CN111917317B - Flexible direct current converter capable of blocking direct current fault, submodule and protection method of flexible direct current converter - Google Patents

Flexible direct current converter capable of blocking direct current fault, submodule and protection method of flexible direct current converter Download PDF

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Publication number
CN111917317B
CN111917317B CN202010636620.XA CN202010636620A CN111917317B CN 111917317 B CN111917317 B CN 111917317B CN 202010636620 A CN202010636620 A CN 202010636620A CN 111917317 B CN111917317 B CN 111917317B
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bridge
full
turn
control type
diode
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CN111917317A (en
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李睿
彭程
蔡旭
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Shanghai Jiaotong University
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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
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • 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
    • H02M1/00—Details of apparatus for conversion
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • 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
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0048—Circuits or arrangements for reducing losses
    • 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
    • H02M1/00—Details of apparatus for conversion
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • H02M1/325—Means for protecting converters other than automatic disconnection with means for allowing continuous operation despite a fault, i.e. fault tolerant converters
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B70/00—Technologies for an efficient end-user side electric power management and consumption
    • Y02B70/10—Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

Abstract

The invention discloses a flexible direct current converter capable of blocking direct current faults, a submodule and a protection method thereof, wherein the submodule comprises: the four-full-control type turn-off device comprises four fully-control type turn-off devices, four diodes, two bidirectional thyristors, two additional diodes and two capacitors; the first and second fully-controlled turn-off devices and the first capacitor are sequentially connected into a loop; the third full-control type turn-off device, the first bidirectional thyristor, the fourth full-control type turn-off device, the second bidirectional thyristor and the second capacitor are sequentially connected into a loop; the diodes are connected with the fully-controlled turn-off device in an anti-parallel mode respectively; the cathode of the first additional diode is connected with the anode of the first capacitor; the cathode of the second additional diode is connected with the anode of the first additional diode, and the anode of the second additional diode is connected with the second bidirectional thyristor, the cathode of the second capacitor and the negative output terminal; the terminal connected with the second and the first full-control type turn-off device is used as a positive output terminal. The invention has the direct current side fault blocking capability, the voltage stress born by each device is unchanged, and the conduction loss is lower.

Description

Flexible direct current converter capable of blocking direct current fault, submodule and protection method of flexible direct current converter
Technical Field
The invention relates to the technical field of power transmission and distribution of a power system, in particular to a flexible direct current converter capable of blocking direct current faults, a submodule and a protection method of the submodule.
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.
The traditional modularized multi-level converter, the direct current converter and the direct current transformer adopt a sub-module topology of a half-bridge structure, and due to uncontrolled pulse control of an anti-parallel diode of a lower tube, a fault current path can be formed when a short-circuit fault occurs on a direct current side, so that overcurrent is caused, the converter, the direct current converter or a direct current transformer system is damaged, and huge cost loss is caused. 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.
In the prior art, the following MMC submodule topologies with fault-ride-through capability have been proposed:
(1) the patent with the application number of CN201410400214.8 provides a submodule structure with a mixed half-bridge structure and a full-bridge structure, and the full-bridge submodule is used for realizing fault blocking of a current converter. However, the cost and the loss are high due to the large number of full-bridge sub-module fully-controlled switches.
(2) The MMC submodule topology proposed in CN201510416707.5 uses fewer devices, but also has higher conduction loss and cost due to the larger number of fully-controlled switching devices in the current path.
Disclosure of Invention
The invention provides a soft direct current converter, a submodule and a protection method thereof which can block direct current faults, aiming at the problems in the prior art, and the fault blocking of direct current side short circuit can be realized without adding an additional fully-controlled turn-off device.
In order to solve the technical problems, the invention is realized by the following technical scheme:
according to a first aspect of the invention, there is provided a sub-module, which is a double half-bridge sub-module, comprising in particular a first half-bridge configuration, a second half-bridge configuration and a freewheel path, wherein,
the first half-bridge structure is composed of a first full-control type turn-off device, a first diode, a second full-control type turn-off device, a second diode and a first capacitor, wherein the first full-control type turn-off device is connected with a terminal on the high-voltage side and is connected with the anode of the first capacitor, the first full-control type turn-off device is connected with a terminal on the low-voltage side and is connected with a terminal on the high-voltage side and is used as a positive output terminal of the first half-bridge structure, the second full-control type turn-off device is connected with a terminal on the low-voltage side and is connected with the cathode of the first capacitor and is used as a negative output terminal of the first half-bridge structure, and the first diode and the second diode are respectively connected with the first full-control type turn-off device and the second full-control type turn-off device in anti-parallel;
the second half-bridge structure consists of a third full-control type switching device, a third diode, a fourth full-control type turn-off device, a fourth diode, a plurality of bidirectional thyristors connected in series with the third full-control type turn-off device and the fourth diode, and a second capacitor, wherein the terminal of the third full-control type turn-off device connected with the low-voltage side is connected with one end of the first bidirectional thyristor, the other end of the first bidirectional thyristor is connected with the negative electrode of the second capacitor, and the terminal of the third full-control type turn-off device connected with the high-voltage side is connected with one end of the second bidirectional thyristor and is used as the negative output terminal of the second half-bridge structure; the other end of the second bidirectional thyristor is connected with a fourth fully-controlled turn-off device and a low-voltage side terminal, a high-voltage side terminal of the fourth fully-controlled turn-off device is connected with the anode of a second capacitor and is simultaneously used as a positive output terminal of a second half-bridge structure, and a third diode and a fourth diode are respectively connected with the third fully-controlled turn-off device and the fourth fully-controlled turn-off device in an anti-parallel mode;
the continuous flow path is connected to the positive polarity end of the capacitor of the first half-bridge module from the negative output terminal of the second half-bridge structure, the negative output terminal of the first half-bridge is connected to the positive output terminal of the second half-bridge, and the positive output terminal of the first half-bridge and the negative output terminal of the second half-bridge are respectively used as the positive output terminal and the negative output terminal of the sub-module.
Optionally, the triac in the second half-bridge structure comprises a first triac and a second triac; the freewheel path further includes a first additional diode, a second additional diode;
the first full-control type turn-off device, the second full-control type turn-off device and the first capacitor are sequentially connected to form a loop;
the third full-control type turn-off device, the first bidirectional thyristor, the fourth full-control type turn-off device, the second bidirectional thyristor and the second capacitor are sequentially connected to form a loop;
the first diode, the second diode, the third diode and the fourth diode are respectively connected with the first full-control turn-off device, the second full-control turn-off device, the third full-control turn-off device and the fourth full-control turn-off device in an anti-parallel mode;
the cathode of the first additional diode is connected with the anode of the first capacitor; the cathode of the second additional diode is connected with the anode of the first additional diode, and the anode of the second additional diode is connected with the second bidirectional thyristor, the cathode of the second capacitor and the negative output terminal; the terminal of the second full-control type turn-off device connected with the first full-control type turn-off device is used as a positive output terminal; the terminal of the second fully-controlled turn-off device connected to the cathode of the first capacitor is connected to the terminal of the fourth turn-off device connected to the first triac, or,
the cathode of the second additional diode is connected with the anode of the first additional diode, and the anode of the second additional diode is connected with the second bidirectional thyristor, the third turn-off device and the negative output terminal; the terminal of the second full-control type turn-off device connected with the first full-control type turn-off device is used as a positive output terminal; and the terminal of the second fully-controlled turn-off device connected with the cathode of the first capacitor is connected with the terminal of the fourth turn-off device connected with the anode of the second capacitor.
On the basis that all the double-half-bridge sub-modules provided by the invention adopt devices which are already used for building a modular multilevel converter system, the conduction loss of the double-half-bridge sub-modules is lower than that of the existing hybrid bridge sub-modules and clamping bi-sub-modules which have fault blocking capability and lowest conduction loss (the conduction loss of the hybrid bridge sub-modules is basically the same as that of the clamping bi-sub-modules). At present, the conduction loss of the field of the modular multilevel converter is the lowest or half-bridge submodule, but the field of the modular multilevel converter does not have fault blocking capability. Therefore, on the basis of having the fault blocking capability, the invention has the advantages that the conduction loss is lower than that of other topologies having the fault blocking capability, and the device cost is lower.
In addition, before the double-half-bridge sub-module provided by the invention is inserted into an additional device, the module can be regarded as two half-bridge sub-modules which are connected in series, so that the voltage stress borne by the device is the sub-module capacitance voltage; after the bidirectional thyristors are connected in series, the thyristors are kept in a conducting state during normal work, so the thyristors basically do not bear voltage stress, and when a fault occurs, the thyristors are sequentially turned off, so the thyristors and the fully-controlled turn-off devices connected in series with the thyristors bear fault voltage together, and the maximum voltage possibly appearing during the fault in the sub-modules is the sum of the maximum voltages of two capacitors, so after voltage division (the parallel capacitors are possibly complemented to ensure voltage sharing), the thyristors and the fully-controlled turn-off devices can bear the same voltage, namely one time of the maximum capacitor voltage. The additional diodes are directly connected in series to ensure that each tube only needs to bear one time of capacitor voltage. Therefore, the maximum voltage born by all devices in the module is the maximum voltage of the sub-module capacitor, and the device type selection is convenient.
Preferably, the first fully-controlled turn-off device is an insulated gate bipolar transistor; and/or the presence of a gas in the gas,
the second fully-controlled turn-off device is an insulated gate bipolar transistor; and/or the presence of a gas in the gas,
the third fully-controlled turn-off device is an insulated gate bipolar transistor; and/or the presence of a gas in the gas,
the fourth fully-controlled turn-off device is an insulated gate bipolar transistor.
Preferably, when the circuit operates in a normal state, the first and second triacs are both in a conducting state;
the first additional diode and the second additional diode are kept in a cut-off state due to the fact that the first additional diode and the second additional diode bear back pressure, and no circuit is added.
According to a second aspect of the present invention, there is provided a method for protecting the sub-module, when a system using the sub-module encounters a dc-side fault condition and needs to latch up a module to block a fault current, then:
two bidirectional thyristors in the second half-bridge of the submodule and all fully-controlled turn-off devices in the submodule are blocked in sequence;
the turn-off sequence of the two triacs is related to their position, requiring an earlier blocking pulse when the triac is connected in series with the fully controlled turn-off device controlling the bypass of the second half-bridge structure, and requiring a later blocking pulse when the triac is connected in series with the fully controlled turn-off device controlling the access to the second half-bridge structure.
According to a third aspect of the present invention, there is also provided a converter, comprising: three phase units, each phase unit comprising: the bridge comprises an upper bridge arm and a lower bridge arm, wherein each bridge arm comprises a plurality of double-half-bridge sub-modules connected in series;
the number of the double half-bridge sub-modules connected in series with the upper bridge arm and the lower bridge arm of each phase unit is the same;
the upper bridge arm and the lower bridge arm of each phase unit are respectively connected with a current-limiting reactor in series;
each phase unit is as follows from top to bottom: all double-half-bridge sub-modules of the upper bridge arm, the reactor of the lower bridge arm and all double-half-bridge sub-modules of the lower bridge arm;
the joint of the upper bridge arm and the lower bridge arm of each phase unit is externally connected with a three-phase alternating current voltage;
and the positive output terminal of the uppermost double-half-bridge submodule of the upper bridge arm is connected with the positive electrode of the direct current bus, and the negative output terminal of the lowermost double-half-bridge submodule of the lower bridge arm is connected with the negative electrode of the direct current bus.
Preferably, in the converter, when a bipolar short-circuit fault is detected on the dc side in the dc transmission system, the trigger pulses of the second triac, the first triac and all the fully-controlled turn-off devices in each of the dual half-bridge submodules are sequentially blocked;
a fault current will flow from the negative output terminal and along the second additional diode, the first capacitor and the second diode and finally out from the positive output terminal.
Preferably, in the converter, when the fault is a temporary fault on the direct current side in the direct current transmission system, the trigger pulses of the second bidirectional thyristor, the first bidirectional thyristor and all the fully-controlled turn-off devices in each double half-bridge submodule are sequentially blocked;
when the alternating current side cuts off and trips, and after a certain time, a second bidirectional thyristor, a first bidirectional thyristor and all fully-controlled turn-off devices in each double half-bridge submodule are sequentially turned on;
if the direct current side has no overcurrent, the alternating current side is reclosed, and the fault is cleared after the reclosing is successful;
if the direct current side has overcurrent, the second bidirectional thyristor, the first bidirectional thyristor and all the rest fully-controlled turn-off devices of each double-half-bridge submodule are sequentially turned off again;
when more than three times of overcurrent occurs on the direct current side, permanent faults are considered to occur.
Preferably, in the direct current transmission system, when the fault is a permanent fault on the direct current side, the trigger pulses of the second bidirectional thyristor, the first bidirectional thyristor and all the fully-controlled turn-off devices of each double half-bridge submodule are sequentially blocked;
waiting for the current breaking of the alternating current side and the direct current side, tripping an alternating current side breaker after the current breaking of the alternating current side, tripping a direct current side switch after the current breaking of the direct current side, and performing direct current line maintenance after the direct current side switch is switched off; after the fault is repaired, the direct-current side switch is closed, and the second bidirectional thyristor, the first bidirectional thyristor and all the fully-controlled turn-off devices of each double-half-bridge submodule are sequentially turned on;
if no overcurrent phenomenon occurs on the direct current side, reclosing the alternating current side, and recovering the normal operation of the converter;
if the overcurrent phenomenon occurs on the direct current side, the line fault is not cleared, the trigger pulses of the second bidirectional thyristor, the first bidirectional thyristor and all the fully-controlled turn-off devices of each double-half-bridge submodule are blocked again in sequence, and the alternating current side switch and the direct current side switch are disconnected for maintenance.
Preferably, when a converter station including the converter needs to be started from an alternating current side, a bidirectional thyristor trigger pulse is supplied by the converter station, then uncontrolled rectification is performed, and after the voltage of each sub-module in the converter reaches 30% of the rated capacitance voltage, a controllable rectification stage is performed until the capacitance voltage of each double-half-bridge sub-module reaches the rated capacitance voltage, and then power transmission is performed.
According to a fourth aspect of the present invention, there is also provided a dc-dc converter, comprising: three phase cells, each of the phase cells comprising: the three bridge arms are in Y-shaped connection, and non-common ends are respectively connected with a positive electrode of a high-voltage side bus, a positive electrode of a low-voltage side bus and the ground;
the bridge arm connected with the positive pole of the high-voltage side bus is recorded as a high-voltage side bridge arm, the bridge arm connected with the positive pole of the low-voltage side bus is recorded as a low-voltage side bridge arm, and the bridge arm connected with the ground is recorded as an auxiliary bridge arm; wherein the content of the first and second substances,
each of the high side bridge arm and the auxiliary bridge arm includes: a plurality of half-bridge sub-modules connected in series and an inductor;
the low-voltage side bridge arm includes: a plurality of double half-bridge sub-modules connected in series.
Preferably, in the dc power transmission system, when a bipolar short-circuit fault is detected on the high-voltage side or the low-voltage side, the second bidirectional thyristor and the first bidirectional thyristor in each of the double half-bridge sub-modules, all the fully-controlled turn-off devices in the double half-bridge sub-modules, and the trigger pulses of the fully-controlled turn-off devices in the half-bridge sub-modules in the high-voltage side bridge arm and the auxiliary bridge arm are sequentially blocked to realize fault blocking.
According to a fifth aspect of the present invention, there is also provided a direct current transformer, comprising: the system comprises a first subsystem, a three-phase power frequency transformer and a second subsystem which are sequentially connected; wherein the content of the first and second substances,
the first subsystem and the second subsystem respectively include: three phase units, each phase unit comprising: an upper bridge arm and a lower bridge arm; each bridge arm comprises a plurality of double half-bridge submodules connected in series;
the number of the double half-bridge sub-modules of each phase unit, which are connected in series with the upper bridge arm and the lower bridge arm, is the same;
each phase unit comprises the following components in sequence from top to bottom: all double-half-bridge sub-modules of the upper bridge arm, the reactor of the lower bridge arm and all double-half-bridge sub-modules of the lower bridge arm;
the joint of the upper bridge arm and the lower bridge arm of each phase unit is connected with the three-phase power frequency transformer;
and the positive output terminal of the uppermost double-half-bridge submodule of the upper bridge arm of each phase unit of the first subsystem and the second subsystem is connected with the positive electrode of the direct current bus of the corresponding subsystem, and the negative output terminal of the lowermost double-half-bridge submodule of the lower bridge arm is connected with the negative electrode of the direct current bus of the corresponding subsystem.
Preferably, in the direct-current transmission system, when a bipolar short-circuit fault is detected on the direct-current side of the first subsystem and/or the second subsystem, the gate trigger pulses of the second bidirectional thyristors, the first bidirectional thyristors and all the fully-controlled switches of all the double-half-bridge submodules in the first subsystem and/or the second subsystem are sequentially blocked;
all the fully-controlled switches comprise: the first full-control type turn-off device, the second full-control type turn-off device, the third full-control type turn-off device and the fourth full-control type turn-off device;
after waiting for the fault current to return to zero, the dc side switch is opened for service.
Preferably, when the first subsystem or the second subsystem needs to be started from the AC side: the converter station supplies two bidirectional thyristor trigger pulses in each double half-bridge submodule in the subsystem, the AC side uncontrolled rectification charging starts, and after all capacitor voltages in the double half-bridge submodules reach 30% of corresponding rated values, a controllable rectification mode is entered; and then entering a normal working state after the capacitor voltage reaches a rated value.
Compared with the prior art, the embodiment of the invention has at least one of the following advantages:
(1) according to the flexible direct current converter capable of blocking direct current faults, the submodule and the protection method of the flexible direct current converter, through adding the uncontrolled device and the semi-controlled device, compared with a traditional half-bridge submodule, fault blocking of direct current side short circuit can be achieved without adding an additional fully-controlled turn-off device, and compared with a traditional hybrid bridge module, cost is reduced;
(2) according to the flexible direct current converter capable of blocking direct current faults, the submodule and the protection method of the flexible direct current converter, conduction is achieved through the first bidirectional thyristor and the second thyristor, and under the normal working condition, the extra semiconductor device on a current path is the bidirectional thyristor, so that conduction loss is lower.
(3) According to the flexible direct current converter, the submodule and the protection method thereof capable of blocking the direct current fault, all controllable device pulses in the submodule are blocked during the fault, a capacitor discharge current path is blocked, and after all fully-controlled devices are turned off, the path of discharging to a bridge arm from all capacitors is blocked, so that the capacitors cannot generate a discharge process of a large current any more, only self-discharge of the capacitors (equivalent parallel resistance through the capacitors) exists, the process is extremely slow, so that the capacitor voltage of the submodule can be kept during the fault, and the power supply recovery speed is high;
(4) according to the flexible direct current converter capable of blocking direct current faults, the submodule and the protection method thereof, the number of devices is selected according to voltage stress, and the turn-off sequence of the driving pulse is reasonably controlled, so that the maximum voltage born by all the devices in the module is the maximum voltage of the submodule capacitor, and the device type selection is facilitated;
(5) according to the flexible direct current converter capable of blocking direct current faults, the submodule and the protection method of the flexible direct current converter, when the direct current side faults occur, the control pulses can be blocked in sequence, and therefore the requirement on triggering simultaneity is low.
(6) The submodule protection method provided by the invention is suitable for the submodule in which the thyristor and the fully-controlled turn-off device are connected in series to block reverse current, and the requirement on the simultaneity of pulse blocking is lower by the protection method.
Drawings
Embodiments of the invention are further described below with reference to the accompanying drawings:
FIG. 1 is a schematic diagram of a dual half-bridge sub-module in accordance with a preferred embodiment of the present invention;
FIG. 2 is a flow chart of a sub-module structure and a corresponding protection method according to an embodiment of the present invention;
fig. 3 is a topology of a converter according to an embodiment of the present invention;
fig. 4 is an equivalent circuit diagram of a converter according to an embodiment of the present invention after being controlled by a switching tube under a dc fault;
fig. 5 is a flowchart of an ac-side start strategy of a converter according to an embodiment of the present invention;
FIG. 6 shows a topology of a DC-DC converter according to an embodiment of the present invention;
fig. 7 is an equivalent circuit diagram of the dc-dc converter after being controlled by the switching tube under the high-side dc fault according to an embodiment of the present invention;
fig. 8 is an equivalent circuit diagram of the dc-dc converter after being controlled by the switching tube under the high-side dc fault according to an embodiment of the present invention;
FIG. 9 shows a topology of a DC transformer according to an embodiment of the present invention;
fig. 10 is an equivalent circuit diagram of a dc transformer controlled by a switching tube under a dc fault according to an embodiment of the present invention;
fig. 11 is a flowchart of an ac-side startup strategy of a dc transformer 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 variations and modifications can be made by persons skilled in the art without departing from the spirit of the invention. All falling within the scope of the present invention.
Fig. 1 shows a schematic diagram of a dual half-bridge sub-module according to a preferred embodiment of the present invention.
Referring to fig. 1, the dual half-bridge sub-module of the present embodiment includes: a first fully-controlled turn-off device T1, a first diode D1, a second fully-controlled turn-off device T2, a second diode D2, a third fully-controlled turn-off device T3, a third diode D3, a fourth fully-controlled turn-off device T4, a fourth diode D4, a first TRIAC1, a second TRIAC2, a first additional diode Da1, a second additional diode Da2, a first capacitor C1, a second capacitor C2, a first fully-controlled turn-off device T1, a second fully-controlled turn-off device T2, and a first capacitor C1 are sequentially connected to form a loop, a third fully-controlled turn-off device T3, a first TRIAC1, a fourth fully-controlled turn-off device T1, a second bidirectional thyristor TRIAC1, a fourth fully-controlled turn-off device T1, a first diode D1, a first fully-controlled turn-off device T1, a first diode D1, a fourth diode D1, a first diode D1, a second diode 1, a third diode D1, a fourth diode 1, a third diode D1, a third diode 1, a fourth diode C1, a third diode C1 and a third diode C1 are sequentially connected in sequence to form a fourth diode 367, a fourth diode 1, a second diode C1, a second diode 1, a fourth diode C1, a second diode C367, a second diode C1, a second, A second full-control turn-off device T2, a third full-control turn-off device T3 and a fourth full-control turn-off device T4 are connected in inverse parallel, the cathode of the first additional diode Da1 is connected with the anode of the first capacitor C1, the cathode of the second additional diode Da2 is connected with the anode of the first additional diode Da1, the anode of the second additional diode Da2 is connected with the second TRIAC2, the cathode of the second capacitor C2 and the negative output terminal, the terminal of the second full-control turn-off device T2 connected with the first full-control turn-off device T1 serves as the positive output terminal, and the terminal connected with the cathode of the first capacitor C1 is connected with the fourth turn-off device T4 and the first bidirectional thyristor T4
The terminal to which TRIAC1 is connected.
In this embodiment, the first fully-controlled turn-off device, the second fully-controlled turn-off device, the third fully-controlled turn-off device, and the fourth fully-controlled turn-off device are insulated gate bipolar transistors.
Under the normal working condition of the double-half-bridge submodule on the direct-current side, the first TRIAC1 and the second TRIAC2 are both in a conducting state, and the whole module is equivalent to two half-bridge modules which are connected in series, so that three levels of 0, VC and 2VC can be output. Under normal conditions, the additional diodes Da1 and Da2 are in an off state due to being subjected to reverse voltage with the amplitude of at least 0.5VC, and therefore no loss is generated.
Since thyristors of the same voltage class have a lower turn-on voltage drop than insulated gate bipolar transistors and diodes. The analysis of a data manual of the existing device can show that the double-half-bridge submodule provided by the invention has lower conduction loss than all the existing submodules which are formed by adopting the traditional insulated gate bipolar transistors and diodes and have fault blocking capability.
Fig. 2 is a flow chart of a sub-module structure and a corresponding protection method according to an embodiment of the invention.
Referring to fig. 2, the dual-half-bridge sub-module adopted in the present embodiment adopts another structure, but it has no essential difference from the sub-module structure shown in fig. 1, and for the dual-half-bridge sub-module shown in fig. 2, the protection method is as follows:
in a modularized multi-level converter, a direct current converter or a direct current transformer system based on the double half-bridge sub-modules, when a bipolar short circuit fault occurs on a certain direct current side, the bidirectional thyristor TRIAC2 connected in series with a fully-controlled turn-off device T4 for bypassing the second half-bridge structure is turned off first, then the bidirectional thyristor TRIAC1 connected in series with a fully-controlled turn-off device T3 for accessing the second half-bridge structure is turned off, and finally the used fully-controlled turn-off devices T1-T4 are turned off.
Fig. 3 shows a topology of a converter according to an embodiment of the present invention.
Referring to fig. 3, the inverter of the present embodiment is a low-loss modular multilevel inverter, which includes: each unit is divided into an upper bridge arm and a lower bridge arm, each bridge arm comprises a plurality of double-half-bridge sub-modules connected in series, and the number of the double-half-bridge sub-modules connected in series with the upper bridge arm and the lower bridge arm in 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 double-half-bridge sub-modules of the upper bridge arm, the reactor of the lower bridge arm and all double-half-bridge sub-modules of the lower bridge arm; and the joint of the upper bridge arm and the lower bridge arm of each phase is externally connected with three-phase alternating current voltage, the positive output terminal of the uppermost double-half-bridge submodule of the upper bridge arm is connected with the positive electrode of the direct current bus, and the negative output terminal of the lowermost double-half-bridge submodule of the lower bridge arm is connected with the negative electrode of the direct current bus.
Referring to fig. 1 and 3, fig. 3 is a three-phase modular multilevel converter structure, in which each double half-bridge submodule of each bridge arm is composed of the double half-bridge submodules shown in fig. 1. The converter has the characteristic of low loss while having the fault blocking capability, and can improve the efficiency of an electric energy conversion system.
Fig. 4 is an equivalent circuit diagram of a low-loss modular multilevel converter sub-module according to an embodiment of the present invention after being controlled by a switching tube under a dc fault. In the direct-current transmission system of the converter, when a bipolar short-circuit fault is detected on a direct-current side, the second TRIAC2, the first TRIAC1 and all the remaining full-control switches including full-control type turn-off devices T1-T4 are immediately turned off in sequence, and fault current flows into the submodule topology from a negative output terminal and then flows out through the second additional diode Da2, the first additional diode Da1, the first capacitor C1 and finally the second diode D2.
Specifically, when a fault occurs on the dc side, after all controllable switches are sequentially blocked, when current flows from the positive output terminal of each double-half-bridge submodule, the double-half-bridge submodule is equivalent to two diodes, one thyristor and 2 capacitors connected in series (at this time, because of the existence of forward current, the first bidirectional thyristor cannot be directly turned off, and when the current drops to 0, the first bidirectional thyristor is naturally turned off); when current flows in from the second output terminal 2, the double half-bridge submodule is equivalent to three diodes and a capacitor connected in series.
When a dc double short circuit fault occurs, the fault current direction should eventually flow from the negative output terminal because the dc side voltage is lower than the series connection of the capacitor voltages of all the double half-bridge sub-modules in each phase unit. One current path of the converter system is marked in fig. 3. The path of the fault current is Da2- > Da1- > C1- > D2. The voltage rated value of a direct current side is Vdc, the system modulation ratio is m, the voltage at two ends of a first additional diode Da1 is VDa1, the voltage at two ends of a second additional diode Da2 is VDa2, the voltage at two ends of a second diode D2 is VD2, each bridge arm is provided with N double-half-bridge sub-modules, the voltage of each independent capacitor is VC, and the peak value of each phase voltage of a power grid is Vgm. The operation characteristics of the modular multilevel converter can be obtained as follows:
Vdc＝2NVC
Figure BDA0002568311890000091
in the fault state, taking the current path shown in fig. 3, we can obtain:
Figure BDA0002568311890000092
VAB＝2NVC+2N·(VDa1+VDa2+VD2)
this gives:
Figure BDA0002568311890000101
and, in general, the modulation ratio m is not more than 1, so:
VDa1+VDa2+VD2＜0
i.e. the three diodes will eventually be in a reverse biased state and the current will be blocked.
When the fault is a direct current temporary fault, the specific process is as follows: sequentially turning off a second TRIAC2, a first TRIAC1 and the rest of all fully-controlled turn-off devices T1-T4, waiting for a certain time after the alternating current side is cut off and tripped, sequentially turning on the TRIAC2 and the TRIAC1, recovering the normal working states of all the fully-controlled turn-off devices, carrying out reclosing on the alternating current side if no overcurrent phenomenon occurs on the direct current side, and indicating that the fault is cleared after the reclosing is successful; if overcurrent occurs, the second TRIAC2, the first TRIAC2 and all the remaining fully-controlled turn-off devices T1-T4 are turned off again in sequence; when more than three times of overcurrent occur, permanent faults are considered to occur.
In addition, in the direct current transmission system, when the fault is a direct current permanent fault, the specific process is as follows: sequentially turning off the second TRIAC2, the first TRIAC1 and the rest of all the full-control switches, including the full-control type turn-off devices T1-T4; 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 and the alternating current side switch are closed, the first bidirectional thyristor TRIAC1 and the second bidirectional thyristor TRIAC2 are sequentially started, and then the normal working states of all the fully-controlled turn-off devices are recovered.
Fig. 5 is a flowchart of an ac-side startup strategy of a low-loss modular multilevel converter according to an embodiment of the invention. After the two bidirectional thyristors are conducted, the rest devices participating in the circuit in the module are completely the same as the structure of the traditional half-bridge module, so that the module can be started from the alternating current side completely according to the traditional half-bridge modular multilevel converter. Specifically, when the converter needs to be started from the alternating current side, uncontrolled rectification is performed first. And when the voltage of the sub-module of each bridge arm in the multi-level modular multi-level 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 a converter may comprise more than one converter due to various connection modes of the converter station, and the starting mode of each converter on the alternating current side is the same.
Fig. 6 is a structural diagram of a low-loss modular multilevel dc-dc converter according to an embodiment of the invention. Specifically, the three-phase modular multilevel direct current converter comprises three phase units, each phase unit is divided into three bridge arms, the three bridge arms are connected in a Y shape, and a non-common end is respectively connected with a positive electrode of a high-voltage side bus, a positive electrode of a low-voltage side bus and the ground. The bridge arm connected with the positive pole of the low-voltage side bus is marked as a low-voltage side bridge arm, and the bridge arm connected with the ground is marked as an auxiliary bridge arm. Wherein each bridge arm of the high-voltage side bridge arm and the auxiliary bridge arm comprises: a plurality of conventional half-bridge modules HBSM in series and an inductor, the low-side bridge arm comprising: a number of series connected double half-bridge sub-modules SCRDSM as shown in figure 1.
In the three-phase modular multilevel direct-current converter, when a bipolar short-circuit fault occurs on the high-voltage side or the low-voltage side in a direct-current transmission system, the trigger pulses of the second TRIAC2, the first TRIAC1 and all the fully-controlled turn-off devices are blocked in sequence, and then fault blocking can be achieved.
Fig. 7 is an equivalent circuit diagram of a multilevel dc-dc converter controlled by a switching tube under a high-side dc fault according to an embodiment of the present invention. Referring to fig. 7, after the short-circuit fault occurs on the high-voltage side, the trigger pulses of the second TRIAC2, the first TRIAC1 and all the fully-controlled turn-off devices are blocked in sequence, and then the equivalent circuit is shown in the figure. For the above-mentioned multilevel dc-dc converter submodule in the low-voltage side bridge arm of the converter, when the current flows from the positive output terminal, the submodule is equivalent to three diodes and a capacitor connected in series; when current flows from the negative output terminal, the submodule is equivalent to two diodes, a bidirectional thyristor and two capacitors which are connected in series.
When a short-circuit fault occurs on the high-voltage side, the current direction should be from the negative output terminal since the high-voltage side voltage is lower than the series connection of all the sub-module capacitor voltages in each phase unit. One current path of the modular multilevel dc-dc converter is marked in fig. 7. For the multi-level DC-DC converter sub-modules in the low-side bridge arm of the converter, the path of the fault current is Da2- > Da1- > C1- > D2. The rated voltage value of the high-voltage side is VH, the rated voltage value of the high-voltage side is VL, the voltage of two ends of a diode D1 is VD1, the voltage of two ends of a diode D2 is VD2, the voltage of two ends of a diode D3 is VD3, the voltage of two ends of an additional diode Da1 is VDa1, the voltage of two ends of an additional diode Da2 is VDa2, the voltage of two ends of a TRIAC TRIAC1 is VTRIAC1, and a half-bridge module is used for a high-voltage side bridge arm and an auxiliary bridge arm, so that the names of devices in the module are added with h to represent a half bridge. The voltage drops of diodes D1h and D2h in the submodules are respectively recorded as VD1h and VD2h, each high-voltage side bridge arm is provided with NH submodules, each auxiliary bridge arm is provided with NA submodules, each low-voltage side bridge arm is provided with NL submodules, and the voltage of each submodule is VC. And (3) performing single-phase analysis, wherein a direct-current component of the output voltage of the high-voltage side bridge arm is represented by VH _ DC, a direct-current component of the output voltage of the low-voltage side bridge arm is represented by VL _ DC, and a direct-current component of the output voltage of the auxiliary bridge arm is represented by VA _ DC. The operation characteristics of the modular multilevel DC-DC converter can be obtained as follows:
VH＝VH_DC+VA_DC
VL＝VL_DC+VA_DC
VH-VL＝VH_DC-VL_DC
when the high-voltage side direct current has a fault, the current path shown in fig. 7 is taken, and the average voltage drop of the diode D2 and the freewheeling diodes Da1 and Da2 in each submodule of the low-voltage side bridge arms can be obtained by neglecting the voltage drop of the inductor:
Figure BDA0002568311890000111
selecting to satisfy: vL-NL·VCThe number of the low-voltage side sub-modules is less than 0, so that the fault current can be blocked when the high-voltage side direct current is in short circuit. It is easy to prove that if a fault current path is selected to pass through the auxiliary bridge arm, the condition is easier to satisfy. This is because: if the current flows through the auxiliary bridge arm, the current inevitably flows in from the positive electrode of the capacitor of the sub-module of the auxiliary bridge arm through D1h, so that each sub-module of the auxiliary bridge arm outputs VC to block the fault. When the modular multilevel converter operates in a normal state, a system formed by the high-voltage side bridge arm and the auxiliary bridge arm has the same structure as the modular multilevel converter, so that the voltage of a direct-current bus is lower than the sum of the capacitor voltages of all sub-modules of each phase.
Fig. 8 is an equivalent circuit diagram of the multilevel dc-dc converter controlled by the switching tube under the low-side dc fault according to an embodiment of the present invention. When the low-voltage side has a direct-current short-circuit fault, the current path shown in fig. 8 is taken, and the average voltage drop across the D1h diode of each submodule on the high-voltage side can be obtained by neglecting the voltage drop across the inductor as:
Figure BDA0002568311890000121
selecting to satisfy: vH-(2NL+NH)·VCThe number of the high-voltage side half-bridge submodules is less than 0, so that the fault current can be blocked when the high-voltage side direct current is in short circuit. It is easy to prove that if a fault current path is selected to flow through the auxiliary bridge arm, the conditions are more easily satisfied. The reason is the same as the principle that the auxiliary bridge arm does not run fault current when the high-voltage side fails
As can be seen from the above analysis process, the fault ride-through method can be implemented.
In summary, in the low-loss modular multilevel dc-dc converter in the above embodiments of the present invention, the fault blocking of the dc-side short circuit can be achieved by controlling the on/off of the switch module.
Fig. 9 is a schematic diagram of a low-loss modular multilevel dc transformer with fault blocking capability according to an embodiment of the present invention. The modular multilevel dc transformer includes: the system comprises a first subsystem, a three-phase power frequency Transformer transform 1 and a second subsystem which are connected in sequence. The first subsystem and the second subsystem respectively comprise: each phase unit is divided into an upper bridge arm and a lower bridge arm, each bridge arm comprises a plurality of double-half-bridge sub-modules connected in series, and the number of the double-half-bridge sub-modules connected in series with the upper bridge arm and the lower bridge arm in 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 double-half-bridge sub-modules of the upper bridge arm, the reactor of the lower bridge arm and all double-half-bridge sub-modules of the lower bridge arm; and the joint of the upper and lower bridge arms of each phase is externally connected with a three-phase power frequency transformer winding, the positive output terminal of the uppermost double-half-bridge 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 negative output terminal of the lowermost double-half-bridge submodule of the lower bridge arm is connected with the negative electrode of the direct current bus of the subsystem.
Referring to fig. 1 and 9, in the modular multilevel dc transformer structure, each sub-module of each bridge arm is composed of the double half-bridge sub-modules shown in fig. 1. The modular multilevel DC transformer is integrally composed of two modular multilevel converters (subsystems).
Fig. 10 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 according to an embodiment of the present invention. When a short-circuit fault occurs on the direct-current side of one side, the trigger pulses of the second TRIAC2, the first TRIAC1 and the rest of all the fully-controlled turn-off devices T1 to T4 in each submodule are blocked in sequence, so that the fault blocking of the system on the side can be realized, and at the moment, an equivalent circuit of the modular multilevel converter system on the side is shown in fig. 10.
Further, when the fault is a permanent dc fault on any side, the specific process is as follows: sequentially blocking trigger pulses of all second TRIAC2, all first TRIAC1 and all remaining fully-controlled turn-off devices 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 thyristor is supplied with trigger pulse firstly, then the current converters on two sides operate under the condition of zero active power, whether the direct current side overcurrent phenomenon still occurs is observed, if the direct current side overcurrent does not occur, the fault is considered to be eliminated, and the circuit breaker on the alternating current side can be closed to restore the system to operate.
When the fault is any side direct current temporary fault, the specific process is as follows: and sequentially blocking trigger pulses of all second TRIACs 2, all first TRIACs 1 and all remaining fully-controlled turn-off devices in the direct-current transformer system, waiting for the direct-current side current 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 or not, and if the direct-current side overcurrent does not occur, considering that the fault is eliminated, so that the system operation can be recovered.
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. 11 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 charging at the ac side without controlled rectification, and after the capacitor voltage reaches a suitable value, the subsystem enters a controlled rectification 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 embodiments were chosen and described in order to best explain the principles of the invention and the practical application, and not to limit the invention. Any modifications and variations within the scope of the description, which may occur to those skilled in the art, are intended to be within the scope of the invention.

Claims (13)

1. A submodule, characterized in that the submodule is a double half-bridge submodule comprising a first half-bridge configuration, a second half-bridge configuration and a free-wheeling path, wherein,
the first half-bridge structure is composed of a first full-control type turn-off device, a first diode, a second full-control type turn-off device, a second diode and a first capacitor, wherein the first full-control type turn-off device is connected with a terminal on the high-voltage side and is connected with the anode of the first capacitor, the first full-control type turn-off device is connected with a terminal on the low-voltage side and is connected with a terminal on the high-voltage side and is used as a positive output terminal of the first half-bridge structure, the second full-control type turn-off device is connected with a terminal on the low-voltage side and is connected with the cathode of the first capacitor and is used as a negative output terminal of the first half-bridge structure, and the first diode and the second diode are respectively connected with the first full-control type turn-off device and the second full-control type turn-off device in anti-parallel;
the second half-bridge structure consists of a third full-control type turn-off device, a third diode, a fourth full-control type turn-off device, a fourth diode, a plurality of bidirectional thyristors and a second capacitor, wherein the bidirectional thyristors and the second capacitor are connected in series with the third full-control type turn-off device; the other end of the second bidirectional thyristor is connected with a fourth fully-controlled turn-off device and a low-voltage side terminal, a high-voltage side terminal of the fourth fully-controlled turn-off device is connected with the anode of a second capacitor and is simultaneously used as a positive output terminal of a second half-bridge structure, and a third diode and a fourth diode are respectively connected with the third fully-controlled turn-off device and the fourth fully-controlled turn-off device in an anti-parallel mode;
the continuous flow path is connected to the positive polarity end of the capacitor of the first half-bridge module from the negative output terminal of the second half-bridge structure, the negative output terminal of the first half-bridge is connected to the positive output terminal of the second half-bridge, and the positive output terminal of the first half-bridge and the negative output terminal of the second half-bridge are respectively used as the positive output terminal and the negative output terminal of the sub-module;
the bidirectional thyristor comprises a first bidirectional thyristor and a second bidirectional thyristor; the freewheel path includes a first additional diode, a second additional diode;
the first full-control type turn-off device, the second full-control type turn-off device and the first capacitor are sequentially connected to form a loop;
the third full-control type turn-off device, the first bidirectional thyristor, the fourth full-control type turn-off device, the second bidirectional thyristor and the second capacitor are sequentially connected to form a loop;
the cathode of the first additional diode is connected with the anode of the first capacitor, the cathode of the second additional diode is connected with the anode of the first additional diode, and the anode of the second additional diode is connected with the second bidirectional thyristor, the third fully-controlled turn-off device and the negative output terminal; the terminal of the second full-control type turn-off device connected with the first full-control type turn-off device is used as a positive output terminal; and the terminal of the second fully-controlled turn-off device connected with the cathode of the first capacitor is connected with the terminal of the fourth fully-controlled turn-off device connected with the anode of the second capacitor.
2. The submodule of claim 1, wherein when the circuit is operating in the normal state, the first and second triacs are both in the conducting state;
the first additional diode and the second additional diode are kept in a cut-off state due to the fact that the first additional diode and the second additional diode bear back pressure, and no circuit is added.
3. A submodule, characterized in that the submodule is a double half-bridge submodule comprising a first half-bridge configuration, a second half-bridge configuration and a free-wheeling path, wherein,
the first half-bridge structure is composed of a first full-control type turn-off device, a first diode, a second full-control type turn-off device, a second diode and a first capacitor, wherein the first full-control type turn-off device is connected with a terminal on the high-voltage side and is connected with the anode of the first capacitor, the first full-control type turn-off device is connected with a terminal on the low-voltage side and is connected with a terminal on the high-voltage side and is used as a positive output terminal of the first half-bridge structure, the second full-control type turn-off device is connected with a terminal on the low-voltage side and is connected with the cathode of the first capacitor and is used as a negative output terminal of the first half-bridge structure, and the first diode and the second diode are respectively connected with the first full-control type turn-off device and the second full-control type turn-off device in anti-parallel;
the second half-bridge structure consists of a third full-control type turn-off device, a third diode, a fourth full-control type turn-off device, a fourth diode, a plurality of bidirectional thyristors and a second capacitor, wherein the bidirectional thyristors are connected with the fourth diode in series and comprise a first bidirectional thyristor and a second bidirectional thyristor; the freewheel path includes a first additional diode, a second additional diode;
the first full-control type turn-off device, the second full-control type turn-off device and the first capacitor are sequentially connected to form a loop;
the third full-control type turn-off device, the first bidirectional thyristor, the fourth full-control type turn-off device, the second bidirectional thyristor and the second capacitor are sequentially connected to form a loop;
the first diode, the second diode, the third diode and the fourth diode are respectively connected with the first full-control type turn-off device, the second full-control type turn-off device, the third full-control type turn-off device and the fourth full-control type turn-off device in an anti-parallel mode;
the cathode of the first additional diode is connected with the anode of the first capacitor; the cathode of the second additional diode is connected with the anode of the first additional diode, and the anode of the second additional diode is connected with the second bidirectional thyristor, the cathode of the second capacitor and the negative output terminal of the submodule; the terminal of the second full-control type turn-off device connected with the first full-control type turn-off device is used as a positive output terminal of the sub-module; and the terminal of the second fully-controlled turn-off device connected with the negative electrode of the first capacitor is connected with the terminal of the fourth fully-controlled turn-off device connected with the first bidirectional thyristor.
4. The submodule of claim 3, wherein when the circuit is operating in the normal state, the first and second triacs are both in the conducting state;
the first additional diode and the second additional diode are kept in a cut-off state due to the fact that the first additional diode and the second additional diode bear back pressure, and no circuit is added.
5. A method for protecting a sub-module according to any one of claims 1 to 4, wherein when a system using the sub-module according to any one of claims 1 to 4 encounters a DC-side fault condition and needs to latch up the module to block the fault current, then:
two bidirectional thyristors in the second half-bridge of the submodule and all fully-controlled turn-off devices in the submodule are blocked in sequence;
the turn-off sequence of the two triacs is related to their position, requiring an earlier blocking pulse when the triac is connected in series with the fully controlled turn-off device controlling the bypass of the second half-bridge structure, and requiring a later blocking pulse when the triac is connected in series with the fully controlled turn-off device controlling the access to the second half-bridge structure.
6. A converter, comprising: three phase units, each phase unit comprising: the bridge comprises an upper bridge arm and a lower bridge arm, wherein each bridge arm comprises a plurality of double-half-bridge sub-modules connected in series;
the double half-bridge sub-module is as claimed in any one of claims 1 to 4;
the number of the double half-bridge sub-modules connected in series with the upper bridge arm and the lower bridge arm of each phase unit is the same;
the upper bridge arm and the lower bridge arm of each phase unit are respectively connected with a current-limiting reactor in series;
each phase unit is as follows from top to bottom: all double-half-bridge sub-modules of the upper bridge arm, the reactor of the lower bridge arm and all double-half-bridge sub-modules of the lower bridge arm;
the joint of the upper bridge arm and the lower bridge arm of each phase unit is externally connected with a three-phase alternating current voltage;
and the positive output terminal of the uppermost double-half-bridge submodule of the upper bridge arm is connected with the positive electrode of the direct current bus, and the negative output terminal of the lowermost double-half-bridge submodule of the lower bridge arm is connected with the negative electrode of the direct current bus.
7. The converter according to claim 6, wherein in the DC power transmission system, when a bipolar short-circuit fault is detected on the DC side, the trigger pulses of the second triac, the first triac and all the fully-controlled turn-off devices in each of the double half-bridge submodules are sequentially blocked;
a fault current will flow from the negative output terminal and along the second additional diode, the first capacitor and the second diode and finally out from the positive output terminal.
8. The converter according to claim 6, wherein in the DC power transmission system, when the fault is a temporary DC side fault, the trigger pulses of the second triac, the first triac and all the fully-controlled turn-off devices in each of the double half-bridge submodules are sequentially blocked;
when the alternating current side cuts off and trips, and after a certain time, a second bidirectional thyristor, a first bidirectional thyristor and all fully-controlled turn-off devices in each double half-bridge submodule are sequentially turned on;
if the direct current side has no overcurrent, the alternating current side is reclosed, and the fault is cleared after the reclosing is successful;
if the direct current side has overcurrent, the second bidirectional thyristor, the first bidirectional thyristor and all the rest fully-controlled turn-off devices of each double-half-bridge submodule are sequentially turned off again;
when more than three times of overcurrent occurs on the direct current side, permanent faults are considered to occur.
9. The converter according to claim 8, wherein in the dc power transmission system, when the fault is a permanent dc-side fault, the trigger pulses of the second triac, the first triac and all the fully-controlled turn-off devices of each of the double half-bridge submodules are sequentially blocked;
waiting for the current breaking of the alternating current side and the direct current side, tripping an alternating current side breaker after the current breaking of the alternating current side, tripping a direct current side switch after the current breaking of the direct current side, and performing direct current line maintenance after the direct current side switch is switched off; after the fault is repaired, the direct-current side switch is closed, and the second bidirectional thyristor, the first bidirectional thyristor and all the fully-controlled turn-off devices of each double-half-bridge submodule are sequentially turned on;
if no overcurrent phenomenon occurs on the direct current side, reclosing the alternating current side, and recovering the normal operation of the converter;
if the overcurrent phenomenon occurs on the direct current side, the line fault is not cleared, the trigger pulses of the second bidirectional thyristor, the first bidirectional thyristor and all the fully-controlled turn-off devices of each double-half-bridge submodule are blocked again in sequence, and the alternating current side switch and the direct current side switch are disconnected for maintenance.
10. A dc-dc converter, comprising: three phase cells, each of the phase cells comprising: the three bridge arms are in Y-shaped connection, and non-common ends are respectively connected with a positive electrode of a high-voltage side bus, a positive electrode of a low-voltage side bus and the ground;
the bridge arm connected with the positive pole of the high-voltage side bus is recorded as a high-voltage side bridge arm, the bridge arm connected with the positive pole of the low-voltage side bus is recorded as a low-voltage side bridge arm, and the bridge arm connected with the ground is recorded as an auxiliary bridge arm; wherein the content of the first and second substances,
each of the high side bridge arm and the auxiliary bridge arm includes: a plurality of half-bridge sub-modules connected in series and an inductor;
the low-voltage side bridge arm includes: a plurality of double half-bridge sub-modules connected in series;
the double half-bridge sub-module is as claimed in any one of claims 1 to 4.
11. The dc-dc converter according to claim 10, wherein when a bipolar short-circuit fault on the high-voltage side or the low-voltage side is detected in the dc transmission system, the trigger pulses of the second triac, the first triac, all the fully-controlled turn-off devices in the dual-half-bridge submodules, and the fully-controlled turn-off devices in the half-bridge submodules in the high-voltage side bridge arm and the auxiliary bridge arm are sequentially blocked to realize fault blocking.
12. A direct current transformer, comprising: the system comprises a first subsystem, a three-phase power frequency transformer and a second subsystem which are sequentially connected; wherein the content of the first and second substances,
the first subsystem and the second subsystem respectively include: three phase units, each phase unit comprising: an upper bridge arm and a lower bridge arm; each bridge arm comprises a plurality of double half-bridge submodules connected in series;
the double half-bridge sub-module is as claimed in any one of claims 1 to 4;
the number of the double half-bridge sub-modules of each phase unit, which are connected in series with the upper bridge arm and the lower bridge arm, is the same;
each phase unit comprises the following components in sequence from top to bottom: all double-half-bridge sub-modules of the upper bridge arm, the reactor of the lower bridge arm and all double-half-bridge sub-modules of the lower bridge arm;
the joint of the upper bridge arm and the lower bridge arm of each phase unit is connected with the three-phase power frequency transformer;
and the positive output terminal of the uppermost double-half-bridge submodule of the upper bridge arm of each phase unit of the first subsystem and the second subsystem is connected with the positive electrode of the direct current bus of the corresponding subsystem, and the negative output terminal of the lowermost double-half-bridge submodule of the lower bridge arm is connected with the negative electrode of the direct current bus of the corresponding subsystem.
13. The direct current transformer according to claim 12, wherein in the direct current transmission system, when a bipolar short-circuit fault is detected on the direct current side of the first subsystem and/or the second subsystem, the gate trigger pulses of the second bidirectional thyristors, the first bidirectional thyristors and all fully-controlled switches of all the double-half-bridge submodules in the first subsystem and/or the second subsystem are sequentially blocked;
all the fully-controlled switches comprise: the first full-control type turn-off device, the second full-control type turn-off device, the third full-control type turn-off device and the fourth full-control type turn-off device;
after waiting for the fault current to return to zero, the dc side switch is opened for service.
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