WO2015123922A1

WO2015123922A1 - Current converter module unit, current converter, direct-current transmission system and control method

Info

Publication number: WO2015123922A1
Application number: PCT/CN2014/076926
Authority: WO
Inventors: 谢晔源; 曹冬明; 邵震霞; 董云龙
Original assignee: 南京南瑞继保电气有限公司; 南京南瑞继保工程技术有限公司
Priority date: 2014-02-20
Filing date: 2014-05-07
Publication date: 2015-08-27
Also published as: CN104868748B; CN104868748A

Abstract

A current converter module unit, a converter, a direct-current transmission system and a control method. In the converter module unit, the positive electrode of a first switch module (T1, D1) is connected to the positive electrode of a first energy storage element (C1), and the negative electrode of a second switch module (T2, D2) is connected to the negative electrode of the first energy storage element; the negative electrode of a diode (D6) is connected to the positive electrode of the first switch module, the positive electrode of the diode is respectively connected to the negative electrode of a second energy storage element (C2) and the negative electrode of a fourth switch module (T4, D4), the positive electrode of the fourth switch module is connected to the negative electrode of a third switch module (T3, D3), and the positive electrode of the second energy storage element is connected to the positive electrode of the third switch module; and the positive electrode of the third switch module is connected to the positive electrode of a fifth switch module (T5, D5), and the negative electrode of the fifth switch module is connected to the negative electrode of the second switch module. This structure can provide a favourable direct-current side fault ride-through performance, thereby overcoming the limitation and deficiency of structures such as a full-bridge module and a clamping double-module, and having better economical and technological performances.

Description

Inverter unit, inverter, direct current transmission system and control method

Technical field

The invention belongs to the field of flexible direct current transmission, and particularly relates to an inverter module unit, a modular multi-level converter based on the converter module unit, a direct current transmission system, and the aforementioned converter module unit, and more modular Level converter and control method of DC transmission system. Background technique

The core of a flexible HVDC system is a voltage source converter based on a fully controlled device. Multilevel technology is the preferred solution for implementing high voltage, large capacity voltage source converters. Compared to two-level converters, multilevel converters can use high voltage devices to achieve high voltage level outputs without the need for direct series connection of switching devices. In recent years, the emergence of Modular Multilevel Converters (MMCs) has enabled multilevel converters to be successfully used in flexible DC transmission. The converter of the modular multi-level converter adopts a modular design, consisting of several identical basic unit modules connected in series, each of which is called an inverter module unit, by adding a series module in the inverter. The number and current levels can be applied to different voltage and power levels.

However, the traditional half-bridge module unit has inherent defects that cannot effectively deal with DC faults. When the DC side of the inverter fails, the anti-parallel freewheeling diode of the full control device easily constitutes an energy feeding loop that directly connects the fault point to the AC system. It is not possible to rely solely on the converter action to complete the DC side fault current clearing. It can only rely on the AC equipment to cut off the connection with the AC system. However, this method has a slow response speed, a complicated restart operation timing, and a long system recovery time. The problem limits the engineering application of traditional half-bridge module converters.

In order to improve the DC fault ride-through capability of the MMC inverter, the basic module structure evolves two units: a full-bridge module and a clamped dual-module, both of which can implement DC fault clearing and traversing of the converter, but both There are their own shortcomings:

The full-bridge module is mainly composed of four full-control switches and DC-supported capacitors. Compared with the traditional half-bridge modules, the switching devices used are twice as large. When the module outputs DC capacitor voltage or bypass, there are two full-control switches flowing at the same time. Current and loss are also doubled.

Clamped dual module cascades two traditional half-bridge modules with one full control switch and two clamp diodes Compared with the full bridge module, the number of full control switches is effectively reduced, and the running loss is reduced. However, the unit has two problems: First, when the clamped dual modules are in all fully controlled open and close lock states, due to the topology The structure of the diode is asymmetrical. The equivalent capacitance of each bridge arm is inconsistent when the AC charging is started, and the rising rate of the capacitor voltage is inconsistent, which makes the high-potential energy-carrying and monitoring consistency of the entire converter poor. The system causes misjudgment; the second is that in the case of DC fault, the two DC support capacitors of the clamped dual module are connected in parallel between the voltage of the AC system and the fault point, the DC voltage of the module rises slowly, and the dynamic response time of clearing the fault is longer. .

In view of the above analysis, the inventors have made research and improvement on the structure of the converter module unit, and the present invention has been produced.

Summary of the invention

The object of the present invention is to provide an inverter module unit and a control method thereof, which can provide good DC-side fault ride-through performance, overcome the limitations and disadvantages of the full-bridge module and the clamp dual-module structure, and are economical and technical. Both have better performance.

In order to achieve the above object, the solution of the present invention is:

An inverter module unit includes two energy storage elements, five switch modules and one diode, wherein a negative pole of the first switch module is connected to a positive pole of the second switch module, and a positive pole of the first switch module is connected to the first energy storage a positive pole of the component, a negative pole of the second switch module is connected to a negative pole of the first energy storage component; a negative pole of the diode is connected to a positive pole of the first switch module, and a positive pole of the diode is respectively connected to a negative pole of the second energy storage component and a negative pole of the fourth switch module, The anode of the fourth switch module is connected to the anode of the third switch module, the anode of the second energy storage component is connected to the anode of the third switch module, and the anode of the third switch module is also connected to the anode of the fifth switch module. a cathode of the fifth switch module is connected to a cathode of the second switch module; and a cathode of the first switch module is used as a first terminal of the converter module unit, and a cathode of the third switch module is used as a second of the converter module unit Lead end.

Each of the above switch modules includes a switch tube and a freewheeling diode connected in anti-parallel thereto, and the positive pole of the switch tube is taken as the positive pole of the switch module, and the negative pole of the switch tube is used as the negative pole of the switch module.

The above switch tube uses a semiconductor device that can be turned off.

The above switch tube adopts IGBT, IGCT, GT0 or MOSFET; when IGBT is used, its collector serves as the positive pole of the switch tube, and its emitter serves as the cathode of the switch tube; when IGCT or GT0 is used, its anode serves as the switch a positive electrode of the tube, the cathode of which is the negative electrode of the switching tube; when a MOSFET is used, Its drain serves as the positive electrode of the switching transistor, and its source serves as the negative electrode of the switching transistor.

The above energy storage element uses a capacitor.

A control method for an inverter module unit as described above, which controls the operation of the converter module unit in the following six working states:

Forward current charging state: Under the forward current, the switching tubes in the first, fourth and fifth switching modules are controlled to be turned on, and the switching tubes in the second and third switching modules are controlled to be turned off;

Forward current bypass state: Under the forward current, the switch tubes in the second, third and fifth switch modules are controlled to be turned on, and the switch tubes in the first and fourth switch modules are controlled to be turned off;

Negative current discharge state: Under the negative current, the switch tubes in the first, fourth and fifth switch modules are controlled to be turned on, and the switch tubes in the second and third switch modules are controlled to be turned off;

Negative current bypass state: Under the negative current, the switch tubes in the second, third and fifth switch modules are controlled to be turned on, and the switch tubes in the first and fourth switch modules are controlled to be turned off;

Forward current blocking state: Under the forward current, the switching tubes in the five switching modules are all turned off; Negative current blocking state: In the negative current, the switching tubes in the five switching modules are all turned off. A modular multilevel converter comprising an upper arm and a lower arm, each of the upper and lower arms comprising at least two inverter module units as previously described being cascaded to each other, All the inverter module units in the same bridge arm are connected in the same direction, and the first outlet end of the first inverter module unit in the upper bridge arm serves as the positive point of the modular multilevel converter, and the lower arm The second terminal of the last converter module unit serves as the negative point of the modular multilevel converter, the positive and negative points are used to access the DC network; and the last commutation in the upper arm The second terminal of the module unit is connected to the first terminal of the first converter module unit of the lower arm as an AC terminal of the modular multilevel converter for accessing the AC network .

At least one reactor is connected in series in each of the upper and lower arms.

In the above upper and lower arms, a bypass device is also connected in parallel between the first and second terminals of each of the inverter module units.

The above converter is composed of a hybrid converter module unit.

A unipolar direct current transmission system comprising a rectifier and an inverter, the rectifier being constructed by three parallel modular multilevel converters as described above, the three modular multilevel converters The AC terminals are respectively connected to the three phases of the AC system or the load; the inverter is composed of three further modular multilevel converters as described above, the three modular multilevel commutations The communication endpoints are connected to the AC system Or three phases of the load; the positive poles of the three modular multilevel converters in the rectifier are connected to the positive poles of the three modular multilevel converters in the inverter via a DC connection, three of the rectifiers The negative point of the modular multilevel converter and the negative point of the three modular multilevel converters in the inverter are also connected via a DC link.

A method for controlling a unipolar direct current transmission system as described above includes the following steps:

( al ) to determine whether the DC side is faulty, if it is faulty, go to step (a2);

( a2 ) When a DC fault occurs, a turn-off signal is applied to the switch tube in each converter module unit at the pole where the fault is located;

( a3 ) After the DC fault is removed, restart each inverter module unit.

A bipolar direct current transmission system comprising a rectifying side and an inverting side, wherein the rectifying side comprises 2n rectifiers connected in series with each other, η is a natural number, and each of the rectifiers comprises three modules as described above in parallel with each other. In the multi-level converter, in each rectifier, the AC terminals of the three modular multi-level converters are respectively connected to the three phases of the AC system or the load, and the positive pole is short-circuited to serve as the anode of the rectifier, and the anode Short-circuited as the negative pole of the rectifier;

The inverter side includes another 2n inverters connected in series with each other in the same direction, and η is a natural number, and each of the inverters includes three modular multilevel converters as described above, which are connected in parallel with each other. In each inverter, the AC terminals of the three modular multilevel converters are respectively connected to the three phases of the AC system or the load, and the positive pole is shorted to be the positive pole of the inverter, and the negative pole is shorted. As the negative electrode of the inverter;

The 2n rectifiers are connected in series in the same direction, and the 2n inverters are connected in series in the same direction, one of the neutral points of the two series lines is grounded, or the neutral points of the two series lines are connected by a DC connection; The anode of the first rectifier in the side is connected to the anode of the first inverter in the inverter side through a DC connection, and the cathode of the last rectifier in the rectifier side is connected to the cathode of the last inverter in the inverter side through a DC connection.

The above DC connection refers to an overhead line connection, an underground cable connection, a submarine cable connection or a hard connection. A filter is connected between the positive side of the rectifying side and the neutral point, or between the negative electrode and the neutral point; or between the positive side of the inverter side and the neutral point, or between the negative electrode and the neutral point.

A control method for a bipolar direct current transmission system as described above includes the following steps:

( b l ) Determine if the DC side is faulty, if it is faulty, go to step (b2);

(b2) When a DC fault occurs, a turn-off signal is applied to the switch tube in each inverter module unit at the pole where the fault is located; (b3) After the DC fault is removed, restart each inverter module unit.

After adopting the above scheme, the present invention has the following beneficial effects by adopting a novel inverter module unit structure:

(1) Combining two half-bridge modules in the form of a switch module and a diode cross-connection, constituting the inverter module unit structure of the present invention, and providing a good DC fault ride-through capability;

(2) The converter module unit can realize the function of multi-level voltage source and clear DC short-circuit fault, and uses less devices, has small running loss, and has engineering practical value;

(3) In the inverter start-up and fault-locked state, all diode loops of the converter module unit are symmetrical with respect to the two terminals, thus providing at least two beneficial characteristics: First, the starting state is from AC When charging, the equivalent capacitance of each bridge arm is consistent, and the rising rate of the capacitor voltage is basically the same, so that the high-potential energy-capacity and monitoring of the entire converter are more consistent and coordinated. Second, in the fault-locked state, the commutation The two capacitors of the module unit are connected in series between the AC power supply and the fault point. When the fault occurs, the DC voltage is rapidly raised, the DC short-circuit fault is quickly cleared, and the dynamic response is fast;

(4) The inverter composed of the converter module unit has a basic control strategy compatible with the inverter constituted by the conventional half bridge module, and has portability;

(5) The inverter composed of the converter module unit, because it comprises five power semiconductor switch modules and two capacitors, can output 0, Vc, 2Vc at two terminals of the module through an advanced control strategy. The level is more flexible than the 0 and Vc levels of the traditional half-bridge module converter. DRAWINGS

1 is a unit diagram of an inverter module unit of the present invention;

2 is a schematic diagram of a charging condition of an inverter module unit under forward current according to the present invention; FIG. 3 is a schematic diagram of a bypass condition of a converter module unit under forward current according to the present invention; FIG. 5 is a schematic diagram of a bypass condition of an inverter module unit under a negative current according to the present invention; FIG. 5 is a schematic diagram of a bypass condition of a converter module unit under a negative current according to the present invention; Schematic diagram of the operating condition of the converter module unit flowing through the forward current under the full blocking of the switch tube;

7 is a schematic diagram of a working condition of an inverter module unit flowing through a negative current under full blocking of a switch tube;

Figure 8 is a unit diagram of a modular multilevel converter of the present invention; Figure 9 is a unit diagram of a first embodiment of a direct current power transmission system of the present invention;

Figure 10 is a schematic view showing the operation of a modular multilevel converter for clearing a DC side short circuit according to the present invention; and Figure 11 is a block diagram showing a second embodiment of a DC power transmission system according to the present invention. detailed description

The technical solution of the present invention will be described in detail below with reference to the accompanying drawings.

As shown in FIG. 1 , the present invention provides an inverter module unit including two energy storage elements Cl C2 5 switch modules and a diode D6 , wherein a negative pole of the first switch module is connected to a positive pole of the second switch module, The anode of the first switch module is connected to the anode of the first energy storage component, the cathode of the second switch module is connected to the cathode of the first energy storage component C1; the cathode of the diode D6 is connected to the anode of the first switch module, and the anode of the diode D6 is connected to the anode a cathode of the second energy storage component C2 and a cathode of the fourth switch module, wherein a positive pole of the fourth switch module is connected to a cathode of the third switch module, and a cathode of the second energy storage component C2 is connected to a cathode of the third switch module; The anode of the third switch module is further connected to the anode of the fifth switch module, and the cathode of the fifth switch module is connected to the cathode of the second switch module; and the cathode of the first switch module is used as the first terminal of the converter module unit, The second anode of the third switch module is taken as the second lead of the inverter module unit. In this embodiment, the energy storage element Cl C2 can adopt a capacitor. For each of the switch modules, the switch tubes T1-T5 and the diodes D1-D5 (also referred to as "freewheeling diodes") that are connected in anti-parallel thereto are connected, and the negative pole of the freewheeling diode and the corresponding switch tube The positive pole is connected, the positive pole of the freewheeling diode is connected to the negative pole of the corresponding switch tube, and the positive pole of the switch tube is taken as the positive pole of the switch module, and the negative pole of the switch tube is used as the negative pole of the switch module; the switch tube can be used for all a power semiconductor device having a turn-off function (ie, turning off the semiconductor device), when the switching transistor adopts an IGBT, with its collector as the positive electrode of the switching transistor and its emitter as the negative electrode of the switching transistor; When the switch tube adopts IGCT or GT0, its anode is used as the positive pole of the switch tube, and its cathode is used as the cathode of the switch tube; when the switch tube uses the MOSFET, its drain is used as the anode of the switch tube. Taking its source as the negative electrode of the switching tube.

In conjunction with the circuit architecture shown in FIG. 1, the present invention also provides a control method for the converter module unit, wherein the control method can control the inverter module unit to operate in six working states: a forward current charging state, The current bypass state, the negative current discharge state, the negative current bypass state, the forward current blocking state, and the negative current blocking state. (1) The switching tubes Tl, Τ4, and Τ5 are turned on, and the switching tubes Τ2 and Τ3 are turned off. Under the forward current, the freewheeling diodes D1, D5, and D4 are turned on in turn to charge the energy storage components C1 and C2. The converter module unit enters a forward current charging state, as shown in FIG. 2;

(2) The switching tubes T2, Τ3, and Τ5 are turned on, and the switching tubes T1 and Τ4 are turned off. Under the forward current, the freewheeling diode D5 is turned on, bypassing the energy storage components C1 and C2, and the converter module The unit enters the forward current bypass state, as shown in Figure 3;

(3) The switching tubes Tl, Τ4, and Τ5 are turned on, and the switching tubes Τ2 and Τ3 are added with a shutdown signal. Under the negative current, the energy storage elements C1 and C2 are discharged, and the converter module unit enters a negative current discharging state. As shown in Figure 4;

(4) Switching tube Τ2, Τ3, Τ5 plus turn-on signal, switching tube T1 and Τ4 plus turn-off signal, under negative current, freewheeling diodes D3, D2 turn on sequentially, bypass energy storage components C1 and C2, change The flow module unit enters a negative current bypass state, as shown in FIG. 5;

(5) The switching tubes T1 to T5 are connected with a shutdown signal. Under the forward current, the freewheeling diodes D1, D5, and D4 are turned on in turn, the energy storage components C1 and C2 are charged, and the converter module unit enters the forward current blocking state. , As shown in Figure 6;

(6) The switching tubes T1 to T5 are connected with a shutdown signal. Under the negative current, the freewheeling diodes D3, D6, D2 are turned on in turn, the energy storage components C1 and C2 are charged, and the converter module unit enters the negative current blocking state. , as shown in Figure 7.

As shown in FIG. 8, the present invention further provides a modular multilevel converter comprising the aforementioned converter module unit, comprising an upper arm and a lower arm, wherein the upper and lower arms each include Cascading at least two inverter module units, and the number of converter module units included in the upper and lower arms may be the same or different, and the specific circuit structure of each converter module unit may be the same, or Different; in the same bridge arm (upper arm or lower arm), the second outlet of the previous converter module unit is connected to the first outlet of the latter inverter module unit, and the first one is changed a first outlet end of the flow module unit as a first end of the upper/lower bridge arm, and a second outlet end of the last inverter module unit as a second end of the upper/lower bridge arm, The first end of the upper arm acts as the positive point of the modular multilevel converter for accessing the DC network, and the second end of the lower arm acts as the negative point of the modular multilevel converter for Connected to the DC network, while the second end of the upper arm is down A first arm end connected together to a terminal of the modular multi-Uac level AC inverter, AC to the access network.

In this embodiment, the upper and lower bridge arms may also be respectively connected in series with a reactor, and the reactor may be connected in series Any position of the first and second ends, usually the reactor is connected in series at the AC end point. Since one reactor can be regarded as a plurality of sub-reactors connected in series, the number of the reactors is not limited as long as the upper/lower bridge arms are The total reactance of the bridge reaches the corresponding requirement of the bridge arm.

As a preferred solution, a switch or other bypass device may be connected in parallel between the first and second terminals of each of the converter module units for use in the event of a fault in one of the inverter module units. Exit to increase the availability of modular multilevel converters.

As shown in FIG. 12, as a preferred solution, the modular multilevel converter is formed by cascading the converter module unit and other topology converter module units, that is, by mixing The streamer module unit is cascaded.

As shown in FIG. 11, the present invention further provides a unipolar direct current power transmission system including a rectifier and an inverter, the rectifier including three aforementioned modular multilevel converters connected in parallel with each other, the three modular The AC terminals of the multilevel converter are respectively connected to the three phases of the AC system or the load; the inverter comprises three further three modular multilevel converters connected in parallel with each other, the three modular multilevels The AC terminals of the inverter are respectively connected to the three phases of the AC system or the load; the positive poles of the three modular multilevel converters in the rectifier and the positive of the three modular multilevel converters in the inverter Pole through f

For connection, the negative point of the three modular multilevel converters in the rectifier and the negative point of the three modular multilevel converters in the inverter are also connected via a DC link.

Such a connection structure is called a pseudo bipolar system, that is, one of the poles of the pole is connected to the DC link, and the cathode is also connected to the DC link, and the power is transmitted from the AC side to the DC side through the rectifier, and then through the inverter Electrical energy is transmitted from the DC side to the AC side.

For the foregoing unipolar direct current transmission system, the present invention also provides a control method for quickly clearing a direct current fault without interrupting the direct current transmission. The control method includes the following steps:

( a3 ) After the DC fault is removed, restart each inverter module unit.

The invention provides a bipolar direct current transmission system, comprising a rectifying side and an inverting side, wherein the rectifying side comprises 2n rectifiers connected in series with each other, η is a natural number, and each of the rectifiers comprises three aforementioned modular modules connected in parallel with each other. In the multi-level converter, in each rectifier, the AC terminals of the three modular multi-level converters are respectively connected to the three phases of the AC system or the load, and the positive pole is short-circuited to serve as the anode of the rectifier, and the anode The short-circuit is used as the negative pole of the rectifier; the inverter side includes another 2n inverters connected in series with each other in the same direction, and η is a natural number, and each of the inverters includes three aforementioned modular modules connected in parallel with each other. Level converter, in each inverter, the AC terminals of the three modular multilevel converters are respectively connected to the three phases of the AC system or the load, and the positive pole is shorted to serve as the anode of the inverter The negative pole is short-circuited and serves as the negative pole of the inverter; the anode of the first rectifier in the rectifying side is connected to the anode of the first inverter in the inverter side through a DC connection, and the anode of the last rectifier in the rectifying side passes The DC connection is connected to the negative pole of the last inverter in the inverter side; meanwhile, the 2n rectifiers are connected in series in the same direction to form a series line, and the 2n inverters are connected in series in the same direction to form another series line, so that the two lines The neutral point of one of the series lines is grounded, or the neutral points of the two series lines are connected by a DC connection.

Figure 9 shows the circuit structure diagram of η = 1. The power is transmitted from the AC side to the DC side through the rectification side, and the power is transmitted from the DC side to the AC side through the inverter side.

In this embodiment, the DC connection can be either an overhead line, an underground cable or a submarine cable, or a hard connection. The so-called hard connection refers to the positive point of the modular multi-level converter directly on both sides. Or the negative point is docked to form a so-called back-to-back system.

As a preferred solution, for the rectification side or the inverter side, a filter may be connected in parallel between the positive and neutral points on the rectification side (or the inverter side) or between the negative and neutral points.

For the foregoing bipolar direct current transmission system, the present invention also provides a control method for quickly clearing a direct current fault without interrupting the direct current transmission. The control method includes the following steps:

(bl) to determine whether the DC side is faulty, if it is faulty, go to step (b2);

(b2) When a DC fault occurs, a turn-off signal is applied to the switch tube in each converter module unit at the pole where the fault is located;

(b3) After the DC fault is removed, restart each inverter module unit.

It should be noted that, based on the foregoing single/bipolar direct current transmission system, the architecture of the multi-terminal direct current transmission system can be extended, and the rectifier pole or/and the inverter pole in the single pole direct current transmission system shown in FIG. 11 can be set. For a plurality of, and parallel with each other, the rectification side or/and the inverter side in the bipolar direct current transmission system shown in FIG. 9 may be arranged in plurality and connected in parallel; the rectifier-inverter on both sides of the aforementioned DC connection The number of inverters and rectifier sides can be the same or different. It only needs to match the transformation parameters. The idea of extending a single/bipolar direct current transmission system into a multi-terminal direct current transmission system is clear and will not be described in detail herein.

For the above-mentioned multiple DC transmission systems, when the overhead connection uses the overhead line, the short-circuit fault often occurs. The prior art scheme can only be connected by disconnecting the AC side due to the short-circuit current feeding loop. Ways to eliminate short-circuit faults, protect equipment and power transmission security, this way will interrupt the entire power transmission, which will bring serious economic losses. The beneficial effects of using the converter module unit proposed in the present case to form the above-mentioned DC power transmission system are as follows: When the DC side is short-circuited, the self-clearing of the DC-side fault and the restart of the fault pole can be realized by the converter control, and the entire improvement is remarkably improved. The performance of the system, thus breaking through the limitations of the current application range.

The DC side fault self-clearing principle is: the positive pole point A of the modular multilevel converter has a short circuit flowing through the fault current, the converter blocks all the switching tubes, and the fault current passes through the diodes D2 and D3 of each converter module unit. The D6 circuit charges the C1 and C2 in series as shown in Figure 10. Thus, the total output of all series converter module units is equivalent to the DC voltage source Vdc. When Vdc is greater than the peak value of the external applied voltage when the short circuit is applied, the DC fault current is automatically cleared, and then the DC fault is quickly recovered.

The inverter module unit of the present invention can also be used for a voltage source type inverter-like device such as a reactive power compensation device, an active or reactive power generating device, and a power feedback.

The above embodiments are only for explaining the technical idea of the present invention, and the scope of protection of the present invention is not limited thereto. Any changes made based on the technical solutions according to the technical idea of the present invention fall within the protection scope of the present invention. Inside.

Claims

claims

1. A converter module unit, characterized by: including two energy storage components, 5 switch modules and a diode, wherein the negative electrode of the first switch module is connected to the positive electrode of the second switch module, The anode is connected to the anode of the first energy storage element, the cathode of the second switch module is connected to the cathode of the first energy storage element; the cathode of the diode is connected to the anode of the first switch module, and the anode of the diode is connected to the cathode of the second energy storage element and the cathode of the third energy storage element respectively. The negative electrode of the fourth switch module is connected to the negative electrode of the third switch module. The positive electrode of the second energy storage element is connected to the positive electrode of the third switch module. The positive electrode of the third switch module is also connected to the fifth switch module. The positive pole of the switch module, and the negative pole of the fifth switch module is connected to the negative pole of the second switch module; the negative pole of the first switch module is used as the first outlet of the converter module unit, and the negative pole of the third switch module is used as the commutation The second terminal of the converter module unit.

2. A converter module unit according to claim 1, characterized in that: each switch module includes a switch tube and a freewheeling diode connected in reverse parallel with it, and the anode of the switch tube is used as the switch. The positive pole of the module takes the negative pole of the switch tube as the negative pole of the switch module where it is located.

3. A converter module unit according to claim 2, characterized in that: the switch tube adopts a semiconductor device that can be turned off.

4. A converter module unit according to claim 2, characterized in that: the switching tube adopts IGBT, IGCT, GT0 or MOSFET; when IGBT is used, its collector is used as the positive electrode of the switching tube, and The emitter is used as the cathode of the switch tube; when IGCT or GT0 is used, its anode is used as the anode of the switch tube, and its cathode is used as the cathode of the switch tube; when MOSFET is used, its drain is used as the anode of the switch tube. , and its source is used as the negative electrode of the switch tube.

5. The converter module unit according to claim 1 or 2, characterized in that: the energy storage element uses a capacitor.

6. A method for controlling a converter module unit as claimed in claim 2, characterized in that the converter module unit is controlled to work in the following six working states:

Forward current charging state: Under forward current, control the switch tubes in the first, fourth, and fifth switch modules to turn on, and control the switch tubes in the second and third switch modules to turn off;

Forward current bypass state: Under forward current, control the switch tubes in the second, third, and fifth switch modules to turn on, and control the switch tubes in the first and fourth switch modules to turn off;

Negative current discharge state: Under negative current, control the switch tubes in the first, fourth, and fifth switch modules Turn on, control the switch tubes in the second and third switch modules to turn off;

Negative current bypass state: Under negative current, control the switch tubes in the second, third, and fifth switch modules to turn on, and control the switch tubes in the first and fourth switch modules to turn off;

Forward current latching state: Under forward current, all switch tubes in the 5 switch modules are controlled to be turned off; Negative current latching state: Under negative current, all switch tubes in the 5 switch modules are controlled to be turned off.

7. A modular multi-level converter, characterized in that: it includes an upper bridge arm and a lower bridge arm, and the upper and lower bridge arms each include at least two mutually cascaded converters as claimed in claim 1 , all the converter module units in the same bridge arm are connected in the same direction, the first lead-out end of the first converter module unit in the upper bridge arm is used as the positive pole point of the modular multi-level converter, and the lower The second lead-out end of the last converter module unit in the bridge arm serves as the negative pole point of the modular multi-level converter, and the positive and negative pole points are used to connect to the DC network; while the last terminal in the upper bridge arm The second lead-out end of one converter module unit is connected to the first lead-out end of the first converter module unit in the lower arm, serving as the AC end point of the modular multi-level converter for access. in the communication network.

8. A modular multi-level converter according to claim 7, characterized in that: at least one reactor is connected in series in the upper and lower bridge arms.

9. A modular multi-level converter as claimed in claim 7, characterized in that: in the upper and lower bridge arms, there is a gap between the first and second lead-out terminals of each converter module unit. Parallel bypass device.

10. A modular multi-level converter according to claim 7, characterized in that: it is composed of hybrid converter module units cascaded.

11. A unipolar DC transmission system, characterized in that: it includes a rectifier and an inverter, and the rectifier is composed of three modular multi-level converters as claimed in claim 7 connected in parallel, and the three The AC endpoints of the modular multi-level converter are respectively connected to the three phases of the AC system or the load; the inverter is composed of three other modular multi-level converters as claimed in claim 7 connected in parallel, The AC end points of the three modular multi-level converters are respectively connected to the three phases of the AC system or load; the positive pole points of the three modular multi-level converters in the rectifier are connected to the three modular multi-level converters in the inverter. The positive pole points of the multi-level converters are connected via a DC connection, and the negative pole points of the three modular multi-level converters in the rectifier are also connected to the negative pole points of the three modular multi-level converters in the inverter. DC connection to connect.

12. A control method for a unipolar DC transmission system as claimed in claim 11, characterized by comprising the following steps:

(al) Determine whether the DC side is faulty. If there is a fault, go to step (a2); (a2) When a DC fault occurs, a shutdown signal is applied to the switching tube in each converter module unit at the pole where the fault is located;

(a3) After the DC fault is eliminated, restart each converter module unit.

13. A bipolar DC transmission system, characterized in that: it includes a rectifier side and an inverter side. The rectifier side includes 2n rectifiers connected in series with each other in the same direction. n is a natural number. Each of the rectifiers includes three rectifiers connected in parallel. The modular multi-level converter as claimed in claim 7, in each rectifier, the AC end points of the three modular multi-level converters are respectively connected to the three phases of the AC system or the load, and the positive pole points are short-circuited. After that, it is used as the positive pole of the rectifier, and after the negative pole is short-circuited, it is used as the negative pole of the rectifier;

The inverter side includes another 2n inverters connected in series with each other in the same direction, n is a natural number, and each of the inverters includes three modular multi-level commutation as claimed in claim 7 connected in parallel with each other. In each inverter, the AC end points of the three modular multi-level converters are connected to the three phases of the AC system or load respectively. The positive pole points are short-circuited as the positive pole of the inverter, and the negative pole points are short-circuited. After being connected, it serves as the negative pole of the inverter; the aforementioned 2n rectifiers are connected in series in the same direction, the aforementioned 2n inverters are connected in series in the same direction, one of the neutral points of the two series lines is grounded, or the neutral points of the two series lines are connected through a DC connection; the positive pole of the first rectifier on the rectifier side is connected to the positive pole of the first inverter on the inverter side through a DC connection, and the negative pole of the last rectifier on the rectifier side is connected to the inverter side through a DC connection. The negative pole of the last inverter.

14. A bipolar DC transmission system as claimed in claim 13, characterized in that: the DC connection refers to an overhead line connection, an underground cable connection, a submarine cable connection or a hard connection.

15. A bipolar DC transmission system as claimed in claim 13, characterized in that: a filter is connected in parallel between the positive pole and the neutral point on the rectifier side, or between the negative pole and the neutral point; or on the inverter side A filter is connected in parallel between the positive pole and the neutral point, or between the negative pole and the neutral point.

16. A control method for a bipolar DC transmission system as claimed in claim 13, characterized by comprising the following steps:

(b l) Determine whether the DC side is faulty. If there is a fault, go to step (b2);

(b2) When a DC fault occurs, a shutdown signal is applied to the switching tube in each converter module unit at the pole where the fault is located;

(b3) After the DC fault is eliminated, restart each converter module unit.