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CN114006544B - Active commutation unit and hybrid converter topological structure - Google Patents

Active commutation unit and hybrid converter topological structure Download PDF

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Publication number
CN114006544B
CN114006544B CN202111485868.1A CN202111485868A CN114006544B CN 114006544 B CN114006544 B CN 114006544B CN 202111485868 A CN202111485868 A CN 202111485868A CN 114006544 B CN114006544 B CN 114006544B
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China
Prior art keywords
branch
buffer
parallel
diode
power device
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CN202111485868.1A
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CN114006544A (en
Inventor
杨俊�
李婷婷
王蒲瑞
欧阳文敏
贺冬珊
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Global Energy Interconnection Research Institute
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Global Energy Interconnection Research Institute
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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
    • 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
    • H02M7/00—Conversion of AC power input into DC power output; Conversion of DC power input into AC power output
    • H02M7/66—Conversion of AC power input into DC power output; Conversion of DC power input into AC power output with possibility of reversal
    • H02M7/68—Conversion of AC power input into DC power output; Conversion of DC power input into AC power output with possibility of reversal by static converters
    • H02M7/72—Conversion of AC power input into DC power output; Conversion of DC power input into AC power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/75—Conversion of AC power input into DC power output; Conversion of DC power input into AC power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means
    • H02M7/757—Conversion of AC power input into DC power output; Conversion of DC power input into AC power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only
    • H02M7/7575—Conversion of AC power input into DC power output; Conversion of DC power input into AC power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only for high voltage direct transmission link
    • 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]

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Rectifiers (AREA)
  • Inverter Devices (AREA)

Abstract

The invention discloses an active phase-change unit and a hybrid converter topological structure, wherein the active phase-change unit comprises: a main branch, on which a thyristor valve is arranged; the auxiliary branch is connected with the main branch in parallel, and is provided with a first control valve which has the functions of controllably switching off forward current and blocking forward and reverse voltage; a second control valve connected with the thyristor valve or with the first control valve, comprising at least one power unit comprising: the first branch is sequentially connected with a first diode and a first power device in series; the second branch is connected with the first branch in parallel, and a second power device and a second diode are sequentially connected in series on the second branch; the first branch and the second branch form a full bridge, and the first power device and the second power device are power electronic devices with turn-off functions. By implementing the invention, the active commutation of each bridge arm is realized, the commutation failure is avoided, and the stable and safe operation of the power grid is ensured.

Description

Active commutation unit and hybrid converter topological structure
Technical Field
The invention relates to the technical field of current conversion in power electronics, in particular to an active current conversion unit and a hybrid current converter topological structure.
Background
The traditional power grid commutation high-voltage direct current (line commutated converter high voltage direct current, LCC-HVDC) power transmission system has the advantages of long-distance large-capacity power transmission, controllable active power and the like, and is widely applied worldwide. The converter is used as core equipment for direct current transmission, is a core functional unit for realizing alternating current and direct current electric energy conversion, and the operation reliability of the converter determines the operation reliability of an extra-high voltage direct current power grid to a great extent.
At present, a capacitive commutation converter or a hybrid converter formed by connecting a turn-off device and a thyristor in series is generally adopted to realize the conversion of alternating current and direct current electric energy. The capacitor commutation converter increases the valve commutation voltage time area through capacitor voltage to ensure reliable turn-off, but a controllable capacitor module is formed by combining a power electronic switch and a capacitor to realize controllable capacitor input and voltage direction, and a single-stage capacitor is required to take a larger value in order to ensure reliable commutation, so that the voltage and current stress of a thyristor which is a core component is increased, and the implementation difficulty of the topological structure engineering is higher; the turn-off device and the thyristor are connected in series to form the hybrid current converter, so that each bridge arm of the current converter has turn-off capability, commutation failure is avoided, but because of large transmission capacity of conventional direct current transmission, each bridge arm of the current converter bears high voltage and high current, and the realization cost and difficulty are high. Because the traditional converter mostly adopts semi-controlled device thyristors as core components to form a six-pulse bridge converter topology, each bridge arm is formed by connecting multistage thyristors and buffer components thereof in series, and the thyristors do not have self-turn-off capability, the phase conversion failure easily occurs under the conditions of AC system faults and the like, so that the direct current is increased rapidly and the direct current transmission power is lost rapidly and greatly, and the stable and safe operation of a power grid is influenced.
Disclosure of Invention
In view of the above, the embodiment of the invention provides an active commutation unit and a hybrid converter topology structure to solve the problem that the stable operation of a power grid is affected due to commutation failure.
According to a first aspect, an embodiment of the present invention provides an active commutation unit, disposed in a bridge arm circuit of a converter, with one end connected to an output end of a converter transformer and the other end connected to a dc bus, the active commutation unit including: a main branch circuit, wherein a thyristor valve is arranged on the main branch circuit; the auxiliary branch is connected with the main branch in parallel, and is provided with a first control valve which has a forward current controllable turn-off function and a forward and reverse voltage blocking function; a second control valve connected with the thyristor valve of the main branch or with the first control valve of the auxiliary branch, the second control valve comprising at least one power unit comprising: the first branch is sequentially connected with a first diode and a first power device in series; the second branch is connected with the first branch in parallel, and a second power device and a second diode are sequentially connected in series on the second branch; the first branch and the second branch form a full bridge, and the first power device and the second power device are power electronic devices with turn-off functions.
With reference to the first aspect, in a first implementation manner of the first aspect, the power unit further includes: and one end of the capacitor element is connected between the first diode and the first power device, and the other end of the capacitor element is connected between the second power device and the second diode.
With reference to the first implementation manner of the first aspect, in a second implementation manner of the first aspect, the power unit further includes: and the protection element is arranged in parallel with the second branch and the first branch and is used for transient overvoltage protection.
With reference to the second embodiment of the first aspect, in a third embodiment of the first aspect, the protection element is a lightning arrester.
With reference to the first aspect, in a fourth implementation manner of the first aspect, the power unit further includes: at least one buffer component, wherein the buffer component is arranged in the power device in parallel; the cushioning member includes: a first buffer branch composed of a second capacitive element; or, the first resistor and the third capacitor element are connected in series with the second buffer branch; or, a third buffer branch connected in parallel with the first resistor and the third capacitor element; or the first resistor is connected in parallel with the third diode and then connected in series with the fourth capacitor element to form a fourth buffer branch; or, the second resistor and the fifth capacitor element are connected in parallel and then connected in series with the fourth diode to form a fifth buffer branch; or a sixth buffer branch consisting of lightning arresters; or, a seventh buffer branch formed by a plurality of the first buffer branch, the second buffer branch, the third buffer branch, the fourth buffer branch, the fifth buffer branch and the sixth buffer branch in parallel is provided.
With reference to the fourth implementation manner of the first aspect, in a fifth implementation manner of the first aspect, the buffer component is disposed in parallel to two ends of the first diode and two ends of the second diode.
With reference to the fourth implementation manner of the first aspect, in a sixth implementation manner of the first aspect, the buffer components are disposed in parallel at two ends of the first power device and two ends of the second power device.
With reference to the fourth embodiment of the first aspect, in a seventh embodiment of the first aspect, the buffer component is disposed in parallel at two ends of the first diode, two ends of the second diode, two ends of the first power device, and two ends of the second power device.
With reference to the fourth embodiment of the first aspect, in an eighth embodiment of the first aspect, the buffer members are disposed in parallel at both ends of the first branch and both ends of the second branch.
According to a second aspect, an embodiment of the present invention provides a hybrid converter topology, where the topology is connected to an ac power grid through a converter transformer, the topology includes a three-phase six-leg circuit, each leg includes an upper leg and a lower leg, and at least one upper leg or lower leg is provided with an active commutation unit according to the first aspect or any implementation manner of the first aspect.
The technical scheme of the invention has the following advantages:
1. The active phase-change unit comprises a main branch and an auxiliary branch which are connected in parallel, and a second control valve arranged on the main branch or the auxiliary branch, wherein the main branch is provided with a thyristor valve, the auxiliary branch is provided with a first control valve with forward and reverse voltage blocking capability, the second control valve is connected with the thyristor valve on the main branch or the first control valve on the auxiliary branch, the active phase-change unit comprises at least one power unit, the power unit comprises a first branch and a second branch which are arranged in parallel, and the first branch and the second branch form a full bridge. The first branch is sequentially connected with a first diode and a first power device in series, the second branch is sequentially connected with a second power device and a second diode in series, the first power device and the second power device are power electronic devices with turn-off functions, and therefore the power devices have controllable turn-off of forward current and blocking of forward and reverse voltage, the second control valve has unidirectional voltage output or unidirectional controllable turn-off capacity, the second control valve is guaranteed to have larger through-flow capacity, normal operation current is borne, and current is transferred from the main branch to the auxiliary branch through the second control valve. The active commutation unit controls the forward turn-off voltage through the second control valve to prolong the reverse recovery time of the main branch thyristor valve, so that the main branch thyristor valve is ensured to be reliably turned off, the active commutation of each bridge arm is further realized, the commutation failure is avoided, and the stable and safe operation of the power grid is ensured.
2. The hybrid converter topological structure provided by the embodiment of the invention comprises a three-phase six-bridge-arm circuit, wherein each phase of bridge arm respectively comprises an upper bridge arm and a lower bridge arm, and at least one upper bridge arm or lower bridge arm is provided with an active phase-changing unit. The second control valve in the active phase-change unit can switch off the main branch current in advance and provide reverse voltage, so that the phase-change voltage-time area of the main branch thyristor valve is increased, the reliable switch-off of the main branch thyristor valve is ensured, the problem of phase-change failure is avoided, and the stable and safe operation of a power grid is ensured.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are needed in the description of the embodiments or the prior art will be briefly described, and it is obvious that the drawings in the description below are some embodiments of the present invention, and other drawings can be obtained according to the drawings without inventive effort for a person skilled in the art.
FIG. 1 is a block diagram of an active commutation cell according to an embodiment of the present invention;
FIG. 2 is a block diagram of a second control valve according to an embodiment of the present invention;
FIG. 3 is another block diagram of the second control valve of an embodiment of the present invention;
FIG. 4 is another block diagram of the second control valve of an embodiment of the present invention;
FIG. 5 is another block diagram of the second control valve of an embodiment of the present invention;
FIG. 6 is a block diagram of the structure of a cushioning component of an embodiment of the present invention;
FIG. 7 is another block diagram of the second control valve of an embodiment of the present invention;
FIG. 8 is another block diagram of the second control valve of an embodiment of the present invention;
FIG. 9 is another block diagram of the second control valve of the embodiment of the present invention;
FIG. 10 is another block diagram of the second control valve of the embodiment of the present invention;
fig. 11 is a block diagram of a thyristor valve according to an embodiment of the invention;
FIG. 12 is a block diagram of a first control valve according to an embodiment of the present invention;
fig. 13 is a hybrid converter topology of an embodiment of the invention;
fig. 14 is another topology of a hybrid converter according to an embodiment of the invention;
fig. 15 is another topology of a hybrid converter according to an embodiment of the invention;
FIG. 16 is a current flow path for a normal operating condition of an embodiment of the present invention;
FIG. 17 is a trigger control sequence for a normal operating condition of an embodiment of the present invention;
FIG. 18 is a schematic diagram of the current path of the main leg commutating to the auxiliary leg according to an embodiment of the present invention;
FIG. 19 is a trigger control sequence for a commutation failure or short circuit fault in accordance with an embodiment of the present invention;
fig. 20 is a control trigger timing of detecting commutation failure or short-circuit failure in advance according to an embodiment of the present invention.
Detailed Description
For the purpose of making the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is apparent that the described embodiments are some embodiments of the present invention, but not all embodiments of the present invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
The converter is used as core equipment for direct current transmission, is a core functional unit for realizing alternating current and direct current electric energy conversion, and the operation reliability of the converter determines the operation reliability of an extra-high voltage direct current power grid to a great extent. However, as the traditional converter mostly adopts semi-controlled device thyristors as core components to form a six-pulse bridge converter topology, each bridge arm is formed by connecting multistage thyristors and buffer components thereof in series, and the thyristors do not have self-turn-off capability, the converter failure easily occurs under the conditions of AC system faults and the like, so that the DC current is increased rapidly and the DC transmission power is lost rapidly and greatly, and the stable and safe operation of a power grid is influenced.
Based on the above, the technical scheme of the invention utilizes the advantages that the thyristor, the first control valve and the second control valve can be turned off, adopts two branches to be connected in parallel, can provide reverse voltage and an auxiliary branch with self-turn-off capability on the basis of the main branch to realize reliable turn-off of the main branch and active phase change of the whole bridge arm, thereby realizing the auxiliary phase change function in a short time, avoiding phase change failure and ensuring stable and safe operation of the power grid.
According to an embodiment of the present invention, an embodiment of an active commutation cell is provided, which is arranged in a bridge arm circuit of a converter. One end of the active commutation unit is connected with the output end of the converter transformer, and the other end is connected with the direct current bus, as shown in fig. 1, the active commutation unit comprises: a main branch and an auxiliary branch. Wherein the main branch is provided with a thyristor valve V11, as shown in fig. 11, the thyristor valve V11 comprises at least one thyristor J1 and a buffer member 12 connected in parallel or in series with the thyristor J1, respectively, wherein the at least one thyristor J1 is arranged in series, the buffer member 12 being for a thyristor device from being damaged by high voltage and high current.
The auxiliary branch is connected with the main branch in parallel, a first control valve V13 is arranged on the auxiliary branch along the direction from the output end of the converter transformer to the direct current bus, and the first control valve V13 has a forward current controllable turn-off function and a forward and reverse voltage blocking function. As shown in fig. 12, the first control valve V13 includes a power device W and a thyristor J2 that are arranged in series, where the thyristor J2 has a reverse blocking function, the power device W has a controllable turn-off function of forward current and a blocking function of forward and reverse voltage, and the power device W is a power electronic device having a turn-off function, and the power electronic device is one or more of IGBT, IGCT, IEGT, GTO or MOSFET or other turn-off devices.
The first control valve V13 is a high-voltage shutoff valve, and of course, the first control valve V13 may also be a series connection of a plurality of power devices and thyristors, so long as the topology form of the function of controllably shutting off forward current and blocking forward and reverse voltage can be realized, and the topology form of the first control valve V13 is not limited in the present application.
The second control valve V12 may be connected to the thyristor valve V11 of the main branch, or may be connected to the first control valve V13 of the auxiliary branch, where the first control valve V12 is a low-voltage shutoff valve having unidirectional voltage controllable output capability or unidirectional controllable shutoff function, and as shown in fig. 2, the second control valve V12 includes: at least one power cell 11, the power cell 11 comprising: a first branch and a second branch.
The first branch is sequentially connected with a first diode D1 and a first power device W1 in series; the second branch is connected with the first branch in parallel, the second branch is sequentially connected with a second power device W2 and a second diode D2 in series, and the first branch and the second branch form a full bridge. Specifically, the power unit 11 includes a first connection end O1 and a second connection end O2, the anode of the first diode D1 is connected with the first connection end O1, the cathode of the first diode D1 is connected with one end of the first power device W1, the other end of the first power device W1 is connected with the second connection end O2, one end of the second power device W2 is connected with the first connection end O1, the other end of the second power device W2 is connected with the anode of the second diode D2, the cathode of the second diode D2 is connected with the second connection end O2, and the first diode D1, the first power device W1, the second power device W2 and the second diode D2 form a full bridge. The first power device W1 and the second power device W2 are power electronic devices with a turn-off function, and the power electronic devices are one or more of IGBT, IGCT, IEGT, GTO or MOSFET and other turn-off devices. The power unit 11 can realize controllable turn-off of forward current and blocking of forward and reverse voltage, so that the first control valve V12 has a function of outputting controllable turn-off of unidirectional voltage, so as to turn off the current of the main branch and provide reverse voltage for the current, and ensure that the thyristor valve of the main branch has enough turn-off time to be reliably turned off.
The active phase-change unit provided by the embodiment comprises a main branch and an auxiliary branch which are connected in parallel, and a second control valve arranged on the main branch or the auxiliary branch, wherein the main branch is provided with a thyristor valve, the auxiliary branch is provided with a first control valve with forward and reverse voltage blocking capability, the second control valve is connected with the thyristor valve of the main branch or the first control valve of the auxiliary branch, the active phase-change unit comprises at least one power unit, the power unit comprises a first branch and a second branch which are arranged in parallel, and the first branch and the second branch form a full bridge. The first branch is sequentially connected with a first diode and a first power device in series, the second branch is sequentially connected with a second power device and a second diode in series, the first power device and the second power device are power electronic devices with turn-off functions, and therefore the power devices have controllable turn-off of forward current and blocking of forward and reverse voltage, the second control valve has unidirectional voltage output or unidirectional controllable turn-off capacity, the second control valve is guaranteed to have larger through-flow capacity, normal operation current is borne, and current is transferred from the main branch to the auxiliary branch through the second control valve. The active commutation unit controls the forward turn-off voltage through the second control valve to prolong the reverse recovery time of the main branch thyristor valve, so that the main branch thyristor valve is ensured to be reliably turned off, the active commutation of each bridge arm is further realized, the commutation failure is avoided, and the stable and safe operation of the power grid is ensured.
Optionally, as shown in fig. 3, the power unit 11 may further include: a first capacitive element C1. One end of the first capacitive element C1 is connected between the first diode D1 and the first power device W1, and the other end of the first capacitive element C1 is connected between the second power device W2 and the second diode D2.
Optionally, as shown in fig. 4, the power unit 11 may further include: and a protective element B. The protection element B is used for transient overvoltage protection. Specifically, the protection element B is arranged in parallel with the second branch and the first branch.
As shown in fig. 5, the protection element B may further have one end connected to the first connection terminal O1 of the first power unit 11 and the other end connected to the second connection terminal O2 of the last power unit 11.
Alternatively, the protection element B may be a lightning arrester, but may be any other element capable of achieving overvoltage protection, which is not specifically limited herein.
Optionally, the power unit 11 may further include: at least one buffer component 12, the buffer component 12 is arranged in parallel in the power unit 11, so as to avoid the damage of the second control valve caused by high voltage and high current, and ensure the stable operation of the second control valve V12. As shown in fig. 6, the buffer member 12 is formed of one or more types of members such as a capacitor, a resistive-capacitive circuit, a diode, an inductor, and a lightning arrester.
Specifically, the buffer member 12 may be a first buffer branch composed of a second capacitive element; a second buffer branch connected in series by a first resistor and a third capacitive element; a third buffer branch connected in parallel by a first resistor and a third capacitive element; a fourth buffer branch RCD1 formed by connecting a first resistor and a third diode in parallel and connecting a fourth capacitor element in series; a fifth buffer branch RCD2 formed by connecting a second resistor and a fifth capacitor in parallel and then connecting a fourth diode in series; the sixth buffer branch is also formed by lightning arresters; the buffer circuit may further include a seventh buffer branch formed by connecting a plurality of the first buffer branch, the second buffer branch, the third buffer branch, the fourth buffer branch, the fifth buffer branch, and the sixth buffer branch in parallel.
Alternatively, as shown in fig. 7, the buffer member 12 may be disposed in parallel to both ends of the first diode D1 and both ends of the second diode D2, respectively.
Alternatively, as shown in fig. 8, the buffer members 12 may be disposed in parallel at both ends of the first power device W1 and both ends of the second power device W2, respectively.
Alternatively, as shown in fig. 9, the buffer member 12 may be respectively disposed in parallel at both ends of the first diode D1, both ends of the second diode D2, both ends of the first power device W1, and both ends of the second power device W2.
Alternatively, as shown in fig. 10, the buffer member 12 may be disposed in parallel at both ends of the first branch and both ends of the second branch.
The second control valve V12 is a low-voltage shutoff valve, and has a unidirectional voltage controllable output capability or a unidirectional controllable shutoff function, and is mainly used for shutting off the main branch current and providing reverse voltage for the main branch current, so that the thyristor valve of the main branch is ensured to have enough shutoff time for reliable shutoff. The topology of the second control valve V12 is not limited in the present application, and any topology may be used as long as it has a unidirectional voltage controllable output or unidirectional controllable shut-off function. The topology of the power unit may be the matching of the power electronic device without reverse blocking function and the diode, the power electronic device without reverse blocking function, the single-stage diode and the buffer component may be matched to form a multistage serial structure, the multistage power electronic device without reverse blocking function and the buffer component combination thereof may be serial to the multistage diode and the buffer component combination thereof, or the multistage power electronic device without reverse blocking function and the multistage diode may be alternatively serial to each other, or of course, other topology may be used, which is not limited herein specifically, and the skilled person may determine according to the actual needs.
According to an embodiment of the invention, a hybrid converter topology is provided, which is connected to an ac power grid through a converter transformer. The topological structure of the hybrid converter comprises a three-phase six-bridge arm circuit, each phase bridge arm comprises an upper bridge arm and a lower bridge arm, and at least one upper bridge arm or lower bridge arm of the hybrid converter is provided with the active phase-changing unit described in the embodiment.
Specifically, the hybrid converter topology as depicted in fig. 13 includes 3 upper legs and 3 lower legs. Each active commutation unit is used as a converter valve, and the hybrid converter topology structure of forced commutation described in fig. 13 includes a converter valve V1, a converter valve V2, a converter valve V3, a converter valve V4, a converter valve V5 and a converter valve V6. The main branches of the 3 upper bridge arms respectively comprise thyristor valves V11, V31, V51 and second control valves V12, V32 and V52, and the auxiliary branches of the 3 upper bridge arms respectively comprise first control valves V13, V33 and V53. The main branches of the 3 lower bridge arms respectively comprise thyristor valves V21, V41, V61 and second control valves V22, V42 and V62, and the auxiliary branches of the 3 lower bridge arms respectively comprise first control valves V23, V43 and V63. The thyristor valve, the first control valve and the second control valve are controlled to be turned off and turned on by controlling the trigger control system.
Specifically, the hybrid converter topology as depicted in fig. 14 includes 3 upper legs and 3 lower legs. Each active commutation unit is used as a converter valve, and the topology structure of the forced commutation hybrid converter shown in fig. 14 includes a converter valve V1, a converter valve V2, a converter valve V3, a converter valve V4, a converter valve V5 and a converter valve V6. The main branches of the 3 upper bridge arms respectively comprise thyristor valves V11, V31 and V51, and the auxiliary branches of the 3 upper bridge arms respectively comprise first control valves V13, V33 and V53 and second control valves V12, V32 and V52. The main branches of the 3 lower bridge arms respectively comprise thyristor valves V21, V41 and V61, and the auxiliary branches of the 3 lower bridge arms respectively comprise first control valves V23, V43 and V63 and second control valves V22, V42 and V62. The thyristor valve, the first control valve and the second control valve are controlled to be turned off and turned on by controlling the trigger control system.
Specifically, the hybrid converter topology shown in fig. 15 includes 3 upper legs and 3 lower legs. Each active commutation unit is used as a converter valve, and the topology structure of the forced commutation hybrid converter shown in fig. 15 includes a converter valve V1, a converter valve V2, a converter valve V3, a converter valve V4, a converter valve V5 and a converter valve V6. The main branches of the 3 upper bridge arms respectively comprise thyristor valves V11, V31 and V51, and the auxiliary branches of the 3 upper bridge arms respectively comprise first control valves V13, V33 and V53. The main branches of the 3 lower bridge arms respectively comprise thyristor valves V21, V41 and V61, and the auxiliary branches of the 3 lower bridge arms respectively comprise first control valves V23, V43 and V63. One ends of the commutation control valves Va2, vb2 and Vc2 are respectively connected between the upper bridge arm and the lower bridge arm, the other ends are respectively connected to the output ends of the converter transformer, the commutation control valves Va2, vb2 and Vc2 can have a forward and reverse current turn-off function and a forward and reverse voltage output function at the same time, and the commutation control valves can be formed by forward and reverse series connection of the second control valves V12 in the above embodiment so as to be shared by the upper bridge arm and the lower bridge arm of each phase, so that the number of the second control valves is reduced, and the topological structure of the hybrid converter is simplified. The thyristor valve, the first control valve and the second control valve are controlled to be turned off and turned on by the control trigger control system, so that the reliable turn-off of the main branch and the active phase change of the whole bridge arm are realized.
According to the hybrid converter topological structure, the reverse voltage and the auxiliary branch with the self-turn-off capability can be provided on the basis of the thyristor valve in parallel, so that the reliable turn-off of the main branch and the active phase change of the whole bridge arm are realized, namely, the turn-off control valve is introduced for each bridge arm.
The hybrid converter topological structure provided by the embodiment comprises three-phase six-bridge-arm circuits, wherein each phase of bridge arm respectively comprises an upper bridge arm and a lower bridge arm, and at least one upper bridge arm or lower bridge arm is provided with an active phase-changing unit. The second control valve in the active phase-change unit can switch off the main branch current in advance, and simultaneously provides reverse voltage, so that the phase-change voltage-time area of the main branch thyristor valve is increased, the reliable switch-off of the main branch thyristor valve is ensured, the problem of phase-change failure is avoided, and the stable and safe operation of a power grid is ensured.
In this embodiment, a commutation control method is provided, which can be used in the hybrid converter topology structure described above, taking the setting of the second control valve V12 in the auxiliary branch as an example, and the commutation control method includes the following steps:
(1) And switching on a thyristor valve of a main branch of an ith bridge arm of the topological structure of the hybrid converter.
(2) And the first control valve and the second control valve of the auxiliary branch of the ith bridge arm of the topological structure of the hybrid converter are conducted.
(3) And switching off the first control valve and the second control valve of the auxiliary branch of the ith bridge arm of the topological structure of the hybrid converter.
(4) After a control period, the thyristor valve of the main branch of the ith bridge arm of the topological structure of the hybrid converter is conducted, wherein i is E [1,6].
Specifically, as shown in fig. 16, which shows the valve current flow path of the hybrid converter topology under normal operation conditions, the main branch is periodically subjected to voltage and current stress, and the auxiliary branch is always in an off state, and only subjected to voltage stress when the thyristor valve of the main branch is turned off. The first control valve and the second control valve of the auxiliary branch of the ith bridge arm of the hybrid converter topological structure are kept in an off state, and the thyristor valve of the main branch of the ith bridge arm of the hybrid converter topological structure is conducted, so that the hybrid converter topological structure can work in a normal phase-change operation mode, namely in a temporary phase-change operation mode, the auxiliary branch is in an off state when the hybrid converter normally operates, only bears voltage stress, and the increase of converter loss in long-term operation is reduced.
When commutation failure or alternating current short circuit failure occurs, a first control valve and a second control valve of an auxiliary branch of an ith bridge arm of the hybrid converter topological structure are conducted; and (3) forcedly transferring the current of the main branch to the auxiliary branch, and when the current transfer is completed, switching off a first control valve and a second control valve of the auxiliary branch of the ith bridge arm of the topological structure of the hybrid current converter to realize forcedly commutation of the hybrid current converter. After a control period, the step of turning on the thyristor valve of the main branch of the ith bridge arm of the topological structure of the hybrid converter is returned, and the main branch is continued to independently and normally operate, so that the auxiliary branch is ensured to bear the turn-off voltage stress only when in failure, the loss of the device is reduced, and the service life of the device is prolonged.
Fig. 17 shows a trigger control timing in the normal operation mode, in which t 0 represents an initial trigger timing.
FIG. 18 shows the primary to secondary branch commutation, in which the primary branch begins to undergo voltage stress, and the process is divided into three phases, FIG. 18 (left) shows the primary to secondary branch commutation phase, in which the secondary branch receives a trigger signal to conduct, and then the secondary branch V12 and V13 valves receive a conduction signal to divert the current of the primary branch to the secondary branch and apply a reverse voltage to the primary branch; FIG. 18 (middle) is a secondary leg current flow stage in which the primary leg has been fully turned off and the primary leg current has been fully diverted to the secondary leg; fig. 18 (right) shows the auxiliary branch off phase, in which the auxiliary branch V13 valve is turned off when the off signal is received, and the V1 valve is in the off state for receiving the forward voltage, and then the V12 valve is turned off before or simultaneously with the V11 valve of the main branch in the next control cycle. The operation process can be put into operation when the commutation failure or the predicted commutation failure occurs.
Fig. 19 is a timing of trigger control of the hybrid converter topology upon commutation failure or ac short circuit failure. In fig. 19, after failure of commutation from the V1 valve to the V3 valve is monitored at time t f, the auxiliary branch V13 valve is turned on when the first preset time period Δt 1 passes, the auxiliary branch V12 valve is turned on when the second preset time period Δt 2 passes, and a process of commutation from the main branch to the auxiliary branch is performed, and Δt 2≥Δt1 is equal to or greater than 0. The main branch current I11 gradually decreases to zero, the auxiliary branch current I12 gradually increases, the auxiliary branch V13 valve is turned off after the third preset time period Deltat 3 is passed, the time from zero crossing of the main branch current to the turning-off of the V13 valve is the turning-off time t off of the thyristor valve, and t off is larger than the minimum turning-off time of the thyristor valve to ensure that the thyristor valve V11 has enough time to turn off. After the auxiliary branch V13 valve is closed, the auxiliary branch current commutates to the V3 valve until reaching the direct current Id, so as to finish the commutation from the V1 valve to the V3 valve, successfully resist the commutation failure fault, and then the auxiliary branch V12 valve is closed before the V11 valve of the next control period is opened. When the commutation failure is predicted to occur or the commutation failure is detected to occur, the operation mode is started, the commutation failure can be successfully avoided, the operation mode is stopped when the commutation process of the converter is recovered to be normal, the auxiliary branch is kept in a cut-off state, and the main branch independently and normally operates.
When the commutation fails or the short circuit fails, the topological structure of the hybrid current converter is controlled to start the operation mode of forced commutation, the occurrence of the commutation failure is avoided, the operation mode of forced commutation is exited when the commutation process of the hybrid current converter is recovered to be normal, the auxiliary branch is kept in an off state, and the main branch is independently and normally operated, so that the auxiliary branch is ensured to bear the off voltage stress only when in failure, the loss of a device is reduced, and the service life of the device is prolonged.
Fig. 20 shows control trigger timing when the topology of the hybrid converter detects commutation failure or short-circuit failure in advance, and each valve control trigger timing when the main branch and the auxiliary branch of the V1 valve periodically operate alternately, and the specific operation process is shown in fig. 18. And at the commutation starting time of the V1 valve and the V3 valve, namely, the time delay of the V1 valve trigger pulse Sg1 is 120 degrees, or the auxiliary branch V13 valve is triggered nearby the moment, and the auxiliary branch V12 valve is opened in a short time (for example, 1s, 5s and the like), so that the commutation from the main branch to the auxiliary branch is realized. After the main branch current crosses zero, the main branch V11 valve is turned off and bears reverse voltage, the time from the main branch current crossing zero to the auxiliary branch V13 valve being turned off is the turn-off time t off of the thyristor valve, and the time t off is larger than the minimum turn-off time of the thyristor valve to ensure reliable turn-off, so that the V1 valve current is completely transferred to the auxiliary branch, the auxiliary branch V13 valve starts to be turned off after the delta t, the V1 valve starts to bear forward voltage, and then the auxiliary branch V12 valve is turned off before or simultaneously with the turn-on of the V11 valve in the next working period. In the operation mode, the main branch and the auxiliary branch in the bridge arm of the topological structure of the hybrid converter with forced commutation periodically run alternately, so that the hybrid converter can be in a small turn-off angle operation mode without predicting commutation failure on the basis of having the capability of resisting commutation failure, and reactive power consumption of the hybrid converter is reduced.
According to the commutation control method, through periodic alternate operation of the main branch and the auxiliary branch, commutation failure can be resisted, and prediction of the commutation failure is not needed. Meanwhile, the operation mode that the hybrid current converter works at a small off angle is ensured, and reactive power consumption of the hybrid current converter is reduced.
Although embodiments of the present invention have been described in connection with the accompanying drawings, various modifications and variations may be made by those skilled in the art without departing from the spirit and scope of the invention, and such modifications and variations fall within the scope of the invention as defined by the appended claims.

Claims (8)

1. An active commutation unit, which is disposed in a bridge arm circuit of a converter, one end of which is connected to an output end of a converter transformer, and the other end of which is connected to a dc bus, and the active commutation unit is characterized in that the active commutation unit comprises:
A main branch circuit, wherein a thyristor valve is arranged on the main branch circuit;
the auxiliary branch is connected with the main branch in parallel, and is provided with a first control valve which has a forward current controllable turn-off function and a forward and reverse voltage blocking function;
a second control valve connected with the thyristor valve of the main branch or with the first control valve of the auxiliary branch, the second control valve comprising at least one power unit comprising:
The first branch is sequentially connected with a first diode and a first power device in series;
The second branch is connected with the first branch in parallel, and a second power device and a second diode are sequentially connected in series on the second branch;
the first branch and the second branch form a full bridge, and the first power device and the second power device are power electronic devices with a turn-off function;
The power unit further includes: a first capacitive element having one end connected between the first diode and the first power device and the other end connected between the second power device and the second diode;
The power unit further includes: at least one buffer component, wherein the buffer component is arranged in the power device in parallel;
The cushioning member includes: a first buffer branch composed of a second capacitive element; or, the first resistor and the third capacitor element are connected in series with the second buffer branch; or, a third buffer branch connected in parallel with the first resistor and the third capacitor element; or the first resistor is connected in parallel with the third diode and then connected in series with the fourth capacitor element to form a fourth buffer branch; or, the second resistor and the fifth capacitor element are connected in parallel and then connected in series with the fourth diode to form a fifth buffer branch; or a sixth buffer branch consisting of lightning arresters; or, a seventh buffer branch formed by a plurality of the first buffer branch, the second buffer branch, the third buffer branch, the fourth buffer branch, the fifth buffer branch and the sixth buffer branch in parallel is provided.
2. The active commutation cell of claim 1, wherein the power cell further comprises:
and the protection element is arranged in parallel with the second branch and the first branch and is used for transient overvoltage protection.
3. The active commutation cell of claim 2, wherein the protection element is a lightning arrester.
4. The active commutation cell of claim 1, wherein the buffer member is disposed in parallel across the first diode and across the second diode.
5. The active commutation cell of claim 1, wherein the buffer members are disposed in parallel across the first power device and across the second power device.
6. The active commutation cell of claim 1, wherein the buffer component is disposed in parallel across the first diode, across the second diode, across the first power device, and across the second power device.
7. The active commutation cell of claim 1, wherein the buffer members are disposed in parallel at both ends of the first leg and both ends of the second leg.
8. A hybrid converter topology for access to an ac grid via a converter transformer, the topology comprising a three-phase six-leg circuit, each leg comprising an upper leg and a lower leg, respectively, characterized in that at least one upper leg or lower leg is provided with an active commutation unit according to any one of claims 1-7.
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