CN111082689A - Neutral point voltage balance control method for neutral point clamped three-level inverter - Google Patents
Neutral point voltage balance control method for neutral point clamped three-level inverter Download PDFInfo
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- CN111082689A CN111082689A CN201911380475.7A CN201911380475A CN111082689A CN 111082689 A CN111082689 A CN 111082689A CN 201911380475 A CN201911380475 A CN 201911380475A CN 111082689 A CN111082689 A CN 111082689A
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/483—Converters with outputs that each can have more than two voltages levels
- H02M7/487—Neutral point clamped inverters
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Abstract
The invention provides a neutral point voltage balance control method of a neutral point clamped three-level inverter, S1, according to the direction of three-phase current of a load, determining neutral point voltage of a load circuit and neutral point current corresponding to the neutral point voltage, and selecting a redundant vector distribution coefficient beneficial to neutral point voltage balance; and S2, changing the action time distribution of the redundant vector by adjusting the distribution coefficient of the redundant vector to realize the control of the midpoint balance voltage. The midpoint voltage balance control method of the midpoint clamping type three-level inverter calculates the midpoint current required by the required midpoint voltage balance by using the midpoint voltage deviation, and then realizes the control of the midpoint voltage balance by accurately adjusting the action time distribution of the small vector pair, namely adjusting the distribution factor of the redundant vector pair.
Description
Technical Field
The invention belongs to the field of power electronic control, and particularly relates to a neutral point voltage balance control method of a neutral point clamped three-level inverter.
Background
The neutral point clamped three-level inverter obtains the neutral point through two capacitors on the direct current side, under an ideal condition, the voltage on each capacitor is half of the voltage of a direct current bus, but the voltages of the two capacitors on the direct current bus have deviation due to factors such as capacitance value errors of the capacitors, inconsistent switching devices, three-phase unbalanced operation and the like, so that the neutral point voltage of the direct current bus is unbalanced, the output waveform is distorted, and the output low-order harmonic content is increased. The shift of the midpoint potential can increase the voltage stress of the device, causing overvoltage damage of the device.
Currently, common methods for midpoint voltage balance control include a hardware method and a modulation method. The hardware method mainly comprises the following four steps: 1. two independent direct current voltage sources are adopted for power supply, and a multi-tap transformer and two three-phase rectification circuits are usually adopted for power supply. 2. The method is characterized in that an additional converter is used for injecting or extracting current to the midpoint, and the method controls the on-off of a power switch tube of the additional converter according to the requirement of midpoint balance control, injects or extracts current to the midpoint and counteracts the influence of load current on midpoint balance. 3. And a three-level Boost circuit is connected to the direct current side of the NPC three-level inverter, and the Boost circuit is used for controlling the current flowing into or out of the midpoint and controlling the voltage balance of the midpoint. According to the method, the neutral point voltage balance control is realized by adding the hardware circuit, so that the hardware cost of the system is increased, and the neutral point voltage control effect is poor when the working state of the inverter changes.
Disclosure of Invention
In view of this, the present invention is directed to a midpoint voltage balance control method for a midpoint clamping type three-level inverter, which selects a redundant vector switching state favorable for midpoint voltage balance according to the direction of three-phase current of a load, and realizes midpoint voltage balance control by accurately adjusting the action time distribution of the redundant vector.
In order to achieve the purpose, the technical scheme of the invention is realized as follows:
a neutral point voltage balance control method of a neutral point clamped three-level inverter comprises the following steps:
s1, determining the midpoint voltage of the load circuit and the midpoint current corresponding to the midpoint voltage according to the direction of the three-phase current of the load, and selecting a redundant vector distribution coefficient beneficial to midpoint voltage balance;
and S2, changing the action time distribution of the redundant vector by adjusting the distribution coefficient of the redundant vector to realize the control of the midpoint balance voltage.
Further, the method for calculating the midpoint current in S1 is as follows: setting the DC bus voltage u with half of the capacitor voltage at the initial timedcAnd/2, the midpoint current is io, the voltages of the capacitors C1 and C2 are
In the formula uc1And uc2Respectively an upper bridge arm capacitance voltage and a lower bridge arm capacitance voltage, C is a bus capacitance value i1And i2Respectively an upper bridge arm capacitance current and a lower bridge arm capacitance current, and t is action time.
The deviation of the two DC capacitor voltages in each control cycle is
The voltages of the capacitors C1 and C2 are collected at the beginning of the switching period, and the midpoint current required for midpoint voltage balance can be calculated as
io＝-CΔuc/Ts
Further, at TsIn the control period, load three-phase current ia、ib、icAnd keeping the current constant, and adjusting the current flowing through the midpoint in the switching period by distributing the action time of the redundancy vector.
Further, the redundant vectors comprise positive redundant vectors and negative redundant vectors, and the time distribution coefficient of the positive redundant vectors is set to be kbThen the negative redundancy vector time distribution coefficient is 1-kbWherein k isb∈[0,1]。
Further, in the pulse width modulation method without adding the midpoint voltage balance control, the action time of the positive redundant vector and the action time of the negative redundant vector are equal, and each of the positive redundant vector and the negative redundant vector occupies 1/2 of the duty ratio of the small vector, namely kb＝0.5；
According to the calculated midpoint current value, the middle redundancy vector distribution coefficient of each region can be obtained as
In the formula (d)sFor negative redundant vector duty cycle, isvCurrent corresponding to negative redundancy vector, drsvFor positive redundant vector duty cycle, irsvCurrent corresponding to a positive redundancy vector, dmIs the medium vector duty cycle, imThe current corresponding to the medium vector.
Furthermore, after the midpoint voltage balance control is added in the pulse width modulation, the switching sequence is kept unchanged, only the relative action time of the positive redundant vector and the negative redundant vector participating in the midpoint voltage balance control is changed, and k is adjustedbThe magnitude of the midpoint current, and thus the midpoint voltage balance, can be controlled.
Further, the method for solving the deviation of the midpoint voltage from the equilibrium point position occurring in S1 is as follows: by applying appropriate midpoint currents or charges, so that during a control period TsThe voltage of the inner midpoint changes to the balance point as much as possible;
further, the method for solving the problem of unequal initial voltages of the capacitors occurring in S1 is as follows: by applying appropriate midpoint currents or charges, so that during a control period TsThe inner midpoint voltage changes toward the equilibrium point.
Compared with the prior art, the neutral point voltage balance control method of the neutral point clamped three-level inverter has the following advantages:
the midpoint voltage balance control method of the midpoint clamping type three-level inverter calculates the midpoint current required by the required midpoint voltage balance by using the midpoint voltage deviation, and then realizes the control of the midpoint voltage balance by accurately adjusting the action time distribution of the small vector pair, namely adjusting the distribution factor of the redundant vector pair.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with the description, serve to explain the invention and not to limit the invention. In the drawings:
fig. 1 is a schematic diagram illustrating charging and discharging of a dc bus capacitor according to a midpoint voltage balance control method of a midpoint clamping type three-level inverter according to an embodiment of the present invention;
fig. 2 is a schematic diagram of an action sequence of an electric voltage balance control vector in the method for controlling the midpoint voltage balance of the midpoint clamping type three-level inverter according to the embodiment of the invention.
Detailed Description
It should be noted that the embodiments and features of the embodiments may be combined with each other without conflict.
In the description of the present invention, it is to be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", and the like, indicate orientations or positional relationships based on those shown in the drawings, and are used only for convenience in describing the present invention and for simplicity in description, and do not indicate or imply that the referenced devices or elements must have a particular orientation, be constructed and operated in a particular orientation, and thus, are not to be construed as limiting the present invention. Furthermore, the terms "first", "second", etc. are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first," "second," etc. may explicitly or implicitly include one or more of that feature. In the description of the present invention, "a plurality" means two or more unless otherwise specified.
In the description of the present invention, it should be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meaning of the above terms in the present invention can be understood by those of ordinary skill in the art through specific situations.
The present invention will be described in detail below with reference to the embodiments with reference to the attached drawings.
As shown in fig. 1 and fig. 2, a method for controlling the midpoint voltage balance of a midpoint clamping type three-level inverter includes the following steps:
s1, determining the midpoint voltage of the load circuit and the midpoint current corresponding to the midpoint voltage according to the direction of the three-phase current of the load, and selecting a redundant vector distribution coefficient beneficial to midpoint voltage balance;
and S2, changing the action time distribution of the redundant vector by adjusting the distribution coefficient of the redundant vector to realize the control of the midpoint balance voltage.
The method for calculating the midpoint current in S1 is as follows: setting the DC bus voltage u with half of the capacitor voltage at the initial timedcAnd/2, the midpoint current is io, the voltages of the capacitors C1 and C2 are
In the formula uc1And uc2Respectively an upper bridge arm capacitance voltage and a lower bridge arm capacitance voltage, C is a bus capacitance value i1And i2Respectively an upper bridge arm capacitance current and a lower bridge arm capacitance current, and t is action time.
The deviation of the two DC capacitor voltages in each control cycle is
The voltages of the capacitors C1 and C2 are collected at the beginning of the switching period, and the midpoint current required for midpoint voltage balance can be calculated as
io＝-CΔuc/Ts
At TsIn the control period, load three-phase current ia、ib、icAnd keeping the current constant, and adjusting the current flowing through the midpoint in the switching period by distributing the action time of the redundancy vector.
The redundancy vector comprises a positive redundancy vector and a negative redundancy vector, and the time distribution coefficient of the positive redundancy vector is set to be kbThen the negative redundancy vector time distribution coefficient is 1-kbWherein k isb∈[0,1]。
In the pulse width modulation method without adding the midpoint voltage balance control, the action time of the positive redundant vector and the action time of the negative redundant vector are equal, and each of the positive redundant vector and the negative redundant vector occupies 1/2 of the duty ratio of the small vector, namely kb＝0.5；
According to the calculated midpoint current value, the middle redundancy vector distribution coefficient of each region can be obtained as
In the formula (d)sFor negative redundant vector duty cycle, isvCurrent corresponding to negative redundancy vector, drsvFor positive redundant vector duty cycle, irsvCurrent corresponding to a positive redundancy vector, dmIs the medium vector duty cycle, imThe current corresponding to the medium vector.
After the midpoint voltage balance control is added in the pulse width modulation, the switching sequence is kept unchanged, only the relative action time of the positive redundant vector and the negative redundant vector participating in the midpoint voltage balance control is changed, and k is adjustedbThe magnitude of the midpoint current, and thus the midpoint voltage balance, can be controlled.
The method for solving the deviation of the midpoint voltage from the equilibrium point position occurring in S1 is as follows: by applying appropriate midpoint currents or charges, so that during a control period TsThe voltage of the inner midpoint changes to the balance point as much as possible;
the method for solving the problem of unequal initial voltages of the capacitors generated in the step S1 is as follows: by applying appropriate midpoint currents or charges, so that during a control period TsThe inner midpoint voltage changes toward the equilibrium point.
FIG. 1 shows DC bus capacitance and midpoint currentAssuming that the initial capacitor voltage is half of the DC bus voltage udcAnd/2, the midpoint current is io, the voltages of the capacitors C1 and C2 are
In the formula uc1And uc2Respectively an upper bridge arm capacitance voltage and a lower bridge arm capacitance voltage, C is a bus capacitance value i1And i2Respectively an upper bridge arm capacitance current and a lower bridge arm capacitance current, and t is action time.
The deviation of the two DC capacitor voltages in each control cycle is
The voltages of the capacitors C1 and C2 are collected at the beginning of the switching period, and the midpoint current required for midpoint voltage balance can be calculated as
io＝-CΔuc/Ts(3)
Suppose that in the Ts control period, three-phase current i is loadeda、ib、icThe current flowing through the midpoint in the switching cycle can be adjusted by assigning the action time of the redundancy vector, which remains unchanged. Dividing the redundant vector into a positive redundant vector and a negative redundant vector, and setting the time distribution coefficient of the positive redundant vector as kb and the time distribution coefficient of the negative redundant vector as 1-kbWherein k isb∈[0,1]. In the pulse width modulation method without adding the midpoint voltage balance control, the action time of the positive and negative small vectors is equal, and each takes 1/2 of the duty ratio of the small vectors, namely kb0.5. According to the calculated midpoint current value, the middle redundancy vector distribution coefficient of each region can be obtained as
In the formula (d)sFor negative redundant vector duty cycle, isvCurrent corresponding to negative redundancy vector，drsvFor positive redundant vector duty cycle, irsvCurrent corresponding to a positive redundancy vector, dmIs the medium vector duty cycle, imThe current corresponding to the medium vector.
After the pulse width modulation method is added with the midpoint voltage balance control, the switching sequence is kept unchanged, and only the relative action time of positive and negative small vectors participating in the midpoint voltage balance control is changed. As can be seen from the foregoing analysis, by adjusting kbThe magnitude of the midpoint current, and thus the midpoint voltage balance, can be controlled. Meanwhile, as can be seen from the previous analysis, kbIs in the value range of [0,1 ]]When k isbIs out of [0,1 ]]In the range, the midpoint current cannot be controlled by adjusting the relative action time of the positive and negative small vectors, so that the midpoint voltage balance cannot be controlled.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.
Claims (8)
1. A neutral point voltage balance control method of a neutral point clamped three-level inverter is characterized by comprising the following steps:
s1, determining the midpoint voltage of the load circuit and the midpoint current corresponding to the midpoint voltage according to the direction of the three-phase current of the load, and selecting a redundant vector distribution coefficient beneficial to midpoint voltage balance;
and S2, changing the action time distribution of the redundant vector by adjusting the distribution coefficient of the redundant vector to realize the control of the midpoint balance voltage.
2. The method of claim 1, wherein the method comprises: the method for calculating the midpoint current in S1 is as follows: setting the DC bus voltage u with half of the capacitor voltage at the initial timedcAnd/2, the midpoint current is io, the voltages of the capacitors C1 and C2 are
In the formula uc1And uc2Respectively an upper bridge arm capacitance voltage and a lower bridge arm capacitance voltage, C is a bus capacitance value i1And i2Respectively an upper bridge arm capacitance current and a lower bridge arm capacitance current, and t is action time.
The deviation of the two DC capacitor voltages in each control cycle is
The voltages of the capacitors C1 and C2 are collected at the beginning of the switching period, and the midpoint current required for midpoint voltage balance can be calculated as
io＝-CΔuc/Ts。
3. The method of claim 2, wherein the method comprises: at TsIn the control period, load three-phase current ia、ib、icAnd keeping the current constant, and adjusting the current flowing through the midpoint in the switching period by distributing the action time of the redundancy vector.
4. The method of claim 3, wherein the method comprises: the redundancy vector comprises a positive redundancy vector and a negative redundancy vector, and the time distribution coefficient of the positive redundancy vector is set to be kbThen the negative redundancy vector time distribution coefficient is 1-kbWherein k isb∈[0,1]。
5. The method of claim 4, wherein the method comprises: in the pulse width modulation method without adding midpoint voltage balance control, the action time of positive redundant vector and negative redundant vector is equal, and each small vector occupiesSpace ratio 1/2, i.e. kb＝0.5；
According to the calculated midpoint current value, the middle redundancy vector distribution coefficient of each region can be obtained as
In the formula (d)sFor negative redundant vector duty cycle, isvCurrent corresponding to negative redundancy vector, drsvFor positive redundant vector duty cycle, irsvCurrent corresponding to a positive redundancy vector, dmIs the medium vector duty cycle, imThe current corresponding to the medium vector.
6. The method of claim 4, wherein the method comprises: after the midpoint voltage balance control is added in the pulse width modulation, the switching sequence is kept unchanged, only the relative action time of the positive redundant vector and the negative redundant vector participating in the midpoint voltage balance control is changed, and k is adjustedbThe magnitude of the midpoint current, and thus the midpoint voltage balance, can be controlled.
7. The method of claim 1, wherein the method comprises: the method for solving the deviation of the midpoint voltage from the equilibrium point position occurring in S1 is as follows: by applying appropriate midpoint currents or charges, so that during a control period TsThe inner midpoint voltage is varied as far as possible toward the equilibrium point.
8. The method of claim 1, wherein the method comprises: the method for solving the problem of unequal initial voltages of the capacitors generated in the step S1 is as follows: by applying appropriate midpoint currents or charges, so that during a control period TsThe inner midpoint voltage changes toward the equilibrium point.
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