CN112217409A - Variable carrier pulse width modulation system and method of three-phase four-bridge arm voltage type inverter - Google Patents

Abstract

The invention provides a variable carrier pulse width modulation method of a three-phase four-bridge arm voltage type inverter, which comprises the following steps of: acquiring a three-phase output sinusoidal current expected value of the inverter, and sampling a three-phase output current instantaneous value of the inverter in real time; subtracting the actual instantaneous current value from the three-phase current expected value of the inverter in each control sampling period to obtain a three-phase current deviation value; calculating the three-phase current control quantity; selecting a three-phase carrier signal required by pulse width modulation according to the three-phase current control quantity; and comparing each phase modulation signal with the corresponding each phase carrier signal to obtain a three-phase switch driving signal with variable pulse width, and using the three-phase switch driving signal to control the on-off operation of a three-phase bridge arm of the inverter circuit. The input end of the N-phase bridge arm controller is connected with the driving signals of the three-phase bridge arm, and the driving signals of the N-phase bridge arm are generated according to the driving signals of the three-phase bridge arm. The invention effectively reduces the electromagnetic interference to the earth leakage current and the like caused by the common mode disturbance voltage of the neutral point.

Description

Variable carrier pulse width modulation system and method for three-phase four-leg voltage inverter

technical field

The invention belongs to the technical field of inverter pulse width modulation, and in particular relates to a variable carrier pulse width modulation system and method of a three-phase four-arm voltage inverter.

Background technique

Energy issues have always been an important factor restricting the development of a human society. Every major progress in society is inseparable from the improvement and replacement of energy. Energy saving and environmental protection will be an important option for sustainable human development and avoiding catastrophic climate change. The use of electric energy in the world today accounts for about 40% of the total energy.

On the power consumption side, the motor (especially the three-phase motor) is a high-energy-consuming power equipment with a large amount of applications and a wide range of applications. For example, according to statistics, my country's motor power consumption accounts for about 60% to 70% of the total industrial power consumption. At present, about 90% of electric machines in the world use asynchronous motors, of which small asynchronous motors account for more than 70%. In the total load of the power system, the electricity consumption of asynchronous motors accounts for a large proportion. In my country, the electricity consumption of asynchronous motors accounts for more than 60% of the total load. However, a large number of industrial equipment, such as fans, pumps and traditional industrial sewing machines, machining equipment, etc., mostly use the scheme of constant speed transmission of asynchronous motors, resulting in generally low efficiency of AC motors. Moreover, in industrial sewing machines and mechanical processing equipment, clutches and friction plates are often used to adjust the speed, resulting in a large amount of standby loss and braking energy consumption. If the frequency conversion speed control device based on the power electronic inverter is used to drive the motor equipment, the industrial user can at least save more than 18% of the electric energy on the existing basis. Therefore, many countries in the world and my country are actively encouraging industrial enterprises to promote advanced means such as frequency conversion speed regulation to achieve energy saving, and at the same time, it can also significantly improve the working performance of electric drive equipment. In addition, mobile devices represented by electric vehicles also rely on the inverter interface to connect to the grid for charging and discharging.

On the power generation side, the use of clean and renewable energy (including wind energy, solar energy, hydrogen energy, etc.) to generate electricity is an important means for countries around the world to ensure energy security, reduce carbon emissions and avoid global warming. In the past ten years, my country's energy structure has been continuously improved and optimized, and the proportion of clean energy (including wind energy, solar energy, hydrogen energy, etc.) has been rapidly increased. Whether it is grid-connected power generation or off-grid power generation, power electronic inverters are an indispensable power interface for clean energy power generation, and a key device for fully and high-quality development and utilization of clean energy.

In medium and high power applications, three-phase voltage inverters are the core components for motor speed regulation and clean energy power supply. and safety have a decisive impact. As a three-phase voltage inverter, the three-phase four-leg voltage inverter can flexibly provide three-phase four-wire system, three-phase three-wire power and even single-phase interface for the system, and can use the auxiliary bridge arm in three-phase unbalanced conditions, etc. It is widely used in electric vehicle charging and discharging, new energy grid-connected power generation and motor drive. The current three-phase four-arm voltage inverter often uses pulse width modulation technology to achieve accurate control of the output voltage or current, and its pulse width modulator generally uses a fixed carrier signal. However, inverters using single-carrier PWM schemes (such as sinusoidal PWM, space voltage vector modulation, discontinuous PWM, etc.) are used to drive star-connected three-phase loads (especially three-phase balanced asynchronous motors). ) or transformer, the amplitude of the common-mode disturbance voltage of the star-connected neutral point will be as high as half of the inverter DC bus voltage, which will bring serious electromagnetic interference such as neutral point-to-ground leakage current and insulation performance degradation. And other issues.

SUMMARY OF THE INVENTION

The purpose of the present invention is to solve the above-mentioned deficiencies in the background technology, and to provide a variable carrier pulse width modulation system and method for a three-phase four-arm voltage inverter, which can effectively reduce the common mode disturbance voltage at the neutral point. Electromagnetic interference such as ground leakage current and insulation degradation.

The technical scheme adopted in the present invention is: a variable carrier pulse width modulation method of a three-phase four-arm voltage inverter, which is characterized by comprising the following steps:

A. Obtain the expected value of the three-phase output sinusoidal current of the inverter, and sample the instantaneous value of the three-phase output current of the inverter in real time;

B. In each control sampling period, subtract the three-phase current expected value of the inverter and the detected actual instantaneous current value to obtain the three-phase current deviation value;

C. Calculate the three-phase current control amount according to the three-phase current deviation value as the three-phase modulation signal;

D. Select the three-phase carrier signal required by the pulse width modulation according to the three-phase current control quantity;

E. Compare the modulation signal of each phase with the corresponding carrier signal of each phase to generate a three-phase switch drive signal with pulse width variation, and use the three-phase switch drive signal to control the on-off operation of the three-phase bridge arm of the inverter circuit .

In the above technical solution, the three-phase carrier signal described in the step D is formed by the combination of two carrier signal sources; and carrier 2.

In the above technical solution, the step D specifically includes the following steps: sorting the three-phase current control quantities according to the numerical value of each phase control quantity, determining the grouping type of the control quantity, and then according to the grouping type of the control quantity, by looking up the table from The carrier selection table 1 or the carrier selection table 2 is changed and selected to obtain a three-phase carrier.

In the above technical solution, in the step A, the expected value of the output current of each phase of the inverter is:

分别为逆变器A、B、C三相输出电流期望值的有效值，

分别为A、B、C三相输出电流期望值的相位角；ω为输出电流的角频率。in,

are the effective values of the expected values of the three-phase output currents of inverters A, B, and C, respectively,

are the phase angles of the expected values of the three-phase output currents of A, B, and C, respectively; ω is the angular frequency of the output current.

In the above technical solution, in the step A, the instantaneous values of the three-phase expected output currents of the inverters A, B, and C are respectively:

分别为A、B、C三相输出电流瞬时值的相位角。Among them, I _a , I _b , I _c , are the effective values of the instantaneous values of the three-phase output currents of the inverters A, B, and C, respectively,

are the phase angles of the instantaneous values of the three-phase output currents of A, B, and C, respectively.

In the above technical solution, in the step E, the three-phase modulation signal and the corresponding three-phase carrier signal are subtracted to obtain a three-phase signal; The phase square wave signal is used to control the on-off of the three-phase bridge arm of the inverter circuit; when any phase difference value signal is greater than or equal to zero, the generated switch drive signal S*+=1 will turn on the upper bridge of the corresponding phase bridge arm arm and disconnect the lower bridge arm; on the contrary, when any phase difference value signal is less than zero, the generated switch drive signal S*+=0 will disconnect the upper bridge arm of the corresponding phase bridge arm and turn on the lower bridge arm; wherein * is A or B or C, SA+ refers to the switch drive signal of the A-phase bridge arm, SB+ refers to the switch drive signal of the B-phase bridge arm, and SC+ refers to the switch drive signal of the C-phase bridge arm.

In the above technical solution, when the inverter is running with three phases and four wires, the N bridge arm switch controller checks the N phase bridge arm switch control table in real time according to the state of the three phase switch drive signal to generate the N bridge arm switch drive signal; When the inverter is in three-phase three-wire operation, all switches of the N bridge arm are set to be off, and the inverter is directly connected to the three-phase three-wire load or the transformer to operate normally.

In the above technical solution, the judgment logic of the carrier selection table 1 is as follows:

When v _a >v _b ≥ v _c , the A-phase carrier selects carrier 1, the B-phase carrier selects carrier 2, and the C-phase carrier selects carrier 1;

When v _c ≥ v _b > v _a , the A-phase carrier selects carrier 2, the B-phase carrier selects carrier 1, and the C-phase carrier selects carrier 2;

When v _b ≥ v _a > v _c , the A-phase carrier selects carrier 1, the B-phase carrier selects carrier 2, and the C-phase carrier selects carrier 2;

When v _c > v _a ≥ v _b , the A-phase carrier selects carrier 2, the B-phase carrier selects carrier 1, and the C-phase carrier selects carrier 1;

When v _b > v _c ≥ v _a , the A-phase carrier selects carrier 1, the B-phase carrier selects carrier 1, and the C-phase carrier selects carrier 2;

When v _a ≥ v _c > v _b , the A-phase carrier selects carrier 2, the B-phase carrier selects carrier 2, and the C-phase carrier selects carrier 1;

The judgment logic of the carrier selection table 2 is as follows:

When v _a >v _b ≥ v _c , the A-phase carrier selects carrier 2, the B-phase carrier selects carrier 1, and the C-phase carrier selects carrier 2;

When v _c ≥ v _b > v _a , the A-phase carrier selects carrier 1, the B-phase carrier selects carrier 2, and the C-phase carrier selects carrier 1;

When v _b ≥ v _a > v _c , the A-phase carrier selects carrier 2, the B-phase carrier selects carrier 1, and the C-phase carrier selects carrier 1;

When v _c > v _a ≥ v _b , the A-phase carrier selects carrier 1, the B-phase carrier selects carrier 2, and the C-phase carrier selects carrier 2;

When v _b > v _c ≥ v _a , the A-phase carrier selects carrier 2, the B-phase carrier selects carrier 2, and the C-phase carrier selects carrier 1;

When v _a ≥ v _c > v _b , the A-phase carrier selects carrier 1, the B-phase carrier selects carrier 1, and the C-phase carrier selects carrier 2;

Among them, v _a refers to the A-phase current control amount, v _b refers to the A-phase current control amount, and _vc refers to the C-phase current control amount.

In the above technical solution, the judgment logic of the N-phase bridge arm switch control table is as follows: wherein, SN+ refers to the switch drive signal of the N-phase bridge arm;

When SA+=0, SB+=0, SC+=0, SN+=1, the upper bridge arm of the N-phase bridge arm is turned on and the lower bridge arm is disconnected;

When SA+=0, SB+=0, SC+=1, SN+=1, the upper bridge arm of the N-phase bridge arm is turned on and the lower bridge arm is disconnected;

When SA+=0, SB+=1, SC+=0, SN+=1, the upper bridge arm of the N-phase bridge arm is turned on and the lower bridge arm is disconnected;

When SA+=0, SB+=1, SC+=1, SN+=1, the upper bridge arm of the N-phase bridge arm is disconnected and the lower bridge arm is turned on;

When SA+=1, SB+=0, SC+=1, SN+=0, the upper bridge arm of the N-phase bridge arm is turned on and the lower bridge arm is disconnected;

When SA+=1, SB+=0, SC+=0, SN+=1, the upper bridge arm of the N-phase bridge arm is disconnected and the lower bridge arm is turned on;

When SA+=1, SB+=1, SC+=0, SN+=0, the upper bridge arm of the N-phase bridge arm is disconnected and the lower bridge arm is turned on;

When SA+=1, SB+=1, SC+=1, SN+=0, the upper bridge arm of the N-phase bridge arm is disconnected and the lower bridge arm is turned on.

The invention provides a variable carrier pulse width modulation system of a three-phase four-arm voltage inverter, which is characterized by comprising a current controller, a pulse width modulator, an N-phase bridge controller; a three-phase voltage inverter The expected value of the output current of each phase and the instantaneous value of the output current are input to the current controller; the current controller calculates the three-phase current control amount according to the current deviation as a three-phase modulation signal and outputs it to the pulse width modulator; the pulse width controller A three-phase carrier signal is connected to the input terminal of the PWM controller; the comparator inside the pulse width controller compares the input modulation signal of each phase with the corresponding carrier signal of each phase, and generates a drive signal for the three-phase bridge arm; the N-phase bridge arm controls The input terminal of the device is connected to the drive signal of the three-phase bridge arm, and generates the drive signal of the N-phase bridge arm according to the drive signal of the three-phase bridge arm.

The beneficial effects of the present invention are as follows: the present invention provides a variable carrier sinusoidal pulse width modulation scheme for the inverter to eliminate the interference of the common mode voltage of the three-phase four-bridge inverter power supply, which is different from the conventional single-carrier sinusoidal pulse width modulation scheme. Compared with the three-phase four-wire operation of the system, the modulation scheme provided by the present invention achieves the same operating performance (such as linear modulation range, DC voltage utilization, output current waveform quality, etc.) In the case of overmodulation, the magnitude of the common mode disturbance voltage of the load neutral point connected to the N bridge arm can be significantly reduced, and the neutral point voltage can be equal to or close to zero in the three-phase basic balance and linear modulation range. Therefore, the electromagnetic interference caused by the leakage current of the neutral point to the ground is effectively reduced; when all the N-arm switches are turned off, the inverter can directly operate with three-phase three-wire power supply, and when the load or When the transformer is connected to a three-phase star, it can also greatly reduce the common mode voltage disturbance of the neutral point of the star connection and reduce the electromagnetic interference of the system.

Description of drawings

Fig. 1 is the system connection schematic diagram of the present invention;

Figure 2 is a schematic diagram of a three-phase four-leg voltage inverter;

3 is a schematic diagram of a carrier signal 1;

4 is a schematic diagram of a carrier signal 2;

5 is a schematic diagram of carrier selection table 1;

6 is a schematic diagram of carrier selection table 2;

FIG. 7 is a schematic diagram of waveforms of a specific embodiment.

Detailed ways

The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, so as to facilitate a clear understanding of the present invention, but they do not limit the present invention.

The invention provides a variable carrier pulse width modulation method of a three-phase four-arm voltage inverter, which is characterized by comprising the following steps:

A. Obtain the expected value of the three-phase output sinusoidal current of the inverter, and sample the instantaneous value of the three-phase output current of the inverter in real time;

B. In each control sampling period, subtract the three-phase current expected value of the inverter and the detected actual instantaneous current value to obtain the three-phase current deviation value;

C. Calculate the three-phase current control amount according to the three-phase current deviation value as the three-phase modulation signal;

D. Select the three-phase carrier signal required by the pulse width modulation according to the three-phase current control quantity;

E. Compare the modulation signal of each phase with the corresponding carrier signal of each phase to generate a three-phase switch drive signal with pulse width variation, and use the three-phase switch drive signal to control the on-off operation of the three-phase bridge arm of the inverter circuit .

As shown in FIG. 1 , the present invention provides a variable carrier pulse width modulation system of a three-phase four-arm voltage inverter, which is characterized by comprising a current controller, a pulse width modulator, and an N-phase bridge arm controller; The expected value of each phase output current and the instantaneous value of the output current of the three-phase voltage inverter are input to the current controller; the current controller calculates the three-phase current control amount according to the current deviation as a three-phase modulation signal and outputs it to the pulse width modulation The input terminal of the pulse width controller is connected with a three-phase carrier signal; the comparator inside the pulse width controller compares the input modulation signal of each phase with the corresponding carrier signal of each phase, and generates a drive signal for the three-phase bridge arm. ; The input end of the N-phase bridge arm controller is connected to the drive signal of the three-phase bridge arm, and generates the drive signal of the N-phase bridge arm according to the drive signal of the three-phase bridge arm.

The purpose of the present invention is to provide a variable carrier sinusoidal pulse width modulation method for a three-phase four-bridge voltage inverter that can eliminate the interference of the common mode voltage of the power supply. The carrier signal sources with an intersection of 180 degrees are combined according to the provided selection algorithm, and the switch on and off of the N bridge arm is determined by the switch states of the A, B, and C phases. Compared with the single-carrier sine PWM, the three-phase four-leg inverter adopts the proposed variable-carrier sine PWM, not only the main operating technical indicators (such as DC voltage utilization, output current waveform quality, switching loss, etc.) Basically the same, it can also significantly reduce the neutral point common mode disturbance voltage amplitude of the star-connected three-phase load/transformer, and achieve zero neutral point common mode disturbance voltage in the case of three-phase balance, thereby effectively reducing the neutral point. Electromagnetic interference such as ground leakage current and insulation degradation caused by common mode disturbance voltage at the neutral point.

As shown in FIG. 2 , the three-phase four-leg voltage inverter of the present invention includes a power conversion circuit, a filter circuit, a controller, and a three-phase star load/transformer.

The output end of the inverter is connected with a three-phase star load, and the neutral point of the star connection of the load is n.

The power conversion circuit includes an A bridge arm, a B bridge arm, a C bridge arm and an N bridge arm connected in parallel, and each bridge arm is composed of an upper bridge arm switch and a lower bridge arm switch in series. The output of the power conversion circuit is connected to the three-phase star load through the filter circuit.

The controller calculates and gives the expected value of the three-phase output sinusoidal current of the inverter according to the power demand, and the controller samples the instantaneous value of the three-phase output current of the inverter in real time. As shown in Figure 1, the current controller subtracts the three-phase expected current values of A, B, and C from the detected actual instantaneous current value in each sampling period to obtain the three-phase current deviation value, and then the current controller calculates the three-phase current deviation according to the current deviation. out its three-phase current control amount.

Among them, the expected value of the output current of each phase of the three-phase voltage inverter is:

分别为逆变器A、B、C三相输出电流期望值的有效值，

分别为A、B、C三相输出电流期望值的相位角；ω为输出电流的角频率。in,

are the effective values of the expected values of the three-phase output currents of inverters A, B, and C, respectively,

are the phase angles of the expected values of the three-phase output currents of A, B, and C, respectively; ω is the angular frequency of the output current.

The instantaneous values of the three-phase expected output currents of inverters A, B, and C are:

分别为A、B、C三相输出电流瞬时值的相位角。Among them, I _a , I _b , and I _c are the effective values of the instantaneous values of the three-phase output currents of inverters A, B, and C, respectively,

are the phase angles of the instantaneous values of the three-phase output currents of A, B, and C, respectively.

分别与检测到的实际电流i_a,i_b,i_c相减，形成偏差，电流控制器根据偏差计算出电流控制量v_a,v_b,v_c。The controller converts the desired output currents of the three phases A, B and C in each control cycle

They are respectively subtracted from the detected actual currents i _a , i _b , _ic to form a deviation, and the current controller calculates the current control variables v _a , v _b , v _c according to the deviation.

The three-phase current control quantities v _a , v _b , and v _c are directly transmitted to the pulse width modulator as three-phase modulation signals.

The three-phase current control quantities v _a , v _b , and v _c are also used as input quantities to select the three-phase carrier signal: first, the controller converts the three-phase current control quantities v _a , v _b , v _c according to the value of each phase control quantity The size is sorted high and low to determine its grouping type, and then according to its grouping type, the corresponding three-phase carrier is selected from the carrier selection table 1 (or carrier selection table 2) by looking up the table. Bipolar triangular (or sawtooth) signal source carrier 1 and carrier 2 whose frequency and phase differ by 180 degrees are combined.

In the embodiment shown in FIG. 3, in the period of t1-t2, if the carrier is selected according to Table 1, the A-phase and C-phase carriers are selected as carrier 1, and the B-phase carrier is selected as carrier 2; if the carrier is selected according to Table 2 , then the A-phase and C-phase carriers select carrier 2, and the B-phase carrier selects carrier 1.

The pulse width modulator subtracts the input A, B, C three-phase modulation signals and the corresponding A, B, C three-phase carrier signals to obtain three phase difference signals. The three-phase difference value signal is compared with zero by the comparator, thereby generating a three-phase square wave signal with varying pulse width to control the on-off of the three-phase bridge arm of the inverter circuit.

In the embodiment, when any phase difference value signal is greater than or equal to zero, the generated driving signal S*+=1 (where *=A, B or C) will turn on the upper bridge arm of the corresponding phase bridge arm and disconnect the lower bridge arm On the contrary, when any phase difference value signal is less than zero, the generated drive signal of the phase bridge arm will disconnect the upper bridge arm of the corresponding phase bridge arm and turn on the lower bridge arm.

When the inverter is in three-phase four-wire operation, the N-bridge switch controller searches for the switch drive signal of the N-bridge arm in real time according to the provided switch list according to the states of the three-phase switch drive signals of A, B, and C, for example, When the drive signals of the three-phase bridge arm switches of A, B, and C are SA+=0, SB+=0 and SC+=1, respectively, the switch signal of the N arm will be SN+=1.

When the inverter is in three-phase three-wire operation, all switches of the N bridge arm are set to be off, that is, SN+=SN-=0.

The invention can greatly reduce the three-phase connection of the star connection without changing the main performance indicators of the three-phase voltage inverter system (such as linear modulation range, DC bus voltage utilization rate, output voltage/current waveform quality, switching loss, etc.). Phase load/transformer neutral point common mode voltage disturbance: When the inverter is powered by a three-phase four-wire system, the variable carrier sinusoidal modulation can change the neutral point of a three-phase balanced star load/transformer within the linear modulation range The common mode disturbance voltage is eliminated to zero, and the midpoint common mode disturbance voltage amplitude can also be reduced to a quarter of the DC bus voltage value under saturation modulation; when the inverter is powered by a three-phase three-wire system, it can also be Reduces the neutral point common mode disturbance voltage amplitude of a three-phase balanced star load/transformer to one sixth of the DC bus voltage value. Therefore, the variable carrier sinusoidal pulse width modulation proposed by the present invention can improve the electromagnetic compatibility and insulation characteristics of star-shaped three-phase loads (especially three-phase balanced motors) or transformers, which is of great significance.

Contents not described in detail in this specification belong to the prior art known to those skilled in the art.

Claims (10)

1. A variable carrier pulse width modulation method of a three-phase four-bridge arm voltage type inverter is characterized by comprising the following steps:

A. acquiring a three-phase output sinusoidal current expected value of the inverter, and sampling a three-phase output current instantaneous value of the inverter in real time;

B. subtracting the actual instantaneous current value from the three-phase current expected value of the inverter in each control sampling period to obtain a three-phase current deviation value;

C. calculating three-phase current control quantity according to the three-phase current deviation value to serve as a three-phase modulation signal;

D. selecting a three-phase carrier signal required by pulse width modulation according to the three-phase current control quantity;

E. and comparing each phase modulation signal with each corresponding phase carrier signal to generate a three-phase switch driving signal with variable pulse width, and using the three-phase switch driving signal to control the on-off operation of a three-phase bridge arm of the inverter circuit.

2. The method for modulating the variable carrier pulse width of the three-phase four-leg voltage-type inverter according to claim 1, wherein the method comprises the following steps: the three-phase carrier signal in the step D is formed by combining two paths of carrier signal sources; the two carrier signal sources consist of a bipolar triangular or sawtooth carrier 1 and a carrier 2 which have the same frequency and 180-degree phase difference.

3. The method for modulating the variable carrier pulse width of the three-phase four-leg voltage-type inverter according to claim 2, wherein the method comprises the following steps: the step D specifically comprises the following steps: and then, the three-phase carrier waves are obtained by changing and selecting from the carrier wave selection table 1 or the carrier wave selection table 2 through table look-up according to the grouping type of the control quantity.

4. The method for modulating the variable carrier pulse width of the three-phase four-leg voltage-type inverter according to claim 1, wherein the method comprises the following steps: in the step a, the expected value of each phase output current of the inverter is:

wherein,

effective values for the desired values of the three-phase output current from inverter A, B, C,

phase angles that are respectively A, B, C three-phase output current expected values; ω is the angular frequency of the output current.

5. The method for modulating the variable carrier pulse width of the three-phase four-leg voltage-type inverter according to claim 1, wherein the method comprises the following steps: in step a, instantaneous values of three-phase desired output currents of the inverter A, B, C are:

wherein, I_a,I_b,I_cEffective values of the three-phase output current transients of inverter A, B, C,

phase angles of A, B, C three-phase output current transients, respectively; ω is the angular frequency of the output current.

6. The method for modulating the variable carrier pulse width of the three-phase four-leg voltage-type inverter according to claim 1, wherein the method comprises the following steps: in the step E, subtracting the three-phase modulation signal from the corresponding three-phase carrier signal to obtain a three-phase difference signal; the three-phase difference signal is compared with zero, so that a three-phase square wave signal with variable pulse width is generated to control the on-off of a three-phase bridge arm of the inverter circuit; when any phase difference signal is greater than or equal to zero, the generated switch driving signal S & ltx + & gt is 1, and the upper bridge arm of the corresponding phase bridge arm is switched on and the lower bridge arm is switched off; on the contrary, when any phase difference value signal is less than zero, the generated switch driving signal S + is 0, the upper bridge arm of the corresponding phase bridge arm is disconnected, and the lower bridge arm is switched on; and SA + refers to a switch driving signal of the bridge arm of the phase A, SB + refers to a switch driving signal of the bridge arm of the phase B, and SC + refers to a switch driving signal of the bridge arm of the phase C.

7. The method for modulating the variable carrier pulse width of the three-phase four-leg voltage-source inverter according to claim 6, wherein the method comprises the following steps: when the inverter operates in three-phase four-wire mode, the N-bridge arm switch controller checks an N-phase bridge arm switch control table in real time according to the state of the three-phase switch driving signal to generate a switch driving signal of an N-bridge arm; when the inverter operates in three-phase and three-wire mode, all switches of the N bridge arms are set to be turned off, and the inverter is directly connected to a three-phase and three-wire load or a transformer to normally operate.

8. The method for modulating the variable carrier pulse width of the three-phase four-leg voltage-source inverter according to claim 3, wherein the judgment logic of the carrier selection table 1 is as follows:

when v is_a>v_b≥v_cWhen the carrier wave is a carrier wave 1, the carrier wave of the phase A is a carrier wave 2, and the carrier wave of the phase C is a carrier wave 1;

when v is_c≥v_b>v_aWhen the carrier wave of A phase is 2, BThe phase carrier wave is carrier wave 1, and the C phase carrier wave is carrier wave 2;

when v is_b≥v_a>v_cWhen the carrier wave is a carrier wave 1, the carrier wave 2 is a carrier wave 2, and the carrier wave 2 is a carrier wave 2;

when v is_c>v_a≥v_bWhen the carrier wave is a carrier wave 2, the carrier wave 1 is selected as the carrier wave of the phase A, and the carrier wave 1 is selected as the carrier wave of the phase C;

when v is_b>v_c≥v_aWhen the carrier wave is a carrier wave 1, the carrier wave 1 is selected as the A-phase carrier wave, and the carrier wave 2 is selected as the C-phase carrier wave;

when v is_a≥v_c>v_bWhen the carrier wave is a carrier wave 2, the carrier wave 2 is a carrier wave 2, and the carrier wave 1 is a carrier wave 1;

the judgment logic of the carrier selection table 2 is as follows:

when v is_a>v_b≥v_cWhen the carrier wave is a carrier wave 2, the carrier wave 1 is a carrier wave of the phase A, and the carrier wave 2 is a carrier wave of the phase C;

when v is_c≥v_b>v_aWhen the carrier wave is a carrier wave 1, the carrier wave of the phase A is a carrier wave 2, and the carrier wave of the phase C is a carrier wave 1;

when v is_b≥v_a>v_cWhen the carrier wave is a carrier wave 2, the carrier wave 1 is selected as the carrier wave of the phase A, and the carrier wave 1 is selected as the carrier wave of the phase C;

when v is_c>v_a≥v_bWhen the carrier wave is a carrier wave 1, the carrier wave 2 is a carrier wave 2, and the carrier wave 2 is a carrier wave 2;

when v is_b>v_c≥v_aWhen the carrier wave is a carrier wave 2, the carrier wave 2 is a carrier wave 2, and the carrier wave 1 is a carrier wave 1;

when v is_a≥v_c>v_bWhen the carrier wave is a carrier wave 1, the carrier wave 1 is selected as the A-phase carrier wave, and the carrier wave 2 is selected as the C-phase carrier wave;

wherein v is_aMeans phase A current control quantity, v_bMeans phase A current control quantity, v_cRefers to the C-phase current control amount.

9. The method of claim 7, wherein the logic for determining the N-phase bridge arm switch control table is as follows: wherein, SN + refers to a switch driving signal of the N-phase bridge arm;

when SA + (0), SB + (0) and SC + (0), SN + (1), the upper bridge arm of the N-phase bridge arm is switched on and the lower bridge arm is switched off;

when SA + (0), SB + (0) and SC + (1), SN + (1) and the upper bridge arm and the lower bridge arm of the N-phase bridge arm are switched on and off;

when SA + (0), SB + (1) and SC + (0), SN + (1), the upper bridge arm of the N-phase bridge arm is switched on and the lower bridge arm is switched off;

when SA + (0), SB + (1) and SC + (1), SN + (0) and the upper bridge arm of the N-phase bridge arm are disconnected and the lower bridge arm is switched on;

when SA + ═ 1, SB + ═ 0 and SC + ═ 1, SN + ═ 0, the upper bridge arm of the N-phase bridge arm is switched on and the lower bridge arm is switched off;

when SA + ═ 1, SB + ═ 0 and SC + ═ 0, SN + ═ 1, the upper bridge arm of the N-phase bridge arm is disconnected and the lower bridge arm is switched on;

when SA + (1), SB + (1) and SC + (0), SN + (0), the upper bridge arm of the N-phase bridge arm is disconnected and the lower bridge arm is switched on;

when SA + ═ 1, SB + ═ 1, and SC + ═ 1, SN + ═ 0, the upper arm of the N-phase arm is turned off and the lower arm is turned on.

10. A variable carrier pulse width modulation system of a three-phase four-bridge arm voltage type inverter is characterized by comprising a current controller, a pulse width modulator and an N-phase bridge arm controller; the expected value of each phase output current of the three-phase voltage type inverter and the instantaneous value of the output current are input into a current controller; the current controller calculates three-phase current control quantity as a three-phase modulation signal according to the current deviation and outputs the three-phase modulation signal to the pulse width modulator; the input end of the pulse width controller is connected with a three-phase carrier signal; a comparator in the pulse width controller compares the input phase modulation signals with corresponding phase carrier signals to generate driving signals of a three-phase bridge arm; the input end of the N-phase bridge arm controller is connected with the driving signals of the three-phase bridge arm, and the driving signals of the N-phase bridge arm are generated according to the driving signals of the three-phase bridge arm.

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