Method for improving stability of deep sea wind power through two-port light converter transmission system

Abstract

The application discloses a stability improvement method of a deep-sea wind power two-port light converter sending-out system, which introduces a voltage feedforward path containing a phase compensator and a low-pass filter into an MMC differential mode control loop in the two-port light converter; the phase characteristics of an oscillation risk frequency band are used to calculate a compensator parameter feasible region and set a filter cutoff frequency, so that targeted phase compensation and impedance reshaping are realized, and the damping of the two-port light converter is effectively enhanced. The method significantly improves the impedance characteristics of the two-port light converter in the oscillation risk frequency band, and provides a reliable technical path for improving the stability of the two-port light converter.

Description

Technical Field

[0001] This invention belongs to the field of offshore wind power DC transmission technology, specifically relating to a method for improving the stability of a deep-sea wind transmission system via a dual-port lightweight converter. Background Technology

[0002] Modular multilevel converters (MMCs) have been widely used in deep-sea wind power high-voltage direct current (HVDC) transmission projects due to their large transmission capacity and excellent harmonic performance. However, with the continuous increase in transmission capacity and distance, the number of sub-modules required by MMCs has increased significantly, leading to a sharp rise in the size and cost of offshore converter platforms, thus restricting their economic viability. To promote the lightweighting of offshore platforms, researchers have proposed an HVDC transmission scheme based on diode rectifier units (DRUs). This topology is compact and can effectively reduce platform construction costs. However, as passive devices, DRUs rely on a stable AC power grid for reliable power transmission, thus making it impossible to directly use conventional grid-connected wind turbines. A grid-based control system must be used to autonomously establish AC voltage. Currently, key technologies such as grid-based wind turbine control, black start, and multi-machine coordination applicable to DRU-HVDC systems are still in the exploratory stage, and mature and reliable engineering solutions are lacking.

[0003] To balance control flexibility and economy, recent research has proposed a two-port lightweight converter topology consisting of a cascaded MMC and DRU. This structure utilizes the DRU to transmit the main power to reduce construction costs, while a small-capacity MMC maintains AC bus voltage stability, allowing wind turbines to continue using mature grid-following control strategies. However, the coupling of the DRU and MMC introduces complex dynamic interaction problems, increasing the difficulty of small-signal stability analysis and making the system more prone to broadband oscillations under offshore wind power fluctuations, posing a potential threat to stable system operation. Therefore, an impedance reshaping method for two-port lightweight converters is urgently needed to improve the system's adaptability to offshore wind power fluctuations and fully realize the engineering benefits of two-port lightweight converters.

[0004] Through research and investigation, it was found that the existing technologies for impedance reshaping methods of dual-port lightweight converters are not perfect. The literature [Analysis of the sub-supersynchronous interactive instability mechanism of wind farm flexible DC interconnection system and its impedance optimization control method. Proceedings of the CSEE, 1-16] adopts active damping control of parallel virtual impedance to optimize the damping characteristics of MMC; the literature [Research on the oscillation mechanism and suppression strategy of wind farm through MMC-HVDC transmission system. Hefei University of Technology, 2022] analyzes the sources of oscillation risk in offshore wind power through flexible DC transmission system in different frequency bands, and adopts virtual admittance controller to improve the phase frequency characteristics of MMC-HVDC system in the oscillation risk frequency band.

[0005] In summary, existing research largely focuses on optimizing the impedance characteristics of MMCs through impedance reshaping to enhance system stability. However, due to significant differences between two-port lightweight converters and MMCs in terms of operating mechanisms, dynamic response, and control structures, the aforementioned impedance reshaping strategies based on a single MMC are limited in their applicability to two-port lightweight converters and cannot be directly applied to effectively improve the overall system stability. Summary of the Invention

[0006] In view of the above, the present invention provides a method for improving the stability of a deep-sea wind transmission system via a dual-port lightweight converter. By introducing a voltage feedforward path containing a phase compensator and a low-pass filter into the MMC differential mode control loop, targeted phase compensation and impedance reshaping are achieved, effectively enhancing the damping of the dual-port lightweight converter and providing a reliable technical path for improving the operational stability of the dual-port lightweight converter.

[0007] A method for improving the stability of a deep-sea wind transmission system via a dual-port lightweight converter includes the following steps:

[0008] (1) Establish the impedance model of the two-port lightweight converter and analyze its damping characteristics to determine the oscillation risk frequency band that needs to be compensated;

[0009] (2) In the differential control loop of MMC, a voltage feedforward path is introduced at the output of the current inner loop. This voltage feedforward path is composed of a low-pass filter and a phase compensator connected in sequence from input to output.

[0010] (3) Based on the impedance phase characteristics of the oscillation risk frequency band, determine the cutoff frequency of the low-pass filter and the feasible region of the time constant of the phase compensator.

[0011] (4) Impedance reshaping was performed using a voltage feedforward path consisting of a phase compensator and a low-pass filter with determined parameters. The impedance characteristic curves of the dual-port lightweight converter before and after reshaping were plotted to verify the impedance reshaping effect.

[0012] Furthermore, the dual-port lightweight converter consists of a DRU connected in parallel to the AC side and an MMC connected in series to the DC side. The DRU is an uncontrolled rectifier whose commutation depends on a stable AC bus voltage. The MMC uses... V / f (Voltage / Frequency) control is used to construct the AC bus voltage amplitude and frequency, and impedance reshaping can only be performed in the MMC control loop.

[0013] Furthermore, in the deep-sea wind transmission system via a dual-port lightweight converter, the system's broadband oscillations originate from the interaction between the weak damping of the dual-port lightweight converter and the negative damping of the wind turbine. Impedance reshaping is necessary to improve the weak damping of the dual-port lightweight converter and mitigate the oscillation risk. The phase compensator improves the overall impedance characteristics of the dual-port lightweight converter by weakening the weak damping characteristics of the MMC, thereby increasing the system's phase margin and enhancing the operational stability of the dual-port lightweight converter under sea wind power fluctuations. The low-pass filter is used to limit the impedance reshaping frequency range and reduce the impact on the impedance characteristics in the mid-to-high frequency range.

[0014] Furthermore, the specific implementation of the improved MMC differential mode control loop in step (2) is as follows:

[0015] First, calculate the current reference value of the MMC using the following expression:

[0016]

[0017] in: and These are the d-axis current reference values ​​and q-axis current reference values ​​for the MMC, respectively. and These are the d-axis voltage reference values ​​and q-axis voltage reference values ​​(given values) of the MMC, respectively. and These are the d-axis and q-axis voltages of the MMC (obtained from the three-phase AC voltages of the MMC via Park transformation). The transfer function for PI (proportional-integral) control. The voltage decoupling coefficient;

[0018] Then, and The inputs to their respective voltage feedforward paths yield d-axis differential mode modulation compensation components. and d-axis differential mode modulation compensation component ;

[0019] Finally, the differential modulation signal of MMC is calculated using the following expression:

[0020]

[0021] in: and These are the d-axis differential modulation signal and the q-axis differential modulation signal of the MMC, respectively. and These are the d-axis and q-axis currents of the MMC (obtained from the three-phase AC currents of the MMC via Park transformation), respectively. This is the current decoupling coefficient.

[0022] Furthermore, the transfer function of the low-pass filter The expression is as follows:

[0023]

[0024] in: ω c The cutoff angular frequency (preferably set to 100Hz) is the cutoff angular frequency. s For the Laplace operator.

[0025] Furthermore, the transfer function of the phase compensator The expression is as follows:

[0026]

[0027] in: T 1 and T 2 is a time constant and satisfies T 1> T 2, s For the Laplace operator.

[0028] Furthermore, the feasible region of the time constant of the phase compensator in step (3) is determined by the following relationship:

[0029]

[0030] in: Z re-Hybrid The impedance of the reshaped two-port lightweight converter, To reshape the impedance of a two-port lightweight converter that ignores weak damping sources before reshaping (set it as the target to be achieved after compensation), angle ( ) indicates the phase angle of the impedance and the time constant. T 1 and T The setting of 2 must satisfy the above relationship.

[0031] A computer device includes a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the aforementioned method for improving the stability of the deep-sea wind transmission system via a dual-port lightweight converter.

[0032] A computer-readable storage medium storing a computer program, which, when executed by a processor, implements the aforementioned method for improving the stability of a deep-sea wind transmission system via a dual-port lightweight converter.

[0033] This invention introduces a voltage feedforward path containing a phase compensator and a low-pass filter into the MMC differential mode control loop of a two-port lightweight converter. Based on the phase characteristics of the oscillation-prone frequency band, the feasible region of the compensator parameters is calculated, and the filter cutoff frequency is set, thereby achieving targeted phase compensation and impedance reshaping, effectively enhancing the damping of the two-port lightweight converter. This method significantly improves the impedance characteristics of the two-port lightweight converter in the oscillation-prone frequency band, providing a reliable technical path for improving the stability of two-port lightweight converters. Attached Figure Description

[0034] Figure 1 A schematic diagram of the topology and control system for sending deep-sea winds out via a dual-port lightweight converter.

[0035] Figure 2 This is a schematic diagram of the MMC differential mode control loop structure based on phase compensation.

[0036] Figure 3 This is a schematic diagram of the feasible region of the time constant of the phase compensator.

[0037] Figure 4 This is a schematic diagram comparing the impedance characteristics of MMC before and after reshaping.

[0038] Figure 5 This is a schematic diagram comparing the impedance characteristics of a two-port lightweight converter before and after remodeling.

[0039] Figure 6 This is a schematic diagram illustrating the improved system stability after impedance reshaping. Detailed Implementation

[0040] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0041] This embodiment provides a method for improving the stability of deep-sea wind delivered via a dual-port lightweight converter, including the following steps:

[0042] (1) Establish the impedance model of the two-port lightweight converter and analyze its damping characteristics to determine the oscillation risk frequency band that needs to be compensated.

[0043] like Figure 1As shown, the electrical energy from the deep-sea wind farm is collected at the offshore substation and then transmitted to the offshore converter platform via AC cables. The dual-port lightweight converter includes a diode rectifier unit connected in parallel on the AC side and a modular multilevel converter connected in series on the DC side. Therefore, it is necessary to establish the small-signal coupling relationship between the AC and DC sides of the diode rectifier and the modular multilevel converter separately, and then derive the impedance of the dual-port lightweight converter. Z Hybrid .

[0044] The small-signal characteristics of a 12-pulse DRU in the frequency domain can be expressed as:

[0045]

[0046] In the formula: f 0 indicates a rated frequency of 50Hz. f p Indicates the positive-sequence perturbation frequency. f n This indicates the negative-sequence perturbation frequency. f i This is expressed as a summation index frequency, which sequentially takes all valid discrete values ​​in the frequency domain; S k express k Phase diode switching function, Δ i dc For the DC current disturbance component, Δ u dc-DRU For the DC voltage disturbance component of the DRU, Δ i ap-DRU With Δ i an-DRU These represent the positive and negative sequence current disturbance components of phase a on the AC side of the DRU, respectively.

[0047] In MMC, an average value model is used, while also considering the small-signal characteristics of the MMC differential / common mode control loop. Taking the upper arm of phase a as an example, the following will be observed:

[0048]

[0049] In the formula: i base , u base and U dc These correspond to the base values ​​of AC current, AC voltage, and DC voltage, respectively; Δ i u and Δ v u These represent the current / voltage disturbance components of the upper bridge arm of phase a, respectively. I u and Vu Indicates its steady-state component; Y Δl This represents the admittance of the bridge arm resistance and inductance; Z Dc This indicates the impedance of the submodule capacitor; T i , T v and T v These correspond to the transfer functions of the current loop, voltage loop, and circulating current suppression, respectively; Δ v g Indicates the AC side voltage disturbance of the MMC; Δ m u The disturbance component of the total modulation signal of the bridge arm. M u Indicates its steady-state component; Δ m v Δ m c These represent the disturbance components of the differential-mode modulated signal and the common-mode modulated signal, respectively; Δ u dc-MMC This represents the DC voltage disturbance component of the MMC.

[0050] In offshore wind power transmission systems via dual-port lightweight converters, the wideband oscillations of the system originate from the interaction between the weak damping of the dual-port lightweight converter and the negative damping of the wind turbine. Impedance reshaping is needed to improve the weak damping of the dual-port lightweight converter in order to avoid the oscillation risk.

[0051] (2) In the differential mode control loop of the MMC, a voltage feedforward path is introduced at the output of the inner current loop, and a phase compensator is connected in series in the feedforward path. H With low-pass filter .

[0052] In a two-port lightweight converter, the DRU is an uncontrolled rectifier device whose commutation depends on a stable AC bus voltage. In this case, the MMC is used. V / f Control is used to establish the AC bus voltage amplitude and frequency; impedance reshaping can only be performed within the MMC control loop. This is to reduce the impact on the original MMC... V / f To mitigate the impact of the control structure, this implementation adds a voltage feedforward path to the output of the inner current loop to compensate for the phase shift caused by the double PI multiplication stage in the control loop. The reshaped MMC differential control loop is as follows: Figure 2 As shown.

[0053] The voltage feedforward path consists of a phase compensator. H With low-pass filter Composition, phase compensator HBy weakening the weak damping characteristics of the MMC, the overall impedance characteristics of the DRU-MMC dual-port lightweight converter are improved, thereby increasing the system phase margin and enhancing the operational stability of the dual-port lightweight converter under offshore wind power fluctuations; low-pass filter Used to limit the impedance reshaping frequency range and reduce the impact on impedance characteristics in the mid-to-high frequency range.

[0054] Phase compensator H The expression can be defined as:

[0055]

[0056] In the formula: T 1 and T 2 represents the time constant, which must satisfy... T 1> T 2.

[0057] low-pass filter The expression is:

[0058]

[0059] In the formula: ω c This is the cutoff angular frequency.

[0060] Considering that a low-pass filter will introduce additional phase shift, improper phase frequency characteristics of the filter may cause problems for the compensator. H Reversing inductive and capacitive properties actually worsens system stability; therefore, a first-order low-pass filter with a gradual phase angle change is chosen here.

[0061] (3) Determine the low-pass filter based on the impedance-phase characteristics of the target oscillation frequency band. Find the cutoff frequency and calculate the phase compensator. H The feasible region of the time constant.

[0062] If compensator H time constant T 1 and T Improper configuration can lead to two problems: first, insufficient phase compensation makes it difficult to effectively improve the weak damping characteristics of the two-port lightweight converter impedance in the original frequency band; second, over-compensation of the phase may introduce new weak damping regions in other frequency bands. Either way, it will trigger system oscillation risks. Therefore, a constraint is set: within the target oscillation neighborhood frequency band, the deviation between the compensated impedance phase and the reference should not exceed 15°. (Phase compensator...) H The feasible region of the time constant can be calculated by the following expression:

[0063]

[0064] in: Z re-Hybrid The impedance of the reshaped two-port lightweight converter, To reshape the impedance of a two-port lightweight converter that ignores weak damping before reshaping (setting it as the target to be achieved after compensation); the time constant is plotted based on this criterion. T 1 and T The feasible region of 2, such as Figure 3 As shown, by selecting a time constant within the feasible region, the phase compensation effect is sufficient and the overcompensation degree is low, which can suppress the subsynchronous / supersynchronous oscillation of the system.

[0065] (4) Using a phase compensator with determined parameters H and low-pass filter Impedance reshaping was performed using a voltage feedforward path, and the impedance characteristics of the two-port lightweight converter before and after reshaping were plotted to verify the impedance reshaping effect.

[0066] Figure 4 and Figure 5 The figures show the MMC impedance and the impedance of the dual-port lightweight converter before and after reshaping. It can be seen that the weak damping characteristics of the dual-port lightweight converter will be significantly improved after adopting the impedance reshaping strategy based on phase compensation.

[0067] To further verify the effectiveness of the impedance reshaping strategy, we added a phase compensation stage in MATLAB / Simulink and simulated the system operation under offshore wind power fluctuations. The results are as follows: Figure 6 As shown: After the phase compensation circuit is enabled, the DC side current... i dc The oscillation components were quickly suppressed, and the system returned to stability; the power of the offshore wind turbines was further increased. P WF The system remained stable even at power levels of 0.7 pu and 1.0 pu, demonstrating its ability to operate stably across the entire power range.

[0068] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A method for improving the stability of a deep-sea wind transmission system via a dual-port lightweight converter, characterized in that, Includes the following steps: (1) Establish the impedance model of the two-port lightweight converter and analyze its damping characteristics to determine the oscillation risk frequency band that needs to be compensated; (2) In the differential control loop of MMC, a voltage feedforward path is introduced at the output of the current inner loop. This voltage feedforward path is composed of a low-pass filter and a phase compensator connected in sequence from input to output. (3) Based on the impedance phase characteristics of the oscillation risk frequency band, determine the cutoff frequency of the low-pass filter and the feasible region of the time constant of the phase compensator. (4) Impedance reshaping was performed using a voltage feedforward path consisting of a phase compensator and a low-pass filter with determined parameters. The impedance characteristic curves of the dual-port lightweight converter before and after reshaping were plotted to verify the impedance reshaping effect. In a deep-sea wind transmission system using a dual-port lightweight converter, the system's broadband oscillations originate from the interaction between the weak damping of the dual-port lightweight converter and the negative damping of the wind turbine. Impedance reshaping is necessary to improve the weak damping of the dual-port lightweight converter and mitigate the oscillation risk. The phase compensator improves the overall impedance characteristics of the dual-port lightweight converter by weakening the weak damping characteristics of the MMC, thereby increasing the system's phase margin and enhancing the operational stability of the dual-port lightweight converter under offshore wind power fluctuations. The low-pass filter is used to limit the impedance reshaping frequency range and reduce the impact on the impedance characteristics in the mid-to-high frequency range. The dual-port lightweight converter consists of a DRU connected in parallel on the AC side and a MMC connected in series on the DC side. The DRU is an uncontrolled rectifier whose commutation depends on a stable AC bus voltage. The MMC uses... V / f Control is used to construct the AC bus voltage amplitude and frequency; impedance reshaping can only be performed within the MMC control loop.

2. The method for improving the stability of the deep-sea wind transmission system via a dual-port lightweight converter as described in claim 1, characterized in that, The specific implementation of the improved MMC differential mode control loop in step (2) is as follows: First, calculate the current reference value of the MMC using the following expression: in: and These are the d-axis current reference values ​​and q-axis current reference values ​​for the MMC, respectively. and These are the d-axis voltage reference values ​​and q-axis voltage reference values ​​for the MMC, respectively. and These are the d-axis and q-axis voltages of the MMC, respectively. The transfer function for PI control. The voltage decoupling coefficient; Then, and The inputs to their respective voltage feedforward paths yield d-axis differential mode modulation compensation components. and d-axis differential mode modulation compensation component ; Finally, the differential modulation signal of MMC is calculated using the following expression: in: and These are the d-axis differential modulation signal and the q-axis differential modulation signal of the MMC, respectively. and These are the d-axis current and q-axis current of the MMC, respectively. This is the current decoupling coefficient.

3. The method for improving the stability of the deep-sea wind transmission system via a dual-port lightweight converter as described in claim 1, characterized in that, The transfer function of the low-pass filter The expression is as follows: in: ω c The cutoff angular frequency, s For the Laplace operator.

4. The method for improving the stability of the deep-sea wind transmission system via a dual-port lightweight converter as described in claim 1, characterized in that, The transfer function of the phase compensator The expression is as follows: in: T 1 and T 2 is a time constant and satisfies T 1> T 2, s For the Laplace operator.

5. The method for improving the stability of the deep-sea wind transmission system via a dual-port lightweight converter according to claim 4, characterized in that, The feasible region of the time constant of the phase compensator in step (3) is determined by the following relationship: in: Z re-Hybrid The impedance of the reshaped two-port lightweight converter, To reshape the impedance of a two-port lightweight converter that ignores the source of weak damping before reshaping, angle ( ) indicates the phase angle of the impedance and the time constant. T 1 and T The setting of 2 must satisfy the above relationship.

6. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that: The processor is used to execute the computer program to implement the stability improvement method for the deep-sea wind transmission system via a dual-port lightweight converter as described in any one of claims 1 to 5.

7. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by the processor, it implements the stability improvement method for the deep-sea wind transmission system via a dual-port lightweight converter as described in any one of claims 1 to 5.

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