Network source coordination control system and method of offshore wind power flexible direct current power transmission system

Abstract

The invention provides a network source coordination control system and method for an offshore wind power flexible direct current transmission system, and the system comprises a receiving end converter control module which controls the output frequency of a receiving end converter through a power ring by simulating the synchronous operation mechanism of a virtual synchronous generator; the voltage loop controls the output voltage of the receiving end converter; the current reference value of the current loop is output and given through the voltage loop, and current tracking is realized through PI control; the sending-end converter control module is used for reflecting the active demand information of the system on the frequency of the sending-end offshore wind power collection alternating-current system to realize active support control of the offshore wind turbine generator to the flexible direct-current power transmission system; and the offshore wind turbine generator control module is used for participating in system frequency modulation control through the wind turbine generator when the wind turbine generator runs in a deloading mode under the normal condition and the power is unbalanced based on the variable-speed wind turbine generator control strategy.

Description

Technical Field

[0001] The present invention belongs to the technical field of offshore wind power, and in particular relates to a network-source coordinated control system and method for an offshore wind power flexible direct current transmission system. Background Art

[0002] Judging from the current global offshore wind power development and planning, offshore wind power development is gradually advancing from nearshore to deep sea areas. Considering the long-distance and large-capacity power transmission needs, high-voltage flexible direct current transmission is the main way for large-scale deep sea wind power to access the power grid. In this operating scenario, offshore wind power exhibits random and fluctuating characteristics and power generation without inertia. Similarly, the flexible direct current transmission system also has the characteristics of fast transient process and no inertia. In addition, offshore wind power and the flexible direct current transmission system are coupled with each other, and the stability control of the overall system is more complicated.

[0003] The active support capability of the offshore wind power flexible direct current transmission system for the receiving power grid needs to take into account the randomness and volatility of deep-sea wind power and various operating constraints of the converter. At present, the receiving converter with constant direct current voltage control is conducive to the stability of the flexible direct current transmission system, but its external characteristics relative to the receiving power grid are equivalent to a controlled current source, and it cannot actively participate in the active support of the receiving power grid, which is not conducive to the frequency stability of the receiving power grid, especially under the access conditions of local weak nodes of the coastal power grid, which puts forward higher requirements for the planning, design, and operation control of the current power grid. Summary of the invention

[0004] The embodiments of the present invention provide a network-source coordinated control system and method for an offshore wind power flexible DC transmission system to solve the problem that a conventional offshore wind power flexible DC transmission system uses a fixed DC voltage to control a receiving-end converter, cannot actively participate in the active support of a receiving-end power grid, and is not conducive to the frequency stability of the receiving-end power grid.

[0005] In order to solve the above technical problems, the embodiments of the present invention provide the following technical solutions: According to a first aspect of an embodiment of the present invention, a network-source coordination control system for an offshore wind power flexible direct current transmission system is provided, comprising: The receiving-end converter control module includes a power loop, a voltage loop and a current loop, wherein the power loop controls the output frequency of the receiving-end converter by simulating the synchronous operation mechanism of the virtual synchronous generator; the voltage loop controls the output voltage of the receiving-end converter; the current reference value of the current loop is given by the voltage loop output, and current tracking is achieved through PI control; The sending-end converter control module is used to reflect the system active power demand information on the sending-end offshore wind power collection AC system frequency based on the droop relationship between the DC capacitor voltage energy change and the sending-end island system frequency, so as to realize the active power support control of the offshore wind turbine to the flexible DC transmission system; The offshore wind turbine control module is used based on the variable speed wind turbine control strategy. It takes the electromagnetic power output by the wind turbine, the wind turbine rotor speed and the grid frequency as input, and sends the power reference value and pitch angle reference value of the rotor-side converter of the controlled object as output. The wind turbine operates at reduced load under normal circumstances, and participates in the system frequency control through the unit when power imbalance occurs.

[0006] Furthermore, the receiving-end converter control module obtains an active power frequency control equation based on the rotor motion equation of the synchronous generator by introducing inertia response control into the converter control; obtains a frequency modulation controller representation by introducing a system frequency difference response function into the virtual synchronous generator speed regulator; and obtains a transfer function of the receiving-end converter control module based on the obtained active power frequency control equation and the frequency modulation controller representation.

[0007] Furthermore, the transfer function of the receiving-end converter control module is specifically expressed as follows:

[0008] in, H vsg represents the flexible virtual inertia time constant, ω vsg represents the flexible direct virtual angular frequency, ω g represents the system angular frequency, ω ref represents the angular frequency reference value, K d represents the load damping factor, P in represents the input active power, P out Represents the output active power, P ref Represents the active power reference value, R D represents the active power slip factor, Represents the frequency domain.

[0009] Furthermore, the control model of the receiving-end converter is specifically expressed as follows:

[0010] Among them, ω vsg represents the flexible virtual angular frequency, s represents the frequency domain, is the voltage phase of the receiving-end converter.

[0011] Furthermore, the control model of the sending-end converter control module is specifically expressed as follows by reflecting the system active power demand information on the frequency of the sending-end offshore wind power collection AC system:

[0012] in, is the droop coefficient between DC capacitor energy storage and frequency, is the DC capacitor voltage, is the initial value of the DC capacitor voltage, is the equivalent DC capacitance of the sending-end converter, is the initial value of angular frequency, is the voltage phase of the sending-end converter.

[0013] Furthermore, the offshore wind turbine control module includes a rotor speed reference value setting unit, a pitch angle reference value setting unit, an additional active power fast setting unit and a speed protection unit, wherein the rotor speed reference value setting unit determines the rotor speed reference value based on the maximum power point tracking method; the pitch angle reference value setting unit sets the pitch angle reference value by constructing a power pitch angle curve based on the system frequency state and the primary frequency modulation participation factor using a variable parameter coordinated control method; the additional active power fast setting unit introduces an inertia participation factor, utilizes the inertia participation factor stored in the machine to ensure that the rotor speed does not exceed the limit, and locks the fast inertia response control when the unit rotor speed is too low to prevent the wind turbine speed from exceeding the limit.

[0014] Furthermore, the offshore wind turbine control module makes the rotor speed reach the maximum value at high wind speed when the system frequency is disturbed, and the rotor speed control at medium and low wind speeds is specifically expressed as follows:

[0015]

[0016] in, is the initial value of the rotor speed; is the target value; is the actual given value; b is a given rate of change; is the proportionality coefficient; is the integration coefficient, is the primary frequency modulation participation factor, is the frequency state, is the current output level of the unit; is the rated power of the unit.

[0017] Furthermore, when the system frequency is disturbed, the offshore wind turbine control module controls the pitch angle by constructing a power pitch angle curve based on the system frequency state and the primary frequency modulation participation factor at high wind speeds, and uses virtual standby to participate in frequency modulation; when the wind speed is medium and low, the power pitch angle curve is controlled by constructing a power pitch angle curve based on the system frequency state and the primary frequency modulation participation factor.

[0018] Furthermore, the pitch angle control is specifically expressed as follows:

[0019] in, β 0 is the initial value of the pitch angle; β 1 is the target value; β ref0 is the actual given value; c is a given rate of change; K 3 is the proportionality coefficient; K 4 is the integral coefficient, is the primary frequency modulation participation factor, It is the frequency status.

[0020] According to a second aspect of an embodiment of the present invention, a network source coordinated control method for an offshore wind power flexible DC power transmission system is provided, which is based on the above-mentioned network source coordinated control system for an offshore wind power flexible DC power transmission system, and includes: For the receiving-end converter, the power loop is used to control the output frequency of the receiving-end converter by simulating the synchronous operation mechanism of the virtual synchronous generator, the output voltage of the receiving-end converter is controlled by the voltage loop, the current reference value of the given current loop is output by the voltage loop, and current tracking is achieved through PI control; For the sending-end converter, based on the droop relationship between the voltage energy change of the DC capacitor and the frequency of the sending-end island system, the system active demand information is reflected in the frequency of the sending-end offshore wind power collection AC system, realizing the active support control of the offshore wind turbines on the flexible DC transmission system; For offshore wind turbines, based on the variable speed wind turbine control strategy, the electromagnetic power output by the wind turbine, the wind turbine rotor speed and the grid frequency are used as input, and the power reference value and pitch angle reference value sent to the rotor-side converter of the controlled object are output. The wind turbine operates at reduced load under normal circumstances, and participates in system frequency control through the unit when power imbalance occurs.

[0021] Compared with the prior art, the present invention has the following beneficial effects: The scheme described in the present invention provides a network-source coordinated control system and method for an offshore wind power flexible direct current transmission system. In the scheme, for a receiving-end converter, a virtual synchronous generator control technology is used to simulate the synchronous operation mechanism of a synchronous generator in a converter control loop, so that the device has frequency regulation, inertia and damping characteristics similar to those of a synchronous generator, and, based on a physical mechanism driven by power imbalance, by establishing a proportional relationship between the receiving-end frequency change and the direct current voltage, active frequency support for the receiving-end power grid is achieved, so that the flexible direct current transmission system has the ability to actively support the power grid; at the same time, based on the rotor motion equation of the synchronous generator, inertia response control is introduced into the control of the converter, thereby obtaining an active power frequency control equation, and by introducing a system frequency difference response function in the virtual synchronous generator speed regulator, a frequency modulation controller is formed through an active power frequency slip control loop, and a corresponding controller transfer function is given.

[0022] The scheme of the present invention adopts frequency control based on the change of DC capacitor voltage energy at the sending-end converter of the flexible DC power transmission system, establishes a droop relationship between the change of DC voltage energy and the frequency of the sending-end island system, and realizes active support control of the flexible DC power transmission system by the offshore wind farm by reflecting the active demand information of the system on the frequency of the sending-end offshore wind power collection AC system; The scheme described in the present invention is based on the strategy of variable speed wind turbine control in the active frequency response control of offshore wind turbines. The preset functional modules are used to realize load reduction operation of the unit under normal circumstances, and the unit participates in system frequency regulation control when power imbalance occurs in the system. At the same time, by introducing the inertia participation factor, the kinetic energy stored in the rotating mass block is used to quickly participate in system frequency regulation.

[0023] Advantages of additional aspects of the present invention will be given in part in the following description, and in part will become obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0025] Figure 1 The present invention is a schematic diagram of the structure of a network-source coordinated control system for an offshore wind power flexible direct current transmission system described in an embodiment of the present invention. DETAILED DESCRIPTION

[0026] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0027] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0028] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0029] After introducing the technical solutions of the embodiments of the present invention, various non-limiting implementation methods of the present application are described in detail below.

[0030] Terminology explanation: Sending-end island system: refers to an isolated power system formed in which a power source (such as a large power plant or a distributed power cluster) is disconnected from the main power grid and supplies power to local loads independently. In such a system, frequency stability is a key issue because it cannot rely on the strong inertia and frequency regulation resources of the main power grid to maintain frequency stability like a grid-connected system.

[0031] Additional active power: refers to the additional active power in addition to meeting the basic load demand. It is usually introduced to achieve specific system control goals or to cope with power fluctuations in the system.

[0032] Virtual reserve: refers to a concept that appears in modern power systems, especially in systems that contain a large number of distributed energy resources (DERs) and power electronic equipment. It mainly uses the rapid response capabilities of energy storage systems, flexible and controllable distributed power sources (such as electric vehicles with energy storage functions, adjustable distributed power generation units) and smart inverters, and builds a resource similar to traditional backup power through advanced control strategies. Unlike traditional backup, virtual backup is not necessarily a physical device dedicated to backup, but realizes the backup function through the integration and optimized control of existing resources.

[0033] Rotating mass: refers to the part of an object that has a certain mass and rotates around an axis in a rotating system. Rotating mass is present in many mechanical and electrical devices, such as the rotor of a motor, the wheel of a generator, etc. From a physical point of view, the main characteristic of a rotating mass is that it has a moment of inertia, which is a physical quantity that measures the inertia of a rotating object. It depends on the mass distribution of the mass and the position of the rotation axis.

[0034] In one or more embodiments, Figure 1 As shown, this embodiment provides a network-source coordination control system for an offshore wind power flexible DC transmission system, which specifically includes the following modules: The receiving-end converter control module includes a power loop, a voltage loop and a current loop, wherein the power loop controls the output frequency of the receiving-end converter by simulating the synchronous operation mechanism of the virtual synchronous generator; the voltage loop controls the output voltage of the receiving-end converter; the current reference value of the current loop is given by the voltage loop output, and current tracking is achieved through PI control; In a specific implementation, the receiving-end converter control module is based on the rotor motion equation of the synchronous generator, and obtains the active power frequency control equation by introducing inertia response control into the converter control; obtains the frequency modulation controller representation by introducing the system frequency difference response function into the virtual synchronous generator speed regulator; and obtains the transfer function of the receiving-end converter control module based on the obtained active power frequency control equation and the frequency modulation controller representation.

[0035] Specifically, Figure 1 As shown, the receiving-end converter adopts a new type of grid controller, in which the power loop controls the output frequency of the receiving-end converter through a virtual synchronous generator link, the voltage loop controls the output voltage of the receiving-end converter, the current reference value of the current loop is given by the voltage loop output, and current tracking is achieved through PI control.

[0036] It should be noted that the virtual synchronous generator control technology commonly used in grid-connected converters simulates the synchronous operation mechanism of the synchronous generator in the converter control loop, so that the device has frequency regulation, inertia and damping characteristics similar to those of the synchronous generator. Based on the physical mechanism of power imbalance drive, the proportional relationship between the frequency change at the receiving end and the DC voltage is established to achieve active frequency support for the receiving end grid. At this time, the flexible DC system has the ability to actively support the grid. Based on the rotor motion equation of the synchronous generator, the inertia response control is introduced into the control algorithm of the converter, thereby obtaining the active power frequency control equation (1). In addition, the virtual synchronous generator speed regulator introduces the system frequency difference response function, in which the active power frequency slip control loop constitutes a frequency modulation controller, and its principle is shown in (2).

[0037] (1) (2) in, H vsg represents the flexible virtual inertia time constant, ω vsg represents the flexible direct virtual angular frequency, ω g represents the system angular frequency, ωref represents the angular frequency reference value, K d represents the load damping factor, P in represents the input active power, P out Represents the output active power, P ref Represents the active power reference value, R D Represents the active power slip factor.

[0038] Then the transfer function of the new network controller of the flexible DC onshore receiving end converter is shown as (3).

[0039] (3) Where s represents the frequency domain.

[0040] Then the control mathematical model of the receiving-end converter is specifically expressed as follows:

[0041] The DC voltage of the receiving-end converter satisfies equation (4): (4) Among them, U dc is the DC capacitor voltage, is the equivalent DC capacitance of the receiving-end converter.

[0042] The sending-end converter control module is used to reflect the system active power demand information on the sending-end offshore wind power collection AC system frequency based on the droop relationship between the DC capacitor voltage energy change and the sending-end island system frequency, so as to realize the active power support control of the offshore wind turbine to the flexible DC transmission system; In the specific implementation, Figure 1 As shown, the flexible DC offshore converter based on DC capacitor voltage ∆E dc - f Control, by establishing a proportional relationship between the DC voltage and the frequency of the island system at the sending end, realizes the electrical conduction of the DC state quantity change to the AC system at the sending end.

[0043] Specifically, the converter at the sending end of the flexible DC transmission system adopts a DC capacitor voltage energy change ∆ E dc - f Control, establish the droop relationship between the DC voltage energy change and the frequency of the island system at the sending end, so as to reflect the system active demand information on the frequency of the offshore wind power collection AC system at the sending end, thereby realizing the active support control of the offshore wind farm to the flexible DC transmission system. ∆Edc - f The control mathematical model is shown in formula (5).

[0044] (5) in, is the droop coefficient between DC capacitor energy storage and frequency, is the DC capacitor voltage, is the initial value of the DC capacitor voltage, is the equivalent DC capacitance of the sending-end converter, is the initial value of angular frequency, is the voltage phase of the sending-end converter.

[0045] The offshore wind turbine control module is used based on the variable speed wind turbine control strategy. It takes the electromagnetic power output by the wind turbine, the wind turbine rotor speed and the grid frequency as input, and sends the power reference value and pitch angle reference value of the rotor-side converter of the controlled object as output. The wind turbine operates at reduced load under normal circumstances, and participates in the system frequency control through the unit when power imbalance occurs.

[0046] In a specific implementation, the offshore wind turbine control module includes a rotor speed reference value setting unit, a pitch angle reference value setting unit, an additional active power fast setting unit and a speed protection unit, wherein the rotor speed reference value setting unit determines the rotor speed reference value based on the maximum power point tracking method; the pitch angle reference value setting unit sets the pitch angle reference value by constructing a power pitch angle curve based on the system frequency state and the primary frequency modulation participation factor using a variable parameter coordinated control method; the additional active power fast setting unit introduces an inertia participation factor, utilizes the stored inertia response control to prevent the wind turbine speed from exceeding the limit, and locks the fast inertia response control when the unit rotor speed is too low to prevent the wind turbine speed from exceeding the limit.

[0047] In a specific implementation, when the system frequency is disturbed, the offshore wind turbine control module controls the pitch angle by constructing a power pitch angle curve based on the system frequency state and the primary frequency modulation participation factor at high wind speeds, and uses virtual standby to participate in frequency modulation; when the wind speed is medium and low, the pitch angle is controlled by constructing a power pitch angle curve based on the system frequency state and the primary frequency modulation participation factor.

[0048] Specifically, Figure 1 As shown in the figure, the offshore wind turbine adopts active frequency response control, and its input quantity adopts the output electromagnetic power which is easy to measure in engineering practice. P E , wind turbine rotor speed ω r and grid frequencyf The output is the power reference value given to the rotor-side converter of the controlled object. P ref and pitch angle reference value β ref The frequency controller is based on the control strategy of variable speed wind turbines. It realizes load reduction operation of the unit under normal conditions through additional functional modules, and enables the unit to participate in system frequency regulation control when power imbalance occurs in the system. Specifically, by adding module ② pitch angle reference value setting module, module ③ additional active power fast setting module and module ④ speed protection module, the modified module ① rotor speed reference value setting module is obtained by improving the conventional wind power control method MPT (maximum power point tracking) control module, and the rest are the inherent structures and functions of the conventional wind power control method.

[0049] Among them, the improvement of the conventional wind power control method MPT (maximum power point tracking) control module is specifically: by setting the reserve coefficient and utilizing the rotor speed control, the wind turbine is operated at a non-maximum power point tracking point, so that the wind turbine has continuous active support capability.

[0050] In the specific implementation, inertia influence control is performed based on module ③ by introducing the influence of inertia participation factor. Its specific function is to use the kinetic energy stored in the rotating mass block to quickly participate in the system frequency modulation, as shown in formula (6).

[0051] (6) in, is the additional inertia response active power, is the inertia response coefficient, To reflect the inertia participation factor of the unit kinetic energy, it is given by formula (7).

[0052] (7) Module ④ is used to control the fan rotor speed not to exceed the limit during the dynamic process, and to lock the fast inertia response control when the unit rotor speed is too low to prevent the fan speed from exceeding the limit.

[0053] The offshore wind turbine unit adopts primary frequency modulation analog control, which is realized by the coordinated control of modules ①, ②, ③ and ④. The unit first has load reduction operation. When the frequency of the offshore AC system is disturbed, the rotor speed reaches the maximum value at high wind speed; at medium and low wind speeds, based on the frequency change state and the primary frequency modulation participation factor, the rotor speed is controlled by the technical means of variable parameter coordinated control operation curve, as shown in formula (8).

[0054] (8) in, is the initial value of the rotor speed; is the target value; is the actual given value; b is a given rate of change; is the proportionality coefficient; is the integration coefficient, is the frequency state, is the primary frequency modulation participation factor, obtained by formula (9).

[0055] (9) in, is the current output level of the unit; is the rated power of the unit.

[0056] Module ② Pitch angle reference value control algorithm: When the frequency of the offshore AC system is disturbed, the pitch angle is controlled by means of a variable parameter coordinated control power pitch angle curve based on the system frequency state and the primary frequency modulation participation factor at high wind speed, and the virtual standby is used to participate in frequency modulation; at medium and low wind speeds, the coordinated speed control is based on the system frequency state and the primary frequency modulation participation factor, and the pitch angle is controlled by means of a variable parameter coordinated control power pitch angle curve to control the frequency of the offshore grid-connected AC system, as shown in formula (10).

[0057] (10) in, β 0 is the initial value of the pitch angle; β 1 is the target value; β ref0 is the actual given value; c is a given rate of change; K 3 is the proportionality coefficient; K 4 is the integration coefficient.

[0058] In one or more implementations, this embodiment provides a network source coordinated control method for an offshore wind power flexible DC transmission system, which is based on the above-mentioned network source coordinated control system of an offshore wind power flexible DC transmission system, including: For the receiving-end converter, the power loop is used to control the output frequency of the receiving-end converter by simulating the synchronous operation mechanism of the virtual synchronous generator, the output voltage of the receiving-end converter is controlled by the voltage loop, the current reference value of the given current loop is output by the voltage loop, and current tracking is achieved through PI control; For the sending-end converter, based on the droop relationship between the voltage energy change of the DC capacitor and the frequency of the sending-end island system, the system active demand information is reflected in the frequency of the sending-end offshore wind power collection AC system, realizing the active support control of the offshore wind turbines on the flexible DC transmission system; For offshore wind turbines, based on the variable speed wind turbine control strategy, the electromagnetic power output by the wind turbine, the wind turbine rotor speed and the grid frequency are used as input, and the power reference value and pitch angle reference value sent to the rotor-side converter of the controlled object are output. The wind turbine operates at reduced load under normal circumstances, and participates in system frequency control through the unit when power imbalance occurs.

[0059] It can be understood that the relevant technical details of this embodiment have been described in detail in the method embodiment, so they will not be repeated here.

[0060] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A network source coordination control system for an offshore wind power flexible direct current transmission system, characterized in that: include: The receiving-end converter control module includes a power loop, a voltage loop and a current loop, wherein the power loop controls the output frequency of the receiving-end converter by simulating the synchronous operation mechanism of the virtual synchronous generator; the voltage loop controls the output voltage of the receiving-end converter; the current reference value of the current loop is given by the voltage loop output, and current tracking is achieved through PI control; The sending-end converter control module is used to reflect the system active power demand information on the sending-end offshore wind power collection AC system frequency based on the droop relationship between the DC capacitor voltage energy change and the sending-end island system frequency, so as to realize the active power support control of the offshore wind turbine to the flexible DC transmission system; The offshore wind turbine control module is used based on the variable speed wind turbine control strategy. It takes the electromagnetic power output by the wind turbine, the wind turbine rotor speed and the grid frequency as input, and sends the power reference value and pitch angle reference value of the rotor-side converter of the controlled object as output. The wind turbine operates at reduced load under normal circumstances, and participates in the system frequency control through the unit when power imbalance occurs.

2. A network-source coordinated control system for an offshore wind power flexible direct current transmission system according to claim 1, characterized in that: The receiving-end converter control module is based on the rotor motion equation of the synchronous generator. By introducing inertia response control into the converter control, an active power frequency control equation is obtained; by introducing the system frequency difference response function into the virtual synchronous generator speed regulator, a frequency modulation controller representation is obtained; based on the obtained active power frequency control equation and frequency modulation controller representation, a transfer function of the receiving-end converter control module is obtained.

3. A network source coordination control system for an offshore wind power flexible DC transmission system as claimed in claim 2, characterized in that: The transfer function of the receiving-end converter control module is specifically expressed as follows: in, H vsg represents the flexible virtual inertia time constant, ω vsg represents the flexible direct virtual angular frequency, ω g represents the system angular frequency, ω ref represents the angular frequency reference value, K d represents the load damping coefficient, P in represents the input active power, P out Represents the output active power, P ref Represents the active power reference value, R D represents the active power slip factor, s Represents the frequency domain.

4. A network source coordination control system for an offshore wind power flexible direct current transmission system according to claim 1, characterized in that: The control model of the receiving-end converter is specifically expressed as follows: Among them, ω vsg represents the flexible virtual angular frequency, s represents the frequency domain, is the voltage phase of the receiving-end converter.

5. A network-source coordinated control system for an offshore wind power flexible direct current transmission system according to claim 1, characterized in that: The sending-end converter control module reflects the system active power demand information on the sending-end offshore wind power collection AC system frequency, and its control model is specifically expressed as follows: in, is the droop coefficient between DC capacitor energy storage and frequency, is the DC capacitor voltage, is the initial value of the DC capacitor voltage, is the equivalent DC capacitance of the sending-end converter, is the initial value of angular frequency, is the voltage phase of the sending-end converter.

6. A network-source coordinated control system for an offshore wind power flexible direct current transmission system according to claim 1, characterized in that: The offshore wind turbine control module includes a rotor speed reference value setting unit, a pitch angle reference value setting unit, an additional active power fast setting unit and a speed protection unit, wherein the rotor speed reference value setting unit determines the rotor speed reference value based on the maximum power point tracking method; the pitch angle reference value setting unit sets the pitch angle reference value by constructing a power pitch angle curve based on the system frequency state and the primary frequency modulation participation factor using a variable parameter coordinated control method; the additional active power fast setting unit introduces an inertia participation factor, utilizes the stored inertia response control to prevent the wind turbine speed from exceeding the limit, and locks the fast inertia response control when the unit rotor speed is too low to prevent the wind turbine speed from exceeding the limit.

7. A network-source coordinated control system for an offshore wind power flexible direct current transmission system according to claim 1, characterized in that: The offshore wind turbine control module makes the rotor speed reach the maximum value at high wind speed when the system frequency is disturbed, and the rotor speed control at medium and low wind speeds is specifically expressed as follows: in, is the initial value of the rotor speed; is the target value; is the actual given value; b is a given rate of change; is the proportionality coefficient; is the integration coefficient, is the primary frequency modulation participation factor, is the frequency state, is the current output level of the unit; is the rated power of the unit.

8. A network-source coordinated control system for an offshore wind power flexible direct current transmission system according to claim 1, characterized in that: The offshore wind turbine control module controls the pitch angle by constructing a power pitch angle curve using a variable parameter coordinated control method based on the system frequency state and the primary frequency modulation participation factor at high wind speed when the system frequency is disturbed, and uses virtual standby to participate in frequency modulation; and controls the pitch angle by constructing a power pitch angle curve using a variable parameter coordinated control method based on the system frequency state and the primary frequency modulation participation factor at medium and low wind speeds.

9. A network-source coordinated control system for an offshore wind power flexible direct current transmission system as claimed in claim 8, characterized in that: The pitch angle control is specifically expressed as follows: in, β 0 is the initial value of the pitch angle; β 1 is the target value; β ref0 is the actual given value; c is a given rate of change; K 3 is the proportionality coefficient; K 4 is the integral coefficient, is the primary frequency modulation participation factor, It is the frequency status.

10. A network source coordinated control method for an offshore wind power flexible direct current transmission system, characterized in that: The network-source coordination control system of an offshore wind power flexible direct current transmission system according to any one of claims 1 to 9 comprises: For the receiving-end converter, the power loop is used to control the output frequency of the receiving-end converter by simulating the synchronous operation mechanism of the virtual synchronous generator, the output voltage of the receiving-end converter is controlled by the voltage loop, the current reference value of the given current loop is output by the voltage loop, and current tracking is achieved through PI control; For the sending-end converter, based on the droop relationship between the voltage energy change of the DC capacitor and the frequency of the sending-end island system, the system active demand information is reflected in the frequency of the sending-end offshore wind power collection AC system, realizing the active support control of the offshore wind turbines on the flexible DC transmission system; For offshore wind turbines, based on the variable speed wind turbine control strategy, the electromagnetic power output by the wind turbine, the wind turbine rotor speed and the grid frequency are used as input, and the power reference value and pitch angle reference value sent to the rotor-side converter of the controlled object are output. The wind turbine operates at reduced load under normal circumstances, and participates in system frequency control through the unit when power imbalance occurs.

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