Network control and active energy management method for offshore wind power system based on dr-mmc parallel hybrid topology and offshore wind power system

Abstract

The application discloses a method for grid connection control and active energy management of an offshore wind power system based on a DR-MMC parallel hybrid topology and an offshore wind power system, and belongs to the technical field of offshore wind power high-voltage direct current transmission. In order to solve the problems of weak frequency support, dependence on communication and non-smooth switching of the existing DR-MMC parallel hybrid transmission system, the following is included: offshore MMC adopts AC / DC grid connection control, and adjusts and controls offshore AC voltage and DC voltage; the onshore MMC adopts virtual synchronous machine control, and adjusts active power according to the grid frequency. When the onshore grid frequency is disturbed, the onshore MMC adjusts the output power according to the frequency deviation, the offshore MMC changes the active power transmitted by the DR line by adjusting the offshore AC voltage, so as to adjust the MMC line power, maintain the stability of the DC voltage and support the grid frequency. An active energy management strategy is adopted to realize smooth switching in normal, wind power surplus and wind power shortage conditions. The application is used in the field of offshore wind power high-voltage direct current transmission.

Description

Technical Field

[0001] This invention relates to a grid control and active energy management method for offshore wind power systems based on DR-MMC parallel hybrid topology, and to an offshore wind power system, belonging to the field of offshore wind power high voltage DC transmission technology. Background Technology

[0002] In recent years, offshore wind power has rapidly developed towards deeper waters and larger scale. Modular multilevel converter-based high-voltage direct current (MMC-HVDC) transmission systems have become the mainstream technology for long-distance, large-scale offshore wind power grid connection due to their advantages such as high voltage levels, strong scalability, no need for AC filters, and flexible control. However, MMC converter stations are large and heavy, requiring the construction of large offshore platforms, leading to a significant increase in initial investment and subsequent operation and maintenance costs.

[0003] To reduce the cost of transmitting offshore wind power, high-voltage direct current (DR-HVDC) transmission systems based on diode rectifiers have attracted widespread attention due to their light weight, small size, low platform requirements, and significant cost advantages. Existing research has preliminarily verified the technical feasibility and fault ride-through capability of DR-HVDC systems. However, DR-HVDC systems require all wind turbines in offshore wind farms to adopt grid-fed (GFM) control to autonomously establish offshore AC voltage and frequency. This necessitates large-scale retrofitting of existing conventional grid-fed (GFL) wind turbines, presenting significant technical challenges and resulting in poor black-start capability, thus limiting their engineering application.

[0004] To balance the flexibility and controllability of MMC with the economy of DR, a hybrid transmission topology combining DR and MMC is proposed. The structure where DR and MMC are connected in parallel on both the AC and DC sides is considered more practical for engineering applications. In this parallel hybrid topology, the small-capacity MMC is responsible for establishing the AC voltage and frequency at sea, providing reactive power compensation and harmonic suppression, and supporting black start; the DR handles most of the power transmission, thereby reducing the overall system cost. However, existing control strategies for DR-MMC parallel hybrid topologies still have the following technical shortcomings:

[0005] 1. Insufficient frequency support capacity of the power grid: In existing solutions, onshore converter stations typically employ constant DC voltage control or constant power control, which cannot provide inertial and damping support for the onshore AC power grid. When the onshore power grid experiences frequency disturbances, offshore wind power cannot actively participate in frequency regulation, making it difficult to adapt to weak grid scenarios with a high proportion of new energy sources.

[0006] 2. Power distribution relies on communication: Power distribution between the MMC and DR at sea usually relies on inter-station communication coordination, which increases system complexity, control delay and communication failure risk, and reduces the system's autonomy and reliability.

[0007] 3. Unstable switching between operating conditions: Under different operating conditions (such as normal, power shortage, and power surplus) such as wind power fluctuations and changes in onshore load, the existing control strategy lacks a unified active energy management mechanism, making it difficult to achieve smooth and seamless switching between control modes. This can easily lead to problems such as DC voltage exceeding limits, power oscillation, and even instability.

[0008] Therefore, there is an urgent need to propose a control method for DR-MMC parallel hybrid transmission systems that can fully leverage the cost advantages of DR, provide active frequency support for onshore power grids, achieve autonomous power allocation without communication, and adapt to smooth operation under all operating conditions. Summary of the Invention

[0009] The purpose of this invention is to solve the problems of insufficient frequency support capability of existing DR-MMC parallel hybrid transmission systems on land, poor reliability due to reliance on communication for power distribution, and unstable switching of control modes under different operating conditions. It provides a method for grid control and active energy management of offshore wind power systems based on DR-MMC parallel hybrid topology, as well as an offshore wind power system.

[0010] The method for grid control and active energy management of offshore wind power systems based on DR-MMC parallel hybrid topology described in this invention includes:

[0011] The offshore modular multilevel converter (MMC) adopts AC / DC grid control, which simultaneously regulates the AC voltage of the offshore AC collection network and the high-voltage DC voltage of the MMC transmission line.

[0012] The onshore modular multilevel converter (MMC) uses virtual synchronous generator control to adjust its active power command in real time according to the frequency of the onshore AC power grid.

[0013] When frequency disturbances occur in the onshore AC power grid, the onshore modular multilevel converter (MMC) adjusts its output power according to the frequency deviation. At the same time, the offshore modular multilevel converter (MMC) adjusts the active power transmitted by the diode rectifier (DR) transmission line by regulating the offshore AC voltage, thereby adjusting the active power transmitted by the MMC transmission line, maintaining the DC voltage stability of the MMC transmission line and supporting the frequency of the onshore AC power grid.

[0014] An active energy management strategy is adopted to achieve smooth switching of control targets under normal operating conditions, wind power surplus operating conditions, and wind power shortage operating conditions.

[0015] Preferably, the offshore modular multilevel converter (MMC) adopts AC / DC grid control, specifically including:

[0016] The deviation between the MMC DC voltage reference value and the measured DC voltage is adjusted by a proportional-integral controller, and the output is limited to serve as the d-axis voltage reference component, while the q-axis voltage reference component is set to 0.

[0017] The AC voltage medium-voltage controller regulates the marine AC voltage within a preset range, and the AC voltage control output is connected to the inner loop controller to complete current regulation.

[0018] The phase angle of the modulated wave is generated by integrating at a constant frequency.

[0019] Preferably, output limiting Per-unit value, default range is ;

[0020] The phase angle of the modulated wave is determined by Constant frequency integral generation.

[0021] Preferably, the active energy management strategy specifically includes:

[0022] Under normal operating conditions, the onshore MMC injects power into the onshore AC grid according to the virtual synchronous generator control; the offshore MMC automatically distributes wind power between the MMC transmission line and the DR transmission line by adjusting the offshore AC voltage, and the DR transmission line acts as a relaxation node to balance the power difference.

[0023] In wind power deficit conditions, when the power command controlled by the virtual synchronous generator exceeds the actual available power of the wind farm and the DC voltage of the MMC transmission line is lower than the first threshold, the onshore MMC switches from virtual synchronous generator control to DC voltage control to stabilize the DC voltage at the first threshold; the DC voltage controller of the offshore MMC is saturated, its d-axis voltage reference value is set to the lower limit to prevent DR from conducting, and all wind power is transmitted through the MMC transmission line.

[0024] In the case of wind power surplus, when the DC voltage of the DR transmission line increases, causing the AC voltage at sea to rise, and the DC voltage of the MMC transmission line exceeds the second threshold, the onshore MMC switches from virtual synchronous generator control to DC voltage control to stabilize the DC voltage at the second threshold; the DC voltage controller of the offshore MMC saturates and switches to AC voltage control mode to stabilize the offshore AC voltage at the upper limit, the DR transmission line operates at maximum power, and the MMC transmission line acts as a relaxation node to absorb the surplus power.

[0025] The offshore wind power system based on DR-MMC parallel hybrid topology described in this invention includes:

[0026] The first offshore wind farm, OWF1, has its AC output connected to the AC side of the offshore modular multilevel converter (MMC).

[0027] The second offshore wind farm, OWF2, has its AC output connected to the AC side of the offshore diode rectifier DR.

[0028] The marine modular multilevel converter (MMC) and the marine diode rectifier (DR) are interconnected on the AC side via an AC submarine cable (Cab1).

[0029] The DC side of the offshore modular multilevel converter (MMC) is connected to the first high-voltage direct current transmission line, and the onshore end of the line is connected to the first onshore MMC, which is connected to the first onshore AC power grid.

[0030] The DC side of the marine diode rectifier DR is connected to the second high-voltage DC transmission line, and the onshore end of the line is connected to the second onshore MMC, which is connected to the second onshore AC power grid.

[0031] The marine modular multilevel converter (MMC) is configured to use AC / DC grid control, and simultaneously regulate the AC voltage of the marine AC collection network and the DC voltage of the first high-voltage DC transmission line.

[0032] The first onshore MMC is configured to use virtual synchronous generator control to adjust its active power command in real time according to the frequency of the first onshore AC grid;

[0033] When the first onshore AC power grid experiences frequency disturbances, the first onshore MMC adjusts its output power according to the frequency deviation. At the same time, the offshore modular multilevel converter (MMC) adjusts the offshore AC voltage to change the active power transmitted by the offshore diode rectifier (DR) transmission line, thereby adjusting the active power transmitted by the first high-voltage direct current (HVDC) transmission line, maintaining the DC voltage stability of the first HVDC transmission line and supporting the frequency of the first onshore AC power grid.

[0034] Advantages of the present invention: The offshore wind power system based on DR-MMC parallel hybrid topology, the method for grid control and active energy management, and the offshore wind power system provided by the present invention have the following advantages:

[0035] 1. Significantly improves the frequency support capability of the onshore power grid. In this invention, the onshore MMC uses VSG control instead of traditional droop control. When load disturbances occur in the onshore power grid, VSG control reduces the frequency deviation from 0.2Hz in droop control to 0.04Hz, significantly improving frequency regulation performance. During transient processes, the onshore AC voltage fluctuation is minimal, and the system recovery time is shortened from approximately 6s in droop control to 3s. Under sudden load reduction conditions, the frequency overshoot decreases from 0.19Hz in droop control to 0.04Hz in VSG control. Simultaneously, supported by the power of MMC2 under VSG control, the output power adjustment of the synchronous generator decreases from approximately 250MW in droop control to approximately 45MW, effectively reducing the regulation burden on the synchronous generator.

[0036] 2. Achieving autonomous power allocation and smooth switching across all operating conditions without communication. In this invention, the offshore MMC employs AC / DC grid control. When the onshore power grid frequency fluctuates, the offshore MMC can rapidly redistribute power between the DR and MMC transmission lines by slightly adjusting the offshore AC voltage (adjustment range 0.9~1.1 pu), maintaining the stability of the DC voltage of the MMC transmission lines. The entire process requires no inter-station communication. The active energy management method proposed in this invention, under power deficit conditions, when the DC voltage of the MMC transmission lines is below 0.98 pu, automatically switches the onshore MMC from VSG control to DC voltage control, stabilizing the DC voltage at 0.98 pu. Under power surplus conditions, when the DC voltage exceeds 1.02 pu, it switches to DC voltage control, stabilizing the DC voltage at 1.02 pu, achieving seamless switching of control targets under normal operating conditions, power deficit conditions, and power surplus conditions.

[0037] 3. Compatible with conventional grid-connected wind turbines, reducing engineering modification costs. In this invention, the offshore MMC adopts the submitted DC grid-connected control, simultaneously regulating the offshore AC collection grid voltage and the MMC transmission line high-voltage DC voltage. Offshore wind farms do not need to be modified to grid-connected control and can directly use conventional grid-connected wind turbines, avoiding the high modification costs of customized wind turbines and the technical difficulties of black start. Attached Figure Description

[0038] Figure 1 This is a topology diagram of the DR-MMC parallel hybrid topology described in this invention;

[0039] Figure 2 This is a control block diagram for the AC / DC grid control of the Modular Multilevel Converter (MMC) at sea.

[0040] Figure 3 This is a flowchart of the control strategy for land-based MMC;

[0041] Figure 4 This is the equivalent circuit diagram of the DR-MMC parallel hybrid transmission system;

[0042] Figure 5 It is a Bode plot of the transfer function from the MMC DC voltage reference value to the measured DC voltage;

[0043] Figure 6 It is a Bode plot of the transfer function from the load disturbance of the onshore power grid to the frequency domain disturbance of the active power output of the offshore MMC;

[0044] Figure 7 This is a waveform diagram of the system response when wind power increases under normal operating conditions;

[0045] Figure 8 These are system response waveforms under power deficit and power surplus conditions.

[0046] Figure 9 This is a waveform comparison of the system response of droop control and VSG control under sudden load increase conditions;

[0047] Figure 10 This is a waveform comparison of the system response of droop control and VSG control under sudden load reduction conditions. Detailed Implementation

[0048] The technical solutions of 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.

[0049] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0050] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the scope of the invention.

[0051] Example 1:

[0052] The following is combined with Figures 1-10 This embodiment describes a method for grid control and active energy management of offshore wind power systems based on DR-MMC parallel hybrid topology, which includes:

[0053] The offshore modular multilevel converter (MMC) adopts AC / DC grid control, which simultaneously regulates the AC voltage of the offshore AC collection network and the high-voltage DC voltage of the MMC transmission line.

[0054] The onshore modular multilevel converter (MMC) uses virtual synchronous generator control to adjust its active power command in real time according to the frequency of the onshore AC power grid.

[0055] When frequency disturbances occur in the onshore AC power grid, the onshore modular multilevel converter (MMC) adjusts its output power according to the frequency deviation. At the same time, the offshore modular multilevel converter (MMC) adjusts the active power transmitted by the diode rectifier (DR) transmission line by regulating the offshore AC voltage, thereby adjusting the active power transmitted by the MMC transmission line, maintaining the DC voltage stability of the MMC transmission line and supporting the frequency of the onshore AC power grid.

[0056] An active energy management strategy is adopted to achieve smooth switching of control targets under normal operating conditions, wind power surplus operating conditions, and wind power shortage operating conditions.

[0057] Furthermore, the marine modular multilevel converter (MMC) adopts AC / DC grid control, specifically including:

[0058] The deviation between the MMC DC voltage reference value and the measured DC voltage is adjusted by a proportional-integral controller, and the output is limited to serve as the d-axis voltage reference component, while the q-axis voltage reference component is set to 0.

[0059] The AC voltage medium-voltage controller regulates the marine AC voltage within a preset range, and the AC voltage control output is connected to the inner loop controller to complete current regulation.

[0060] The phase angle of the modulated wave is generated by integrating at a constant frequency.

[0061] Furthermore, output limiting. Per-unit value, default range is ;

[0062] The phase angle of the modulated wave is determined by Constant frequency integral generation.

[0063] Furthermore, the transfer function from the MMC DC voltage reference value to the measured DC voltage. satisfy:

[0064] ;

[0065] in, This represents the frequency domain disturbance component of the measured DC voltage of the marine MMC. The frequency domain disturbance component representing the DC voltage reference value of the marine MMC. This represents the transfer function from DC voltage disturbance to output power change. This represents the equivalent DC capacitance of an MMC transmission line. This represents the steady-state operating point value of the DC voltage of the marine MMC. This represents the Laplace operator.

[0066] Furthermore, the active energy management strategy specifically includes:

[0067] Under normal operating conditions, the onshore MMC injects power into the onshore AC grid according to the virtual synchronous generator control; the offshore MMC automatically distributes wind power between the MMC transmission line and the DR transmission line by adjusting the offshore AC voltage, and the DR transmission line acts as a relaxation node to balance the power difference.

[0068] In wind power deficit conditions, when the power command controlled by the virtual synchronous generator exceeds the actual available power of the wind farm and the DC voltage of the MMC transmission line is lower than the first threshold, the onshore MMC switches from virtual synchronous generator control to DC voltage control to stabilize the DC voltage at the first threshold; the DC voltage controller of the offshore MMC is saturated, its d-axis voltage reference value is set to the lower limit to prevent DR from conducting, and all wind power is transmitted through the MMC transmission line.

[0069] In the case of wind power surplus, when the DC voltage of the DR transmission line increases, causing the AC voltage at sea to rise, and the DC voltage of the MMC transmission line exceeds the second threshold, the onshore MMC switches from virtual synchronous generator control to DC voltage control to stabilize the DC voltage at the second threshold; the DC voltage controller of the offshore MMC saturates and switches to AC voltage control mode to stabilize the offshore AC voltage at the upper limit, the DR transmission line operates at maximum power, and the MMC transmission line acts as a relaxation node to absorb the surplus power.

[0070] Furthermore, the onshore modular multilevel converter (MMC) employs virtual synchronous generator control, specifically including: using virtual synchronous generator control to simulate the swing equation of a synchronous generator, providing virtual inertia and damping support for the onshore AC power grid, and its transfer function... for:

[0071] ;

[0072] in, This represents the change in active power output by the onshore MMC. This refers to the frequency deviation of the onshore AC power grid.

[0073] This is the proportional coefficient for droop control;

[0074] This is virtual inertia; The damping coefficient; For gain; This is the rated angular frequency;

[0075] Indicates droop control; This indicates virtual synchronous generator control.

[0076] Furthermore, onshore power grid load disturbances Frequency deviation to onshore AC power grid transfer function satisfy:

[0077] ;

[0078] in, Represents the transfer function of a synchronous generator. This represents the damping coefficient of the synchronous generator. This represents the inertia constant of a synchronous generator;

[0079] Frequency deviation from land-based AC power grid Transfer function to offshore modular multilevel converter (MMC) satisfy:

[0080] ;

[0081] in, This represents the frequency domain disturbance component of the active power output by the marine MMC. The frequency domain disturbance component representing the angular frequency of an onshore AC power grid; This represents the droop control transfer function of the virtual synchronous generator (VSG) in onshore MMC. , , This represents a coefficient determined by the controller parameters and the steady-state operating point;

[0082] Indicates droop control; Indicates virtual synchronous generator control;

[0083] according to and Obtaining load disturbances from onshore power grids Frequency domain disturbance of active power output by offshore MMC transfer function for:

[0084] .

[0085] Furthermore, the small-signal disturbance of the active power output of the marine modular multilevel converter (MMC) satisfies:

[0086] ;

[0087] in, This represents the small-signal disturbance variation in the active power output of the marine MMC. This represents the disturbance variation of the d-axis voltage reference component of the offshore MMC. This represents the steady-state operating point value of the d-axis current of the marine MMC. This represents the steady-state operating point value of the DC voltage of the marine MMC. This represents the disturbance change in the d-axis current of the marine MMC.

[0088] Furthermore, the DC voltage of the offshore diode rectifier (DR) satisfy:

[0089] ;

[0090] Power of marine diode rectifier (DR) satisfy:

[0091] ;

[0092] thereby obtaining for:

[0093] ;

[0094] in, This indicates the voltage at the land end of the DR transmission line. Indicates the DC current of the marine DR. This indicates the impedance of the DR transmission line.

[0095] Example 2:

[0096] The offshore wind power system based on DR-MMC parallel hybrid topology described in this embodiment, such as Figure 1 As shown, it includes:

[0097] The first offshore wind farm, OWF1, has its AC output connected to the AC side of the offshore modular multilevel converter (MMC).

[0098] The second offshore wind farm, OWF2, has its AC output connected to the AC side of the offshore diode rectifier DR.

[0099] The marine modular multilevel converter (MMC) and the marine diode rectifier (DR) are interconnected on the AC side via an AC submarine cable (Cab1).

[0100] The DC side of the offshore modular multilevel converter (MMC) is connected to the first high-voltage direct current transmission line, and the onshore end of the line is connected to the first onshore MMC, which is connected to the first onshore AC power grid.

[0101] The DC side of the marine diode rectifier DR is connected to the second high-voltage DC transmission line, and the onshore end of the line is connected to the second onshore MMC, which is connected to the second onshore AC power grid.

[0102] The marine modular multilevel converter (MMC) is configured to use AC / DC grid control, and simultaneously regulate the AC voltage of the marine AC collection network and the DC voltage of the first high-voltage DC transmission line.

[0103] The first onshore MMC is configured to use virtual synchronous generator control to adjust its active power command in real time according to the frequency of the first onshore AC grid;

[0104] When the first onshore AC power grid experiences frequency disturbances, the first onshore MMC adjusts its output power according to the frequency deviation. At the same time, the offshore modular multilevel converter (MMC) adjusts the offshore AC voltage to change the active power transmitted by the offshore diode rectifier (DR) transmission line, thereby adjusting the active power transmitted by the first high-voltage direct current (HVDC) transmission line, maintaining the DC voltage stability of the first HVDC transmission line and supporting the frequency of the first onshore AC power grid.

[0105] In this invention, such as Figure 1 As shown, the DR-MMC parallel hybrid high-voltage direct current transmission system provided by this invention includes: a first offshore wind farm OWF1 and a second offshore wind farm OWF2. The AC output terminal of OWF1 is connected to the AC side of the offshore modular multilevel converter (MMC), and the AC output terminal of OWF2 is connected to the AC side of the offshore diode rectifier (DR). The AC sides of the offshore MMC and the offshore DR are interconnected via an AC submarine cable (Cab1) to achieve parallel operation.

[0106] The DC side of the offshore MMC is connected to the first high-voltage direct current transmission line (MMC transmission line), and the onshore end of this line is connected to the first onshore MMC (i.e., onshore MMC2). The first onshore MMC is connected to the first onshore AC power grid (grid 1). The DC side of the offshore DR is connected to the second high-voltage direct current transmission line (DR transmission line), and the onshore end of this line is connected to the second onshore MMC (i.e., onshore MMC3). The second onshore MMC is connected to the second onshore AC power grid (grid 2).

[0107] The offshore DR uses a 12-pulse diode rectifier bridge, with reactive power compensation and harmonic suppression filters configured on its AC side. The offshore MMC, onshore MMC2, and onshore MMC3 all use a half-bridge submodule topology. The wind turbines in OWF1, which connects to the AC side of the offshore MMC, and OWF2, which connects to the AC side of the offshore DR, all use conventional grid-connected control and do not need to be modified to grid-connected control.

[0108] Specific implementation of control methods:

[0109] (I) Offshore MMC AC / DC Grid Control

[0110] like Figure 2 As shown, the offshore MMC employs an AC / DC grid control system, simultaneously regulating the AC voltage of the offshore AC power grid and the high-voltage DC voltage of the MMC transmission lines. The specific control process is as follows:

[0111] MMC DC voltage reference value Compared with the measured DC voltage The deviation is adjusted by a proportional-integral controller, and the output, after being limited, is used as the d-axis voltage reference component. q-axis voltage reference component Set to 0. The AC voltage medium-voltage controller regulates the offshore AC voltage within the range of 0.9~1.1 pu, and the AC voltage control output is connected to the inner loop controller to complete current regulation. The modulation wave phase angle is generated by direct integration at a constant frequency of 50Hz.

[0112] Through the above control, the offshore MMC can simultaneously stabilize the DC voltage of the MMC transmission line and the AC voltage of the offshore AC grid, and is compatible with conventional grid-connected wind turbines.

[0113] (II) Land-based MMC Control Strategy

[0114] like Figure 3 As shown, the first onshore MMC (connected to grid 1) adopts virtual synchronous generator (VSG) control, simulating the swing equation of a synchronous generator to provide inertial and damping support for the first onshore AC grid. Its active power command is adjusted in real time according to the frequency of the first onshore AC grid. The transfer function of VSG control is:

[0115]

[0116] in, This represents the change in active power output by the onshore MMC. This refers to the frequency deviation of the onshore AC power grid.

[0117] This is the proportional coefficient for droop control;

[0118] This is virtual inertia; The damping coefficient; For gain; This is the rated angular frequency;

[0119] Indicates droop control; This indicates virtual synchronous generator control.

[0120] The second onshore MMC (connected to the grid 2) adopts conventional DC voltage control to stabilize the high-voltage DC voltage of the DR transmission line.

[0121] (III) Active Energy Management Strategy

[0122] This invention employs an active energy management strategy to achieve smooth switching of control objectives under normal operating conditions, wind power deficit operating conditions, and wind power surplus operating conditions.

[0123] Under normal operating conditions: Onshore MMC injects power into the onshore AC grid according to VSG control; offshore MMC automatically distributes wind power between MMC transmission lines and DR transmission lines by adjusting the offshore AC voltage, with DR transmission lines acting as relaxation nodes to balance the power difference.

[0124] Wind power deficit condition: When the power command controlled by VSG exceeds the actual available power of the wind farm, and the DC voltage of the MMC transmission line is lower than the first threshold (0.98 pu), the onshore MMC switches from VSG control to DC voltage control to stabilize the DC voltage at the first threshold; the DC voltage controller of the offshore MMC is saturated, and its d-axis voltage reference value is set to the lower limit (0.9 pu), preventing DR from conducting, and all wind power is transmitted through the MMC transmission line.

[0125] Wind power surplus operation: When the DC voltage of the DR transmission line increases, causing the AC voltage at sea to rise, and the DC voltage of the MMC transmission line exceeds the second threshold (1.02 pu), the onshore MMC switches from VSG control to DC voltage control to stabilize the DC voltage at the second threshold; the DC voltage controller of the offshore MMC saturates and switches to AC voltage control mode to stabilize the offshore AC voltage at the upper limit (1.1 pu). The DR transmission line operates at maximum power, and the MMC transmission line acts as a relaxation node to absorb the surplus power.

[0126] Frequency disturbance response:

[0127] When frequency disturbances occur in the onshore AC power grid, the first onshore MMC, controlled by VSG, adjusts its output power according to the frequency deviation to provide frequency support for the onshore grid. Simultaneously, the offshore MMC slightly adjusts the offshore AC voltage, changing the active power transmitted by the DR transmission line, thereby adjusting the active power transmitted by the MMC transmission line and maintaining the DC voltage stability of the MMC transmission line. The entire process achieves autonomous allocation of wind power between the two HVDC transmission lines without the need for inter-station communication.

[0128] Simulation verification:

[0129] To verify the effectiveness of the control strategy proposed in this invention, a system was built in the PSCAD / EMTDC environment as follows: Figure 1 The DR-MMC hybrid high-voltage direct current transmission system shown was simulated and verified.

[0130] (I) Frequency Domain Analysis of Transfer Function

[0131] like Figure 5 As shown, the Bode plot of the transfer function H_{dc}(s)Hdc(s) from the MMC DC voltage reference value to the measured DC voltage shows that the gain across the entire frequency band is close to 0dB and the phase change is extremely small, verifying the attenuation-free and accurate tracking of the DC voltage command.

[0132] like Figure 6As shown, the Bode plot of the transfer function H_3(s)H3(s) from the onshore power grid load disturbance to the active power output of the offshore MMC shows that, compared with droop control, VSG control increases the system bandwidth from 0.17Hz to 0.51Hz, and responds to disturbances faster and more effectively, confirming the dynamic advantages of the VSG strategy.

[0133] (II) Verification of Active Energy Control

[0134] like Figure 7 As shown, when the wind power increases under normal operating conditions, the offshore MMC slightly increases the offshore AC voltage from 64.5kV to 68.1kV, and the DR transmission line power increases from 983MW to 1953MW. The power increment is mainly transmitted by the DR transmission line, and the system quickly stabilizes after the transient.

[0135] like Figure 8 As shown, the system achieves smooth mode switching across the entire power range under both power deficit and power surplus conditions. When there is a power deficit, the DR transmission line's power drops to zero, and all wind power is transmitted through the MMC transmission line. When there is a power surplus, the DR transmission line operates at maximum power, and the MMC transmission line acts as a relaxation node to absorb the remaining power.

[0136] (III) Verification of Active Frequency Support for Onshore Power Grid

[0137] like Figure 9 As shown, under the condition of sudden load increase, the VSG control reduces the frequency deviation from 0.2Hz under droop control to 0.04Hz, and the synchronous generator output increases by only 43MW (256MW under droop control), and the recovery time is shortened from about 6s to 3s.

[0138] like Figure 10 As shown, under the condition of sudden load reduction, the VSG control reduces the frequency overshoot from 0.19Hz to 0.04Hz under droop control, and the synchronous generator power is adjusted by only 46MW (253MW under droop control).

[0139] The simulation results above fully verify the effectiveness of the control strategy proposed in this invention.

[0140] The inventive principle of this invention:

[0141] This invention is based on a DR-MMC parallel hybrid topology. Through the coordinated operation of AC / DC grid control of the offshore MMC and VSG control of the onshore MMC, combined with an active energy management strategy, it achieves efficient transmission of offshore wind power and active frequency support for the onshore power grid. Its core inventive principle is as follows:

[0142] 1. Control principle of AC / DC grid connection for offshore MMC

[0143] The offshore MMC simultaneously undertakes two control objectives: first, to stabilize the DC voltage of the MMC transmission lines, and second, to establish and regulate the AC voltage of the offshore AC collection network.

[0144] For DC voltage control, the deviation between the DC voltage reference value and the measured value is adjusted by a PI controller and used as the d-axis voltage reference component. Rapid adjustment of the DC voltage is achieved through inner-loop current control. For AC voltage control, the marine AC voltage is adjusted to the range of 0.9~1.1 pu using a medium-voltage controller.

[0145] The core advantage of this control system is that the offshore MMC, as a voltage source, can provide stable voltage and frequency support for the offshore AC power grid, enabling offshore wind turbines to adopt conventional grid-following control without the need to be modified to grid-connected control, which greatly reduces the difficulty and cost of engineering implementation.

[0146] 2. VSG Control and Frequency Support Principles of Land-based MMC

[0147] Traditional droop control only provides a static droop response at frequency, failing to offer inertia and damping to the power grid. This invention employs VSG control, simulating the swing equations of a synchronous generator. This enables the onshore MMC to possess inertia and damping characteristics similar to a synchronous generator. When frequency disturbances occur in the onshore power grid, the VSG-controlled onshore MMC can respond quickly, automatically adjusting its active power output to provide frequency support for the vulnerable grid.

[0148] Transfer function analysis shows that VSG control increases the system bandwidth from 0.17Hz to 0.51Hz and provides a faster dynamic response. Frequency domain Bode plots show that VSG control has smaller phase lag in the mid-frequency band and better phase characteristics in the high-frequency band, significantly improving system stability and transient predictability.

[0149] 3. Principle of autonomous power allocation without communication

[0150] This invention achieves autonomous power allocation through local control between the marine MMC and the onshore MMC, eliminating the need for inter-station communication. The principle is as follows:

[0151] When the frequency of the onshore power grid changes, the onshore MMC controlled by the VSG automatically adjusts its output power, causing a deviation in the DC voltage of the MMC transmission lines. The offshore MMC senses this deviation through its DC voltage controller and automatically adjusts the offshore AC voltage.

[0152] Changes in the offshore AC voltage directly alter the transmission power of the DR transmission line (the DR DC voltage and the offshore AC voltage satisfy:

[0153]

[0154] This redistributes wind power between the two transmission lines, maintaining the DC voltage stability of the MMC transmission lines.

[0155] This mechanism utilizes the natural coupling relationship of electrical quantities (DC voltage and AC voltage) to transmit control signals, achieving closed-loop autonomous regulation of "land-based frequency change → land-based MMC power adjustment → MMC line DC voltage change → offshore MMC AC voltage adjustment → DR line power change".

[0156] 4. Operating condition switching principle of active energy management

[0157] To address the volatility of offshore wind power and the variability of onshore loads, this invention designs an active energy management strategy to achieve seamless switching between three operating conditions:

[0158] Normal operating condition: The DR transmission line acts as a relaxation node to balance the difference between the total wind power and the VSG power command.

[0159] Power deficit condition: When wind power is insufficient, the DC voltage controller of the offshore MMC saturates, the d-axis voltage reference value drops to the lower limit (0.9 pu), and the DR (radio frequency converter) is automatically shut off due to the low AC voltage. All wind power is transmitted through the MMC lines. When the onshore MMC detects that the DC voltage is below 0.98 pu, it automatically switches to DC voltage control mode.

[0160] Excess power condition: When wind power is in excess, the DR transmission line saturates, causing its DC voltage to rise, which in turn leads to a rise in the offshore AC voltage. Once the DC voltage controller of the offshore MMC reaches saturation, it switches to AC voltage control mode to stabilize the offshore AC voltage at 1.1 pu. When the onshore MMC detects that the DC voltage exceeds 1.02 pu, it switches back to DC voltage control mode.

[0161] This strategy utilizes the controller's saturation characteristics and voltage threshold detection to achieve autonomous judgment and smooth transition of operating conditions, without the need for complex state machines or communication coordination.

[0162] 5. System-level collaboration principle

[0163] From a system perspective, this invention achieves coordinated operation of DR and MMC transmission lines through the following mechanism:

[0164] Power flow balance: When losses are ignored, the system power flow satisfies:

[0165] ,

[0166] in The frequency is determined by the VSG control based on the land-based frequency. It is indirectly controlled by the marine MMC by adjusting the AC voltage.

[0167] Voltage Coordination: AC Voltage Controlled by Marine MMC Simultaneously affects the DC voltage of DR The power transmission capacity of the MMC line forms an electrical coupling between the two lines.

[0168] Frequency-power closed loop: Land-based frequency disturbance → Land-based MMC power response → MMC line DC voltage change → Marine MMC AC voltage regulation → DR line power change → MMC line power adjustment → Land-based frequency recovery, forming a complete closed-loop regulation link.

[0169] In summary, this invention achieves efficient, stable, and autonomous operation of the DR-MMC parallel hybrid transmission system by organically integrating the AC / DC grid control of the offshore MMC, the VSG control of the onshore MMC, and the active energy management strategy, while providing active frequency support capabilities for the weak onshore power grid.

[0170] While the invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. A method for grid control and active energy management of offshore wind power systems based on DR-MMC parallel hybrid topology, characterized in that, It includes: The offshore modular multilevel converter (MMC) adopts AC / DC grid control, which simultaneously regulates the AC voltage of the offshore AC collection network and the high-voltage DC voltage of the MMC transmission line. The onshore modular multilevel converter (MMC) uses virtual synchronous generator control to adjust its active power command in real time according to the frequency of the onshore AC power grid. When frequency disturbances occur in the onshore AC power grid, the onshore modular multilevel converter (MMC) adjusts its output power according to the frequency deviation. At the same time, the offshore modular multilevel converter (MMC) adjusts the active power transmitted by the diode rectifier (DR) transmission line by regulating the offshore AC voltage, thereby adjusting the active power transmitted by the MMC transmission line, maintaining the DC voltage stability of the MMC transmission line and supporting the frequency of the onshore AC power grid. An active energy management strategy is adopted to achieve smooth switching of control targets under normal operating conditions, wind power surplus operating conditions, and wind power shortage operating conditions.

2. The method for grid control and active energy management of offshore wind power systems based on DR-MMC parallel hybrid topology according to claim 1, characterized in that, The marine modular multilevel converter (MMC) adopts AC / DC grid control, specifically including: The deviation between the MMC DC voltage reference value and the measured DC voltage is adjusted by a proportional-integral controller, and the output is limited to serve as the d-axis voltage reference component, while the q-axis voltage reference component is set to 0. The AC voltage medium-voltage controller regulates the marine AC voltage within a preset range, and the AC voltage control output is connected to the inner loop controller to complete current regulation. The phase angle of the modulated wave is generated by integrating at a constant frequency.

3. The method for grid control and active energy management of offshore wind power systems based on DR-MMC parallel hybrid topology according to claim 2, characterized in that, Output limiting Per-unit value, default range is ; The phase angle of the modulated wave is determined by Constant frequency integral generation.

4. The method for grid control and active energy management of offshore wind power systems based on DR-MMC parallel hybrid topology according to claim 2, characterized in that, Transfer function from MMC DC voltage reference value to measured DC voltage satisfy: ； in, This represents the frequency domain disturbance component of the measured DC voltage of the marine MMC. The frequency domain disturbance component representing the DC voltage reference value of the marine MMC. This represents the transfer function from DC voltage disturbance to output power change. This represents the equivalent DC capacitance of an MMC transmission line. This represents the steady-state operating point value of the DC voltage of the marine MMC. This represents the Laplace operator.

5. The method for grid control and active energy management of offshore wind power systems based on DR-MMC parallel hybrid topology according to claim 1, characterized in that, The active energy management strategy specifically includes: Under normal operating conditions, the onshore MMC injects power into the onshore AC grid according to the virtual synchronous generator control; the offshore MMC automatically distributes wind power between the MMC transmission line and the DR transmission line by adjusting the offshore AC voltage, and the DR transmission line acts as a relaxation node to balance the power difference. In wind power deficit conditions, when the power command controlled by the virtual synchronous generator exceeds the actual available power of the wind farm and the DC voltage of the MMC transmission line is lower than the first threshold, the onshore MMC switches from virtual synchronous generator control to DC voltage control to stabilize the DC voltage at the first threshold; the DC voltage controller of the offshore MMC is saturated, its d-axis voltage reference value is set to the lower limit to prevent DR from conducting, and all wind power is transmitted through the MMC transmission line. In the case of wind power surplus, when the DC voltage of the DR transmission line increases, causing the AC voltage at sea to rise, and the DC voltage of the MMC transmission line exceeds the second threshold, the onshore MMC switches from virtual synchronous generator control to DC voltage control to stabilize the DC voltage at the second threshold; the DC voltage controller of the offshore MMC saturates and switches to AC voltage control mode to stabilize the offshore AC voltage at the upper limit, the DR transmission line operates at maximum power, and the MMC transmission line acts as a relaxation node to absorb the surplus power.

6. The method for grid control and active energy management of offshore wind power systems based on DR-MMC parallel hybrid topology according to claim 1, characterized in that, The onshore modular multilevel converter (MMC) employs virtual synchronous generator control, specifically including: using virtual synchronous generator control to simulate the swing equation of a synchronous generator, providing virtual inertia and damping support for the onshore AC power grid, and its transfer function... for: ； in, This represents the change in active power output by the onshore MMC. This refers to the frequency deviation of the onshore AC power grid. This is the proportional coefficient for droop control; This is virtual inertia; The damping coefficient; For gain; This is the rated angular frequency; Indicates droop control; This indicates virtual synchronous generator control.

7. The method for grid control and active energy management of offshore wind power systems based on DR-MMC parallel hybrid topology according to claim 6, characterized in that, Onshore power grid load disturbance Frequency deviation to onshore AC power grid transfer function satisfy: ； in, Represents the transfer function of a synchronous generator. This represents the damping coefficient of the synchronous generator. This represents the inertia constant of a synchronous generator; Frequency deviation from land-based AC power grid Transfer function to offshore modular multilevel converter (MMC) satisfy: ； in, This represents the frequency domain disturbance component of the active power output by the marine MMC. The frequency domain disturbance component representing the angular frequency of an onshore AC power grid; This represents the droop control transfer function of the virtual synchronous generator (VSG) in onshore MMC. , , This represents a coefficient determined by the controller parameters and the steady-state operating point; Indicates droop control; Indicates virtual synchronous generator control; according to and Obtaining load disturbances from onshore power grids Frequency domain disturbance of active power output by offshore MMC transfer function for: 。 8. The method for grid control and active energy management of offshore wind power systems based on DR-MMC parallel hybrid topology according to claim 1, characterized in that, The small-signal disturbance of the active power output of the marine modular multilevel converter (MMC) satisfies: ； in, This represents the small-signal disturbance variation in the active power output of the marine MMC. This represents the disturbance variation of the d-axis voltage reference component of the offshore MMC. This represents the steady-state operating point value of the d-axis current of the marine MMC. This represents the steady-state operating point value of the DC voltage of the marine MMC. This represents the disturbance change in the d-axis current of the marine MMC.

9. The method for grid control and active energy management of offshore wind power systems based on DR-MMC parallel hybrid topology according to claim 1, characterized in that, DC voltage of marine diode rectifier DR satisfy: ； Power of marine diode rectifier (DR) satisfy: ； thereby obtaining for: ； in, This indicates the onshore voltage of the DR transmission line. Indicates the DC current of the marine DR. This indicates the impedance of the DR transmission line.

10. An offshore wind power system based on a DR-MMC parallel hybrid topology for implementing the grid control and active energy management method according to any one of claims 1-9, characterized in that, It includes: The first offshore wind farm, OWF1, has its AC output connected to the AC side of the offshore modular multilevel converter (MMC). The second offshore wind farm, OWF2, has its AC output connected to the AC side of the offshore diode rectifier DR. The marine modular multilevel converter (MMC) and the marine diode rectifier (DR) are interconnected on the AC side via an AC submarine cable (Cab1). The DC side of the offshore modular multilevel converter (MMC) is connected to the first high-voltage direct current transmission line, and the onshore end of the line is connected to the first onshore MMC, which is connected to the first onshore AC power grid. The DC side of the marine diode rectifier DR is connected to the second high-voltage DC transmission line, and the onshore end of the line is connected to the second onshore MMC, which is connected to the second onshore AC power grid. The marine modular multilevel converter (MMC) is configured to use AC / DC grid control, and simultaneously regulate the AC voltage of the marine AC collection network and the DC voltage of the first high-voltage DC transmission line. The first onshore MMC is configured to use virtual synchronous generator control to adjust its active power command in real time according to the frequency of the first onshore AC grid; When the first onshore AC power grid experiences frequency disturbances, the first onshore MMC adjusts its output power according to the frequency deviation. At the same time, the offshore modular multilevel converter (MMC) adjusts the offshore AC voltage to change the active power transmitted by the offshore diode rectifier (DR) transmission line, thereby adjusting the active power transmitted by the first high-voltage direct current (HVDC) transmission line, maintaining the DC voltage stability of the first HVDC transmission line and supporting the frequency of the first onshore AC power grid.

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