CN108023494B - Modular multilevel converter and submodule structure thereof - Google Patents

Modular multilevel converter and submodule structure thereof Download PDF

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Publication number
CN108023494B
CN108023494B CN201610951470.5A CN201610951470A CN108023494B CN 108023494 B CN108023494 B CN 108023494B CN 201610951470 A CN201610951470 A CN 201610951470A CN 108023494 B CN108023494 B CN 108023494B
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switching tube
branch
switching
capacitor
tube
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CN108023494A (en
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迟永宁
李琰
李武华
孙蔚
杨贺雅
董玉斐
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Zhejiang University ZJU
State Grid Corp of China SGCC
China Electric Power Research Institute Co Ltd CEPRI
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Zhejiang University ZJU
State Grid Corp of China SGCC
China Electric Power Research Institute Co Ltd CEPRI
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H02M7/4835—Converters with outputs that each can have more than two voltages levels comprising two or more cells, each including a switchable capacitor, the capacitors having a nominal charge voltage which corresponds to a given fraction of the input voltage, and the capacitors being selectively connected in series to determine the instantaneous output voltage

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Abstract

The invention provides a modular multilevel converter and a submodule structure thereof, wherein: the multi-level converter includes: the inductance and the submodule structure which is mutually cascaded are connected in series to form a bridge arm, and the two groups of bridge arms are connected in series to form a phase unit; the sub-module structure includes: the circuit comprises a positive end branch, a negative end branch, a capacitor branch and a bidirectional power switch branch; the positive end branch is connected with the capacitor branch in parallel, and two ends of the bidirectional power switch branch are respectively connected with the middle points of the positive end branch and the capacitor branch; one end of the negative end branch is connected with the positive end branch in parallel with the capacitor branch, and the other end of the negative end branch is connected with the midpoint of the capacitor branch. The technical scheme provided by the invention is applied to an MMC-HVDC system, the autonomous protection of direct current faults can be realized, and due to the symmetry of the structure, the output characteristics of the sub-module structure are symmetrical about the current direction in a locking mode, and the good symmetry is favorable for maintaining the current stress balance of power devices and capacitors in the sub-module.

Description

Modular multilevel converter and submodule structure thereof
Technical Field
The invention relates to the technical field of flexible direct current power transmission and distribution, in particular to a modular multilevel converter and a submodule structure thereof.
Background
Modular Multilevel Converters (MMC) have been introduced since 2002, and increasingly become one of the most promising Converter topologies in a High Voltage Direct Current (HVDC) power transmission system due to the characteristics of High modularity, good output waveform quality, low step Voltage, low device switching frequency, and the like. Currently, most MMC-HVDC projects which are put into operation adopt a Half-Bridge Sub-Module (HBSM) structure, the structure has the advantages of small quantity of power devices, low system cost and high operation efficiency, but when a direct current side fails, a current converter does not have active protection capacity, and the fault needs to be isolated by means of alternating current relay protection equipment.
A dc-side short-circuit fault is a common fault in dc transmission, especially in overhead lines. Currently, there are three main ways to handle dc side faults: 1) the connection between the fault and the alternating current system is cut off through alternating current equipment such as an alternating current circuit breaker, an alternating current fuse and the like; 2) blocking-up the connection of a fault to a converter by means of a direct current device such as a direct current breaker or the like; 3) the isolation of the direct current side fault is realized through the switching action of a power semiconductor device in the converter. However, the first processing mode has long response time and complex restart of the alternating current equipment due to mechanical limitation; the second treatment mode has the defects that the direct current breaker is not mature in technology and high in manufacturing cost, and is difficult to apply to actual engineering; compared with the former two modes, the third mode has quick response time and strong capability of recovering normal operation of the system after the fault, so the method becomes an effective solution for processing the short-circuit fault on the DC side of the MMC-HVDC.
Currently, the Sub-Module structures capable of achieving dc fault protection are typically Full Bridge Sub-Module (FBSM) and clamped Double Sub-Module (CDSM) structures. The number of power semiconductor devices of the full-bridge submodule is twice that of the half-bridge submodule, so that the system cost is increased, and the operation efficiency of the converter is reduced; although the clamping dual sub-module has a simple structure, the fault-tolerant capability is poor, and once the clamping switch tube damages the sub-module, the normal output of the level cannot be realized, so that the normal operation of the whole system is influenced.
Therefore, there is a need to provide a solution to the needs of the prior art.
Disclosure of Invention
In order to overcome the above-mentioned deficiencies of the prior art, the present invention provides a modular multilevel converter and a sub-module structure thereof, wherein: the modular multilevel converter comprises an inductor and a submodule structure which are mutually cascaded, wherein the inductor and the submodule structure are connected in series to form a bridge arm, and two groups of bridge arms are connected in series to form a phase unit.
The sub-module structure includes: the circuit comprises a positive end branch, a negative end branch, a capacitor branch and a bidirectional power switch branch; the positive end branch is connected with the capacitor branch in parallel, and two ends of the bidirectional power switch branch are respectively connected with the middle points of the positive end branch and the capacitor branch; one end of the negative end branch is connected with the positive end branch in parallel with the capacitor branch, and the other end of the negative end branch is connected with the midpoint of the capacitor branch.
The positive terminal branch includes: two switching tubes S1 and S2 with inverse diodes; the emitter of the first switch tube S1 is connected to the collector of the second switch tube S2.
The negative terminal branch includes: two switching tubes S3 and S4 with inverse diodes; the emitter of the third switch tube S3 is connected to the collector of the fourth switch tube S4.
The capacitance branch circuit comprises: two capacitors C1 and C2 connected in series with each other;
the positive terminal of the first capacitor C1 is connected to the collector of the first switch tube S1, the negative terminal of the second capacitor C2 is connected to the emitters of the second switch tube S2 and the fourth switch tube S4, and the collector of the third switch tube S3 is connected to the middle node of the capacitor branch.
The bidirectional power switch branch comprises: two switching tubes S5 and S6 with inverse diodes; one end of the bidirectional power switch branch is connected with the collector of the fifth switch tube S5, the other end of the bidirectional power switch branch is connected with the collector of the sixth switch tube S6, and the emitter of the fifth switch tube S5 is connected with the emitter of the sixth switch tube S6.
Another preferred embodiment of the bi-directional power switch branch comprises: two parallel switch tubes S5 and S6; the collector of the fifth switching tube S5 is connected to the emitter of the sixth switching tube S6.
The base electrode of the switching tube with the inverse diode receives a switching control signal provided by external equipment; the switch tube with the anti-parallel diode is an IGBT type switch tube.
The operation modes of the sub-module structure comprise a normal mode, a lockout mode or a STATCOM mode;
the normal mode includes: three level modes of +2E, +1E and 0; the STATCOM mode includes: three level modes of +1E, 0 and-1E.
The normal mode and the STATCOM mode include 6 switch states:
a first switching state: the first switch tube S1, the fourth switch tube S4 and the sixth switch tube S6 are turned on; the second switch tube S2, the third switch tube S3 and the fifth switch tube S5 are turned off;
a second switching state: the first switch tube S1, the third switch tube S3 and the sixth switch tube S6 are turned on; the second switch tube S2, the fourth switch tube S4 and the fifth switch tube S5 are turned off;
third switch state: the fourth switching tube S4, the fifth switching tube S5 and the sixth switching tube S6 are turned on; the first switch tube S1, the second switch tube S2 and the third switch tube S3 are turned off;
fourth switching state: the third switch tube S3, the fifth switch tube S5 and the sixth switch tube S6 are turned on; the first switch tube S1, the second switch tube S2 and the fourth switch tube S4 are turned off;
fifth switching state: the second switch tube S2, the fourth switch tube S4 and the fifth switch tube S5 are turned on; the first switch tube S1, the third switch tube S3 and the sixth switch tube S6 are turned off;
sixth switching state: the second switching tube S2, the third switching tube S3 and the fifth switching tube S5 are turned on; the first switch tube S1, the fourth switch tube S4 and the sixth switch tube S6 are turned off;
the latching mode includes 2 switch states:
seventh switching state: the gate signals of all the switch tubes are turned off, and when the current flows from the positive end to the negative end, the current charges the first capacitor C1 and the second capacitor C2 through the anti-parallel diodes in the first switch tube S1 and the fourth switch tube S4;
eighth switching state: when the gate signals of all the switch tubes are turned off and the current flows from the negative terminal to the positive terminal, the current charges the second capacitor C2 through the anti-parallel diode in the second switch tube S2 and the third switch tube S3.
Compared with the closest prior art, the technical scheme provided by the invention has the following beneficial effects:
1. in a locking mode, all the sub-module capacitors of the technical scheme provided by the invention are put into a bridge arm, the sub-module capacitors are charged to generate reverse electromotive force, and the function of isolating direct-current side faults is achieved;
2. in an MMC-HVDC system, the technical scheme provided by the invention can realize the autonomous protection of direct current faults, and due to the symmetry of the structure, the output characteristics of the sub-module structure are symmetrical about the current direction in a locking mode, and the good symmetry is favorable for maintaining the current stress balance of power devices and capacitors in the sub-module;
3. in the HCMC-HVDC system, the technical scheme provided by the invention has lower conduction loss than a full-bridge sub-module structure, and the operating efficiency of the system is improved.
Drawings
Fig. 1 is a topology structure diagram of a single-ended three-phase modular multilevel converter of the present invention;
FIG. 2 is a schematic topology diagram of an MMC sub-module structure according to the present invention;
FIG. 3 is another schematic topology diagram of the MMC sub-module structure of the present invention;
FIG. 4 is a schematic diagram of a first switch state of the MMC sub-module structure of the present invention;
FIG. 5 is a schematic diagram of a second switching state of the MMC sub-module structure of the present invention;
FIG. 6 is a third switch state diagram of the MMC sub-module structure of the present invention;
FIG. 7 is a fourth switching state diagram of the MMC sub-module structure of the present invention;
FIG. 8 is a fifth switch state diagram of the MMC sub-module structure of the present invention;
FIG. 9 is a sixth switch state diagram of the MMC sub-module structure of the present invention;
FIG. 10 is a seventh switch state diagram of the MMC sub-module structure of the present invention;
FIG. 11 is a schematic diagram of an eighth switching state of the MMC sub-module structure of the present invention.
Detailed Description
The technical scheme of the invention is further explained in detail by combining the drawings in the specification.
The invention discloses a modular multilevel converter submodule structure with a fault ride-through function, which adjusts the current stress balance of 2 capacitors and 8 power switch tubes in a submodule through redundant switch states; three levels can be output in a normal mode, so that the level integration level of the sub-module is improved; in a locking mode, all the sub-module capacitors are put into a bridge arm, the sub-module capacitors are charged to generate reverse electromotive force, and the function of isolating direct-current side faults is achieved. The submodule structure is applied to an MMC-HVDC system, the autonomous protection of direct current faults can be realized, and due to the symmetry of the structure, the output characteristics of the submodule structure are symmetrical about the current direction in a locking mode, and the good symmetry is favorable for maintaining the current stress balance of power devices and capacitors in the submodule; the full-bridge sub-module structure is applied to the HCMC-HVDC system, has lower conduction loss than a full-bridge sub-module structure, and improves the operation efficiency of the system.
The sub-module structure of the invention comprises: 4 switching tubes S1-S4 with inverse diode, 2 capacitors C1-C2 and 1 bidirectional power switch, wherein: an emitter of the switch tube S1 is connected with a collector of the switch tube S2 and is a positive end of the sub-module structure, a collector of the switch tube S1 is connected with a positive end of the capacitor C1, an emitter of the switch tube S2 is connected with a negative end of the capacitor C2 and an emitter of the switch tube S4, a negative end of the capacitor C1 is connected with a positive end of the capacitor C2 and is used as a middle node, a collector of the switch tube S3 is connected with the middle node, an emitter of the switch tube S3 is connected with a collector of the switch tube S4 and is a negative end of the sub-module structure, one end of the bidirectional switch tube is connected with the positive end of the sub-module, and the other end of the bidirectional switch tube is connected. The bases of the four switching tubes S1-S4 all receive switching control signals provided by external equipment, and 4 switching tubes S1-S4 with anti-parallel diodes all adopt IGBTs.
The bidirectional power switch consists of 2 switching tubes S5-S6 with anti-parallel diodes; the collector of the switch tube S5 is one end of a bidirectional power switch, the emitter of the switch tube S5 is connected to the emitter of the switch tube S6, the collector of the switch tube S6 is the other end of the bidirectional power switch, and the bases of 2 switch tubes S5 to S6 all receive switch control signals provided by external devices.
The bidirectional power switch consists of 2 switch tubes S5-S6 without a reverse diode; the collector of the switch tube S5 is connected to the emitter of the switch tube S6 and is one end of a bidirectional power switch, the emitter of the switch tube S5 is connected to the collector of the switch tube S6 and is the other end of the bidirectional power switch, the bases of 2 switch tubes S5 to S6 all receive a switch control signal provided by an external device, and 2 switch tubes S5 to S6 with a diode are all IGBTs.
The MMC sub-module structure has three operation modes (a normal mode, a locking mode and a STATCOM mode); the +2E, +1E and 0 three levels can be output in the normal mode, and the level integration level of the submodule is improved; in a locking mode, all the sub-module capacitors are put into a bridge arm, the sub-module capacitors are charged to generate reverse electromotive force, and the function of isolating direct-current side faults is achieved; under the STATCOM mode, three levels of +1E, 0 and-1E can be output, and reactive power support of the alternating current system is achieved. The on-state loss of the MMC submodule is lower than that of an MMC full-bridge submodule structure, so that the system operation efficiency of MMC-HVDC is improved; the negative level can be actively output, and the fault processing is more flexible than that of a clamping dual-sub module.
As shown in fig. 1, a basic unit of a single-ended three-phase Modular Multilevel Converter (MMC) is a Sub-Module (SM), N Sub-modules are cascaded in series with a bridge arm inductor to form a bridge arm, and an upper bridge arm and a lower bridge arm are cascaded in series to form a phase unit. The three-phase MMC current converter comprises three phase units, 6 bridge arms and 6N sub-modules. The DC side bus voltage is UdcThe three-phase voltage on the AC side is ua、ubAnd ucAnd the point O is a zero potential reference point.
The MMC submodule structure shown in FIG. 2 comprises four switching tubes S1-S4 with inverse diodes, two capacitors C1-C2 and 1 bidirectional power switch; wherein:
an emitter of the switch tube S1 is connected with a collector of the switch tube S2 and is a positive end of a sub-module structure, a collector of the switch tube S1 is connected with a positive end of a capacitor C1, an emitter of the switch tube S2 is connected with a negative end of a capacitor C2 and an emitter of a switch tube S4, a negative end of a capacitor C1 is connected with a positive end of a capacitor C2 and is used as a middle node, a collector of the switch tube S3 is connected with the middle node, an emitter of the switch tube S3 is connected with a collector of the switch tube S4 and is a negative end of the sub-module structure, one end of the bidirectional switch tube is connected with the positive end of the sub-module, the other end of the bidirectional switch tube is connected with the middle node, and 4 switch tubes S1-S4 with anti-parallel diodes.
The bidirectional power switch in fig. 2 is composed of 2 switch tubes S5-S6 with anti-parallel diodes; the two ends of the bidirectional power switch are respectively a collector of a switch tube S5 and a collector of a switch tube S6, the collector of the switch tube S5 is connected with the positive electrode end, the emitter of the switch tube S5 is connected with the emitter of the switch tube S6, the collector of the switch tube S6 is connected with the middle node, and bases of the two switch tubes S5-S6 receive switch control signals provided by external equipment.
Alternatively, the bidirectional power switch consists of 2 switching tubes S5-S6 without a reverse diode; specifically, as shown in fig. 3, the collector of the switch tube S5 is connected to the emitter of the switch tube S6 and is one end of the bidirectional power switch, and the emitter of the switch tube S5 is connected to the collector of the switch tube S6 and is the other end of the bidirectional power switch. The switching tubes S5-S6 are all IGBTs.
Fig. 4 to 11 are current flow diagrams in different switching states of the MMC submodule structure of the present embodiment. 4-9 are diagrams of the current flow for the 6 switch states in the normal mode or STATCOM mode; fig. 10-11 are graphs showing the current flow for 2 switch states in the latch-up mode. The following table is a switching state table for an MMC submodule structure of a modular multilevel converter with active fault ride-through capability.
Figure BDA0001140306770000061
Figure BDA0001140306770000071
The switching state table in the table gives the switching states in a normal mode, a STATCOM mode and a locking mode, in the normal mode, the sub-modules are switched between the switching states of 1-5 and are equivalently connected in series with two half-bridge sub-modules, in the STATCOM mode, the sub-modules are switched between the switching states of 2-6 and are equivalently connected with one full-bridge sub-module.
The switch states in the above table will be described in detail with reference to the drawings.
As shown in fig. 4, switch state 1: s1, S4 and S6 are on; s2, S3, and S5 are off. Capacitors C1 and C2 are put into a bridge arm through switching tubes S1 and S4, AB output voltage USM of the sub-modules is the sum of voltages on the capacitors C1 and C2, the theoretical value is +2E, and E is the theoretical value of the capacitor voltage of each sub-module. In this switching state, current flows in both directions, and the direction of the current determines the charging and discharging states of the sub-module capacitors C1 and C2.
As shown in fig. 5, switch state 2: s1, S3 and S6 are on; s2, S4, and S5 are off. Capacitor C1 is fed to the arm through switching tube S1 and switching tube S3, and capacitor C2 is bypassed. The output voltage USM of the sub-module AB is the voltage value on the capacitor C1, and the theoretical value is + E. In this switching state, current flows in both directions, and the direction of the current determines the charging and discharging state of the sub-module capacitor C1.
As shown in fig. 6, switch state 3: s4, S5 and S6 are on; s1, S2, and S3 are off. Capacitor C2 is fed to the arm through the bidirectional switch tube and switch tube S4, and capacitor C1 is bypassed. The output voltage USM of the sub-module AB is the voltage value on the capacitor C2, and the theoretical value is + E. In this switching state, current flows in both directions, and the direction of the current determines the charging and discharging state of the sub-module capacitor C2. The switch state 2 and the switch state 3 have the same external characteristics of the sub-modules, the output voltage USM is + E, and the capacitors C1 and C2 can be charged and discharged by the two switch states respectively, so that the voltage balance of the capacitors C1 and C2 is maintained.
As shown in fig. 7, switch state 4: s3, S5 and S6 are on; s1, S2, and S4 are off. Capacitors C1 and C2 are bypassed and the output voltage USM of sub-module AB is zero. The current flows bidirectionally through the bidirectional switch tube and the switch tube S3.
As shown in fig. 8, switch state 5: s2, S4 and S5 are on; s1, S3, and S6 are off. Capacitors C1 and C2 are bypassed and the output voltage USM of sub-module AB is zero. Current flows bidirectionally through switching tubes S2 and S4.
As shown in fig. 9, switch state 6: s2, S3 and S5 are on; s1, S4, and S6 are off. Capacitor C2 is bypassed and sub-module AB outputs voltage USM at-E. Current flows bidirectionally through switching tubes S2 and S3.
As shown in fig. 10, switch state 7: the gate signals of all the power semiconductor devices are turned off, the current direction flows from the A end to the B end of the submodule, the current charges capacitors C1 and C2 through IGBT reverse diodes in switching tubes S1 and S4, and the output voltage USM of the submodule is +2E of the sum of the voltages on the capacitors C1 and C2.
As shown in fig. 11, switch state 8: the gate signals of all the power semiconductor devices are turned off, the current direction flows from the terminal B to the terminal A of the submodule, the current charges a capacitor C2 through IGBT reverse parallel diodes in switching tubes S2 and S3, and the output voltage USM of the submodule is-E.
Finally, it should be noted that: the above embodiments are only intended to illustrate the technical solution of the present invention and not to limit the same, and a person of ordinary skill in the art can make modifications or equivalents to the specific embodiments of the present invention with reference to the above embodiments, and such modifications or equivalents without departing from the spirit and scope of the present invention are within the scope of the claims of the present invention as set forth in the claims.

Claims (3)

1. A sub-module structure of a modular multilevel converter is characterized in that,
the sub-module structure comprises: the circuit comprises a positive end branch, a negative end branch, a capacitor branch and a bidirectional power switch branch; the positive end branch is connected with the capacitor branch in parallel, and two ends of the bidirectional power switch branch are respectively connected with intermediate nodes of the positive end branch and the capacitor branch;
one end of the negative end branch is connected with the positive end branch and the capacitor branch in parallel, and the other end of the negative end branch is connected with the middle node of the capacitor branch;
the positive terminal branch includes: two switching tubes with inverse diodes (S1 and S2);
the emitter of the first switch tube (S1) is connected with the collector of the second switch tube (S2);
the negative terminal branch includes:
two switching tubes with inverse diodes (S3 and S4); the emitter of the third switching tube (S3) is connected with the collector of the fourth switching tube (S4);
the capacitance branch circuit comprises: two capacitors (C1 and C2) connected in series with each other;
the positive terminal of a first capacitor (C1) is connected with the collector of the first switching tube (S1), the negative terminal of a second capacitor (C2) is connected with the emitters of the second switching tube (S2) and the fourth switching tube (S4), and the collector of the third switching tube (S3) is connected with the middle node of the capacitor branch;
the bidirectional power switch branch comprises: two switching tubes with inverse diodes (S5 and S6);
the collector of the fifth switching tube (S5) is one end of the bidirectional power switch branch and is connected with the middle node of the positive end branch; the collector of the sixth switching tube (S6) is the other end of the bidirectional power switch branch and is connected with the middle node of the capacitor branch; the emitter of the fifth switching tube (S5) is connected with the emitter of the sixth switching tube (S6);
the base electrode of the switching tube with the anti-parallel diode receives a switching control signal provided by external equipment;
the switch tube with the anti-parallel diode is an IGBT type switch tube;
the operation modes of the sub-module structure comprise a normal mode, a lockout mode or a STATCOM mode;
the normal mode includes: three level modes of +2E, +1E and 0;
the lockout mode includes: two level modes of +2E and-E;
the STATCOM mode includes: three level modes of +1E, 0 and-1E;
the normal mode and the STATCOM mode comprise 6 switch states:
a first switching state: the first switching tube (S1), the fourth switching tube (S4) and the sixth switching tube (S6) are turned on; the second switching tube (S2), the third switching tube (S3) and the fifth switching tube (S5) are turned off;
a second switching state: the first switching tube (S1), the third switching tube (S3) and the sixth switching tube (S6) are turned on; the second switching tube (S2), the fourth switching tube (S4) and the fifth switching tube (S5) are turned off;
third switch state: the fourth switching tube (S4), the fifth switching tube (S5) and the sixth switching tube (S6) are turned on; the first switching tube (S1), the second switching tube (S2) and the third switching tube (S3) are turned off;
fourth switching state: the third switching tube (S3), the fifth switching tube (S5) and the sixth switching tube (S6) are turned on; the first switching tube (S1), the second switching tube (S2) and the fourth switching tube (S4) are turned off;
fifth switching state: the second switching tube (S2), the fourth switching tube (S4) and the fifth switching tube (S5) are turned on; the first switching tube (S1), the third switching tube (S3) and the sixth switching tube (S6) are turned off;
sixth switching state: the second switching tube (S2), the third switching tube (S3) and the fifth switching tube (S5) are turned on; the first switching tube (S1), the fourth switching tube (S4) and the sixth switching tube (S6) are turned off;
the latching mode includes 2 switch states:
seventh switching state: turning off gate signals of all switching tubes, when the current flows from the positive terminal to the negative terminal, the current charges the first capacitor (C1) and the second capacitor (C2) through anti-parallel diodes in the first switching tube (S1) and the fourth switching tube (S4);
eighth switching state: and turning off gate signals of all the switching tubes, wherein when the current flows from the negative terminal to the positive terminal, the current flows through the second switching tube (S2) and a reverse diode in the third switching tube (S3) to charge the second capacitor (C2).
2. The sub-module architecture of claim 1, wherein the bi-directional power switch legs comprise: two switching tubes (S5 and S6) connected in parallel;
the collector of the fifth switching tube (S5) is connected with the emitter of the sixth switching tube (S6);
the emitter of the fifth switching tube (S5) is connected with the collector of the sixth switching tube (S6).
3. A modular multilevel converter comprising an inductor, characterized in that: the modular structure of any one of claims 1-2 further comprising cascaded sub-modules, wherein the inductors are connected in series with the sub-modules to form bridge arms, and wherein two sets of the bridge arms are connected in series to form phase units.
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