CN106787876B - Modularized multi-level converter and high-voltage valve group earth fault protection method thereof - Google Patents

Modularized multi-level converter and high-voltage valve group earth fault protection method thereof Download PDF

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Publication number
CN106787876B
CN106787876B CN201611105372.6A CN201611105372A CN106787876B CN 106787876 B CN106787876 B CN 106787876B CN 201611105372 A CN201611105372 A CN 201611105372A CN 106787876 B CN106787876 B CN 106787876B
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China
Prior art keywords
voltage
bridge arm
valve
bypass
valve bank
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CN106787876A (en
Inventor
林卫星
刘伟增
盛俊毅
侯丹
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Tbea Xi'an Flexible Power T&d Co ltd
TBEA Xinjiang Sunoasis Co Ltd
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Tbea Xi'an Flexible Power T&d Co ltd
TBEA Xinjiang Sunoasis Co Ltd
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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
    • 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
    • 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
    • H02M7/4835—Converters with outputs that each can have more than two voltages levels comprising two or more cells, each including a switchable capacitor, the capacitors having a nominal charge voltage which corresponds to a given fraction of the input voltage, and the capacitors being selectively connected in series to determine the instantaneous output voltage
    • 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
    • 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 modularized multi-level converter and the method for protecting the high-voltage valve group of the modularized multi-level converter from the ground fault are simple in structure, low in cost, convenient to control and accurate in protection value setting. The high-pressure valve bank and the low-pressure valve bank are connected in series on the direct current side and connected in parallel on the alternating current side; each phase of the high-pressure valve bank and the low-pressure valve bank consists of an upper bridge arm and a lower bridge arm, and the upper bridge arm and the lower bridge arm consist of a plurality of blocking sub-modules and a plurality of half-bridge sub-modules in cascade connection; or each of the plurality of blocking sub-modules is formed by cascading; bypasses which are controlled to be switched on and off by thyristors are respectively arranged on the upper bridge arm and the lower bridge arm; when the high-voltage valve bank fails to the ground, a trigger signal is applied to the thyristor in the bypass, so that fault current flows through the bypass, and the fault current is prevented from flowing through the sub-module to charge the capacitance of the sub-module. Through the bypass formed by corresponding bridge arms, when the high-voltage valve bank fails to the ground, overvoltage protection can be provided for the submodule by reasonably controlling the switching state of the thyristor in the bypass.

Description

Modularized multi-level converter and high-voltage valve group earth fault protection method thereof
Technical Field
The invention relates to the technical field of direct current transmission, in particular to a modularized multi-level converter and a high-voltage valve group earth fault protection method thereof.
Background
The modular multilevel converter (Modular Multilevel Converter, MMC) technology has the advantages of modular structure, easy expansion and the like, and has been widely applied to the field of direct current transmission since the proposal.
As MMC technology matures, MMCs will evolve towards higher voltage levels and greater power. In high-voltage and high-power MMCs (e.g., +/-800 kV,5000MW MMC), the valve blocks of the MMC usually adopt a scheme of connecting the high-voltage valve blocks and the low-voltage valve blocks in series so as to reduce the requirement on the rated power of a single converter, and the normal operation of the non-fault valve blocks can be maintained when the single valve blocks are out of operation due to faults.
One serious problem with the high and low voltage valve bank serial schemes is that when the dc negative bus of the high voltage valve bank or the low voltage end of the upper bridge arm of the high voltage valve bank fails to ground, the high voltage valve bank or the upper bridge arm of the high voltage valve bank will bear a rated ground-to-ground dc voltage, which will charge the fault valve bank/the fault bridge arm, resulting in a sudden rise in the capacitance voltage of the submodule of the fault valve bank or the fault bridge arm, thereby damaging the fault valve bank/the bridge arm.
The existing technology for dealing with such faults is generally to connect lightning arresters in parallel to the high and low voltage ends of each bridge arm or each sub-module, so as to provide overvoltage protection for the bridge arm or the sub-module. The scheme has the defect that the rated current and the energy requirement on the lightning arrester are high, so that the system cost is increased, and in the scheme, how to set the protection level of the lightning arrester is a great difficulty. The lightning arrester protection level is set higher, so that effective overvoltage protection cannot be provided for bridge arms/submodules, when the lightning arrester protection level is set lower, a certain current flows through the lightning arrester in a steady state, the loss of the system is increased, the cost is high, and the protection fixed value is difficult to set.
Disclosure of Invention
Aiming at the problems existing in the prior art, the invention provides the modularized multi-level converter and the high-voltage valve group earth fault protection method thereof, which have the advantages of simple structure, low cost, convenient control and accurate protection value setting.
The invention is realized by the following technical scheme:
the invention relates to a modularized multi-level converter, which comprises a high-voltage valve group and a low-voltage valve group which are connected in series on a direct current side and connected in parallel on an alternating current side; each phase of the high-pressure valve bank and the low-pressure valve bank consists of an upper bridge arm and a lower bridge arm, and each of the upper bridge arm and the lower bridge arm consists of a plurality of blocking sub-modules and a plurality of half-bridge sub-modules in cascade connection;
Bypasses which are controlled to be switched on and off by thyristors are respectively arranged on the upper bridge arm and the lower bridge arm; when the high-voltage valve bank fails to the ground, a trigger signal is applied to the thyristor in the bypass, so that fault current flows through the bypass, and the fault current is prevented from flowing through the sub-module to charge the capacitance of the sub-module.
Preferably, the bypass comprises bypass branches corresponding to the submodules one by one; the bypass branch comprises two thyristors connected in parallel in opposite directions of each blocking type submodule voltage output end or one bidirectional thyristor connected in parallel with the output end, and one thyristor or two thyristors connected in parallel in opposite directions of each half-bridge submodule voltage output end or one bidirectional thyristor; the anode of the one thyristor is connected with the high-voltage output end of the half-bridge sub-module, and the cathode of the one thyristor is connected with the low-voltage output end of the half-bridge sub-module.
Preferably, the bypass comprises bypass branches corresponding to the valve sections one by one, and the bypass branches comprise a bidirectional thyristor valve connected in parallel between a high-voltage output end and a low-voltage output end of each valve section formed by connecting blocking-type submodules in series, and a unidirectional thyristor valve or a bidirectional thyristor valve connected in parallel between the high-voltage output end and the low-voltage output end of each valve section formed by connecting half-bridge type submodules in series; the anode of the one-way thyristor valve is connected with the high-voltage output end of the valve section of the corresponding half-bridge submodule, and the cathode of the one-way thyristor valve is connected with the low-voltage output end of the valve section of the corresponding half-bridge submodule.
Preferably, the bypass comprises bypass branches corresponding to each group in the valve section one by one; dividing valve sections formed by connecting blocking sub-modules in series into a plurality of groups; dividing valve sections formed by connecting half-bridge submodules in series into a plurality of groups;
the bypass branch comprises a bidirectional thyristor valve connected with the output end of each group of blocking type submodule in parallel, and a unidirectional thyristor valve or a bidirectional thyristor valve connected with the output end of each group of half-bridge type submodule in parallel; the anode of the one unidirectional thyristor valve is connected with the high-voltage output end corresponding to each group of half-bridge submodules, and the cathode of the one unidirectional thyristor valve is connected with the low-voltage output end corresponding to each group of half-bridge submodules.
The invention relates to a modularized multi-level converter, which is characterized in that a high-voltage valve bank and a low-voltage valve bank are connected in series on a direct current side and in parallel on an alternating current side; each phase of the high-pressure valve bank and the low-pressure valve bank consists of an upper bridge arm and a lower bridge arm, and the upper bridge arm and the lower bridge arm are formed by cascading a plurality of blocking-type submodules;
bypasses controlled by thyristors are respectively arranged on the upper bridge arm and the lower bridge arm; when the high-voltage valve bank fails to the ground, a trigger signal is applied to the thyristor in the bypass, so that fault current flows through the bypass, and the fault current is prevented from flowing through the sub-module to charge the capacitance of the sub-module.
Preferably, the bypass comprises bypass branches corresponding to the blocking sub-modules one by one; the bypass branch comprises a thyristor or two thyristors or a bidirectional thyristor which are connected in parallel at the voltage output end of each blocking submodule in an inverse parallel manner; the anode of a thyristor is connected with the high-voltage output end of the blocking type sub-module, and the cathode of the thyristor is connected with the low-voltage output end of the blocking type sub-module.
Preferably, the bypass comprises bypass branches corresponding to the valve sections one by one, and the bypass branches comprise a unidirectional thyristor valve or a bidirectional thyristor valve which are connected in parallel between a high-voltage output end and a low-voltage output end of each valve section formed by connecting blocking-type submodules in series; the anode of the unidirectional thyristor valve is connected with the high-voltage output end of the corresponding blocking type submodule valve section, and the cathode of the unidirectional thyristor valve is connected with the low-voltage output end of the corresponding blocking type submodule valve section.
Further, the bypass comprises bypass branches corresponding to each group in the valve section one by one; dividing valve sections formed by connecting blocking sub-modules in series into a plurality of groups;
the bypass branch comprises a unidirectional thyristor valve or a bidirectional thyristor valve which are connected in parallel with the output ends of each group of blocking sub-modules; the anode of the one-way thyristor valve is connected with the high-voltage output end of the corresponding blocking type submodule, and the cathode of the one-way thyristor valve is connected with the low-voltage output end of the valve section of the corresponding blocking type submodule.
Still further, the unidirectional thyristor valve is formed by connecting a plurality of thyristors in series; the bidirectional thyristor valve is formed by connecting a plurality of anti-parallel thyristors in series, or is formed by connecting a plurality of bidirectional thyristors in series, or is formed by connecting two anti-parallel unidirectional thyristor valves in parallel.
Still further, characterized by that, there is a current-limiting resistor in each bypass branch, the current-limiting resistor is connected in series in the low-voltage output end or the high-voltage output end of the bypass branch correspondingly.
Still further, the resistance of the current limiting resistor can prevent the bypass thyristor from overcurrent, prevent the alternating current breaker of the modularized multi-level converter from being broken due to the overcurrent, and enable the voltage drop of the current limiting resistor to be lower than the sum of the capacitance voltages of all the sub-modules connected in parallel.
Further, the blocking type sub-module is a modularized multi-level converter power module with the capability of blocking direct current fault current, and any one of a full-bridge sub-module, a clamping double sub-module, a diode clamping sub-module, a self-resistance sub-module and a crosslinking sub-module can be adopted.
The invention relates to a high-voltage valve group earth fault protection method of a modularized multi-level converter, which is characterized in that bypasses which are controlled to be switched on and switched off by thyristors are respectively established on an upper bridge arm and a lower bridge arm of each phase in the modularized multi-level converter; when the ground fault occurs at any point of the upper bridge arm of any phase of the high-voltage valve bank, trigger pulses are applied to thyristors in bypasses of the upper bridge arms of all phases of the high-voltage valve bank, so that the corresponding bypasses of the upper bridge arms of all phases of the high-voltage valve bank are conducted, and fault current flows through the bypasses.
Preferably, the method further comprises the step of maintaining thyristors in bypasses of the lower bridge arm of each phase of the high-voltage valve bank and the bridge arms of the low-voltage valve bank in a locking state.
Preferably, the trigger pulse is applied to the thyristors in the bypass of the upper bridge arm of each phase of the high-voltage valve group continuously or only once.
Preferably, specifically, after the ground fault of any point of the upper bridge arm of any phase of the high-voltage valve bank is monitored, trigger pulse is applied to the thyristors which are controlled to be conducted forward in the bypass of the upper bridge arm of each phase of the high-voltage valve bank only once; and maintaining the locking of thyristors in all other bypasses of the high-voltage valve bank and the low-voltage valve bank, and locking all full-control power electronic device trigger pulses of the high-voltage valve bank and the low-voltage valve bank.
Further, the bypass comprises thyristors for controlling reverse conduction, when the ground fault of any point of the upper bridge arm of any phase of the high-voltage valve bank is monitored, trigger pulses are only applied to the thyristors for controlling forward conduction in the bypass of the upper bridge arm of each phase of the high-voltage valve bank once, and when the thyristors for controlling forward conduction in the bypass of the upper bridge arm of the fault phase are monitored to be in a conducting state all the time, the trigger pulses are continuously applied to the thyristors for controlling reverse conduction in the bypass of the upper bridge arm of each phase; and maintaining the locking of thyristors in all other bypasses of the high-voltage valve bank and the low-voltage valve bank, and locking all full-control power electronic device trigger pulses of the high-voltage valve bank and the low-voltage valve bank.
The invention relates to a high-voltage valve group earth fault protection method of a modularized multi-level converter, which is characterized in that bypasses which are controlled to be switched on and switched off by thyristors are respectively established on an upper bridge arm and a lower bridge arm of each phase in the modularized multi-level converter; when the ground fault of any point of the lower bridge arm of the high-voltage valve bank occurs, trigger pulses are applied to thyristors in bypasses of the bridge arms of all phases of the high-voltage valve bank, so that the corresponding bypasses of the bridge arms of all phases of the high-voltage valve bank are conducted, and fault current flows through the bypasses.
Preferably, the method further comprises the step of maintaining thyristors in the bypass of each phase of the bridge arm of the low-voltage valve bank and each bridge arm of the low-voltage valve bank in a locking state.
Preferably, the trigger pulse is applied to the thyristors in the bypass of each phase leg of the high voltage valve bank continuously or only once.
Preferably, specifically, after the occurrence of the ground fault of any point of the lower bridge arm of any phase of the high-voltage valve bank is monitored, trigger pulse is only applied to the thyristors which are controlled to be positively conducted in the bypass of each phase of the bridge arm of the high-voltage valve bank once; and maintaining all bypass thyristors of the low-voltage valve bank in a blocking state, and blocking all full-control power electronic device trigger pulses of the high-voltage valve bank and the low-voltage valve bank.
Further, the bypass comprises thyristors for controlling reverse conduction, after the occurrence of the ground fault of any point of a lower bridge arm of any phase of the high-voltage valve bank is monitored, trigger pulses are applied to thyristors for controlling forward conduction in the bypass of each phase of bridge arm of the high-voltage valve bank only once, and when the fact that the thyristors for controlling forward conduction in the bypass of the bridge arm of the fault phase are always in a conduction state is monitored, trigger pulses are continuously applied to thyristors for controlling reverse conduction in the bypasses of the other two phases of bridge arms; and maintaining all bypass thyristors of the low-voltage valve bank in a blocking state, and blocking all full-control power electronic device trigger pulses of the high-voltage valve bank and the low-voltage valve bank.
Preferably, the system also comprises trigger pulses for locking all the full-control power electronic devices in the upper bridge arm and the lower bridge arm of each phase of the high-voltage valve group.
Preferably, the system also comprises trigger pulses for locking all the full-control power electronic devices in the upper bridge arm and the lower bridge arm of each phase of the low-voltage valve group.
Preferably, when the direct current side of the modularized multi-level converter is connected with a phase control converter formed by thyristors, after the high-voltage valve bank fails to the ground,
the phase-controlled inverter is locked out,
or the phase control converter is switched to an inversion operation state,
or the current command value of the phase control converter is adjusted to be zero or negative value.
Preferably, the method further comprises opening an ac circuit breaker in the modular multilevel converter.
Compared with the prior art, the invention has the following beneficial technical effects:
according to the modularized multi-level converter and the high-voltage valve bank earth fault protection method thereof, only two thyristors or a thyristor which are connected in parallel in opposite directions are required to be connected in parallel at the output end of each sub-module to form a bypass corresponding to each bridge arm, and overvoltage protection can be provided for the sub-module by reasonably controlling the switching state of the thyristor in the bypass when the high-voltage valve bank earth fault occurs. On average, only one or two bypasses consisting of thyristors are additionally added on each sub-module, so that the cost is low, reliable protection can be provided for high-voltage and low-voltage valve groups, and the high-voltage and low-voltage valve group serial scheme is convenient to popularize and put into practical use; compared with the protection scheme adopting the lightning arrester, the high-capacity lightning arrester is not needed, so that the cost is greatly reduced; meanwhile, the protection level of the lightning arrester is not required to be set, and the problems that the effective protection cannot be provided for the submodule due to the fact that the protection level is set too high in the lightning arrester scheme and the steady-state operation loss is caused due to the fact that the protection level is set too low are avoided.
Drawings
Fig. 1 is a block diagram of a hybrid MMC with a blocking sub-module and a half-bridge sub-module for isolating a high-voltage valve block from ground faults according to an embodiment of the present invention.
Fig. 2 is a diagram of a mixed MMC structure composed of a blocking type submodule and a half-bridge type submodule with a current limiting resistor, which is provided with an isolation high-voltage valve group to ground fault in the embodiment of the invention.
Fig. 3 is a diagram of a mixed MMC structure composed of a blocking-type submodule and a half-bridge-type submodule with a thyristor valve for bypass in the example of the invention, which is provided with a high-voltage valve group for isolating a ground fault.
Fig. 4 is a diagram of a mixed MMC structure composed of a blocking sub-module and a half-bridge sub-module with a bypass of a current limiting resistor and a thyristor valve with a high-voltage valve set for isolating a ground fault in an example of the invention.
Fig. 5 is a block diagram of a mixed MMC formed by a blocking type submodule and a half-bridge type submodule, wherein the bypass thyristor corresponds to the submodule and is divided into a plurality of groups, and the groups are used for isolating a high-voltage valve group from ground faults.
Fig. 6 is a diagram of a mixed MMC structure formed by a blocking type submodule and a half-bridge type submodule, wherein the submodule components corresponding to thyristors in the bypass are multiple groups and are provided with current limiting resistors, and the mixed MMC structure is used for isolating a ground fault of a high-voltage valve group.
Fig. 7 is a block diagram of an MMC with a blocking-type submodule for isolating a high-voltage valve block from ground faults in an example of the present invention.
Fig. 8 is a block diagram of an MMC with current limiting resistor with a blocking sub-module for isolating a high voltage valve block from ground faults in an example of the present invention.
Fig. 9 is a block diagram of an MMC with a blocking-type submodule for isolating a high-voltage valve group from a ground fault, in which the thyristor valve in the bypass is connected in parallel to the output end of the bridge arm in the embodiment of the invention.
Fig. 10 is a block diagram of an MMC with a blocking-type submodule for isolating a high-voltage valve group from a ground fault, which is connected in parallel to an output end of a bridge arm after a thyristor valve and a current limiting resistor in a bypass in an example of the invention.
FIG. 11 is a block diagram of an MMC with corresponding sub-modules of the bypass thyristor divided into multiple groups.
FIG. 12 is a block diagram of an MMC with current limiting resistors and blocking submodules with corresponding submodules of thyristors in the bypass according to an example of the present invention.
Fig. 13 is a half-bridge type sub-module topology as described in an example of the invention.
Fig. 14 (a) to (e) are various types of blocking-type submodule topologies described in examples of the present invention.
Fig. 15 is a dc power transmission topology of an example of the present invention, comprising a phase-controlled converter and a modular multilevel converter according to the present invention.
Fig. 16 is a current path of the hybrid converter formed by the half-bridge submodule and the full-bridge submodule after the scheme of the invention is adopted in the example of the invention when the upper bridge arm of the high-voltage valve bank a fails to ground.
Fig. 17 shows the current paths of the hybrid converter consisting of the half-bridge submodule and the full-bridge submodule, in which the Quan Qiaozi module includes only one bypass thyristor, when the upper bridge arm of the high-voltage valve group a fails to ground.
Fig. 18 is a current path of the modular multilevel converter with high voltage block to ground fault isolation capability in accordance with an example of the present invention when the upper leg of the high voltage block a phase fails to ground.
Fig. 19 is a hybrid dc power transmission topology constructed from modular multilevel converters and phase-controlled converters, built on PSCAD/EMTDC, according to an example of the present invention, where the modular multilevel converters are configured with high and low voltage valve banks in series.
Fig. 20 is a waveform of dc fault current obtained by simulation of the structure of fig. 19.
Fig. 21 is a graph showing the average capacitance voltage of the full-bridge sub-module and the half-bridge sub-module of each bridge arm of the high-voltage valve block of fig. 19.
Fig. 22 is a graph showing the average capacitance voltage of the full-bridge sub-module and the half-bridge sub-module of each bridge arm of the low-voltage valve block of fig. 19.
Fig. 23 is a valve side ac current of the high pressure valve block of fig. 19 construction.
Fig. 24 is a graph of the current flowing through each antiparallel thyristor of the upper leg of the high voltage block of fig. 19.
In the figure: the anti-parallel thyristor pair 1, the bypass thyristor 2, the alternating current breaker 3, the high-voltage valve bank 4, the low-voltage valve bank 5, the low-voltage side 6 of the upper bridge arm of the high-voltage valve bank, the negative direct current bus 7 of the high-voltage valve bank, the upper bridge arm 8 of the high-voltage valve bank A phase, the upper bridge arm 9 of the high-voltage valve bank B phase, the upper bridge arm 10 of the high-voltage valve bank C phase, the phase control converter 11, the bidirectional thyristor valve 12, the current limiting resistor 13 and the bypass thyristor valve 14.
Detailed Description
For the purpose of making apparent the objects, technical solutions and advantages of the present invention, the present invention will be described in further detail with reference to the following specific embodiments, and it should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the present invention. In addition, the technical features of the embodiments of the present invention described below may be combined with each other as long as they do not collide with each other.
According to the flexible direct current transmission system provided by the embodiment of the invention, the protection method for the high-voltage valve bank to the ground fault in the modularized multi-level converter can solve the problem of the overvoltage of the submodule capacitor existing in the ground fault of the high-voltage valve bank when the existing high-voltage valve bank and the existing low-voltage valve are operated in series.
The invention relates to a modularized multi-level converter, which is a mixed modularized multi-level converter formed by blocking type sub-modules and half-bridge sub-modules, wherein two thyristors which are connected in inverse parallel are connected in parallel at the voltage output end of each blocking type sub-module to form a bypass branch, one thyristor is connected in parallel at the voltage output end of each half-bridge sub-module to form a bypass branch, the anode of the thyristor is connected with the high-voltage output end of the half-bridge sub-module, and the cathode of the thyristor is connected with the low-voltage output end of the half-bridge sub-module; when the high-voltage valve bank fails to the ground, a trigger signal is applied to the thyristors in the bypass, so that fault current flows through the thyristors in the bypass, and the fault current is prevented from flowing through the submodules to charge the submodule capacitors. The voltage output end of each half-bridge sub-module is connected with two thyristors in parallel in an anti-parallel mode to form a bypass branch, and the voltage output end of each blocking sub-module is connected with one thyristor or two thyristors in parallel in an anti-parallel mode to form a bypass branch.
The invention relates to a modularized multi-level converter, which is a mixed modularized multi-level converter formed by blocking type sub-modules and half-bridge sub-modules, wherein the voltage output end of each blocking type sub-module is connected in parallel with a bypass branch formed by connecting two thyristors which are connected in parallel in opposite directions with a current limiting resistor in series; the bypass branch corresponding to the bridge arm is formed by the corresponding bypass branch in the same bridge arm, and a trigger signal is applied to the thyristor in the bypass, so that fault current flows through the bypass thyristor, and the fault current is prevented from flowing through the sub-module to charge the capacitance of the sub-module.
The invention relates to a modularized multi-level converter, which is a hybrid modularized multi-level converter formed by blocking type submodules and half-bridge submodules, wherein a bidirectional thyristor valve is connected in parallel between high-voltage output ends and low-voltage output ends of valve sections formed by the blocking type submodules in series to form a bypass branch, the bidirectional thyristor valve is formed by connecting a plurality of anti-parallel thyristors in series or connecting two anti-parallel unidirectional thyristor valves in parallel, a unidirectional thyristor valve is connected in parallel between the high-voltage output ends and the low-voltage output ends of the valve sections formed by the half-bridge submodules in series to form a bypass branch, the unidirectional thyristor valve is formed by connecting a plurality of thyristors in series, the anode of the unidirectional thyristor valve is connected with the high-voltage output end of the valve section of the half-bridge submodule, and the cathode of the unidirectional thyristor valve is connected with the low-voltage output end of the valve section of the half-bridge submodule; when the high-voltage valve bank fails to the ground, a trigger signal is applied to the thyristor in the bypass, so that fault current flows through the bypass thyristor, and the fault current is prevented from flowing through the sub-module to charge the capacitance of the sub-module.
The bidirectional thyristor valve and the current-limiting resistor are connected in series and then connected in parallel to a valve section formed by connecting the blocking type submodules in series, and the unidirectional thyristor valve and the current-limiting resistor are connected in series and then connected in parallel to a valve section formed by connecting the half-bridge submodules in series.
The valve sections formed by the blocking type submodules can be divided into a plurality of groups, and the output end of each group is connected with a bidirectional thyristor valve group or a series combination formed by the bidirectional thyristor valve group and a current limiting resistor in parallel. The valve sections formed by the half-bridge submodules are divided into a plurality of groups, and the output end of each group is connected with a unidirectional thyristor valve group or a series combination formed by the unidirectional thyristor valve group and a current limiting resistor in parallel.
For the mixed modular multilevel converter, when the ground fault of any point of the upper bridge arm of any phase of the high-voltage valve bank occurs, the method comprises the following steps,
and triggering pulse is applied to the bypass thyristor of each phase upper bridge arm of the high-voltage valve bank.
And maintaining bypass thyristors of each lower bridge arm of the high-voltage valve group and each lower bridge arm of the high-voltage valve group in a locking state.
And locking trigger pulses of all full-control power electronic devices of the upper bridge arm and the lower bridge arm of each phase of the high-voltage valve group.
When the ground fault of any point of the upper bridge arm of any phase of the high-voltage valve bank occurs, the trigger pulse of all the full-control power electronic devices of the upper bridge arm and the lower bridge arm of each phase of the low valve is blocked.
Wherein the trigger pulse is applied continuously or only once on the thyristor.
Specifically, after a fault is detected, trigger pulses are applied to thyristors which are controlled to be conducted in the forward direction in the bypasses of the upper bridge arms of the high-voltage valve groups once, and when the thyristors which are controlled to be conducted in the forward direction in the bypasses of the upper bridge arms of the fault phases are detected to be in a conducting state all the time, trigger pulses are continuously applied to thyristors which are controlled to be conducted in the reverse direction in the bypasses of the upper bridge arms of the fault phases; and maintaining the locking of thyristors in all other bypasses of the high-voltage valve bank and the low-voltage valve bank and locking all full-control power electronic device trigger pulses of the high-voltage valve bank and the low-voltage valve bank.
For the modularized multi-level converter, when the ground fault of any point of the lower bridge arm of the high-voltage valve bank occurs, the method comprises the following steps,
and triggering pulses are applied to bypass thyristors of each phase of bridge arm of the high-voltage valve bank.
And maintaining the bypass thyristors of each bridge arm of the low valve in a locking state.
And locking trigger pulses of all full-control power electronic devices of the upper bridge arm and the lower bridge arm of each phase of the high-voltage valve group.
And locking trigger pulses of all full-control power electronic devices of upper and lower bridge arms of each phase of the low valve.
Wherein the trigger pulse is applied continuously or only once on the thyristor.
Specifically, after a fault is detected, trigger pulses are applied to thyristors which are controlled to be conducted in the forward directions in bypasses of all phase bridge arms of the high-voltage valve bank only once, when the thyristors which are controlled to be conducted in the forward directions in the bypasses of the fault phase bridge arms are always in a conducting state, trigger pulses are continuously applied to thyristors which are controlled to be conducted in the reverse directions in the bypasses of the other two phase bridge arms, all bypass thyristors of the low-voltage valve bank are maintained to be in a locking state, and all full-control type power electronic device trigger pulses of the high-voltage valve bank and the low-voltage valve bank are locked.
The invention relates to a modularized multi-level converter, which is a modularized multi-level converter formed by blocking type submodules, wherein a thyristor is connected in parallel to the voltage output end of each blocking type submodule to form a bypass branch, the anode of the thyristor is connected with the high-voltage output end of the blocking type submodule, and the cathode of the thyristor is connected with the low-voltage output end of the blocking type submodule; when the high-voltage valve group fails to the ground, a trigger signal is applied to the bypass thyristor, so that fault current flows through the bypass thyristor, and the fault current is prevented from flowing through the sub-module to charge the sub-module capacitor.
The bypass branch is formed by connecting the thyristor with the current-limiting resistor in series and then connecting the thyristor with the voltage output port of the blocking type submodule in parallel.
Preferably, a plurality of thyristors are connected in series to form a thyristor valve, the high voltage of the thyristor valve is connected with the high voltage output end of each bridge arm, and the low voltage of the thyristor valve is connected with the low voltage output end of each bridge arm to form a bypass corresponding to the bridge arm.
The thyristor valve and the current limiting resistor can be connected in series and then connected to the voltage output end of each bridge arm in parallel to form a bypass branch.
When the ground fault occurs at any point of the upper bridge arm of the high-voltage valve bank, the method comprises the following steps of applying a trigger pulse to bypass thyristors of all upper bridge arms of the high-voltage valve bank once, maintaining the bypass thyristors of the lower bridge arm of the high-voltage valve bank and all low valves to be in a blocking state, and locking the modularized multi-level converter.
When the ground fault occurs at any point of the lower bridge arm of the high-voltage valve bank, the method comprises the following steps of applying a trigger pulse to bypass thyristors of all the high-voltage valve bank once, maintaining all the bypass thyristors of the low-voltage valve to be in a blocking state, and locking the modularized multi-level converter.
In this example, the blocking sub-module is a modular multilevel converter power module with the capability of blocking direct current fault current, and various known topologies such as a full-bridge sub-module, a clamping double sub-module, a diode clamping sub-module, a self-blocking sub-module, and a crosslinking sub-module can be adopted.
When the direct current side of the modularized multi-level converter is connected with a phase control converter formed by thyristors, after a high-voltage valve bank fails to the ground, the phase control converter is locked or is switched to an inversion running state or the current instruction value of the phase control converter is adjusted to be zero or negative, so that the residual energy on the direct current transmission line is quickly absorbed, and the current-resistant requirement on the bypass thyristors is reduced.
And when the occurrence of the high-voltage valve bank to ground fault is detected, the alternating current circuit breaker of the modularized multi-level converter is disconnected.
The selection principle of the resistance value of the current limiting resistor is that the bypass thyristor does not generate overcurrent, the alternating current breaker is not opened due to the overcurrent, and the voltage drop on the current limiting resistor is lower than the sum of capacitance voltages of submodules of the submodules, the valve sections or the bridge arms which are connected in parallel.
The details are as follows.
Fig. 1 illustrates a modular multilevel converter topology employing the protection scheme of the present invention, which is formed by a high voltage valve block 4 and a low voltage valve block 5 in series, which reduces the capacity and insulation requirements of each ac transformer and thus reduces costs compared to a single converter valve scheme. Each bridge arm of the high-voltage and low-voltage valve group in fig. 1 is formed by connecting one or more blocking sub-modules and half-bridge sub-modules in series, and only one blocking sub-module and one half-bridge sub-module are illustrated in each bridge arm in fig. 1 for simplifying drawing.
But the series scheme of the high-pressure valve and the low-pressure valve has potential safety hazards. When the low-voltage side 6 of the upper bridge arm of the high-voltage valve bank or the negative direct-current bus 7 of the high-voltage valve bank has a ground fault, the polar line ground voltage of the direct-current power transmission line is applied to the upper bridge arm of the high-voltage valve bank or the high-voltage valve bank, and the sum of the capacitance voltages of all the submodules of each phase of the high-voltage valve bank is only 1/2 of the rated polar line ground voltage, so that the sum of the capacitance voltages of all the submodules of each phase of the high-voltage valve bank is equal to the rated polar line ground voltage, and after the residual energy storage on the direct-current power transmission line and the direct-current inductor is considered, when the low-voltage side 6 of the upper bridge arm of the high-voltage valve bank or the negative direct-current bus 7 of the high-voltage valve bank has a fault, even if all the full-control power electronic devices of the high-voltage valve bank are blocked, the submodules of the upper bridge arm of the high-voltage valve bank or the upper bridge arm and the lower bridge arm of each phase of the high-voltage valve bank will still be charged to the high voltage so as to damage the submodules.
In order to solve the above-mentioned problem, as shown in fig. 1 in this example, a structure of protecting a high-voltage valve block of a hybrid MMC composed of a blocking sub-module and a half-bridge sub-module from a ground fault is given. The ratio of the number of blocking sub-modules to half-bridge sub-modules in fig. 1 is approximately 1:1. the protection structure designed by the invention is characterized in that: the voltage output end of each blocking type sub-module is connected in parallel with an anti-parallel thyristor pair 1 formed by two thyristors in anti-parallel connection or connected in parallel with a bidirectional thyristor; the voltage output end of each half-bridge sub-module is connected with a bypass thyristor 2 in parallel, the anode of the thyristor connected with the half-bridge sub-module in parallel is connected with the high voltage output end of the half-bridge sub-module, and the cathode is connected with the low voltage output end of the half-bridge sub-module. Taking the low-voltage output end of the upper bridge arm of the high-voltage valve bank A, namely the ground fault of the upper bridge arm low-voltage side 6 of the high-voltage valve bank A as an example, when the ground fault of the upper bridge arm low-voltage side 6 of the high-voltage valve bank A occurs, trigger pulses are applied to bypass thyristors of all sub-modules of the upper bridge arm 8 of the high-voltage valve bank A, the upper bridge arm 9 of the B phase and the upper bridge arm 10 of the C phase, so that fault current flows through the bypass thyristors to avoid the fault current from charging capacitors of the sub-modules. When the fault point 6d or 7d of the negative direct current bus 7 of the upper bridge arm low voltage side 6 or the high voltage valve bank corresponding to the high voltage valve bank has a ground fault, the trigger pulse of all the full-control power electronic devices of the high voltage valve bank and the low voltage valve bank is recommended to be locked so as to prevent the sub-module capacitor from discharging through the fault point 6d or the fault point 7 d. When the fault point 6d has a ground fault, bypass thyristors of sub-modules of lower bridge arms of each phase of the high-voltage valve bank and all bridge arms of the low-voltage valve bank are kept in a blocking state, so that a short-circuit reactance value between an alternating current side and the fault point 6d is reduced, and current of the alternating current side is reduced. Similarly, when the fault point 7d has a ground fault, it is preferable to maintain the bypass thyristors of the submodules of all the bridge arms of the low-voltage valve bank in a blocking state.
In order to further reduce the fault current, fig. 2 shows the improvement of fig. 1, which is that after each anti-parallel thyristor pair 1 or each bypass thyristor 2 is connected in series with a current limiting resistor 13, it is connected in parallel to the output of the sub-module, so as to reduce the direct current fault current and the alternating current fault current at the same time, reduce the current-resisting requirement of the bypass thyristors and reduce the disturbance to the alternating current power grid. The resistance of the current limiting resistor 13 is preferably selected reasonably, if the resistance is too small, the current limiting effect is not obvious, if the resistance is too large, the voltage drop on the current limiting resistor is too high, and each submodule can bear overvoltage. The selection principle of the current limiting resistor 13 is recommended to make the bypass thyristor not over-current and make the ac breaker not open due to over-current and make the voltage drop across the current limiting resistor lower than the sub-modules it is connected in parallel with.
Fig. 3-4 are the same type of schemes of fig. 1-2. The difference between fig. 3 and fig. 1 is that the anti-parallel thyristor pair 1 or the bypass thyristor 2 is not directly connected in parallel to each sub-module, but a plurality of anti-parallel thyristor pairs are connected in series to obtain a bi-directional bypass thyristor valve 12 and then connected in parallel to a series combination formed by the blocking sub-modules, and a plurality of bypass thyristors are connected in series to obtain a bypass thyristor valve 14 and then connected in parallel to a series combination formed by the half-bridge sub-modules. The lower left corner of fig. 3 shows two implementations of the bi-directional bypass thyristor valve 12. One is obtained by antiparallel connection of two unidirectional bypass thyristor valves 14, as shown at 12 a. The other is obtained by connecting a plurality of anti-parallel thyristors 1 or a plurality of bidirectional thyristors in series, as shown in 12 b.
Similarly, the scheme in fig. 4 is that after the bidirectional bypass thyristor valve 12 is connected in series with the current-limiting resistor 13, the bidirectional bypass thyristor valve 12 is connected in parallel to the series combination formed by the blocking-type submodule, and after the unidirectional bypass thyristor valve 14 is connected with the current-limiting resistor string 13, the bidirectional bypass thyristor valve 14 is connected in parallel to the series combination formed by the half-bridge submodule. The selection principle of each current limiting resistor 13 in fig. 4 is that the bypass thyristor does not generate overcurrent, the alternating current breaker is not opened due to the overcurrent, and the voltage drop of the current limiting resistor is lower than the sum of rated capacitor voltages of the submodules connected in parallel.
Compared with the schemes of fig. 1-2, the scheme of fig. 3-4 has the advantage of facilitating centralized management of bypass thyristors and current limiting resistors, so that the topology and layout of the existing sub-modules are not required to be changed, the disadvantage is that the bidirectional bypass thyristor valve 12 and the unidirectional bypass thyristor valve 14 are formed by connecting a plurality of thyristors in series, and when the number of thyristors connected in series is large, the problem of difficult voltage equalizing of the series connection of the thyristors exists.
Fig. 5-6 are the same type of solution as fig. 3-4, and fig. 5 and 6 depict only the topology of the high-pressure valve block 4 in order to reduce the size of the figure. The principle of fig. 5-6 is identical to that of fig. 3-4, except that the blocking type submodules are divided into a plurality of groups, the output end of each blocking type submodule is connected in parallel with a bidirectional bypass thyristor valve 12 or a series combination of the bidirectional bypass thyristor valve 12 and a current limiting resistor 13, the half-bridge submodules are also divided into a plurality of groups, and the output end of each half-bridge submodule is connected in parallel with a unidirectional bypass thyristor valve 14 or a series combination of the unidirectional bypass thyristor valve 14 and the current limiting resistor 13. The advantage of the scheme of fig. 5-6 over the scheme of fig. 3-4 is that the number of thyristors in series between the bi-directional bypass thyristor valve 12 and the unidirectional bypass thyristor valve 14 can be reduced, thereby reducing the voltage sharing difficulty of the bypass thyristor.
The selection principle of each current limiting resistor 13 in fig. 6 is that the bypass crystal gate valve does not generate overcurrent, the alternating current breaker is not opened due to the overcurrent, and the voltage drop of the current limiting resistor is lower than the sum of rated capacitor voltages of the submodules connected in parallel.
A further solution presented in this example, as shown in fig. 7-8, is a structure of protection against ground faults for the high-voltage valve block of the MMC constituted by the blocking-type submodules. In fig. 7, the voltage output end of each blocking type sub-module is reversely connected with a bypass thyristor 2 in parallel, the anode of the bypass thyristor 2 is connected with the high voltage output end of the sub-module, and the cathode of the bypass thyristor 2 is connected with the low voltage output end of the sub-module. When the ground faults of the low-voltage output ends of the upper bridge arm of the high-voltage valve bank are monitored, trigger pulses are applied to the bypass thyristors 2 of all sub-modules of the fault bridge arm once, so that fault currents circulate through the bypass thyristors 2, all other bypass thyristors 2 are maintained to be in a locking state, and trigger signals of all full-control power electronic devices of the high-voltage valve bank are locked. After the zero crossing of the direct current fault current, the bypass thyristor 2 will be automatically turned off.
Fig. 8 is similar to fig. 7, except that each bypass thyristor 2 is also connected in series with a current limiting resistor 13 to reduce the fault current during a dc fault.
Fig. 9 is similar to fig. 7, except that each bypass thyristor is not connected to each sub-module, but a plurality of bypass thyristors 2 are connected in series to form a unidirectional bypass thyristor valve 14, which is connected in parallel to the high and low voltage output terminals of the bridge arm. The advantage of the topology of fig. 9 over the topology of fig. 7 is that the structure and design of each sub-module need not be changed, facilitating centralized management of bypass thyristor valve banks. The disadvantage brought by the method is that a certain voltage equalizing difficulty exists after a plurality of thyristors are connected in series.
Fig. 10 is similar to fig. 9 except that each bypass thyristor valve 14 is also connected in series with a current limiting resistor 13 so as to speed up the reduction of the fault current to zero and limit the magnitude of the fault current.
Fig. 11 shows a bypass thyristor valve 14 formed by dividing the blocking submodule of each bridge arm into a plurality of groups, and connecting one or more thyristors in parallel to each group. The performance of fig. 11 is between fig. 7 and fig. 9, avoiding both changing the topology of each sub-module and avoiding the voltage sharing difficulty after too many bypass thyristors are connected in series.
Fig. 12 is similar to fig. 11 except that each bypass thyristor valve 14 is also connected in series with a current limiting resistor 13 to reduce the magnitude of the fault current. The method comprises the steps of carrying out a first treatment on the surface of the
Fig. 13 illustrates a topology of half-bridge sub-modules. Fig. 14 (a) - (e) illustrate topologies of blocking-type submodules, and fig. 14 (a) - (e) are full-bridge submodule topologies, clamping double submodule topologies, diode clamping submodules, self-blocking submodules, and crosslinking submodules, respectively. Fig. 13-14 are prior art, and details thereof are not repeated.
In this example, as shown in fig. 15, a two-terminal dc transmission system topology is provided, which is composed of a unipolar positive polarity phase control converter 11 and a unipolar positive polarity hybrid modular multilevel converter. The modularized multi-level converter consists of a high-voltage valve bank 4 and a low-voltage valve bank 5. When the upper bridge arm low voltage end 6 of the high voltage valve bank 4 has a ground fault, trigger pulses are applied to the bypass thyristors 1 of all sub-modules of the A, B, C three-phase upper bridge arm of the high voltage valve bank 4, the bypass thyristors of all sub-modules of all the lower bridge arms of the high voltage valve bank and all the bridge arms of the low voltage valve bank are maintained in a blocking state, and the trigger pulses of all the full-control power electronic devices of the high voltage valve bank and the low voltage valve bank are blocked.
In this example, fig. 16 illustrates current paths of each phase when the low-voltage output end of the upper bridge arm of the high-voltage valve group a fails to ground, taking a hybrid MMC formed by a half-bridge submodule and a full-bridge submodule as an example. Wherein the proportion of each bridge arm half-bridge submodule to Quan Qiaozi module is 1: about 1. In fig. 16, elements through which current flows are indicated by black lines, and elements in a closed state are indicated by gray lines. In the fault, the bypass thyristors of the upper bridge arm of the three phases of the high-voltage valve bank A, B, C are triggered, the bypass thyristors of the other bridge arms are kept in a locking state, the modularized multi-level converter of the fault pole (only the positive pole is drawn in fig. 16) is locked, and the modularized multi-level converter of the non-fault pole (the negative pole is not drawn in fig. 16) still keeps in a normal running state. Fig. 16 shows that fault current will flow through the bypass thyristors of the A, B, C three-phase upper leg, and that fault current will not charge the submodule capacitor and will not cause the submodule capacitor overvoltage problem.
Fig. 17 illustrates the current path of the high voltage valve bank to ground fault when the bypass thyristor of the Quan Qiaozi module is a single thyristor. The ratio of full-bridge sub-module to half-bridge sub-module in fig. 17 is approximately 1:1. when the upper bridge arm of the phase A of the high-voltage valve bank has a ground fault, the direct current fault current path is consistent with that of fig. 16. In addition to the dc fault current, the ac side will also pass current as indicated by the B, A two-phase solid arrows in fig. 17. It will be appreciated that the ac side will charge the full-bridge sub-module of the upper leg of phase B, so the scheme of fig. 17, where the full-bridge sub-module contains only a single bypass thyristor, is a technically infeasible scheme.
Fig. 18 illustrates ac side potential fault current paths when the upper arm of the modular multilevel converter high voltage valve bank a phase of each bridge arm has a ground fault, with the sub-modules of each bridge arm being full bridge sub-modules. Taking B, A two phases of the high-voltage valve bank as an example, the capacitor voltages of all the sub-modules of the B-phase upper bridge arm of the high-voltage valve bank are reversely connected to the current path. The potential fault current path illustrated in fig. 18 is not capable of flowing current because the sum of the capacitance voltages of the left and right sub-modules of each leg is higher than the peak value of the B, A two-phase line voltage. When all the sub-modules of the bridge arm are blocking sub-modules, only one bypass thyristor is needed to be reversely connected in parallel at the output end of each sub-module, and the anode and the cathode of the bypass thyristor are respectively connected with the high-voltage output end and the low-voltage output end of the sub-module. When the high-voltage valve bank upper bridge arm is in ground fault or the high-voltage valve bank low-voltage direct current bus is in ground fault, a trigger pulse is applied to bypass thyristors of all sub-modules of the A-phase upper bridge arm, the B-phase upper bridge arm and the C-phase upper bridge arm of the high-voltage valve bank once, so that direct current fault current flows through the bypass thyristors, and the direct current fault current is prevented from charging capacitors of the sub-modules.
To verify the technical feasibility of the design scheme of the invention, a simulation example is built under PSCAD/EMTDC, as shown in FIG. 19. In fig. 19, LCC represents a phase-controlled converter, mmc_high is a High-voltage valve group of a modular multilevel converter, and mmc_low is a Low-voltage valve group of the modular multilevel converter. The high and low pressure valve sets adopt the topology shown in fig. 16.
Fig. 20 to 24 show the corresponding simulation results. And when the applied fault is 0.8s, the high-voltage valve group A phase low-voltage end of the modularized multi-level converter has a ground fault. After the fault is detected, all bypass thyristors of the A-phase, B-phase and C-phase upper bridge arms of the high-voltage valve bank are continuously applied with trigger signals, all bypass thyristors of the A-phase, B-phase and C-phase lower bridge arms and all bypass thyristors of the low-voltage valve bank are kept in a locking state, and all full-control power electronic devices of the high-voltage and low-voltage valve banks are locked after the fault is detected. The phase-controlled inverter will also latch its trigger pulse after detecting the fault.
Fig. 20 shows a dc current waveform, and it is known that the dc current can be reduced to zero within 15ms after the fault occurs. Fig. 21 and 22 show average capacitance voltages of full-bridge sub-modules and half-bridge sub-modules of each phase bridge arm of the high-voltage valve bank, and it is known that under the design scheme of the present invention, when the high-voltage valve bank fails to ground, the capacitance of each sub-module will not generate overvoltage. In the valve side ac current of fig. 23, it is known that in the design of the present invention, there is no significant ac overcurrent when the high-voltage valve block fails to ground. Fig. 24 shows bypass thyristors for each phase of upper bridge arm of the high voltage valve block, and it is known that no significant overcurrent flows through each bypass thyristor.

Claims (20)

1. A high-voltage valve group earth fault protection method of a modularized multi-level converter is characterized in that,
the modularized multi-level converter comprises a high-voltage valve bank and a low-voltage valve bank which are connected in series on a direct current side and connected in parallel on an alternating current side; each phase of the high-pressure valve bank and the low-pressure valve bank consists of an upper bridge arm and a lower bridge arm; the upper bridge arm and the lower bridge arm are formed by cascading a plurality of blocking sub-modules and a plurality of half-bridge sub-modules; or the upper bridge arm and the lower bridge arm are formed by cascading a plurality of blocking-type submodules;
bypasses which are controlled to be switched on and off by thyristors are respectively arranged on the upper bridge arm and the lower bridge arm; when the high-voltage valve bank fails to the ground, a trigger signal is applied to the thyristor in the bypass, so that fault current flows through the bypass, and the fault current is prevented from flowing through the sub-module to charge the capacitance of the sub-module;
the method comprises the steps of respectively establishing bypasses which are controlled to be switched on and switched off by thyristors on an upper bridge arm and a lower bridge arm of each phase in the modularized multi-level converter; when the ground fault occurs at any point of the upper bridge arm of any phase of the high-voltage valve bank, trigger pulses are applied to thyristors in bypasses of the upper bridge arms of all phases of the high-voltage valve bank, so that the corresponding bypasses of the upper bridge arms of all phases of the high-voltage valve bank are conducted, and fault current flows through the bypasses;
Specifically, after the ground fault of any point of the upper bridge arm of any phase of the high-voltage valve bank is monitored, trigger pulse is applied to the thyristors which are controlled to be positively conducted in the bypass of the upper bridge arm of each phase of the high-voltage valve bank only once; maintaining the locking of thyristors in all other bypasses of the high-voltage valve bank and the low-voltage valve bank, and locking all full-control power electronic device trigger pulses of the high-voltage valve bank and the low-voltage valve bank;
the bypass comprises thyristors for controlling reverse conduction, when the ground fault of any point of the upper bridge arm of the high-voltage valve bank is monitored, trigger pulses are applied to the thyristors for controlling forward conduction in the bypass of the upper bridge arm of each phase of the high-voltage valve bank only once, and when the thyristors for controlling forward conduction in the bypass of the upper bridge arm of the fault phase are monitored to be in a conducting state all the time, trigger pulses are continuously applied to the thyristors for controlling reverse conduction in the bypass of the upper bridge arm of each phase; and maintaining the locking of thyristors in all other bypasses of the high-voltage valve bank and the low-voltage valve bank, and locking all full-control power electronic device trigger pulses of the high-voltage valve bank and the low-voltage valve bank.
2. The method of claim 1, further comprising maintaining thyristors in the bypass of each phase lower leg of the high voltage block and each leg of the low voltage block in a blocked state.
3. The method for protecting a high-voltage valve bank of a modular multilevel converter according to claim 1, wherein the triggering pulse is applied to the thyristors in the bypass of the upper bridge arm of each phase of the high-voltage valve bank continuously or only once.
4. A high-voltage valve group earth fault protection method of a modularized multi-level converter is characterized in that,
the modularized multi-level converter comprises a high-voltage valve bank and a low-voltage valve bank which are connected in series on a direct current side and connected in parallel on an alternating current side; each phase of the high-pressure valve bank and the low-pressure valve bank consists of an upper bridge arm and a lower bridge arm; the upper bridge arm and the lower bridge arm are formed by cascading a plurality of blocking sub-modules and a plurality of half-bridge sub-modules; or the upper bridge arm and the lower bridge arm are formed by cascading a plurality of blocking-type submodules;
bypasses which are controlled to be switched on and off by thyristors are respectively arranged on the upper bridge arm and the lower bridge arm; when the high-voltage valve bank fails to the ground, a trigger signal is applied to the thyristor in the bypass, so that fault current flows through the bypass, and the fault current is prevented from flowing through the sub-module to charge the capacitance of the sub-module;
the method comprises the steps of respectively establishing bypasses which are controlled to be switched on and switched off by thyristors on an upper bridge arm and a lower bridge arm of each phase in the modularized multi-level converter; when the ground fault of any point of the lower bridge arm of the high-voltage valve bank occurs, trigger pulses are applied to thyristors in bypasses of the bridge arms of all phases of the high-voltage valve bank, so that the corresponding bypasses of the bridge arms of all phases of the high-voltage valve bank are conducted, and fault current flows through the bypasses;
Specifically, after the occurrence of the ground fault of any point of the lower bridge arm of any phase of the high-voltage valve bank is monitored, trigger pulse is only applied to thyristors which are controlled to be positively conducted in the bypass of each phase of the bridge arm of the high-voltage valve bank once; maintaining all bypass thyristors of the low-voltage valve bank in a blocking state, and blocking all full-control power electronic device trigger pulses of the high-voltage valve bank and the low-voltage valve bank;
the bypass comprises thyristors for controlling reverse conduction, after any point of a lower bridge arm of the high-voltage valve bank is monitored to be grounded, trigger pulses are applied to thyristors for controlling forward conduction in the bypass of each phase bridge arm of the high-voltage valve bank only once, and when the thyristors for controlling forward conduction in the bypass of the fault phase bridge arm are monitored to be in a conduction state all the time, trigger pulses are continuously applied to the thyristors for controlling reverse conduction in the bypasses of the other two phase bridge arms; and maintaining all bypass thyristors of the low-voltage valve bank in a blocking state, and blocking all full-control power electronic device trigger pulses of the high-voltage valve bank and the low-voltage valve bank.
5. The method of claim 4, further comprising maintaining thyristors in the bypass of each phase leg of the low voltage block and each leg of the low voltage block in a blocking state.
6. The method for protecting a high-voltage valve block of a modular multilevel converter according to claim 4, wherein the triggering pulse is applied to the thyristor in the bypass of each phase leg of the high-voltage valve block continuously or only once.
7. The method for protecting a high-voltage valve bank of a modular multilevel converter from ground faults according to claim 1 or 4, further comprising locking trigger pulses of all fully-controlled power electronic devices in an upper bridge arm and a lower bridge arm of each phase of the high-voltage valve bank.
8. The method for protecting a high-voltage valve bank of a modular multilevel converter from ground faults according to claim 1 or 4, further comprising locking trigger pulses of all fully-controlled power electronic devices in an upper bridge arm and a lower bridge arm of each phase of the low-voltage valve bank.
9. The method for protecting a high-voltage valve group of a modular multilevel converter against a ground fault according to claim 1 or 4, wherein, when a phase-controlled converter comprising thyristors is connected to the dc side of the modular multilevel converter, after the high-voltage valve group against the ground fault,
the phase-controlled inverter is locked out,
or the phase control converter is switched to an inversion operation state,
Or the current command value of the phase control converter is adjusted to be zero or negative value.
10. A method of protecting a high voltage valve bank of a modular multilevel converter against ground faults according to claim 1 or 4, further comprising opening an ac circuit breaker in the modular multilevel converter.
11. The method for protecting a high-voltage valve bank of a modular multilevel converter against ground faults according to claim 1 or 4, wherein when an upper bridge arm and a lower bridge arm are formed by cascading a plurality of blocking sub-modules and a plurality of half-bridge sub-modules, the bypass comprises bypass branches corresponding to the sub-modules one by one; the bypass branch comprises two thyristors connected in parallel in opposite directions of each blocking type submodule voltage output end or one bidirectional thyristor connected in parallel with the output end, and one thyristor or two thyristors connected in parallel in opposite directions of each half-bridge submodule voltage output end or one bidirectional thyristor; the anode of the one thyristor is connected with the high-voltage output end of the half-bridge sub-module, and the cathode of the one thyristor is connected with the low-voltage output end of the half-bridge sub-module.
12. The method for protecting a high-voltage valve block of a modular multilevel converter according to claim 1 or 4, wherein when an upper bridge arm and a lower bridge arm are each formed by cascading a plurality of blocking type submodules and a plurality of half-bridge submodules, the bypass comprises bypass branches corresponding to valve segments one by one, and each bypass branch comprises a bidirectional thyristor valve connected in parallel between a high-voltage output end and a low-voltage output end of each valve segment formed by connecting the blocking type submodules in series, and a unidirectional thyristor valve or a bidirectional thyristor valve connected in parallel between the high-voltage output end and the low-voltage output end of each valve segment formed by connecting the half-bridge type submodules in series; the anode of the one-way thyristor valve is connected with the high-voltage output end of the valve section of the corresponding half-bridge submodule, and the cathode of the one-way thyristor valve is connected with the low-voltage output end of the valve section of the corresponding half-bridge submodule.
13. The method for protecting a high-voltage valve bank of a modular multilevel converter against ground faults according to claim 1 or 4, wherein when an upper bridge arm and a lower bridge arm are formed by cascading a plurality of blocking sub-modules and a plurality of half-bridge sub-modules, the bypass comprises bypass branches corresponding to each group in a valve section one by one; dividing valve sections formed by connecting blocking sub-modules in series into a plurality of groups; dividing valve sections formed by connecting half-bridge submodules in series into a plurality of groups;
the bypass branch comprises a bidirectional thyristor valve connected with the output end of each group of blocking type submodule in parallel, and a unidirectional thyristor valve or a bidirectional thyristor valve connected with the output end of each group of half-bridge type submodule in parallel; the anode of the one unidirectional thyristor valve is connected with the high-voltage output end corresponding to each group of half-bridge submodules, and the cathode of the one unidirectional thyristor valve is connected with the low-voltage output end corresponding to each group of half-bridge submodules.
14. The method for protecting a high-voltage valve bank of a modular multilevel converter against ground faults according to claim 1 or 4, wherein when an upper bridge arm and a lower bridge arm are formed by cascading a plurality of blocking type submodules, the bypass comprises bypass branches corresponding to the blocking type submodules one by one; the bypass branch comprises a thyristor or two thyristors or a bidirectional thyristor which are connected in parallel at the voltage output end of each blocking submodule in an inverse parallel manner; the anode of a thyristor is connected with the high-voltage output end of the blocking type sub-module, and the cathode of the thyristor is connected with the low-voltage output end of the blocking type sub-module.
15. The method for protecting a high-voltage valve block of a modular multilevel converter according to claim 1 or 4, wherein when an upper bridge arm and a lower bridge arm are formed by cascading a plurality of blocking type submodules, the bypass comprises bypass branches corresponding to valve segments one by one, and each bypass branch comprises a unidirectional thyristor valve or a bidirectional thyristor valve connected in parallel between a high-voltage output end and a low-voltage output end of each valve segment formed by connecting the blocking type submodules in series; the anode of the unidirectional thyristor valve is connected with the high-voltage output end of the corresponding blocking type submodule valve section, and the cathode of the unidirectional thyristor valve is connected with the low-voltage output end of the corresponding blocking type submodule valve section.
16. The method for protecting a high-voltage valve block of a modular multilevel converter against ground faults according to claim 15, wherein when an upper bridge arm and a lower bridge arm are formed by cascading a plurality of blocking sub-modules, the bypass comprises bypass branches corresponding to each group in a valve section one by one; dividing valve sections formed by connecting blocking sub-modules in series into a plurality of groups;
the bypass branch comprises a unidirectional thyristor valve or a bidirectional thyristor valve which are connected in parallel with the output ends of each group of blocking sub-modules; the anode of the one-way thyristor valve is connected with the high-voltage output end of the corresponding blocking type submodule, and the cathode of the one-way thyristor valve is connected with the low-voltage output end of the valve section of the corresponding blocking type submodule.
17. The method for protecting a high-voltage valve block of a modular multilevel converter against ground faults according to claim 16, wherein the unidirectional thyristor valve is formed by connecting a plurality of thyristors in series; the bidirectional thyristor valve is formed by connecting a plurality of anti-parallel thyristors in series, or is formed by connecting a plurality of bidirectional thyristors in series, or is formed by connecting two anti-parallel unidirectional thyristor valves in parallel.
18. The method for protecting a high-voltage valve block of a modular multilevel converter against ground faults according to claim 16, wherein a current limiting resistor is respectively connected in series in each bypass branch, and the current limiting resistor is correspondingly connected in series with a low-voltage output end or a high-voltage output end of the bypass branch.
19. The method for protecting a high-voltage valve block of a modular multilevel converter from ground faults according to claim 18, wherein the resistance of the current limiting resistor is such that the bypass thyristor does not generate overcurrent, the alternating current circuit breaker of the modular multilevel converter is not broken due to the overcurrent, and the voltage drop across the current limiting resistor is lower than the sum of the capacitance voltages of all the sub-modules connected in parallel.
20. The method for protecting a high-voltage valve bank of a modular multilevel converter against a ground fault according to claim 1 or 4, wherein the blocking type submodule is a modular multilevel converter power module with a capability of blocking direct-current fault current, and any one of a full-bridge submodule, a clamping double submodule, a diode clamping submodule, a self-blocking type submodule and a crosslinking submodule is adopted.
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