A hybrid carrier modulation method for NPC three-level inverter with extended range

Abstract

The present invention discloses an NPC three-level inverter interval extension hybrid carrier modulation method. Within the fundamental wave period of the inverter output, the distribution of the zero-sequence voltage and the reference voltage under the modulation of the zero-sequence voltage injection carrier modulation method is analyzed, and the working range of the reference voltage is divided into a midpoint voltage uncontrollable range and a midpoint voltage controllable range; according to the distribution, the zero-sequence voltage injection carrier modulation method and the dual modulation wave carrier modulation method are mixed to construct a hybrid carrier modulation method to perform hybrid modulation on the inverter. The present invention expands the action range of the zero-sequence voltage injection carrier modulation method, performs extended hybrid carrier modulation, optimizes the switching frequency, determines the optimal extension mode, and minimizes the capacitor voltage fluctuation amplitude when the switching frequency optimization capability is the same.

Description

Technical Field

[0001] The present invention belongs to the field of power converter modulation for driving motors and is an inverter modulation method, which relates to an NPC three-level inverter interval extension hybrid carrier modulation method based on hybrid carrier modulation to achieve balanced control of the switching frequency and midpoint voltage of the NPC three-level inverter. Background Art

[0002] Neutral-point-clamped (NPC) three-level inverters are widely used in photovoltaic wind power generation, traction drive, ultra-deep well lifting and other fields, solving many problems faced by medium and high voltage and high power occasions. Compared with two-level inverters, three-level inverters have the advantages of small voltage stress of switching devices, high output voltage quality, and high inverter efficiency.

[0003] The traditional sinusoidal pulse width modulation method and space vector pulse width modulation method (SVPWM) acting on the NPC three-level inverter will cause unbalanced charging and discharging of the DC side capacitor. Therefore, the balance of the midpoint voltage has become the main research problem of the inverter. The control methods based on carrier modulation to solve this problem mainly include: Zero Sequence Voltage Injection Carrier-Based Pulse Width Modulation (ZSV-CBPWM) method, Double Modulation Wave Carrier-Based Pulse Width Modulation (DMW-CBPWM) method and Hybrid Carrier-Based Pulse Width Modulation (HPWM) method. Among them, the ZSV-CBPWM method realizes the balanced control of the midpoint voltage by superimposing the zero-sequence voltage on the reference voltage. The use of three-phase symmetrical zero-sequence voltage not only improves the utilization rate of the DC voltage but also keeps the line voltage unchanged. And when the zero-sequence voltage presents a change form of three times the fundamental frequency, the harmonic content of the output voltage can be effectively reduced. The DMW-CBPWM method decomposes a single modulation wave into positive and negative modulation waves in a specific way, solving the problem of low-frequency fluctuations in capacitor voltage under arbitrary modulation degree and power factor. The disadvantage is that the switching frequency of the inverter increases significantly. It is worth noting that although the traditional DMW-CBPWM can reduce the midpoint voltage fluctuation, it will cause the capacitor voltage to continuously shift. The HPWM method is a modulation method in which multiple carrier modulation methods work together. According to the characteristics of various modulation methods and mixed in a specific way, the balanced control of the midpoint voltage and switching frequency is achieved. The disadvantage is that it is greatly affected by the modulation degree and power factor, which will lead to a decrease in the midpoint voltage balancing ability or the switching frequency optimization ability.

[0004] Nowadays, scholars have proposed many optimization control methods and hybrid modulation methods based on ZSV-CBPWM and DMW-CBPWM. Although they can reduce the fluctuation of the mid-point voltage to a certain extent, there are certain constraints between the inverter switching frequency and the mid-point voltage balance performance. Summary of the invention

[0005] In order to solve the problems existing in the background technology, the present invention provides an NPC three-level inverter interval extension hybrid carrier modulation method. Based on the HPWM modulation method, the present invention solves the problem of too high switching frequency when the NPC three-level inverter operates in a high modulation index or low power factor condition, and determines the optimal extension interval distribution mode. The capacitor voltage fluctuation amplitude is minimized when the switching frequency optimization capability is the same.

[0006] The technical solution of the present invention is:

[0007] The present invention sets up a hybrid carrier modulation in which a zero-sequence voltage injection carrier modulation method ZSV-CBPWM and a dual modulation wave carrier modulation method DMW-CBPWM work together. When working at a high modulation index or a low power factor, the switching frequency of the inverter is significantly increased, and the switching frequency is optimized by extending the action range of the zero-sequence voltage injection carrier modulation method ZSV-CBPWM.

[0008] The method specifically comprises the following steps:

[0009] 1) In the fundamental wave period of the NPC three-level inverter output, the distribution of zero-sequence voltage and reference voltage under the zero-sequence voltage injection carrier modulation method ZSV-CBPWM modulation is analyzed, and the working range of the reference voltage is divided into the midpoint voltage uncontrollable range and the midpoint voltage controllable range;

[0010] The NPC three-level inverter includes a DC side, an AC side and a three-phase bridge arm. The DC side includes two support capacitors and an input voltage. The two support capacitors are connected in series to the two ends of the input voltage on the DC side, and the voltage between the two support capacitors is used as the midpoint voltage. Each phase bridge arm is composed of four power switch tubes and two clamping diodes. The four power switch tubes are connected in series, and the two clamping diodes are connected in series and then connected in parallel to the two power switch tubes in the middle. The AC side includes three loads respectively connected between the two power switch tubes in the middle of each phase bridge arm.

[0011] The uncontrollable midpoint voltage interval refers to a reference voltage interval in which the midpoint voltage of the NPC three-level inverter cannot be controlled to zero due to reasons such as excessive modulation or too low power factor.

[0012] 2) According to the distribution of the controllable and uncontrollable intervals of the midpoint voltage, the zero-sequence voltage injection carrier modulation method ZSV-CBPWM and the dual modulation wave carrier modulation method DMW-CBPWM are mixed to construct a hybrid carrier modulation method to perform hybrid modulation on the NPC three-level inverter.

[0013] The step 1) is specifically as follows: dividing the sinusoidal reference voltage within a unit fundamental wave period into six regions, each region being spaced 60° apart;

[0014] In each area, the position angle of the midpoint voltage uncontrollable interval is calculated by inversely solving the following formula, and then the width θ2-θ1 of the midpoint voltage uncontrollable interval is calculated to determine the midpoint voltage uncontrollable interval:

[0015]

[0016]

[0017]

[0018] Among them, θ0 represents the phase angle segmentation threshold of the zero-sequence voltage; θ1 and θ2 represent the position angles on both sides of the uncontrollable interval of the midpoint voltage in the area, represents the power factor, θ represents the phase angle of the reference voltage, and m represents the modulation degree;

[0019] Determining the position angle and width of the midpoint voltage uncontrollable interval means determining the midpoint voltage uncontrollable interval. According to the position angle and width of the midpoint voltage uncontrollable interval, the midpoint voltage uncontrollable interval within the unit fundamental wave period is set as:

[0020] θ1+(n-1)*60°≤θ≤θ2+(n-1)*60°

[0021] Where n represents the ordinal number of the region;

[0022] The remaining part of the region except the midpoint voltage uncontrollable interval is the midpoint voltage controllable interval.

[0023] In each area, the zero-sequence voltage of the zero-sequence voltage injection carrier modulation method ZSV-CBPWM is calculated according to the reference voltage according to the following formula, and the zero-sequence voltage is divided into two sections:

[0024]

[0025]

[0026] Among them, v 01 represents the first zero-sequence voltage injected into the reference voltage, v 02 represents the second zero-sequence voltage injected into the reference voltage; represents the power factor, θ represents the phase angle of the reference voltage, and m represents the modulation degree;

[0027] According to the following formula, a unique zero-sequence voltage with midpoint voltage balance is set in the area. 0z Injected into the sinusoidal reference signal, the reference voltage signal of ZSV-CBPWM is obtained, which is expressed as:

[0028]

[0029]

[0030] Where θ0 represents the phase angle segmentation threshold of the zero-sequence voltage; the first zero-sequence voltage v 01 With the second zero sequence voltage v 02 Intersect at the phase angle segment threshold θ0.

[0031] When the modulation index m is greater than the maximum modulation index m that can achieve midpoint voltage balance cmax , and the power factor When the zero-sequence voltage of the zero-sequence voltage injection carrier modulation method ZSV-CBPWM is equal to the pre-known zero-sequence voltage boundary value -1-v C There is an intersection point, and the uncontrollable range of the midpoint voltage is determined by inverse calculation based on the relationship at the intersection point.

[0032] The step 2) is specifically as follows for each area after being divided into six areas:

[0033] When the phase angle of the reference voltage is within the controllable range of the midpoint voltage, all modulation is performed using only the zero-sequence voltage injection carrier modulation method ZSV-CBPWM;

[0034] When the phase angle of the reference voltage is within the uncontrollable interval of the midpoint voltage, the dual modulation wave carrier modulation method DMW-CBPWM and the zero-sequence voltage injection carrier modulation method ZSV-CBPWM are used for hybrid modulation. Specifically, the uncontrollable interval of the midpoint voltage is divided into an extended interval and a non-extended interval, and then:

[0035] When the phase angle of the reference voltage is in the extended interval, the zero-sequence voltage injection carrier modulation method ZSV-CBPWM is used for modulation;

[0036] When the phase angle of the reference voltage is in the non-extended interval, the dual modulation wave carrier modulation method DMW-CBPWM is used for modulation.

[0037] When the dual modulation wave carrier modulation method DMW-CBPWM is used in step 2), an offset compensation voltage is additionally applied in the action interval of the dual modulation wave carrier modulation method DMW-CBPWM.

[0038] The offset compensation voltage v os The settings are processed according to the following formula:

[0039] v os = k*(v C2 -v C1 )*i x

[0040] Among them, i x is the three-phase load current, x represents the phase sequence of the NPC three-level inverter, x∈{A, B, C}, A, B, C are the three phases of the NPC three-level inverter respectively; k is the closed-loop gain parameter, which is used to control the size of the compensation amount; v C1 and v C2 are the upper support capacitor voltage and the lower support capacitor voltage on the DC side of the NPC three-level inverter.

[0041] In the dual modulation wave carrier modulation method DMW-CBPWM modulation, the offset compensation voltage v os applied to the reference voltage.

[0042] The expansion interval width Δθ can be obtained by inversely solving the following formula:

[0043]

[0044] Among them, N switching represents the number of switch state changes of the interval extension hybrid carrier modulation method of the present invention within a unit fundamental wave period, f c represents the carrier frequency, f0 represents the fundamental frequency of the reference voltage;

[0045] Then, the position angles θ3 and θ4 on both sides of the extended interval within the uncontrollable interval of the midpoint voltage are set according to the following formula to determine the extended interval:

[0046]

[0047] α+β=1

[0048] Among them, θ3 and θ4 represent the position angles on both sides of the extended interval within the uncontrollable interval of the midpoint voltage, and α and β represent the left extended interval width parameters and the right extended interval width parameters, respectively;

[0049] Then, the non-extended interval is determined according to the extended interval:

[0050] θ3+(n-1)*60°≤θ≤θ4+(n-1)*60°

[0051] Where n is the ordinal number of the region.

[0052] The ratio of the left expansion interval width parameter α to the right expansion interval width parameter β is used as the distribution pattern between the expansion interval and the non-expanded interval. In the specific implementation, considering the influence of modulation degree and power factor on the fluctuation amplitude of midpoint voltage, the simulation data is integrated and analyzed to determine the distribution pattern between the expansion interval and the non-expanded interval.

[0053] The present invention constructs a hybrid carrier modulation method based on the advantages and disadvantages of the zero-sequence voltage injection carrier modulation method and the dual modulation wave carrier modulation method, which not only achieves the complete balance of the midpoint voltage but also appropriately reduces the switching frequency. However, as the modulation index increases or the power factor decreases, the switching frequency of the hybrid carrier modulation method gradually increases.

[0054] The present invention expands the action range of the zero-sequence voltage injection carrier modulation method, proposes an NPC three-level inverter interval expansion hybrid carrier modulation method, optimizes the switching frequency, determines the optimal expansion mode, and minimizes the capacitor voltage fluctuation amplitude when the switching frequency optimization capability is the same.

[0055] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0056] (1) The present invention uses the switching frequency as an optimization index and can adjust the width of the uncontrollable interval according to the midpoint voltage fluctuation amplitude or the inverter switching frequency limit;

[0057] (2) The present invention analyzes the capacitor voltage fluctuation under different expansion interval distribution modes and determines the optimal expansion interval distribution mode, which minimizes the fluctuation of the midpoint voltage when the switching frequency optimization capability remains constant. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 It is the topological structure diagram of NPC three-level converter;

[0059] Figure 2 It is a three-phase sinusoidal reference voltage area division diagram;

[0060] Figure 3 It is the sub-region division diagram within region I;

[0061] Figure 4a m = 0.8, Time 0z Distribution and v C1 Change graph;

[0062] Figure 4b m = 0.95, Time 0z Distribution and v C1 Change graph;

[0063] Figure 5a The left side is the distribution diagram of the expansion mode and the carrier modulation method interval;

[0064] Figure 5b The right side is the distribution diagram of the expansion mode and the carrier modulation method interval;

[0065] Figure 5c It is the interval distribution diagram of the two-side extension mode and the carrier modulation method;

[0066] Figure 6 for The width diagram of the uncontrollable interval of midpoint voltage;

[0067] Figure 7a The capacitance voltage fluctuation amplitude v is when the modulation index is constant and Δθ1 changes caf Schematic diagram of the results;

[0068] Figure 7b is the capacitor voltage fluctuation amplitude v when the power factor angle is constant and Δθ1 changes caf Schematic diagram of the results;

[0069] Figure 8 This is the algorithm flow chart of the OEM-HPWM method. DETAILED DESCRIPTION

[0070] In order to make the objectives, technical solutions and advantages of the present invention easier to understand, the specific details are further described below in conjunction with the accompanying drawings.

[0071] Embodiments of the present invention are as follows:

[0072] NPC type three-level inverter topology structure Figure 1 As shown, V dc is the DC side input voltage, C1 and C2 are DC side support capacitors, i NP is the midpoint current, i A 、i B 、i C is the three-phase load current. Each phase bridge arm consists of four power switches and two clamping diodes, with three effective switching states. Therefore, there are 3 3 = 27 switch states.

[0073] The three-phase reference voltage of the three-level inverter is:

[0074]

[0075] Among them, v A 、v B 、v C is the three-phase reference voltage, m is the modulation index, θ is the phase angle of the reference voltage.

[0076] If a three-phase symmetrical load is used, the three-phase load current i x for:

[0077]

[0078] Among them, I m is the phase current amplitude, is the power factor.

[0079] The ZSV-CBPWM method can improve the midpoint voltage fluctuation problem without affecting the output line voltage by injecting the three-phase symmetrical zero-sequence voltage v0. After the zero-sequence voltage v0 is injected, the three-phase reference voltage is transformed into:

[0080]

[0081] Among them, v A0 、v B0 、v C0 are the three-phase reference voltages after the zero-sequence voltage v0 is injected, and the zero-sequence voltage v0 extends the linear modulation area to [0,1].

[0082] The average midpoint current under the ZSV-CBPWM method is expressed as:

[0083]

[0084] in, Represents the average midpoint current.

[0085] The average midpoint current The size of is affected by the zero-sequence voltage v0. Choosing a suitable zero-sequence voltage is the key to controlling the midpoint voltage.

[0086] Considering the influence of DC bus voltage, the value range of zero-sequence voltage is set as:

[0087] -1-v min ≤v0≤1-v max (5)

[0088] Among them, v min Indicates the minimum value of three-phase sinusoidal voltage, v min =min(v A ,v B ,v C );v max Indicates the maximum value of three-phase sinusoidal voltage, v max =max(v A ,v B ,v C ). min(·) and max(·) are the minimum / maximum value functions.

[0089] Under the DMW-CBPWM method, when the algebraic sum of the three-phase load current is zero, the midpoint voltage balance in any working mode can be achieved. First, a zero-sequence voltage v is injected into the three-phase sinusoidal reference voltage to improve the DC voltage utilization rate. z , to obtain the modulation wave signal of the DMW-CBPWM method:

[0090] v z =-(v max +v min ) / 2 (6)

[0091] At this time, the three-phase reference voltage v xz , x∈{A, B, C} is expressed as:

[0092]

[0093] Among them, v A0 、v B0 、v C0 are the zero-sequence voltage v to improve the DC voltage utilization rate. z The three-phase reference voltage after injection;

[0094] Then, v xz Decompose into positive and negative dual modulation waves, and meet the conditions:

[0095]

[0096] in, Indicates the injected zero-sequence voltage v z After the positive reference voltage, is a positive modulation wave, It indicates the injected zero sequence voltage v z After the negative reference voltage, is a negative modulation wave; v xz Indicates the injected zero-sequence voltage v z The three-phase reference voltage after is x∈{A,B,C}.

[0097] The average midpoint current expression is established as:

[0098]

[0099] In the formula, is the duty cycle of the x-phase switch state "O". The switch state "O" refers to the switch state corresponding to the two inner power devices of any phase bridge arm of the three-level inverter being turned on and the two outer power devices being turned off.

[0100] make right and To solve, the dual modulation wave combination is expressed as:

[0101]

[0102] Then as Figure 2 As shown, the three-phase sinusoidal reference voltage within a unit fundamental wave period is divided into six regions, and each region is spaced 60° apart.

[0103] In the specific implementation, taking region I as an example, the NPC three-level inverter has three phases A, B, and C, the maximum value of the reference voltage of phase A is 0°, the minimum value of the reference voltage of phase C is 60°, and the region from 0° to 60° is region I. A 、-v B and -v C The region can be further divided into four sub-regions R k , k∈{1,2,3,4}, such as Figure 3 shown.

[0104] Table 1 shows the different sub-areas in region I when |v x0 | and the corresponding average midpoint current expression. When the zero sequence voltage is located in sub-regions R3 and R2, let v0 = v 01 、v0=v 02 ,but It is always 0, indicating that the midpoint voltage is controllable in this area.

[0105] Table 1 |v in different sub-regions x0 | and average midpoint current

[0106]

[0107] Table 1 01 and v 02 Respectively expressed as:

[0108]

[0109] The only zero-sequence voltage that achieves midpoint voltage balance in region I is calculated as:

[0110]

[0111] Among them, v 01 With v 02 Intersect at point

[0112] Similarly, the zero-sequence voltages of the remaining five regions are solved and injected into the three-phase reference voltage signal, thereby obtaining the modulation signal of the ZSV-CBPWM method. In summary, the uncontrollable interval of the midpoint voltage within a unit fundamental wave period is defined as:

[0113] θ1+(n-1)*60°≤θ≤θ2+(n-1)*60° (13)

[0114] Wherein, θ is the fundamental wave phase angle; n={1, 2, ..., 6} is the region where θ is located.

[0115] When θ satisfies conditional formula (13), the DMW-CBPWM method can be used, otherwise the ZSV-CBPWM method is used to form the preliminary hybrid modulation method HPWM of the present invention.

[0116] When the DMW-CBPWM method is applied to the NPC three-level inverter, it will cause a continuous offset of the midpoint voltage. The offset compensation voltage v is set according to the following formula: os Apply offset compensation voltage v to the modulation signal of DMW-CBPWM os To suppress, the calculation formula is:

[0117] v os = k*(v C2 -v C1 )*i x (14)

[0118] Among them, i x is the three-phase load current, x∈{A, B, C}; k is the closed-loop gain, which is used to control the size of the compensation amount; v C1 and v C2 is the upper and lower support capacitor voltage on the DC side.

[0119] The new dual modulation wave is expressed as:

[0120]

[0121] in, They represent the injected offset compensation voltage v os After that, the positive and negative reference voltage signals.

[0122] Then set the expansion interval width Δθ:

[0123] For any region ( Figure 2 As shown), the upper limit of the expansion interval width must be smaller than the width of the midpoint voltage uncontrollable interval NPVUC interval θ2-θ1, and the lower limit can be greater than zero when the carrier frequency is high enough, taking into account the influence of the carrier frequency.

[0124] In a specific implementation, the value range of the width Δθ of the expansion interval in region I is:

[0125] 0<Δθ<θ2-θ1 (16)

[0126] Among them, θ1 and θ2 are the left and right position angles of the midpoint voltage uncontrollable interval NPVUC.

[0127] If any one-phase modulation signal has two expansion intervals within a unit fundamental wave period, the total expansion width is 2Δθ. For a three-phase modulation signal, the total expansion width is 6Δθ.

[0128] In a specific implementation, the width Δθ of the extended interval is set according to the switching frequency and the midpoint voltage fluctuation amplitude, and the relationship between the number of switch state changes of the interval extended hybrid carrier modulation method of the present invention and the width Δθ of the extended interval within a unit fundamental wave period is established as follows:

[0129]

[0130] It can be seen from the above formula that when the operating conditions are constant, the position angles θ1 and θ2 on both sides of the uncontrollable interval of the midpoint voltage remain unchanged. If the width of the extended interval is increased, the number of switching actions of the inverter within a unit fundamental wave period will decrease.

[0131] When the width of the expansion interval remains the same, the change of the DC side capacitor voltage when the ZSV-CBPWM method acts on the inverter is analyzed to study the influence of the expansion interval distribution pattern on the midpoint voltage, such as Figure 4a and Figure 4b As shown in the figure, the capacitor voltage v on the DC side in the NPVUC interval under different working conditions C1 The offset rate changes all the time, and the offset rate in the middle area is significantly higher than that on the left and right sides. Therefore, assuming that the DMW-CBPWM method is used in the middle area and the ZSV-CBPWM method is used on the left and right sides, the maximum voltage balancing ability of the NPC three-level inverter can be achieved. Based on the above assumptions, three expansion interval distribution modes are proposed for research, namely: left expansion mode, right expansion mode and both sides expansion mode, such as Figure 5a , Figure 5b and Figure 5c shown.

[0132] Figure 6 Shown The change curve of the width of the uncontrollable interval NPVUC of the midpoint voltage in region I. When m>0.7, the width of the NPVUC interval increases nonlinearly with the increase of the modulation index. When the modulation index is ≤0.7, the midpoint voltage is controllable in the entire fundamental wave period. The above assumption is further analyzed by taking the unit area expansion interval width Δθ=20° as an example.

[0133] Table 2 and Table 3 show the three expansion methods in V dc =200V, total impedance is 15Ω, the capacitor voltage fluctuation amplitude v at different modulation degrees and power factorsCfa The simulation verification analysis is carried out to prove the generality of the above conclusions. Under certain working conditions, the width of the uncontrollable interval of the midpoint voltage is smaller than the expansion interval, and the interval cannot be expanded. In Table 2, it is indicated by "--". For working conditions that meet the research conditions, it is found that the capacitor voltage fluctuation amplitude corresponding to the expansion mode on both sides is smaller, and it is easier to control the midpoint voltage.

[0134] Table 2 Low The corresponding v of the three expansion interval distribution modes for different m Cfa

[0135]

[0136] Table 3 High The corresponding v of the three expansion interval distribution modes for different m Cfa

[0137]

[0138]

[0139] Table Note: The data in the table are the capacitor voltage fluctuation amplitudes corresponding to the left expansion mode-both sides expansion mode-right expansion mode. “--” means that the working condition does not meet the research conditions.

[0140] The left and right expansion interval widths Δθ1 and Δθ2 are quantitatively set: when the expansion interval width Δθ is determined, Δθ1 and Δθ2 are adjusted by setting the values ​​of parameters α and β, as shown in the following formula:

[0141]

[0142] α+β=1

[0143] Among them, Δθ1 and Δθ2 represent the width of the expansion interval on the left and right sides respectively.

[0144] Assuming that the ratio of α increases, Δθ1 will increase and Δθ2 will decrease. At this time, as Δθ1 changes, the non-expanded interval will move within the midpoint voltage uncontrollable interval NPVUC.

[0145] The left and right position angles θ3 and θ4 of the extended interval in area I are calculated as:

[0146]

[0147] Finally, the distribution range of the non-extended interval within the fundamental wave period is obtained as follows:

[0148] θ3+(n-1)*60°≤θ≤θ4+(n-1)*60° (20)

[0149] When θ satisfies formula (20), DMW-CBPWM modulation is adopted; otherwise, ZSV-CBPWM modulation is adopted, thereby forming the final interval extension hybrid carrier modulation method of the present invention.

[0150] Figure 7a and Figure 7b The figure shows the capacitance voltage fluctuation amplitude corresponding to the interval extension hybrid carrier modulation method when the width of the left extension interval changes under different working modes. Figure 7a As shown. The capacitor voltage fluctuation amplitude v Cfa Maintaining the trend of decreasing first and then increasing, for any power factor angle, as Δθ1 increases, v Cfa First it decreases and then increases. For any Δθ1, when the modulation index increases, v Cfa When the power factor angle is constant, Figure 7b As shown. For any modulation degree, as the width of the left expansion interval Δθ1 increases, the change trend of the capacitor voltage is similar to the former. For any Δθ1, when the power factor angle increases v Cfa There are varying degrees of increase.

[0151] Will Figure 7a and Figure 7b The points with the lowest voltage fluctuation amplitude in the inner surface are connected to obtain the curve of the minimum fluctuation amplitude of the midpoint voltage. It can be seen that when the width of the left and right expansion intervals Δθ1:Δθ2=α:β=3:2, the capacitor voltage fluctuation amplitude is the smallest, and the expansion interval width ratio is not affected by the modulation degree and power factor. Therefore, this expansion mode is defined as the optimal expansion mode, and the corresponding hybrid modulation method is defined as OEM-HPWM.

[0152] The flow chart of the interval extension hybrid carrier modulation method OEM-HPWM under the optimal extended interval distribution mode is as follows: Figure 8 As shown, the implementation steps are as follows:

[0153] 1) Obtaining a modulation signal of a zero-sequence voltage injection carrier modulation method;

[0154] In order to facilitate the mixing of methods, equation (3) is decomposed into positive and negative modulation waves The decomposition principles are as follows:

[0155]

[0156] 2) Obtaining a modulation signal of a dual modulation wave carrier modulation method and applying a compensation voltage;

[0157] 3) Select and set the expansion interval width Δθ according to the switching frequency or capacitor voltage fluctuation amplitude, and set the expansion interval distribution mode, that is, Δθ1:Δθ2=α:β=3:2, and solve the left and right expansion interval widths.

[0158] 4) Determine the position angles θ1 and θ2 of the uncontrollable interval of the midpoint voltage by looking up the table based on the modulation index and power factor.

[0159] 5) The concatenation equation (19) is used to solve the extended interval position angles θ3 and θ4 in region I, and then the distribution of the non-extended intervals in the fundamental period is determined and the above two methods are mixed.

[0160] The present invention is not limited to the above-described embodiments. The above description of the specific embodiments is only to illustrate the effectiveness and feasibility of the present method, and is not specific and restrictive. All modifications, form nesting, improvements, etc. made within the spirit and principles of the present invention are within the scope of protection of the present invention.

Claims

1. A hybrid carrier modulation method for an NPC three-level inverter with interval extension, characterized in that: The method specifically comprises the following steps: 1) In the fundamental wave period of the NPC three-level inverter output, the distribution of zero-sequence voltage and reference voltage under the zero-sequence voltage injection carrier modulation method ZSV-CBPWM modulation is analyzed, and the working range of the reference voltage is divided into the midpoint voltage uncontrollable range and the midpoint voltage controllable range; 2) According to the distribution of the midpoint voltage controllable interval and the midpoint voltage uncontrollable interval, the zero-sequence voltage injection carrier modulation method ZSV-CBPWM and the dual modulation wave carrier modulation method DMW-CBPWM are mixed to construct a hybrid carrier modulation method to perform hybrid modulation on the NPC three-level inverter; When the phase angle of the reference voltage is within the controllable range of the midpoint voltage, the zero-sequence voltage injection carrier modulation method ZSV-CBPWM is used for modulation; When the phase angle of the reference voltage is within the uncontrollable interval of the midpoint voltage, the dual modulation wave carrier modulation method DMW-CBPWM and the zero-sequence voltage injection carrier modulation method ZSV-CBPWM are used for hybrid modulation. Specifically, the uncontrollable interval of the midpoint voltage is divided into an extended interval and a non-extended interval, and then: When the phase angle of the reference voltage is in the extended interval, the zero-sequence voltage injection carrier modulation method ZSV-CBPWM is used for modulation; When the phase angle of the reference voltage is in the non-extended interval, the dual modulation wave carrier modulation method DMW-CBPWM is used for modulation; The expansion interval width Dq can be obtained by inverse calculation according to the following formula: Among them, N switching Indicates the number of switching state changes within a unit fundamental wave period, f c represents the carrier frequency, f0 represents the fundamental frequency of the reference voltage, θ1 and θ2 represent the position angles on both sides of the uncontrollable interval of the midpoint voltage in the region; Then, the position angles θ3 and θ4 on both sides of the extended interval within the uncontrollable interval of the midpoint voltage are set according to the following formula to determine the extended interval: α+β＝1 Among them, θ3 and θ4 represent the position angles on both sides of the extended interval within the uncontrollable interval of the midpoint voltage, and α and b represent the left extended interval width parameter and the right extended interval width parameter, respectively; Then, the non-extended interval is determined according to the extended interval: Where n is the ordinal number of the region.

2. The NPC three-level inverter interval extension hybrid carrier modulation method according to claim 1, characterized in that: Step 1) is specifically as follows: dividing the reference voltage within the unit fundamental wave period into six regions, each region having an interval of 60°; In each area, the position angle of the midpoint voltage uncontrollable interval is calculated by inversely solving the following formula, and then the width θ2-θ1 of the midpoint voltage uncontrollable interval is calculated to determine the midpoint voltage uncontrollable interval: Among them, q0 represents the phase angle segmentation threshold of the zero-sequence voltage; θ1 and θ2 represent the position angles on both sides of the uncontrollable interval of the midpoint voltage in the area, represents the power factor, q represents the phase angle of the reference voltage, and m represents the modulation degree; According to the position angle and width of the midpoint voltage uncontrollable interval, the midpoint voltage uncontrollable interval within the unit fundamental wave cycle is set as: Where n represents the ordinal number of the region; The remaining part of the region except the midpoint voltage uncontrollable interval is the midpoint voltage controllable interval.

3. The NPC three-level inverter interval extension hybrid carrier modulation method according to claim 1, characterized in that: When the dual modulation wave carrier modulation method DMW-CBPWM is used in step 2), an offset compensation voltage is additionally applied in the action interval of the dual modulation wave carrier modulation method DMW-CBPWM.

4. The NPC three-level inverter interval extension hybrid carrier modulation method according to claim 3, characterized in that: The offset compensation voltage v os The settings are processed according to the following formula: v os =k*(v C2 -v C1 )*i x Among them, i x is the three-phase load current, x represents the phase sequence of the NPC three-level inverter, x∈{A, B, C}, A, B, C are the three phases of the NPC three-level inverter respectively; k is the closed-loop gain parameter; v C1 and v C2 are the upper support capacitor voltage and the lower support capacitor voltage on the DC side of the NPC three-level inverter.

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