CN111276942B - Bridge arm current stress reduction method for offshore wind power flexible direct-transmission system - Google Patents

Bridge arm current stress reduction method for offshore wind power flexible direct-transmission system Download PDF

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CN111276942B
CN111276942B CN202010225794.7A CN202010225794A CN111276942B CN 111276942 B CN111276942 B CN 111276942B CN 202010225794 A CN202010225794 A CN 202010225794A CN 111276942 B CN111276942 B CN 111276942B
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bridge arm
current
criterion
counter
mmc
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CN111276942A (en
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张军
林卫星
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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
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/10—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers
    • H02H7/12—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers
    • H02H7/122—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers for inverters, i.e. dc/ac converters
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H1/00—Details of emergency protective circuit arrangements
    • H02H1/0007—Details of emergency protective circuit arrangements concerning the detecting means
    • 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
    • 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
    • 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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Abstract

The invention discloses a bridge arm current stress reduction method for an offshore wind power soft direct-sending-out system, which takes a serious fault criterion as a judgment condition of MMC emergency locking. According to the operation and fault characteristics of the system, a serious fault criterion YP aiming at bridge arm overcurrent is designed, and the serious criterion YP consists of two different principles which are complementary to each other, namely YP1 and YP 2. The YP1 is a serious fault criterion based on the change rate of the bridge arm current, and the YP2 is a serious fault criterion based on the surplus current of the bridge arm. One of the two criteria is triggered at will, YP is started, and MMC is locked quickly. According to the scheme for reducing the bridge arm current of the offshore wind power soft and direct sending system, the fault current continuous development time is reduced mainly through the advance judgment of serious faults, and the current stress of the bridge arm is effectively reduced on the basis, so that the safety margin of the offshore wind power system is improved.

Description

Bridge arm current stress reduction method for offshore wind power flexible direct-transmission system
Technical Field
The invention belongs to the technical field of new energy and electric power engineering, and particularly relates to a bridge arm current stress reduction method for an offshore wind power soft direct-output system.
Background
With the increasing energy problem, the development and utilization of new clean energy has become a hot issue in the world. With the continuous progress of science and technology, the offshore wind power generation capacity is continuously expanded, and the development of offshore wind power plants has important significance for solving the energy crisis. Offshore wind power grid-connected operation becomes the most effective mode for large-scale utilization of wind energy, and direct current transmission is suitable for large-capacity and long-distance electric energy transmission. With the requirements of longer transmission distance and larger transmission capacity, the direct current transmission plays an important role in the development and utilization of offshore wind farms. 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.
With continuous operation of onshore flexible direct-current transmission projects, the MMC technology is gradually mature, but the offshore converter station has the characteristics of high maintenance cost, long period, high difficulty and the like. The design of the offshore converter station is always a key technology in a high-capacity offshore wind power flexible direct current transmission system, and a plurality of problems exist at present. The safety and stability of the flexible direct current transmission MMC determine the reliability of the whole flexible direct current transmission system. Due to the limitation of the self characteristics of the fully-controlled power electronic device, the safe operation of the flexible-direct MMC is always disturbed by the relatively high current stress level and the low safety margin of the flexible-direct MMC. In the offshore wind power flexible direct-transmission system, the problem of current stress is more severe.
At present, no engineering example of an offshore platform for connecting offshore wind power flexible direct current to an electric power system exists domestically, the effect of suppressing the MMC current stress level from the angles of one-time complete set design and improvement of device current and water is limited, and the economical efficiency and the engineering practicability are poor.
Disclosure of Invention
The invention provides a bridge arm current stress reduction method for an offshore wind power soft and direct sending-out system, which can effectively reduce the current stress level of a bridge arm and cannot influence the normal operation of the offshore wind power soft and direct sending-out system.
In order to achieve the purpose, the bridge arm current stress reduction method for the offshore wind power soft and direct sending-out system takes a serious fault criterion YP as a judgment basis for rapid MMC locking, and after a serious fault occurs in the system, the MMC is locked after the serious fault criterion YP is triggered and started;
the serious fault criterion YP consists of a criterion A and a criterion B which have different principles, and if any one of the criterion A and the criterion B is triggered to be started, the serious fault criterion YP is triggered to be started;
the criterion A is a severe fault criterion YP1 based on the change rate of the bridge arm current, and the criterion B is a severe fault criterion YP2 based on the surplus current of the bridge arm.
Further, the method comprisesWhen the current period variation quantity delta I of the bridge armt2When the protection constant value DDI of the bridge arm current change rate is exceeded, the count value in the continuous sampling period exceeds the counting constant value ND, and the enable signal of the counter is SN1 which is equal to 1, the serious fault criterion YP1 based on the bridge arm current change rate is triggered, the output signal is 1, otherwise, the serious fault criterion YP1 based on the bridge arm current change rate is triggered, and the output signal is 0;
the enabling signal of the counter is SN1, the running state of the MMC is used as the enabling signal, when the MMC is in the unlocking state, SN1 is 1, and the counter is started; when the MMC is in the locked state, SN1 is 0, and the counter is not started.
Further, the calculation of the output signal of the critical fault criterion YP1 based on the bridge arm current change rate comprises the following steps:
step 1, calculating current period variation quantity delta I according to formula (1)t2：
Figure BDA0002427593740000021
Wherein, It2For the peak current value of each sampling period, It1The current peak value of the last sampling period is obtained, and T is the sampling period of the valve control system;
step 2, changing the current period by delta It2Compared with the protection fixed value DDI, the comparison result CP1 is used as a clear signal of the counter:
when Δ It2 < DDI, CP1 is 1, the counter is always in a zero-clearing state, the count result Nt is zero and is less than ND, and therefore the counter output result YP1 is zero;
when Δ It2 is not less than DDI, CP1 is 0, when the counter enable signal SN1 is 1, the counter starts counting, each sampling period is a counting period, and when the count result Nt is greater than ND, the counter output result YP1 is 1; when the enable signal SN1 is 0, the counter does not start the counting function, the count result is zero, and the counter output result YP1 is 0.
Further, when the bridge arm surplus current InyWhen the protection constant value Iset4 of the surplus current of the bridge arm is more than or equal to the protection constant value Iset4, based on the surplus current of the bridge armThe triggering output signal of the re-fault criterion YP2 is 1, otherwise, the triggering output signal of the YP2 is 0; surplus current I of bridge armnyIs defined as: i isny＝∑Iup-∑Idown(ii) a Wherein, IupFor each phase upper arm current, IdownFor each phase lower leg current.
Further, the bridge arm current change rate protection fixed value DDI is 3000A/ms-6000A/ms.
Further, bridge arm current surplus current protection constant value Iset4Is 500A-800A.
Compared with the prior art, the invention has at least the following beneficial technical effects:
1. the method and the device are designed according to the criterion of the fault causing the serious overcurrent of the MMC, the continuous development time of the fault current is reduced by quickly judging the serious fault, the current stress of a bridge arm is effectively reduced on the basis, and the current stress of the bridge arm can be effectively reduced, so that the safety margin of the offshore wind power system is improved.
2. The invention adopts two criteria of bridge arm current change rate and bridge arm surplus current as complementation, and can completely and accurately identify faults; the MMC current stress can be effectively reduced, the safety of equipment and a system is improved, and the design of the MMC is facilitated.
Under the condition of partial faults such as two short-circuit grounding faults at the valve side and the like, the existing protection strategy and current change rate protection cannot meet the safety requirement of electrical stress; if the two short circuit grounds at the direct current side adopt the existing bridge arm protection strategy and bridge arm surplus current protection, the MMC current stress can not meet the safety requirement.
3. The bridge arm overcurrent protection is improved aiming at the prior bridge arm overcurrent protection, any primary equipment and measuring equipment are not required to be added in the implementation process, namely, any cost is not increased, the implementation process is simple, and the economical efficiency is better in the actual operation of the converter station.
Further, the bridge arm current change rate protection fixed value DDI is 3000A/ms-6000A/ms; bridge arm current surplus current protection setting value Iset4500A to 800A; severe fault criterion YP1 based on bridge arm current change rate and severe fault criterion based on bridge arm surplus currentYP2 is a structural fault criterion, the bridge arm current change rate protection fixed value DDI set by the two criteria has larger difference with the bridge arm current change rate in the operating modes of non-serious fault, fault ride-through and the like, and the bridge arm current surplus current protection fixed value Iset4And the surplus current of the bridge arm under the operation modes of non-serious faults, fault ride-through and the like is greatly different, so that under the operation modes of non-serious faults, fault ride-through and the like, two criteria cannot be triggered, and the normal operation of the system is ensured.
Drawings
FIG. 1 illustrates a typical topology of an offshore wind power soft-direct export system;
FIG. 2 shows a bridge arm current development trend in an offshore wind power soft and direct sending-out system under a fault;
FIG. 3 illustrates a critical fault criterion based on bridge arm current rate of change;
FIG. 4 illustrates a critical fault criterion based on bridge arm surplus current;
FIG. 5 illustrates a bridge arm current stress reduction scheme based on a critical fault criterion.
Detailed Description
In order to make the objects and technical solutions of the present invention clearer and easier to understand. The present invention will be described in further detail with reference to the following drawings and examples, wherein the specific examples are provided for illustrative purposes only and are not intended to limit the present invention.
The scheme is designed after comprehensively and deeply analyzing the offshore wind soft direct-sending system, the current stress level of a bridge arm can be effectively reduced by adopting the scheme, the normal operation is not influenced, 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 of engineering implementation is very low, and the engineering implementation has better economical efficiency.
A severe fault criterion is added in a basic bridge arm overcurrent protection strategy of an MMC valve control system, and when a fault occurs and is obtained through the function of the severe criterion, the fault belongs to a fault causing serious MMC overcurrent, a criterion enabling signal YP is 1, and enabling signals YP are 0 under other conditions. When the output of the serious criterion function signal YP is 1, directly triggering MMC locking protection; when the YP output is 0, this function is not activated. The scheme can not increase the area of a primary system and an offshore platform, the technical cost of engineering realization is very low, and the economic efficiency is better.
The bridge arm current reduction scheme designed by the invention takes a serious fault criterion as a judgment condition of MMC emergency locking. According to the operation and fault characteristics of the system, a severe fault criterion YP for causing bridge arm overcurrent is designed, and the severe fault criterion YP simultaneously consists of a criterion YP1 and a criterion YP2 which have different principles and are mutually complemented. The criterion YP1 is a serious fault criterion based on the change rate of the bridge arm current, and the criterion YP2 is a serious fault criterion based on the surplus current of the bridge arm. If the criterion YP1 and the criterion YP2 are triggered randomly, the serious fault criterion YP is started, and the MMC is locked quickly. The invention mainly reduces the continuous development time of fault current by judging serious faults in advance, and effectively reduces the current stress of a bridge arm on the basis, thereby improving the safety margin of the offshore wind power system.
Firstly, a scheme design principle for reducing the MMC current stress in the offshore wind power flexible direct-sending system is described by combining with a figure 1, wherein the offshore wind power flexible direct-sending system comprises an offshore wind power plant, an alternating current collecting system, an alternating current booster station, an offshore converter station, a direct current transmission system and a land converter station, and the MMC is located in the offshore converter station.
Alternating current multi-circuit submarine cables are adopted in alternating current lines of the offshore wind power station and the offshore converter station, and direct current one-circuit submarine cables are adopted in direct current transmission lines. Due to the factors of high cost of the offshore platform, high fault rate of the direct current submarine cable, high price and the like, the symmetrical monopole MMC topological structure is more suitable for being adopted by an offshore wind power flexible direct-sending-out system. By adopting the system scheme, the areas of the convertor station equipment and the offshore platform can be effectively reduced, and the system has better economy.
When the offshore wind power soft direct-sending system breaks down and causes the bridge arm of the MMC to be in overcurrent, the MMC performs locking protection when the current of the bridge arm exceeds a protection fixed value ID, and due to the fact that the detection and valve control system is delayed, the MMC can really complete locking after the overcurrent happens for a period of time.
As shown in fig. 2, when a serious failure occurs in the MMC at time t0, the bridge arm current of the MMC increases rapidly; bridge arm current I delayed by TDarmWhen the valve control protection fixed value ID is exceeded, the bridge arm locking protection is triggered; passing of the latch-up time TLAfter time delay, the bridge arm is locked, and the fault current in the bridge arm reaches a peak value Ip; re-delay TSThe current in the bridge arm decays to 0.
According to the MMC control protection strategy, when the peak value of the bridge arm current reaches a valve control protection fixed value ID, 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.
The locking time T is caused by factors of measuring equipment and other hardware in the offshore wind power soft and direct sending-out systemLGenerally, the optimization space is limited due to the fact that the optimization space is relatively fixed. Under serious fault, the current rises to valve control protection constant value IDAt the latch-up time TLThe current of the inner bridge arm continuously and rapidly rises. When a serious fault occurs, the current peak value may exceed the safety value of MMC current stress, and the operation of the offshore wind power flexible direct-transmission system is threatened.
As 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 relying on the traditional bridge arm overcurrent protection, especially the maintenance and repair cycle of the MMC equipment in the offshore converter station is long, and the reduction of the MMC current stress is very critical to the safety and stability of the system.
The invention designs a serious fault criterion aiming at the condition causing the serious overcurrent of the bridge arm. The critical failure criteria are described below in conjunction with fig. 3 and 4.
FIG. 3 illustrates a critical fault criterion based on bridge arm current rate of change.
Bridge arm current enters a valve control system through sampling, and the MMC locks the front bridge arm current to flow through a power device in the MMC. 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 rated current I of the power deviceN. The bridge arm current change rate protection constant value DDI and the counting constant value ND are obtained by carrying out a large amount of analysis and simulation demonstration on the system. The bridge arm current change rate protection fixed value DDI is 3000A/ms-6000A/ms; the value range of the counting fixed value ND is 5-10.
The enable signal of the counter is SN1, and the operation state of the MMC is used as the enable signal. When the MMC is in an unlocking state, SN1 is equal to 1, and a counter is started; when the MMC is in the locked state, SN1 is equal to 0, the counter is not started, and the count result output is zero.
The sampling period of the valve control system is T, and the current peak value I of each sampling period ist2Current peak value I of last sampling periodt1Calculating by using a formula (1) to obtain the bridge arm current period variation delta It2：
Figure BDA0002427593740000071
Then the current period is changed by delta I through a comparator At2Comparing with the current change rate fixed value DDI, and taking the output result CP1 of the comparator as a zero clearing signal of the counter:
when the current period variation Δ It2 is smaller than the bridge arm current variation rate fixed value DDI, CP1 is 1, the counter is always in the zero clearing state, the count result Nt is zero and is smaller than the count fixed value ND, and therefore the comparator B output result YP1 is zero.
When the current period variation of the delta It2 is larger than or equal to the fixed value DDI of the bridge arm current variation rate, the CP1 is equal to 0:
when the counter enable signal SN1 is 1, the counter starts counting with each sampling period as a count period, and when the count result Nt is greater than the count fixed value ND, the comparator B outputs a result YP1 of 1;
when the enable signal SN1 is 0, the counter does not start the counting function, the count result is zero, and the counter output result YP1 is 0.
When a system has a serious fault, the impedance in a fault loop is relatively low, so that the increase rate of fault current is very high, and whether the serious fault occurs in a station can be quickly judged by adopting the rate detection function shown in fig. 3.
The adoption of current rate protection alone may not allow rapid identification and determination of serious faults within a station for certain faults or for certain times when faults occur. Through the deep research of the system, the current rate judgment and the bridge arm surplus current judgment can completely and accurately identify the serious fault in the station.
FIG. 4 illustrates a critical fault criterion based on bridge arm surplus current.
Surplus current I of bridge armnyIs defined as:
Iny＝∑Iup-∑Idown
in the above formula, IupRepresenting the upper arm current of each phase, IdownRepresenting the lower bridge arm current of each phase; in the steady-state operation process, surplus current I of a bridge armnySubstantially equal to zero. In a severe in-station fault, the bridge arm surplus current Iny increases rapidly.
The bridge arm surplus current protection setting value I is obtained by carrying out a large amount of analysis and simulation demonstration on the systemset4。
In the running process of the system, the upper bridge arm current I adopted in the ith periodDiAnd lower bridge arm current IUiObtaining real-time bridge arm surplus current I in the ith sampling period by difference calculationnyi。
When surplus current I of bridge armnyi< surplus current protection setting value I of bridge armset4Output signal YP2 is 0; and the surplus current protection of the bridge arm is not triggered.
When surplus current I of bridge armnyProtection constant value I for excessive current of bridge armset4Output signal YP2 is 1; and judging that the surplus current of the bridge arm exceeds a protection fixed value and judging that a serious fault exists in the system, thereby triggering the surplus current protection function of the bridge arm.
Bridge arm current surplus current protection setting value Iset4Is 500A-800A. By combining the two judgment methods of fig. 3 and fig. 4, the serious fault in the station can be completely and accurately judged and identified. In the invention, electricity is simultaneously designedThe current change rate, namely the speed change protection YP1 function and the bridge arm surplus current protection YP2 are two serious fault criteria.
Through simulation and analysis of the offshore wind power soft and direct sending-out system, two functions are in a mutual complementary relationship in actual operation. The critical fault criterion YP designed in the present invention is therefore the logical or generation of YP1 and YP 2.
FIG. 5 illustrates a method for bridge arm current stress reduction based on a critical fault criterion.
In the method, after the fault occurs, whether the fault is a serious in-station fault is judged according to a serious fault criterion YP.
The critical fault criterion YP is triggered by triggering of the criterion YP1 or the criterion YP 2. According to fig. 3 and 4, after a serious fault occurs in the station, the bridge arm current rate protection or the bridge arm surplus current protection can be triggered rapidly according to the system characteristics. Therefore, the fault development time TD is greatly reduced, the bridge arm electric peak value IP is effectively reduced, and the bridge arm current stress is effectively reduced.
And meanwhile, two protection modes are adopted, so that serious faults in the station can be completely and accurately identified.
When YP is 1, MMC locking protection is immediately carried out, and locking time T is passedLAnd completing converter locking.
When the YP is 0, the fault ride-through is required to be carried out on the system, or the fault of the system is a non-serious fault, so that the serious influence of bridge arm current stress cannot be generated. And the bridge arm overcurrent protection of the MMC is processed according to the existing bridge arm overcurrent protection strategy.
Aiming at the characteristics of high reliability requirement, high maintenance cost and the like of an MMC in an offshore wind power soft direct-sending-out system, the problem that the effect of a scheme for reducing the current stress of a bridge arm by adopting one-time system optimization, compression locking time and the like in the existing engineering is limited is solved, and the bridge arm current reduction scheme based on the serious fault criterion is provided. According to the scheme, after detailed analysis and simulation are carried out on a system, the fault working condition causing serious overcurrent of a bridge arm is directly identified and judged, and MMC locking protection is carried out on the basis.
After the scheme for reducing the bridge arm current stress of the offshore wind power soft and direct sending-out system is adopted, the MMC current stress is effectively reduced, the safety of equipment and the system is improved, and the design of the MMC is facilitated. In addition, no primary equipment and measuring equipment are added in the implementation process of the scheme, the implementation process is simple, and the economical efficiency is better in the actual operation of the converter station.
The above embodiments are only used for illustrating the technical solutions of the present invention and not for limiting the same, and after reading the present application, those skilled in the art will make various modifications or alterations of the present invention with reference to the above embodiments within the scope of the claims of the present invention.

Claims (3)

1. A bridge arm current stress reduction method for an offshore wind power soft and direct sending-out system is characterized in that a severe fault criterion YP is used as a judgment basis for rapid MMC locking, and after a system has a severe fault, an MMC is locked after the severe fault criterion YP is triggered and started;
the serious fault criterion YP consists of a criterion A and a criterion B which have different principles, and if any one of the criterion A and the criterion B is triggered, the serious fault criterion YP is triggered and started;
the criterion A is a serious fault criterion YP1 based on the change rate of the bridge arm current, and the criterion B is a serious fault criterion YP2 based on the surplus current of the bridge arm;
when the current period of the bridge arm changes by delta It2When the protection constant value DDI of the bridge arm current change rate is exceeded, the count value in the continuous sampling period exceeds the counting constant value ND, and the enable signal of the counter is SN1 which is equal to 1, the serious fault criterion YP1 based on the bridge arm current change rate is triggered, the output signal is 1, otherwise, the serious fault criterion YP1 based on the bridge arm current change rate is triggered, and the output signal is 0;
the enabling signal of the counter is SN1, the running state of the MMC is used as the enabling signal, when the MMC is in the unlocking state, SN1 is 1, and the counter is started; when the MMC is in a locked state, SN1 is equal to 0, and the counter is not started;
the calculation of the output signal of the severe fault criterion YP1 based on the bridge arm current change rate comprises the following steps:
step 1, calculating current period variation quantity delta I according to formula (1)t2：
Figure FDA0003508363360000011
Wherein, It2For the peak current value of each sampling period, It1The current peak value of the last sampling period is shown, and T is the sampling period of the valve control system;
step 2, changing the current period by delta It2Compared with the protection fixed value DDI, the comparison result CP1 is used as a clear signal of the counter:
when Δ It2 is less than DDI, CP1 is 1, the counter is always in a zero clearing state, the count value Nt in the continuous sampling period is zero and is less than ND, and therefore the counter output result YP1 is zero;
when Δ It2 is greater than or equal to DDI, CP1 is 0, when the counter enable signal SN1 is 1, the counter starts counting, each sampling period is a counting period, and when the count value Nt in successive sampling periods is greater than ND, the counter output result YP1 is 1; when the enable signal SN1 is 0, the counter does not start the counting function, the counting result is zero, and the counter output result YP1 is 0;
when surplus current I of bridge armnyWhen the protection constant value Iset4 of the surplus current of the bridge arm is larger than or equal to the protection constant value Iset4, triggering an output signal to be 1 based on the serious fault criterion YP2 of the surplus current of the bridge arm, otherwise triggering an output signal to be 0 by YP 2;
surplus current I of bridge armnyIs defined as: i isny＝∑Iup-∑Idown(ii) a Wherein, IupFor each phase upper arm current, IdownFor each phase lower leg current.
2. The method for reducing the stress of the bridge arm current for the offshore wind power flexible direct-sending system according to claim 1, wherein the bridge arm current change rate protection setting value DDI is 3000A/ms-6000A/ms.
3. The method of claim 1A bridge arm current stress reduction method for an offshore wind power flexible direct-transmission system is characterized in that a bridge arm surplus current protection fixed value Iset4Is 500A-800A.
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