Frequency modulation method for offshore wind power through low frequency power transmission system based on frequency linear mapping

Abstract

This invention discloses a frequency regulation method for offshore wind power via a low-frequency transmission system based on frequency linear mapping. Building upon existing frequency regulation control for offshore wind power via a flexible DC transmission system, this method introduces a frequency linear mapping coefficient for the offshore wind power via low-frequency transmission system. This directly transmits the frequency changes of the onshore power grid to the offshore wind farm, thereby re-establishing the frequency relationship between the offshore wind farm and the onshore power grid. Simultaneously, using the mapped frequency change signal, a droop coefficient is applied to coordinate the power control of the wind turbines, providing support for the frequency of the onshore power grid. By utilizing frequency linear mapping, the frequency connection between the offshore low-frequency system and the onshore power frequency AC system is re-established, enabling the offshore wind farm to provide some support for the frequency recovery of the onshore power frequency AC system.

Description

Technical Field

[0001] This invention relates to a frequency regulation method for offshore wind power transmitted through a low-frequency power transmission system based on frequency linear mapping, and belongs to the field of power grid frequency regulation control technology. Background Technology

[0002] In recent years, the growing contradiction between the demands of rapid social development and energy scarcity has led countries around the world to place increasing emphasis on the development and utilization of new energy sources. Offshore wind power, with its abundant wind energy resources and land-saving advantages, has attracted widespread attention.

[0003] Currently, the main forms of offshore wind power transmission and grid connection include high-voltage alternating current (HVAC) transmission, flexible direct current (DC) transmission, and flexible low-frequency transmission. Among these, small-scale nearshore wind farms often use HVAC transmission. This method is technologically mature, low-cost, and has numerous practical projects for reference. However, when transmitting power to deep-sea wind farms, the capacitive effect of cables becomes a significant problem, and there is coupling between the offshore wind farm and the onshore power grid, making fault isolation impossible. Therefore, considering the relationship between transmission distance and capacitive effect, some researchers have developed another transmission technology—flexible DC transmission—based on fully controllable power electronic devices. This transmission method fully utilizes the advantages of fully controllable devices and both AC and DC power, effectively improving the system's transmission capacity and distance, and solving the problem of large-scale offshore wind power transmission. However, this transmission method requires a large number of power electronic devices and necessitates the construction of offshore and onshore frequency converters, inevitably increasing the system construction cost.

[0004] To address the problems of the two aforementioned power transmission methods, researchers have proposed a novel power transmission technology suitable for offshore areas: Low Frequency Alternating Current (LFAC). This system consists of an offshore wind farm, a low-frequency transformer, submarine cables, an onshore frequency converter, and the main power grid. Unlike traditional offshore wind farm grid connection methods, the LFAC system directly transmits 16.7Hz AC power from the wind farm. This lower frequency significantly reduces the capacitive effect of the cables. However, because the offshore LFAC system has a frequency inconsistency with the onshore power grid, the low-frequency converter and the grid-side converter in the frequency converter need to be decoupled. This also isolates the offshore wind farm from the onshore power grid's frequency, making the offshore wind farm unable to respond to changes in the onshore grid's frequency.

[0005] Similar issues have arisen in offshore wind power connected to the grid via flexible direct current (LFAC). This method uses DC voltage as the signal transmission medium for frequency changes in the onshore power grid, re-establishing the connection between the grid frequency and the wind farm frequency, allowing the offshore wind farm to respond to grid frequency variations. However, due to reactive power losses in AC transmission systems, the voltage at each wind turbine in the wind farm cannot be guaranteed to be consistent. Therefore, for LFAC systems, which are essentially AC, the method of using voltage as the frequency change signal carrier in flexible direct current (LFAC) transmission systems cannot be directly adopted. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a frequency regulation method for offshore wind power transmission via a low-frequency power grid based on frequency linear mapping, thereby solving the decoupling problem between the offshore low-frequency power grid and the onshore power frequency power grid in existing offshore wind power transmission via low-frequency power grid systems.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A frequency regulation method for offshore wind power via a low-frequency transmission system based on frequency linear mapping includes the following steps:

[0009] A model of the control strategies on both sides of a back-to-back converter is constructed, wherein the rectifier side adopts constant voltage and constant frequency control, and the inverter side adopts constant DC voltage and constant AC voltage control.

[0010] Using a droop control strategy, calculate the coefficient by which frequency changes on the grid side are mapped to frequency changes on the low-frequency side;

[0011] By coordinating the power control of wind turbine units through droop control coefficients, support is provided for the frequency recovery of the main power grid.

[0012] Furthermore, the aforementioned models of the control strategies on both sides of the back-to-back converter include a low-frequency side converter control strategy model and a grid-side converter control strategy model.

[0013] The low-frequency side converter adopts a control strategy of constant AC voltage and constant frequency;

[0014] The grid-side converter adopts a control strategy of constant DC voltage and constant AC voltage, based on dual-loop control of outer loop voltage and inner loop current. The control steps of the outer loop voltage include:

[0015] The DC capacitor voltage and the converter output voltage amplitude are controlled to generate reference signals for the d-axis and q-axis currents;

[0016] The output current dq signal, after PI control, 2s / 3r transformation, and PWM modulation, generates the gate trigger pulse signal for the grid-side converter. The 2s / 3r transformation matrix is ​​as follows:

[0017]

[0018] In the formula, V a V b V c These represent the three-phase voltages; V d V q V0 represents the transformed voltage direct-axis component, voltage quadrature-axis component, and voltage zero-axis component.

[0019] Furthermore, the aforementioned steps for calculating the frequency change coefficient mapped from the grid-side frequency change to the low-frequency side using a droop control strategy include:

[0020] According to the principles of power flow and power conservation within the converter:

[0021]

[0022] In the formula, P WF P represents the power transmitted from an offshore wind farm to the frequency converter station via submarine cables and transformers. c P represents the power stored in a DC capacitor. grid This indicates the power transmitted to the onshore power grid converter;

[0023] Based on the dynamic characteristics of a capacitor:

[0024]

[0025] In the formula, C represents the capacitance, and V dc This represents the instantaneous value of the capacitor voltage;

[0026] The conclusion is ;

[0027] Analogous to the relationship between the mechanical power, electromagnetic power, and frequency of a synchronous generator set:

[0028]

[0029] In the formula, G dc f is the virtual inertia of the DC capacitor. ins For the instantaneous frequency of the large onshore power grid;

[0030] make Integrating both sides with respect to time t and applying Taylor expansion at the initial value of the capacitor voltage, we have:

[0031] ,

[0032] In the formula, V dc0 Indicates the initial value of the capacitor voltage; Δf gr This represents the change in frequency of the large onshore power grid;

[0033] Similarly, based on the relationship between the low-frequency side frequency and the change in DC capacitor voltage, we have:

[0034]

[0035] In the formula, f WF This represents the value after the low-frequency side frequency change, k2 represents the proportionality coefficient between the low-frequency side frequency change and the capacitor voltage change, and f0 represents the low-frequency side frequency reference value. This indicates the voltage change of a DC capacitor;

[0036] Continuous and have to:

[0037]

[0038]

[0039] In the formula, K LFAC This represents the coefficient that maps frequency changes on the grid side to frequency changes on the low-frequency side.

[0040] Furthermore, based on the aforementioned coefficient that maps frequency changes on the grid side to frequency changes on the low-frequency side, an upper limit action value f is introduced. high With lower limit action value f low ,in:

[0041]

[0042] In the formula, f1 represents the reference frequency of the onshore power grid; α is the fluctuation amplitude.

[0043] Furthermore, the aforementioned α takes the value of 0.2%.

[0044] Furthermore, the aforementioned steps for coordinated control of wind turbine power through droop control coefficients include:

[0045] Transmitting onshore power grid frequency signals to offshore wind farms;

[0046] Based on the frequency-active power droop control, the frequency signal is converted into a reference value for the active power of the wind farm.

[0047] The power controller is used to adjust the actual output active power of offshore wind farms in real time, targeting MPPT control and frequency-active power droop control.

[0048] Furthermore, the aforementioned frequency-active power droop control strategy is as follows:

[0049]

[0050] In the formula, ΔP represents the change in active power of the wind turbine; K WF The coefficient is based on the active power and frequency droop control of the wind turbine. This indicates the instantaneous frequency of the onshore power grid.

[0051] The beneficial effects achieved by this invention are as follows:

[0052] This invention, building upon existing frequency regulation control of offshore wind power via flexible DC transmission systems, addresses the mapping of grid-side frequency changes introduced by offshore wind power through low-frequency transmission systems to the low-frequency side. This involves directly transmitting the frequency change signal from the onshore power grid to the offshore wind farm, thereby re-establishing the frequency relationship between the offshore wind farm and the onshore power grid. Simultaneously, utilizing the mapped frequency change signal, a droop control coefficient is used to coordinate the power control of the wind turbines, providing support for the onshore power grid frequency. This invention leverages linear frequency mapping to re-establish the frequency connection between the offshore low-frequency system and the onshore power frequency AC system, enabling the offshore wind farm to provide some support for the frequency recovery of the onshore power frequency AC system. Attached Figure Description

[0053] Figure 1 This is a control block diagram for the low-frequency side converter.

[0054] Figure 2 Block diagram of grid-side converter control strategy;

[0055] Figure 3 This is a diagram showing the power flow inside the converter.

[0056] Figure 4 (a) is the droop characteristic curve;

[0057] Figure 4 (b) shows the droop characteristic curve after adding the dead zone;

[0058] Figure 5 Frequency coordination control strategy for wind turbine units;

[0059] Figure 6 This is a topology diagram of an offshore wind farm connected to the LFAC grid.

[0060] Figure 7 A comparison chart of the active power output of a single wind turbine measured with and without the present invention;

[0061] Figure 8 A comparison chart of onshore power grid frequencies measured using the present invention and those not using the present invention;

[0062] Figure 9 A comparison chart of the rate of change of onshore power grid frequency measured with and without the present invention;

[0063] Figure 10 This invention maps the frequency changes on the low-frequency side of the onshore power grid to the frequency changes measured by the present invention. Detailed Implementation

[0064] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0065] This embodiment discloses a frequency regulation control method for offshore wind power transmitted through a low-frequency power transmission system based on frequency linear mapping. The specific steps are as follows:

[0066] The first step is to construct the control strategy model for both sides of the back-to-back converter. The rectifier side employs constant voltage and constant frequency control, while the inverter side employs constant DC voltage and constant AC voltage control. The back-to-back converter consists of a low-frequency side converter and a grid-side converter. The primary purpose of the low-frequency side converter is to receive and transmit power generated by offshore wind farms. However, due to wind speed fluctuations and the limitations of reactive power losses in the AC system, if the low-frequency side converter does not employ constant AC voltage and constant frequency control, the frequency and voltage stability of the low-frequency transmission system cannot be maintained. Therefore, this side converter needs to employ constant voltage and constant frequency control. Its simplified control block diagram is shown below. Figure 1 As shown in the control chart, the 3r / 2s transformation matrix, the 2s / 3r inverse transformation matrix, and the Mean mathematical model are shown in (1), (2), and (3), respectively:

[0067] (1)

[0068] (2)

[0069] (3)

[0070] In the formula, V a V b V c These represent the three-phase voltages; V d V q V0 represents the direct-axis component, quadrature-axis component, and zero-axis component of the transformed voltage; V represents the amplitude of the AC voltage.

[0071] Meanwhile, due to the presence of inductive components in the low-frequency system, the AC voltage will change accordingly when the power delivered by the wind farm fluctuates. Therefore, constant AC voltage control is adopted on this side to ensure that the AC voltage amplitude and phase angle remain unchanged. Furthermore, based on the relationship between active power and frequency, controlling the frequency on this side to be constant not only provides a reference for the frequency of each wind turbine but also ensures the stability of the power received at the frequency converter station.

[0072] The grid-side converter is directly connected to the low-frequency converter via a DC capacitor. Because of its energy storage characteristics, the DC capacitor provides a certain degree of system frequency regulation and can maintain voltage stability during power transmission. Therefore, the grid-side converter control strategy employs constant DC voltage and constant AC voltage control.

[0073] Therefore, the grid-side converter adopts a dual-loop control based on outer-loop voltage and inner-loop current. The PI-based outer-loop voltage control primarily controls the DC capacitor voltage and the converter output voltage amplitude, while simultaneously generating reference signals for the d-axis and q-axis currents. Finally, the output current dq signal, along with PI control, 2s / 3r conversion, and PWM modulation, generates the gate trigger pulse signal for the grid-side converter. The specific control strategy is as follows: Figure 2 As shown in the figure. V dc , Indicates the instantaneous value and reference value of the capacitor voltage; V ac , I ac This indicates the instantaneous value of the AC bus voltage, the reference value, and the instantaneous value of the AC current at the converter output terminal; , The d-axis and q-axis reference values ​​of the outer loop voltage control output are represented; the 3r / 2s and 2s / 3r transformation matrices are the same as those in equations (1) and (2).

[0074] The second step involves using the novel droop control strategy described in this invention to calculate the correlation coefficient of the frequency mapping, thereby mapping the frequency variation on the onshore power grid side to the offshore low-frequency transmission system. Assume the power transmitted from the offshore wind farm to the frequency converter station via submarine cables and transformers is P. WF The DC capacitor stores power P. c The power transmitted to the onshore power grid converter is P. grid The power flow inside the converter is as follows Figure 3 As shown, according to Figure 3 According to the principle of power conservation:

[0075] (4)

[0076] Based on the dynamic characteristics of a capacitor:

[0077] (5)

[0078] In the formula, C represents the capacitance, and the unit is F.

[0079] Then, from equations (4) and (5), we have:

[0080] (6)

[0081] Analogous to the relationship between the mechanical power, electromagnetic power, and frequency of a synchronous generator set:

[0082] (7)

[0083] In the formula, G dc f is the virtual inertia of the DC capacitor. ins This refers to the instantaneous frequency of the large onshore power grid.

[0084] Integrating both sides of equation (7) with respect to time t and performing a Taylor expansion at the initial value of the capacitor voltage, we have:

[0085] (8)

[0086] (9)

[0087] In the formula, V dc0 Indicates the initial value of the capacitor voltage; Δf gr This represents the change in frequency of the large onshore power grid;

[0088] Similarly, based on the relationship between the low-frequency side frequency and the change in DC capacitor voltage, we have:

[0089] (10)

[0090] In the formula, due to the change in DC capacitor voltage, the frequency on the low-frequency side will change, therefore f WF The value represents the low-frequency side after the frequency change; k2 represents the proportionality coefficient between the low-frequency side frequency change and the capacitor voltage change; f0 represents the low-frequency side frequency reference value (16.7Hz in this article). This represents the voltage change of a DC capacitor.

[0091] From equations (8) and (10), we have:

[0092] (11)

[0093] (12)

[0094] In the formula, K LFAC This represents the coefficient by which the frequency change on the grid side is mapped to the frequency change on the low-frequency side. At this point, from equation (11), it can be found that the change in the onshore grid frequency is linearly related to the change in the wind farm frequency, such as... Figure 4 As shown in (a). In the figure, f1 represents the reference frequency of the onshore power grid; f2 represents the instantaneous frequency of the offshore wind farm.

[0095] Therefore, when the frequency of the onshore power grid fluctuates under stable operating conditions, the frequency of the offshore wind farm will also change immediately, causing the wind turbine power controller to activate. To avoid the controller becoming overly sensitive, this paper adds a dead zone to equation (11), that is, introduces an upper limit action value f.high With lower limit action value f low ,like Figure 4 As shown in (b), this ensures that offshore wind farms are not affected by static fluctuations in the frequency of the onshore power grid, thus making the system more stable.

[0096] However, the selection of the deadband limit range directly affects the controller's dynamic response. If the deadband value is too small, the controller becomes overly sensitive; if it is too large, the controller responds slowly. Therefore, considering both the controller's response speed and actual performance, and in accordance with relevant provisions in the national wind power grid connection technical standards, the upper limit action value f is set... high With lower limit action value f low Set them to:

[0097] (13)

[0098] In the formula, α is a constant value, representing the fluctuation amplitude (0.2% in this paper).

[0099] The third step is to coordinate the power of the wind turbine through the droop control coefficient to support the frequency recovery of the power grid. That is, according to the relationship between active power and frequency, if the wind farm is to support the frequency change of the power grid, the wind farm must be able to adjust the active power output in real time. Therefore, the droop control shown in Equation (14) is used to establish the relationship between active power and frequency.

[0100] (14)

[0101] In the formula, ΔP represents the change in active power of the wind turbine; K WF The coefficient is based on the active power and frequency droop control of the wind turbine. This indicates the instantaneous frequency of the onshore power grid.

[0102] Therefore, the offshore wind power frequency coordination control strategy adopted in this invention is as follows: Figure 5 As shown, firstly, the frequency signal from the onshore power grid is transmitted to the offshore wind farm using the frequency mapping coefficients proposed above. Secondly, this frequency signal is converted into a reference value for the active power of the wind farm based on frequency-active power droop control. Finally, through the power controller, the actual output active power of the offshore wind farm is adjusted in real time according to MPPT control and frequency-active power droop control, thereby responding to changes in the frequency of the onshore power grid and providing some support for its frequency recovery.

[0103] Finally, to verify the effectiveness of the frequency regulation control method for offshore wind power via a low-frequency transmission system based on frequency linear mapping proposed in this invention, the method was tested on the Matlab / Simulink platform according to the following... Figure 6The simulation model of the offshore wind farm shown is built using the LFAC grid topology diagram. The offshore wind farm is represented by a single wind turbine based on a permanent magnet synchronous motor; the back-to-back frequency converter stations are connected to the AC system via step-down transformers. Other necessary simulation parameters are shown in Table 1.

[0104] Table 1 System Simulation Parameters

[0105]

[0106] In the simulation, the grid-side load power consumption was 4.33MW; the rated power of a single offshore wind turbine was 5.2MW; and the onshore power grid was considered as a balancing node. At t=6s, due to a sudden disturbance, the frequency of the onshore power grid dropped by approximately 0.5Hz. To facilitate a comparative analysis of the effects of different control strategies on the response to frequency changes in the onshore power grid, this paper applies the present invention to the MPPT control of the wind turbine (referred to as Simulation 2). Simultaneously, a control experiment was conducted without using the method proposed in this invention (referred to as Simulation 1).

[0107] Depend on Figure 7 The active power output waveform of the wind turbine shows that, under the conditions of Simulation 1, when the frequency of the onshore power grid drops at t=6s, the offshore wind turbine cannot directly respond to the frequency change of the power grid due to the decoupling control of the onshore frequency converter. Therefore, the active power output of the wind turbine itself remains unchanged. However, after the onshore power grid itself adjusts, the frequency of the onshore power grid recovers to 50Hz after about 1 second, and its frequency change is as follows: Figure 8 , Figure 9 As shown in the figure. Compared with Simulation 1, Simulation 2 introduces the frequency mapping strategy proposed in this paper for the wind turbine. That is, when the frequency of the onshore power grid fluctuates, the frequency change can be mapped to the frequency change on the low-frequency side. The mapping result is shown in the figure. Figure 10 As shown. Meanwhile, Figure 7 and Figure 10 The simulation waveforms also show that the wind turbine can immediately adjust its active power output according to the mapped frequency change, providing some support for the restoration of the onshore power grid frequency, thereby accelerating the restoration of the onshore power grid frequency.

[0108] In summary, the frequency regulation control method for offshore wind power via a low-frequency transmission system based on frequency linear mapping proposed in this invention utilizes a novel droop control strategy and introduces a frequency mapping coefficient. This coefficient can directly map the frequency variation of the onshore power frequency AC system to the offshore low-frequency system, thereby re-establishing the frequency connection between the two systems and effectively solving the frequency decoupling problem between the systems. This enables offshore wind farms to participate in the frequency regulation of the onshore power frequency AC system and provides some support for its frequency recovery.

[0109] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A frequency regulation method for offshore wind power via a low-frequency transmission system based on frequency linear mapping, characterized in that, Includes the following steps: A model of the control strategies on both sides of a back-to-back converter is constructed, wherein the rectifier side adopts constant voltage and constant frequency control, and the inverter side adopts constant DC voltage and constant AC voltage control. Using a droop control strategy, the coefficients by which grid-side frequency changes are mapped to low-frequency side frequency changes are calculated, specifically including: The step of calculating the frequency change coefficient mapped from the grid-side frequency change to the low-frequency side using the droop control strategy includes: According to the principles of power flow and power conservation within the converter: ； In the formula, P WF P represents the power transmitted from an offshore wind farm to the frequency converter station via submarine cables and transformers. c P represents the power stored in a DC capacitor. grid This indicates the power transmitted to the onshore power grid converter; Based on the dynamic characteristics of a capacitor: ； In the formula, C represents the capacitance, and V dc This represents the instantaneous value of the capacitor voltage; The conclusion is ; Analogous to the relationship between the mechanical power, electromagnetic power, and frequency of a synchronous generator set: ； In the formula, G dc f is the virtual inertia of the DC capacitor. ins The instantaneous frequency of the onshore power grid; make Integrating both sides with respect to time t and using Taylor expansion at the initial value of the capacitor voltage, we have: ， ； In the formula, V dc0 Δf represents the initial value of the capacitor voltage. gr This represents the change in frequency of the large onshore power grid; Similarly, based on the relationship between the low-frequency side frequency and the change in DC capacitor voltage, we have: ； In the formula, f WF This represents the value after the low-frequency side frequency change, k2 represents the proportionality coefficient between the low-frequency side frequency change and the capacitor voltage change, and f0 represents the low-frequency side frequency reference value. This indicates the voltage change of a DC capacitor; Continuous and have to: ； ； In the formula, K LFAC The coefficient representing the mapping of frequency changes on the grid side to frequency changes on the low-frequency side; By coordinating the power control of wind turbine units through droop control coefficients, support is provided for the frequency recovery of the main power grid.

2. The frequency regulation method for offshore wind power via a low-frequency transmission system based on frequency linear mapping according to claim 1, characterized in that, The model of the control strategy on both sides of the back-to-back converter includes a low-frequency side converter control strategy model and a grid-side converter control strategy model. The low-frequency side converter adopts a control strategy of constant AC voltage and constant frequency. The grid-side converter adopts a control strategy of constant DC voltage and constant AC voltage, based on dual-loop control of outer loop voltage and inner loop current. The control steps of the outer loop voltage include: The DC capacitor voltage and the converter output voltage amplitude are controlled to generate reference signals for the d-axis and q-axis currents; The output current dq signal is controlled by a PI controller, transformed by a 2s / 3r converter, and modulated by PWM to generate the gate trigger pulse signal for the grid-side converter. The 2s / 3r transformation matrix is ​​as follows: ； In the formula, V a V b V c These represent the three-phase voltages; V d V q V0 represents the transformed voltage direct-axis component, voltage quadrature-axis component, and voltage zero-axis component.

3. The frequency regulation method for offshore wind power via a low-frequency transmission system based on frequency linear mapping according to claim 1, characterized in that, An upper limit action value f is introduced based on the frequency change coefficient that maps grid-side frequency changes to low-frequency-side frequency changes. high With lower limit action value f low ,in: ； In the formula, f1 represents the reference frequency of the onshore power grid; α is the fluctuation amplitude.

4. The frequency regulation method for offshore wind power via a low-frequency transmission system based on frequency linear mapping according to claim 3, characterized in that, The value of α is 0.2%.

5. The frequency regulation method for offshore wind power via a low-frequency transmission system based on frequency linear mapping according to claim 1, characterized in that, The steps for coordinated control of wind turbine power through droop control coefficient include: Transmitting onshore power grid frequency signals to offshore wind farms; Based on the frequency-active power droop control, the frequency signal is converted into a reference value for the active power of the wind farm. The power controller is used to adjust the actual output active power of offshore wind farms in real time, targeting MPPT control and frequency-active power droop control.

6. The frequency regulation method for offshore wind power via a low-frequency transmission system based on frequency linear mapping according to claim 5, characterized in that, The frequency-active power droop control strategy is as follows: ； In the formula, ΔP represents the change in active power of the wind turbine; K WF The coefficient is based on the active power and frequency droop control of the wind turbine. This indicates the instantaneous frequency of the onshore power grid.

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