CN103326608B - One seed module, facies unit, voltage-source type multilevel converter and control method - Google Patents

Abstract

The invention discloses a sub-module, which includes a first branch and a second branch connected in parallel; the first branch includes an energy storage element and at least one turn-off device connected in series, and at least one turn-off device is forward connected in series in the first branch; the second branch includes at least one charging circuit and at least two turn-off devices, all the turn-off devices are connected in series with each other, and at least one turn-off device is in reverse series with the other turn-off devices , and the charging circuit is connected in parallel with the turn-off device, and any turn-off device in reverse series is between the two ends of a certain charging circuit. This structure can inhibit or even prevent the AC system from injecting fault current into the DC network when the DC fault blocks the converter through the reverse series connection of the turn-off devices. The invention also discloses a control method of the aforementioned sub-module, a phase unit composed of the sub-module and a control method, and a voltage source multilevel converter composed of the phase unit and a control method.

Description

Submodule, phase unit, voltage source multilevel converter and control method

technical field

The invention belongs to the field of power electronics, and in particular relates to a voltage source type multilevel converter and its submodules, phase unit structure and control method.

Background technique

Modular multilevel converter is a new type of converter suitable for high-voltage applications that has attracted much attention in recent years. It adopts a cascading method of sub-modules, and by separately controlling the state of each sub-module, the AC voltage output by the converter can be approached to a sine wave, thereby reducing the harmonic content in the output voltage. Its appearance solves the problem of series voltage equalization in the two-level voltage source converter, and has broad application prospects.

Marquardt Rainer's "Distributed Energy Storage and Converter Circuit" first mentioned a modular multilevel converter (MMC) (patent application publication number: DE10103031A), the sub-module of which uses a half-bridge and a capacitor Composed in parallel, the output port of the sub-module can be controlled to generate two levels of capacitor voltage or zero voltage. In 2010, the TransBay project, the world's first flexible DC transmission project using this topology, which was undertaken by Siemens, was successfully put into operation, proving the engineering feasibility of this converter topology.

ABB has modified the structure based on the modular multilevel converter topology, and proposed a cascaded two-level modular multilevel topology (patent application publication number: US20100328977A1). The difference between the converter and the above-mentioned modular multilevel converter is that the sub-modules are connected in the opposite way.

The disadvantage of the above two modular multilevel converters is that when the DC network fails, the AC network can provide fault current to the fault point through the diodes of the sub-modules, thereby causing overcurrent on the DC side.

ALSTOM also proposed a new type of topology (patent application publication number: US20120113699A1), called the bridge arm switching multilevel converter (AAMC), each bridge arm of the converter is composed of N full bridges The modules are cascaded to form a controllable voltage source, and M IGBT switches are connected in series as bridge arm switching switches. The converter puts the cascaded full-bridge sub-modules of the upper and lower bridge arms into the AC network in turn by connecting M IGBT switches in series. Provide a path for the fault current without causing overcurrent on the DC side. However, this converter control method is more complicated and needs engineering verification.

Contents of the invention

The object of the present invention is to provide a sub-module, a phase unit, a voltage source multilevel converter and a control method, which can block the converter when a DC fault occurs through the reverse series connection of the turn-off devices in the sub-module Suppresses or even prevents the AC system from injecting fault currents into the DC network.

In order to achieve the above object, the technical scheme adopted in the present invention is:

A sub-module, including a first branch and a second branch connected in parallel;

The first branch includes an energy storage element and at least one first turn-off device connected in series, each first turn-off device has a diode connected in anti-parallel, and at least one first turn-off device is forwardly connected in series In the first branch; when there are at least two first turn-off devices, and at least one first turn-off device is in reverse series with other first turn-off devices, the first branch also includes at least one charging circuit , the charging circuit is connected in parallel with the first turn-off device, and any one of the first turn-off devices in reverse series is between two ends of a certain charging circuit; and the energy storage element and the first turn-off device The connection relationship can only have the following four situations: the positive pole of the energy storage element is connected to the positive pole of the first turn-off device in forward series connection, the positive pole of the energy storage element is connected to the negative pole of the first turn-off device in reverse series connection, The negative pole of the energy element is connected to the negative pole of the first shut-off device connected forwardly, or the negative pole of the energy storage element is connected to the positive pole of the first shut-off device connected in reverse series;

The second branch includes at least one charging circuit and at least two second turn-off devices, each second turn-off device has a diode connected in antiparallel, all the second turn-off devices are connected in series, and at least one second turn-off device The shutdown device is connected in reverse series with other second shutdown devices, and the charging circuit is connected in parallel with the second shutdown device, and any second shutdown device connected in reverse series is at both ends of a certain charging circuit. between.

The above-mentioned charging circuit adopts any one of the following six structures: only normally closed nodes are used; only charging resistors are used; normally closed nodes are connected in series with charging resistors; diodes are reversely connected in series with normally closed nodes; diodes reversely connected in series with charging resistors; After the diode is reversed, it is connected in series with the normally closed node and the charging resistor.

A method for controlling the sub-module as described above, controlling the sub-module to work in the following three states: the on-state, controlling the conduction of all the first turn-off devices in the first branch, and controlling the All the second turn-off devices are turned off; in the off state, all the first turn-off devices in the first branch are controlled to be turned off, and all the second turn-off devices in the second branch are controlled to be turned on; in the blocking state, All the first turn-off devices in the first branch and all the second turn-off devices in the second branch are controlled to be turned off.

When a normally closed node is connected in series in the charging circuit, the control sub-module first opens the normally closed node before it works in the on or off state.

A phase unit, comprising an upper bridge arm and a lower bridge arm, the upper and lower bridge arms each include at least two sub-modules as claimed in claim 1 or 2 and at least one reactor connected in series, the same All sub-modules in the bridge arm are connected in the same direction, and the sub-modules in the upper and lower bridge arms are connected in opposite directions, and one end of the upper and lower bridge arms is respectively used as the first and second DC terminals of the phase unit for access In the DC network, the other ends of the upper and lower bridge arms are short-circuited with each other as the AC terminal of the phase unit to be connected to the AC network.

A phase unit, the number of sub-modules and reactors included in the upper and lower bridge arms is the same or different.

A control method for a phase unit as described above, which controls each sub-module in the phase unit to work in the following three states: conduction state, controls all the first turn-off devices in the first branch to conduct, controls the first All the second turn-off devices in the two branches are turned off; in the off state, all the first turn-off devices in the first branch are controlled to turn off, and all the second turn-off devices in the two branches are controlled to turn on On; in the blocking state, all the first turn-off devices in the first branch and all the second turn-off devices in the second branch are controlled to be turned off.

When a normally closed node is connected in series in the charging circuit of a certain sub-module in the phase unit, before controlling the sub-module to work in the on or off state, the normally closed node is first opened.

A voltage source multilevel converter includes at least one phase unit as described above.

A control method for the above-mentioned voltage source multilevel converter, which controls the sub-modules in the phase units that make up the converter to work in the following three states: conduction state, and controls all the sub-modules in the first branch The first turn-off device is turned on, and all the second turn-off devices in the second branch are controlled to be turned off; in the off state, all the first turn-off devices in the first branch are controlled to be turned off, and the two branches are controlled. All the second turn-off devices in the circuit are turned on; in the blocking state, all the first turn-off devices in the first branch and all the second turn-off devices in the second branch are controlled to be turned off.

After adopting the above scheme, the beneficial effects of the present invention are:

(1) When the DC network is faulty, it is possible to suppress or even prevent the AC network from providing fault current to the fault point by turning off the turn-off devices in all sub-modules;

(2) When the charging circuit includes a charging resistor, the charging resistor on the AC side of the converter can be canceled.

Description of drawings

Fig. 1 is the schematic diagram of the first embodiment of the neutron module of the present invention;

Fig. 2 is a schematic diagram of the charging circuit of the neutron module of the present invention;

Fig. 3 is the schematic diagram of the second embodiment of the neutron module of the present invention;

Fig. 4 is the schematic diagram of the third embodiment of the neutron module of the present invention;

Fig. 5 is a schematic diagram of the fourth embodiment of the neutron module of the present invention;

Fig. 6 is a schematic diagram of the control method of the sub-module shown in Fig. 1;

Fig. 7 is a schematic diagram of the connection of the voltage source multilevel converter in the present invention.

detailed description

The present invention provides a sub-module for forming a voltage source multilevel converter, which includes a first branch and a second branch connected in parallel, and the two parallel connection points of the two branches are used as two output terminals. Used to connect an external circuit, wherein the first branch includes an energy storage element and at least one turn-off device connected in series, and each turn-off device has a diode in antiparallel connection, when there are at least two turn-off devices , at least one turn-off device is in reverse series with other turn-off devices, and at this time the first branch also includes at least one charging circuit, which is connected in parallel with the turn-off device, and the principle of connection is to ensure that any one of the turn-off devices The turn-off devices connected in series are all between the two ends of a certain charging circuit; and the connection relationship between the energy storage element and the turn-off device can only have the following four situations: the positive electrode of the energy storage element is connected in forward series Turn off the positive pole of the device, the positive pole of the energy storage element is connected to the negative pole of the reverse-series turn-off device, the negative pole of the energy storage element is connected to the negative pole of the forward-connected turn-off device, or the negative pole of the energy storage element is connected in reverse series The anode of the turn-off device; the second branch includes at least one charging circuit and at least two turn-off devices, each turn-off device has a diode connected in antiparallel, all turn-off devices are connected in series, and at least one turn-off device The shutdown device is connected in reverse series with other shutdown devices, and the charging circuit is connected in parallel with the shutdown device. The principle of connection is to ensure that any reverse-series shutdown device is between the two ends of a certain charging circuit. , which will be described in detail below in conjunction with specific embodiments.

When the turn-off device adopts IGBT, the positive pole is its collector, and the negative pole is its emitter; when the turn-off device uses IGCT or GTO, the positive pole is its anode, and the negative pole is its anode. Cathode; when the turn-off device adopts a MOSFET, the positive pole is its drain, and the negative pole is its source.

First, as shown in FIG. 2 , it is an implementation form of the charging circuit 19. Between the two terminals X3 and X4 of the charging circuit 19, only a normally closed node 191 or a charging resistor 192 may be used, or a normally closed node 191 may be used. It is connected in series with the charging resistor 192, and a series structure of the diode 193 and any one or two devices in the normally closed node 191 and the charging resistor 192 can be used, that is, there are 6 realization structures in total. Figure 2 shows all three devices. In the adopted structure, the diodes 193 need to be connected in reverse series. It should be noted that no matter what combination is used to realize the structure, each device is connected in series, so its positional relationship is not limited, and the structure shown in FIG. 2 is only an example. The function of the normally closed node is to open the normally closed node during normal operation, so that the blocked sub-module can prevent the AC network from providing fault current to the fault point; the function of the charging resistor is to limit the charging current during the charging process of the converter; The function of the diode is to limit the direction of the charging current when the charging circuit is connected in parallel with the device that can be turned off. The charging circuit can be composed of different combinations of normally closed nodes, charging resistors and diodes in series according to the required functions.

Several implementation circuits of sub-modules provided by the present invention will be given below and described in detail.

As shown in Fig. 1, it is an implementation circuit of the neutron module of the present invention, wherein the first branch includes a device 11 that can be turned off, a diode 12 and a capacitor 10 as an energy storage element, wherein the diode 12 is connected in antiparallel to the Both ends of the device 11 are turned off, and the device 11 and the capacitor 10 are connected in series, and the positive electrode of the device 11 is connected to the positive electrode of the capacitor, and the negative electrode of the device 11 is connected to the output terminal X1 of the sub-module. The negative pole of the capacitor 10 is connected to the output terminal X2 of the sub-module; the second branch includes the turn-off devices 13, 15, diodes 14, 16 and a charging circuit 19, wherein the diodes 14, 16 are respectively anti-parallel connected to the turn-off device 13 15, the two ends of 15, the device 13 that can be turned off is connected forwardly, and the device 15 that can be turned off is connected in reverse, and the "forward" and "reverse" here are based on the direction of the capacitor 10; the charging circuit 19 is connected in parallel The two ends of the turn-off device 15 are used for charging other sub-modules. The structure of the charging circuit 19 has been introduced above and will not be described in detail here.

Cooperating with the structure in Figure 1, the present invention also provides a control method for the sub-module, as shown in Figure 6, the control method can control the sub-module to work in three working states: on state, off state and locked state.

Conduction state: the turn-off device 11 is turned on, the turn-off devices 13 and 15 are turned off, and the capacitor 10 is connected through the diode 12 (as shown in Figure 6(a)) or the turn-off device 11 (as shown in Figure 6(d) Enter between the two output ports X1 and X2 of the sub-module. At this time, the voltage between X1 and X2 is the voltage of the capacitor 10. The current can flow in from X1 and flow out from X2, or flow in from X2 and flow out from X1;

Off state: the turn-off device 11 is turned off, the turn-off devices 13 and 15 are turned on, the capacitor 10 is bypassed, the output voltage between the output ports X1 and X2 is 0, the current can flow in from X1, and pass through the turn-off device 13 and diode 16, and then flow out from X2, see Figure 6(b), or flow in from X2, pass through the device 15 and diode 14 that can be turned off, and then flow out from X1;

Locking state: If there is a normally closed node 191 in series in the charging circuit 19, the normally closed node 191 needs to be opened first when locking; the turn-off devices 11, 13, and 15 are all turned off, and the current can flow in from X1 and pass through Diode 12 and capacitor 10 flow out from X2, as shown in Figure 6(c); in Figure 6(f), there are two situations: if there is a normally closed node in series in the charging circuit 19, due to the normally closed node Pull it apart, the current cannot flow in from X2 and flow out from X1, and if there is no normally closed node in series in the charging circuit 19, then no matter whether the charging circuit only uses the charging resistor 192, or uses the charging resistor 192 in series with the diode 193, the current can be It flows in from X2, passes through charging circuit 19 and diode 14 in turn, and then flows out through X1.

As shown in Figure 3, it is the second implementation structure of the sub-module provided by the present invention, and the number and types of components used in it are the same as the structure shown in Figure 1: the first branch includes a turn-off device 21, a diode 22 and a capacitor 20. The second branch includes turn-off devices 23, 25, diodes 24, 26, and a charging circuit 19. The difference from the structure shown in FIG. 1 is that the charging circuit 19 is connected in parallel at both ends of the second branch; this implementation structure The control method is the same as the structure shown in Figure 1, and will not be repeated here.

Shown in Fig. 4 is the third kind of implementation structure of the submodule of the present invention, and the second branch includes turn-off devices 33, 35, diodes 34, 36 and charging circuit 19, and the connection relationship of the second branch is the same as that in Fig. 1 The connection relationship of the second branch is the same and will not be described in detail; the first branch includes a turn-off device 31, a diode 32 and a capacitor 33, wherein the diode 32 is connected in antiparallel to both ends of the turn-off device 31, and the capacitor 30 The positive pole is connected to the output terminal X1 of the sub-module, the negative pole is connected to the negative pole of the turn-off device 31, and the positive pole of the turn-off device 31 is connected to the output terminal X2, that is, the turn-off device 31 is reversely connected in series in the circuit. The control method of this implementation structure is the same as the structure in Figure 1. When the sub-module is in the on state, the turn-off device 31 is on, and the turn-off devices 33 and 35 are off; when the sub-module is in the off state, the turn-off The device 31 is turned off, and the turn-off devices 33, 35 are turned on; when the sub-module is in a locked state, the turn-off devices 31, 33, 35 are all turned off. The specific current flow will not be described in detail here.

Figure 5 shows the fourth implementation structure of the sub-module of the present invention, the second branch includes turn-off devices 45, 47, diodes 46, 48 and charging circuit 19, and the connection relationship of the second branch is the same as that of Fig. 1 The connection relationship of the second branch is the same and will not be described in detail; the first branch includes turn-off devices 41, 43, diodes 42, 44, charging circuit 19 and capacitor 40, and the diodes 42, 44 are connected in antiparallel to the turn-off device respectively. The two ends of the off devices 41, 43, the turn-off devices 43, 41 and the capacitor 40 are connected in series in sequence, and the turn-off device 43 is reversely connected in series, the turn-off device 41 is forward connected in series, and the charging circuit 19 is connected to the reverse series connection. Shutdown devices 43 are connected in parallel. The control method of this implementation structure is the same as the control method shown in Figure 6. It has three working states: in the on state, all the devices that can be turned off in the first branch are turned on, and all the devices that can be turned off in the second branch are turned on. In the off state, all the turn-off devices in the first branch are turned off, and all the turn-off devices in the second branch are turned on. In the blocking state, all the turn-off devices connected in the sub-module The off devices are all turned off, and before the control enters the locked state, if there is a normally closed node in series in the charging circuit, the normally closed node must be opened first. At this time, the current cannot flow in from the output port X1 or flow out from the output port X2, nor Inflow from X2 and outflow from X1.

After adopting the aforementioned structure, when the sub-module is not charging, the normally closed node is closed, and other sub-modules can be charged through the charging circuit and diode; When the module is charged, the normally closed node can be opened through the corresponding control circuit.

As shown in Figure 7, it is a circuit diagram of a phase unit composed of the aforementioned sub-modules, wherein the phase unit includes an upper bridge arm 3 and a lower bridge arm 4, and the upper and lower bridge arms include at least two connected in series. sub-module 1 (SM) and at least one reactor 2, and the number of sub-modules 1 and reactors 2 contained in the upper and lower bridge arms can be the same or different, and the specific circuit structure of each sub-module 1 can be the same or can also be Can be different; in the same bridge arm (upper bridge arm 3 or lower bridge arm 4), all sub-modules 1 are connected in the same direction, and the connection direction of the sub-modules 1 in the upper and lower bridge arms is opposite; the upper bridge arm 3 One end of the lower bridge arm 4 is used as the first DC terminal 6 of the phase unit to be connected to the DC network, one end of the lower bridge arm 4 is used as the second DC terminal 7 of the phase unit to be connected to the DC network, and The other ends of the upper bridge arm 3 and the lower bridge arm 4 are connected together to serve as the AC terminal 5 of the phase unit for accessing the AC network. It should be noted that, for the upper bridge arm 3 or the lower bridge arm 4, there is no restriction on the series position of the sub-module 1 and the reactor 2, and since one reactor can be regarded as composed of multiple sub-reactors in series, therefore The number of the reactors is not limited, as long as the total reactance in a bridge arm meets the corresponding requirements of the bridge arm.

The present invention also provides a voltage source multilevel converter, comprising at least one phase unit as shown in FIG. 7 , and the number of the phase units can be determined according to the number of AC terminals of the AC system. The converter inhibits or prevents the AC network from supplying fault current to the fault point of the DC network by shutting off the turn-off devices in all sub-modules. When a ground fault occurs in the DC network, for example, when the first DC terminal 6 is grounded in FIG. flow out, so the AC network cannot supply the fault current to the fault point.

The above embodiments are only to illustrate the technical ideas of the present invention, and cannot limit the protection scope of the present invention with this. All technical ideas proposed according to the present invention, any changes made on the basis of technical solutions, all fall within the protection scope of the present invention. Inside.

Claims (5)

1. A control method for a submodule, said submodule comprising a first branch and a second branch connected in parallel; The first branch includes an energy storage element and at least one first turn-off device connected in series, each first turn-off device has a diode connected in anti-parallel, and at least one first turn-off device is forwardly connected in series In the first branch; when there are at least two first turn-off devices, and at least one first turn-off device is in reverse series with other first turn-off devices, the first branch also includes at least one charging circuit , the charging circuit is connected in parallel with the first turn-off device, and any one of the first turn-off devices in reverse series is between two ends of a certain charging circuit; and the energy storage element and the first turn-off device The connection relationship can only have the following four situations: the positive pole of the energy storage element is connected to the positive pole of the first turn-off device in forward series connection, the positive pole of the energy storage element is connected to the negative pole of the first turn-off device in reverse series connection, The negative pole of the energy element is connected to the negative pole of the first shut-off device connected forwardly, or the negative pole of the energy storage element is connected to the positive pole of the first shut-off device connected in reverse series; The second branch includes at least one charging circuit and at least two second turn-off devices, each second turn-off device has a diode connected in antiparallel, all the second turn-off devices are connected in series, and at least one second turn-off device The shutdown device is connected in reverse series with other second shutdown devices, and the charging circuit is connected in parallel with the second shutdown device, and any second shutdown device connected in reverse series is at both ends of a certain charging circuit. between; It is characterized in that the control sub-module works in the following three states: on-state, controlling all the first turn-off devices in the first branch to turn on, and controlling all the second turn-off devices in the second branch to turn off ;Off state, control all the first turn-off devices in the first branch to turn off, control all the second turn-off devices in the second branch to turn on; block state, control all the first turn-off devices in the first branch A turn-off device and all second turn-off devices in the second branch are turned off. 2. The control method of a sub-module as claimed in claim 1, characterized in that: when a normally closed node is connected in series in the charging circuit, before the control sub-module works in the on or off state, the normally closed node is first opened. closed point. 3. A control method for a phase unit, the phase unit includes an upper bridge arm and a lower bridge arm, and each of the upper bridge arm and the lower bridge arm includes at least two sub-modules and at least one reactor connected in series, the All the sub-modules in the upper bridge arm and the lower bridge arm are connected in the same direction, and the sub-modules in the upper bridge arm and the lower bridge arm are connected in opposite directions, and one end of the upper bridge arm and the lower bridge arm are respectively used as the first 2. Two DC terminals for connecting to the DC network, and the other ends of the upper bridge arm and the lower bridge arm are short-circuited with each other as the AC terminal of the phase unit for connecting to the AC network; The sub-module includes a first branch and a second branch connected in parallel; The first branch includes an energy storage element and at least one first turn-off device connected in series, each first turn-off device has a diode connected in anti-parallel, and at least one first turn-off device is forwardly connected in series In the first branch; when there are at least two first turn-off devices, and at least one first turn-off device is in reverse series with other first turn-off devices, the first branch also includes at least one charging circuit , the charging circuit is connected in parallel with the first turn-off device, and any one of the first turn-off devices in reverse series is between two ends of a certain charging circuit; and the energy storage element and the first turn-off device The connection relationship can only have the following four situations: the positive pole of the energy storage element is connected to the positive pole of the first turn-off device in forward series connection, the positive pole of the energy storage element is connected to the negative pole of the first turn-off device in reverse series connection, The negative pole of the energy element is connected to the negative pole of the first shut-off device connected forwardly, or the negative pole of the energy storage element is connected to the positive pole of the first shut-off device connected in reverse series; The second branch includes at least one charging circuit and at least two second turn-off devices, each second turn-off device has a diode connected in antiparallel, all the second turn-off devices are connected in series, and at least one second turn-off device The shutdown device is connected in reverse series with other second shutdown devices, and the charging circuit is connected in parallel with the second shutdown device, and any second shutdown device connected in reverse series is at both ends of a certain charging circuit. between; It is characterized in that each sub-module in the phase unit is controlled to work in the following three states: conduction state, controls all the first turn-off devices in the first branch to be turned on, controls all the second turn-off devices in the second branch The shutdown device is turned off; in the off state, all the first turn-off devices in the first branch are controlled to be turned off, and all the second turn-off devices in the second branch are controlled to be turned on; in the blocking state, the first branch is controlled All first turn-off devices in the branch and all second turn-off devices in the second branch are turned off. 4. The control method of a phase unit as claimed in claim 3, characterized in that: when a certain sub-module charging circuit in the phase unit has a normally closed node in series, the sub-module is controlled to work on or off. Before the off state, first pull the normally closed node. 5. A control method for a voltage source multilevel converter, said voltage source multilevel converter comprising at least one phase unit, said phase unit comprising an upper bridge arm and a lower bridge arm, said upper bridge arm Both the bridge arm and the lower bridge arm include at least two submodules and at least one reactor connected in series, all the submodules in the upper bridge arm and the lower bridge arm are connected in the same direction, and the upper bridge arm and the lower bridge arm The connection directions of the sub-modules are opposite, and one end of the upper bridge arm and the lower bridge arm are respectively used as the first and second DC terminals of the phase unit to connect to the DC network, while the other ends of the upper bridge arm and the lower bridge arm are short to each other. Connected as the AC endpoint of the phase unit to access the AC network; The sub-module includes a first branch and a second branch connected in parallel; The first branch includes an energy storage element and at least one first turn-off device connected in series, each first turn-off device has a diode connected in anti-parallel, and at least one first turn-off device is forwardly connected in series In the first branch; when there are at least two first turn-off devices, and at least one first turn-off device is in reverse series with other first turn-off devices, the first branch also includes at least one charging circuit , the charging circuit is connected in parallel with the first turn-off device, and any one of the first turn-off devices in reverse series is between two ends of a certain charging circuit; and the energy storage element and the first turn-off device The connection relationship can only have the following four situations: the positive pole of the energy storage element is connected to the positive pole of the first turn-off device in forward series connection, the positive pole of the energy storage element is connected to the negative pole of the first turn-off device in reverse series connection, The negative pole of the energy element is connected to the negative pole of the first shut-off device connected forwardly, or the negative pole of the energy storage element is connected to the positive pole of the first shut-off device connected in reverse series; The second branch includes at least one charging circuit and at least two second turn-off devices, each second turn-off device has a diode connected in antiparallel, all the second turn-off devices are connected in series, and at least one second turn-off device The shutdown device is connected in reverse series with other second shutdown devices, and the charging circuit is connected in parallel with the second shutdown device, and any second shutdown device connected in reverse series is at both ends of a certain charging circuit. between; It is characterized in that each sub-module in the phase unit composing the converter is controlled to work in the following three states: conduction state, controlling the conduction of all the first turn-off devices in the first branch, and controlling the second branch All the second turn-off devices in the first branch are turned off; in the off state, all the first turn-off devices in the first branch are controlled to be turned off, and all the second turn-off devices in the second branch are controlled to be turned on; latch state, controlling all the first turn-off devices in the first branch and all the second turn-off devices in the second branch to be turned off.