CN110995039B - A low-loss modular multilevel converter and its parameter design method - Google Patents

Abstract

The present invention provides a low-loss modular multi-level converter, comprising three phase units, each unit is divided into upper and lower bridge arms, each bridge arm includes several serially connected sub-modules, and the sub-modules are composed of two half-phase units. a bridge structure, four capacitors and two freewheeling diodes, the first half-bridge includes a first switch module and a second switch module, the second half-bridge includes a third switch module and a fourth switch module; the first half-bridge The negative pole of the switch module is connected to the positive pole of the second switch module, the negative pole of the second switch module is connected to the positive pole of the third switch module, and the negative pole of the third switch module is connected to the positive pole of the fourth switch module. Positive connected. The present invention also provides a method for designing the above inverter parameters. The invention can realize the fault blocking of the DC side short circuit by controlling the switching module on and off without increasing the loss.

Description

A low-loss modular multilevel converter and its parameter design method

technical field

The invention relates to the technical field of power transmission and distribution of power systems, in particular to a low-loss modular multilevel converter with fault blocking capability and a parameter design method thereof.

Background technique

With the increase of renewable energy power generation, the integration of renewable energy into the grid has become a very important research direction.

Flexible DC transmission technology provides a solution to the grid connection of renewable energy, and has strong technical advantages. Among them, the flexible DC transmission technology using modular multi-level converters has better maintainability and expandability than traditional two-level and three-level converters, and there is no series voltage equalization or parallel connection of switch tubes. flow sharing problem. The structure of each sub-module of the modular multi-level converter is relatively simple, the control is easy, and due to the characteristics of easy expansion of modularization, it is especially suitable for the field of high-voltage direct current transmission. The number of levels output by the modular multi-level converter can be adjusted by adjusting the number of sub-modules in the bridge arm, and the voltage on the DC side can also be controlled by adjusting the number of sub-modules per phase. The voltage level and output harmonic content of the system constituted by the current transformer can be effectively controlled.

The traditional modular multi-level converter adopts the sub-module topology of the half-bridge structure, and the half-bridge sub-module topology is not controlled by the control pulse due to the anti-parallel diodes of the lower tube, and when a short-circuit fault occurs on the DC side, the AC side will be damaged. The freewheeling circuit of the DC fault point, thereby transmitting the short-circuit fault to the AC side, which affects the stability of the power grid. Therefore, the fault current needs to be blocked by DC, AC circuit breakers or other means. However, there is currently no mature technology for DC circuit breakers, and the cost is too expensive; AC circuit breakers require a long response time, and the converter valve device needs to withstand a large current stress before the AC circuit breaker is disconnected, which is very easy to cause device damage. of damage. Therefore, a sub-module topology is required, and the control of this sub-module can block the DC side fault.

A common problem with the existing sub-module topologies with fault blocking capability is that in the normal working state of the converter, there will be an additional switching device in each sub-module located on the normal current path, and the device is in a normally open state. When a fault occurs, the switching device is turned off, allowing current to flow from another path to achieve the effect of fault current blocking or limiting. This extra switching device will increase the conduction loss of the system, resulting in a loss of resources.

SUMMARY OF THE INVENTION

In view of the defects of the prior art, the purpose of the present invention is to provide a low-loss modular multi-level converter with fault blocking capability and a parameter design method thereof, which can realize the fault resistance of the DC side short circuit without additional switching devices. break.

According to a first aspect of the present invention, a low-loss modular multi-level converter is provided, comprising three phase units, each unit is divided into upper and lower bridge arms, each bridge arm includes a number of sub-modules connected in series, each The number of sub-modules connected in series on the upper and lower bridge arms of the phase is the same; the upper and lower bridge arms are respectively connected in series with current limiting reactors, each phase from top to bottom: all sub-modules of the upper bridge arm, upper bridge arm reactors, and lower bridge arm reactances and all sub-modules of the lower bridge arm; and the connection of the upper and lower bridge arms of each phase is connected to a three-phase AC voltage. The two output terminals are connected to the negative pole of the DC bus; in each bridge arm, the sub-module consists of two half-bridge structures, four capacitors and two freewheeling diodes, wherein:

In the half-bridge structure, the first half-bridge includes a first switch module and a second switch module, the second half-bridge includes a third switch module and a fourth switch module; the negative pole of the first switch module is connected to the The positive pole of the second switch module is connected to the positive pole of the second switch module, the negative pole of the second switch module is connected to the positive pole of the third switch module, and the negative pole of the third switch module is connected to the positive pole of the fourth switch module;

Among the four capacitors, the positive electrode of the first capacitor is connected to the positive electrode of the first switch module; the negative electrode of the first capacitor is connected to the positive electrode of the second capacitor; the negative electrode of the second capacitor is connected to the positive electrode of the second capacitor. The negative pole of the second switch module is connected to the negative pole of the second switch module; the positive pole of the third capacitor is connected to the positive pole of the third switch module; the negative pole of the third capacitor is connected to the positive pole of the fourth capacitor, and the fourth capacitor The negative pole is connected to the negative pole of the fourth switch module;

Among the two freewheeling diodes, the positive electrode of the first freewheeling diode is connected to the negative electrode of the first capacitor, and the negative electrode of the first freewheeling diode is connected to the positive electrode of the fourth switch module. The anode of the second freewheeling diode is connected to the anode of the second switch module, and the cathode of the second freewheeling diode is connected to the cathode of the third capacitor;

The node between the negative pole of the first switch module and the positive pole of the second switch module is used as the first output terminal of the entire sub-module; the negative pole of the third switch module and the positive pole of the fourth switch module are connected. The node between them is used as the second output terminal of the whole sub-module.

Optionally, the first output terminal is connected to the output port of the first half-bridge structure and the cathode of the second freewheeling diode, and the second output terminal is connected to the output port of the second half-bridge structure and the first Anode of the freewheeling diode.

Optionally, the first switch module and the fourth switch module are both composed of an insulated gate bipolar transistor and a diode in anti-parallel connection.

Optionally, the second switch module and the third switch module are both reverse resistance switch modules. Further, the second switch module is composed of a first reverse-resistance insulated gate bipolar transistor and a second reverse-resistance insulated gate bipolar transistor connected in anti-parallel with it; similarly, the third switch module is composed of a third It is composed of a reverse resistance type insulated gate bipolar transistor and a fourth reverse resistance type insulated gate bipolar transistor connected in anti-parallel with it.

Optionally, in the above-mentioned converter of the present invention, under normal working conditions, the negative electrode of the second switch module is connected to the second reverse-resistance insulated gate bipolar transistor of the first output terminal and the positive electrode of the third switch module is connected. The fourth reverse-resistance insulated gate bipolar transistor of the second output terminal remains in an on state; the two freewheeling diodes remain in an off state due to the reverse voltage, and are not added to the circuit, so that no conduction loss occurs.

Optionally, in the DC power transmission system of the above-mentioned converter of the present invention, when a bipolar short-circuit fault is detected on the DC side, all fully-controlled switches are immediately turned off, and the fully-controlled switches include the first switch module, the Insulated gate bipolar transistors in the fourth switch module, and reverse-resistance insulated gate bipolar transistors in the second switch module and the third switch module; the fault current will flow into each bridge arm from the second output terminal The sub-module topology in the circuit flows out through the second freewheeling diode, the second capacitor, the third capacitor and the first freewheeling diode.

Optionally, in the above-mentioned converter of the present invention, when the fault is a DC permanent fault: turn off all the full-control switches, disconnect the AC circuit breaker and the DC side switch after the AC side current returns to zero, and perform maintenance; Afterwards, the DC side switch is closed, the AC side is reclosed, and then the second and fourth reverse-resistance insulated gate bipolar transistors are turned on to restore the normal working state of the first, second, third, and fourth switch modules.

Optionally, in the above-mentioned converter of the present invention, when the fault is a DC temporary fault: turn off all full-control switches, wait for the DC side current to return to zero, wait for a certain period of time after the fault is cleared, and then turn on the second and fourth inverters. Resistive insulated gate bipolar transistor, restore the normal working state of the first, second, third and fourth switch modules, if there is no overcurrent phenomenon, then reclosing, after the reclosing is successful, it means the fault has been cleared; If overcurrent occurs, all full-control switches will be turned off again; when overcurrent occurs more than three times, it is considered that a permanent fault has occurred.

According to a second aspect of the present invention, a parameter design method for a low-loss modular multilevel converter is provided, including:

Determine the limiting relationship between the capacitance and inductance and the number of bridge arm sub-modules according to the circulating current resonance relationship of the converter;

According to the withstand voltage of the selected semiconductor device, the number of sub-modules of each bridge arm is designed, and then the capacitance of each sub-module is selected according to the relationship between the system capacity and the system energy storage;

Finally, the inductance value of the system is determined according to the capacitor and the number of sub-modules of the bridge arm.

Under normal conditions, the series-connected half-bridge sub-module structure is adopted, so that when generating each unit level, the current only needs to flow through one switching device, which can reduce the conduction loss; The nature of the fuse blocks the current path, and through the additional freewheeling loop, the fault current is directed to the positive pole of the sub-module capacitor, so as to use the sub-module capacitor voltage to achieve fault blocking. At the same time, the voltage stress of some switching tubes can be reduced by splitting the capacitors of the sub-modules and connecting the freewheeling diodes to the split points.

Compared with the prior art, the present invention has the following beneficial effects:

The above-mentioned low-loss modular multi-level converter of the present invention adopts a sub-module topology structure different from the sub-module topology in each bridge arm of the prior art. The bridge sub-modules have the same number of conduction elements and thus have similar conduction losses. Therefore, there is no need to rely on additional switching devices, and the conduction loss of the system will not be increased.

The above-mentioned low-loss modular multi-level converter of the present invention can realize fault isolation in the case of DC fault by controlling the state of the switch modules of the sub-modules in each bridge arm, and the isolation speed is fast.

The above-mentioned low-loss modular multi-level converter of the present invention can maintain the capacitor voltage of the sub-modules in each bridge arm in the event of a fault, and restore power supply quickly.

The above-mentioned low-loss modular multilevel converter of the present invention has a start-up process on the AC side similar to that of a traditional half-bridge system, and is simple to control.

Description of drawings

Other features, objects and advantages of the present invention will become more apparent by reading the detailed description of non-limiting embodiments with reference to the following drawings:

1 is a schematic diagram of a low-loss modular multilevel converter in an embodiment of the present invention;

2 is a sub-module topology of a low-loss modular multilevel converter in an embodiment of the present invention;

FIG. 3 is an equivalent circuit diagram of a low-loss modular multilevel converter sub-module controlled by a switch tube under a DC fault in an embodiment of the present invention;

4 is a flow chart of an AC side startup strategy of a low-loss modular multilevel converter in an embodiment of the present invention;

Fig. 5 is the deduction idea of the sub-module topology structure of the low-loss modular multi-level converter in an embodiment of the present invention;

FIG. 6 is a flowchart of a parameter design method for a low-loss modular multilevel converter in an embodiment of the present invention.

Detailed ways

The present invention will be described in detail below with reference to specific embodiments. The following examples will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be pointed out that for those skilled in the art, without departing from the concept of the present invention, several modifications and improvements can be made, which all belong to the protection scope of the present invention.

FIG. 1 is a schematic diagram of a low-loss modular multilevel converter in an embodiment of the present invention.

Referring to Figure 1, the low-loss modular multi-level converter includes three phase units, each unit is divided into upper and lower bridge arms, each bridge arm includes several serially connected sub-modules, and the upper and lower bridge arms of each phase are connected in series. The number of sub-modules is the same; the upper and lower arms are connected in series with current-limiting reactors, each phase from top to bottom: all sub-modules of the upper arm, reactors of the upper arm, reactors of the lower arm, and all sub-modules of the lower arm module; and the connection between the upper and lower bridge arms of each phase is connected to a three-phase AC voltage, the first output terminal of the top sub-module topology of the upper bridge arm is connected to the positive pole of the DC bus, and the second output terminal of the lowermost terminal module of the lower bridge arm is connected to the DC bus. Negative connection.

FIG. 2 is a sub-module topology of a low-loss modular multilevel converter in an embodiment of the present invention.

Referring to FIG. 2 , the sub-module of each bridge arm in the multi-level DC-DC converter is composed of two half-bridge structures, four capacitors C ₁ -C ₄ and two freewheeling diodes D ₃ -D ₄ .

In the two half-bridge structures, the first half-bridge includes a first switch module and a second switch module; the negative pole of the _first switch module T1 is connected to the positive pole of the second switch module. The second half bridge includes a third switch module and a fourth switch module; the negative pole of the third switch module is connected to the positive pole of the fourth switch module; the positive pole of the third switch module is connected to the negative pole of the second switch module. Specifically, as shown in FIG. ₂ , the first switch module is composed of an insulated gate bipolar transistor T1 and _a diode D1 in antiparallel; the fourth switch module is composed of _an insulated gate bipolar transistor T2 and a diode D2 in _antiparallel The second switch module is a reverse resistance type switch module, which is composed of a first reverse resistance type insulated gate bipolar transistor _TR1 and a second reverse resistance type insulated gate bipolar transistor _TR2 connected in anti-parallel with it. The third switch The module is also a reverse resistance type switch module, which is composed of a third reverse resistance type insulated gate bipolar transistor _TR3 and a fourth reverse resistance type insulated gate bipolar transistor _TR4 connected in anti-parallel with it.

Among the four capacitors, the positive electrode of the first capacitor _C1 is connected to the positive electrode of the first switch module T1 _; the negative electrode of the first capacitor _C1 is connected to the positive electrode of the _second capacitor C2 _; The negative electrode is connected to the negative electrode of the second switch module; the positive electrode of the _third capacitor C3 is connected to the positive electrode of the third switch module; the negative electrode of the _third capacitor C3 is connected to the positive electrode of the fourth capacitor _C4 ; the fourth capacitor C3 is connected to the positive electrode of the fourth capacitor C4; The negative electrode of _C4 is connected to the negative electrode of the fourth switch module; the positive electrode _D3 of the first freewheeling diode is connected to the negative electrode of the first capacitor _C1; the negative electrode of the first freewheeling diode _D3 is connected to the positive electrode of the fourth switching module ; _The positive pole of the second freewheeling diode D4 is connected to the positive pole of the second switch module _; the negative pole of the second freewheeling diode D4 is connected to the negative pole of the _third capacitor C3.

In the multi-level converter sub-module of the above embodiment, the node between the negative pole of the first switch module and the positive pole of the second switch module is the first output terminal 1; the node between the negative pole of the third switch module and the positive pole of the fourth switch module is the first output terminal 1; node as the second output terminal 2. The first output terminal 1 is connected to the output port of a half-bridge structure and the cathode of the second freewheeling diode D4, and the second output terminal ₂ is connected to the output port of the other half-bridge structure and the first freewheeling diode _D3 anode.

Under the normal working conditions of the DC side of the above sub-modules, the _TR2 and _TR4 tubes in the second switch module and the third switch module are in the normally on state, which is equivalent to the anti-parallel diodes of _TR1 and _TR3 , and the whole module is equivalent to Two half-bridge modules are connected in series, so three levels of 0, V _C and 2V _C can be output. Under normal operating conditions, the freewheeling diodes D ₃ and D ₄ are in an off state because they bear at least a reverse voltage with an amplitude of 0.5V _C , so no loss occurs.

Under normal working conditions, when the sub-module generates three levels, the current only passes through two semiconductor switching devices, which is the same as the number of switching devices that current flows through when two series-connected half-bridge modules are in normal operation. From the data sheet analysis of existing devices, it can be concluded that the newly proposed sub-module has lower conduction losses than all existing sub-modules with fault blocking capability.

Referring to FIG. 1 and FIG. 2 , FIG. 1 is a three-phase modular multi-level converter structure, wherein each sub-module of each bridge arm is composed of the sub-module shown in FIG. 2 . The converter does not need to rely on additional switching devices, and has the characteristics of low loss while having the fault blocking capability, which can realize fault blocking of the DC side short circuit.

FIG. 3 is an equivalent circuit diagram of a low-loss modular multilevel converter sub-module controlled by a switch tube under a DC fault in an embodiment of the present invention. Modular multilevel converter In the DC transmission system, when a bipolar short-circuit fault is detected on the DC side, all fully-controlled switches are immediately turned off. The fully-controlled switches include insulated gate bipolar transistors T ₁ and T ₃ , reverse resistance type Insulated gate bipolar transistors T _{R1 ˜T R4} , the fault current will flow into the sub-module topology from the second output terminal and pass through the second freewheeling diode D ₄ , the second capacitor C2 , the third capacitor C3 and the first Freewheeling diode _D3 flows out.

Specifically, when the DC side fails and all the controllable switches are blocked and the current flows in from the first output terminal 1, the sub-module is equivalent to two diodes and four capacitors in series; when the current flows from the second output terminal 1 When terminal 2 flows in, the submodule is equivalent to two diodes and two capacitors in series.

When a DC bipolar short-circuit fault occurs, since the DC side voltage is lower than the series connection of the capacitor voltages of all sub-modules in each phase unit, the current direction should flow from the second output terminal 2. One current path of the modular multilevel converter system is marked in FIG. 3 . The path of the fault current is D ₃ ->C ₁ ->C ₂ ->D ₄ . Suppose the rated DC side voltage is V _dc , the system modulation ratio is m, the voltage across the first freewheeling diode _{D3 is V D3 ,} the voltage across the second _freewheeling diode D4 is V _D4 , and each bridge arm has N Each sub-module, the voltage of each independent capacitor is 0.5V _C , and the peak value of each phase voltage of the grid is V _gm . It can be obtained from the operating characteristics of the modular multilevel converter:

V _dc =N·4·0.5V _C =2NV _C

In the fault state, taking the current path shown in Figure 3, we can get:

V _AB =2N·2·0.5V _C +2N·(V _D3 +V _D4 )

Therefore:

In general, the modulation ratio m is not greater than 1, so:

V _D3 +V _D4 <0

That is, the two freewheeling diodes are in a reverse biased state, and the current will be blocked.

In addition, in the DC power transmission system, when the fault is a DC permanent fault, the specific process is: turn off all fully-controlled switches (the fully-controlled switches include insulated gate bipolar transistors T ₁ and T ₃ , reverse-resistance insulated gate bipolar Transistors T _R1 ~ T _R4 ); disconnect the AC circuit breaker and the DC side switch after the current on the AC side returns to zero for maintenance. After the fault is repaired, the DC side switch is closed, the AC side is reclosed, and then the reverse-resistance insulated gate bipolar transistors _TR2 and _TR4 are turned on to restore the normal working states of the first to fourth switch modules.

When the fault is a DC temporary fault, the specific process is: turn off all fully-controlled switches (the fully-controlled switches include insulated gate bipolar transistors T ₁ and T ₂ , and reverse-resistance insulated gate bipolar transistors _TR1 to _TR4 ), Wait for the DC side current to return to zero, wait for a certain time after the fault is cleared, turn on the reverse-resistance insulated gate bipolar transistors _TR2 and _TR4 , and restore the normal working state of the first to fourth switch modules. If there is no overcurrent phenomenon, Then reclosing can be carried out. After the reclosing is successful, it means that the fault has been cleared; if there is an overcurrent, all the full-control switches will be turned off again. When three overcurrents occur, it is considered that a permanent fault has occurred. Perform power outage repairs.

FIG. 4 is a flowchart of an AC side startup strategy of a low-loss modular multilevel converter in an embodiment of the present invention. Since the current flowing into each sub-module from the first output terminal 1 and the second output terminal 2 can charge the sub-module, and other types of sub-modules are not mixed, there is no need to charge in groups at the beginning of the controllable rectification phase, and you can directly press The active and reactive power are given as 0, and the voltage is charged to the rated value. Specifically, when the modular multilevel converter needs to be started from the AC side, the startup strategy is basically the same as that of the half-bridge structure, and uncontrolled rectification is performed first. When the sub-module voltage of each bridge arm in the multi-level DC-DC converter reaches 30% of the rated capacitor voltage, it enters the controllable rectification stage until it reaches the rated capacitor voltage, and then starts power transmission. Further, the converter station including the converter may contain more than one of the modular multi-level converters because of the various wiring modes of the converter station, and each of the converters is activated on the AC side. the same way.

FIG. 5 is the design idea of the sub-module topology structure of the low-loss modular multi-level converter with fault ride-through capability of the present invention. In a multi-sub-module system, since the switching frequency can be reduced to a relatively low level, and the conduction loss basically does not change with the switching frequency, reducing the conduction loss is beneficial to reducing the total system loss. Among the existing modules, the half-bridge sub-module has the smallest conduction loss, but does not have fault blocking capability. The key point of the design idea is to keep the conduction loss low and to have the fault blocking capability. Specifically, a reverse-resistance IGBT is introduced into the sub-module, and its on-voltage drop is lower than that of the series structure of a conventional IGBT and a diode to achieve lower conduction loss under normal conditions. Under normal conditions, the series-connected half-bridge sub-module structure is adopted, so that when generating each unit level, the current only needs to flow through one switching device, which can reduce the conduction loss; The nature of the fault blocks the current path, and through the additional freewheeling loop D ₃ ->C ₁ -> C ₂ -> D ₄ , the fault current is directed to the positive poles of the sub-module capacitors C ₂ and C ₃ , so as to utilize the sub-module The capacitor voltage achieves fault blocking. At the same time, the voltage stress of some switching tubes can be reduced by splitting the sub-module capacitors and connecting the freewheeling diode to the splitting point. In this embodiment, the characteristics of the reverse resistance IGBT that can block reverse voltage and current are used, and it is applied to the occasion where the freewheeling diode path needs to be blocked, so as to realize the DC side fault of the modular multilevel converter. Blocking; by splitting the capacitor, the voltage that the remaining IGBT tubes in the module need to withstand in the event of a fault can be effectively reduced, thereby making it possible to build a module with a higher voltage level.

FIG. 6 is a flowchart of a parameter design method for a low-loss modular multilevel converter in an embodiment of the present invention. Referring to FIG. 6 , first, the limiting relationship between the capacitance and inductance and the number of bridge arm sub-modules is determined according to the circulating current resonance relationship of the half-bridge modular multilevel converter system. Then, according to the withstand voltage of the selected semiconductor device, the number of sub-modules of each bridge arm can be designed, and then the capacitance of each sub-module can be selected according to the relationship between the system capacity and the system energy storage. Finally, the inductance value of the system is determined according to the capacitor and the number of sub-modules of the bridge arm.

Specifically, the parameter design method of the low-loss modular multilevel converter is as follows:

First determine the DC side voltage level V _dc of the modular multilevel converter, and then set aside a margin of 1 times or more according to the voltage stress V _IGBT of the reverse-resistance IGBT device to be used, thereby The number N of bridge arm submodules can be obtained. Assuming that there is a double margin, the reverse resistance type insulated gate bipolar transistors _TR1 , _TR2 , _TR3 , _TR4 can be selected to have a withstand voltage of 2V _IGBT , insulated gate bipolar transistors T ₁ , T ₂ and diode D ₁ , D ₂ , D ₃ , D ₄ have a withstand voltage of 3V _IGBT .

After obtaining the number of sub-modules of the bridge arm, the angular frequency formula of the circulating current resonance can be used (m is the modulation ratio, and k is the k-th circulating current):

The parameters of the inductance L of the bridge arm inductance and the capacitance C of the sub-module capacitor in the modular multilevel converter are selected so that the double-frequency circulating current will not cause resonance. It can be seen from the formula that the higher the number of circulating currents, the lower the corresponding resonant frequency. Therefore, it is only necessary to ensure that the double-frequency circulating current does not resonate to ensure that all circulating currents do not resonate.

Note that the C parameter is regarded as the equivalent capacitance value obtained by connecting two capacitors C ₁ , C ₂ or C ₃ and C ₄ in series. Since C ₁ , C ₂ , C ₃ and C ₄ need to be selected from the same value as Capacitance value, so that the actual capacitance value of the selected capacitor can be obtained as 2C.

From this we get:

According to the actual system, the constraint of the product of the inductor-capacitor value can be obtained by determining the double-frequency angular frequency.

The selection of the capacitance value can be determined according to the ratio of the total energy E _total stored by the inverter to the rated capacity S. If t _ES is given, the specific equivalent capacitance value C can be determined, and the relationship is as follows:

E _total =S·t _ES

After the equivalent capacitance value is selected, a capacitor with a capacitance value of 2C can be selected according to half of the withstand voltage, and then the inductance can be selected according to the aforementioned product constraints. Thereby, the parameters of the low-loss modular multilevel converter are determined.

Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the above-mentioned specific embodiments, and those skilled in the art can make various variations or modifications within the scope of the claims, which do not affect the essential content of the present invention.

Claims (10)

1. a low-loss modular multilevel converter is characterized in that: comprise three phase units, each unit is divided into upper and lower bridge arms, and each bridge arm comprises several submodules connected in series, each phase upper and lower The number of sub-modules connected in series with the bridge arms is the same; the upper and lower bridge arms are connected in series with current-limiting reactors respectively, and each phase from top to bottom is: all sub-modules of the upper bridge arm, the upper bridge arm reactor, the lower bridge arm reactor, the lower bridge arm All sub-modules of the arm; and three-phase AC voltage is externally connected to the connection of the upper and lower arms of each phase, the first output terminal of the top sub-module topology of the upper arm is connected to the positive pole of the DC bus, and the second output terminal of the bottom terminal module of the lower arm Connect to the negative pole of the DC bus; In each bridge arm, the sub-module consists of two half-bridge structures, four capacitors and two freewheeling diodes, wherein: In the half-bridge structure, the first half-bridge includes a first switch module and a second switch module, the second half-bridge includes a third switch module and a fourth switch module; the negative pole of the first switch module is connected to the The positive pole of the second switch module is connected to the positive pole of the second switch module, the negative pole of the second switch module is connected to the positive pole of the third switch module, and the negative pole of the third switch module is connected to the positive pole of the fourth switch module; the second switch module is connected to the positive pole of the fourth switch module; The switch module and the third switch module are both reverse resistance switch modules; Among the four capacitors, the positive electrode of the first capacitor is connected to the positive electrode of the first switch module; the negative electrode of the first capacitor is connected to the positive electrode of the second capacitor; the negative electrode of the second capacitor is connected to the positive electrode of the second capacitor. The negative pole of the second switch module is connected to the negative pole of the second switch module; the positive pole of the third capacitor is connected to the positive pole of the third switch module; the negative pole of the third capacitor is connected to the positive pole of the fourth capacitor, and the fourth capacitor The negative pole is connected to the negative pole of the fourth switch module; Among the two freewheeling diodes, the negative electrode of the first freewheeling diode is connected to the negative electrode of the first capacitor, and the positive electrode of the first freewheeling diode is connected to the positive electrode of the fourth switch module. The cathode of the second freewheeling diode is connected to the anode of the second switch module, and the anode of the second freewheeling diode is connected to the cathode of the third capacitor; The node between the negative pole of the first switch module and the positive pole of the second switch module is used as the first output terminal of the entire sub-module; the negative pole of the third switch module and the positive pole of the fourth switch module are connected. The node between them is used as the second output terminal of the whole sub-module. 2. The low-loss modular multilevel converter according to claim 1, wherein the first output terminal is connected to the output port of the first half-bridge structure and the cathode of the second freewheeling diode, The second output terminal is connected to the output port of the second half-bridge structure and the anode of the first freewheeling diode. 3 . The low-loss modular multilevel converter according to claim 1 , wherein the first switch module and the fourth switch module are anti-parallel connected by an insulated gate bipolar transistor and a diode. 4 . composition. 4 . The low-loss modular multi-level converter according to claim 3 , wherein the second switch module is composed of a first reverse-resistance insulated gate bipolar transistor and a second reverse-parallel connection therewith. 5 . Resistive insulated gate bipolar transistor composition; The third switch module is composed of a third reverse resistance type insulated gate bipolar transistor and a fourth reverse resistance type insulated gate bipolar transistor connected in anti-parallel with it. 5 . The low-loss modular multi-level converter according to claim 4 , wherein under normal working conditions, the negative pole of the second switch module is connected to the second reverse resistance type insulation of the first output terminal. 6 . The gate bipolar transistor and the fourth reverse-resistance insulated gate bipolar transistor whose positive pole of the third switch module is connected to the second output terminal remain in an on state; the two freewheeling diodes remain in an off state due to the reverse voltage, and are not in an on state. Join the circuit so that there is no conduction loss. 6. The low-loss modular multi-level converter according to claim 4, characterized in that: when the converter is in a DC transmission system, when a bipolar short-circuit fault is detected on the DC side, it immediately shuts down all the The fully-controlled switch includes an insulated gate bipolar transistor in the first switch module and the fourth switch module, and a reverse-resistance insulation in the second switch module and the third switch module. gated bipolar transistor; fault current will flow from the second output terminal into the sub-module topology in each leg through the second freewheeling diode, the second capacitor, the third capacitor and the first freewheeling diode outflow. 7. The low-loss modular multilevel converter according to claim 6, characterized in that: when the fault is a DC permanent fault: turn off all full-control switches, and disconnect the AC after the current on the AC side returns to zero The circuit breaker and DC side switch shall be repaired; after the fault is repaired, the DC side switch shall be closed, the AC side shall be reclosed, and then the second and fourth reverse-resistance insulated gate bipolar transistors shall be turned on to restore the first, second and third , the normal working state of the fourth switch module. 8. The low-loss modular multi-level converter according to claim 6, wherein: when the fault is a DC temporary fault: turn off all full-control switches, wait for the DC side current to return to zero, and after the fault is cleared After waiting for a certain period of time, turn on the second and fourth reverse-resistance insulated gate bipolar transistors to restore the normal working state of the first, second, third and fourth switch modules. If there is no overcurrent phenomenon, perform reclosing , after the reclosing is successful, it means that the fault has been cleared; if there is an overcurrent, all the full-control switches will be turned off again; when the overcurrent occurs more than three times, it is considered that a permanent fault has occurred. 9 . The low-loss modular multilevel converter according to claim 6 , wherein when the converter station containing the converter needs to be started from the AC side, uncontrolled rectification is performed first, and when the converter is changed. 10 . After the voltage of each sub-module in the rectifier reaches 30% of the rated capacitor voltage, it enters the controllable rectification stage until it reaches the rated capacitor voltage, and then starts power transmission. 10. A parameter design method for a low-loss modular multilevel converter according to any one of claims 1-9, characterized in that: comprising: Determine the limiting relationship between the capacitance and inductance and the number of bridge arm sub-modules according to the circulating current resonance relationship of the converter; According to the withstand voltage of the selected semiconductor device, the number of sub-modules of each bridge arm is designed, and then the capacitance of each sub-module is selected according to the relationship between the system capacity and the system energy storage; Finally, the inductance value of the system is determined according to the capacitor and the number of sub-modules of the bridge arm.

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