阅读说明：本技术 具备故障阻断能力的低损耗的模块化多电平直流变压器 (Low-loss modular multilevel DC transformer with fault blocking capability ) 是由 李睿 彭程 蔡旭 于 2019-11-25 设计创作，主要内容包括：本发明提供一种具备故障阻断能力的低损耗的模块化多电平直流变压器,包括两个子系统和一个三相工频变压器,每个子系统包括三个相单元,每一个相单元分上、下桥臂,每个桥臂包括若干个串联的子模块,所述子模块拓扑由两个半桥结构、四个电容和两个续流二极管组成,所述半桥结构中,第一个半桥包括第一开关模块和第二开关模块,第二个半桥包括第三开关模块和第四开关模块；所述第一开关模块的负极与所述第二开关模块的正极相连,所述第二开关模块的负极与所述第三开关模块的正极相连,所述第三开关模块的负极与所述第四开关模块的正极相连。本发明通过控制开关模块的开断即可实现直流侧短路的故障阻断,同时不会增加损耗。(The invention provides a low-loss modular multilevel DC transformer with fault blocking capability, which comprises two subsystems and a three-phase power frequency transformer, wherein each subsystem comprises three phase units, each phase unit is divided into an upper bridge arm and a lower bridge arm, each bridge arm comprises a plurality of serially connected submodules, the submodule topology comprises two half-bridge structures, four capacitors and two freewheeling diodes, in each half-bridge structure, a first half-bridge comprises a first switch module and a second switch module, and a second half-bridge comprises a third switch module and a fourth switch module; the negative electrode of the first switch module is connected with the positive electrode of the second switch module, the negative electrode of the second switch module is connected with the positive electrode of the third switch module, and the negative electrode of the third switch module is connected with the positive electrode of the fourth switch module. According to the invention, the fault blocking of the direct-current side short circuit can be realized by controlling the on-off of the switch module, and meanwhile, the loss is not increased.)

1. A low-loss modular multilevel DC transformer with fault blocking capability comprises two subsystems and a three-phase power frequency transformer, wherein each subsystem comprises three phase units, each phase unit is divided into an upper bridge arm and a lower bridge arm, and the low-loss modular multilevel DC transformer is characterized in that: each bridge arm comprises a plurality of serially connected sub-modules, and the number of the serially connected sub-modules of the upper bridge arm and the lower bridge arm of each phase is the same; each phase is, from top to bottom: all the sub-modules of the upper bridge arm, the upper bridge arm reactor, the lower bridge arm reactor and all the sub-modules of the lower bridge arm; the connection part of the upper bridge arm and the lower bridge arm of each phase is externally connected with a three-phase power frequency transformer winding, the first output terminal of the uppermost submodule of the upper bridge arm of each phase of each subsystem is connected with the positive electrode of the direct current bus of the subsystem, and the second output terminal of the lowermost submodule of the lower bridge arm is connected with the negative electrode of the direct current bus of the subsystem;

in each bridge arm, the submodule consists of two half-bridge structures, four capacitors and two freewheeling diodes, wherein:

in the half-bridge structure, a first half-bridge comprises a first switch module and a second switch module, and a second half-bridge comprises a third switch module and a fourth switch module; the negative electrode of the first switch module is connected with the positive electrode of the second switch module, the negative electrode of the second switch module is connected with the positive electrode of the third switch module, and the negative electrode of the third switch module is connected with the positive electrode of the fourth switch module;

the positive electrode of a first capacitor in the four capacitors is connected with the positive electrode of the first switch module; the negative electrode of the first capacitor is connected with the positive electrode of the second capacitor; the negative electrode of the second capacitor is connected with the negative electrode of the second switch module; the anode of the third capacitor is connected with the anode of the third switch module; the negative electrode of the third capacitor is connected with the positive electrode of a fourth capacitor, and the negative electrode of the fourth capacitor is connected with the negative electrode of the fourth switch module;

in the two freewheeling diodes, the anode of the first freewheeling diode is connected with the cathode of the first capacitor, the cathode of the first freewheeling diode is connected with the anode of the fourth switch module, the anode of the second freewheeling diode is connected with the anode of the second switch module, and the cathode of the second freewheeling diode is connected with the cathode of the third capacitor;

a node between the negative electrode of the first switch module and the positive electrode of the second switch module is used as a first output terminal of the whole sub-module; and a node between the cathode of the third switch module and the anode of the fourth switch module is used as a second output terminal of the whole sub-module.

2. The low-loss modular multilevel dc transformer with fault blocking capability of claim 1, wherein: the first output terminal is connected to an output port of the first half-bridge arrangement and to a cathode of the second freewheeling diode, and the second output terminal is connected to an output port of the second half-bridge arrangement and to an anode of the first freewheeling diode.

3. The low-loss modular multilevel dc transformer with fault blocking capability of claim 1, wherein: the first switch module and the fourth switch module are composed of an insulated gate bipolar transistor and a diode which are connected in an anti-parallel mode.

4. The low-loss modular multilevel dc transformer with fault blocking capability of claim 3, wherein: the second switch module and the third switch module are both reverse resistance type switch modules.

5. The low-loss modular multilevel dc transformer with fault blocking capability of claim 4, wherein: the second switch module consists of a first reverse-resistance type insulated gate bipolar transistor and a second reverse-resistance type insulated gate bipolar transistor which is connected with the first reverse-resistance type insulated gate bipolar transistor in an anti-parallel mode;

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 with the third reverse-resistance type insulated gate bipolar transistor in an anti-parallel mode.

6. The low-loss modular multilevel dc transformer with fault blocking capability of claim 5, wherein: under the normal working condition, the second reverse-resistance insulated gate bipolar transistor with the negative electrode of the second switch module connected with the first output terminal and the fourth reverse-resistance insulated gate bipolar transistor with the positive electrode of the third switch module connected with the second output terminal keep the conducting state; the two freewheeling diodes are kept in an off state due to the fact that the two freewheeling diodes bear reverse voltage, and no circuit is added, so that conduction loss is not generated.

7. The low-loss modular multilevel dc transformer with fault blocking capability of claim 5, wherein: in a direct-current transmission system, when a bipolar short-circuit fault is detected on the direct-current side of a subsystem on one side, all full-control switches in the system are immediately turned off, and the fault blocking of the system on the one side is realized; the fully-controlled switch comprises insulated gate bipolar transistors in the first switch module and the fourth switch module, and reverse-resistance insulated gate bipolar transistors in the second switch module and the third switch module.

8. The low-loss modular multilevel dc transformer with fault blocking capability of claim 7, wherein: when the fault is any side direct current permanent fault:

turning off all control switches of all sub-modules in the transformer;

after the current is cut off, the direct current side knife switch is disconnected, after the fault is repaired, the current converters on the two sides are enabled to operate under the condition of zero active power, whether the direct current side overcurrent phenomenon still occurs or not is observed, if the direct current side overcurrent does not occur, the fault is considered to be eliminated, and the operation of the alternating current side circuit breaker recovery system can be closed.

9. The low-loss modular multilevel dc transformer with fault blocking capability of claim 7, wherein: when the fault is any side direct current temporary fault:

turning off all the full-control switches;

and waiting for the return-to-zero of the current at the direct current side, and when the current returns to zero and reaches a set time, operating the system under the zero active power given condition, observing whether the over-current phenomenon at the direct current side still occurs, and if the over-current at the direct current side does not occur, determining that the fault is eliminated, so that the operation of the system can be recovered.

10. A low-loss modular multilevel dc transformer with fault blocking capability according to any of claims 1-9, characterized by: when one-side subsystem needs to be started from the AC side:

firstly, uncontrolled rectification charging is started on an alternating current side, and a controllable rectification mode is entered after all capacitor voltages of the sub-modules reach 30% of corresponding rated values; and then entering a normal working state after the capacitor voltage reaches a rated value.

Technical Field

The invention relates to the technical field of power transmission and distribution of a power system, in particular to a low-loss modular multilevel DC transformer with fault blocking capability.

Background

With the increase of the power generation amount of renewable energy sources, the integration of renewable energy sources becomes the next very important research direction.

The flexible direct-current transmission technology provides a solution for solving the renewable energy grid connection, and has strong technical advantages. Concepts such as multi-terminal dc and dc grids have also been proposed and are beginning to be applied to practical systems.

In a dc network, there are often some dc-dc converters for boosting the dc voltage or reversing the voltage polarity. In the situation that the direct current voltage on the two sides needs to be isolated. An isolated dc transformer is typically used.

The structure of the dc transformer is relatively various, and among them, there is a dc transformer using the operation principle of the modular multilevel converter. The high-voltage high-power direct-current transformer based on the modular multilevel converter can be realized by connecting the two modular multilevel converters through the power frequency transformer.

The traditional direct current transformer based on the modular multilevel converter adopts a submodule topology of a half-bridge structure, and due to the fact that an anti-parallel diode of a lower tube is not controlled by pulses, a follow current loop from an alternating current side to a direct current fault point can be formed when a short circuit fault occurs on a direct current side of the submodule topology, even if a normal side converter is rapidly locked, energy stored in a bridge arm inductor of the fault side converter can form a loop by using a follow current diode, and long-time overcurrent of the follow current diode in the loop is caused. Thereby easily causing damage to the freewheel diode. It is therefore common to configure converter systems with sub-modules having fault blocking capabilities.

The conventional submodule topology with the fault blocking capability generally has a problem that an additional switching device is positioned on a normal current path in each submodule under the normal working state of a converter, and the device is in a normally open state. When a fault occurs, the switching device is turned off, thereby allowing current to flow from the other path to achieve the effect of fault current blocking or limiting. This extra switching device will increase the conduction losses of the system, resulting in a loss of resources.

Through retrieval, the Chinese invention patent application number: 201910726553.8, the patent discloses a flexible direct current transmission DC/DC converter with fault blocking capability, which comprises a thyristor string T11, a thyristor string T12, a thyristor string T21, a thyristor string T22, a half-bridge submodule string and an inductor L, wherein the half-bridge submodule string comprises a half-bridge submodule SM 1-a half-bridge submodule SMN; the thyristor strings T11 and T21 are connected in parallel in an inverse manner and are respectively connected with the positive electrode of the direct-current low-voltage side and the input stage of the half-bridge submodule SM 1; the thyristor strings T12 and T22 are connected in parallel in an inverse manner and are respectively connected with the positive electrode of the direct-current high-voltage side and the input stage of the half-bridge submodule SM 1; the current output end of the half-bridge submodule SMN is connected with the first end of the inductor L; and the second end of the inductor L is connected with the direct-current low-voltage side negative electrode and the direct-current high-voltage side negative electrode at the same time.

However, the above patents have the following disadvantages: under the normal operation state, the driving signals of each thyristor must be kept at the same time, the voltage-sharing of each thyristor during operation must be ensured, and higher requirements are provided for a driving circuit and a voltage-sharing circuit.

Disclosure of Invention

Aiming at the defects of the prior art, the invention aims to provide a low-loss modular multilevel direct current transformer with fault blocking capability, which can realize fault blocking of short circuit at a direct current side and has higher reliability.

According to the purpose of the invention, the low-loss modular multilevel DC transformer with the fault blocking capability comprises two subsystems and a three-phase power frequency transformer, wherein each subsystem comprises three phase units, each phase unit is divided into an upper bridge arm and a lower bridge arm, each bridge arm comprises a plurality of serially-connected submodules, and the number of the serially-connected submodules of the upper bridge arm and the lower bridge arm of each phase is the same; the upper bridge arm and the lower bridge arm are respectively connected with a current-limiting reactor in series, and each phase comprises from top to bottom: all the sub-modules of the upper bridge arm, the upper bridge arm reactor, the lower bridge arm reactor and all the sub-modules of the lower bridge arm; the connection part of the upper bridge arm and the lower bridge arm of each phase is externally connected with the three-phase power frequency transformer winding, the first output terminal of the topology of the uppermost sub-module of the upper bridge arm of each phase of each subsystem is connected with the positive electrode of the direct current bus of the subsystem, and the second output terminal of the lowermost sub-module of the lower bridge arm is connected with the negative electrode of the direct current bus of the subsystem;

in each bridge arm, the submodule consists of two half-bridge structures, four capacitors and two freewheeling diodes, wherein:

in the half-bridge structure, a first half-bridge comprises a first switch module and a second switch module, and a second half-bridge comprises a third switch module and a fourth switch module; the negative electrode of the first switch module is connected with the positive electrode of the second switch module, the negative electrode of the second switch module is connected with the positive electrode of the third switch module, and the negative electrode of the third switch module is connected with the positive electrode of the fourth switch module;

the positive electrode of a first capacitor in the four capacitors is connected with the positive electrode of the first switch module; the negative electrode of the first capacitor is connected with the positive electrode of the second capacitor; the negative electrode of the second capacitor is connected with the negative electrode of the second switch module; the anode of the third capacitor is connected with the anode of the third switch module; the negative electrode of the third capacitor is connected with the positive electrode of a fourth capacitor, and the negative electrode of the fourth capacitor is connected with the negative electrode of the fourth switch module;

in the two freewheeling diodes, the anode of the first freewheeling diode is connected with the cathode of the first capacitor, the cathode of the first freewheeling diode is connected with the anode of the fourth switch module, the anode of the second freewheeling diode is connected with the anode of the second switch module, and the cathode of the second freewheeling diode is connected with the cathode of the third capacitor;

a node between the negative electrode of the first switch module and the positive electrode of the second switch module is used as a first output terminal of the whole sub-module; and a node between the cathode of the third switch module and the anode of the fourth switch module is used as a second output terminal of the whole sub-module.

Optionally, the first output terminal is connected to an output of the first half-bridge arrangement and to a cathode of the second freewheeling diode, and the second output terminal is connected to an output of the second half-bridge arrangement and to an anode of the first 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 type insulated gate bipolar transistor and a second reverse-resistance type insulated gate bipolar transistor connected with the first reverse-resistance type insulated gate bipolar transistor in an anti-parallel mode; 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 with the third reverse-resistance type insulated gate bipolar transistor in an anti-parallel mode.

Optionally, under normal operation, the second reverse-blocking insulated gate bipolar transistor with the cathode of the second switch module connected to the first output terminal and the fourth reverse-blocking insulated gate bipolar transistor with the anode of the third switch module connected to the second output terminal are kept in a conducting state; the two freewheeling diodes are kept in an off state due to the fact that the two freewheeling diodes bear reverse voltage, and no circuit is added, so that conduction loss is not generated.

Optionally, in the direct-current power transmission system, when it is detected that a bipolar short-circuit fault occurs at the direct-current side of a subsystem at one side, all the fully-controlled switches in the system are immediately turned off, that is, fault blocking of the system at the side is realized; the fully-controlled switch comprises insulated gate bipolar transistors in the first switch module and the fourth switch module, and reverse-resistance insulated gate bipolar transistors in the second switch module and the third switch module.

Optionally, when the fault is any side direct current permanent fault: turning off all control switches of all sub-modules in the transformer; after the current is cut off, the direct current side knife switch is disconnected, after the fault is repaired, the current converters on the two sides are enabled to operate under the condition of zero active power, whether the direct current side overcurrent phenomenon still occurs or not is observed, if the direct current side overcurrent does not occur, the fault is considered to be eliminated, and the operation of the alternating current side circuit breaker recovery system can be closed.

Optionally, when the fault is any side direct current temporary fault: turning off all the full-control switches; and waiting for the return-to-zero of the current at the direct current side, and when the current returns to zero and reaches a set time, operating the system under the zero active power given condition, observing whether the over-current phenomenon at the direct current side still occurs, and if the over-current at the direct current side does not occur, determining that the fault is eliminated, so that the operation of the system can be recovered.

Alternatively, when a side subsystem needs to be started from the ac side:

firstly, uncontrolled rectification charging is started on an alternating current side, and a controllable rectification mode is entered after all capacitor voltages of the sub-modules reach 30% of corresponding rated values; and then entering a normal working state after the capacitor voltage reaches a rated value.

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

in a normal state, the topology of the low-loss modular multilevel converter with fault blocking capability has the same number of conducting elements as that of a modular multilevel converter based on a traditional half-bridge submodule, and therefore, the topology of the low-loss modular multilevel converter with fault blocking capability has similar conducting loss.

The low-loss modular multilevel DC transformer with the fault blocking capability adopts a modular structure, the work of each module is relatively independent, simultaneous triggering is not needed, and a single module can be bypassed after a fault occurs, so that the modular multilevel DC transformer with the fault blocking capability has higher reliability.

According to the low-loss modular multilevel DC transformer with the fault blocking capability, the fault isolation under the condition of short-circuit fault of the DC sides at two sides can be realized by controlling the state of the switch module, and the isolation speed is high.

The low-loss modular multilevel DC transformer with the fault blocking capability can keep the sub-module capacitor voltage during fault and has high power restoration speed.

According to the low-loss modular multilevel DC transformer with the fault blocking capability, the starting process of the AC side of the subsystem on any side is similar to that of a modular multilevel DC transformer system based on a traditional half bridge, and the control is simple.

Drawings

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

fig. 1 is a schematic diagram of a low-loss modular multilevel dc transformer with fault blocking capability according to an embodiment of the present invention;

fig. 2 is a sub-module topology of a low-loss modular multilevel dc transformer with fault blocking capability according to an embodiment of the present invention;

fig. 3 is an equivalent circuit diagram of a low-loss modular multilevel dc transformer submodule with fault blocking capability under dc fault control by a switching tube in an embodiment of the present invention;

fig. 4 is a flowchart of a one-end ac-side startup strategy of a low-loss modular multilevel dc transformer with fault blocking capability according to an embodiment of the present invention.

Detailed Description

The present invention will be described in detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the invention, but are not intended to limit the invention in any way. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the spirit of the invention, which falls within the scope of the invention.

Fig. 1 is a schematic diagram of a low-loss modular multilevel dc transformer with fault blocking capability according to an embodiment of the present invention. In the figure, the modular multilevel DC Transformer comprises two subsystems and a three-phase power frequency Transformer transducer 1, wherein each subsystem comprises three phase units, each phase unit is divided into an upper bridge arm and a lower bridge arm, each bridge arm comprises a plurality of serially-connected sub-modules, and the number of the serially-connected sub-modules of the upper bridge arm and the lower bridge arm of each phase is the same; the upper bridge arm and the lower bridge arm are respectively connected with a current-limiting reactor in series, and each phase comprises from top to bottom: all the sub-modules of the upper bridge arm, the upper bridge arm reactor, the lower bridge arm reactor and all the sub-modules of the lower bridge arm; and the connection part of the upper bridge arm and the lower bridge arm of each phase is externally connected with the three-phase power frequency transformer winding, the first output terminal of the topology of the uppermost sub-module of the upper bridge arm of each phase of each subsystem is connected with the positive electrode of the direct current bus of the subsystem, and the second output terminal of the lowermost sub-module of the lower bridge arm is connected with the negative electrode of the direct current bus of the subsystem.

Fig. 2 is a low-loss modular multilevel dc transformer submodule topology with fault blocking capability according to an embodiment of the present invention. Referring to fig. 2, each bridge arm of the multilevel dc-dc converter has a sub-module formed by two half-bridge structures and four capacitors C₁～C₄And two freewheeling diodes D₃～D₄And (4) forming.

In the two half-bridge structures, a first half bridge comprises a first switch module and a second switch module; first switch module T₁Is connected to the anode of the second switch module. The second half-bridge comprises a third switching module and a fourth switching module; the negative electrode of the third switch module is connected with the positive electrode of the fourth switch module; and the anode of the third switch module is connected with the cathode of the second switch module. In particular, with reference to fig. 2, the first switching module consists of an insulated gate bipolar transistor T₁And a diode D₁Anti-parallel connection; the fourth switch module is composed of an insulated gate bipolar transistor T₂And a diode D₂Anti-parallel connection; the second switch module is a reverse-resistance switch module composed of a first reverse-resistance insulated gate bipolar transistor T_R1And a second reverse-blocking insulated gate bipolar transistor T connected in inverse parallel therewith_R2The third switch module is a reverse-resistance switch module and consists of a third reverse-resistance insulated gate bipolar transistor T_R3And a fourth reverse-blocking insulated gate bipolar transistor T connected in inverse parallel therewith_R4And (4) forming.

Of the four capacitors, the first capacitor C₁Positive pole and first switch module T₁The positive electrodes of the two electrodes are connected; a first capacitor C₁Negative pole of and a second capacitor C₂The positive electrodes of the two electrodes are connected; a second capacitor C₂The negative electrode of the first switch module is connected with the negative electrode of the second switch module; third capacitor C₃The anode of the first switch module is connected with the anode of the second switch module; third capacitor C₃Negative pole of and a fourth capacitor C₄The positive electrodes of the two electrodes are connected; first, theFour capacitors C₄The negative electrode of the fourth switching module is connected with the negative electrode of the fourth switching module; first freewheeling diode anode D₃And a first capacitor C₁The negative electrodes are connected; first freewheeling diode D₃The negative electrode of the second switch module is connected with the positive electrode of the fourth switch module; second freewheeling diode D₄The positive pole of the first switch module is connected with the positive pole of the second switch module; second freewheeling diode D₄Negative pole and third capacitor C₃Are connected with each other.

In the multilevel converter sub-module of the embodiment, a node between the cathode of the first switch module and the anode of the second switch module is a first output terminal 1; the node between the negative pole of the third switching module and the positive pole of the fourth switching module serves as the second output terminal 2. Wherein the first output terminal 1 is connected to an output port of a half-bridge configuration and a second freewheeling diode D₄A second output terminal 2 is connected to the output of the other half-bridge configuration and to a first freewheeling diode D₃Of (2) an anode.

Under the normal working condition of the sub-modules at the direct current side, T in the second switch module and the third switch module_R2And T_R4The tube is in a normally open state, equivalent to T_R1And T_R3The whole module is equivalent to two half-bridge modules which are connected in series, so that 0, V can be output_C，2V_CThree levels. Under normal operating conditions, the freewheeling diode D₃And D₄Due to at least 0.5V of amplitude_CThe reverse voltage of (2) is in an off state, and thus no loss is generated.

Under normal working conditions, when the submodule generates 3 levels, current only passes through 2 semiconductor switching devices, and the number of the switching devices through which the current flows is the same as that of the switching devices of the two half-bridge modules connected in series when the half-bridge modules work normally. From the analysis of the data sheet of the existing device, it can be concluded that the newly proposed sub-module has a lower conduction loss than all existing sub-modules with fault blocking capability.

Referring to fig. 1 and 2, each submodule of each bridge arm is composed of the improved submodule shown in fig. 2. The modular multilevel DC transformer is integrally composed of two modular multilevel converters (subsystems).

Fig. 3 is an equivalent circuit diagram of a low-loss modular multilevel dc transformer submodule with fault blocking capability under dc fault control by a switching tube in an embodiment of the present invention. And after the direct current side at one side has a short-circuit fault and all the controllable switches are blocked, the equivalent circuit of the modular multilevel converter system at one side is obtained. In a direct-current transmission system, when a bipolar short-circuit fault on the direct-current side of a subsystem on one side is detected, all full-control switches in the system are immediately turned off, wherein the full-control switches specifically comprise insulated gate bipolar transistors in a first switch module and a fourth switch module, and reverse-resistance insulated gate bipolar transistors in a second switch module and a third switch module, so that the fault blocking of the system on the side can be realized.

Further, when the fault is a permanent dc fault on any side, the specific process is as follows: turning off all control switches of all sub-modules in the direct current transformer system; after the current is cut off, the direct current side knife switch is disconnected, after the fault is repaired, the current converters on the two sides are enabled to operate under the condition of zero active power, whether the direct current side overcurrent phenomenon still occurs or not is observed, if the direct current side overcurrent does not occur, the fault is considered to be eliminated, and the operation of the alternating current side circuit breaker recovery system can be closed.

When the fault is any side direct current temporary fault, the specific process is as follows: and turning off all full-control switches, including the first insulated gate bipolar transistor, the second insulated gate bipolar transistor, the first reverse-resistance insulated gate bipolar transistor, the second reverse-resistance insulated gate bipolar transistor, the third reverse-resistance insulated gate bipolar transistor and the fourth reverse-resistance insulated gate bipolar transistor, waiting for the current at the direct current side to return to zero, and after the current returns to zero and reaches a specific time, enabling the system to operate under the condition of zero active power, observing whether the direct current side overcurrent phenomenon still occurs, and if the direct current side overcurrent does not occur, considering that the fault is eliminated, and recovering the system operation.

It can be seen from the figure that, no matter whether the current path is 1 or 2, the current will flow through the capacitor, so that the current can be rapidly attenuated to 0, and the switching tube only needs to bear short-time overcurrent. Thereby functioning as a protection system.

Fig. 4 is a flowchart of a start strategy of an ac side at one end of a low-loss modular multilevel dc transformer with fault blocking capability according to an embodiment of the present invention, when a subsystem at one side needs to be started from the ac side, the subsystem may be started by uncontrolled rectifying charging at the ac side, and after a capacitor voltage reaches a suitable value, the subsystem enters a controllable rectifying mode; and then entering a normal working state after the capacitor voltage reaches a rated value. As all the modules in the system have the same structure, grouping operation is not needed at the beginning stage of controllable rectification, and the control is simple.

The foregoing description of specific embodiments of the present invention has been presented. It is to be understood that the present invention is not limited to the specific embodiments described above, and that various changes and modifications may be made by one skilled in the art within the scope of the appended claims without departing from the spirit of the invention.