Neutral point potential control method for NPC four-leg inverter with low switching frequency

Abstract

The present invention discloses a midpoint potential control method of an NPC four-bridge arm inverter with low switching frequency, comprising the following steps: S1, obtaining a three-phase reference voltage, comparing the three-phase reference voltage with a neutral bridge arm reference voltage, and obtaining the maximum and minimum values ​​of the four reference voltages; S2, establishing two positron modulation waves of each bridge arm, deriving a general expression of the two positron modulation waves of each bridge arm, and calculating the optimal bias voltage and the two optimal positron modulation waves of each bridge arm; S3, calculating the modulation wave adjustment value of the midpoint current adjustable bridge arm; S4, obtaining the adjusted two positron modulation waves of each bridge arm according to the two optimal positron modulation waves of each bridge arm and the modulation wave adjustment value; S5, comparing with a single carrier to generate a pulse signal for controlling the on and off of a power switch tube. The present invention adopts the above-mentioned midpoint potential control method, and has a lower switching frequency and similar midpoint potential low-frequency oscillation control capability.

Description

Technical Field

[0001] The present invention relates to the field of electric power and electronic technology, and in particular to a method for controlling a midpoint potential of an NPC four-bridge-arm inverter with a low switching frequency. Background Art

[0002] As an AC power source, the inverter can be used to provide high-quality output power to the load. With the increase in power levels and the emergence and application of a large number of single-phase, unbalanced and nonlinear loads, the neutral point clamped (NPC) four-leg inverter has become a structure with broad application prospects in high-power unbalanced systems.

[0003] The low-frequency oscillation and offset of the midpoint potential are inherent problems of the NPC three-level inverter and are two main aspects of the midpoint potential control. First, the low-frequency oscillation of the midpoint potential will increase the voltage stress of the power switch tube and the capacitance of the capacitor, and generate low-order harmonics in the output voltage; second, the midpoint potential offset will cause the midpoint potential to deviate from the equilibrium value, resulting in the DC side voltage being added to one capacitor, which will eventually damage the capacitor and the power switch tube. Therefore, the midpoint potential is the main control target of the NPC three-level inverter. At present, the modulation methods for the midpoint potential control of the NPC three-leg inverter are mainly divided into space vector modulation and carrier modulation. In the space vector modulation method, the amplitude of the low-frequency oscillation of the midpoint potential can be reduced by adjusting the duty cycle of the redundant small vector. When the control ability of the small vector is insufficient, the low-frequency oscillation of the midpoint potential is generally completely eliminated by constructing a virtual vector. In the carrier modulation method, the amplitude of the low-frequency oscillation of the midpoint potential is reduced by injecting a zero-sequence component into the modulation wave. The dual modulation wave and dual carrier method in the carrier modulation method can completely eliminate the low-frequency oscillation of the midpoint potential. Since the topology of the four-arm inverter is different from that of the three-arm inverter, and the modulation strategy and method are directly related to the inverter topology, the above three-arm modulation method cannot be directly used in the four-arm inverter.

[0004] The NPC four-bridge inverter can provide a neutral current flow path and has a strong load capacity. However, due to its complex structure and large number of vectors, current research on it mainly focuses on algorithm implementation and structural simplification, and there is less research on its performance improvement. The NPC four-bridge space vector modulation method is divided into two types: based on abc Natural coordinate system and based on avg There are two coordinate systems. LG Franquelo et al. proposed a abc More literature research is based on avg F. Rojas et al. proposed two methods to simplify the process of identifying the position of small tetrahedrons. avgCoordinate system algorithm. Shao Hongjun et al. proposed a dimensionality reduction modulation strategy. First, the virtual space vector modulation method is applied to the first three bridge arms to obtain the phase duty ratio of the first three bridge arms. Then, according to the three-phase output voltage symmetry and zero average midpoint current constraints, the phase duty ratio of the neutral bridge arm is determined. M. Bouzidi et al. proposed a method based on rst Coordinate system space vector modulation method, space vector in rst The expression in the coordinate system is simpler. When judging the spatial position of the reference vector, the tetrahedron judgment step is omitted, thereby simplifying the modulation step. At present, there are few research papers on the NPC four-arm carrier modulation method. J. Kim et al. studied an optimal sequence carrier modulation method suitable for NPC four-arm converters. By adding a suitable bias voltage, the optimal switching sequence can be obtained, thereby reducing the harmonic content in the output voltage. However, this method does not consider the midpoint potential control problem. Chen Weiqiang et al. proposed an optimal sequence carrier modulation method with midpoint potential balance control capability. While maintaining the lowest harmonic content, it can reduce the amplitude of the low-frequency oscillation of the midpoint potential. M. Tsai et al. proposed a three-dimensional neutral-point oscillation elimination PWM method (3-D NPOEPWM for short). This method obtains two modulation waves with correction values ​​by judging the positive and negative values ​​of the terminal voltage, which can completely eliminate the low-frequency oscillation of the midpoint potential. This method also directly gives the terminal voltages of the four bridge arms. Although it can ensure that one of the four bridge arms is switched once in one switching cycle, the other three bridge arms are switched twice each, and the switching frequency is high. Summary of the invention

[0005] The object of the present invention is to provide a method for controlling the midpoint potential of an NPC four-bridge-arm inverter with a low switching frequency, so as to solve the problems mentioned in the background technology.

[0006] To achieve the above object, the present invention provides a method for controlling the midpoint potential of an NPC four-bridge-arm inverter with a low switching frequency, comprising the following steps:

[0007] S1. Obtain a three-phase reference voltage, compare the three-phase reference voltage with a neutral bridge arm reference voltage, and obtain a maximum value and a minimum value of four reference voltages;

[0008] S2. Establish two positron modulation waves for each bridge arm, derive a general expression for the two positron modulation waves for each bridge arm, and calculate an optimal bias voltage and two optimal positron modulation waves for each bridge arm;

[0009] S3, calculating the modulation wave adjustment value of the midpoint current adjustable bridge arm;

[0010] S4, obtaining two adjusted positron modulation waves of each bridge arm according to the two optimal positron modulation waves of each bridge arm and the modulation wave adjustment value;

[0011] S5, compare with the single carrier to generate a pulse signal to control the on and off of the power switch tube.

[0012] Preferably, the S1 specifically includes:

[0013] S11, taking the symmetry of the load three-phase output voltage as the control target, according to the deadbeat control method, avg The three-phase reference voltage in the coordinate system is avg The three-phase reference voltage in the coordinate system is transformed to obtain abc Three-phase reference voltage in the coordinate system;

[0014] S12. Obtain the maximum and minimum values ​​of the four reference voltages using the following calculation formula:

[0015] ;

[0016] in, and Respectively a Mutually, b Harmony c Phase three-phase reference voltage, Represents the neutral bridge arm reference voltage, which is zero; and Respectively represent the maximum and minimum values ​​of the four reference voltages.

[0017] Preferably, the S11 specifically includes:

[0018] sampling The three-phase load voltage, three-phase load current and three-phase inductor current are then obtained through the deadbeat control method. avg The three-phase reference voltage expression in the coordinate system is:

[0019] ;

[0020] in, 、 and Respectively The three-phase reference voltage at the moment α , β and c Axis component; L and C Respectively represent the filter inductor and capacitor; T s Indicates the sampling period; 、 and Respectively k The three-phase load voltage reference value at the moment α , β and c Axis component; 、 and Respectively k The three-phase load voltage sampling value at the moment α , β and c Axis component; 、 and Respectively k The three-phase load current sampling value at the moment α , β and c Axis component; 、 and Respectively k The three-phase inductor current sampling value at the moment α , β and c Axis component;

[0021] Will avg The three-phase reference voltage in the coordinate system is transformed into abc The conversion formula of the reference voltage in the coordinate system is as follows:

[0022] .

[0023] Preferably, the S2 specifically comprises the following steps:

[0024] S21, establish two positron modulation waves for each bridge arm, and the two positron modulation waves are respectively used and Indicates that ,and , ;

[0025] S22. Taking the average midpoint current equal to zero as the constraint condition, derive the general expression of the two positron modulation waves of each bridge arm;

[0026] S23. Taking low switching times as the optimization goal, an optimal bias voltage and two optimal positron modulation waves for each bridge arm are obtained.

[0027] Preferably, the S22 is specifically:

[0028] The output switch state of each bridge arm is set to be generated in the following way: the two positron modulation waves of each bridge arm and Carrier v carrier Compare, add the comparison results and then subtract 1 to equal the switching state of the bridge arm, and then get the relationship between the phase duty cycle of each bridge arm and the two positive sub-modulation waves as follows:

[0029] ;

[0030] in, express i The bridge arm p "The phase duty cycle of the state; express i The bridge arm o "The phase duty cycle of the state; express i Bridge arm Phase duty cycle of the state;

[0031] According to the relationship between the terminal voltage and the reference voltage, the terminal voltages of the four bridge arms are obtained as follows:

[0032] ;

[0033] in, and Respectively a , b , c and The terminal voltages of the four bridge arms are Also called bias voltage;

[0034] According to the volt-second balance principle, we get:

[0035] ;

[0036] in, express The terminal voltage of the bridge arm, and ;

[0037] According to the relationship between the phase duty cycle of each bridge arm and the two positron modulation waves, the relationship between the terminal voltage of each bridge arm and the two positron modulation waves is obtained as follows:

[0038] ;

[0039] According to the relationship between the phase duty cycle of each bridge arm and the two positive sub-modulation waves, the midpoint current is obtained. The relationship with the positron modulation wave is:

[0040] ;

[0041] Since the output currents of the four bridge arms satisfy:

[0042] ;

[0043] Let the four bridge arms o "The duty cycle of the two states is equal, then , then we get:

[0044] ;

[0045] in, Represents the difference between the two positron modulation waves of each bridge arm;

[0046] The general expression of the two positron modulation waves in each bridge arm is obtained by calculation:

[0047] ;

[0048] in, for The reference voltage of the bridge arm, .

[0049] Preferably, the S23 is specifically:

[0050] In each switching cycle, let the maximum positron modulation wave be equal to 1 and the minimum positron modulation wave be equal to 0, and the following formula is obtained:

[0051] ;

[0052] in, represents the largest positron modulation wave; represents the smallest positron modulation wave;

[0053] Then the optimal bias voltage is obtained and the difference between the two positron modulation waves of each bridge arm They are:

[0054] ;

[0055] ;

[0056] Substituting the above two formulas into the general expression of the two positron modulation waves of each bridge arm, the two optimal positron modulation waves of each bridge arm are obtained. and They are:

[0057] .

[0058] Preferably, the specific steps of S3 are:

[0059] S31, defining a midpoint current adjustable bridge arm and a non-adjustable bridge arm;

[0060] S32. Select one bridge arm from another bridge arm with adjustable midpoint current as the adjustment bridge arm, and calculate the " o "State phase duty cycle adjustment value ;

[0061] S33, by adjusting the two positron modulation waves of the regulating bridge arm, the regulating bridge arm " o "The state phase duty cycle adjustment value, and then the relationship between the positive sub-modulation wave adjustment value and the phase duty cycle adjustment value is obtained as follows:

[0062] ;

[0063] in, Indicates the adjustment value of the bridge arm positron modulation wave.

[0064] S31, defining a midpoint current adjustable bridge arm and a non-adjustable bridge arm;

[0065] S32. Select one bridge arm from the two midpoint current adjustable bridge arms as the adjustment bridge arm, and calculate the " o "State phase duty cycle adjustment value ;

[0066] S33, by adjusting the two positron modulation waves of the regulating bridge arm, the regulating bridge arm " o "The state phase duty cycle adjustment value, and then get the relationship between the positive sub-modulation wave adjustment value and the phase duty cycle adjustment value:

[0067] ;

[0068] in, Indicates the adjustment value of the bridge arm positron modulation wave.

[0069] Preferably, S4 is specifically as follows: the two positron modulation waves of the non-adjustable bridge arm are calculated according to the formula of the two optimal positron modulation waves of each bridge arm, and the two positron modulation waves of the adjustable bridge arm are calculated according to the formula of the two optimal positron modulation waves of each bridge arm and then adjusted to obtain the following expression:

[0070] ;

[0071] in, and Represents two positron modulation waves after adjusting the bridge arm.

[0072] Preferably, S5 is specifically as follows: comparing the two positive sub-modulation waves of each bridge arm with the same carrier respectively to obtain the switching state of each bridge arm, and then obtaining four control pulses of each bridge arm according to the on-off relationship between the switching state of the bridge arm and the four power switch tubes of each bridge arm.

[0073] Therefore, the present invention adopts the above-mentioned NPC four-bridge-arm inverter midpoint potential control method with low switching frequency, which has the following beneficial effects:

[0074] (1) Two positron modulation waves are established in each bridge arm, increasing the o "The value range of the phase duty cycle is determined by the state, and there is no need to determine the positive or negative sign of the terminal voltage.

[0075] (2) Taking low switching times as the optimization goal, the optimal bias voltage is obtained, and the “ o "State phase duty cycle suppresses midpoint potential shift.

[0076] (3) By comparing the simulation results with the traditional NPC four-arm inverter midpoint potential control method, the present invention has a lower switching frequency and similar midpoint potential low-frequency oscillation control capability, and has the ability to suppress midpoint potential offset.

[0077] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 Schematic diagram of the topological structure of an NPC four-bridge-arm inverter according to an embodiment of the present invention;

[0079] Figure 2 A structural schematic diagram of a method for generating a bridge arm output switch state according to an embodiment of the present invention;

[0080] Figure 3 For the embodiment of the present invention a A schematic diagram of the generation method of the bridge arm output switch state;

[0081] Figure 4 A schematic diagram of an output state sequence of an adjustable bridge arm and a non-adjustable bridge arm according to an embodiment of the present invention;

[0082] Figure 5 A schematic diagram of the phase duty cycle before and after the adjustment of the bridge arm according to an embodiment of the present invention;

[0083] Figure 6 It is a schematic diagram of adjusting two positron modulation waves of the adjustment bridge arm according to an embodiment of the present invention;

[0084] Figure 7 is a flow chart of a point potential control method in an embodiment of the present invention;

[0085] Figure 8 The steady-state waveform of the 3-D NPOEPWM method with group A load, (a) is the three-phase load voltage, (b) is the three-phase load current, (c) is the voltage of two capacitors on the DC side, and (d) is the driving pulse of the power switch tube on the four bridge arms;

[0086] Figure 9 The steady-state waveform diagram of the embodiment of the present invention with group A load, (a) is the three-phase load voltage, (b) is the three-phase load current, (c) is the voltage of two capacitors on the DC side, and (d) is the driving pulse of the power switch tube of the upper bridge arm of the four bridge arms;

[0087] Figure 10 1 is a transient waveform diagram when the load group B is transformed to the load group A according to an embodiment of the present invention, (a) is the three-phase load voltage, (b) is the three-phase load current, and (c) is the voltage of two capacitors on the DC side;

[0088] Reference numerals

[0089] 1. The bridge arm cannot be adjusted; 2. The bridge arm can be adjusted. DETAILED DESCRIPTION

[0090] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. The components of the embodiments of the present invention generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0091] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0092] Example

[0093] like Figure 1 As shown, the structures of the four bridge arms are the same. a , b and c For the three-phase bridge arm, bridge arm It is a neutral bridge arm. Each bridge arm can output three levels, namely V dc / 2, 0, and - V dc / 2 , divided by the reference value V dc / 2 The per-unit values ​​of the output level are 1, 0 and -1, and the corresponding switch states are " p "," o "and" n ”.

[0094] Reference Figure 7 The present invention provides a method for controlling the midpoint potential of an NPC four-bridge-arm inverter with a low switching frequency, which specifically comprises the following steps:

[0095] S1, obtain the three-phase reference voltage, compare the three-phase reference voltage with the neutral bridge arm reference voltage, and obtain the maximum and minimum values ​​of the four reference voltages. Specifically, the following steps are included:

[0096] S11, taking the symmetry of the load three-phase output voltage as the control target, according to the deadbeat control method, avg The three-phase reference voltage in the coordinate system is avg The three-phase reference voltage in the coordinate system is transformed to obtain abc The three-phase reference voltage in the coordinate system. Specifically:

[0097] sampling k The three-phase load voltage, three-phase load current and three-phase inductor current are calculated using the deadbeat control method. avg Three-phase reference voltage in coordinate system.

[0098] Obtained by deadbeat control avg The three-phase reference voltage expression in the coordinate system is:

[0099] (1)

[0100] in, 、 and Respectively k+1 The three-phase reference voltage at the moment α , β and c Axis component; L and C Respectively represent the filter inductor and capacitor; T s Indicates the sampling period; 、 and Respectively kThe three-phase load voltage reference value at the moment α , β and c Axis component; 、 and Respectively k The three-phase load voltage sampling value at the moment α , β and c Axis component; 、 and Respectively k The three-phase load current sampling value at the moment α , β and c Axis component; 、 and Respectively k The three-phase inductor current sampling value at the moment α , β and c Axis component.

[0101] Will avg The three-phase reference voltage in the coordinate system is obtained by coordinate transformation abc The reference voltage in the coordinate system, the transformation formula is as follows:

[0102] (2)

[0103] in, and Respectively a Mutually, b Harmony c Phase three-phase reference voltage.

[0104] S12. Four reference voltages are formed by the three-phase reference voltages and the neutral bridge arm reference voltage, and the maximum value and the minimum value of the four reference voltages are obtained by calculation.

[0105] The maximum and minimum values ​​of the reference voltage are:

[0106] (3)

[0107] in, Represents the neutral bridge arm reference voltage, which is zero; and Respectively represent the maximum and minimum values ​​of the four reference voltages.

[0108] S2. Establish two positron modulation waves for each bridge arm. Based on the volt-second balance principle and with the average midpoint current equal to zero as the constraint condition, derive the general expression of the two positron modulation waves for each bridge arm. With low switching times as the optimization goal, obtain the optimal bias voltage and the two optimal positron modulation waves for each bridge arm. Specifically:

[0109] S21, establish two positron modulation waves for each bridge arm, and the two positron modulation waves are respectively used and indicates that, and ， .

[0110] S22. Derive the general expression of the two positron modulation waves in each bridge arm.

[0111] The output switch state of each bridge arm is set to be generated in the following way: the two positron modulation waves of each bridge arm and Carrier v carrier Compare, add the comparison results and then subtract 1 to equal the switch state of the bridge arm, such as Figure 2 As shown, according to the above settings, the relationship between the phase duty cycle of each bridge arm and the two positive sub-modulation waves is:

[0112] (4)

[0113] in, express i The bridge arm p "The phase duty cycle of the state; express i The bridge arm o "The phase duty cycle of the state; express i The bridge arm n "The phase duty cycle of the state. The above carrier v carrier is a triangle wave between [0,1], and the comparison logic is: (or ) is greater than or equal to the carrier, the comparison result is 1, otherwise the comparison result is 0.

[0114] According to the relationship between the terminal voltage and the reference voltage, the terminal voltages of the four bridge arms are obtained:

[0115] (5)

[0116] in, and Respectively a , b ,c and The terminal voltages of the four bridge arms are Also called bias voltage.

[0117] According to the volt-second balance principle, we have

[0118] (6)

[0119] in, express The terminal voltage of the bridge arm, and .

[0120] From formulas (4) and (6), the relationship between the terminal voltage of each bridge arm and the two positive sub-modulation waves is obtained as follows:

[0121] (7)

[0122] Figure 3 express a The terminal voltage of the bridge arm and the two positron modulation waves of the bridge arm.

[0123] Among them, the midpoint current The relationship with the positron modulation wave is:

[0124] (8)

[0125] The output current of the four bridge arms meets:

[0126] (9)

[0127] in, Respectively a , b , c and Output current of the four bridge arms.

[0128] Let the four bridge arms o "The duty cycle of the two states is equal, so that the ,Right now

[0129] (10)

[0130] in Represents the difference between the two positron modulation waves of each bridge arm.

[0131] From formulas (5), (7) and (10), we can obtain:

[0132] (11)

[0133] The general expression of the two positron modulation waves in each bridge arm is obtained from formula (11):

[0134] (12)

[0135] in, for The reference voltage of the bridge arm, .

[0136] S23. Taking the low switching number as the constraint condition, the optimal bias voltage and two optimal positron modulation waves of each bridge arm are derived.

[0137] In each switching cycle, let the maximum positron modulation wave be equal to 1 and the minimum positron modulation wave be equal to 0, that is,

[0138] (13)

[0139] in, represents the largest positron modulation wave; represents the minimum positron modulation wave, and its expressions are:

[0140] (14)

[0141] The optimal bias voltage is obtained from formula (13): and They are:

[0142] (15)

[0143] (16)

[0144] Substituting equations (15) and (16) into equation (12), we can obtain two optimal positron modulation waves for each bridge arm:

[0145] (17)

[0146] S3, calculate the modulation wave adjustment value of the midpoint current adjustable bridge arm 2. The specific steps are:

[0147] S31, define the midpoint current adjustable bridge arm 2 and the non-adjustable bridge arm 1. Specifically:

[0148] The midpoint current adjustable bridge arm 2 refers to the bridge arm in which both positive sub-modulation waves are neither 1 nor 0, and the output state sequence of this bridge arm is " p ” → “ o ” → “ n ” → “ o ” → “ p ". The midpoint current cannot be adjusted. The bridge arm 1 refers to the bridge arm in which one of the two positive sub-modulation waves is 1 or 0. The output state sequence of this bridge arm is " p” → “ o ” → “ p "or" o ” → “ n ” → “ o ". There are two adjustable bridge arms 2 and two non-adjustable bridge arms 1 in the four bridge arms. Figure 4 It is the output state sequence of the adjustable bridge arm 2 and the non-adjustable bridge arm 1.

[0149] S32, calculate the midpoint current to adjust the bridge arm 2 o "State phase duty cycle adjustment value, specifically:

[0150] Select one bridge arm from the two midpoint current adjustable bridge arms 2 as the adjustment bridge arm, and the relationship between the phase duty ratios of the adjustment bridge arm before and after adjustment is:

[0151] (18)

[0152] in, d mid_p , d mid_o and d mid_n Respectively represent the " p "," o "and" n "The phase duty cycle of the state; d mid_p ' , d mid_o ' and d mid_n ' Respectively represent the " p "," o "and" n "The phase duty cycle of the state; Indicates the adjustment bridge arm " o "State phase duty cycle adjustment value.

[0153] like Figure 5 , sample the voltage of the upper and lower capacitors on the DC side, so that the midpoint current generated by the regulating bridge arm is equal to the midpoint compensation current, and get The expression is:

[0154] (19)

[0155] in, Indicates the midpoint compensation current; Indicates the output current of the regulating bridge arm; C1 and C 2Respectively represent the capacitance values ​​of the upper and lower capacitors on the DC side; and Respectively represent the voltage of the upper and lower capacitors on the DC side.

[0156] According to formula (19), The following constraints must be met:

[0157] (20)

[0158] S33, by adjusting the two positron modulation waves of the regulating bridge arm, the above-mentioned regulating bridge arm " o "The relationship between the state phase duty cycle adjustment value, the positron modulation wave adjustment value and the phase duty cycle adjustment value is:

[0159] (twenty one)

[0160] in, Indicates the adjustment value of the bridge arm positron modulation wave.

[0161] After adjustment Move Down , Move Up ,like Figure 6 shown.

[0162] S4, according to the two optimal positron modulation waves of each bridge arm and the modulation wave adjustment value, obtain the two adjusted positron modulation waves of each bridge arm. Specifically:

[0163] The two positron modulation waves of the non-adjusted bridge arm are calculated according to formula (17), and the two positron modulation waves of the adjusted bridge arm are calculated according to formula (17) and then adjusted to obtain the expression:

[0164] (twenty two)

[0165] in, and Represents two positron modulation waves after adjusting the bridge arm.

[0166] S5, compare with the single carrier to generate a pulse signal to control the on and off of the power switch tube, specifically:

[0167] The two positron modulation waves of each bridge arm are compared with the same carrier wave to obtain the switching state of each bridge arm. Then, according to the on-off relationship between the switching state of the bridge arm and the four power switch tubes of each bridge arm, four control pulses of each bridge arm are obtained to complete the control.

[0168] The present invention uses Matlab / Simulink simulation software to compare and analyze the proposed method with the 3-D NPOEPWM method. C1=C2 =470μF, filter inductor L =2mH, filter capacitor C =40μF, fundamental frequency f =50Hz, carrier frequency f s =10kHz, group A load: the three-phase load resistances are 80Ω, 10Ω, 30Ω respectively, group B load: the three-phase load resistances are 30Ω, 10Ω, 10Ω respectively, and the amplitude of the three-phase load voltage is 70V.

[0169] from Figure 8 (a), (b) and Figure 9 (a) and (b) show that both methods can output three-phase symmetrical voltage under unbalanced load conditions. Since the load resistance is unbalanced, the three-phase load current is asymmetrical. Figure 8 (c) and Figure 9 (c) It can be seen that both methods can eliminate the low-frequency oscillation of the mid-point potential, but the 3-D NPOEPWM method produces a mid-point potential offset, while the proposed method can suppress the mid-point potential offset. Figure 8 (d) and Figure 9 (d) is the driving pulse of 8 power switch tubes in the four bridge arms of the two methods. In one fundamental wave cycle, the 8 power switch tubes of the 3-D NPOEPWM method are turned on and off 1400 times, and the proposed method is turned on and off 1200 times, which can reduce the number of switching times, thereby reducing switching losses and improving efficiency.

[0170] like Figure 10 As shown in the figure, at 0.06s, the load is switched from group B to group A. As can be seen from the figure, the proposed method has good transient performance, can quickly transition to a new stable state when the load changes, can maintain the symmetry of the three-phase load voltage, and eliminate the low-frequency oscillation and offset of the midpoint potential.

[0171] Therefore, the present invention adopts the above-mentioned NPC four-arm inverter midpoint potential control method with low switching frequency, has lower switching frequency and similar midpoint potential low-frequency oscillation control capability, and has midpoint potential deviation suppression capability.

[0172] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for controlling the midpoint potential of an NPC four-bridge-arm inverter with a low switching frequency, characterized in that: The following steps are involved: S1. Obtain a three-phase reference voltage, compare the three-phase reference voltage with a neutral bridge arm reference voltage, and obtain a maximum value and a minimum value of four reference voltages; S2. Establish two positron modulation waves for each bridge arm, derive a general expression for the two positron modulation waves for each bridge arm, and calculate an optimal bias voltage and two optimal positron modulation waves for each bridge arm; S3, calculating the modulation wave adjustment value of the midpoint current adjustable bridge arm; S4, obtaining two adjusted positron modulation waves of each bridge arm according to the two optimal positron modulation waves of each bridge arm and the modulation wave adjustment value; S5, compare with the single carrier to generate a pulse signal for controlling the on and off of the power switch tube; The S2 specifically includes the following steps: S21, establish two positron modulation waves for each bridge arm, and the two positron modulation waves are respectively used and Indicates that ,and , ; S22. Taking the average midpoint current equal to zero as the constraint condition, derive the general expression of the two positron modulation waves of each bridge arm; S23, taking low switching times as the optimization goal, obtaining the optimal bias voltage and two optimal positron modulation waves for each bridge arm; The S22 is specifically: The output switch state of each bridge arm is set to be generated in the following way: the two positron modulation waves of each bridge arm and Carrier Compare, add the comparison results and then subtract 1 to equal the switching state of the bridge arm, and then get the relationship between the phase duty cycle of each bridge arm and the two positive sub-modulation waves as follows: ； in, express Bridge arm "The phase duty cycle of the state; express Bridge arm "The phase duty cycle of the state; express Bridge arm "The phase duty cycle of the state; According to the relationship between the terminal voltage and the reference voltage, the terminal voltages of the four bridge arms are obtained as follows: ； in, and Respectively a , b , c and The terminal voltages of the four bridge arms are Also called bias voltage; According to the volt-second balance principle, we get: ； in, express The terminal voltage of the bridge arm, and ; According to the relationship between the phase duty cycle of each bridge arm and the two positron modulation waves, the relationship between the terminal voltage of each bridge arm and the two positron modulation waves is obtained as follows: ； According to the relationship between the phase duty cycle of each bridge arm and the two positive sub-modulation waves, the midpoint current is obtained. The relationship with the positron modulation wave is: ； Since the output currents of the four bridge arms satisfy: ； Let the four bridge arms" o "The duty cycle of the two states is equal, then , then we get: ； in, Represents the difference between the two positron modulation waves of each bridge arm; The general expression of the two positron modulation waves in each bridge arm is obtained by calculation: ； in, for The reference voltage of the bridge arm, ; The S23 is specifically: In each switching cycle, let the maximum positron modulation wave be equal to 1 and the minimum positron modulation wave be equal to 0, and the following formula is obtained: ； in, represents the largest positron modulation wave; represents the smallest positron modulation wave; Then the optimal bias voltage is obtained and the difference between the two positron modulation waves of each bridge arm They are: ； ； Substituting the above two formulas into the general expression of the two positron modulation waves of each bridge arm, the two optimal positron modulation waves of each bridge arm are obtained. and They are: 。 2. The method for controlling the midpoint potential of an NPC four-bridge-arm inverter with a low switching frequency according to claim 1, characterized in that: The S1 specifically includes: S11, taking the symmetry of the load three-phase output voltage as the control target, according to the deadbeat control method, The three-phase reference voltage in the coordinate system is The three-phase reference voltage in the coordinate system is transformed to obtain abc Three-phase reference voltage in the coordinate system; S12. Obtain the maximum and minimum values ​​of the four reference voltages using the following calculation formula: ； in, and Respectively a Mutually, b Harmony c Phase three-phase reference voltage, Represents the neutral bridge arm reference voltage, which is zero; and Respectively represent the maximum and minimum values ​​of the four reference voltages.

3. The method for controlling the midpoint potential of an NPC four-bridge-arm inverter with a low switching frequency according to claim 2, characterized in that: The S11 specifically includes: sampling k The three-phase load voltage, three-phase load current and three-phase inductor current are then obtained through the deadbeat control method. The three-phase reference voltage expression in the coordinate system is: ； in, 、 and Respectively The three-phase reference voltage at the moment α , β and γ Axis component; L and C Respectively represent the filter inductor and capacitor; T s Indicates the sampling period; 、 and Respectively k The three-phase load voltage reference value at the moment α , β and γ Axis component; 、 and Respectively k The three-phase load voltage sampling value at the moment α , β and γ Axis component; 、 and Respectively k The three-phase load current sampling value at the moment α , β and γ Axis component; 、 and Respectively k The three-phase inductor current sampling value at the moment α , β and γ Axis component; Will The three-phase reference voltage in the coordinate system is transformed into abc The conversion formula of the reference voltage in the coordinate system is as follows: 。 4. The method for controlling the midpoint potential of an NPC four-bridge-arm inverter with a low switching frequency according to claim 1, characterized in that: The specific steps of S3 are: S31, defining a midpoint current adjustable bridge arm and a non-adjustable bridge arm; S32. Select one bridge arm from the two midpoint current adjustable bridge arms as the adjustment bridge arm, and calculate the adjustment bridge arm's o "State phase duty cycle adjustment value ; S33, by adjusting the two positron modulation waves of the regulating bridge arm, the regulating bridge arm in S32 is obtained" o "The state phase duty cycle adjustment value, and then the relationship between the positive sub-modulation wave adjustment value and the phase duty cycle adjustment value is obtained as follows: ； in, Indicates the adjustment value of the bridge arm positron modulation wave.

5. The method for controlling the midpoint potential of an NPC four-bridge-arm inverter with a low switching frequency according to claim 4, characterized in that: The S4 is specifically as follows: the two positron modulation waves of the non-adjusted bridge arm are calculated according to the formula of the two optimal positron modulation waves of each bridge arm, and the two positron modulation waves of the adjusted bridge arm are calculated according to the formula of the two optimal positron modulation waves of each bridge arm and then adjusted to obtain the following expression: ； in, and Represents two positron modulation waves after adjusting the bridge arm.

6. The method for controlling the midpoint potential of an NPC four-bridge-arm inverter with a low switching frequency according to claim 1, characterized in that: The S5 is specifically as follows: the two positron modulation waves of each bridge arm are compared with the same carrier respectively to obtain the switch state of each bridge arm, and then the four control pulses of each bridge arm are obtained according to the switch state of the bridge arm and the on-off relationship of the four power switch tubes of each bridge arm.

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