flag is also determined according to the values ​​of the phase angle position, the midpoint voltage and the output current, including:

[0089] When the phase angle position is in the phase angle region of 0° to 60°, 120° to 180°, or 240° to 300°, when the product of the midpoint voltage and the output current is greater than zero, the vector flag is determined to be negative, and when the product of the midpoint voltage and the output current is less than zero, the vector flag is determined to be positive;

[0090] When the phase angle position is in the phase angle region of 60° to 120°, 180° to 240°, or 300° to 360°, when the product of the midpoint voltage and the output current is greater than zero, the vector flag is determined to be positive, and when the product of the midpoint voltage and the output current is less than zero, the vector flag is determined to be negative.

[0091] The calculation process of the midpoint voltage is as follows:

[0092] V neu =V Cap_up -V Cap_dn ;

[0093] V neu Represents the midpoint voltage, V cap_up is the upper capacitor voltage on the DC side of the three-level inverter, V cap_dn is the capacitor voltage at the lower end of the DC side of the three-level inverter.

[0094] Among them, the output current is determined according to the phase angle position and the three-phase current, specifically including:

[0095] When the phase angle position is in the phase angle region of 0° to 60° or 180° to 240°, the output current is determined to be the B-phase output current of the three-phase inverter;

[0096] When the phase angle position is in the phase angle region of 60° to 120° or 240° to 300°, determining that the output current is the A-phase output current of the three-phase inverter;

[0097] When the phase angle position is in the phase angle region of 120° to 180° or 300° to 360°, the output current is determined to be the C-phase output current of the three-phase inverter.

[0098] Specifically, the process of determining the zero-sequence component according to the vector flag and the value of the current three-phase modulation wave includes:

[0099] When the vector mark is positive and the sum of the amplitudes of the current three-phase modulation wave is greater than zero, or when the vector mark is negative and the sum of the amplitudes of the current three-phase modulation wave is less than zero, the value of the zero-sequence component is determined to be zero;

[0100] When the vector flag is positive and the sum of the amplitudes of each phase of the current three-phase modulation wave is less than zero, the value of the zero-sequence component is determined to be K;

[0101] When the vector flag is negative and the sum of the amplitudes of the phases of the current three-phase modulation wave is greater than zero, the value of the zero-sequence component is determined to be -K, where K is the amplitude of the three-phase modulation wave.

[0102] The embodiment of the present application discloses a specific square wave modulation midpoint voltage control method. Compared with the previous embodiment, this embodiment further explains and optimizes the technical solution.

[0103] The embodiment of the present application uses PSIM software to build a three-level inverter model, and uses simulation to verify the effectiveness of the midpoint voltage balance control method of the present invention based on square wave modulation implemented by the carrier. The simulation conditions of the embodiment are: simulation step size 1us, DC side voltage 5000V, DC side capacitance 16.2mF, carrier frequency and fundamental frequency are consistent, and the output load is a 1.6Ω resistor in series with a 1.6mH inductor.

[0104] See also Figure 5 , which is the DC side voltage, phase voltage, modulation wave and output current when the initial voltage of the upper capacitor of the DC side is 4000V, the initial voltage of the lower capacitor is 1000V, the modulation ratio is 0.8, the fundamental frequency is 120Hz, and the square wave modulation is not added with the midpoint voltage balance control method. Analysis Figure 5 It can be seen that when the modulation strategy of the three-level NPC inverter is square wave modulation based on carrier and the midpoint voltage is unbalanced, if an effective midpoint voltage balance control method is not added, the deviation value of the upper and lower voltages on the DC side will always exist. The unbalanced midpoint voltage will cause the output current distortion of the three-level NPC inverter and increase the voltage stress of the switching device, affecting the safe operation of the three-level NPC inverter. Therefore, in order to improve the reliability of the three-level NPC inverter when using square wave modulation, an effective midpoint voltage balance control method must be added to it.

[0105] See also Figure 6 , is the DC side voltage, phase voltage, modulation wave and output current when the DC side upper capacitor initial voltage is 4000V, the lower capacitor initial voltage is 1000V, the modulation ratio is 0.8, the fundamental frequency is 120Hz, and the square wave modulation is added to the midpoint voltage balance control method of the present application. Figure 6 and Figure 5, when the voltage at the upper end of the DC side is greater than the voltage at the lower end, after adding the midpoint voltage balance control method of the present invention, the midpoint voltage deviation value of the three-level NPC inverter is reduced to within 3% of the DC side voltage value within 0.1s, so the method of the present application can effectively control the midpoint voltage balance when the voltage at the upper end of the DC side is greater than the voltage at the lower end. In addition, the method of the present application can ensure the symmetry of the output current during the process of controlling the midpoint voltage balance, thereby minimizing the adverse effects of the midpoint voltage deviation, and improving the safety and reliability of the three-level NPC inverter when using square wave modulation.

[0106] See also Figure 7 , which is the DC side voltage, phase voltage, modulation wave and output current when the DC side upper capacitor initial voltage is 1000V, the lower capacitor initial voltage is 4000V, the modulation ratio is 0.8, the fundamental frequency is 120Hz, and the square wave modulation is added to the midpoint voltage balance control method of the present application. Figure 7 It shows that when the voltage at the upper end of the DC side is less than the voltage at the lower end, after adding the midpoint voltage balance control method of the present application, the midpoint voltage deviation value of the three-level NPC inverter is reduced to within 3% of the DC side voltage value within 0.1s. Therefore, the method of the present application can effectively control the midpoint voltage balance when the voltage at the upper end of the DC side is less than the voltage at the lower end.

[0107] See also Figure 8 , the DC side voltage, phase voltage, modulation wave, output current and modulation ratio when the DC side upper capacitor initial voltage is 4000V, the lower capacitor initial voltage is 1000V, the modulation ratio is increased from 0.6 to 1.1, the fundamental frequency is 120Hz, and the square wave modulation is added to the DC side voltage, phase voltage, modulation wave, output current and modulation ratio when the midpoint voltage balance control method of the present application is controlled. Figure 8 It can be seen that when the midpoint voltage is unbalanced and the modulation ratio is constantly changing, the method of the present application can control the midpoint voltage of the three-level NPC inverter to quickly restore balance, and can maintain the symmetry of the output current during the midpoint voltage balance control process, thereby improving the safety and reliability of the three-level NPC inverter under changing modulation ratios.

[0108] See also Fig. 9 , the DC side voltage, phase voltage, modulation wave, output current and fundamental frequency when the DC side upper capacitor initial voltage is 1000V, the lower capacitor initial voltage is 4000V, the modulation ratio is 0.8, the fundamental frequency is increased from 120Hz to 130Hz, and the square wave modulation is added to the DC side voltage, phase voltage, modulation wave, output current and fundamental frequency when the midpoint voltage balance control method of the present application is controlled. Fig. 9 It can be seen that when the midpoint voltage is unbalanced and the fundamental frequency is constantly changing, the method of the present application can control the midpoint voltage of the three-level NPC inverter to quickly restore balance, and can maintain the symmetry of the output current during the midpoint voltage balance control process, thereby improving the safety and reliability of the three-level NPC inverter under changing fundamental frequency.

[0109] Further analysis Figures 6 to 9 In the embodiment of the present invention, the method of the present invention controls the midpoint voltage balance during square wave modulation by injecting a specific zero-sequence component into the three-phase modulation wave, and does not require the use of a PI controller in the balance control process. Therefore, the method of the present invention also has the advantages of simple calculation and easy implementation.

[0110] See also Figures 5 to 9 As shown, the results of the embodiment verify the effectiveness of the midpoint voltage balance control method of square wave modulation based on carrier wave implementation of the present application. When the voltage at the upper end of the DC side is greater than or less than the voltage at the lower end of the DC side, when the modulation ratio and the fundamental frequency are fixed or changed, the method of the present application can control the midpoint voltage balance during square wave modulation by injecting a specific zero-sequence component into the three-phase modulation wave, and can maintain the symmetry of the output current during the midpoint voltage balance control process. The method of the present application overcomes the defect that the traditional midpoint voltage balance control method cannot be applied to square wave modulation, and improves the safety and reliability of the three-level inverter during square wave modulation. In addition, the method of the present application is based on carrier wave implementation and does not require the use of a PI controller during the control process. It also has the advantages of simple calculation and easy implementation.

[0111] Correspondingly, the present application also discloses a square wave modulation midpoint voltage control system, which is applied to a three-level inverter, see Fig.10 As shown, including:

[0112] Acquisition module 1, used to obtain the value of the current three-phase modulation wave;

[0113] A first determination module 2, used to determine the output voltage vector type according to the value of the current three-phase modulation wave;

[0114] A judging module 3 is used to judge whether the output voltage vector type is a preset output voltage vector type; if so, triggering a second determining module 4;

[0115] A second determination module 4 is used to determine the zero-sequence component according to the vector flag and the value of the current three-phase modulation wave;

[0116] The action module 5 is used to inject the zero-sequence component into the current three-phase modulation wave to control the midpoint voltage balance of the three-level inverter.

[0117] The present application determines the output voltage vector type of the three-level inverter by the value of the current three-phase modulation wave. When the output voltage vector type is a preset output voltage vector type, the zero-sequence component is determined according to the vector flag and the value of the current three-phase modulation wave, and the zero-sequence component is injected into the current three-phase modulation wave, thereby controlling the midpoint voltage to restore balance, improving the reliability of the three-level inverter during square wave modulation, and achieving midpoint voltage balance.

[0118] In some specific embodiments, the first determining module 2 is specifically configured to:

[0119] When the amplitudes of the phases of the current three-phase modulation wave are equal, determining that the output voltage vector type of the three-level inverter is a first vector type;

[0120] When the sum of the amplitudes of the phases of the current three-phase modulation wave is zero, determining that the output voltage vector type of the three-level inverter is a second vector type;

[0121] When the sum of the absolute values ​​of the amplitudes of the phases of the current three-phase modulation wave is 3K, determining that the output voltage vector type of the three-level inverter is a third vector type;

[0122] When the sum of the absolute values ​​of the amplitudes of each phase of the current three-phase modulation wave is K or 2K, it is determined that the output voltage vector type of the three-level inverter is a fourth vector type, and the fourth vector type is the preset output voltage vector type.

[0123] Wherein, K represents the amplitude of the three-phase modulation wave, and the corresponding value range of the three-phase modulation wave is -K to K.

[0124] In some specific embodiments, the square wave modulated midpoint voltage control system also includes a third determination module, which is used to determine the vector flag according to the phase angle position, midpoint voltage and output current values ​​before determining the zero-sequence component according to the vector flag and the value of the current three-phase modulation wave.

[0125] In some specific embodiments, the third determination module is specifically used to:

[0126] When the phase angle position is in the phase angle region of 0° to 60°, 120° to 180°, or 240° to 300°, when the product of the midpoint voltage and the output current is greater than zero, the vector flag is determined to be negative, and when the product of the midpoint voltage and the output current is less than zero, the vector flag is determined to be positive;

[0127] When the phase angle position is in the phase angle region of 60° to 120° or 180° to 240° or 300° to 360°, when the product of the midpoint voltage and the output current is greater than zero, the vector flag is determined to be positive, and when the product of the midpoint voltage and the output current is less than zero, the vector flag is determined to be negative.

[0128] In some specific embodiments, the square wave modulation midpoint voltage control system further includes a fourth determination module, which is used to determine the output current according to the phase angle position and the three-phase current, and is specifically used to:

[0129] When the phase angle position is in the phase angle region of 0° to 60° or 180° to 240°, the output current is determined to be the B-phase output current of the three-phase inverter;

[0130] When the phase angle position is in the phase angle region of 60° to 120° or 240° to 300°, the output current is determined to be the A-phase output current of the three-phase inverter;

[0131] When the phase angle position is in the phase angle region of 120° to 180° or 300° to 360°, the output current is determined to be the C-phase output current of the three-phase inverter.

[0132] In some specific embodiments, the second determining module 4 is specifically configured to:

[0133] When the vector position is positive and the sum of the amplitudes of the current three-phase modulation waves is greater than zero, or when the vector position is negative and the sum of the current three-phase modulation waves is less than zero, the value of the zero-sequence component is determined to be zero;

[0134] When the vector position is positive and the sum of the amplitudes of the current three-phase modulation wave is less than zero, the value of the zero-sequence component is determined to be K;

[0135] When the vector position is negative and the sum of the amplitudes of the current three-phase modulation wave is greater than zero, the value of the zero-sequence component is determined to be -K; wherein K is the amplitude of the three-phase modulation wave.

[0136] In some specific embodiments, the action module 5 is used to inject a zero-sequence component into the current three-phase modulation wave to control the midpoint voltage balance of the three-level inverter, specifically for:

[0137] The way to inject the zero-sequence component into the current three-phase modulation wave is as follows:

[0138]

[0139] Among them, U ban_neu is the zero sequence component, U am2 is the target A phase modulation wave after the zero sequence component is injected, U bm2 is the target B phase modulation wave after the zero sequence component is injected, U cm2 is the target C phase modulation wave after injecting zero sequence component, U am is the current A phase modulation wave, U bm is the current B phase modulation wave, U cm is the current C phase modulation wave.

[0140] Correspondingly, the embodiment of the present application further discloses a square wave modulation midpoint voltage control device, comprising:

[0141] Memory for storing computer programs;

[0142] A processor is used to implement the steps of the square wave modulation midpoint voltage control method as described in any of the above embodiments when executing the computer program.

[0143] Correspondingly, an embodiment of the present application further discloses a readable storage medium, on which a computer program is stored. When the computer program is executed by a processor, the steps of the square wave modulation midpoint voltage control method in any of the above embodiments are implemented.

[0144] For details about the square wave modulation midpoint voltage control method, please refer to the relevant description in the above embodiment, which will not be repeated here.

[0145] Among them, the square wave modulation midpoint voltage control device and the readable storage medium in this embodiment have the same technical effects as the square wave modulation midpoint voltage control method in the above embodiment, and will not be repeated here.

[0146] Finally, it should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the presence of other identical elements in the process, method, article or device including the elements.

[0147] The above is a detailed introduction to a square wave modulated midpoint voltage control method, system and related components provided by the present application. Specific examples are used in this article to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the method of the present invention and its core idea; at the same time, for general technical personnel in this field, according to the idea of ​​the present application, there will be changes in the specific implementation method and application scope. In summary, the content of this specification should not be understood as a limitation on the present application.

Claims

1. A square wave modulation midpoint voltage control method, characterized in that: Applicable to three-level inverters, including: Get the value of the current three-phase modulation wave; When the amplitudes of the phases of the current three-phase modulation wave are equal, determining that the output voltage vector type of the three-level inverter is a first vector type; When the sum of the amplitudes of the phases of the current three-phase modulation wave is zero, determining that the output voltage vector type of the three-level inverter is a second vector type; When the sum of the absolute values ​​of the amplitudes of the phases of the current three-phase modulation wave is 3K, determining that the output voltage vector type of the three-level inverter is a third vector type; When the sum of the absolute values ​​of the amplitudes of the phases of the current three-phase modulation wave is K or 2K, determining that the output voltage vector type of the three-level inverter is a fourth vector type; Determining whether the output voltage vector type is a preset output voltage vector type; If so, determining the zero-sequence component according to the vector flag and the value of the current three-phase modulation wave; Injecting the zero-sequence component into the current three-phase modulation wave to control the midpoint voltage balance of the three-level inverter; Wherein, determining the zero-sequence component according to the vector flag and the value of the current three-phase modulation wave includes: When the vector flag is positive and the sum of the amplitudes of each phase of the current three-phase modulation wave is less than zero, determining that the value of the zero-sequence component is K; When the vector flag is negative and the sum of the amplitudes of each phase of the current three-phase modulation wave is greater than zero, the value of the zero-sequence component is determined to be -K; K represents the amplitude of the three-phase modulation wave.

2. The square wave modulation midpoint voltage control method according to claim 1, characterized in that: The fourth vector type is a preset output voltage vector type; the value range of the three-phase modulation wave is -K to K.

3. The square wave modulation midpoint voltage control method according to claim 1, characterized in that: Before the process of determining the zero-sequence component according to the vector flag and the value of the current three-phase modulation wave, it also includes determining the vector flag according to the values ​​of the phase angle position, the midpoint voltage and the output current.

4. The square wave modulation midpoint voltage control method according to claim 3, characterized in that: Determining the vector flag according to the phase angle position, the midpoint voltage and the output current value includes: When the phase angle position is in the phase angle region of 0° to 60° or 120° to 180° or 240° to 300°, when the product of the midpoint voltage and the output current is greater than zero, the vector flag is determined to be negative, and when the product of the midpoint voltage and the output current is less than zero, the vector flag is determined to be positive; When the phase angle position is in the phase angle region of 60° to 120° or 180° to 240° or 300° to 360°, when the product of the midpoint voltage and the output current is greater than zero, the vector flag is determined to be positive, and when the product of the midpoint voltage and the output current is less than zero, the vector flag is determined to be negative.

5. The square wave modulation midpoint voltage control method according to claim 4, characterized in that: The output current is determined according to the phase angle position and the three-phase current, including: When the phase angle position is in a phase angle region of 0° to 60° or 180° to 240°, determining that the output current is a B-phase output current of the three-level inverter; When the phase angle position is in a phase angle region of 60° to 120° or 240° to 300°, determining that the output current is an A-phase output current of the three-level inverter; When the phase angle position is in a phase angle region of 120° to 180° or 300° to 360°, the output current is determined to be a C-phase output current of the three-level inverter.

6. The square wave modulation midpoint voltage control method according to claim 5, characterized in that: The determining of the zero-sequence component according to the vector flag and the value of the current three-phase modulation wave comprises: When the vector flag is positive and the sum of the amplitudes of each phase of the current three-phase modulation wave is greater than zero, or when the vector flag is negative and the sum of the amplitudes of each phase of the current three-phase modulation wave is less than zero, the value of the zero-sequence component is determined to be zero.

7. The square wave modulation midpoint voltage control method according to any one of claims 1 to 6, characterized in that: Injecting the zero-sequence component into the current three-phase modulation wave to control the midpoint voltage balance of the three-level inverter includes: The method of injecting the zero-sequence component into the current three-phase modulation wave is: Among them, U ban_neu is the zero sequence component, U am2 is the target A phase modulation wave after the zero sequence component is injected, U bm2 is the target B phase modulation wave after the zero sequence component is injected, U cm2 is the target C phase modulation wave after injecting zero sequence component, U am is the current A phase modulation wave, U bm is the current B phase modulation wave, U cm is the current C phase modulation wave.

8. A square wave modulated midpoint voltage control system, characterized in that: Applicable to three-level inverters, including: An acquisition module is used to obtain the value of the current three-phase modulation wave; A first determination module is used to determine that the output voltage vector type of the three-level inverter is a first vector type when the amplitudes of the current three-phase modulation waves are equal; determine that the output voltage vector type of the three-level inverter is a second vector type when the sum of the amplitudes of the current three-phase modulation waves is zero; determine that the output voltage vector type of the three-level inverter is a third vector type when the absolute value sum of the amplitudes of the current three-phase modulation waves is 3K; determine that the output voltage vector type of the three-level inverter is a fourth vector type when the absolute value sum of the amplitudes of the current three-phase modulation waves is K or 2K; A judging module, used for judging whether the output voltage vector type is a preset output voltage vector type; The second determination module is used to determine the zero-sequence component according to the vector flag and the value of the current three-phase modulation wave; wherein, the zero-sequence component is determined according to the vector flag and the value of the current three-phase modulation wave, including: when the vector flag is positive and the sum of the amplitudes of each phase of the current three-phase modulation wave is less than zero, the value of the zero-sequence component is determined to be K; when the vector flag is negative and the sum of the amplitudes of each phase of the current three-phase modulation wave is greater than zero, the value of the zero-sequence component is determined to be -K; K represents the amplitude of the three-phase modulation wave; An action module is used to inject the zero-sequence component into the current three-phase modulation wave to control the midpoint voltage balance of the three-level inverter.

9. A square wave modulated midpoint voltage control device, characterized in that: include: Memory for storing computer programs; A processor, configured to implement the steps of the square wave modulation midpoint voltage control method as claimed in any one of claims 1 to 7 when executing the computer program.

10. A readable storage medium, characterized in that: The readable storage medium stores a computer program, and when the computer program is executed by a processor, the steps of the square wave modulation midpoint voltage control method according to any one of claims 1 to 7 are implemented.

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