CN113131444A - Bridge arm current stress reduction method and system for flexible direct current transmission system - Google Patents
Bridge arm current stress reduction method and system for flexible direct current transmission system Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/10—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers
- H02H7/12—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers
- H02H7/122—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers for inverters, i.e. dc/ac converters
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/10—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers
- H02H7/12—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers
- H02H7/125—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers for rectifiers
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/10—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers
- H02H7/12—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers
- H02H7/125—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers for rectifiers
- H02H7/1255—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers for rectifiers responsive to internal faults, e.g. by monitoring ripple in output voltage
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- 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
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]
Abstract
The invention relates to a method and a system for reducing bridge arm current stress of a flexible direct current transmission system, belonging to the technical field of new energy and power engineering, wherein the method comprises the steps of collecting an upper bridge arm current and a lower bridge arm current of each phase of a three-phase bridge arm of a converter valve at the same moment; the method comprises the steps of calculating bridge arm surplus current of a three-phase bridge arm of the converter valve, reflecting the three-phase unbalance degree of the converter valve through the bridge arm surplus current, executing bridge arm overcurrent protection action if the bridge arm surplus current is larger than a preset surplus current protection fixed value, and executing the bridge arm overcurrent protection action through conventional overcurrent protection if the bridge arm surplus current is not larger than the preset surplus current protection fixed value, so that the problems of poor current stress suppression effect and high cost of the bridge arm of the converter valve in the prior art are solved.
Description
Technical Field
The invention relates to a method and a system for reducing bridge arm current stress of a flexible direct current transmission system, and belongs to the technical field of new energy and electric power engineering.
Background
The flexible direct current transmission is a new generation high-voltage direct current transmission technology adopting a voltage source type converter, and a flexible direct current transmission system generally comprises two-end converter stations, a direct current transmission line and the like.
In the application of the flexible direct current transmission power transmission system, as offshore wind power grid-connected operation becomes the most effective mode for utilizing wind energy in a large scale, direct current transmission is also widely applied to grid-connected consumption of offshore wind power plant wind energy. Compared with a conventional high-voltage direct-current transmission and voltage source type converter (VSC-HVDC) with two levels and three levels, the offshore wind power flexible direct-current access system based on the Modular Multilevel Converter (MMC) is more suitable for a long-distance and large-scale offshore wind power access system.
The current stress, which is the ratio of the current at which the device actually operates to the rated specification, generally does not exceed 80%. Thus, in actual operation of the converter, reducing current stress can significantly improve system stability when a fault occurs.
In the conventional bridge arm overcurrent protection, whether bridge arm current exceeds a set threshold value is generally judged, if so, corresponding protection action is executed, and fault current does not flow in a bridge arm through the locking process of a current converter, so that protection is realized.
As shown in fig. 1, is a typical topology of an offshore wind power flexible direct-out (transmission) system. The offshore wind power flexible direct-transmission system consists of an offshore wind power plant, an alternating current collecting system (not shown in the figure), a booster station (alternating current), an offshore converter station, a direct current transmission system and an onshore converter station, wherein a converter valve is positioned in the offshore converter station.
Fig. 2 shows the development trend of bridge arm current in the offshore wind power flexible direct-transmission system under a fault. When t is shown in FIG. 20After the MMC has a serious fault at any moment, the bridge arm current is rapidly increased; through TDTime delay IarmExceeding the valve-controlled protection constant value IDWhen the bridge arm is locked, the protection is triggered; through TLTime delay, the bridge arm completes locking, and the fault current reaches the peak value Ip(ii) a Through TSAnd delaying, and the current attenuation in the bridge arm is 0.
According to the conventional MMC control protection strategy, when the current peak value of the bridge arm reaches a protection constant value IDAnd the MMC performs locking protection. After the locking protection action is finished, the current is cut off through flow, and the fault current in the bridge arm begins to drop.
Due to factors in the measuring equipment and other hardware in the system, TLGenerally, the optimization space is limited due to the fact that the optimization space is relatively fixed. Under severe fault, current rises to IDThe fault can be detected after the fault is detected, and the fault detection has longer time delay and then TLThe delayed internal current continues to rise rapidly. When a critical fault occurs, the current peak may exceed the safe value of the converter valve current stress, threatening the operation of the system.
And for the offshore wind power flexible direct-output system, the distributed capacitance parameter in the system is larger due to the submarine cables adopted on the alternating current side and the direct current side. After the fault occurs, the fault current development and distribution characteristics are far more serious than those of the conventional flexible direct current converter station.
Therefore, the risk may exist only by depending on the traditional bridge arm overcurrent protection, meanwhile, converter valve equipment in the offshore converter station has the characteristic of long maintenance and overhaul period, and the reduction of the current stress of the converter valve is very critical to the safety and stability of the system.
At present, aiming at the problem of bridge arm overcurrent protection, a generally adopted solution is to perform converter valve current stress level suppression through one-time complete design and angle improvement of device current stress level and the like, for example, a scheme of reducing bridge arm current stress by adopting one-time system optimization, compression locking time and the like is adopted, but the effect is limited, and the economical efficiency and the engineering practicability are poor.
Disclosure of Invention
The invention aims to provide a method and a system for reducing bridge arm current stress of a flexible direct-current transmission system, and aims to solve the problems of poor effect of inhibiting bridge arm current stress of a converter valve and high cost in the prior art.
In order to achieve the purpose, the technical scheme of the invention is as follows: the invention provides a bridge arm current stress reduction method of a flexible direct current transmission system, which comprises the following steps:
1) collecting an upper bridge arm current and a lower bridge arm current of each phase of a three-phase bridge arm of the converter valve at the same moment;
2) calculating the surplus current of a bridge arm of a three-phase bridge arm of the converter valve, wherein the surplus current I of the bridge arm of the three-phase bridge armnyThe calculation formula of (2) is as follows:
Inyi＝IUi-IDi
wherein, IUiFor each phase upper arm current, IDiFor each phase lower leg current, InyiFor the surplus current of the bridge arm of each phase of bridge arm,
the surplus current vector of the bridge arm of each phase of bridge arm is obtained;
3) if the surplus current of the bridge arm is larger than a preset surplus current protection fixed value IsetThen, thenAnd executing the overcurrent protection action of the bridge arm.
According to the invention, the unbalanced degree of three phases of the converter valve is obtained by calculating the surplus current of the bridge arm of the three-phase bridge arm of the converter valve, the surplus current protection fixed value is set, the relation between the surplus current and the protection fixed value is compared, the fault is judged in advance and the protection action is executed, when the fault occurs in the station, the protection action can be triggered rapidly, so that the continuous development time of the fault current is reduced, the current peak value of the bridge arm is reduced, the current stress level of the bridge arm is effectively reduced, the normal operation is not influenced, the safety margin of an offshore wind power system is improved, the operation is easy in the practical engineering, the realization process is simple, the area increase of a primary system and a platform is not caused, the technical cost of the engineering realization is very low, and the economic.
Further, in order to comprehensively and accurately judge and identify serious faults in the station and improve the running reliability of the system, the method also comprises the following steps of judging whether to perform bridge arm overcurrent protection action according to bridge arm current:
(1) calculating the maximum value of the bridge arm current of a three-phase bridge arm of the converter valve, wherein the maximum value I of the bridge arm current of the three-phase bridge armFThe calculation formula of (2) is as follows:
IF＝max{|IUi|、|IDi|}
(2) when the bridge arm current IFGreater than bridge arm current overcurrent definite value IDAnd then, carrying out the overcurrent protection action of the bridge arm.
Further, the bridge arm overcurrent protection is used for controlling the converter to be locked.
The invention also provides a system for reducing the bridge arm current stress of the flexible direct-current power transmission system, which comprises a processor and a memory, wherein the memory is stored with a computer program, and the processor executes the computer program to realize the following steps:
1) collecting an upper bridge arm current and a lower bridge arm current of each phase of a three-phase bridge arm of the converter valve at the same moment;
2) calculating the surplus current of a bridge arm of a three-phase bridge arm of the converter valve, wherein the surplus current I of the bridge arm of the three-phase bridge armnyMeter (2)The calculation formula is as follows:
Inyi＝IUi-IDi
wherein, IUiFor each phase upper arm current, IDiFor each phase lower leg current, InyiFor the surplus current of the bridge arm of each phase of bridge arm,
the surplus current vector of the bridge arm of each phase of bridge arm is obtained;
3) if the surplus current of the bridge arm is larger than a preset surplus current protection fixed value IsetAnd executing the overcurrent protection action of the bridge arm.
According to the invention, the surplus current fault criterion is added in the valve control system, the bridge arm surplus current of the three-phase bridge arm of the converter valve is calculated, the unbalanced degree of the three phases of the converter valve is obtained, the surplus current protection fixed value is set, the fault is judged in advance and the protection action is executed by comparing the relation between the surplus current and the protection fixed value, when the fault occurs in a station, the protection action can be triggered rapidly, so that the fault current continuous development time is reduced, the bridge arm current peak value is reduced, the current stress level of the bridge arm is effectively reduced, the normal operation is not influenced, the safety margin of an offshore wind power system is improved, the operation is easy in the actual engineering, the implementation process is simple, the area of a primary system and the area of an offshore platform are not increased, the technical cost for engineering implementation is very low, and the economic performance.
Further, in order to comprehensively and accurately judge and identify serious faults in the station and improve the running reliability of the system, the method also comprises the following steps of judging whether to perform bridge arm overcurrent protection action according to bridge arm current:
(1) calculating the maximum value of the bridge arm current of a three-phase bridge arm of the converter valve, wherein the maximum value I of the bridge arm current of the three-phase bridge armFThe calculation formula of (2) is as follows:
IF＝max{|IUi|、|IDi|}
(2) when the bridge arm current IFGreater than bridge arm current overcurrent definite value IDAnd then, carrying out the overcurrent protection action of the bridge arm.
Further, the bridge arm overcurrent protection is used for controlling the converter to be locked.
Drawings
FIG. 1 is a typical topology structure diagram of an existing offshore wind power flexible-direct power transmission system in the invention;
FIG. 2 shows the bridge arm current development trend in the offshore wind power flexible-direct power transmission system under the fault condition of the invention;
fig. 3 is a schematic diagram of fault protection based on the surplus current of the bridge arm in embodiment 1 of the method of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. The components of embodiments of the present invention generally described and illustrated in the figures herein may be arranged and designed in a wide variety of different configurations.
Thus, the following detailed description of the embodiments of the present invention, presented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments of the present invention without making any creative effort, shall fall within the protection scope of the present invention.
The features and properties of the present invention are described in further detail below with reference to examples.
Method example 1:
as shown in fig. 3, in this embodiment, taking overcurrent protection of a converter in the offshore wind power flexible-direct power transmission system as an example, a severe fault criterion based on surplus current of a bridge arm is added to the valve control system, and the fault current of the bridge arm is determined in advance, so that the continuous development time of the fault current is reduced, the current stress of the bridge arm is effectively reduced, and the safety margin of the offshore wind power flexible-direct power transmission system is effectively improved.
The valve control system collects the upper bridge arm current I of each phase of the three-phase bridge arms of the converter valve at the same momentUiLower bridge arm current IDiAnd calculating the surplus current I of the bridge arm of each phase of bridge armnyi，Inyi＝IUi-IDi；
Then, surplus current vector of bridge arm of three-phase bridge arm
Carrying out vector operation to obtain the surplus current of the bridge arm of the three-phase bridge arm of the converter valve, wherein the surplus current of the bridge arm of the three-phase bridge arm is calculated according to the formula:
in the formula, according to kirchhoff's current law, in the steady-state operation process, the bridge arm current I acquired in each period is measuredDi、IUiCalculating to obtain the real-time bridge arm surplus current I of each phase in each sampling periodnyiThe surplus current corresponds to the amplitude variation of each phase of alternating current, and the three-phase current is subjected to vector summation according to the phase angle of each phase at the current acquisition moment of each phase of bridge arm to obtain InySubstantially equal to zero, when the three phases are in equilibrium. However, under a severe fault in a station, when an overcurrent condition occurs in a certain bridge arm, the alternating current corresponding to the fault is caused to change, so that the original three-phase balance state is broken, and then the calculated I is obtainednyIs reflected in the value of (A), in particular will be represented as InyIs rapidly increased, and thus, in the present embodiment, by InyThe value of (a) represents the three-phase unbalance degree of the converter valve.
In this embodiment, the bridge arm current surplus current protection fixed value is obtained by modeling the offshore wind power transmission system, and performing simulation analysis and experimental verificationIset. In this embodiment, in the simulation experiment, in order to select the optimal protection fixed value, the following conditions may be set in advance: comprehensively considering whether the time for executing the overcurrent protection action is proper or not, namely after judging that the surplus current of the bridge arm reaches the surplus protection fixed value, controlling the moment for executing the protection action to be earlier than the moment for triggering the bridge arm locking protection in the existing valve control protection, thereby being capable of protecting the serious fault in advance, and realizing that the peak value which can be reached by the current is smaller than the peak value I of the bridge arm current in the existing serious fault within the delay time for completing the locking of the bridge arm when executing the protection actionpAnd the fault development time is effectively reduced. Meanwhile, the surplus current protection fixed value is selected by considering the practical application of the engineering, and cannot be set to be too small, so that the protection action is frequently executed in the running process of the system. It should be noted that the specific simulation test is as follows: firstly, a primary main loop model and a secondary control system model of the system are built in simulation software according to primary complete set design parameters, a wiring mode and a control strategy of the system, then various faults possibly occurring in the system are combed and subjected to fault scanning under all working conditions, the minimum value and the maximum value of surplus current under all fault working conditions are obtained through simulation calculation, then a preset surplus current protection fixed value is reasonably selected, the protection action is ensured not to be triggered when the system has non-serious faults and needs to carry out fault ride-through, and the system has serious faults and the protection action is effectively triggered without fault crossing, and finally a secondary protection system model is set up in simulation software to check the correctness of the protection constant value, and the mode of the surplus current protection fixed value obtained through the simulation experiment process is conventional, can be obtained and is also feasible after verification.
In the operation process of the system, after the surplus current of the bridge arm of the three-phase bridge arm is obtained through calculation, the surplus current is compared and judged with a preset surplus current protection fixed value, and the following corresponding processing strategies are executed:
when I isnyIs less than IsetJudging that no serious fault occurs; at the moment, the system needs to carry out fault ride-through, or the fault of the system is a non-serious fault, so that the current stress of a bridge arm cannot be seriously influenced, and the bridgeThe arm surplus current protection is not triggered.
When I isnyIs greater than IsetAnd when the surplus current of the bridge arm exceeds the surplus current protection fixed value, judging that the system has a serious fault in the station, triggering the surplus current protection function of the bridge arm, immediately carrying out MMC locking protection, and after the locking time T, carrying out the MMC locking protectionLAnd completing converter locking.
In this embodiment, the bridge arm overcurrent protection action of the MMC is handled conventionally, so detailed description is omitted here.
Method example 2:
in the embodiment, the in-station serious fault is judged and identified comprehensively and accurately. On the basis of the embodiment 1, a bridge arm current protection function is designed, two fault protection functions of the bridge arm current protection function and the bridge arm surplus current protection function are integrated, and the two fault protection functions complement each other in two ways to perform a serious fault protection action.
In this embodiment, fault determination is further performed based on a serious fault criterion of the bridge arm current, so that overcurrent protection is performed according to a determination result.
And (3) collecting bridge arm current, and blocking the front bridge arm current by the MMC to flow through a power device in the converter valve. In the stable operation process of the system, the peak value of the bridge arm current is a fixed value and is generally smaller than the peak value I of the bridge arm current rated by the systemN。
In this embodiment, similarly to the setting of the surplus current protection fixed value, the bridge arm current protection fixed value I is obtained by performing simulation test verification on the offshore wind power flexible-direct power transmission systemDIn the setting process of the fixed value, protection action for serious faults also needs to be performed, and the requirement for operation efficiency in engineering application is met.
In this embodiment, the current peak values of the bridge arms in each sampling period are evaluated to obtain absolute values, and then the absolute values are sequenced to obtain the maximum value I of the bridge arm currentF。
Then, the maximum value I of the bridge arm current is calculatedFAnd bridge arm current protection constant value IDAnd (3) comparison:
when I isFIs less than IDWhen it is determined thatWhen a serious fault occurs, the current protection of the bridge arm is not triggered;
when I isFIs greater than IDAnd judging that a serious fault occurs, and executing protection action at the moment.
In this embodiment and the method embodiment 1, two protection functions are mutually supplemented and completed in actual operation. And meanwhile, two protection modes are adopted, so that serious faults in the station can be more accurately identified. When protection is carried out, the surplus current of the bridge arm is firstly adopted for fault judgment, when the condition is met, protection action is executed, when the condition is not met, the current of the bridge arm is adopted for fault judgment, when the condition is met, the protection action is executed, but at the moment, the protection fixed value is required to be reasonably set according to the judgment.
The embodiment of the system is as follows:
the embodiment provides a bridge arm current stress reduction system of a flexible direct current power transmission system, which includes a processor and a memory, where the memory stores a computer program for running on the processor, the processor may be implemented by a single chip microcomputer, an FPGA, a DSP, a PLC, or an MCU, and the memory may be a RAM memory, a flash memory, a ROM memory, an EPROM memory, an EEPROM memory, a register, a hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art, and the storage medium may be coupled to the processor, so that the processor can read information from the storage medium, or the storage medium may be a component of the processor. The processor executes the computer program stored in the memory to implement the steps of:
1) collecting three-phase bridge arm current of a converter valve;
2) calculating the deviation of the upper bridge arm current and the lower bridge arm current in each phase of bridge arm current, and calculating the sum of the deviations of the three phases of bridge arms to obtain surplus bridge arm current;
3) and if the surplus current of the bridge arm is greater than the surplus current protection fixed value, executing the overcurrent protection action of the bridge arm.
The implementation of each step has been described in detail in the above method embodiments, and thus is not described herein again.
The above description is only a preferred embodiment of the present invention, and not intended to limit the present invention, the scope of the present invention is defined by the appended claims, and all structural changes that can be made by using the contents of the description and the drawings of the present invention are intended to be embraced therein.
Claims (6)
1. A bridge arm current stress reduction method of a flexible direct current transmission system is characterized by comprising the following steps:
1) collecting an upper bridge arm current and a lower bridge arm current of each phase of a three-phase bridge arm of the converter valve at the same moment;
2) calculating the surplus current of a bridge arm of a three-phase bridge arm of the converter valve, wherein the surplus current I of the bridge arm of the three-phase bridge armnyThe calculation formula of (2) is as follows:
Inyi＝IUi-IDi
wherein, IUiFor each phase upper arm current, IDiFor each phase lower leg current, InyiFor the surplus current of the bridge arm of each phase of bridge arm,
the surplus current vector of the bridge arm of each phase of bridge arm is obtained;
3) if the surplus current of the bridge arm is larger than a preset surplus current protection fixed value IsetAnd executing the overcurrent protection action of the bridge arm.
2. The method for reducing the stress of the bridge arm current of the flexible direct-current transmission system according to claim 1, further comprising the step of determining whether to perform a bridge arm overcurrent protection action according to the bridge arm current:
(1) calculating the maximum value of the bridge arm current of a three-phase bridge arm of the converter valve, wherein the maximum value I of the bridge arm current of the three-phase bridge armFThe calculation formula of (2) is as follows:
IF＝max{|IUi|、|IDi|}
(2) when the bridge arm current IFGreater than bridge arm current overcurrent definite value IDAnd executing the overcurrent protection action of the bridge arm.
3. The method of reducing bridge arm current stress of a flexible direct current transmission system according to claim 1 or 2, characterized in that the bridge arm overcurrent protection is blocked as a control converter.
4. A system for reducing bridge arm current stress of a flexible direct current transmission system is characterized by comprising a processor and a memory, wherein the memory stores a computer program, and the processor executes the computer program to realize the following steps:
1) collecting an upper bridge arm current and a lower bridge arm current of each phase of a three-phase bridge arm of the converter valve at the same moment;
2) calculating the surplus current of a bridge arm of a three-phase bridge arm of the converter valve, wherein the surplus current I of the bridge arm of the three-phase bridge armnyThe calculation formula of (2) is as follows:
Inyi＝IUi-IDi
wherein, IUiFor each phase upper arm current, IDiFor each phase lower leg current, InyiFor the surplus current of the bridge arm of each phase of bridge arm,
the surplus current vector of the bridge arm of each phase of bridge arm is obtained;
3) if the surplus current of the bridge arm is larger than a preset surplus current protection fixed value IsetAnd executing the overcurrent protection action of the bridge arm.
5. The system for reducing the stress of the bridge arm current of the flexible direct-current power transmission system according to claim 1, further comprising a step of determining whether to perform a bridge arm overcurrent protection action according to the bridge arm current:
(1) calculating the maximum value of the bridge arm current of a three-phase bridge arm of the converter valve, wherein the maximum value I of the bridge arm current of the three-phase bridge armFThe calculation formula of (2) is as follows:
IF＝max{|IUi|、|IDi|}
(2) when the bridge arm current IFGreater than bridge arm current overcurrent definite value IDAnd executing the overcurrent protection action of the bridge arm.
6. The system for leg current stress reduction of a flexible direct current transmission system according to claim 1 or 2, characterized in that the leg overcurrent protection is blocked as a control converter.
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Citations (6)
| Publication number | Priority date | Publication date | Assignee | Title |
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