A midpoint potential balance control method and device

Abstract

The present application relates to the field of power electronics, in particular to a kind of midpoint potential balance control method and device.The present application obtains three-phase reference voltage signal by injecting three zero sequence components to original three-phase modulation signal, combined with the matching processing of correction voltage modulation signal and three-phase output current, the midpoint potential and the current signal obtained by matching are output compensation zero sequence component using pre-set controller, and the compensation zero sequence component is injected into three-phase reference voltage signal, the balance control of midpoint potential in three-level inverter is realized, without additional hardware circuit, control logic is simple and reliable, can quickly suppress midpoint potential deviation, effectively reduce the bus capacitor voltage uneven, switch stress uneven and the problem such as the increase of output harmonic caused by midpoint potential fluctuation, while guaranteeing the power quality of inverter output, simplify midpoint potential balance control implementation difficulty, greatly improve the reliability and stability of inverter system.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a method and apparatus for controlling midpoint potential balance. Background Technology

[0002] Against the backdrop of rapid social productivity development and the energy crisis, the demand for efficient power conversion devices is growing stronger. Considering that the performance of existing power devices has not made any breakthrough progress, multilevel technology is currently a good choice for medium- and high-voltage high-power applications. Among them, the most widely used is the neutral-point-clamped three-level inverter, also known as the NPC (Neutral-Point-Clamped) three-level inverter.

[0003] However, NPC-type three-level inverters suffer from midpoint potential fluctuations, which not only increase output harmonics but also pose serious safety hazards. For this topology to operate normally, the voltage at midpoint O must be maintained at half the DC bus voltage. If there is no external hardware control for the voltage across the DC bus capacitors, modulation and control strategies must be designed to achieve voltage balance between the capacitors. While the average midpoint voltage is controllable, low-frequency pulsations still occur under certain operating conditions. This is an inherent problem with NPC three-level inverters, and this drawback forces the NPC converter and DC bus capacitors to withstand these low-frequency pulsations, creating a serious overvoltage breakdown safety hazard.

[0004] Therefore, overcoming the shortcomings of the existing technology is an urgent problem to be solved in this technical field. Summary of the Invention

[0005] The technical problem to be solved by this invention is how to solve the problem of unbalanced point potential in existing three-level inverters.

[0006] The present invention adopts the following technical solution: Firstly, a method for controlling midpoint potential balance is provided, including: The original three-phase modulation signal of the sampled inverter is injected with three zero-sequence components to obtain the three-phase reference voltage signal. A corrected voltage modulation signal is constructed based on the three-phase reference voltage signal; The three-phase output current of the inverter is sampled and matched with the amplitude of the corrected voltage modulation signal in each carrier cycle to obtain a matched current signal; The midpoint potential is obtained, and based on the midpoint potential and the corresponding current signal, the zero-sequence component is compensated through a preset controller. The compensated zero-sequence component is injected into the three-phase reference voltage signal to achieve balanced control of the inverter's midpoint potential.

[0007] Preferably, the original three-phase modulation signal of the sampling inverter is sampled, and three zero-sequence components are injected into the original three-phase modulation signal to obtain a three-phase reference voltage signal, specifically including: The original three-phase modulation signal is: ; Three zero-sequence components are injected into the original three-phase modulation signal. Then, the three-phase reference voltage signal is obtained. as follows: , ; in, The original three-phase voltage modulation signal. The phase voltage amplitude, Let be the angular frequency, and a, b, and c correspond to the three phases.

[0008] Preferably, constructing the corrected voltage modulation signal based on the three-phase reference voltage signal specifically includes: For injecting zero-sequence components The three-phase reference voltage signal obtained afterwards The modified voltage modulation signal is constructed by dividing the voltage into three phases: the maximum voltage phase, the intermediate voltage phase, and the minimum voltage phase, within each carrier cycle. Represented as: ; in, The phase with the maximum voltage. This is the voltage midpoint phase. The phase with the minimum voltage is a, b, and c, which correspond to the three phases.

[0009] Preferably, the sampling inverter's three-phase output current is matched with the amplitude of the corrected voltage modulation signal in each carrier cycle to obtain a matched current signal, specifically including: The three-phase output current of the inverter is respectively matched to the maximum voltage phase, the intermediate voltage phase, and the minimum voltage phase of the corrected voltage modulation signal to obtain the instantaneous current value corresponding to each amplitude level. The current signal includes the instantaneous current value. The three-phase output current of the inverter... , , Represented as: ; The inverter's three-phase output current is mapped to the maximum value phase, the intermediate value phase, and the minimum value phase of the corrected voltage modulation signal to obtain: ; in, This is the peak value of the phase current. The power factor angle, The maximum value of the three-phase reference voltage The corresponding instantaneous value of the phase current, The intermediate value of the three-phase reference voltage The corresponding instantaneous value of the phase current, The minimum value of the three-phase reference voltage phase The corresponding instantaneous value of the phase current.

[0010] Preferably, the step of obtaining the compensated zero-sequence component based on the midpoint potential and the corresponding current signal through a preset controller specifically includes: The midpoint potential is input to the preset controller to obtain the midpoint fluctuation current. Combined with the current corresponding to the minimum original modulation signal voltage value in the matched current signal, the compensated zero-sequence component is obtained after the sign function is used to determine the zero-sequence component. Compensation zero-sequence component The expression is: ; in, For the proportional term coefficient, Here, s represents the coefficients of the integral term, and s represents the complex variables of the Laplace transform. For a sign function, when hour, ;when hour, , Midpoint potential The intermediate value of the three-phase reference voltage The corresponding current signal.

[0011] Preferably, the three-phase reference voltage signal after injecting the zero-sequence component compensation must meet the requirements of the linear modulation region, and the injection lower limit of the zero-sequence component compensation must be met. for: ; Upper limit of injection for compensating zero-order components for: ; in, This is the DC bus voltage. This is the three-phase reference voltage signal.

[0012] Preferably, a compensated zero-sequence component is injected into the three-phase reference voltage signal. Average midpoint current after Represented as: ; in, This is the DC bus voltage. The three-phase reference voltage signal, , , This refers to the three-phase output current of the inverter.

[0013] Preferably, obtaining the midpoint potential specifically includes: The upper and lower capacitor voltages of the DC bus are sampled and obtained. The difference between the voltage of the upper capacitor and the voltage of the lower capacitor is calculated to obtain the midpoint potential.

[0014] Secondly, a midpoint potential balance control device is provided, the midpoint potential balance control device comprising: a processor and a memory for storing processor-executable instructions; The processor is configured to execute the midpoint potential balance control method.

[0015] Thirdly, a non-volatile computer storage medium is provided, the computer storage medium storing computer-executable instructions, which are executed by one or more processors to perform the midpoint potential balance control method described in the first aspect.

[0016] Fourthly, a chip is provided, comprising: a processor and an interface for calling and running a computer program stored in a memory to perform the midpoint potential balance control method as described in the first aspect.

[0017] Fifthly, a computer program product containing instructions is provided, which, when executed on a computer or processor, causes the computer or processor to perform the midpoint potential balance control method as described in the first aspect.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention obtains a three-phase reference voltage signal by injecting three zero-sequence components into the original three-phase modulation signal. It then combines this with a corrected voltage modulation signal and the three-phase output current for matching. The zero-sequence component is compensated by outputting the midpoint potential and the matched current signal via a preset controller, and then injected into the three-phase reference voltage signal. This achieves balanced control of the midpoint potential of the three-level inverter without requiring additional hardware circuitry. The control logic is simple and reliable, quickly suppressing midpoint potential deviation and effectively reducing problems such as uneven bus capacitor voltage, uneven switching transistor stress, and increased output harmonics caused by midpoint potential fluctuations. Furthermore, based on real-time carrier cycle matching and closed-loop compensation, it offers high control accuracy and fast dynamic response, adapting to different load conditions. While ensuring the quality of the inverter's output power, it simplifies the implementation of midpoint potential balance control, significantly improving the reliability and stability of the inverter system. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is an equivalent model diagram of an NPC-type three-level inverter provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of the modified modulation signal waveform under case one of the PD-PWM modulation strategies provided in this embodiment of the invention; Figure 3 This is a schematic diagram of the modified modulation signal waveform under case two of a PD-PWM modulation strategy provided in an embodiment of the present invention; Figure 4 This is a schematic diagram of the corrected modulation signal waveform under case three of a PD-PWM modulation strategy provided in an embodiment of the present invention; Figure 5 This is a schematic diagram of the corrected modulation signal waveform under case four of a PD-PWM modulation strategy provided in an embodiment of the present invention; Figure 6 This is a schematic diagram of calculating the duty cycle of in-phase stacked carrier modulation provided by an embodiment of the present invention; Figure 7 This is a midpoint potential fluctuation provided by an embodiment of the present invention. A schematic diagram showing the relationship between the sign of the zero-sequence component and the phase current required for compensation under different conditions; Figure 8 This invention provides a method for obtaining compensated zero-sequence components. The control block diagram; Figure 9 This is a flowchart illustrating a midpoint potential balance control method provided in an embodiment of the present invention; Figure 10 This invention provides a sign relationship between modulation voltage and compensated zero-sequence component, and an average midpoint current value. Figure 11 This invention provides a method for compensating for zero-sequence components when the modulation ratio m=0.9 and the power factor is unity. A schematic diagram of the injection range; Figure 12 This is a schematic diagram illustrating the compensation range of ICOM and the fluctuation range of ID when the modulation ratios m=0.2, m=0.4, m=0.6, and m=0.9 are unity power factor, provided by an embodiment of the present invention. Figure 13This is a schematic diagram illustrating the compensation range of ICOM and the fluctuation range of id when the modulation ratio m varies between 0 and 1 and the power factor is unit, provided by an embodiment of the present invention. Figure 14 This is a schematic diagram illustrating the compensation range of ICOM and the fluctuation range of ID when the modulation ratio m=0.9 and the power factor angle is -45°, according to an embodiment of the present invention. Figure 15 This is a schematic diagram illustrating the compensation range of ICOM and the fluctuation range of id when the modulation ratio m=0.9 and the power factor angle is 45°, according to an embodiment of the present invention. Figure 16 This is a schematic diagram illustrating the compensation range of ICOM and the fluctuation range of ID when the modulation ratio m=0.9 and the power factor angle is -90°, according to an embodiment of the present invention. Figure 17 This is a schematic diagram illustrating the compensation range of ICOM and the fluctuation range of id when the modulation ratio m=0.9 and the power factor angle is 90°, according to an embodiment of the present invention. Figure 18 This invention provides a modulation ratio m=0.9 and a power factor angle. A diagram illustrating the compensation range of ICOM and the fluctuation range of ID when varying between -180° and 180°. Figure 19 This is a schematic diagram of the structure of a midpoint potential balance control device provided in an embodiment of the present invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0022] Unless the context otherwise requires, throughout the specification and claims, the term "comprising" is interpreted as openly inclusive, meaning "including, but not limited to." In the description of the specification, terms such as "one embodiment," "some embodiments," "exemplary embodiment," "example," "specific example," or "some examples" are intended to indicate that a particular feature, structure, material, or characteristic associated with that embodiment or example is included in at least one embodiment or example of this disclosure. The illustrative representations of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics mentioned may be included in any suitable manner in any one or more embodiments or examples; that is, although they may be incorporated into embodiments or examples using the above terms for reasons such as order and position, it does not limit them to be incorporated in combination by a single embodiment or example.

[0023] In the description of this invention, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of embodiments of this disclosure, unless otherwise stated, "a plurality of" means two or more. Furthermore, for example, the description may use the prefix "A" or "B" to describe the same type of nouns as two independent entities. In this case, the corresponding features defined with "A" and "B" are used only to distinguish between similar entities and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features.

[0024] In describing some embodiments, the terms "coupled," "coupled," and "connected," and their derivative expressions, may be used. For example, the term "connected" may be used in describing some embodiments to indicate that two or more components have direct physical or electrical contact with each other. Similarly, the term "coupled" may be used in describing some embodiments to indicate that two or more components have direct physical or electrical contact. However, the terms "connected" or "coupled" may also refer to two or more components that do not have direct contact with each other but still cooperate or interact with each other, such as "optical coupling," "wireless connection," etc. The embodiments disclosed herein are not necessarily limited to the scope of this invention.

[0025] Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0026] Example 1: This embodiment first explains the principle of the midpoint potential balance control method proposed in this invention.

[0027] In one implementation, the duty cycle is calculated differently under different carrier modulation strategies. First, the modulation rules and duty cycle calculation under phase disposition pulse width modulation (PD-PWM) are derived and analyzed.

[0028] like Figure 1 As shown, under the PD-PWM modulation strategy, the corrected modulation signal in each carrier cycle Depending on its amplitude, the following situations may occur: like Figure 2 Situation 1 as shown Figure 3 In case two shown, a zero-sequence component is added. The compensated modulated signal There will be one phase whose sign is opposite to the other two phases.

[0029] like Figure 4 Situation 3 as shown Figure 5 In case four shown, a zero-sequence component is added. The compensated modulated signal All symbols have the same number.

[0030] In order for the corrected modulation signal to remain within the linear modulation region, the compensated modulation signal must first meet the following requirements: (1) ; (2) .

[0031] Injecting compensation zero-sequence component back, The corresponding bridge arm may have three output states: P, O, and N. However, the P and N states will not appear simultaneously within a carrier cycle. Next, the duty cycle of different output states will be derived and solved based on the PD-PWM modulation strategy.

[0032] The equivalent topology based on the NPC-type three-level inverter is as follows: Figure 1 As shown, when a certain phase bridge arm is operating in state P, its output phase voltage is generated by the capacitor on the DC bus. Provided; when operating in N state, the output phase voltage is supplied by the capacitor below the DC bus. supply.

[0033] like Figure 6 As shown, when the modulated signal after phase compensation is greater than zero ( During one carrier cycle, its output state switches between P and O, with the duty cycle of the P state being... for: (1) The duty cycle of state O at this time for: (2) Similarly, when the modulated signal after phase compensation is less than zero ( During one carrier cycle, its output state switches between O and N, with the duty cycle of the N state being... for: (3) The duty cycle of state O at this time for: (4) Set the capacitor voltage on the DC bus to The lower capacitor voltage is Then the modulated signal The duty cycle of the P state corresponding to the phase is The duty cycle of state N is The modulated signal The duty cycle of the P state corresponding to the phase is The duty cycle of state N is The modulated signal The duty cycle of the P state corresponding to the phase is The duty cycle of state N is For the compensated modulated signal After normalization, the duty cycle is obtained as follows: (5) The preceding text only briefly introduced the need to inject a compensated zero-sequence component into the three-phase modulation signal. To achieve the goal of controlling the average midpoint current to zero within one carrier cycle, the zero-sequence component is now being compensated. The acquisition of [the property] is derived in detail.

[0034] Average midpoint current over one carrier cycle for: (6) Substituting equation (5) into equation (6), we get: (7) Assuming the switching frequency is high enough and the carrier period is short enough, the voltage across the DC bus capacitor is equal to the voltage across the lower capacitor. Reference voltage after injecting compensation for zero-sequence components The corresponding i-phase bridge arm will only have two output states, P and O or O and N, within one carrier cycle. , One of them must be zero. And when At this time, the modulation signal is compared with the upper carrier to obtain the duty cycle of the P state, and its output phase voltage is determined by the capacitor voltage on the DC bus. Provide; when At that time, the modulation signal is compared with the download wave to obtain the N-state duty cycle, and its output phase voltage is determined by the voltage of the capacitor below the DC bus. supply.

[0035] Based on the four cases analyzed above, the corresponding expressions for the average midpoint current can be written as follows: (1) Case 1

[0036] (8) (2) Case 2,

[0037] (9) (3) Scenario 3

[0038] (10) (4) Situation 4

[0039] (11) From equations (8), (9), (10), and (11), it can be found that when the compensated modulation signal is Figure 2 , Figure 3 In cases one and two, the signs of the three-phase modulation signals are not entirely the same. The average neutral point current can be divided into two parts: one part is the constant component introduced by the modulation strategy, and the other part includes the load current of a certain phase and the compensation zero-sequence component. Then it can be changed This is used to compensate for the average midpoint current, thereby controlling the midpoint potential. And when the compensated modulation signal is... Figure 4 , Figure 5 In cases three and four, the three-phase modulation signals are identical, and the average neutral point current does not include the compensated zero-sequence component. This means that no matter what zero-sequence component is injected at this time, it will not affect the average midpoint current, so the average midpoint current cannot be changed in this way, and thus the midpoint potential cannot be controlled.

[0040] According to equations (8) and (9), the controller design is as follows: The expression for compensating the zero-sequence component differs depending on the sign, as detailed below.

[0041] (1) When At this time, the phase currents participating in the synthesis of the average midpoint current are: Average midpoint current The expression is: (12) The compensated zero-sequence component at this time can be derived. The expression is: (13) (2) When At this time, the phase currents participating in the synthesis of the average midpoint current are: Average midpoint current The expression is: (14) The compensated zero-sequence component at this time can be derived. The expression is: (15) The above analysis summarizes the midpoint potential fluctuation. The corresponding sign relationships of the required compensation zero-sequence component and phase current under different conditions are as follows: Figure 7 As shown, by combining equations (13) and (14), we can obtain the compensation zero-sequence component. The control block diagram, such as Figure 8 As shown.

[0042] in, This refers to the load current corresponding to the modulated signal and the two phases with opposite signs within one carrier cycle. The unified expression for the midpoint ripple current is: (16) (17) (18) in, for Midpoint ripple current; for Midpoint ripple current; This refers to the midpoint potential fluctuation. The transfer function of the PI controller used for this compensation strategy, in the controller... This is the proportionality coefficient. Here, s represents the coefficients of the integral term, and s represents the complex variables of the Laplace transform. For a sign function, when hour, ;when hour, .

[0043] Depend on Figure 7 It is not difficult to observe that different midpoint potential fluctuations Required compensation zero-sequence component The symbols are also different. The sign of the current is also related to the current that is generated at this time to compensate for the current. The sign of the phase current is related. As can be seen from the previous derivation, adding... The modulated signal after The sign of the current determines the phase current that participates in generating the compensation midpoint current, and since its sign can be determined, the required compensation zero-sequence component can be obtained. The symbol.

[0044] The compensated zero-order component can be obtained from the above derivation. The final expression is as follows: (19).

[0045] Example 2: Based on the derivation process in Example 1, this example proposes a midpoint potential balance control method. In one implementation, such as... Figure 9 As shown, the method includes: Step 101: Sample the original three-phase modulation signal of the inverter and inject three zero-sequence components into the original three-phase modulation signal to obtain the three-phase reference voltage signal.

[0046] The original three-phase modulation signal is: (20) Three zero-sequence components are injected into the original three-phase modulation signal. Then, the three-phase reference voltage signal is obtained. as follows: (twenty one) (twenty two) in, The original three-phase voltage modulation signal. The phase voltage amplitude, Let be the angular frequency, and a, b, and c correspond to the three phases.

[0047] Step 102: Construct a corrected voltage modulation signal based on the three-phase reference voltage signal.

[0048] Among them, the zero-order component is injected. It will not change the relative magnitudes of the phase reference voltages, even with the injection of zero-sequence components. The three-phase reference voltage signal obtained afterwards The modified voltage modulation signal is constructed by dividing the voltage into three phases: the maximum voltage phase, the intermediate voltage phase, and the minimum voltage phase, within each carrier cycle. Represented as: (twenty three) in, The phase with the maximum voltage. This is the voltage midpoint phase. The phase with the minimum voltage is a, b, and c, which correspond to the three phases.

[0049] Step 103: Sample the three-phase output current of the inverter and match it with the amplitude of the corrected voltage modulation signal in each carrier cycle to obtain a matched current signal.

[0050] Specifically, the three-phase output current of the inverter is matched to the maximum voltage phase, the intermediate voltage phase, and the minimum voltage phase of the corrected voltage modulation signal, respectively, to obtain the instantaneous current value corresponding to each amplitude level. The current signal includes the instantaneous current value. The three-phase output current of the inverter... , , Represented as: (twenty four) The inverter's three-phase output current is mapped to the maximum value phase, the intermediate value phase, and the minimum value phase of the corrected voltage modulation signal to obtain: (25) in, This is the peak value of the phase current. The power factor angle, The maximum value of the three-phase reference voltage The corresponding instantaneous value of the phase current, The intermediate value of the three-phase reference voltage The corresponding instantaneous value of the phase current, The minimum value of the three-phase reference voltage phase The corresponding instantaneous value of the phase current.

[0051] Step 104: Obtain the midpoint potential, and based on the midpoint potential and the corresponding current signal, obtain the compensated zero-sequence component through a preset controller.

[0052] Specifically, obtaining the midpoint potential includes: sampling and obtaining the upper capacitor voltage and the lower capacitor voltage of the DC bus; and calculating the difference between the upper capacitor voltage and the lower capacitor voltage to obtain the midpoint potential.

[0053] In one implementation, according to the derivation process in Example 1, the midpoint potential is input to a preset controller to obtain the midpoint fluctuation current. Combined with the current corresponding to the minimum original modulation signal voltage value in the matched current signal, the compensated zero-sequence component is obtained after determination by the sign function.

[0054] Based on the derivation process in Example 1, the zero-sequence component is compensated. The expression is: ; in, For the proportional term coefficient, Here, s represents the coefficients of the integral term, and s represents the complex variables of the Laplace transform. For a sign function, when hour, ;when hour, , Midpoint potential The intermediate value of the three-phase reference voltage The corresponding current signal.

[0055] Step 105: Inject the compensated zero-sequence component into the three-phase reference voltage signal to achieve balanced control of the inverter's midpoint potential.

[0056] Ultimately, the zero-order component will be compensated. Injected into the three-phase reference voltage signal In this process, the balance of the midpoint potential is controlled.

[0057] In summary, this embodiment obtains a three-phase reference voltage signal by injecting three zero-sequence components into the original three-phase modulation signal. This is then matched with the corrected voltage modulation signal and the three-phase output current. The zero-sequence component is compensated by outputting the midpoint potential and the matched current signal via a preset controller, and then injected into the three-phase reference voltage signal. This achieves balanced control of the midpoint potential of the three-level inverter without requiring additional hardware circuitry. The control logic is simple and reliable, quickly suppressing midpoint potential deviation and effectively reducing problems such as uneven bus capacitor voltage, uneven switching transistor stress, and increased output harmonics caused by midpoint potential fluctuations. Furthermore, based on real-time carrier cycle matching and closed-loop compensation, it offers high control accuracy and fast dynamic response, adapting to different load conditions. While ensuring the quality of the inverter's output power, it simplifies the implementation of midpoint potential balance control, significantly improving the reliability and stability of the inverter system.

[0058] This embodiment first constructs a modified voltage modulation signal. Inject a preset compensated zero-sequence component. The final compensated modulated signal's maximum, median, and minimum values ​​were obtained. : (26) At this point, the final modulated signal has been obtained. By mapping these signals to the three-phase modulation signals A, B, and C, and comparing them with the carrier wave, the switching sequence of the corresponding bridge arm can be obtained.

[0059] It should also be noted that when injecting compensation zero-sequence components... back, The corresponding bridge arm may have three output states: P, O, and N. However, the P and N states will not appear simultaneously within a carrier cycle. Next, the duty cycle of different output states will be derived and solved based on the PD-PWM modulation strategy.

[0060] Based on the topology of an NPC-type three-level inverter, when a phase arm operates in the P state, its output phase voltage is generated by the capacitor on the DC bus. Provided; when operating in N state, the output phase voltage is supplied by the capacitor below the DC bus. supply.

[0061] Duty cycle in state 0 for: (27) As can be seen from the above formula, changing The size can be changed Size, The only controllable part is Therefore, by changing Change size O The duty cycle of the state is used to control the average midpoint current, thereby suppressing midpoint potential fluctuations.

[0062] Injecting a compensated zero-sequence component into the three-phase reference voltage signal Average midpoint current after Represented as: (28) in, This is the DC bus voltage. The three-phase reference voltage signal, , , This refers to the three-phase output current of the inverter.

[0063] In the above formula To i Phase injection triple zero sequence The modulated signal after that. To remove the absolute value sign in the above formula, we will now discuss different cases.

[0064] Case (a) is ,and ; Then add back, , , ,have: (29) As can be seen from equation (29), the average midpoint current It contains the following three parts: Part One: Under three-phase equilibrium, there is .

[0065] Part Two: The midpoint ripple current generated by the PD-PWM modulation strategy is not zero and is an uncontrollable component. It is the root cause of midpoint potential fluctuations. This midpoint ripple current is expressed as... .

[0066] Part Three: , which can be determined by Simplify to This part of the component equation is due to the injection compensation zero-sequence component. The resulting compensation current is expressed as . use To compensate for the midpoint ripple current in the second part Control the average midpoint current This controls the fluctuation of the midpoint potential.

[0067] Representing equation (29) as follows: (30) The subsequent cases b, c, d, e, f, g are similar to case a, and will not be elaborated here. The compensation zero-sequence component and average midpoint current corresponding to the three-phase modulation voltage symbols at different times are listed in tabular form, and the restrictions on the compensation zero-sequence component at different times are derived.

[0068] For compensated zero-sequence components The limiting processing requirement is: add The situations described in points three and four above will not occur, i.e., the three-phase modulation signal will not be affected. , All symbols are of the same sign, by Figure 10 It can be known The constraint of equation (31) must be satisfied in each carrier cycle.

[0069] (31) As the analysis above shows, adding a compensating zero-sequence component... The average midpoint current can be expressed as: (32) in The midpoint current of the fluctuation introduced by the modulation itself The compensation midpoint current is introduced to compensate for the zero-sequence component. According to... Figure 10 By discussing the modulated voltage in different cases within one power frequency cycle, we can obtain... and Unified expression: (33) (34) In equation (33) , To inject three zero-sequence components The three-phase voltage afterwards; , For three-phase current, we have: (35) (36) In equations (35) and (36), The modulation ratio, The peak value of the phase voltage. This is the DC bus voltage value. This is the peak value of the phase current. The phase angle, The power factor angle.

[0070] The above analysis shows that the injected compensation zero-order component will not affect... This will have an impact, and will directly affect Therefore, the magnitude of the compensation zero-sequence component directly affects... The ability to compensate for the average midpoint current. Figure 7 The average midpoint current direction under different midpoint potential fluctuation conditions was also analyzed. To compensate for the midpoint potential, it is necessary to inject a compensation zero-sequence component. Make the average midpoint current The direction changes. Simultaneously, the modulation ratio... Phase current peak value Phase angle Power factor angle The differences will affect , The impacts will be described below.

[0071] In one implementation, the three-phase reference voltage signal after injecting the zero-sequence component compensation must meet the linear modulation region requirements. The injection range of the zero-sequence component compensation is the difference between half of the DC bus voltage value and the maximum and minimum values ​​of the three-phase modulated wave after injecting three zero-sequence components, and the lower limit of the zero-sequence component compensation injection is also specified. for: (37) Upper limit of injection for compensating zero-order components for: (38) in, This is the DC bus voltage. This is the three-phase reference voltage signal.

[0072] It can be seen that the compensation zero-order component The injection range varies with the modulation ratio m It changes with the changes. When the operating state of the NPC-type three-level inverter is determined, The injection range can also be determined. For example, when the DC bus voltage... Peak phase current of grid connection Peak phase current of grid connection modulation ratio When operating at unity power factor, the result can be determined by equations (37) and (38). The upper and lower limits of the injection capability are plotted in the MATLAB simulation platform. The upper and lower limits, such as Figure 11 As shown, The injection range is between these two curves.

[0073] Given the compensated zero-sequence component Under the conditions of upper and lower limits, by Figure 10 By combining equations (33) and (34), the oscillating midpoint current within one power frequency cycle can be obtained. Waveform and compensated midpoint current The upper and lower limits are plotted in the MATLAB simulation platform. Waveform and The upper and lower limits, such as Figure 11 As shown in the figure. It can be observed from the figure that at the modulation ratio... m When the power factor is 0.9, the inverter operates under unity power factor conditions, and the midpoint ripple current at this time is... The neutral point current can be compensated throughout the entire power frequency cycle. The compensation.

[0074] From equations (33) and (34), it can be seen that the modulation ratio m Size will not only affect It will have an impact, and will also affect This has an impact, in order to investigate the modulation ratio m right , The influence of setting the modulation ratio m =0.2、 m =0.4、 m =0.6、 m =0.9, with other conditions remaining unchanged, plot its waveform, as follows. Figure 12 As shown. It can be observed that when the phase current amplitude remains constant, as the modulation ratio continuously increases, The scope of compensation is gradually expanding. The range of fluctuations is gradually decreasing.

[0075] To further investigate whether the proposed midpoint potential balance control method still has balancing capability under different modulation ratios, the modulation ratio was set... m With the power factor fixed at 1 and other conditions remaining constant, plot a 3D graph over one power frequency cycle, varying the power factor between 0 and 1. Figure 13 As shown in the figure. It can be observed from the figure that the modulation ratio, regardless of its magnitude, is within the range of 0 to 1. The fluctuation range will not exceed The upper and lower limits.

[0076] From equations (33) and (34), it can be seen that the power factor angle The size will also affect , The effects of different power factor angles have been discussed previously. right , The impact, such as Figure 14 , Figure 15 , Figure 16 and Figure 17 As shown, set the power factor angle. , , , With other conditions remaining unchanged, plot its waveform. It can be observed that when the NPC-type three-level inverter operates at the power factor angle... At that time, the compensation midpoint current can completely compensate for the fluctuating midpoint current within the power frequency cycle; at the power factor angle At this time, the midpoint current of the fluctuation is in its worst state. At this time, the compensation midpoint current cannot fully compensate for the fluctuation midpoint current within the power frequency cycle, and the uncompensated part appears in the form of a component with a frequency three times the fundamental frequency. Moreover, as the power factor angle increases, the amplitude of this component increases.

[0077] To further investigate whether the proposed midpoint potential balance control method still possesses balancing capability under different power factors, the power factor angle was varied between -180° and 180°, while other parameters remained constant. Three-dimensional graphs were then plotted in the MATLAB simulation platform, as shown below. Figure 18 As shown. From Figure 18 It can be observed that when the power factor angle varies between -180° and 180°, the fluctuating midpoint current... It is a waveform three times the fundamental frequency, at the power factor angle When the fluctuation amplitude reaches its peak, at the power factor angle Depend on toward 0° and The fluctuation amplitude drops to 0 during the change. Furthermore, it can be observed that the compensation midpoint current... The upper and lower limit amplitudes change very little with the power factor angle. When the power factor angle varies between -180° and 180°, the compensation midpoint current cannot fully compensate for the fluctuating midpoint current within one power frequency cycle, and the uncompensated... Some frequencies are three times the fundamental frequency, consistent with the previous analysis.

[0078] Example 3: In Embodiment 2, a midpoint potential balance control method is provided. In this embodiment, a midpoint potential balance control device is proposed, which includes a processor and a memory for storing processor-executable instructions; wherein the processor is configured to execute the midpoint potential balance control method described in Embodiment 1.

[0079] like Figure 19 As shown, the midpoint potential balance control device includes a processor 21 and a memory 22, wherein the processor 21 and the memory 22 can be connected by a bus or other means.

[0080] Processor 21 can be a Central Processing Unit (CPU). Processor 21 can also be other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, or combinations of the above types of chips.

[0081] The memory 22, as a non-transitory computer-readable storage medium, can be used to store non-transitory software programs, non-transitory computer-executable programs, and modules, such as the program instructions / modules corresponding to the midpoint potential balance control method in Embodiment 1 of the present invention. The processor executes various functional applications and training processes by running the non-transitory software programs, instructions, and modules stored in the memory.

[0082] The memory 22 may include a program storage area and a training storage area. The program storage area may store the operating system and applications required for at least one function; the training storage area may store training data created by the processor. Furthermore, the memory may include high-speed random access memory and non-transitory memory, such as at least one disk storage device, flash memory, or other non-transitory solid-state storage device. In some embodiments, the memory 22 may optionally include memory remotely located relative to the processor, which can be connected to the processor via a network. Examples of such networks include, but are not limited to, the Internet, intranets, local area networks, mobile communication networks, and combinations thereof. The one or more modules stored in the memory 22, when executed by the processor 21, perform functions such as... Figure 1 The midpoint potential balance control method in Example 1 is shown. For specific details of the above midpoint potential balance control method, please refer to the relevant documentation. Figure 1 , Figure 2 and Figure 3 The relevant descriptions and effects in the embodiments shown are for reference only and will not be repeated here.

[0083] This embodiment also provides a computer storage medium storing a computer program that can be executed by a processor to perform the midpoint potential balance control method described in Embodiment 1.

[0084] The computer storage medium stores computer-executable instructions, which can execute the midpoint potential balance control method in any of the above method embodiments. The storage medium can be a magnetic disk, optical disk, read-only memory (ROM), random access memory (RAM), flash memory, hard disk drive (HDD), or solid-state drive (SSD), etc.; the storage medium may also include combinations of the above types of memory.

[0085] The specific steps of the midpoint potential balance control method are described in Example 1, and will not be repeated in this example.

[0086] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for controlling midpoint potential balance, characterized in that, include: The original three-phase modulation signal of the sampled inverter is injected with three zero-sequence components to obtain the three-phase reference voltage signal. A corrected voltage modulation signal is constructed based on the three-phase reference voltage signal; The three-phase output current of the inverter is sampled and matched with the amplitude of the corrected voltage modulation signal in each carrier cycle to obtain a matched current signal; The midpoint potential is obtained, and based on the midpoint potential and the corresponding current signal, the zero-sequence component is compensated through a preset controller. The compensated zero-sequence component is injected into the three-phase reference voltage signal to achieve balanced control of the inverter's midpoint potential.

2. The midpoint potential balance control method according to claim 1, characterized in that, The original three-phase modulation signal of the sampling inverter is sampled, and three zero-sequence components are injected into the original three-phase modulation signal to obtain a three-phase reference voltage signal, specifically including: The original three-phase modulation signal is: ； Three zero-sequence components are injected into the original three-phase modulation signal. Then, the three-phase reference voltage signal is obtained. as follows: ， ； in, The original three-phase voltage modulation signal. The phase voltage amplitude, Let be the angular frequency, and a, b, and c correspond to the three phases.

3. The midpoint potential balance control method according to claim 1, characterized in that, The construction of the corrected voltage modulation signal based on the three-phase reference voltage signal specifically includes: For injecting zero-sequence components The three-phase reference voltage signal obtained afterwards The modified voltage modulation signal is constructed by dividing the voltage into three phases: the maximum voltage phase, the intermediate voltage phase, and the minimum voltage phase, within each carrier cycle. Represented as: ； in, The phase with the maximum voltage. This is the voltage midpoint phase. The phase with the minimum voltage is a, b, and c, which correspond to the three phases.

4. The midpoint potential balance control method according to claim 1, characterized in that, The sampling inverter's three-phase output current is matched with the amplitude of the corrected voltage modulation signal in each carrier cycle to obtain a matched current signal, specifically including: The three-phase output current of the inverter is respectively matched to the maximum voltage phase, the intermediate voltage phase, and the minimum voltage phase of the corrected voltage modulation signal to obtain the instantaneous current value corresponding to each amplitude level. The current signal includes the instantaneous current value. The three-phase output current of the inverter... , , Represented as: ； The inverter's three-phase output current is mapped to the maximum value phase, the intermediate value phase, and the minimum value phase of the corrected voltage modulation signal to obtain: ； in, This is the peak value of the phase current. The power factor angle, The maximum value of the three-phase reference voltage The corresponding instantaneous value of the phase current, The intermediate value of the three-phase reference voltage The corresponding instantaneous value of the phase current, The minimum value of the three-phase reference voltage phase The corresponding instantaneous value of the phase current.

5. The midpoint potential balance control method according to claim 4, characterized in that, The process of obtaining the compensated zero-sequence component based on the midpoint potential and the corresponding current signal through a preset controller specifically includes: The midpoint potential is input to the preset controller to obtain the midpoint fluctuation current. Combined with the current corresponding to the minimum original modulation signal voltage value in the matched current signal, the compensated zero-sequence component is obtained after the sign function is used to determine the zero-sequence component. Compensation zero-sequence component The expression is: ； in, For the proportional term coefficient, Here, s represents the coefficients of the integral term, and s represents the complex variables of the Laplace transform. For a sign function, when hour, ;when hour, , Midpoint potential The intermediate value of the three-phase reference voltage The corresponding current signal.

6. The midpoint potential balance control method according to claim 4, characterized in that, The three-phase reference voltage signal after injecting compensation for the zero-sequence component must meet the requirements of the linear modulation region, and the injection lower limit of the compensation for the zero-sequence component must be met. for: ； Upper limit of injection for compensating zero-order components for: ； in, This is the DC bus voltage. This is the three-phase reference voltage signal.

7. The midpoint potential balance control method according to claim 1, characterized in that, Injecting a compensated zero-sequence component into the three-phase reference voltage signal Average midpoint current after Represented as: ； in, This is the DC bus voltage. The three-phase reference voltage signal, , , This refers to the three-phase output current of the inverter.

8. The midpoint potential balance control method according to claim 1, characterized in that, The acquisition of the midpoint potential specifically includes: The upper and lower capacitor voltages of the DC bus are sampled and obtained. The difference between the voltage of the upper capacitor and the voltage of the lower capacitor is calculated to obtain the midpoint potential.

9. A midpoint potential balance control device, characterized in that, The midpoint potential balance control device includes: a processor and a memory for storing processor-executable instructions; The processor is configured to execute the midpoint potential balance control method according to any one of claims 1-8.

10. A non-volatile computer storage medium, characterized in that, The computer storage medium stores computer-executable instructions, which are executed by one or more processors to perform the midpoint potential balance control method according to any one of claims 1-8.

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