CN112886822B

CN112886822B - Neutral point potential balance control method and system based on ANPC full-bridge three-level DAB

Info

Publication number: CN112886822B
Application number: CN202110095840.0A
Authority: CN
Inventors: 张宇; 曹清泉
Original assignee: Huazhong University of Science and Technology
Current assignee: Huazhong University of Science and Technology
Priority date: 2021-01-25
Filing date: 2021-01-25
Publication date: 2022-05-24
Anticipated expiration: 2041-01-25
Also published as: CN112886822A

Abstract

The invention discloses a neutral point potential balance control method and system based on ANPC full-bridge three-level DAB, and belongs to the technical field of multi-level converters. Firstly, a switching mode for realizing the voltage boosting and voltage reducing of the midpoint voltage of the ANPC full-bridge three-level circuit is determined, and the control of the midpoint voltage is realized by selecting the voltage boosting and voltage reducing switching mode. Secondly, sampling the direct current side voltage and the midpoint voltage thereof, comparing the sampled midpoint voltage with an upper limit value and a lower limit value, adopting a voltage reduction switch mode when the midpoint voltage is higher than the upper limit value, and adopting a voltage boosting switch mode when the midpoint voltage is lower than the lower limit value. Otherwise, the switching pattern remains unchanged. The modulation method provided by the invention is simple to calculate, can limit the ripple of the midpoint voltage to a set value, is compatible with various modulation strategies, has quick dynamic response, and can obviously improve the anti-interference capability of the system.

Description

Neutral point potential balance control method and system based on ANPC full-bridge three-level DAB

Technical Field

The invention belongs to the technical field of multi-level DC/DC converters, and particularly relates to a neutral point potential balance control method and system based on ANPC full-bridge three-level DAB.

Background

Three-level topologies are increasingly used due to their superior performance. Among them, the widely used three-level circuit is a midpoint clamping type three-level topology, mainly including: a diode Clamped (NPC) three-level topology, a T-type three-level topology, and an Active Neutral Point Clamped (ANPC) topology, etc. The three-level topologies can be extended to full-bridge topologies. The full-bridge topology can improve the capacity and the voltage grade of the system and has a wide application prospect in high-voltage and high-power occasions.

Due to the parameter dispersion of the switching device and the direct-current side capacitor and the influence of external disturbance, in the full-bridge midpoint-clamped three-level converter, the midpoint voltage of the direct-current bus is easy to disperse, and if no measures are taken, the circuit cannot work normally. Therefore, in order to ensure safe and reliable operation of the full-bridge three-level circuit, it is necessary to ensure stable midpoint potential of the circuit.

In order to effectively solve the problem of unbalanced midpoint potential in the full-bridge three-level circuit, the midpoint voltage needs to be controlled in a closed loop mode.

Disclosure of Invention

Aiming at the defects of the prior art, the invention aims to provide a method and a system for controlling the neutral point potential balance of full-bridge three-level DAB, aiming at solving the problem of neutral point potential unbalance in a full-bridge three-level circuit.

To realizeIn order to achieve the above object, the present invention provides a midpoint potential balance control method based on ANPC full-bridge three-level DAB, where the ANPC full-bridge three-level DAB includes a full-bridge ANPC active bridge, an H-bridge circuit, a transformer and an inductor, the full-bridge ANPC active bridge includes a first dc voltage-dividing capacitor, a second dc voltage-dividing capacitor and two half-bridge ANPC bridge arm circuits, and the two dc voltage-dividing capacitors are connected in series to a dc bus voltage V_dc1The voltage on each DC voltage-dividing capacitor is V_dc1The half-bridge ANPC bridge arm circuit connects the midpoint of the bridge arm of the series-connected switching tubes to the series midpoint of the two direct-current voltage-dividing capacitors through the active switching device, so that the voltage born by each switching tube in each half-bridge ANPC bridge arm circuit is V _dc12; the two half-bridge ANPC bridge arm circuits comprise a first switch tube, a second switch tube, a third switch tube, a fourth switch tube, a fifth switch tube, a sixth switch tube, a seventh switch tube, an eighth switch tube, a ninth switch tube, a tenth switch tube, an eleventh switch tube and a twelfth switch tube; the first direct current voltage division capacitor and the second direct current voltage division capacitor are connected in series; the first switching tube, the second switching tube, the third switching tube and the fourth switching tube are sequentially connected in series in a forward direction, and two ends of the first switching tube, the second switching tube, the third switching tube and the fourth switching tube are respectively connected with the voltage of a direct current bus; the fifth switching tube and the sixth switching tube are connected in series in the forward direction, and two ends of the fifth switching tube and the sixth switching tube are respectively connected with the series midpoint of the first switching tube and the second switching tube and the series midpoint of the third switching tube and the fourth switching tube; the series midpoint of the fifth switching tube and the sixth switching tube is connected with the series midpoint of the first direct current voltage-dividing capacitor and the second direct current voltage-dividing capacitor; the seventh switching tube, the eighth switching tube, the ninth switching tube and the tenth switching tube are sequentially connected in series in the forward direction, and two ends of the seventh switching tube, the eighth switching tube, the ninth switching tube and the tenth switching tube are respectively connected with the direct-current bus voltage; the eleventh switching tube and the twelfth switching tube are connected in series in the forward direction, and two ends of the eleventh switching tube and the twelfth switching tube are respectively connected with the series midpoint of the seventh switching tube and the eighth switching tube and the series midpoint of the ninth switching tube and the tenth switching tube; the serial midpoint of the eleventh switching tube and the twelfth switching tube is connected with the serial midpoint of the first direct current voltage-dividing capacitor and the second direct current voltage-dividing capacitor; the series midpoint of the second switching tube and the third switching tube is connected with one end of an inductor, and the other end of the inductor is connected with one side of the transformerOne end of (a); the series midpoint of the eighth switching tube and the ninth switching tube is connected with the other end of one side of the transformer; the first switch tube, the second switch tube, the third switch tube, the fourth switch tube, the fifth switch tube and the sixth switch tube form a first half-bridge ANPC active bridge arm, and the seventh switch tube, the eighth switch tube, the ninth switch tube, the tenth switch tube, the eleventh switch tube and the twelfth switch tube form a second half-bridge ANPC active bridge arm; output level V of full-bridge ANPC active bridge_ABFor two half-bridge ANPC bridge arm circuits output level V_AAnd V_BA difference of (d); the neutral point potential balance control method based on the ANPC full-bridge three-level DAB comprises the following steps of:

s1, sampling the direct-current side voltage V of the full-bridge three-level circuit in the current switching period_dc1And the voltage of the second direct current voltage-dividing capacitor, and the voltage of the second direct current voltage-dividing capacitor is the midpoint voltage V by taking the negative bus as a zero voltage reference point_mid；

S2, comparing the midpoint voltage with a preset upper limit value and a preset lower limit value, selecting a control mode as a boosting switch mode when the midpoint voltage is smaller than the lower limit value, selecting the control mode as a voltage reduction switch mode when the midpoint voltage is larger than the upper limit value, and selecting the control mode as a voltage boosting switch mode when the midpoint current generates negative charge and increases midpoint voltage in one switch cycle;

and S3, entering the next switching period, and transferring to the step S1 to control the neutral point potential balance through the switching mode conversion.

Further, the output of the full-bridge ANPC active bridge has five levels, which may correspond to 9 switch states. Output voltage V_dc1and-V_dc1Respectively corresponding to the switch states U_PNAnd U_NPOutput voltage V_dc1/2 corresponds to the switch state U_POAnd U_ONOutput voltage-V_dc1/2 corresponds to the switch state U_NOAnd U_OPOutput voltage 0 corresponds to switch state U_PP、U_NNAnd U_OO。

Further, the air conditioner is provided with a fan,according to the output level V of the full-bridge ANPC active bridge_ABDivides a switching cycle into 8 switching states, in boost switching mode, the output level V of the full-bridge ANPC active bridge_ABThe eight switch states in a cycle are in turn: u shape_OO、U_PO、U_PN、U_PO、U_OO、U_OP、U_NP、U_OP。

The output levels of eight switching states of the first half-bridge ANPC bridge arm circuit in one switching period are as follows in sequence: 0. v_dc1/2、V_dc1/2、V_dc1/2、0、0、-V_dc1/2、0；

The output levels of eight switching states of the second half-bridge ANPC bridge arm circuit in one switching period are as follows in sequence: 0.0, -V_dc1/2、0、0、V_dc1/2、V_dc1/2、V_dc1/2。

Further, in the buck switching mode, the output level V of the full-bridge ANPC active bridge_ABThe eight switch states in a cycle are in turn: u shape_OO、U_ON、U_PN、U_ON、U_OO、U_NO、U_NP、U_NO；

The output levels of eight switching states of the first half-bridge ANPC bridge arm circuit in one switching period are as follows in sequence: 0.0, V_dc1/2、0、0、-V_dc1/2、-V_dc1/2、-V_dc1/2；

The output levels of eight switching states of the second half-bridge ANPC bridge arm circuit in one switching period are as follows in sequence: 0. -V_dc1/2、-V_dc1/2、-V_dc1/2、0、0、V_dc1/2、0。

Further, when the output level of the first half-bridge ANPC bridge arm circuit is V_dc1When the voltage exceeds 2, the first switching tube, the second switching tube and the sixth switching tube are conducted; the third switching tube, the fourth switching tube and the fifth switching tube are turned off;

when the output level of the second half-bridge ANPC bridge arm circuit is V_dc1When the voltage exceeds 2, the seventh switching tube, the eighth switching tube and the twelfth switching tube are conducted;the ninth switching tube, the tenth switching tube and the eleventh switching tube are turned off;

when the output level of the first half-bridge ANPC bridge arm circuit is-V_dc1When the voltage exceeds 2, the third switching tube, the fourth switching tube and the fifth switching tube are conducted; the first switch tube, the second switch tube and the sixth switch tube are turned off;

when the output level of the second half-bridge ANPC bridge arm circuit is-V_dc1When the voltage exceeds the first threshold voltage,/2, the ninth switching tube, the tenth switching tube and the eleventh switching tube are conducted; the seventh switching tube, the eighth switching tube and the twelfth switching tube are turned off;

when the output level of the first half-bridge ANPC bridge arm circuit is 0, the second switching tube is conducted with the fifth switching tube, and/or the third switching tube is conducted with the sixth switching tube;

when the output level of the second half-bridge ANPC bridge arm circuit is 0, the eighth switching tube is conducted with the eleventh switching tube, and/or the ninth switching tube is conducted with the twelfth switching tube.

The invention provides a midpoint potential balance control system based on ANPC full-bridge three-level DAB, which comprises: a computer-readable storage medium and a processor;

the computer-readable storage medium is used for storing executable instructions;

the processor is used for reading the executable instructions stored in the computer readable storage medium and executing the midpoint potential balance control method of the full-bridge three-level DAB.

Compared with the prior art, the technical scheme provided by the invention aims at the ANPC full-bridge DAB, and the DC side midpoint potential is kept balanced by sampling the DC side voltage and the midpoint voltage, comparing the midpoint voltage with the upper limit value and the lower limit value and selecting the corresponding boosting and reducing switch modes. First, the present invention can limit the midpoint voltage ripple to a given value. Secondly, the method has the advantages of simple calculation process, quick dynamic response and strong anti-interference capability.

The neutral point potential balance control method can be simply popularized to other neutral point clamping type full-bridge topologies.

Drawings

FIG. 1 is a schematic flow chart of a midpoint potential balance control method provided by the present invention;

FIG. 2 is a three-level full bridge DAB topology based on ANPC according to an embodiment of the present invention;

FIG. 3 is a half-bridge circuit diagram of three-level full-bridge DAB based on ANPC according to an embodiment of the present invention;

FIG. 4(a) is a three-level full-bridge output voltage waveform and a half-bridge output voltage waveform per phase in boost switching mode for ANPC based three-level full-bridge DAB according to an embodiment of the present invention;

FIG. 4(b) is a diagram of three-level full-bridge output voltage waveform and each phase half-bridge output voltage waveform of ANPC-based three-level full-bridge DAB in buck switching mode according to the embodiment of the present invention;

FIG. 5 is a schematic diagram of the midpoint current of three-level full bridge DAB based on ANPC according to an embodiment of the present invention;

FIG. 6 is a simulated waveform diagram of the midpoint voltage of three-level full-bridge DAB based on ANPC under rated power according to the embodiment of the present invention;

FIG. 7 is a simulated waveform diagram of midpoint voltage of three-level full bridge DAB based on ANPC according to another embodiment of the present invention;

fig. 8(a) is a simulated waveform diagram of midpoint voltage of three-level full-bridge DAB based on ANPC according to the embodiment of the present invention under sudden load change;

fig. 8(b) is a simulation waveform diagram of output voltage of three-level full-bridge DAB based on ANPC according to the embodiment of the present invention under sudden load change.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

In the full-bridge three-level circuit, the switching modulation strategy implementation method for neutral point potential balance is shown in fig. 1, a negative bus is used as a reference potential, and a direct current sideThe voltage of the second capacitor is the midpoint voltage V_midThe input voltage at the DC side is V_dc1The control strategy comprises the following specific steps:

Examples

In the three-level full-bridge DAB converter based on the ANPC shown in fig. 2, two half-bridge circuits are named as HBA and HBB respectively, and in the HBA, the output of the bridge arm output point a relative to the midpoint O of the direct-current bus is V_AIn HBB, the output of bridge arm output point B relative to DC bus midpoint O is V_B。

For the ANPC half-bridge circuit shown in FIG. 3, it can output three levels V_dc1/2、0、-V_dc1/2}. Output level V of full-bridge three-level circuit_ABIs the difference of the output levels of the two half-bridge three-level circuits. When the capacitor voltage is balanced, the full-bridge three-level circuit can output five levels { V } in the working process_dc1/2、V_dc1/2、0、-V_dc1/2、-V_dc1And/2, when the two half-bridge three-level circuits output five levels in a combined mode, 9 switch states exist, and the states are shown in the table 1.

TABLE 1

In the above 9 combinations, the full bridge outputs V_dc1and-V_dc1Each having only one switching state, output V_dc1[ 2 ] and-V_dc1And/2 has two switch states respectively, and the output 0 has three switch states.

Output V_dc1and-V_dc1Respectively by switching state U_PNAnd U_NPRepresents, outputs V_dc1/2 switch state U_POAnd U_ONRepresents, outputs-V_dc1/2 switch state U_OPAnd U_NOIndicating that output 0 is in switch state U_PP、U_NNAnd U_OOAnd (4) showing.

Eight switch states of one switch cycle are sequentially selected as U_OO、U_PO、U_PN、U_PO、U_OO、U_OP、U_NPAnd U_OPGenerating a boost switching pattern with eight switching states of one switching cycle being sequentially selected as U_OO、U_ON、U_PN、U_ON、U_OO、U_NO、U_NPAnd U_NOGenerating a buck switching mode. In both modes, the three-level full-bridge output voltage waveform and each half-bridge output voltage waveform are shown in fig. 4(a) and 4 (b).

Bridge arm voltage V output in boost switch mode and buck switch mode_ABThe waveforms are consistent and therefore do not affect DAB control of inductor current. Based on the characteristic that the average value of the midpoint currents generated by the two switching modes in one switching period is not zero, the midpoint currents are equal in magnitude and opposite in direction, and a switching modulation strategy can be formed by selecting the two switching modes.

When switching vector U_PO、U_NO、U_OPAnd U_ONWhen acting, a midpoint current is generated at the midpoint O, the existence of the midpoint current influences the midpoint voltage on the direct current side, the magnitude of the midpoint current is equal to the inductance current, and the direction of the midpoint current depends on the adopted switch state.

As shown in FIG. 2The direction of the arrow is the inductive current i_LIn the forward direction of (1), the current i is taken in the direction of the arrow shown in FIG. 5 as the midpoint_midIn the forward direction of (c).

For the switching state U_POWhen the inductor current i_LWith positive direction and current i at midpoint_midDirection is negative when the inductor current i_LWhen the direction is negative, the midpoint current i_midThe direction is positive.

For the switching state U_OPWhen the inductor current i_LWith positive direction and current i at midpoint_midDirection is positive when the inductor current i_LWhen the direction is negative, the midpoint current i_midThe direction is negative.

For the switching state U_ONWhen the inductor current i_LWith positive direction and current i at midpoint_midDirection is positive when the inductor current i_LWhen the direction is negative, the midpoint current i_midThe direction is negative.

For the switching state U_NOWhen the inductor current i_LWith positive direction and current i at midpoint_midDirection is negative when the inductor current i_LWhen the direction is negative, the midpoint current i_midThe direction is positive.

In other words, the relationship between the midpoint current and the inductor current is:

setting the voltage of the first capacitor at the DC side as V_C1The voltage of the second capacitor on the DC side is V_C2The relationship between the capacitor voltage and the midpoint current is as follows:

when the capacitance values of the first capacitor and the second capacitor on the direct current side are both C, the formula can be written as follows:

midpoint voltage V_midThe relationship with the dc side capacitor voltage is:

and (4) substituting (3) into (4), wherein the relation between the midpoint voltage and the midpoint current is as follows:

the change expression of the midpoint voltage caused by the midpoint current in one switching period is as follows:

where T is a switching period, the change in midpoint voltage is negative when the midpoint current direction is positive, and the change in midpoint voltage is positive when the midpoint current direction is negative.

The correspondence between the change of the midpoint current and the midpoint voltage for the four switching states, obtained by substituting (1) into (6), is shown in table 2:

TABLE 2

In boost switching mode, the change in midpoint voltage over a cycle is:

t₀-t₄for the first half period, t₄-t₈The second half period, the inductor current in the first and second half periods is symmetrical, i.e. equal in magnitude and opposite in direction, so (7) can be written as follows:

similarly, for the buck switching mode, the change of the midpoint voltage in one cycle is:

as can be seen from (8) and (9), the change of the midpoint voltage generated in one switching cycle in the boost switching mode and the buck switching mode is equal in magnitude and opposite in direction. In one cycle of DAB, the inductor current i is normally_LAt t₁-t₂And t₃-t₄The integral over time is greater than zero.

When the midpoint voltage is lower than the lower limit value, the control mode is selected to be a boost switching mode. When the midpoint voltage is higher than the upper limit value, the control mode is selected to be a step-down switching mode, otherwise, the control mode is kept unchanged.

The simulation software is used for carrying out simulation verification of the method of the invention, and simulation parameters are shown in table 3.

TABLE 3

Input voltage V_dc1	200V
		Output voltage V_dc2	80V
Rated power P_N	1kW
		Switching frequency f_s	10kHz
Inductor L	250μH
		DC side capacitor C₁、C₂	990μF
Limit of midpoint voltage	98V～102V

The waveform of the midpoint voltage on the dc side at rated power is shown in fig. 6.

As can be seen from fig. 6, the midpoint voltage on the dc side can be stabilized at 100V, the ripple of the midpoint voltage is 4V, and the simulation is consistent with the theory.

When the dc-side first capacitance is 1200 μ F and the dc-side second capacitance is 990 μ F, the waveform of the midpoint voltage is shown in fig. 7.

As can be seen from fig. 7, the midpoint voltage rapidly recovers the equilibrium at 0.05s, and can be stabilized at 100V, and the ripple of the midpoint voltage is 4V.

When the capacitance of the dc-side capacitor is 990 μ F, the load resistance is suddenly changed from 15 Ω to 7.5 Ω at t ═ 0.3s, and the waveforms of the midpoint voltage and the output voltage are shown in fig. 8(a) and 8 (b). As can be seen from fig. 8(b), the midpoint voltage is kept in an equilibrium state during the dynamic process, and when t is 0.5s, the output voltage is stabilized.

The neutral point potential balance control method can also be simply popularized to other neutral point clamping type full-bridge topologies, such as a DNPC full-bridge topology and a T-type full-bridge topology.

It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims

1. A neutral point potential balance control method based on ANPC full-bridge three-level DAB, the ANPC full-bridge three-level DAB comprises a full-bridge ANPC active bridge, an H bridge circuit, a transformer and an inductor, the full-bridge ANPC active bridge is composed of a first direct current voltage-dividing capacitor, a second direct current voltage-dividing capacitor and two half-bridge ANPC bridge arm circuits, and the two direct current voltage-dividing capacitors are connected in series with a direct current bus voltage V_dc1The voltage on each DC voltage-dividing capacitor is V_dc1The half-bridge ANPC bridge arm circuit connects the midpoint of the bridge arm of the series-connected switching tubes to the series midpoint of the two direct-current voltage-dividing capacitors through the active switching device, so that the voltage born by each switching tube in each half-bridge ANPC bridge arm circuit is V_dc1/2；

The full-bridge ANPC circuit comprises a first switch tube, a second switch tube, a third switch tube, a fourth switch tube, a fifth switch tube, a sixth switch tube, a seventh switch tube, an eighth switch tube, a ninth switch tube, a tenth switch tube, an eleventh switch tube and a twelfth switch tube; the first direct current voltage division capacitor and the second direct current voltage division capacitor are connected in series; the first switching tube, the second switching tube, the third switching tube and the fourth switching tube are sequentially connected in series in a forward direction, and two ends of the first switching tube, the second switching tube, the third switching tube and the fourth switching tube are respectively connected with a direct-current bus voltage; the fifth switching tube and the sixth switching tube are connected in series in the forward direction, and two ends of the fifth switching tube and the sixth switching tube are respectively connected with the series midpoint of the first switching tube and the second switching tube and the series midpoint of the third switching tube and the fourth switching tube; the series midpoint of the fifth switching tube and the sixth switching tube is connected with the series midpoint of the first direct current voltage-dividing capacitor and the second direct current voltage-dividing capacitor; the seventh switching tube, the eighth switching tube, the ninth switching tube and the tenth switching tube are sequentially connected in series in the forward direction, and two ends of the seventh switching tube, the eighth switching tube, the ninth switching tube and the tenth switching tube are respectively connected with the direct-current bus voltage; the eleventh switching tube and the twelfth switching tube are connected in series in the forward direction, and two ends of the eleventh switching tube and the twelfth switching tube are respectively connected with the series midpoint of the seventh switching tube and the eighth switching tube and the series midpoint of the ninth switching tube and the tenth switching tube; the serial midpoint of the eleventh switching tube and the twelfth switching tube is connected with the serial midpoint of the first direct current voltage-dividing capacitor and the second direct current voltage-dividing capacitor; the second switching tube and the third switching tube are connected in seriesThe middle point is connected with one end of an inductor, and the other end of the inductor is connected with one end of one side of the transformer; the series midpoint of the eighth switching tube and the ninth switching tube is connected with the other end of one side of the transformer; the first switch tube, the second switch tube, the third switch tube, the fourth switch tube, the fifth switch tube and the sixth switch tube form a first half-bridge ANPC bridge arm circuit, and the seventh switch tube, the eighth switch tube, the ninth switch tube, the tenth switch tube, the eleventh switch tube and the twelfth switch tube form a second half-bridge ANPC bridge arm circuit; output level V of full-bridge ANPC active bridge_ABFor two half-bridge ANPC bridge arm circuits output level V_AAnd V_BA difference of (d); three levels { V ] are output by each half-bridge ANPC bridge arm circuit_dc1/2、0、-V_dc1/2 }; when the capacitor voltage is balanced, the full-bridge ANPC active bridge outputs five levels { V }_dc1/2、V_dc1/2、0、-V_dc1/2、-V_dc1/2, the five levels correspond to 9 switch states, and the output voltage V_dc1and-V_dc1Respectively corresponding to the switch states U_PNAnd U_NPOutput voltage V_dc1/2 corresponds to the switch state U_POAnd U_ONOutput voltage-V_dc1/2 corresponds to the switch state U_NOAnd U_OPOutput voltage 0 corresponds to switch state U_PP、U_NNAnd U_OO；

The method is characterized in that the neutral point potential balance control method based on ANPC full-bridge three-level DAB comprises the following steps:

S2, comparing the midpoint voltage with a preset upper limit value and a preset lower limit value, selecting a control mode as a boost switch mode when the midpoint voltage is smaller than the lower limit value, selecting a control mode as a buck switch mode when the charge quantity generated by the midpoint current in one switch period is negative and the midpoint voltage rises in one switch period, and selecting a control mode as a buck switch mode when the midpoint voltage is larger than the upper limit value, and selecting a switch period as a buck switch mode when the voltage is lower than the lower limit valueThe electric charge quantity generated by the midpoint current in the period is positive, the midpoint voltage is reduced, otherwise, the switching mode is unchanged; according to the output level V of the full-bridge ANPC active bridge_ABDivides a switching cycle into 8 switching states, in boost switching mode, the output level V of the full-bridge ANPC active bridge_ABThe eight switch states in a cycle are in turn: u shape_OO、U_PO、U_PN、U_PO、U_OO、U_OP、U_NP、U_OP(ii) a The output levels of eight switching states of the first half-bridge ANPC bridge arm circuit in one switching period are as follows in sequence: 0. v_dc1/2、V_dc1/2、V_dc1/2、0、0、-V_dc10/2; the output levels of eight switching states of the second half-bridge ANPC bridge arm circuit in one switching period are as follows in sequence: 0.0, -V_dc1/2、0、0、V_dc1/2、V_dc1/2、V_dc12; in the step-down switching mode, the output level V of the full-bridge ANPC active bridge_ABThe eight switch states in a cycle are in turn: u shape_OO、U_ON、U_PN、U_ON、U_OO、U_NO、U_NP、U_NO(ii) a The output levels of eight switching states of the first half-bridge ANPC bridge arm circuit in one switching period are as follows in sequence: 0.0, V_dc1/2、0、0、-V_dc1/2、-V_dc1/2、-V_dc12; the output levels of eight switching states of the second half-bridge ANPC bridge arm circuit in one switching period are as follows in sequence: 0. -V_dc1/2、-V_dc1/2、-V_dc1/2、0、0、V_dc1/2、0；

2. The method of claim 1,

U_PNwhen the first switch tube, the second switch tube and the sixth switch tube are conducted; the third switching tube, the fourth switching tube and the fifth switching tube are turned off, and the output voltage of the first half-bridge ANPC bridge arm is V_dc1/2, ninthThe switch tube, the tenth switch tube and the eleventh switch tube are conducted; the seventh switching tube, the eighth switching tube and the twelfth switching tube are turned off, and the output voltage of the second half-bridge ANPC bridge arm is-V_dc1/2；

U_NPWhen the first switch tube is connected with the second switch tube, the third switch tube, the fourth switch tube and the fifth switch tube are connected; the first switch tube, the second switch tube and the sixth switch tube are turned off, and the output voltage of the first half-bridge ANPC bridge arm is-V_dc1The seventh switch tube, the eighth switch tube and the twelfth switch tube are conducted; the ninth switching tube, the tenth switching tube and the eleventh switching tube are turned off, and the output voltage of the second half-bridge ANPC bridge arm is V_dc1/2；

U_POWhen the first switch tube, the second switch tube and the sixth switch tube are conducted; the third switching tube, the fourth switching tube and the fifth switching tube are turned off, and the output voltage of the first half-bridge ANPC bridge arm is V_dc1The eighth switching tube is conducted with the eleventh switching tube, and/or the ninth switching tube is conducted with the twelfth switching tube, and the output voltage of the second half-bridge ANPC bridge arm is 0;

U_NOwhen the output voltage of the first half-bridge ANPC bridge arm is-V_dc1The third switching tube, the fourth switching tube and the fifth switching tube are conducted; the first switch tube, the second switch tube and the sixth switch tube are turned off, the eighth switch tube and the eleventh switch tube are turned on, and/or the ninth switch tube and the twelfth switch tube are turned on, and the output voltage of the second half-bridge ANPC bridge arm is 0;

U_OPwhen the bridge is in a state of zero, the second switching tube is conducted with the fifth switching tube, and/or the third switching tube is conducted with the sixth switching tube, the output voltage of the first half-bridge ANPC bridge arm is 0, and the seventh switching tube, the eighth switching tube and the twelfth switching tube are conducted; the ninth switching tube, the tenth switching tube and the eleventh switching tube are turned off, and the output voltage of the second half-bridge ANPC bridge arm is V_dc1/2；

U_ONWhen the first half-bridge ANPC bridge arm is in a state of being connected with the second switch tube, the second switch tube is connected with the fifth switch tube, and/or the third switch tube is connected with the sixth switch tube, the output voltage of the first half-bridge ANPC bridge arm is 0, and the ninth switch tube, the tenth switch tube and the eleventh switch tube are connected; seventh switch tube, eighth switch tube and twelfth switch tubeThe switching tube is turned off, and the output voltage of the second half-bridge ANPC bridge arm is-V_dc1/2；

U_PPWhen the first switch tube, the second switch tube and the sixth switch tube are conducted; the third switching tube, the fourth switching tube and the fifth switching tube are turned off, and the output voltage of the first half-bridge ANPC bridge arm is V_dc1The seventh switch tube, the eighth switch tube and the twelfth switch tube are conducted; the ninth switching tube, the tenth switching tube and the eleventh switching tube are turned off, and the output voltage of the second half-bridge ANPC bridge arm is V_dc1/2；

U_NNWhen the first switch tube is connected with the second switch tube, the third switch tube, the fourth switch tube and the fifth switch tube are connected; the first switch tube, the second switch tube and the sixth switch tube are turned off, and the output voltage of the first half-bridge ANPC bridge arm is-V_dc1The ninth switching tube, the tenth switching tube and the eleventh switching tube are conducted; the seventh switching tube, the eighth switching tube and the twelfth switching tube are turned off, and the output voltage of the second half-bridge ANPC bridge arm is-V_dc1/2；

U_OOWhen the second switch tube is conducted with the fifth switch tube, and/or the third switch tube is conducted with the sixth switch tube, the output voltage of the first half-bridge ANPC bridge arm is 0, the eighth switch tube is conducted with the eleventh switch tube, and/or the ninth switch tube is conducted with the twelfth switch tube, and the output voltage of the second half-bridge ANPC bridge arm is 0.

3. A mid-point potential balance control system of full-bridge three-level DAB, characterized by comprising: a computer-readable storage medium and a processor;

the processor is used for reading the executable instructions stored in the computer readable storage medium and executing the midpoint potential balance control method of the full-bridge three-level DAB as claimed in claim 1 or 2.