A modulation method for three-phase four-bridge-arm three-level inverter to achieve zero midpoint current

Abstract

The invention discloses a three-phase four-bridge arm three-level inverter modulation method for realizing zero midpoint current, belonging to the field of power electronic conversion and control, and the method includes: according to the voltage of each phase AC filter capacitor and the current of each phase AC filter inductor, calculating the differential mode component of the three-phase bridge arm modulation wave and the fourth bridge arm modulation wave before common mode voltage compensation; using the load imbalance of each phase, combined with the modulation wave of the current three-phase bridge arm, calculating the phase where the sub-state switching cycle is output at a constant +1, -1 level, so as to determine the common mode component of the three-phase bridge arm modulation wave; calculating the modulation wave of the three-phase bridge arm and the fourth bridge arm according to the common mode component of the three-phase bridge arm modulation wave, determining the +1, -1, 0 level duty ratio actually output by each phase bridge arm; the three-phase bridge arm modulation wave and the fourth bridge arm modulation wave are respectively intercepted with the corresponding carrier to generate a drive signal of the inverter. The present invention can realize the midpoint voltage balance in the full power factor range and eliminate the low-frequency pulsating component in the midpoint voltage.

Description

Technical Field

[0001] The present invention belongs to the field of power electronic conversion and control, and more specifically, relates to a modulation method for a three-phase four-bridge-arm three-level inverter for realizing zero midpoint current. Background Art

[0002] In order to overcome the incompatibility of unbalanced / non-linear AC loads, the three-phase three-wire (3P3W) inverter can be connected to the neutral line to form a three-phase four-wire (3P4W) inverter. The 3P4W inverter is mainly divided into a three-phase three-bridge-arm four-wire (3P3L4W) inverter and a three-phase four-bridge-arm four-wire (3P4L4W) inverter. Compared with the 3P3L4W inverter, the neutral current of the 3P4L4W inverter flows through the fourth bridge arm instead of the DC side split capacitor, which greatly reduces the volume of the bus capacitor of the 3P4L4W inverter, making it more advantageous in power density.

[0003] For three-level inverters, midpoint voltage balance is a prerequisite for safe and stable operation. Due to factors such as uneven busbar capacitance, inconsistent switching of power devices, and parasitic parameters, the midpoint voltage will inevitably deviate from the rated value. If not promptly managed, the midpoint voltage may further diverge, and even cause the entire system to crash and shut down. Therefore, midpoint voltage balance control is crucial for the safe and stable operation of three-phase four-bridge-arm three-level inverters.

[0004] The basic modulation methods of 3P4L4W inverter mainly include three-dimensional space vector modulation (3D-SVM) and carrier-based pulse width modulation (CB-PWM). Similar to traditional space vector modulation, 3D-SVM also achieves midpoint voltage balance control by adjusting the action time of positive and negative redundant small vectors. The three-dimensional distribution of space vectors makes the implementation process of 3D-SVM very complicated; at the same time, due to the large number of vectors, it is particularly difficult to select a suitable action vector. Compared with 3D-SVM, CB-PWM is intuitive, simple and easy to implement. It can directly control the common mode component in the three-phase modulation wave through the voltage balancing loop to balance the upper and lower split capacitor voltages. Although this basic midpoint voltage balancing control is simple, when the inverter is running under different working conditions, the gain sign of the voltage balancing control needs to be manually modified to ensure the balance of the bus capacitor voltage at any time; at the same time, when the inverter is running under zero power factor conditions, due to the influence of control delay and parasitic parameters, there will inevitably be a small phase disturbance between the phase voltage and the phase current, which makes the actual output power factor of the system fluctuate around zero. However, because the disturbance is unknown and unobservable, the system cannot correctly modify the sign of the voltage-equalizing gain according to the output power factor, resulting in the risk of failure of the voltage-equalizing control. If the disturbance causes the voltage-equalizing loop to form a positive feedback logic, the split capacitor voltage will diverge rapidly in a short period of time, causing the system to crash.

[0005] Although the fourth bridge arm bypasses the neutral current, the voltage pulsation of the DC side split capacitor is still very large when the inverter is running under high imbalance and low power factor conditions, and a large capacitance capacitor is needed to suppress it. This capacitor is generally an electrolytic capacitor. The life of the electrolytic capacitor is about 5,000 hours. When affected by factors such as ambient temperature, current ripple and operating frequency, its service life will be further shortened. Therefore, the electrolytic capacitor has become the main factor affecting the reliability and life of the three-phase four-bridge arm three-level inverter; at the same time, the large size of the electrolytic capacitor also restricts the further improvement of the inverter power density, which is not conducive to the lightweight and miniaturization of the system.

[0006] Therefore, for the three-phase four-bridge-arm three-level inverter, how to overcome the defect of traditional voltage equalization control under zero power factor conditions on the basis of carrier modulation, so as to achieve the balance of the midpoint voltage within the full power factor range; and further reduce the pulsation of the DC side split capacitor voltage to improve the reliability and power density of the system, is of great research significance. Summary of the invention

[0007] In view of the defects of the prior art and the need for improvement, the present invention provides a three-phase four-bridge-arm three-level inverter modulation method for realizing zero midpoint current, the purpose of which is to realize the safe and stable operation of the three-phase four-bridge-arm inverter at full power factor, and further reduce the pulsation of the DC side split capacitor voltage to improve the reliability and power density of the system.

[0008] To achieve the above object, according to one aspect of the present invention, a method for realizing zero-midpoint current three-phase four-bridge-arm three-level inverter modulation is provided, wherein the inverter comprises a three-level three-phase bridge arm, a two-level fourth bridge arm, an LC AC filter and a neutral line inductor, wherein the output end of the three-level three-phase bridge arm is connected to the LC AC filter, and the neutral point of the fourth bridge arm is connected to the neutral point of the AC filter capacitor through the neutral line inductor, and the method comprises: according to the voltage v of each phase AC filter capacitor Cfx And each phase AC filter inductor current i Lfx , calculate the differential mode component v of the three-phase bridge arm modulation wave mxDM The fourth bridge arm modulation wave v before common mode voltage compensation md0 ; According to the load imbalance of each phase λ Lx And the three-phase bridge arm modulation wave v at the previous moment mx_pre , determine the phase where the current switching cycle outputs a constant +1, -1 level, and determine the common mode component v of the three-phase bridge arm modulation wave according to the phase mCM ; v mxDM and v mCM Add together to get the three-phase bridge arm modulation wave v mx , according to λ Lx and v mxCalculate the duty ratio of the actual +1, -1, and 0 levels of the three phases, and use the duty ratio to calculate the positive modulation wave v that is intercepted by the three-phase bridge arm and the upper and lower stacked carriers in real time mxp and negative modulation wave v mxn ; Using v mCM v md0 Perform open-loop compensation of the common-mode voltage to obtain the fourth bridge arm modulation wave v md ;v mxp 、v mxn 、v md They intersect with the corresponding carrier waves respectively to generate driving signals to drive the inverter.

[0009] Furthermore, Lx for:

[0010]

[0011] Among them, I m is the peak current under three-phase full load, RMS 50 (i Lfx ) indicates i Lfx The effective value at a specific frequency of 50 Hz, where x is the phase among the three phases a, b, and c.

[0012] Furthermore, the determination of the phase in which the current switching cycle outputs a constant +1 or -1 level specifically includes: Lx and v mx_pre , calculate the duty cycle d of each phase to meet the constant +1 level output zpx And the duty ratio d of each phase to meet the constant -1 level output znx ; d zpx The phase corresponding to the minimum value and d znx The phase corresponding to the minimum value is the phase that outputs at a constant +1 or -1 level in the current switching cycle, and x is the phase among the three phases a, b, and c.

[0013] Furthermore, v mCM for:

[0014]

[0015] Among them, v maDM 、v mbDM 、v mcDM are the differential mode components of the modulation waves of the a, b, and c phase bridge arms, respectively, La , Lb , Lc are the load imbalance of phases a, b, and c respectively, mind zpx Indicates d zpa d zpb d zpc The minimum value in mindznx Indicates d zna d znb d znc The minimum value in .

[0016] Furthermore, d zpx and d znx They are: d zpx =(1-λ Lx )(1+v mx_pre ), d znx =(1-λ Lx )(1-v mx_pre ).

[0017] Furthermore, the actual duty ratio of +1, -1, 0 levels of each phase is:

[0018]

[0019]

[0020]

[0021] Among them, d px d nx d zx are the +1, -1, and 0 level duty ratios of the x-phase output in the current switching cycle, respectively. z is the minimum duty cycle of the three phases outputting constant +1 level and -1 level respectively in the current switching cycle, d z =min{(1-λ Lx )(1+v mx ),(1-λ Lx )(1-v mx )}.

[0022] Furthermore, v mxp and v mxn They are: v mxp =d px 、v mxn =-d nx .

[0023] According to another aspect of the present invention, a three-phase three-bridge arm three-level inverter modulation method for realizing zero midpoint current is provided, the inverter includes a three-level three-phase bridge arm and an LC AC filter, the LC AC filter is connected to the output end of the three-level three-phase bridge arm, the method includes: according to the AC filter capacitor voltage v of each phase Cfx And each phase AC filter inductor current i Lfx , calculate the differential mode component v of the three-phase bridge arm modulation wave mxDM ; According to the three-phase bridge arm modulation wave v at the previous moment mx_pre, determine the phase where the current switching cycle outputs a constant +1, -1 level, and determine the common mode component v of the three-phase bridge arm modulation wave according to the phase mCM ; v mxDM and v mCM Add together to get the three-phase bridge arm modulation wave v mx , according to v mx Calculate the duty ratio of the actual +1, -1, and 0 levels of the three phases, and use the duty ratio to calculate the positive modulation wave v that is intercepted by the three-phase bridge arm and the upper and lower stacked carriers in real time mxp and negative modulation wave v mxn ;v mxp 、v mxn They intersect with the corresponding carrier waves respectively to generate driving signals to drive the inverter.

[0024] According to another aspect of the present invention, a controller is provided, characterized in that it is used to execute the three-phase four-bridge-arm three-level inverter modulation method for realizing zero mid-point current as described above, or to execute the three-phase three-bridge-arm three-level inverter modulation method for realizing zero mid-point current as described above.

[0025] In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects:

[0026] (1) A modulation method for a three-phase four-bridge-arm three-level inverter to achieve zero midpoint current is provided. By adding -1 level and +1 level in each switching cycle of the positive / negative half-cycle of the three-level bridge arm, an independently adjustable 0-level duty cycle is constructed, which effectively realizes the decoupling control of the midpoint current and the load voltage. The method is based on carrier modulation and has the characteristics of simple control and easy implementation.

[0027] (2) By managing the zero-level duty cycle of the three phases to achieve zero midpoint current control, the problem of failure of traditional voltage-sharing control under zero power factor conditions is fundamentally solved, so that the midpoint voltage is always balanced within the full power factor range, fully improving the reliability of the system;

[0028] (3) A method for suppressing the low-frequency pulsation of the midpoint voltage of a three-phase four-bridge-arm three-level inverter is provided. Zero midpoint current control is achieved by managing the 0-level duty cycle of the three phases to eliminate the low-frequency ripple component of the midpoint current, thereby completely suppressing the low-frequency pulsation of the midpoint voltage, greatly reducing the capacitance of the DC side split capacitor, and thereby improving the power density of the entire system. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 A flow chart of a three-phase four-bridge-arm three-level inverter modulation method for realizing zero midpoint current provided by an embodiment of the present invention;

[0030] Figure 2A working principle diagram of a three-phase four-bridge-arm three-level inverter provided in an embodiment of the present invention;

[0031] Figure 3 A topological structure diagram of a three-phase four-bridge-arm three-level inverter provided in an embodiment of the present invention;

[0032] Figure 4 A phase modulation waveform diagram of the modulation method provided in an embodiment of the present invention;

[0033] Figure 5A The bus capacitor voltage waveform of the three-phase four-bridge three-level inverter with a power factor of 0 when the traditional carrier modulation method is adopted;

[0034] Figure 5B The bus capacitor voltage waveform of a three-phase four-bridge-arm three-level inverter at a power factor of 0 when the modulation method provided by the embodiment of the present invention is adopted;

[0035] Figure 6 This is a control principle diagram using the traditional carrier modulation method;

[0036] Fig. 7A The working waveform of the three-phase four-bridge-arm three-level inverter at full load and a power factor of 1 when the modulation method provided by the embodiment of the present invention is adopted;

[0037] Figure 7B When the modulation method provided by the embodiment of the present invention is adopted, the working waveform of the three-phase four-bridge-arm three-level inverter is under the condition that the a phase is 2 / 3 loaded, the b phase and the c phase are fully loaded, and the power factor is 1;

[0038] Figure 7C When the modulation method provided by the embodiment of the present invention is adopted, the working waveform of the three-phase four-bridge-arm three-level inverter is when phase a is 2 / 3 loaded, phase b and phase c are fully loaded, and the power factor is 0. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0040] In the present invention, the terms "first", "second", etc. (if any) in the present invention and the accompanying drawings are used to distinguish similar objects but not necessarily to describe a specific order or sequence.

[0041] Figure 1 A flow chart of a three-phase four-bridge-arm three-level inverter modulation method for realizing zero midpoint current provided by an embodiment of the present invention. Figure 1 , combined with Figure 2-7C , a three-phase four-bridge-arm three-level inverter modulation method for realizing zero midpoint current in this embodiment is described in detail, and the method includes operations S1 to S5.

[0042] The inverter includes a three-level three-phase bridge arm, a two-level fourth bridge arm, an LC AC filter and a neutral line inductor. The output end of the three-level three-phase bridge arm is connected to the LC AC filter, and the neutral point of the AC filter capacitor is connected to the midpoint of the fourth bridge arm through the neutral line inductor.

[0043] See also Figure 2 , shows the working principle diagram of the three-phase four-bridge three-level inverter, including the main power circuit and the control circuit. The topology of the main power circuit is as follows Figure 3 See Figure 3 The main power circuit uses a three-level three-phase bridge arm, a two-level fourth bridge arm, an LC AC filter (including filter inductance and filter capacitor) and a neutral line inductor L N Among them, the three-level bridge arm adopts a T-type three-level structure, the input end of the three-phase bridge arm is connected to the DC bus, and the output end is connected to an external LC AC filter; the input end of the fourth bridge arm is also connected to the DC bus, and its output end is connected to the neutral line inductor L N Connect to the neutral point N of the three-phase filter capacitor.

[0044] Operate S1, according to the voltage v of each phase AC filter capacitor Cfx And each phase AC filter inductor current i Lfx , calculate the differential mode component v of the three-phase bridge arm modulation wave mxDM The fourth bridge arm modulation wave v before common mode voltage compensation md0 .

[0045] The filter inductor current includes the inductor L fa Current i Lfa 、Inductance L fb Current i Lfb and inductor L fc Current i Lfc The filter capacitor voltage includes the capacitor C fa Voltage Cfa , capacitor C fb Voltage Cfb and capacitor C fc Voltage Cfc .

[0046] Specifically, the differential control loop command value and the filter capacitor voltage v Cfx The difference between the differential mode components in the filter inductor current i Lfx Subtract the differential mode components in the phase to obtain the differential mode component v of the modulation wave of each phase bridge arm mxDM ; for zero and filter capacitor voltage vCfx The difference between the common-mode components in the filter inductor current i Lfx Subtract the common-mode components in the output to obtain the fourth bridge arm modulation wave v before common-mode voltage compensation. md0 .

[0047] See also Figure 2 , the differential mode / common mode variable extraction module extracts the filter capacitor voltage v Cfx The differential mode component v CfxDMfb and the common mode component v CfCMfb , and extract the filter inductor current i Lfx The differential mode component i LfxDMfb and the common mode component i LfCMfb ,x∈{a,b,c}。 Set the differential mode control loop command value to a three-phase symmetrical sine wave v refx , v refx and v CfxDMfb The difference passes through the differential mode regulator G DM (s) after enlargement and i LfxDMfb Subtract, and we get v mxDM ; Set the common mode control loop command value to 0, 0 and v CfxCMfb The difference passes through the differential mode regulator G CM (s) after enlargement and i LfCMfb Subtract, and we get v md0 .

[0048] Operation S2, according to the load imbalance of each phase λ Lx And the three-phase bridge arm modulation wave v at the previous moment mx_pre , determine the phase where the current switching cycle outputs a constant +1, -1 level, and determine the common mode component v of the three-phase bridge arm modulation wave according to the phase mCM .

[0049] According to an embodiment of the present invention, Lx for:

[0050]

[0051] Among them, I m is the peak current under three-phase full load, RMS 50 (i Lfx ) indicates i Lfx The effective value at a specific frequency of 50 Hz, where x is the phase among the three phases a, b, and c.

[0052] According to an embodiment of the present invention, determining the phase in which the current switching cycle outputs a constant +1 / -1 level specifically includes the following sub-operations S21 and S22, and operation S2 also includes sub-operation S23.

[0053] In sub-operation S21, according to λLx and v mx_pre , calculate the duty cycle d of each phase to meet the constant +1 level output zpx And the duty ratio d of each phase to meet the constant -1 level output znx .

[0054] d zpx and d znx They are

[0055] d zpx =(1-λ Lx )(1+v mx_pre )

[0056] d znx =(1-λ Lx )(1-v mx_pre )

[0057] In sub-operation S22, d zpx The phase corresponding to the minimum value and d znx The phase corresponding to the minimum value is the phase that outputs a constant +1 / -1 level in the current switching cycle, and x is the phase among the three phases a, b, and c.

[0058] In sub-operation S23, the common mode component v of the three-phase bridge arm modulation wave is determined according to the phase. mCM , which can be divided into the following nine situations:

[0059] 1)d zpx The phase corresponding to the minimum value is phase a, and phase d znx The phase corresponding to the minimum value is phase a, i.e., phase d zpa =mind zpx And d zna =mind znx hour:

[0060] v mCM =-v maDM

[0061] 2)d zpx The phase corresponding to the minimum value is phase a, and phase d znx The phase corresponding to the minimum value is phase b, that is, phase d zpa =mind zpx And d znb =mind znx hour:

[0062]

[0063] 3)d zpx The phase corresponding to the minimum value is phase a, and phase d znx The phase corresponding to the minimum value is phase c, i.e., phase d zpa =mindzpx And d znc =mind znx hour:

[0064]

[0065] 4)d zpx The phase corresponding to the minimum value is phase b, and phase d znx The phase corresponding to the minimum value is phase a, i.e., phase d zpb =mind zpx And d zna =mind znx hour:

[0066]

[0067] 5)d zpx The phase corresponding to the minimum value is phase b, and phase d znx The phase corresponding to the minimum value is phase b, that is, phase d zpb =mind zpx And d znb =mind znx hour:

[0068] v mCM =-v mbDM

[0069] 6)d zpx The phase corresponding to the minimum value is phase b, and phase d znx The phase corresponding to the minimum value is phase c, i.e., phase d zpb =mind zpx And d znc =mind znx hour:

[0070]

[0071] 7)d zpx The phase corresponding to the minimum value is phase c, and phase d znx The phase corresponding to the minimum value is phase a, i.e., phase d zpc =mind zpx And d zna =mind znx hour:

[0072]

[0073] 8)d zpx The phase corresponding to the minimum value is phase c, and phase d znx The phase corresponding to the minimum value is phase b, that is, phase d zpc =mind zpx And d znb =mind znx hour:

[0074]

[0075] 9)d zpx The phase corresponding to the minimum value is phase c, and phase d znx The phase corresponding to the minimum value is phase c, i.e., phase d zpc =mind zpx And d znc =mind znx hour:

[0076] v mCM =-v mcDM

[0077] Among them, v maDM 、v mbDM 、v mcDM are the differential mode components of the modulation waves of the a, b, and c phase bridge arms, respectively, La , Lb , Lc are the load imbalance of phases a, b, and c respectively, mind zpx Indicates d zpa d zpb d zpc The minimum value in mind znx Indicates d zna d znb d znc The minimum value in .

[0078] Operation S3, v mxDM and v mCM Add together to get the three-phase bridge arm modulation wave v mx , according to λ Lx and v mx Calculate the duty ratio of the actual +1, -1, and 0 levels of the three phases, and use the duty ratio to calculate the positive modulation wave v that is intercepted by the three-phase bridge arm and the upper and lower stacked carriers in real time mxp and negative modulation wave v mxn .

[0079] According to an embodiment of the present invention, the duty cycle of the actual +1 / -1 / 0 level of each phase is:

[0080]

[0081]

[0082]

[0083] Among them, d px d nx d zxare the +1, -1, and 0 level duty ratios of the x-phase output in the current switching cycle, respectively. z is the minimum duty cycle of the three phases outputting constant +1 level and -1 level respectively in the current switching cycle, d z =min{(1-λ Lx )(1+v mx ),(1-λ Lx )(1-v mx )}.

[0084] According to an embodiment of the present invention, v mxp and v mxn They are:

[0085] v mxp =d px

[0086] v mxn =-d nx

[0087] In this embodiment, the midpoint current i mid , each phase inductor current i ox The relationship between them is:

[0088] i mid =d za i oa +d zb i ob +d zc i oc

[0089] Therefore, by adjusting d z , thereby adjusting d za d zb d zc , so that the midpoint current i at any time mid =0.

[0090] Operation S4, using v mCM v md0 Perform open-loop compensation of the common-mode voltage to obtain the fourth bridge arm modulation wave v md .

[0091] Operation S5, v mxp 、v mxn 、v md They intersect with the corresponding carrier waves respectively to generate driving signals to drive the inverter.

[0092] Preferably, the carrier corresponding to the three-level three-phase bridge arm is a stacked carrier; the carrier corresponding to the two-level fourth bridge arm is a triangular carrier. mxp 、v mxnThey are respectively intercepted with the stacked carrier to generate driving signals for the three-level three-phase bridge arm switch tubes; md The symmetrical triangular carrier is intercepted to generate a driving signal for the two-level fourth bridge arm switch tube. In response to the driving signal, the three-phase four-bridge arm three-level inverter operates stably.

[0093] Figure 4 The a-phase modulation waveform diagram of the three-phase four-bridge-arm three-level inverter modulation method for realizing zero midpoint current in an embodiment of the present invention is given. Different from the traditional carrier modulation method, the output voltage waveform of each phase in one switching cycle is a three-level waveform.

[0094] by Figure 3 The main parameter settings in the test example shown are as follows to verify the effectiveness of this method. Rated capacity S = 10kVA, DC input voltage V dc =800V, AC output voltage V Cfx =220V(RMS), output frequency f o =50Hz, the switching frequency of the three-phase inverter f s =10kHz, the switching frequency of the fourth bridge arm f sd =10kHz, DC side split capacitor C busp =C busn =25μF, filter inductor L fx =2.4mF, neutral line inductance L N =2.4mF, filter capacitor C f =10μF.

[0095] Under the above parameter settings, when the traditional carrier modulation method is used, the bus capacitor voltage waveform of the three-phase four-bridge three-level inverter when the power factor is 0 is as follows: Figure 5A As shown, the control principle of the traditional carrier modulation method is as follows Figure 6 When the modulation method provided by the embodiment of the present invention is adopted, the bus capacitor voltage waveform of the three-phase four-bridge-arm three-level inverter when the power factor is 0 is as shown in FIG. Figure 5B shown.

[0096] contrast Figure 5A and Figure 5B It can be seen that: when the inverter works under the condition of power factor 0, when the traditional method is adopted, the bus capacitor voltage diverges, which seriously threatens the normal operation of the system; while when the modulation method provided by the embodiment is adopted, the bus capacitor voltage converges to around 400V, and the voltage balancing effect is good.

[0097] Fig. 7A , Figure 7B , Figure 7CThe working waveforms of the three-phase four-bridge-arm three-level inverter under different loads using the modulation method provided by the embodiment of the present invention are respectively shown. It can be seen that by using the modulation method provided by the embodiment of the present invention, when the three-phase four-bridge-arm three-level inverter operates under an unbalanced load and different power factors, the split capacitor voltage can always maintain a balance. At the same time, the modulation method eliminates the low-frequency pulsation of the capacitor voltage, so that the voltage pulsation is kept small under any working conditions.

[0098] The embodiment of the present invention also provides a three-phase three-bridge arm three-level inverter modulation method for realizing zero midpoint current. The three-phase three-bridge arm three-level inverter includes a three-level three-phase bridge arm and an LC AC filter, and the LC AC filter is connected to the output end of the three-level three-phase bridge arm. The method includes the following operations S1'-operation S4'.

[0099] Operate S1', according to the AC filter capacitor voltage v of each phase Cfx And each phase AC filter inductor current i Lfx , calculate the differential mode component v of the three-phase bridge arm modulation wave mxDM .

[0100] Operation S2', according to the three-phase bridge arm modulation wave v at the previous moment mx_pre , determine the phase where the current switching cycle outputs a constant +1, -1 level, and determine the common mode component v of the three-phase bridge arm modulation wave according to the phase mCM .

[0101] Operation S3', v mxDM and v mCM Add together to get the three-phase bridge arm modulation wave v mx , according to v mx Calculate the duty ratio of the actual +1, -1, and 0 levels of the three phases, and use the duty ratio to calculate the positive modulation wave v that is intercepted by the three-phase bridge arm and the upper and lower stacked carriers in real time mxp and negative modulation wave v mxn .

[0102] Operation S4', v mxp 、v mxn They intersect with the corresponding carrier waves respectively to generate driving signals to drive the inverter.

[0103] In this embodiment, in addition to making the load imbalance of each phase Lx Except for taking 0, the generation process of the driving signal of the three-level three-phase bridge arm is the same as the generation process of the driving signal of the three-level three-phase bridge arm in the above embodiment, which will not be repeated here.

[0104] An embodiment of the present invention further provides a controller for executing the above-mentioned three-phase four-bridge-arm three-level inverter modulation method for realizing zero midpoint current, or for executing the above-mentioned three-phase three-bridge-arm three-level inverter modulation method for realizing zero midpoint current.

[0105] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for realizing modulation of a three-phase four-bridge-arm three-level inverter with zero midpoint current, characterized in that: The inverter includes a three-level three-phase bridge arm, a two-level fourth bridge arm, an LC AC filter and a neutral line inductor, the output end of the three-level three-phase bridge arm is connected to the LC AC filter, and the neutral point of the AC filter capacitor is connected to the middle point of the fourth bridge arm through the neutral line inductor, and the method includes: According to the AC filter capacitor voltage v of each phase Cfx And each phase AC filter inductor current i Lfx , calculate the differential mode component v of the three-phase bridge arm modulation wave mxDM The fourth bridge arm modulation wave v before common mode voltage compensation md0 ; According to the load imbalance of each phase λ Lx And the three-phase bridge arm modulation wave v at the previous moment mx_pre , determine the phase where the current switching cycle outputs a constant +1, -1 level, and determine the common mode component v of the three-phase bridge arm modulation wave according to the phase mCM ; v mxDM and v mCM Add together to get the three-phase bridge arm modulation wave v mx , according to λ Lx and v mx Calculate the duty ratio of the actual +1, -1, and 0 levels of the three phases, and use the duty ratio to calculate the positive modulation wave v that is intercepted by the three-phase bridge arm and the upper and lower stacked carriers in real time mxp and negative modulation wave v mxn ; Using v mCM v md0 Perform open-loop compensation of the common-mode voltage to obtain the fourth bridge arm modulation wave v md ; v mxp 、v mxn 、v md Intersecting with corresponding carrier waves respectively to generate driving signals to drive the inverter; The step of determining the phase in which the current switching cycle outputs a constant +1 or -1 level specifically includes: Lx and v mx_pre , calculate the duty cycle d of each phase to meet the constant +1 level output zpx And the duty ratio d of each phase to meet the constant -1 level output znx ; will d zpx The phase corresponding to the minimum value and d znx The phase corresponding to the minimum value is the phase that outputs at a constant +1 or -1 level in the current switching cycle, and x is the phase among the three phases a, b, and c; v mCM for: Among them, v maDM 、v mbDM 、v mcDM are the differential mode components of the modulation waves of the a, b, and c phase bridge arms, respectively, La , Lb , Lc are the load imbalance of phases a, b, and c respectively, mind zpx Indicates d zpa d zpb d zpc The minimum value in mind znx Indicates d zna d znb d znc The minimum value in .

2. The method according to claim 1, characterized in that λ Lx for: Among them, I m is the peak current under three-phase full load, RMS 50 (i Lfx ) indicates i Lfx The effective value at a specific frequency of 50 Hz, where x is the phase among the three phases a, b, and c.

3. The method according to claim 1, characterized in that d zpx and d znx They are: d zpx =(1-λ Lx )(1+v mx_pre ), d znx =(1-λ Lx )(1-v mx_pre ).

4. The method according to claim 1, characterized in that The actual duty ratio of +1, -1, 0 level of each phase is: Among them, d px d nx d zx are the +1, -1, and 0 level duty ratios of the x-phase output in the current switching cycle, respectively. z is the minimum duty cycle of the three phases outputting constant +1 level and -1 level respectively in the current switching cycle, d z =min{(1-λ Lx )(1+v mx ),(1-λ Lx )(1-v mx )}.

5. The method according to claim 4, characterized in that v mxp and v mxn They are: v mxp =d px 、v mxn =-d nx .

6. A three-phase three-bridge-arm three-level inverter modulation method for realizing zero midpoint current, characterized in that: The inverter includes a three-level three-phase bridge arm and an LC AC filter, the LC AC filter is connected to the output end of the three-level three-phase bridge arm, and the method includes: According to the AC filter capacitor voltage v of each phase Cfx And each phase AC filter inductor current i Lfx , calculate the differential mode component v of the three-phase bridge arm modulation wave mxDM ; According to the three-phase bridge arm modulation wave v at the previous moment mx_pre , determine the phase where the current switching cycle outputs a constant +1, -1 level, and determine the common mode component v of the three-phase bridge arm modulation wave according to the phase mCM ; v mxDM and v mCM Add together to get the three-phase bridge arm modulation wave v mx , according to v mx Calculate the duty ratio of the actual +1, -1, and 0 levels of the three phases, and use the duty ratio to calculate the positive modulation wave v that is intercepted by the three-phase bridge arm and the upper and lower stacked carriers in real time mxp and negative modulation wave v mxn ; v mxp 、v mxn Intersecting with corresponding carrier waves respectively to generate driving signals to drive the inverter; The step of determining the phase in which the current switching cycle outputs a constant +1 or -1 level specifically includes: mx_pre , calculate the duty cycle d of each phase to meet the constant +1 level output zpx And the duty ratio d of each phase to meet the constant -1 level output znx ; will d zpx The phase corresponding to the minimum value and d znx The phase corresponding to the minimum value is the phase that outputs at a constant +1 or -1 level in the current switching cycle, and x is the phase among the three phases a, b, and c; v mCM for: Among them, v maDM 、v mbDM 、v mcDM are the differential mode components of the modulation waves of the a, b, and c phase bridge arms, respectively, La , Lb , Lc are the load imbalance of phases a, b, and c respectively, mind zpx Indicates d zpa d zpb d zpc The minimum value in mind znx Indicates d zna d znb d znc The minimum value in .

7. A controller, characterized in that: Used to execute the three-phase four-bridge-arm three-level inverter modulation method for realizing zero midpoint current as described in any one of claims 1-5, or to execute the three-phase three-bridge-arm three-level inverter modulation method for realizing zero midpoint current as described in claim 6.

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