CN110995039B - Low-loss modular multilevel converter and parameter design method thereof - Google Patents

Low-loss modular multilevel converter and parameter design method thereof Download PDF

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CN110995039B
CN110995039B CN201911168950.4A CN201911168950A CN110995039B CN 110995039 B CN110995039 B CN 110995039B CN 201911168950 A CN201911168950 A CN 201911168950A CN 110995039 B CN110995039 B CN 110995039B
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switch module
bridge arm
capacitor
reverse
insulated gate
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CN110995039A (en
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李睿
彭程
蔡旭
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Shanghai Jiaotong University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06Q—INFORMATION AND COMMUNICATION TECHNOLOGY [ICT] SPECIALLY ADAPTED FOR ADMINISTRATIVE, COMMERCIAL, FINANCIAL, MANAGERIAL OR SUPERVISORY PURPOSES; SYSTEMS OR METHODS SPECIALLY ADAPTED FOR ADMINISTRATIVE, COMMERCIAL, FINANCIAL, MANAGERIAL OR SUPERVISORY PURPOSES, NOT OTHERWISE PROVIDED FOR
    • G06Q50/00—Systems or methods specially adapted for specific business sectors, e.g. utilities or tourism
    • G06Q50/06—Electricity, gas or water supply
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/10—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers
    • H02H7/12—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers
    • H02H7/122—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers for inverters, i.e. dc/ac converters
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/10—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers
    • H02H7/12—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers
    • H02H7/122—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers for inverters, i.e. dc/ac converters
    • H02H7/1222—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers for inverters, i.e. dc/ac converters responsive to abnormalities in the input circuit, e.g. transients in the DC input

Abstract

The invention provides a low-loss modular multilevel converter, which comprises three phase units, wherein each unit is divided into an upper bridge arm and a lower bridge arm, each bridge arm comprises a plurality of serially connected submodules, each submodule consists of two half-bridge structures, four capacitors and two freewheeling diodes, the first half-bridge comprises a first switch module and a second switch module, and the 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 invention also provides a parameter design method of the converter. 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.

Description

Low-loss modular multilevel converter and parameter design method thereof
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 converter with fault blocking capability and a parameter design method thereof.
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. Compared with the traditional two-level and three-level converters, the flexible direct current transmission technology utilizing the modular multilevel converter has better maintainability and expansibility, and the problems of series voltage-sharing and parallel current-sharing of switch tubes do not exist. Each sub-module of the modular multilevel converter has a relatively simple structure and is easy to control, and the modular multilevel converter is particularly suitable for the field of high-voltage direct-current transmission due to the characteristic that the modularization is easy to expand. The number of the output levels of the modular multilevel converter can be adjusted by adjusting the number of the sub-modules in the bridge arm, and the voltage at the direct current side can also be controlled by adjusting the number of the sub-modules in each phase, so that the voltage grade and the output harmonic content of a system formed by the modular multilevel converter can be effectively controlled.
Traditional modularization multilevel converter adopts half-bridge structure's submodule piece topology, and half-bridge submodule piece topology can form the afterflow return circuit of alternating current side to direct current fault point when direct current side takes place the short circuit fault because the uncontrolled pulse control of anti-parallel diode of low tube to transmit the short circuit fault to the alternating current side, cause the influence to electric wire netting stability. It is necessary to interrupt the fault current by means of a dc, ac breaker or other means. However, the direct current circuit breaker has no mature technology at present and is too expensive to manufacture; the alternating current circuit breaker needs longer response time, and a converter valve device needs to bear larger current stress before the alternating current circuit breaker is disconnected, so that the device is easy to damage. Therefore, a sub-module topology is needed, and the dc side fault can be blocked by controlling the sub-module.
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.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a low-loss modular multilevel converter with fault blocking capability and a parameter design method thereof, which can realize fault blocking of a direct-current side short circuit without additional switching devices.
According to a first aspect of the invention, a modular multilevel converter with low loss is provided, which comprises three phase units, wherein each 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; the connection part of the upper bridge arm and the lower bridge arm of each phase is externally connected with three-phase alternating current voltage, a first output terminal of the topology of the submodule at the uppermost end of the upper bridge arm is connected with the positive electrode of the direct current bus, and a second output terminal of the submodule at the lowermost end of the lower bridge arm is connected with the negative electrode of the direct current bus; 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; similarly, 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 the third reverse-resistance type insulated gate bipolar transistor.
Optionally, in the converter according to the present invention, under a normal operation condition, the second reverse-blocking type insulated gate bipolar transistor whose cathode is connected to the first output terminal of the second switch module and the fourth reverse-blocking type insulated gate bipolar transistor whose anode is connected to the second output terminal of the third switch module maintain 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 converter according to the present invention, in the dc power transmission system, when a bipolar short-circuit fault is detected on the dc side, all fully-controlled switches are turned off immediately, where the fully-controlled switches include insulated gate bipolar transistors in the first switch module and the fourth switch module, and reverse-blocking insulated gate bipolar transistors in the second switch module and the third switch module; fault current will flow from the second output terminal into the sub-module topology in each leg and out through the second freewheeling diode, the second capacitor, the third capacitor and the first freewheeling diode.
Optionally, when the fault is a dc permanent fault, the converter of the present invention: turning off all the fully-controlled switches, and turning off the alternating-current circuit breaker and the direct-current side switch after the alternating-current side current returns to zero for maintenance; and after the fault is repaired, the direct current side switch is closed, the alternating current side is reclosed, the second reverse-resistance type insulated gate bipolar transistor and the fourth reverse-resistance type insulated gate bipolar transistor are opened, and the normal working states of the first switch module, the second switch module, the third switch module and the fourth switch module are recovered.
Optionally, when the fault is a temporary dc fault, the converter of the present invention: turning off all the fully-controlled switches, waiting for the current on the direct current side to return to zero, after a certain time after the fault is cleared, starting the second and fourth reverse-resistance type insulated gate bipolar transistors, recovering the normal working states of the first, second, third and fourth switch modules, if no overcurrent phenomenon occurs, carrying out reclosing, and if the reclosing is successful, indicating that the fault is cleared; if the overcurrent occurs, all the full-control switches are turned off again; when more than three times of overcurrent occur, permanent faults are considered to occur.
According to a second aspect of the present invention, there is provided a parameter design method for a low-loss modular multilevel converter, including:
determining the limiting relation between the capacitance and the inductance and the number of bridge arm sub-modules according to the circulating resonance relational expression of the converter;
designing the number of sub-modules of each bridge arm according to the withstand voltage of the selected semiconductor device, and selecting the capacitance of each sub-module according to the relation between the system capacity and the system energy storage;
and finally, determining the inductance value of the system according to the capacitance and the number of the bridge arm sub-modules.
Under normal working conditions, a series half-bridge submodule structure is adopted, so that when each unit level is generated, current only needs to flow through one switching device, and the conduction loss can be reduced; under the working condition of direct-current side fault, the property of a reverse-resistance device is utilized to block a current path, and fault current is led to the anode of the sub-module capacitor through an additionally added follow current loop, so that fault blocking is realized by utilizing sub-module capacitor voltage. Meanwhile, the submodule capacitor is split, and the freewheeling diode is connected to the splitting point, so that the voltage stress of part of the switch tube is reduced.
Compared with the prior art, the invention has the following beneficial effects:
the low-loss modular multilevel converter adopts a sub-module topological structure different from that of each bridge arm in the prior art, and under a normal state, the sub-module topology of the converter has the same number of conducting elements as that of a traditional half-bridge sub-module, so that the converter also has similar conducting loss. Therefore, the conduction loss of the system is not increased without depending on an additional switching device.
According to the low-loss modular multilevel converter, fault isolation under the condition of direct-current fault can be realized by controlling the state of the switch module of the submodule in each bridge arm, and the isolation speed is high.
According to the low-loss modular multilevel converter, the capacitor voltage of the sub-modules in each bridge arm can be kept when the fault occurs, and the power supply recovery speed is high.
The low-loss modular multilevel converter has the advantages that the starting process of the alternating current side is similar to that of a traditional half-bridge system, 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 converter according to an embodiment of the present invention;
fig. 2 is a sub-module topology of a low-loss modular multilevel converter according to an embodiment of the present invention;
fig. 3 is an equivalent circuit diagram of a low-loss modular multilevel converter sub-module under dc fault after being controlled by a switching tube according to an embodiment of the present invention;
fig. 4 is a flowchart of an ac-side startup strategy of a low-loss modular multilevel converter according to an embodiment of the invention;
fig. 5 is a deductive idea of a sub-module topology of a low-loss modular multilevel converter according to an embodiment of the present invention;
fig. 6 is a flow chart of a method for designing parameters of a low-loss modular multilevel converter according to an embodiment of the 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 converter according to an embodiment of the invention.
Referring to fig. 1, the low-loss modular multilevel converter includes three phase units, each unit is divided into an upper bridge arm and a lower bridge arm, each bridge arm includes 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 three-phase alternating current voltage, a first output terminal of the topology of the submodule at the uppermost end of the upper bridge arm is connected with the positive electrode of the direct current bus, and a second output terminal of the submodule at the lowermost end of the lower bridge arm is connected with the negative electrode of the direct current bus.
Fig. 2 is a sub-module topology of a low-loss modular multilevel converter 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. Utensil for cleaning buttockIn 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 TR1And a second reverse-blocking insulated gate bipolar transistor T connected in inverse parallel therewithR2The third switch module is also a reverse-resistance type switch module and consists of a third reverse-resistance type insulated gate bipolar transistor TR3And a fourth reverse-blocking insulated gate bipolar transistor T connected in inverse parallel therewithR4And (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; a fourth capacitor 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 moduleR2And TR4The tube is in a normally open state, equivalent to TR1And TR3The whole module is equivalent to two half-bridge modules which are connected in series, so that 0, V can be outputC，2VCThree levels. Under normal operating conditions, the freewheeling diode D3And D4Due to at least 0.5V of amplitudeCThe 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, fig. 1 is a three-phase modular multilevel converter structure in which each sub-module of each leg is composed of the sub-modules shown in fig. 2. The converter does not need to depend on an additional switching device, has the characteristic of low loss while having fault blocking capability, and can realize fault blocking of direct-current side short circuit.
Fig. 3 is an equivalent circuit diagram of a low-loss modular multilevel converter sub-module under dc fault and controlled by a switching tube according to an embodiment of the present invention. In a direct-current transmission system of the modular multilevel converter, when a bipolar short-circuit fault is detected on a direct-current side, all full-control switches are immediately turned off, wherein the full-control switches comprise insulated gate bipolar transistors T1And T3Reverse blocking type insulated gate bipolar transistor TR1～TR4Fault current will flow from the second output terminal into the sub-module topology through the second freewheeling diode D4A second capacitor C2, a third capacitor C3 and a first freewheeling diode D3And (4) flowing out.
Specifically, when a fault occurs on the direct current side and all controllable switches are blocked, and current flows in from the first output terminal 1, the submodule is equivalent to two diodes and four capacitors which are connected in series; when current flows in from the second output terminal 2, the submodule is equivalent to two diodes and two capacitors connected in series.
When a dc double short circuit fault occurs, the current direction should be from the second output terminal 2, since the dc side voltage is lower than the series connection of all sub-module capacitor voltages in each phase unit. One current path of the modular multilevel converter system is marked in fig. 3. Path of fault current is D3->C1->C2->D4. Setting the rated value of the DC side voltage as VdcSystem modulation ratio of m, first freewheeling diode D3Voltage at both ends is VD3Second freewheeling diode D4Voltage at both ends is VD4Each bridge arm is provided with N sub-modules, and the voltage of each independent capacitor is 0.5VCThe peak value of each phase voltage of the power grid is Vgm. The operation characteristics of the modular multilevel converter can be obtained as follows:
Vdc＝N·4·0.5VC＝2NVC
Figure BDA0002288200480000071
in the fault state, taking the current path shown in fig. 3, we can obtain:
Figure BDA0002288200480000072
VAB＝2N·2·0.5VC+2N·(VD3+VD4)
this gives:
Figure BDA0002288200480000073
and, in general, the modulation ratio m is not more than 1, so:
VD3+VD4＜0
i.e. the two freewheeling diodes are in a reverse-biased state, the current will be blocked.
In addition, in the direct current transmission system, when the fault is a direct current permanent fault, the specific process is as follows: all the fully controlled switches are turned off (the fully controlled switches comprise insulated gate bipolar transistors T)1And T3Reverse blocking type insulated gate bipolar transistor TR1～TR4) (ii) a And after the current on the alternating current side returns to zero, the alternating current breaker and the direct current side switch are disconnected for maintenance. After the fault is repaired, the direct current side switch is closed, the alternating current side is reclosed, and then the reverse-resistance type insulated gate bipolar transistor T is startedR2And TR4And recovering the normal working states of the first to fourth switch modules.
When the fault is a direct current temporary fault, the specific process is as follows: all the fully controlled switches are turned off (the fully controlled switches comprise insulated gate bipolar transistors T)1And T2Reverse blocking type insulated gate bipolar transistor TR1～TR4) Waiting for the current on the DC side to return to zero and waiting for a certain time after the fault is cleared, and then starting the reverse-resistance type insulated gate bipolar transistor TR2And TR4Recovering the normal working states of the first to fourth switch modules, if no overcurrent phenomenon occurs, performing reclosing, and if the reclosing is successful, indicating that the fault is cleared; and if the overcurrent occurs, all the full-control switches are turned off again. After three overcurrents, a permanent fault is considered to have occurred. And carrying out power failure maintenance.
Fig. 4 is a flowchart of an ac-side start strategy of a low-loss modular multilevel converter according to an embodiment of the invention. Since each submodule can charge the submodule by current flowing from the first output terminal 1 and the second output terminal 2, and other submodules are not mixed, the voltage can be directly charged to the rated value in a mode that the active power and the idle power are set to be 0 without grouping charging at the beginning of the controllable rectification stage. Specifically, when the modular multilevel converter needs to be started from the alternating current side, the starting strategy is basically the same as that of a half-bridge structure, and uncontrolled rectification is firstly carried out. And when the voltage of the submodule of each bridge arm in the multi-level direct current converter reaches 30% of the rated capacitance voltage, entering a controllable rectification stage until the voltage reaches the rated capacitance voltage, and starting power transmission. Further, a converter station comprising said converter may comprise more than one said modular multilevel converter due to various connection modes of the converter station, and the ac side starting mode of each said converter is the same.
Fig. 5 is a design concept of the sub-module topology of the low-loss modular multilevel converter with fault-ride-through capability of the present invention. In a multi-submodule system, since the switching frequency can be reduced to a relatively low level, and the conduction loss is basically not changed along with the switching frequency, the reduction of the conduction loss is beneficial to reducing the total loss of the system. The half-bridge sub-module in the existing module has minimum conduction loss but does not have fault blocking capability. The design idea is that the low conduction loss is kept and the fault blocking capability is provided. Specifically, the reverse-resistance type insulated gate bipolar transistor is introduced into the submodule, and the characteristic that the conduction voltage drop of the reverse-resistance type insulated gate bipolar transistor is lower than that of a series structure of a conventional insulated gate bipolar transistor and a diode is utilized, so that low conduction loss in a normal state is realized. Under normal working conditions, a series half-bridge submodule structure is adopted, so that when each unit level is generated, current only needs to flow through one switching device, and the conduction loss can be reduced; under the working condition of direct-current side fault, the property of a reverse-resistance device is utilized to block a current path, and a follow current loop D is additionally added3->C1->C2->D4Directing fault current to sub-module capacitor C2And C3Thereby utilizing the sub-module capacitor voltage to realize fault blocking. Meanwhile, the submodule capacitor is split, and the freewheeling diode is connected to the splitting point, so that the voltage stress of part of the switch tube is reduced. In the embodiment, the characteristic that the reverse-resistance IGBT can block reverse voltage and current is utilized, and the reverse-resistance IGBT is applied to occasions needing to block the access of the freewheeling diode, so that the direct-current side fault blocking of the modular multilevel converter is realized; by splitting the capacitor, the voltage required to be borne by the rest IGBT tubes in the module during fault can be effectively reduced, so that the module with higher voltage level can be constructed.
Fig. 6 is a flow chart of a method for designing parameters of a low-loss modular multilevel converter according to an embodiment of the invention. Referring to fig. 6, firstly, the limiting relation between the capacitance and the inductance and the number of the bridge arm sub-modules is determined according to the circulating resonance relation of the half-bridge modular multilevel converter system. Then, the number of the sub-modules of each bridge arm can be designed according to the withstand voltage of the selected semiconductor device, and the capacitance of each sub-module is selected according to the relation between the system capacity and the system energy storage. And finally, determining the inductance value of the system according to the capacitance and the number of the bridge arm sub-modules.
Specifically, the parameter design method of the low-loss modular multilevel converter is as follows:
firstly, determining the DC side voltage class V of the modular multilevel converterdcAnd then according to the voltage stress V of the prepared reverse-resistance type insulated gate bipolar transistor deviceIGBTAnd reserving 1 time or more of margin, so that the number N of the bridge arm sub-modules can be obtained. Assuming that a double margin is left, the reverse-blocking type insulated gate bipolar transistor T can be selectedR1、TR2、TR3、TR4Withstand voltage of 2VIGBTInsulated gate bipolar transistor T1、T2And a diode D1、D2、D3、D4Withstand voltage of 3VIGBT。
After the number of the bridge arm sub-modules is obtained, an angular frequency formula of circulation resonance (m is a modulation ratio, and k represents a kth circulation) can be utilized:
Figure BDA0002288200480000091
and selecting the inductance value L of the bridge arm inductor and the capacitance value C parameter of the sub-module capacitor in the modular multilevel converter, so that the double frequency circulating current can not cause resonance. According to the formula, the higher the circulation frequency is, the lower the corresponding resonance frequency is, so that the circulation can not resonate only by ensuring that the double-frequency circulation does not resonate.
Note that where the C parameter is considered to be C1、C2Or C3、C4The capacitance value of the equivalent capacitor obtained by connecting two capacitors in series is C1、C2、C3And C4In the selection, care needs to be taken to ensure the same capacitance value, so that the actually selected capacitance value of the capacitor is 2C.
This makes it possible to obtain:
Figure BDA0002288200480000092
according to an actual system, the constraint of the product of the inductance and the capacitance can be obtained by determining the double frequency angular frequency.
The capacitance value can be selected according to the total energy E stored by the invertertotalAnd the rated capacity S. As given by tESThen a specific equivalent capacitance value C can be determined, the relationship is as follows:
Figure BDA0002288200480000101
Etotal＝S·tES
Figure BDA0002288200480000102
after the equivalent capacitance value is selected, the capacitor with the capacitance value of 2C can be selected according to half of the withstand voltage, and then the inductor is selected according to the product constraint. And determining parameters of the low-loss modular multilevel converter.
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.

Claims (10)

1. A low-loss modular multilevel converter, characterized by: the bridge comprises three phase units, each 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; the connection part of the upper bridge arm and the lower bridge arm of each phase is externally connected with three-phase alternating current voltage, a first output terminal of the topology of the submodule at the uppermost end of the upper bridge arm is connected with the positive electrode of the direct current bus, and a second output terminal of the submodule at the lowermost end of the lower bridge arm is connected with the negative electrode of the direct current bus;
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 second switch module and the third switch module are both reverse resistance type switch modules;
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 cathode of the first freewheeling diode is connected with the cathode of the first capacitor, the anode of the first freewheeling diode is connected with the anode of the fourth switch module, the cathode of the second freewheeling diode is connected with the anode of the second switch module, and the anode 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 converter 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 converter 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 converter of claim 3, 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.
5. The low loss modular multilevel converter of claim 4, 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.
6. The low loss modular multilevel converter of claim 4, wherein: in the direct-current transmission system, when a bipolar short-circuit fault is detected on a direct-current side of the converter, all fully-controlled switches are immediately turned off, wherein the fully-controlled switches comprise 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; fault current will flow from the second output terminal into the sub-module topology in each leg and out through the second freewheeling diode, the second capacitor, the third capacitor and the first freewheeling diode.
7. The low loss modular multilevel converter of claim 6, wherein: when the fault is a direct current permanent fault: turning off all the fully-controlled switches, and turning off the alternating-current circuit breaker and the direct-current side switch after the alternating-current side current returns to zero for maintenance; and after the fault is repaired, the direct current side switch is closed, the alternating current side is reclosed, the second reverse-resistance type insulated gate bipolar transistor and the fourth reverse-resistance type insulated gate bipolar transistor are opened, and the normal working states of the first switch module, the second switch module, the third switch module and the fourth switch module are recovered.
8. The low loss modular multilevel converter of claim 6, wherein: when the fault is a direct current temporary fault: turning off all the fully-controlled switches, waiting for the current on the direct current side to return to zero, after a certain time after the fault is cleared, starting the second and fourth reverse-resistance type insulated gate bipolar transistors, recovering the normal working states of the first, second, third and fourth switch modules, if no overcurrent phenomenon occurs, carrying out reclosing, and if the reclosing is successful, indicating that the fault is cleared; if the overcurrent occurs, all the full-control switches are turned off again; when more than three times of overcurrent occur, permanent faults are considered to occur.
9. The low loss modular multilevel converter of claim 6, wherein: when a converter station containing the converter needs to be started from an alternating current side, uncontrolled rectification is carried out firstly, after the voltage of each submodule in the converter reaches 30% of the rated capacitor voltage, a controllable rectification stage is carried out until the voltage reaches the rated capacitor voltage, and then power transmission is carried out.
10. A method for designing parameters of a low loss modular multilevel converter according to any of claims 1-9, characterized by: the method comprises the following steps:
determining the limiting relation between the capacitance and the inductance and the number of bridge arm sub-modules according to the circulating resonance relational expression of the converter;
designing the number of sub-modules of each bridge arm according to the withstand voltage of the selected semiconductor device, and selecting the capacitance of each sub-module according to the relation between the system capacity and the system energy storage;
and finally, determining the inductance value of the system according to the capacitance and the number of the bridge arm sub-modules.
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