Method for active frequency support of large-scale offshore wind power through flexible interconnection system

Abstract

The present application relates to a kind of large-scale offshore wind power through flexible networking system frequency active support's regulation and control method, belong to new energy through flexible HVDC transmission system grid-connected field.The steps include virtual inertia control based on DC capacitor energy storage, virtual inertia control based on wind turbine rotor kinetic energy, primary frequency control strategy, secondary frequency control strategy.Inertia support aspect, by analogy the inertia response characteristics of synchronous generator, active support system inertia using DC capacitor itself energy, and through DC voltage establishes the coupling relationship of wind turbine speed and frequency, proposes the wind farm inertia support collaborative control strategy based on different rotor kinetic energy regulation, to improve the inertia level of receiving end power grid.In frequency deviation adjustment aspect, according to local DC voltage deviation, proposes the primary and secondary frequency modulation strategy of wind farm based on wind turbine variable speed control and pitch angle control, to fully tap the frequency modulation potential of offshore wind farm, improve the frequency stability of system.

Description

Technical Field

[0001] This invention relates to the field of grid connection technology for new energy via flexible DC transmission systems, and particularly to a coordinated control strategy for the frequency of offshore wind power and the active support system of flexible DC transmission, especially a regulation method for active frequency support of large-scale offshore wind power via flexible DC interconnection system, which ensures the frequency stability of the receiving-end power grid. Background Technology

[0002] Modular multilevel converter-based high voltage direct current (MMC-HVDC) technology features fast control speed, flexible operation, and high power supply reliability, meeting the technical requirements for deep-sea wind power aggregation and long-distance transmission. Therefore, the adoption of flexible DC transmission technology provides strong technical support for the development and grid connection of large-scale renewable energy bases. The most typical example of this type of operation is the Zhangbei Flexible DC Demonstration Project currently in operation in my country.

[0003] As the scale of offshore wind power connected to the receiving-end grid via flexible DC transmission continues to increase, some synchronous generators will be gradually replaced, leading to a gradual scarcity of traditional frequency regulation resources. Due to the decoupling of the AC frequency on the offshore wind farm side from the receiving-end grid frequency, offshore wind power cannot actively support the receiving-end grid frequency, resulting in a gradual decrease in system inertia and seriously threatening its safe and stable operation. Domestic and international scholars have conducted extensive research on inertia support and frequency deviation regulation in offshore wind power connected to flexible DC transmission systems. However, most studies focus only on the active frequency support problem of wind farms connected to flexible DC transmission systems using single-unit aggregation models, while research on control strategies that comprehensively consider the differences in response capabilities among multiple wind turbines is limited, and research on wind farms actively participating in secondary frequency regulation is still lacking. Therefore, there is an urgent need to optimize and improve the grid-connected collaborative control strategy for offshore wind power connected to flexible DC transmission systems. Summary of the Invention

[0004] The purpose of this invention is to provide a method for actively supporting the frequency of large-scale offshore wind power connected to a flexible DC grid, solving the aforementioned problems in the prior art. Regarding inertia support, this invention, by analogy with the inertia response characteristics of a synchronous generator, actively supports the system inertia using the energy of the DC capacitor itself. It also establishes a coupling relationship between wind turbine speed and frequency through DC voltage, proposing a collaborative control strategy for wind farm inertia support based on differentiated rotor kinetic energy regulation to improve the inertia level of the receiving-end grid. Regarding frequency deviation regulation, this invention proposes a primary frequency regulation strategy for wind farms based on wind turbine speed control and pitch angle control, based on the local DC voltage deviation. Furthermore, considering the increasing scarcity of traditional frequency regulation resources, a secondary frequency regulation strategy for wind farms based on additional pitch angle control is designed to fully exploit the frequency regulation potential of offshore wind farms and improve the frequency stability of the system.

[0005] The above-mentioned objective of the present invention is achieved through the following technical solution:

[0006] A method for actively frequency-supported regulation of large-scale offshore wind power via a flexible DC-DC interconnection system includes the following steps:

[0007] Step (1) Based on the virtual inertia integrated control of DC capacitor energy storage and wind turbine rotor kinetic energy;

[0008] Step (2) Primary frequency modulation control strategy;

[0009] Step (3) Secondary frequency modulation control strategy.

[0010] The virtual inertia integrated control based on DC capacitor energy storage and wind turbine rotor kinetic energy described in step (1) is as follows: establish the coupling relationship between DC voltage and grid frequency, prioritize DC capacitor to provide inertia support to the receiving end system after frequency change, and directly transmit grid frequency fluctuation information to the wind farm with DC voltage as the carrier. Introduce the rate of change of DC voltage into the active power control link of each wind turbine in the wind farm, so that it actively changes its active power output in response to the frequency change of the receiving end grid. By reasonably designing control parameters, provide necessary inertia support for the system.

[0011] The primary frequency regulation control strategy described in step (2) is as follows: Based on step (1), by introducing the deviation of DC voltage into the speed control and pitch angle control links of each DFIG in the wind farm, the active power output of the wind turbine is adjusted. By reasonably designing control parameters and action logic, the wind farm can actively participate in primary frequency regulation, thereby improving the frequency stability of the receiving-end AC grid.

[0012] The secondary frequency regulation control strategy described in step (3) is to set the target of the secondary frequency regulation control in the voltage-type additional pitch angle control to the rated DC voltage of the flexible DC system. After receiving the dispatch instruction, it will actively participate in the DC voltage regulation of the flexible DC system to eliminate its steady-state deviation, thereby realizing error-free regulation of the receiving end grid frequency.

[0013] The beneficial effects of this invention are as follows: The method for coordinated control of the frequency of a large-scale offshore wind power system connected to a flexible DC grid provides the following advantages: Regarding inertia support, by analogy with the inertia response characteristics of a synchronous generator, the method actively supports the system inertia using the energy of the DC capacitor itself. Furthermore, it establishes a coupling relationship between wind turbine speed and frequency through DC voltage, proposing a coordinated control strategy for wind farm inertia support based on differentiated rotor kinetic energy regulation to improve the inertia level of the receiving-end grid. Regarding frequency deviation regulation, based on the local DC voltage deviation, a primary frequency regulation strategy for wind farms based on wind turbine speed control and pitch angle control is proposed. Considering the increasing scarcity of traditional frequency regulation resources, a secondary frequency regulation strategy for wind farms based on additional pitch angle control is designed to fully exploit the frequency regulation potential of offshore wind farms and improve the frequency stability of the system. Attached Figure Description

[0014] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, illustrate the invention and are used to explain it, but do not constitute an undue limitation of the invention.

[0015] Figure 1 This is a structural diagram of the offshore wind power system connected to a flexible DC power grid according to the present invention;

[0016] Figure 2 This is a diagram illustrating the frequency control strategy of the wind farm active support system of the present invention.

[0017] Figure 3 This is a power characteristic curve of the wind turbine generator of the present invention. Detailed Implementation

[0018] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0020] See Figures 1 to 3As shown, the purpose of this invention is to provide a coordinated control strategy and parameter selection method for virtual inertia control, primary frequency regulation, and secondary frequency regulation in offshore wind power systems connected to flexible DC power grids. This strategy enables the system to exhibit dominant power supply characteristics and provide necessary inertia and frequency support. Regarding inertia support, the invention utilizes DC capacitor energy to provide inertia while establishing a coupling relationship between DC voltage and frequency. The wind farm can then differentiate the utilization of rotor kinetic energy based on the local DC voltage change rate and current rotational speed, effectively suppressing grid frequency changes during initial disturbances while fully utilizing inertia support and damping capabilities. Regarding frequency deviation regulation, by introducing DC voltage deviation into wind turbine speed control and pitch angle control, the frequency response speed and capability of both are combined, enabling the wind farm to possess the primary and secondary frequency regulation functions of a traditional synchronous machine, effectively improving the system's frequency stability.

[0021] The present invention provides a method for active frequency support regulation of large-scale offshore wind power via a flexible DC-DC interconnection system, comprising the following steps: 1. Virtual inertia control based on DC capacitor energy storage.

[0022] The inertial response process of a synchronous machine can be described as follows:

[0023]

[0024] In the formula: H is the inertial time constant of the synchronous machine; ω is the angular frequency of the synchronous machine; P M P represents mechanical power. E d represents electromagnetic power, d is the differential symbol; dω / dt is the rate of change of the synchronous machine's angular frequency.

[0025] The equivalent capacitance on the DC side of the MMC can be used to suppress DC voltage changes by utilizing its dynamic characteristics. That is, when the active power loss of the line is ignored, the change in DC voltage can be used to reflect the power imbalance between the rectifier side and the inverter side, as shown in equation (1-2).

[0026]

[0027] In the formula: C eq The equivalent capacitance of MMC; U dc P is the measured DC voltage. wind P represents the output power of the wind farm. grid d represents the active power transmitted to the grid side; d is the differential symbol; dU dc / dt represents the measured rate of change of DC voltage.

[0028] In order to enable the MMC to possess the inertial response characteristics of a synchronous machine, analogous to the rotor motion equation of a synchronous machine (1-1), P wind This can be considered as the mechanical power of a synchronous machine, which is assumed to be constant here, meaning only the DC capacitor is considered to provide inertia support for the system; Pgrid The DC voltage can be viewed as the electromagnetic power of a synchronous generator. The change in DC voltage can be analogous to the change in the speed of a synchronous machine. The main difference is that the DC capacitor uses its own energy to provide virtual inertia support for the system, thereby suppressing frequency abrupt changes; while the synchronous machine uses its rotor kinetic energy to provide inertia support, directly determining the system frequency. Combining equations (1-1) and (1-2), we can obtain...

[0029]

[0030] In the formula: H MMC C is the virtual inertial time constant of the MMC; eq d is the equivalent capacitance of MMC; d is the differential symbol; df / dt is the rate of change of AC grid frequency; dU dc / dt represents the measured rate of change of DC voltage.

[0031] Integrating both sides of equation (1-3) and simplifying, we can obtain

[0032]

[0033]

[0034] In the formula: H MMC C is the virtual inertial time constant of the MMC; eq The equivalent capacitance of MMC; U dc0 The rated DC voltage under steady-state conditions; ΔU dc =U dc -U dc0 f is the deviation of DC voltage; f is the actual frequency of AC power grid; f0 is the rated frequency of AC power grid; and d is the differential symbol.

[0035] To ensure the safe and stable operation of the system, the DC voltage deviation is usually limited to a small range (around ±5%). Therefore, the quadratic term of the voltage deviation can be ignored. Combining equation (1-5), we can obtain...

[0036]

[0037] In the formula: H MMC C is the virtual inertial time constant of the MMC; eq The equivalent capacitance of the MMC; ΔU dc =U dc -U dc0 The deviation is the DC voltage; Δf = f - f0 is the AC power grid frequency deviation; U dc0 k is the rated DC voltage under steady state. c This is the coupling coefficient between the DC voltage deviation and the frequency deviation.

[0038] As shown in Equation (1-6), by introducing the frequency deviation of the receiving-end grid into the DC voltage control link of the receiving-end converter station on the grid side, as shown in Equation (1-7), a coupling relationship between DC voltage and frequency can be established, enabling it to actively participate in grid frequency regulation while transmitting frequency fluctuation information to the DC side, thus providing response conditions for the wind farm to actively support inertia.

[0039] U dcref =U dc0 +k c Δf (1-7)

[0040] In the formula: U dcref The reference value for the DC voltage of the receiving-end converter station on the grid side; U dc0 k is the rated DC voltage under steady state. c Δf = f - f0 is the coupling coefficient between DC voltage deviation and frequency deviation; Δf = f - f0 is the frequency deviation of the AC power grid.

[0041] From the above equation, we can see that ΔU dc The coupling coefficient k between Δf and c The larger U dcref The larger the value, the easier it is to reach the DC voltage limit, resulting in the DC voltage failing to transmit complete receiving-end grid frequency information. Therefore, the maximum allowable DC voltage deviation value ΔU can be used as a reference. dcmax Maximum deviation from frequency Δf max Design k c The value is

[0042]

[0043] In the formula: k c ΔU is the coupling coefficient between DC voltage deviation and frequency deviation. dcmax The maximum allowable deviation of DC voltage; Δf max This represents the maximum permissible frequency deviation.

[0044] Therefore, the virtual inertial time constant of MMC can be obtained as follows:

[0045]

[0046] In the formula: H MMC C is the virtual inertial time constant of the MMC; eq The equivalent capacitance of MMC; U dc0 The rated DC voltage under steady-state conditions; ΔU dcmax The maximum allowable deviation of DC voltage; Δf max This represents the maximum permissible frequency deviation.

[0047] 2. Virtual Inertia Control Based on Wind Turbine Rotor Kinetic Energy

[0048] When the system is disturbed, the DC voltage and grid frequency coupling relationship established by equation (1-7) is used to transmit grid frequency fluctuation information to the wind farm side via DC voltage. The rate of change (dU) of the DC voltage at the wind farm's sending-end converter station is also transmitted. dc / dt) is introduced into the active power control links of each DFIG in the wind farm, such as Figure 2 As shown in the green dashed box, it actively changes its active power output in response to sudden changes in the frequency of the receiving-end power grid, providing inertia support for the system.

[0049] To enable offshore wind farms to simultaneously meet frequency regulation requirements under varying frequency conditions, a certain amount of power is typically reserved for steady-state operation in overspeed load shedding mode. Let the power point tracking curve equation after overspeed load shedding be...

[0050]

[0051] In the formula: P wind k is the output power of the wind farm. LS ω is the load shearing power tracking coefficient; d This is the current rotational speed.

[0052] When the frequency of the receiving-end power grid increases, the DC voltage will rise in real time, and each DFIG will adjust its voltage according to the rate of change of the DC voltage, dU. dc / dt changes its speed reference value, rapidly reducing the wind farm's output power, as shown in equation (2-2). Since the speed adjustment Δω is usually small, the quadratic term Δω is ignored here. 2 and cubic term Δω 3 Combining equations (1-2) and (2-2), the wind turbine reduces its active power output ΔP. LS The process can be analogized to a virtual capacitor providing inertial support, that is, the wind turbine provides virtual inertial support to the system through rotor kinetic energy, as shown in equation (2-3), and then Δω and dU can be obtained. dc The relationship between / dt is shown in equation (2-4).

[0053]

[0054]

[0055]

[0056] In the formula: P wind ΔP represents the output power of the wind farm. LS Reduced active power output of the wind turbine; k LS ω is the load shearing power tracking coefficient; d Δω is the current speed; ΔC is the speed adjustment amount; ΔC is the speed adjustment amount. vir Add a virtual capacitor to the DC side; Udc d represents the measured DC voltage; d is the differential symbol; dU dc / dt is the measured rate of change of DC voltage; k d This is the coupling coefficient between the speed regulation and the DC voltage change rate.

[0057] Figure 3 This diagram illustrates the active inertial support provided to the wind turbine. The turbine operates in steady state at load shedding point 2. During the initial stage of the rising grid frequency at the receiving end, the unbalanced power is at its maximum. Utilizing the proposed virtual inertial control, the turbine's output power can be rapidly and significantly reduced. This process is as follows: Figure 3 As indicated by the red arrows at points 2 and 3, the rate of frequency rise is effectively reduced. At this point, the mechanical power captured by the fan is greater than the electromagnetic power, causing the rotor speed to increase and providing inertial support to the system through stored kinetic energy; as the speed increases, the mechanical power will gradually decrease, as... Figure 3 ΔP wind As shown, it can also suppress frequency rise, exhibiting damping characteristics similar to a synchronous generator. After the primary frequency regulation action of the synchronous generator in the receiving-end grid, the system frequency deviation will gradually tend to a constant value, dU dc / dt then becomes 0, the wind turbine no longer provides inertial support, and it returns to its steady-state operating point before the disturbance. This process is as follows: Figure 3 The red arrows indicate the points from midpoint 4 to point 2.

[0058] Based on the above analysis, considering the inertia of the active support system of offshore wind farms, equation (1-2) can be further modified to equation (2-5). That is, the wind farm can enhance its inertial effect by increasing the equivalent capacitance of the flexible DC system, so that its rotor kinetic energy and DC capacitor energy can jointly provide inertial support, increasing its equivalent inertial time constant, as shown in equation (2-6), thereby suppressing frequency abrupt changes and improving system frequency stability. Since the inertial response process when the receiving end grid frequency decreases is similar to the above, it will not be repeated here.

[0059]

[0060]

[0061] In the formula: C eq The equivalent capacitance of the MMC; ΔC vir Add a virtual capacitor to the DC side; U dc d represents the measured DC voltage; d is the differential symbol; dU dc / dt is the measured rate of change of DC voltage; P wind P represents the output power of the wind farm. grid The active power transmitted to the grid side; H S U is the virtual inertial time constant of the system; dc0 The rated DC voltage under steady-state conditions; ΔUdcmax The maximum allowable deviation of DC voltage; Δf max This represents the maximum frequency deviation.

[0062] During steady-state operation, the closer the fan rotor speed is to the maximum (or optimal) speed, the smaller the range of speed variation and the less rotor kinetic energy can be stored (or released); conversely, the closer it is to the maximum (or optimal) speed, the more rotor kinetic energy can be stored (or released). Therefore, it is necessary to determine the starting speed ω of the fan based on the variable speed control. d0 Reasonable design of k d This allows for full utilization of the rotor kinetic energy of each DFIG to achieve a coordinated response of virtual inertia.

[0063] Depend on Figure 3 It can be seen that the fan rotor operates at the optimal speed ω opt At this point, the mechanical power captured by the wind turbine reaches its maximum value, and it can exert its maximum damping effect. Therefore, when a disturbance causes a drop in grid frequency, the minimum limit for the wind turbine speed to decrease is ω in order to suppress the frequency drop more quickly. opt When a disturbance causes the grid frequency to rise, the wind turbine increases its rotational speed to store kinetic energy; the maximum increase in rotational speed is ω. max Therefore, this paper designs the adjustable range of the fan speed as [ω]. opt ω max According to equations (6) and (13), Δω and dU dc The frequency change rate df / dt is directly proportional to the frequency change rate k. To prevent damage to the unit due to an excessively large frequency change rate df / dt, k is designed... d When considering parameters, both Δω and df / dt limit constraints must be taken into account.

[0064] Based on dU dc The coupling relationship between / dt and df / dt, when the frequency increases, considers the maximum constraint of the DC voltage change rate (dU). dc / dt) max With ω max The correspondence between them is shown in equation (2-7); similarly, when the frequency decreases, (dU) dc / dt) max With ω opt The correspondence between them. In summary, the coupling coefficient k can be obtained. d The values ​​of are shown in equation (2-8).

[0065]

[0066]

[0067] In the formula: Δω max This represents the maximum speed regulation value; k d The coupling coefficient between the speed regulation and the DC voltage change rate; (dU)dc / dt) max The maximum constraint value for the rate of change of DC voltage; ω max This is the maximum possible increase in rotational speed; ω d ω is the current rotational speed; d0 Δf is the starting speed of the wind turbine variable speed control; Δf is the frequency deviation of the receiving end power grid.

[0068] From equation (2-8), it can be seen that each DFIG can be determined according to ω. d0 Choose its k appropriately d This value enables differentiated utilization of the kinetic energy of each wind turbine rotor under different wind speeds, effectively improving the inertia support capacity of the wind farm.

[0069] 3. Primary frequency modulation control strategy

[0070] To achieve frequency regulation functionality similar to that of a synchronous machine, a coordinated primary frequency regulation control strategy for wind farms was designed based on local DC voltage measurement information, taking into account the differences in response between variable speed control and pitch angle control. This strategy involves controlling the DC voltage deviation (ΔU)... dc By introducing active power control links in each DFIG of the wind farm, the active power output of the wind turbines can be adjusted, enabling the wind farm to actively participate in primary frequency regulation, thereby improving the frequency stability of the receiving-end AC power grid.

[0071] When the DC voltage reaches its dead zone limit ΔU dcth At that time, the wind turbine immediately adjusts its speed to regulate the mechanical power it captures, enabling the wind farm to actively participate in primary frequency regulation, such as... Figure 2 The blue dashed line in the figure shows this. Combining equation (2-2), it can be seen that the frequency deviation (Δf) of the receiving-end power grid is proportional to the DC voltage deviation (ΔU). dc The energy is transferred to the wind farm in the form of Δf∝ΔU, and satisfies Δf∝ΔU. dc ∝Δω∝ΔP LS As shown in equations (3-1) and (3-2), if the wind turbine's rotational speed is within its allowable adjustment range after the first frequency regulation, the wind turbine will operate stably in a new equilibrium state.

[0072]

[0073]

[0074] Where: ΔP LS Reduced active power output of the wind turbine; k LS ω is the load shearing power tracking coefficient; d Δω is the current speed; η is the speed adjustment amount; η is the voltage regulation coefficient; ΔU dc For the deviation of DC voltage; k p This is the coupling coefficient between the speed regulation amount and the DC voltage deviation amount.

[0075] To achieve a reasonable allocation of primary frequency regulation power in the wind farm, combining equations (2-7) and (2-8), we can obtain ΔU dc The coupling coefficient k between Δω and p Values

[0076]

[0077] In the formula: k p ω is the coupling coefficient between the speed regulation and the DC voltage deviation. max This is the maximum possible increase in rotational speed; ω d0 The speed at which the fan starts using variable speed control; ΔU dcmax The maximum allowable deviation of DC voltage; ω opt Δf is the minimum allowable reduction in wind turbine speed; Δf is the frequency deviation of the receiving-end power grid.

[0078] When the fan rotor speed reaches ω max (or ω) opt When the frequency regulation capability of the variable speed control reaches its upper limit, the frequency regulation capability of the fan is maintained by keeping the rotor speed constant, and then the fan is started. Figure 2 The primary frequency regulation mode within the red dashed box is based on voltage-type additional pitch angle control. By superimposing the additional pitch angle Δβ generated by the DC voltage deviation onto the initial reserve pitch angle β0, it further alters the active power output of the wind farm, thereby achieving coordinated operation of wind turbine speed control and additional pitch angle control in primary frequency regulation. This is analogous to the design concept of virtual inertia control parameters based on the kinetic energy of the wind turbine rotor. Figure 2 Coupling coefficient k in medium voltage type additional pitch angle control β Can be defined as

[0079]

[0080] In the formula: k β β0 is the coupling coefficient in voltage-type additional pitch angle control; β0 is the initial reserve pitch angle; ΔU dcmax Δf represents the maximum allowable deviation of the DC voltage; Δf represents the frequency deviation of the receiving-end power grid.

[0081] 4. Secondary frequency modulation control strategy

[0082] As the penetration rate of new energy sources such as offshore wind power continues to increase, traditional synchronous generators will gradually be replaced, significantly reducing the system's frequency regulation capability and weakening its ability to eliminate frequency deviations. Frequency stability may become a major bottleneck in the low-carbon transformation of my country's power system. Compared to inertia and the process of suppressing frequency fluctuations through primary frequency regulation, secondary frequency regulation achieves dynamic frequency balance over a longer timescale, improving the overall system's stability and disturbance immunity. Therefore, this paper further explores the secondary frequency regulation potential of large-scale offshore wind power systems connected to flexible DC grids. Based on local DC voltage measurement information, a secondary frequency regulation strategy based on additional pitch angle control is designed, such as... Figure 2 As shown in the red dashed box, this makes it exhibit the characteristics of a dominant power source, improving the frequency stability of the receiving-end power grid.

[0083] Since the frequency deviation of the receiving-end power grid is directly coupled with the DC voltage, the secondary frequency regulation control target in the voltage-type additional pitch angle control is set to the rated DC voltage U of the flexible DC system. dcref (Ignoring line losses), upon receiving a dispatch command, the wind farm actively participates in the DC voltage regulation of the flexible DC system to eliminate its steady-state deviation, thereby achieving error-free regulation of the receiving-end grid frequency. After the wind farm participates in primary frequency regulation, the grid frequency deviation tends to a constant value. If the wind turbine rotor speed does not reach its limit ω at this time... max (or ω) opt Upon receiving the dispatch command, the DC voltage deviation generates an additional pitch angle Δβ through a proportional-integral circuit. By adjusting the pitch angle alone, the active power output of the wind farm is adjusted, eliminating the frequency deviation remaining after primary frequency regulation. This achieves a secondary frequency regulation function similar to a synchronous machine, while simultaneously restoring the rotor speed to its initial speed, thus restoring the wind farm's inertia support capability. If the wind turbine rotor speed has already reached its limit ω... max (or ω) opt If the unbalanced power is large, it means that the speed needs to be kept constant, that is, the frequency regulation capability of the variable speed control needs to be maintained. Then, the pitch angle is further adjusted through the secondary frequency regulation, so that the variable speed control and the pitch angle control can jointly bear the unbalanced power of the system and maintain the DC voltage stability. While making full use of the wind farm's power regulation capability, the frequency regulation of the receiving end grid is achieved without error.

[0084] 5. Overall Control Strategy Design

[0085] After a sudden frequency change in the receiving-end power grid, the DC capacitor energy provides inertia while establishing a coupling relationship between DC voltage and frequency. The wind farm can then differentiate the utilization of rotor kinetic energy based on the local DC voltage change rate and current speed, effectively suppressing initial grid frequency changes while fully utilizing inertial support and damping capabilities. By introducing DC voltage deviation into wind turbine speed control and pitch angle control, combining the frequency response speed and capabilities of both, the wind farm acquires the primary and secondary frequency regulation functions of a traditional synchronous machine, effectively improving system frequency stability.

[0086] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the invention by those skilled in the art. Any modifications, equivalent substitutions, or improvements made to the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for controlling large-scale offshore wind power through a flexible DC-DC interconnection system with active frequency support, characterized in that: Includes the following steps: Step (1) Virtual inertia integrated control based on DC capacitor energy storage and wind turbine rotor kinetic energy: Establish the coupling relationship between DC voltage and grid frequency. After a frequency change, the DC capacitor will prioritize providing inertial support to the receiving-end system and transmit grid frequency fluctuation information directly to the wind farm via DC voltage. Introduce the rate of change of DC voltage into the active power control of each wind turbine in the wind farm, so that it can actively change its active power output in response to the frequency change of the receiving-end grid. By reasonably designing control parameters, provide the necessary inertial support for the system. Step (2) Primary frequency modulation control strategy: Based on step (1), by introducing the deviation of DC voltage into the speed control and pitch angle control links of each DFIG in the wind farm, the active power output of the wind turbine is adjusted. By rationally designing control parameters and action logic, the wind farm can actively participate in primary frequency regulation, thereby improving the frequency stability of the receiving end AC grid. When the fan rotor speed reaches or When the primary frequency regulation capability based on variable speed control reaches its upper limit, the frequency regulation capability of the wind turbine variable speed control is maintained by keeping its rotor speed constant, and the primary frequency regulation mode based on voltage-type additional pitch angle control is initiated. This is achieved by superimposing the voltage-type additional pitch angle Δβ generated by the DC voltage deviation onto the initial reserve pitch angle β0, further altering the active power output of the wind farm. This achieves coordinated operation of wind turbine variable speed control and voltage-type additional pitch angle control in primary frequency regulation; the coupling coefficient k in voltage-type additional pitch angle control... β Defined as: ； In the formula: k β β0 is the coupling coefficient in voltage-type additional pitch angle control; β0 is the initial reserve pitch angle; ∆U dcmax The maximum allowable DC voltage deviation is given; Δf is the frequency deviation of the receiving-end power grid. Step (3) Secondary frequency modulation control strategy.

2. The method for active frequency support regulation of large-scale offshore wind power via a flexible DC-DC interconnection system according to claim 1, characterized in that: The secondary frequency regulation control strategy described in step (3) is to set the target of the secondary frequency regulation control in the voltage-type additional pitch angle control to the rated DC voltage of the flexible DC system. After receiving the dispatch instruction, it will actively participate in the DC voltage regulation of the flexible DC system to eliminate its steady-state deviation, thereby realizing error-free regulation of the receiving end grid frequency.

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