A parameter optimization method for improving the oscillation stability of a flexible low-frequency transmission system of offshore wind power

Abstract

The application discloses a parameter optimization method for improving the oscillation stability of a marine wind power flexible low-frequency sending-out system, and comprises the following steps: establishing a refined broadband impedance model of the M3C low-frequency side; determining the frequency band in which the marine wind power flexible low-frequency sending-out system has an oscillation risk by using an impedance analysis method; defining an electric reactance coupling degree index for quantifying the coupling degree of the control loop of the power frequency side to the control loop of the low-frequency side; screening the control link with the largest electric reactance coupling degree in the oscillation risk frequency band, and analyzing the influence of the coupling effect on the electric reactance of the M3C low-frequency side in the frequency band; and adjusting the control bandwidth of the control link to change the coupling degree according to the influence, so as to improve the phase margin of the system and make it meet the stability requirement. The application quantitatively analyzes the coupling effect among the M3C control loops, optimizes the parameters of the control link with the largest electric reactance coupling degree in the oscillation risk frequency band, and improves the oscillation stability of the marine wind power flexible low-frequency sending-out system.

Description

Technical Field

[0001] This invention belongs to the field of offshore wind power grid connection control technology, and in particular, it is a parameter optimization method to improve the oscillation stability of offshore wind power flexible low-frequency transmission system. Background Technology

[0002] Flexible low-frequency (LHF) power transmission systems, as a novel technology for long-distance transmission of offshore wind power, effectively reduce engineering costs while ensuring transmission capacity, showing promising application prospects. However, its core conversion equipment—the modular multilevel matrix converter (M3C)—has a highly complex circuit topology and multi-loop coupled control characteristics, leading to broadband oscillation stability issues in offshore wind power LHF transmission systems. Significant coupling exists between the M3C's power frequency side control loop and low-frequency side control loop. This coupling effect alters the impedance characteristics of the M3C's low-frequency side, thus affecting system stability. Traditional parameter design methods struggle to quantify the impact of this coupling, resulting in a lack of guidance for parameter optimization and an inability to specifically address oscillation risks in specific frequency bands.

[0003] Existing parameter optimization methods for the oscillation stability of offshore wind power flexible low-frequency transmission systems still have shortcomings: on the one hand, there is a lack of effective quantitative indicators to measure the coupling degree between the power frequency side control loop and the low-frequency side control loop, making it difficult to identify the key control links that have the greatest impact on system stability in the oscillation risk frequency band; on the other hand, it is not clear how the coupling effect between control loops affects the reactance characteristics of the M3C low-frequency side and thus the system phase margin, and there is a lack of a systematic method from coupling degree analysis to parameter optimization.

[0004] In summary, existing technologies have not yet developed a parameter optimization design method for broadband oscillation stability of offshore wind power flexible low-frequency transmission systems, making it difficult to meet the requirements for improving the oscillation stability of M3C interconnection systems with low-frequency wind farms. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a parameter optimization method for improving the oscillation stability of offshore wind power flexible low-frequency transmission systems. This method defines a reactance coupling index, quantifies the coupling degree between the high-frequency side control loop and the low-frequency side control loop, selects the control loop with the highest reactance coupling degree within the oscillation risk frequency band, and analyzes the impact of its coupling effect on the M3C low-frequency side reactance within this oscillation risk frequency band. Based on this impact, the control bandwidth of the control loop is adjusted to change its coupling degree, thereby improving the system phase margin and ensuring it meets stability requirements.

[0006] To achieve the aforementioned objectives, the technical solution adopted by this invention is: a parameter optimization method for improving the oscillation stability of a flexible low-frequency transmission system for offshore wind power, comprising: Establish a refined broadband impedance model for the low-frequency side of the M3C; Impedance analysis was used to determine the frequency bands where offshore wind power flexible low-frequency transmission systems are at risk of oscillation. Define an electrical coupling index to quantify the degree of coupling between the high-frequency control loop and the low-frequency control loop; The control element with the highest reactance coupling in the oscillation risk frequency band was selected, and its coupling effect on the low-frequency side reactance of M3C in this oscillation risk frequency band was analyzed. Based on the aforementioned effects, the control bandwidth of the control loop is adjusted to change its coupling degree, thereby improving the system phase margin and enabling it to meet stability requirements.

[0007] Furthermore, the refined broadband impedance model of the low-frequency side of the M3C is established using the harmonic state-space method. This model needs to take into account the harmonic effects of the M3C submodule, multi-loop control coupling, and complex harmonic characteristics of the port.

[0008] Furthermore, the impedance analysis method divides the source and load sides of the M3C low-frequency side and the interconnection system, draws the Bode diagram of the source and load sides impedance, and then uses the impedance Bode diagram to perform stability analysis and determine the frequency bands where the offshore wind power flexible low-frequency transmission system has oscillation risks; the interconnection system refers to the system in which the low-frequency wind farm is connected to the M3C low-frequency side via a low-frequency line.

[0009] Furthermore, the frequency band where oscillation risk exists refers to the frequency band where the intersection of the source-side impedance amplitude and the load-side impedance amplitude is located in the impedance Bode plot.

[0010] Furthermore, the reactive coupling degree Defined as:

[0011] In the formula, Im represents the imaginary part. Z ref Detailed model of the low-frequency side impedance of M3C Z simp This is a simplified model of the M3C with neglected low-frequency side impedance and power-frequency side control loop.

[0012] Furthermore, the reactive coupling degree The absolute value of the value characterizes the degree of influence of the coupling effect of the power frequency side control loop on the low frequency side reactance of the M3C, and its sign reflects the direction of the influence of the coupling effect on the reactance properties of the M3C low frequency side. The larger the value, the greater the coupling effect of the power frequency side control loop; This indicates that the coupling effect of the control loop increases the inductive component of the low-frequency side reactance of the M3C. This indicates that the coupling effect of the control loop reduces the inductive component of the low-frequency side reactance of the M3C or enhances its capacitive component.

[0013] Furthermore, the control loop of the power frequency side control loop includes a DC voltage loop and a power frequency side current inner loop.

[0014] Furthermore, the screening of the control link with the largest reactance coupling in the oscillation risk frequency band includes: calculating the reactance coupling of the control link in the power frequency side control loop under different control bandwidths in the oscillation risk frequency band; comparing the degree of change in reactance coupling caused by the change in control bandwidth of each control link, and determining the control link with the largest change as the control link with the largest reactance coupling in the oscillation risk frequency band.

[0015] Furthermore, by adjusting the control bandwidth of the control loop with the largest reactance coupling, the coupling degree between the power frequency side control loop and the low frequency side control loop is changed; the direction of the change in reactance coupling with the control bandwidth adjustment characterizes the direction of the influence of the coupling effect of the control loop on the low frequency side reactance of the M3C.

[0016] Furthermore, based on the direction of influence, the control bandwidth of the control loop is adjusted so that, within the oscillation risk frequency band, the coupling effect of the power frequency side control loop has a capacitive effect on the low-frequency side reactance of the M3C, thereby improving the system phase margin and meeting the stability requirements.

[0017] Compared with existing technologies, the present invention has the following advantages: By establishing a refined broadband impedance model for the low-frequency side of the M3C, the present invention can overcome the shortcomings of traditional models in terms of frequency band coverage, refined characterization of the low-frequency side, and control coupling considerations, providing a more reliable model foundation for the oscillation mechanism analysis and stability assessment of offshore wind power flexible low-frequency transmission systems. The present invention further defines the reactance coupling degree, transforming the coupling effect between various control loops of the M3C from qualitative analysis to quantitative characterization, making the coupling strength between control loops calculable, comparable, and rankable. For the oscillation risk frequency band, the present invention prioritizes optimizing the parameters of the control loop with the highest reactance coupling degree, avoiding traditional global trial-and-error and blind adjustments.

[0018] Through the above modeling, quantification and optimization, the oscillation of the offshore wind power flexible low-frequency transmission system in the relevant frequency band can be effectively suppressed, and the oscillation stability, operational reliability and transmission capacity of the system under the conditions of offshore wind power output fluctuation, grid condition changes and different operating conditions can be improved. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a flowchart of a parameter optimization method for improving the oscillation stability of a flexible low-frequency transmission system for offshore wind power according to the present invention; Figure 2 This is a topology diagram of the flexible low-frequency transmission system for offshore wind power in a specific embodiment of the present invention; Figure 3 This is a schematic diagram of the circuit structure of the M3C system in a specific embodiment of the present invention; Figure 4 This is a schematic diagram of the power frequency side control loop and phase-locked loop of the M3C system in a specific embodiment of the present invention; Figure 5 This is a schematic diagram of the low-frequency control loop of the M3C system in a specific embodiment of the present invention; Figure 6 This is a schematic diagram of the circulating current suppression control loop of the M3C system in a specific embodiment of the present invention; Figure 7 This is a diagram of the refined broadband impedance model of the M3C low-frequency side in a specific embodiment of the present invention; Figure 8 The impedance Bode diagram of the M3C and low-frequency wind farm interconnection system in a specific embodiment of the present invention is shown. Figure 9 This is a schematic diagram illustrating the reactance coupling of each control link on the M3C power frequency side to the impedance on the M3C low frequency side in a specific embodiment of the present invention. Figure 10 This is a Bode plot of the impedance of the low-frequency side of the M3C and the impedance of the low-frequency wind farm interconnection system under different M3C power frequency side current loop bandwidths in a specific embodiment of the present invention. Figure 11 This is a time-domain waveform diagram of the low-frequency side power of the offshore wind power flexible low-frequency transmission system in a specific embodiment of the present invention. Detailed Implementation

[0021] The present invention will be described in detail below with reference to embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0022] like Figure 1The method for parameter optimization to improve the oscillation stability of a flexible low-frequency transmission system for offshore wind power is shown below, and its steps are as follows: S4. Select the control element with the largest reactance coupling in the oscillation risk frequency band, and analyze the impact of its coupling effect on the low-frequency side reactance of M3C in this frequency band. S5. Based on this influence, adjust the control bandwidth of the control loop to change its coupling degree, thereby improving the system phase margin and making it meet the stability requirements.

[0023] S1. Establish a refined broadband impedance model for the low-frequency side of the M3C.

[0024] Specifically, the refined broadband impedance model of the low-frequency side of the M3C is established using the harmonic state-space method. This model needs to fully consider the harmonic effects of the M3C submodule, multi-loop control coupling, and complex harmonic characteristics of the port.

[0025] S2 uses impedance analysis to determine the frequency bands where the offshore wind power flexible low-frequency transmission system is at risk of oscillation.

[0026] Specifically, the impedance analysis method divides the source and load sides of the M3C low-frequency side and the interconnection system, plots Bode diagrams of the source and load impedances, and then uses the impedance Bode diagrams to perform stability analysis and determine the frequency bands where the offshore wind power flexible low-frequency transmission system has oscillation risks. The interconnection system refers to the system connecting the low-frequency wind farm to the M3C low-frequency side via low-frequency lines.

[0027] The frequency band where oscillation risk exists refers to the frequency band where the amplitudes of the source-side impedance and the load-side impedance intersect in the impedance Bode plot.

[0028] S3 defines the reactance coupling index, which is used to quantify the degree of coupling between the high-frequency side control loop and the low-frequency side control loop.

[0029] Specifically, the reactive coupling degree Defined as:

[0030] In the formula: Im represents the imaginary part, Z ref Detailed model of the low-frequency side impedance of M3C Z simp A simplified model of the M3C with neglected low-frequency side impedance and power frequency side control loop; The reactive coupling The absolute value of the value characterizes the degree of influence of the coupling effect of the power frequency side control loop on the low frequency side reactance of the M3C, and its sign reflects the direction of the influence of the coupling effect on the reactance properties of the M3C low frequency side. The larger the value, the greater the coupling effect of the power frequency side control loop; This indicates that the coupling effect of the control loop increases the inductive component of the low-frequency side reactance of the M3C. This indicates that the coupling effect of the control loop reduces the inductive component of the low-frequency side reactance of the M3C or enhances its capacitive component.

[0031] The control loop of the power frequency side control circuit includes a DC voltage loop and a power frequency side current inner loop.

[0032] S4. Select the control element with the largest reactance coupling in the oscillation risk frequency band, and analyze the impact of its coupling effect on the low-frequency side reactance of M3C in this frequency band.

[0033] Specifically, for the control links of the power frequency side control loop, the reactance coupling degree under different control bandwidths in the oscillation risk frequency band is calculated respectively; the degree of change of reactance coupling degree caused by the change of control bandwidth of each control link is compared, and the control link with the largest change is determined as the control link with the largest reactance coupling degree in the oscillation risk frequency band.

[0034] By adjusting the control bandwidth of the control loop with the largest reactance coupling, the coupling degree between the power frequency side control loop and the low frequency side control loop is changed; the direction of the change in reactance coupling with the control bandwidth adjustment characterizes the direction of the influence of the coupling effect of the control loop on the low frequency side reactance of the M3C.

[0035] S5. Based on the aforementioned influence, adjust the control bandwidth of the control loop to change its coupling degree, thereby improving the system phase margin and ensuring it meets stability requirements.

[0036] Specifically, based on the direction of influence, the control bandwidth of the control loop is adjusted so that within the oscillation risk frequency band, the coupling effect of the power frequency side control loop has a capacitive effect on the low-frequency side reactance of the M3C, thereby improving the system phase margin and meeting the stability requirements.

[0037] This embodiment uses Figure 2 The low-frequency power transmission system shown Figure 3 The circuit structure of the M3C system shown is as follows: Figures 4 to 6 Taking the control structure of the M3C system shown as an example, the method of this invention is applied to optimize the parameters for the oscillation stability of the offshore wind power flexible low-frequency transmission system.

[0038] Figure 2 The low-frequency wind farm operates at a frequency of 20Hz, and its voltage is boosted by a transformer and then connected to the low-frequency side of M3C via a low-frequency transmission line.

[0039] Figure 3 middle C arm It is the capacitor of the bridge arm submodule;L arm It is the bridge arm inductor; R arm It is the bridge arm resistance; R is It is the low-frequency side resistor; , , These are the low-frequency side A, B, and C phase currents, respectively. , , These are the voltages of phases A, B, and C on the low-frequency side; , , These are the currents of phases a, b, and c on the power frequency side; , , These are the voltages of phases a, b, and c on the power frequency side; It is the bridge arm current; It is the voltage across the bridge arm capacitor; It is the bridge arm adjustment system. SM is a submodule.

[0040] According to Kirchhoff's voltage and current laws, taking bridge arm Aa as an example, it satisfies: (1) (2) In the formula: It is the neutral point voltage.

[0041] The voltage loop equations of the nine bridge arms can be superimposed to obtain the following: .

[0042] As can be seen from the main circuit, (3) In the formula: ; ; .

[0043] The small-signal form of equation (1) is: (4) In the formula: The small signal form representing the variable.

[0044] Similar to the small-signal forms of equations (1) and (2), considering the mathematical model of the main circuit composed of 9 bridge arms, it can be written in matrix form as follows: (5) In the formula, ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; .

[0045] It is a zero matrix; It is the identity matrix; Represents a diagonal matrix; and These are the transfer function matrices between the bridge arm current and the bridge arm capacitor voltage and the modulation index, respectively. and These are the transfer function matrices between the low-frequency side voltage and the power frequency side voltage and the modulation index, respectively.

[0046] From equation (3), we get: (6) In the formula: ; ; . This is the transfer function matrix between the bridge arm current and the power frequency side output current.

[0047] From equations (5) and (6), we can obtain: (7) In the formula: This is the multidimensional phase admittance matrix on the low-frequency side.

[0048] Figure 4 middle, The three-phase current on the power frequency side (i.e. , and ), The three-phase voltage on the power frequency side (i.e.) , and ), For the reason The phase angle generated by the phase-locked loop (PLL). for of q Shaft voltage; A PI controller for a phase-locked loop. The initial angular frequency of the PLL; , is the average voltage of the bridge arm capacitors; the variable in the upper right corner Indicates a reference value; and respectively power frequency side d shaft current and q shaft current; and respectively power frequency side d shaft voltage and q Shaft voltage; A PI controller for the DC voltage loop of the power frequency side control loop; A PI controller for the current loop of the power frequency side control loop; and These are the feedforward coefficient and decoupling coefficient of the power frequency side control loop, respectively. The power frequency side angular frequency, This is the equivalent inductance on the power frequency side; This is for power frequency side control delay.

[0049] Figure 5 In the low-frequency side AC voltage control loop, For low-frequency side three-phase current, This refers to the three-phase voltage on the low-frequency side. and Low frequency side d shaft current and q shaft current; and Low frequency side d shaft voltage and q Shaft voltage; A PI controller for the AC voltage loop of the low-frequency side AC voltage control loop; A PI controller for the low-frequency AC voltage control loop current loop; and These are the feedforward coefficient and decoupling coefficient of the low-frequency AC voltage control loop, respectively. It is the low-frequency side angle frequency. This is the equivalent inductance on the low-frequency side; This is for low-frequency side control delay.

[0050] Figure 6 In the circulating suppression control loop, , , and These represent the first output of the circulating current suppression control loop. α shaft current, first one β shaft current, second one α shaft current and the second β shaft current; , , and These are the first outputs of the circulating current suppression control loop. α Shaft voltage, first one β Shaft voltage, second one α shaft voltage and the second β Axis voltage, the variable in the upper right corner Indicates a reference value.

[0051] by Figure 6 Taking the circulating current suppression control loop as an example, the bridge arm current in the abc coordinate system is transformed to... In coordinate system: (8) In the formula: ; , , , , , , , , for Bridge arm current in coordinate system.

[0052] Further circulation can be obtained , , and for: (9) From equations (8) and (9), we can obtain: (10) In the formula: .

[0053] From the circulating suppression control loop, we can obtain: (11) In the formula: , , and It is the circulating voltage; A PI controller for circulating current suppression control loop; The control delay is for the circulating flow suppression control loop.

[0054] By performing small-signal linearization on equation (11) and combining it with equation (10), we obtain: (12) In the formula: .

[0055] The M3C modulation can be synthesized from the output voltages of each control loop as shown in equation (13).

[0056] (13) In the formula: ; ; ; This is a reference value for the bridge arm capacitor voltage; , , , , These represent the outputs of the power frequency side control loop. α Shaft voltage, power frequency side control loop output β Shaft voltage, low-frequency side control loop output α Shaft voltage, low-frequency side control loop output β Shaft voltage, zero-sequence voltage output from the low-frequency side control loop.

[0057] Equation (13) can then be expressed as a small-signal form of column vectors: (14) In the formula: ; ; .

[0058] From equations (3), (12), and (14), the transfer function matrix between the bridge arm current generated by the low-frequency loop and the modulation index can be obtained. for: (15) Similarly, the transfer function matrix between the bridge arm capacitor voltage and the modulation index can be derived based on the power frequency side control loop and the low frequency side control loop. Transfer function matrix between low-frequency side voltage and modulation index Transfer function matrix between power frequency side voltage and modulation index Transfer function matrix between bridge arm current and modulation index Substituting this into (7) to solve for the transfer function matrix between small-signal quantities in the harmonic state-space model, we obtain... During the solution process, for the time-invariant linear elements in the M3C system... Expand it into Diagonal matrix formed by frequency shift : (16) For periodic time-varying components in the M3C system A It needs to be expanded into a Toeplitz matrix. A : (17) In the formula: for A The h Second harmonic components.

[0059] for It took into consideration h The harmonic balance equation for the second harmonic satisfies: (18) In the formula: taking phase a as an example, we have , for The h Second harmonic components; , for The h Second harmonic components.

[0060] In equation (18) Reorganized into To satisfy: (19) In the formula: , .

[0061] By performing symmetrical component transformation on each of the quantities in equation (19), we have: (20) In the formula: This is the low-frequency side multidimensional order admittance matrix; , and The low-frequency side is the first h Secondary positive and negative sequence currents; , and The low-frequency side is the first h Secondary positive and negative sequence voltages.

[0062] From (19) and (20), we can obtain (twenty one) In the formula: ; ; ; .

[0063] The low-frequency side multidimensional sequence impedance matrix for: (twenty two) It satisfies: (twenty three) from Extract the first k The two-dimensional order impedance matrix of the secondary impedance for: (twenty four) The M3C submodule has complex internal dynamic characteristics, and there are a series of harmonic components that are integer multiples of 10 Hz in the bridge arm capacitor voltage. Therefore, the fundamental frequency is selected as 10 Hz, and the truncation order is... h The value is selected as 15. Therefore, the low-frequency side two-dimensional sequence impedance is... The two-dimensional order impedance matrix of the second impedance is: (25) Combined with the two-dimensional order impedance matrix of the interconnected system The positive-sequence impedance and negative-sequence impedance of the low-frequency side of the M3C were obtained as follows: (26) Similarly, the positive-sequence impedance and negative-sequence impedance of the interconnected system can be obtained.

[0064] M3C low-frequency side positive sequence impedance and negative sequence impedance frequency sweep verification, such as... Figure 7 As shown, the refined broadband impedance model of the low-frequency side of the M3C established has good consistency with the frequency sweep results, verifying the accuracy of the established model.

[0065] This embodiment uses Figure 8 Taking the Bode plot of the M3C and low-frequency wind farm interconnection system as an example, specifically, the low-frequency side of the M3C is equivalent to the source-side impedance, and the low-frequency wind farm interconnection system is equivalent to the load-side impedance, and the Bode plot of their positive-sequence impedances is drawn. The positive-sequence impedance of the M3C low-frequency side and the positive-sequence impedance of the low-frequency wind farm interconnection system intersect at 59Hz. At this point, the phase of the positive-sequence impedance of the M3C low-frequency side is 63.0°, and the phase of the positive-sequence impedance of the low-frequency wind farm interconnection system is -121.3°, with a system phase margin of -4.3°, which does not meet the stability requirements. Therefore, there is a risk of oscillation at 59Hz. Analysis shows that this oscillation is caused by the interaction between the inductive characteristics of the M3C low-frequency side and the capacitive negative resistance characteristics of the low-frequency wind farm interconnection system. Therefore, the inductive characteristics of the M3C low-frequency side should be weakened.

[0066] This embodiment uses Figure 9Taking the coupling effect of each control element on the M3C's power frequency side to the low frequency side impedance as an example, specifically, based on the reactance coupling degree... The definition calculates the reactive coupling degree of the M3C's power frequency side current inner loop and DC voltage loop under different control bandwidths. , Figure 9 (a) and Figure 9 (b) shows the reactance coupling-frequency curves of the M3C power frequency side current loop and the M3C power frequency side DC voltage loop, respectively. Figure 9 As shown in (a), with the change in the control bandwidth of the M3C power frequency side current loop, the reactance coupling decreases from 0.12 to -0.10; Figure 9 As shown in (b), with the change in the control bandwidth of the M3C DC voltage loop, the reactance coupling decreases from 0.12 to -0.01. Comparison reveals that the change in the control bandwidth of the power frequency side current loop has the greatest impact on the reactance coupling at 59Hz, indicating that it is the control loop with the highest reactance coupling at this oscillation frequency. When the control bandwidth of the power frequency side current loop increases from 220Hz to 240Hz, the reactance coupling at 59Hz changes from negative to positive, indicating that the coupling effect of the power frequency side control loop will enhance the inductive characteristics of the M3C's low-frequency side impedance. Combined with the... Figure 8 Analysis shows that this will worsen system stability; the control bandwidth of the power frequency side current loop decreases from 220Hz to 200Hz, and the reactance coupling at 59Hz is negative and its absolute value increases, indicating that the coupling effect of the power frequency side control loop will weaken the inductive characteristics of the M3C's low-frequency side impedance. Combined with the analysis based on... Figure 8 Analysis shows that this will help improve system stability.

[0067] This embodiment uses Figure 10 Taking the Bode plots of the M3C low-frequency side impedance and the low-frequency wind farm interconnection system impedance under different M3C power frequency side current loop bandwidths as an example, specifically, according to the adjustment principle described in step S5, the power frequency side current loop control bandwidth is reduced to weaken the inductive characteristics of the M3C low-frequency side impedance. Figure 10 It can be seen that as the control bandwidth of the power frequency side current loop increases from 220Hz to 240Hz, the reactance-inductive component of the M3C's low-frequency side impedance in the oscillation risk frequency band will increase accordingly, and the phase margin will increase from... Reduce to As the control bandwidth of the power frequency side current loop decreases from 220Hz to 200Hz, the reactance-inductive component of the M3C's low-frequency side impedance in the oscillation risk frequency band also decreases, and the phase margin decreases from... Increase to , and based on Figure 9 This is consistent with the analysis.

[0068] This embodiment uses Figure 11Taking the low-frequency side power time-domain waveform of the low-frequency transmission system shown as an example, the accuracy of the analysis method for the oscillation stability of offshore wind power transmitted through the flexible low-frequency transmission system in this invention is verified.

[0069] Specifically, before parameter optimization, the control bandwidth of the power frequency side current loop was 220Hz. At 3 seconds, the output of the low-frequency wind farm was increased from 0.5 pu to 1.0 pu. The power waveform gradually oscillated and diverged. FFT analysis revealed an oscillation frequency of 39Hz, corresponding to an AC frequency of 59Hz. Figure 8 The results show that the oscillation stability analysis of the offshore wind power transmission system via flexible low-frequency transmission is consistent. By optimizing the control bandwidth parameters of the power frequency side current loop according to the method of this invention, reducing it from 220Hz to 200Hz, the power waveform oscillations disappeared, and the system returned to stable operation, verifying the effectiveness of the proposed parameter optimization method.

[0070] The embodiments disclosed herein are merely preferred embodiments of the present invention. These embodiments are selected and specifically described in this specification to better explain the principles and practical applications of the present invention, and are not intended to limit the present invention. Any modifications and variations made by those skilled in the art within the scope of this specification should fall within the protection scope of the present invention.

Claims

1. A parameter optimization method for improving the oscillation stability of a flexible low-frequency transmission system for offshore wind power, characterized in that, include: Establish a refined broadband impedance model for the low-frequency side of the M3C; Impedance analysis was used to determine the frequency bands where offshore wind power flexible low-frequency transmission systems are at risk of oscillation. Define an electrical coupling index to quantify the degree of coupling between the high-frequency control loop and the low-frequency control loop; The control element with the highest reactance coupling in the oscillation risk frequency band was selected, and its coupling effect on the low-frequency side reactance of M3C in this oscillation risk frequency band was analyzed. Based on the aforementioned effects, the control bandwidth of the control loop is adjusted to change its coupling degree, thereby improving the system phase margin and enabling it to meet stability requirements.

2. The parameter optimization method for improving the oscillation stability of a flexible low-frequency transmission system for offshore wind power according to claim 1, characterized in that, The refined broadband impedance model of the low-frequency side of the M3C is established using the harmonic state-space method, taking into account the harmonic effects of the M3C submodules, multi-loop control coupling, and complex harmonic characteristics of the ports.

3. The parameter optimization method for improving the oscillation stability of a flexible low-frequency transmission system for offshore wind power according to claim 1, characterized in that, The impedance analysis method divides the source and load sides of the M3C low-frequency side and the interconnection system, draws the Bode diagram of the source and load sides impedance, and then uses the impedance Bode diagram to perform stability analysis and determine the frequency band with oscillation risk; the interconnection system refers to the system in which the low-frequency wind farm is connected to the low-frequency side of the M3C via a low-frequency line.

4. The parameter optimization method for improving the oscillation stability of a flexible low-frequency transmission system for offshore wind power according to claim 1, characterized in that, The frequency band where oscillation risk exists refers to the frequency band where the intersection of the source-side impedance amplitude and the load-side impedance amplitude is located in the impedance Bode plot.

5. The parameter optimization method for improving the oscillation stability of a flexible low-frequency transmission system for offshore wind power according to claim 1, characterized in that, The reactive coupling Defined as: In the formula, Im represents the imaginary part. Z ref Detailed model of the low-frequency side impedance of M3C Z simp This is a simplified model of the M3C with neglected low-frequency side impedance and power-frequency side control loop.

6. The parameter optimization method for improving the oscillation stability of a flexible low-frequency transmission system for offshore wind power according to claim 1, characterized in that, The reactive coupling The absolute value of the value characterizes the degree of influence of the coupling effect of the power frequency side control loop on the low frequency side reactance of the M3C, and its sign reflects the direction of the influence of the coupling effect on the reactance properties of the M3C low frequency side. The larger the value, the greater the coupling effect of the power frequency side control loop; This indicates that the coupling effect of the control loop increases the inductive component of the low-frequency side reactance of the M3C. This indicates that the coupling effect of the control loop reduces the inductive component of the low-frequency side reactance of the M3C or enhances its capacitive component.

7. The parameter optimization method for improving the oscillation stability of a flexible low-frequency transmission system for offshore wind power according to claim 1, characterized in that, The control loop of the power frequency side control circuit includes a DC voltage loop and a power frequency side current inner loop.

8. The parameter optimization method for improving the oscillation stability of a flexible low-frequency transmission system for offshore wind power according to claim 1, characterized in that, The selection of the control element with the highest reactance coupling in the oscillation risk frequency band includes: calculating the reactance coupling of the control element in the power frequency side control loop under different control bandwidths in the oscillation risk frequency band; comparing the degree of change in reactance coupling caused by the change in control bandwidth of each control element, and determining the control element with the largest change as the control element with the highest reactance coupling in the oscillation risk frequency band.

9. The parameter optimization method for improving the oscillation stability of a flexible low-frequency transmission system for offshore wind power according to claim 1, characterized in that, By adjusting the control bandwidth of the control loop with the largest reactance coupling, the coupling degree between the power frequency side control loop and the low frequency side control loop is changed; the direction of the change in reactance coupling with the control bandwidth adjustment characterizes the direction of the influence of the coupling effect of the control loop on the low frequency side reactance of the M3C.

10. The parameter optimization method for improving the oscillation stability of a flexible low-frequency transmission system for offshore wind power according to claim 1, characterized in that, Based on the direction of influence, the control bandwidth of the control loop is adjusted so that, within the oscillation risk frequency band, the coupling effect of the power frequency side control loop has a capacitive effect on the low-frequency side reactance of the M3C, thereby improving the system phase margin and meeting the stability requirements.

Images