A method and device for suppressing overcurrent caused by single-pole disconnection for offshore wind power

Abstract

The present invention discloses a method and device for suppressing overcurrent caused by single-pole disconnection for offshore wind power, belonging to the technical field of flexible DC transmission. The method includes: when the offshore converter station and the onshore converter station detect a single-pole disconnection fault of the positive and negative bipolar cables of the submarine cable, controlling the wind farm to start load shedding after a certain delay; locking the converter inside the faulty pole when the energy inside it rises to the second threshold, starting active energy charging and current limiting control for the converter of the non-faulty pole and lasting for a period of time, and starting active boosting and current limiting control for the onshore converter station and lasting for a period of time; when the energy inside the converter of the non-faulty pole rises to the second threshold, keeping the energy from rising any further, entering the active energy holding state after the active energy charging and current limiting control ends, and performing active energy release after the active energy holding ends. The present invention can effectively suppress the overcurrent amplitude caused by single-pole disconnection in a true bipolar system, and avoid serious faults such as DC overcurrent locking and subsequent large-scale disconnection of new energy.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible DC power transmission for offshore wind power, and more specifically, relates to a method and device for suppressing overcurrent caused by single-pole disconnection for offshore wind power. Background Art

[0002] True bipolar offshore wind power has gradually become the main structure for large-scale deep-sea and far-sea wind power transmission due to its advantages in terms of safety, economy, scalability, etc. The modular multilevel converter (MMC) has characteristics such as modularity, low harmonics, and low losses, and is the mainstream topology of the DC converter applied to the DC external transmission system for long-distance and large-capacity offshore wind power. Under this structure and topology, since the sending-end MMC cannot control the power injected into the DC line by the wind power generation system, once a single-pole disconnection fault occurs in the DC transmission line, the channel of the offshore faulty pole is blocked, which will cause all the power generated by the wind farm to be transferred from the offshore faulty pole to the offshore non-faulty pole, thus triggering overcurrent in the offshore non-faulty pole. If the overcurrent is not suppressed in time, it will trigger the overcurrent protection of the transmission line and the locking of the converter station, ultimately resulting in a large area of disconnection of new energy units and greatly reducing the availability of the new energy power generation system.

[0003] To solve the problem of overcurrent caused by disconnection in true bipolar, the measures that can be taken include two categories: reducing the output power of the wind farm, which is restricted by factors such as communication delay and ramp rate, and its effect is not ideal. Installing energy-consuming devices on the AC side of the offshore converter station. However, due to the high construction difficulty and high cost of the offshore platform, this will greatly reduce the engineering economy.

[0004] With the increase in the DC grid voltage level and transmission capacity for new energy access, there is an urgent need for a method that does not require the introduction of energy-consuming devices and has excellent current-limiting effects to ensure the safe and stable operation of the power system. Summary of the Invention

[0005] In view of the above defects or improvement requirements of the prior art, the present invention provides a method and device for suppressing overcurrent caused by single-pole disconnection for offshore wind power. The purpose is to suppress the overcurrent in the offshore non-faulty pole through the energy and voltage coordination control strategies of the onshore converter station and the offshore converter station after a single-pole disconnection fault occurs in the submarine transmission line of the true bipolar offshore wind power through the flexible DC grid connection system, and avoid serious faults such as DC locking and new energy disconnection caused by overvoltage, thereby solving the technical problem of poor AC overcurrent suppression ability of the offshore wind power through the flexible DC external transmission system.

[0006] To achieve the above object, according to one aspect of the present invention, a method for suppressing overcurrent caused by single-pole disconnection for offshore wind power is provided, which is applied to the true bipolar offshore wind power through the flexible DC grid connection system and includes:

[0007] S1: When the offshore converter station and the onshore converter station detect that the positive or negative submarine cable current meets the current mutation condition, it is determined that a single-pole break fault of the submarine cable occurs; the offshore converter station controls the wind farm to start load shedding after a delay until the power drops to the first threshold;

[0008] Among them, the offshore converter station includes a positive converter and a negative converter. The converter corresponding to the pole where the current mutation occurs is recorded as the faulty pole converter, and the other is recorded as the non-faulty pole converter; during the power drop of the wind farm, the internal energy of the faulty pole converter increases;

[0009] S2: When the internal energy of the faulty pole converter rises to the second threshold, control the faulty pole converter to lock and send an active energy charging and current limiting control signal to the non-faulty pole converter, and send an active boosting and current limiting signal to the onshore converter station; the active energy charging and current limiting control signal is used to make the non-faulty pole converter absorb the power of the wind farm to actively increase the internal energy and last for the first preset time; the active boosting and current limiting signal is used to control the DC submarine cable voltage of the onshore converter station to rise and last for the second preset time;

[0010] S3: Before reaching the first preset time, if the internal energy of the non-faulty pole converter rises to the second threshold, keep the internal energy unchanged until the third preset time after reaching the first preset time; then control the non-faulty pole converter to perform active energy release.

[0011] In one embodiment, the S1 includes:

[0012] S101: Use both the offshore converter station and the onshore converter station to monitor the positive and negative submarine cable currents in real time; when it is detected that the positive or negative submarine cable current suddenly becomes 0, it is determined that a single-pole break fault of the submarine cable occurs;

[0013] S102: Control the offshore converter station to send a load shedding signal to the wind farm so that its power drops evenly to the first threshold.

[0014] In one embodiment, the S102 includes:

[0015] Control the offshore converter station to send the load shedding signal to the wind farm, and when the wind farm receives the load shedding signal, its power drops at a uniform rate;

[0016] When the power of the wind farm is reduced to 0.5 pu, control the output power of the wind farm to remain unchanged.

[0017] In one embodiment, the S2 includes:

[0018] S201: When the internal energy of the faulty pole converter rises to the second threshold, control the faulty pole converter to block, send an active energy charging and current limiting control signal to the non-faulty pole converter, and send an active boost and current limiting signal to the onshore converter station;

[0019] S202: Control the non-faulty pole converter to absorb the power of the wind farm according to the energy command value carried by the active energy charging and current limiting control signal, actively increase the internal energy and continue for the first preset time;

[0020] S203: Control the onshore converter station to increase the DC submarine cable voltage according to the active boost and current limiting signal and continue for the second preset time; the second preset time is less than the first preset time.

[0021] In one embodiment, the energy command value is:

[0022]

[0023] where P WFMn is the real-time power received by the non-faulty pole of the offshore converter station from the wind farm, P WFMnN is the rated transmission power of the non-faulty pole, E WFMn is the rated internal energy of the non-faulty pole of the offshore converter station, and t r is the duration from the start of the active energy charging and current limiting control to the current moment.

[0024] In one embodiment, the S3 includes:

[0025] S301: Before reaching the first preset time, when the internal energy increased by the non-faulty pole converter according to the active energy charging and current limiting control signal reaches the second threshold, keep the current internal energy unchanged;

[0026] S302: When reaching the second preset time, the active energy charging and current limiting control of the non-faulty pole converter ends, control the non-faulty pole converter to enter the active energy holding state and continue for the third preset time;

[0027] S303: When reaching the third preset time, the active energy holding state of the non-faulty pole converter ends, and active energy release is performed until the third threshold is reached.

[0028] In one embodiment, the S303 includes:

[0029] When reaching the third preset time, the active energy holding state of the non-faulty pole converter ends, and active energy release is performed at a uniform rate of 0.002 pu / ms until the internal energy drops to 1.0 pu and the energy remains unchanged.

[0030] According to another aspect of the present invention, a monopolar disconnection overcurrent suppression device is provided, which is applied to a true bipolar offshore wind power grid-connected system via a flexible DC link and is used to execute the above-mentioned monopolar disconnection overcurrent suppression method for offshore wind power, including:

[0031] A fault detection module, configured to determine that a monopolar disconnection fault of the submarine cable occurs when the positive submarine cable current or the negative submarine cable current detected by the offshore converter station and the onshore converter station satisfies the current mutation condition; the offshore converter station controls the wind farm to start load shedding after a delay until the power drops to a first threshold;

[0032] Wherein, the offshore converter station includes a positive converter and a negative converter. The converter corresponding to the pole where the current mutation occurs is denoted as the fault pole converter, and the other is denoted as the non-fault pole converter; during the power reduction process of the wind farm, the internal energy of the fault pole converter increases;

[0033] An energy transfer module, configured to, when the internal energy of the fault pole converter rises to a second threshold, control the fault pole converter to lock and send an active energy charging and current limiting control signal to the non-fault pole converter, and send an active boost and current limiting signal to the onshore converter station; the active energy charging and current limiting control signal is used to make the non-fault pole converter absorb the power of the wind farm to actively increase the internal energy and last for a first preset time; the active boost and current limiting signal is used to control the DC submarine cable voltage of the onshore converter station to rise and last for a second preset time;

[0034] An energy release module, configured to, before reaching the first preset time, if the internal energy of the non-fault pole converter rises to the second threshold, keep the internal energy unchanged until the third preset time after reaching the first preset time; then control the non-fault pole converter to perform active energy release.

[0035] Generally speaking, compared with the prior art by the above technical solution conceived by the present invention, the following beneficial effects can be obtained:

[0036] (1) By the present invention, the magnitude of the DC line current connected to the onshore converter station and the offshore converter station is detected in real time to obtain fault information, reducing the information transmission delay; the offshore fault pole converter can passively absorb the power of the wind farm first after the fault occurs, gaining time for the wind farm to shed load; at the same time, the active energy charging and current limiting control of the offshore converter station, the active boost and current limiting control of the onshore converter station, and the load shedding of the wind farm are jointly coordinated to enhance the overcurrent suppression effect. In this way, the overcurrent of the non-fault pole after the DC single-line disconnection fault can be quickly suppressed, and startup and ramp-up time for the wind farm to actively reduce power and other schemes can be obtained, or the capacity requirement of the energy-consuming device can be reduced. Thus, the technical problem of poor AC overcurrent suppression ability of the offshore wind power flexible DC transmission system is solved.

[0037] (2) The non-faulty pole converter at sea calculates the energy command value for controlling active charge and current limiting according to the real-time wind farm power received, and balances the relationship between overcharging and overcurrent suppression during its charging period. Description of the Drawings

[0038] Figure 1 It is a flowchart of a method for suppressing overcurrent in a single-pole disconnection of a true bipolar offshore wind power through a flexible DC grid connection system for offshore wind power in an embodiment of the present invention;

[0039] Figure 2 It is a topology and control structure diagram of a true bipolar offshore wind power through a flexible DC transmission system in an embodiment of the present invention;

[0040] Figure 3 It is a schematic diagram of the blocking of the faulty pole converter in an embodiment of the present invention;

[0041] Figure 4 It is a schematic diagram of the energy control implementation mode of the non-faulty pole converter in an embodiment of the present invention;

[0042] Figure 5 It is a schematic diagram of the implementation mode of the active boost and current limiting control of the onshore converter station in an embodiment of the present invention;

[0043] Figure 6 It is a schematic diagram of the coordinated control of each converter station and the wind farm in an embodiment of the present invention;

[0044] Figure 7a It is a simulation diagram of the influence effect of the present invention on DC overcurrent when the wind farm starts load shedding with a delay of 80 ms in an embodiment of the present invention;

[0045] Figure 7b It is a simulation diagram of the influence effect of the present invention on DC overcurrent when the wind farm starts load shedding with a delay of 50 ms in an embodiment of the present invention;

[0046] Figure 7c It is a simulation diagram of the influence effect of the present invention on DC overcurrent when the wind farm starts load shedding with a delay of 20 ms in an embodiment of the present invention;

[0047] Figure 8 It is a comparison diagram of the DC overcurrent suppression effects under different control modes in an embodiment of the present invention. Detailed Implementation Modes

[0048] In order to make the objectives, technical solutions and advantages of the present invention more clear and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0049] As Figure 1 shown, the present invention provides a method for suppressing overcurrent caused by single-pole disconnection for offshore wind power, which is applied to a true bipolar offshore wind power system connected to the grid through a flexible DC link, and includes:

[0050] S1: When the positive or negative sea cable current detected by the offshore converter station and the onshore converter station meets the current mutation condition, it is determined that a single-pole disconnection fault of the sea cable occurs; the offshore converter station controls the wind farm to start load shedding after a delay until the power drops to the first threshold;

[0051] Among them, the offshore converter station includes a positive converter and a negative converter. The converter corresponding to the pole where the current mutation occurs is recorded as the faulty pole converter, and the other is recorded as the non-faulty pole converter; during the power reduction process of the wind farm, the internal energy of the faulty pole converter increases;

[0052] S2: When the internal energy of the faulty pole converter rises to the second threshold, control the faulty pole converter to lock and send an active energy charging and current limiting control signal to the non-faulty pole converter, and send an active boost and current limiting signal to the onshore converter station; the active energy charging and current limiting control signal is used to make the non-faulty pole converter absorb the power of the wind farm and actively increase the internal energy and last for the first preset time; the active boost and current limiting signal is used to control the DC sea cable voltage of the onshore converter station to rise and last for the second preset time;

[0053] S3: Before reaching the first preset time, if the internal energy of the non-faulty pole converter rises to the second threshold, keep the internal energy unchanged until the third preset time after reaching the first preset time; then control the non-faulty pole converter to perform active energy release.

[0054] Specifically, S1: As Figure 2 shown, the offshore converter station and the onshore converter station detect the positive and negative sea cable currents in real time. When it is detected that the positive and negative bipolar currents of the sea cable meet the condition that the current of a certain pole suddenly becomes 0, it is judged that a single-pole disconnection fault of the sea cable occurs. The disconnection pole is called the offshore fault pole, and the non-disconnection pole is called the offshore non-fault pole. After judging the fault, the offshore converter station transmits a load shedding signal to the wind farm. After receiving the signal, the wind farm starts to reduce the power at a uniform rate of 2.5 pu / s; when the power of the wind farm is reduced to 0.5 pu, control the output power of the wind farm to remain unchanged.

[0055] S2: As Figure 3As shown, when the internal energy of the offshore fault pole converter rises to the energy threshold of 2.25 pu, the offshore fault pole converter will receive a blocking signal S block , and the offshore fault pole converter will then be blocked; after the offshore fault pole converter is blocked, it transmits an active energy charging and current limiting control signal to the offshore non-fault converter, causing it to absorb the wind farm power and actively increase its internal energy. The energy absorption rate in the active energy charging and current limiting control of the offshore non-fault pole varies in real time according to the wind farm output power. The specific energy command value is:

[0056]

[0057] where P WFMn is the real-time power received by the offshore non-fault pole of the offshore converter station from the offshore wind farm, P WFMnN is the rated transmission power of the offshore non-fault pole, E WFMn is the rated internal energy of the offshore non-fault pole of the offshore converter station, and t r is the time from the start of the active energy charging and current limiting control to the current moment;

[0058] The offshore fault pole converter sends an active boost and current limiting signal to the onshore converter station to control the DC submarine cable voltage to rise; the active energy charging control ends after 500 ms of startup, and the active boost and current limiting control ends after 200 ms of startup.

[0059] S3: When the internal energy of the offshore non-fault pole converter rises to the energy threshold of 2.25 pu, keep the energy command value at 2.25 pu; after the active energy charging and current limiting control of the offshore non-fault pole converter ends, enter 200 ms of active energy holding, and maintain the energy command value at the numerical value of the energy command value at the end of the active energy charging control; after the active energy holding ends, the offshore non-fault pole converter performs uniform active energy release at a rate of 0.002 pu / ms until the internal energy drops to 1.0 pu and then keeps the energy unchanged.

[0060] In one embodiment, S1 includes:

[0061] S101: Both the offshore converter station and the onshore converter station monitor the positive submarine cable current and the negative submarine cable current in real time; when it is detected that the positive submarine cable current or the negative submarine cable current suddenly becomes 0, it is determined that a single-pole submarine cable break fault has occurred;

[0062] S102: Control the offshore converter station to send a load reduction signal to the wind farm so that it uniformly reduces the power to the first threshold.

[0063] In one embodiment, S102 includes:

[0064] Control the offshore converter station to send a load reduction signal to the wind farm. When the wind farm receives the load reduction signal, it reduces the power at a uniform rate;

[0065] When the power of the wind farm is reduced to 0.5 pu, control the output power of the wind farm to remain unchanged.

[0066] In one embodiment, S2 includes:

[0067] S201: When the internal energy of the faulty pole converter rises to the second threshold, control the faulty pole converter to block, and send an active energy charging current limiting control signal to the non-faulty pole converter, and send an active boost current limiting signal to the onshore converter station;

[0068] S202: Control the non-faulty pole converter to absorb the power of the wind farm according to the energy command value carried by the active energy charging current limiting control signal, actively increase the internal energy and continue for the first preset time;

[0069] S203: Control the onshore converter station to increase the DC submarine cable voltage according to the active boost current limiting signal and continue for the second preset time; the second preset time is less than the first preset time.

[0070] In one embodiment, the energy command value is:

[0071]

[0072] Where P WFMn is the real-time power received by the non-faulty pole in the offshore converter station from the wind farm, P WFMnN is the rated transmission power of the non-faulty pole, E WFMn is the rated internal energy of the non-faulty pole in the offshore converter station, t r is the duration from the start of the active energy charging current limiting control to the current moment.

[0073] In one embodiment, S3 includes:

[0074] S301: Before reaching the first preset time, when the internal energy increased by the non-faulty pole converter according to the active energy charging current limiting control signal reaches the second threshold, keep the current internal unchanged;

[0075] S302: When reaching the second preset time, the active energy charging current limiting control of the non-faulty pole converter ends, control the non-faulty pole converter to enter the active energy holding state and continue for the third preset time;

[0076] S303: When reaching the third preset time, the active energy holding state of the non-faulty pole converter ends, and active energy release is performed until the third threshold.

[0077] In one embodiment, S303 includes:

[0078] When the third preset time is reached, the active energy holding state of the non-faulty pole converter ends, and active energy is released at a uniform rate of 0.002 pu / ms until the internal energy drops to 1.0 pu and then the energy remains unchanged.

[0079] The energy control implementation method of the offshore non-faulty pole controller, the active boost and current limiting control implementation method of the onshore converter station, and the schematic diagram of the coordinated control between the converter station and the wind farm for the single-pole disconnection overcurrent suppression method of the true bipolar offshore wind power through flexible DC grid connection system provided by the present invention are respectively as Figure 4 、 Figure 5 and Figure 6 shown.

[0080] To verify the effectiveness of the method provided by the present invention, an offshore wind power through flexible DC transmission system as shown in Figure 2 is built on the PSCAD / EMTDC electromagnetic simulation platform, and the main parameters are shown in Table 1.

[0081] Table 1 Main parameters of the offshore wind power through flexible DC transmission system

[0082] System parameters Numerical value <![CDATA[System DC voltage U dc > ±320 kV <![CDATA[Half-bridge MMC converter station sub-module capacitor C SM > 12500 uF Number of sub - modules N in half - bridge MMC converter station 320*6 <![CDATA[Rated voltage U of sub-module in half-bridge MMC converter station SM > 2 kV <![CDATA[Wind farm output power P wind > 1200 MW

[0083] Simulation setting 1: A short-circuit fault occurs in the negative cable of the offshore wind power through flexible DC grid connection system operating at rated conditions at 3 s. Assume that the time for the offshore converter station to transmit a load reduction signal to the wind farm is 80 ms, 50 ms, and 20 ms respectively. After configuring the present invention, the simulation results of the positive and negative DC currents, DC power, non-faulty DC voltage, internal energies of the positive and negative converters of the offshore converter station, and the output power of the wind farm are respectively as Figure 7a 、 Figure 7b 、 Figure 7c shown.

[0084] From Figure 7a 、 Figure 7b 、 Figure 7c it can be seen that: 1) The control strategies proposed by the present invention can all respond correctly; 2) Through the present invention, the overcurrent of the offshore non-faulty pole can be well suppressed under different communication delays between the wind farm and the offshore converter station; 3) The shorter the delay, the more obvious the suppression effect.

[0085] Simulation settings 2: Six types of control schemes are set respectively: Scheme 1 is only wind farm load shedding; Scheme 2 is wind farm load shedding + active boost current limiting; Scheme 3 is wind farm load shedding + constant slope active energy charging current limiting control; Scheme 4 is wind farm load shedding + variable slope active energy charging current limiting control; Scheme 5 is wind farm load shedding + active boost current limiting + constant slope active energy charging current limiting control; Scheme 6 is wind farm load shedding + active boost current limiting + variable slope active energy charging current limiting control (the complete present invention). A break fault occurs in the negative cable at 3 s, and the time for the offshore converter station to transmit the load shedding signal to the wind farm is 50 ms. The offshore non-fault pole current is as Figure 8 shown.

[0086] As Figure 8 can be seen, applying the scheme of the present invention compared with the traditional scheme (only wind farm load shedding), the overcurrent can be suppressed from 2.12 pu to 1.33 pu. Among the six schemes, the suppression effect is the best, greatly improving the current safety.

[0087] According to another aspect of the present invention, a single-pole disconnection overcurrent suppression device is provided, which is applied to a true bipolar offshore wind power grid-connected system via a flexible DC transmission system and is used to execute the above-mentioned single-pole disconnection overcurrent suppression method for offshore wind power, including:

[0088] A fault detection module, which is used to determine that a single-pole disconnection fault of the submarine cable occurs when the positive submarine cable current or the negative submarine cable current detected by the offshore converter station and the onshore converter station meets the current mutation condition; the offshore converter station controls the wind farm to start load shedding after a delay until the power drops to the first threshold;

[0089] Among them, the offshore converter station includes a positive converter and a negative converter. The converter corresponding to the pole where the current mutation occurs is recorded as the fault pole converter, and the other is recorded as the non-fault pole converter; during the power drop process of the wind farm, the internal energy of the fault pole converter increases;

[0090] An energy transfer module, which is used to control the locking of the fault pole converter and send an active energy charging current limiting control signal to the non-fault pole converter and an active boost current limiting signal to the onshore converter station when the internal energy of the fault pole converter rises to the second threshold; the active energy charging current limiting control signal is used to make the non-fault pole converter absorb the power of the wind farm to actively increase the internal energy and last for the first preset time; the active boost current limiting signal is used to control the DC submarine cable voltage of the onshore converter station to rise and last for the second preset time;

[0091] An energy release module, which is used to keep the internal energy unchanged until the third preset time after the first preset time if the internal energy of the non-fault pole converter rises to the second threshold before reaching the first preset time; then control the non-fault pole converter to perform active energy release.

[0092] Those skilled in the art can easily understand that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for suppressing overcurrent caused by single - pole disconnection for offshore wind power, characterized in that, it is applied to a true bipolar offshore wind power system connected to the grid through a flexible DC transmission system, and includes: S1: When the offshore converter station and the onshore converter station detect that the positive - pole submarine cable current or the negative - pole submarine cable current meets the current mutation condition, it is determined that a single - pole disconnection fault of the submarine cable occurs; the offshore converter station controls the wind farm to start load shedding after a delay until the power drops to the first threshold; Among them, the offshore converter station includes a positive - pole converter and a negative - pole converter. The converter corresponding to the pole where the current mutation occurs is recorded as the fault - pole converter, and the other is recorded as the non - fault - pole converter; during the power drop process of the wind farm, the internal energy of the fault - pole converter increases; S2: When the internal energy of the fault - pole converter rises to the second threshold, control the fault - pole converter to lock, and send an active energy - charging current - limiting control signal to the non - fault - pole converter, and send an active voltage - boosting current - limiting signal to the onshore converter station; the active energy - charging current - limiting control signal is used to make the non - fault - pole converter absorb the power of the wind farm, actively increase the internal energy and last for the first preset time; the active voltage - boosting current - limiting signal is used to control the DC submarine cable voltage of the onshore converter station to rise and last for the second preset time; S3: Before reaching the first preset time, if the internal energy of the non - fault - pole converter rises to the second threshold, keep the internal energy unchanged until the third preset time after reaching the first preset time; then control the non - fault - pole converter to perform active energy release; The S2 includes: S201: When the internal energy of the fault - pole converter rises to the second threshold, control the fault - pole converter to lock, and send an active energy - charging current - limiting control signal to the non - fault - pole converter, and send an active voltage - boosting current - limiting signal to the onshore converter station; S202: Control the non - fault - pole converter to absorb the power of the wind farm according to the energy command value carried by the active energy - charging current - limiting control signal, actively increase the internal energy and last for the first preset time; S203: Control the onshore converter station to increase the DC submarine cable voltage according to the active voltage - boosting current - limiting signal and last for the second preset time; the second preset time is less than the first preset time; The S2 includes: The energy command value is: ; Among them, P WFMn is the real-time power received by the non-faulty pole of the offshore converter station from the wind farm, and P WFMnN is the rated transmission power of the non-faulty pole, and E WFMn is the rated internal energy of the non-faulty pole of the offshore converter station, and t r is the duration from the self-start of the active charge current limiting control to the current moment.

2. The method for suppressing overcurrent caused by single - pole disconnection for offshore wind power according to claim 1, characterized in that, the S1 includes: S101: Use both the offshore converter station and the onshore converter station to monitor the positive - pole submarine cable current and the negative - pole submarine cable current in real time; when it is detected that the positive - pole submarine cable current or the negative - pole submarine cable current suddenly becomes 0, it is determined that a single - pole disconnection fault of the submarine cable occurs; S102: Control the offshore converter station to send a load - shedding signal to the wind farm so that its power drops evenly to the first threshold.

3. The method for suppressing overcurrent caused by single - pole disconnection for offshore wind power according to claim 2, characterized in that, the S102 includes: Control the offshore converter station to send the load - shedding signal to the wind farm, and when the wind farm receives the load - shedding signal, its power drops at a uniform rate; When the power of the wind farm is reduced to 0.5 pu, control the output power of the wind farm to remain unchanged.

4. The method for suppressing overcurrent of single-pole disconnection for offshore wind power as claimed in claim 1, characterized in that S3 includes: S301: Before reaching the first preset time, when the internal energy of the non-faulty pole converter increased according to the active energy charging current limiting control signal reaches the second threshold, keep the current internal state unchanged; S302: When reaching the second preset time, the active energy charging current limiting control of the non-faulty pole converter ends, and control the non-faulty pole converter to enter the active energy holding state and continue for the third preset time; S303: When reaching the third preset time, the active energy holding state of the non-faulty pole converter ends, and perform active energy release until reaching the third threshold.

5. The method for suppressing overcurrent of single-pole disconnection for offshore wind power as claimed in claim 4, characterized in that S303 includes: When reaching the third preset time, the active energy holding state of the non-faulty pole converter ends, and perform active energy release at a uniform rate of 0.002 pu / ms until the internal energy drops to 1.0 pu and then keep the energy unchanged.

6. A device for suppressing overcurrent of single-pole disconnection, which is applied to a true bipolar offshore wind power integrated into the grid system via a flexible DC link, and is used to execute the method for suppressing overcurrent of single-pole disconnection for offshore wind power according to any one of claims 1-5, characterized in that it includes: A fault detection module, which is used to determine that a single-pole disconnection fault of the submarine cable occurs when the positive or negative submarine cable current detected by the offshore converter station and the onshore converter station satisfies the current mutation condition; the offshore converter station controls the wind farm to start load shedding after a delay until the power drops to the first threshold; Wherein, the offshore converter station includes a positive converter and a negative converter, and the converter corresponding to the pole where the current mutation occurs is recorded as the faulty pole converter, and the other is recorded as the non-faulty pole converter; during the power drop process of the wind farm, the internal energy of the faulty pole converter increases; An energy transfer module, which is used to control the faulty pole converter to block when the internal energy of the faulty pole converter rises to the second threshold, and send an active energy charging current limiting control signal to the non-faulty pole converter and an active boost current limiting signal to the onshore converter station; the active energy charging current limiting control signal is used to make the non-faulty pole converter absorb the power of the wind farm and actively increase the internal energy and continue for the first preset time; the active boost current limiting signal is used to control the DC submarine cable voltage of the onshore converter station to rise and continue for the second preset time; An energy release module, which is used to keep the internal energy unchanged if the internal energy of the non-faulty pole converter rises to the second threshold before reaching the first preset time until the third preset time after reaching the first preset time; then control the non-faulty pole converter to perform active energy release.

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