An active support method for a sea wind low-frequency sending-out system based on reverse frequency transmission

Abstract

This invention discloses an active support method for offshore wind low-frequency power transmission systems based on reverse frequency transfer. It first reveals the transient power mismatch mechanism between the decrease in offshore wind farm output power caused by phase-locked loop (PLL) delay and the increase in M3C output power demand. Then, it reconstructs the control strategies of the M3C and the grid-side converter of the wind turbine. By configuring a negative frequency droop coefficient, the polarity of the reactive current induced by the PLL delay is reversed, thereby completely eliminating the power mismatch between the M3C and the offshore wind farm and improving the system's transient stability. This invention not only significantly suppresses power overshoot during active support and prevents over-discharge of the M3C submodule capacitors, but also maintains the overall synthetic inertia support capability for the onshore power grid without increasing the system's energy storage hardware. This invention is applicable to various deep-sea wind power transmission low-frequency AC converter systems via M3C, and can significantly improve system transient stability and power supply reliability.

Description

Technical Field

[0001] This invention belongs to the field of power system stability analysis and advanced integrated control technology, specifically relating to an active support method for a low-frequency offshore wind transmission system based on reverse frequency transmission. Background Technology

[0002] The large-scale cluster development and grid connection of deep-sea wind power has become a key physical path for the evolution of modern new power systems. For large-scale offshore wind farms located hundreds of kilometers from the coastline, traditional power frequency AC transmission technology faces an insurmountable physical transmission distance bottleneck because the huge capacitive charging current of submarine cables easily fills the thermal capacity of the cables. While flexible high-voltage direct current (HVDC) transmission technology has no distance limitations, its offshore converter platforms require a large number of high-voltage insulation facilities and huge DC bus capacitors, resulting in large offshore platforms, extremely high weight, and high construction and maintenance costs.

[0003] Against this backdrop, Flexible Low-Frequency AC Transmission (LFAC) has emerged as a compromise yet highly efficient disruptive alternative. By reducing the transmission frequency, it proportionally mitigates the distributed capacitance charging effect of submarine cables, effectively extending the physical limit of AC transmission distance. Simultaneously, it eliminates the need for massive offshore DC converter station construction, demonstrating immense engineering application potential. In the underlying architecture of the LFAC transmission system, the Modular Multilevel Matrix Converter (M3C) plays a crucial role in AC-AC frequency conversion. The M3C topology consists of nine high-frequency matrix arms, with numerous cascaded H-bridge sub-modules within each arm, enabling flexible conversion of multi-phase AC power at different frequencies directly without the need for an intermediate DC bus. The M3C features complete modularity, ease of expansion to high voltage and large capacity, and superior harmonic characteristics. Therefore, it is widely used not only in the field of traction power electronic transformers for high-speed trains but is also widely recognized as highly suitable for medium / high voltage, high-power offshore wind power integration scenarios.

[0004] However, when deep-sea wind power clusters transmit power through the M3C flexible low-frequency system, their cross-coupling operating environment presents two extremely severe core structural contradictions, which pose the following disruptive challenges to traditional control theory: Firstly, there is a contradiction between the weakening inertia of the onshore receiving-end main power grid and the rigid demand for active support. With the accelerated retirement of traditional large-capacity synchronous rotating generators, the onshore main power grid is gradually evolving towards characteristics of low inertia and low short-circuit ratio. Under these physical conditions, once a large-capacity load surge or generator tripping occurs in the onshore power grid, causing a rapid drop in system frequency, the main power grid objectively and urgently requires offshore wind farms to utilize the rotor kinetic energy of generator units or internal energy storage systems for rapid response, providing emergency active power support and virtual synthetic inertia response to the onshore grid. The traditional approach is to configure high-cost lithium-ion battery energy storage units at the wind farm side to optimize high-frequency and low-frequency output power fluctuations and simulate inertia. However, in offshore low-frequency transmission systems that pursue a minimalist architecture, this method of adding massive physical energy storage not only violates the initial intention of lightweight design but also brings unbearable maintenance costs.

[0005] Secondly, there is a contradiction between the extremely limited energy buffering capacity of the M3C converter and the long-distance weak power grid environment. Due to the stringent "lightweight and compact" engineering design constraints of offshore converter platforms, the volume of the thin-film capacitors configured in each H-bridge submodule inside the M3C is extremely compressed. Therefore, the "energy-to-power ratio" of the entire M3C converter is much lower than that of traditional MMC (Modular Multilevel Converter). This means that during AC-AC frequency and power conversion, the M3C is like a "shallow pool with extremely poor water storage capacity," unable to absorb or buffer the huge power difference between the AC and DC ends during transient processes. To make matters worse, the connection between offshore wind turbines and M3C converter stations is often a submarine cable tens to hundreds of kilometers long, which results in the wind turbine terminals operating in a typical high-impedance "low-frequency weak power grid" environment. In order to maintain steady-state operation in a high-impedance weak grid with complex coupling and resonance characteristics involving multiple inputs and multiple outputs, and to avoid wide-band control instability and negative damping high-frequency oscillations (such as the risk of instability caused by admittance cutoff), the bandwidth parameter of the phase-locked loop (PLL) of the grid-connected wind turbine must be designed to be extremely narrow. This compromise directly results in an extremely slow phase angle tracking response speed, often requiring a stabilization adjustment time of more than 100 milliseconds. Summary of the Invention

[0006] In view of the profound technical pain points that have long existed in the background technology and have restricted the reliability of deep-sea wind power transmission systems, the present invention provides an active support method for offshore low-frequency wind power transmission systems based on reverse frequency transfer. This method can maintain the overall synthetic inertia support capability for the onshore power grid without increasing the system's energy storage hardware, thereby significantly improving the system's transient stability and power supply reliability.

[0007] An active support method for a low-frequency wind transmission system based on reverse frequency transfer includes the following steps: (1) Based on the topology and energy transmission physical characteristics of the offshore wind low-frequency transmission system, the transient power mismatch mechanism between the decrease in offshore wind farm output power and the increase in M3C output power demand caused by the delay of phase-locked loop is revealed through theoretical analysis; (2) Based on the transient power mismatch mechanism, the control strategy of M3C and wind turbine grid-side converter is reconstructed. By configuring a negative frequency droop coefficient, the polarity of reactive current induced by phase-locked loop delay is reversed to completely eliminate the power mismatch between M3C and offshore wind farm and improve the transient stability of the system.

[0008] Furthermore, the offshore wind low-frequency transmission system includes an offshore wind farm, a submarine AC cable, a low-frequency converter station based on M3C, and an onshore power grid. The wind turbines in the offshore wind farm are connected to the AC bus via grid-side converters. One end of the submarine AC cable is connected to the AC bus, and the other end is connected to the low-frequency side of the M3C. The power frequency side of the M3C is connected to the onshore power grid. The M3C adopts a V / f (voltage / frequency ratio) control strategy, and the grid-side converter adopts a grid-following control strategy.

[0009] Further, the specific implementation of step (1) is as follows: First, based on the topology and energy transmission physical characteristics of the system, a frequency coupling control and large-signal model between the offshore low-frequency power grid and the onshore power frequency power grid is constructed to quantify the impact of the frequency drop of the onshore power frequency power grid on the transient support power of the offshore wind farm; then, based on the operating characteristics of the offshore wind farm in the low-frequency weak power grid environment, a grid-following wind turbine model including the dynamic response of the low-bandwidth phase-locked loop is constructed, and the phase error closed-loop transfer function of the phase-locked loop and the dynamic reactive current injection equation of the stator side caused by the sudden frequency disturbance are derived; then, based on the impedance distribution characteristics of the long-distance submarine AC cable and the dynamic reactive current injection equation of the stator side, a dynamic correlation equation between the voltage drop of the wind turbine end side and the transmission power of the offshore wind farm-M3C is constructed, thereby revealing the transient power mismatch mechanism between the decrease in the output power of the offshore wind farm and the increase in the output power demand of the M3C caused by the delay of the phase-locked loop.

[0010] Furthermore, the reconstructed M3C control strategy in step (2) is as follows: First, the PCC (point of common coupling) voltage on the low-frequency side of the M3C is input to the SRF-PLL (synchronous rotating coordinate system phase-locked loop) on the power frequency side to generate the power frequency side angular frequency. oh 1. oh The frequency deviation between 1 and the power frequency is used to obtain the low-frequency side angular frequency control command through droop control. oh 2, and then to oh 2. Integrate to obtain the low-frequency side power angle. i 2; then the low-frequency side voltage reference value The low-frequency dq-axis voltage control command is obtained through dual closed-loop control of voltage and current, and finally based on the power angle... i 2. The coordinate transformation of the low-frequency side dq axis voltage control command is performed to obtain the low-frequency side three-phase voltage control command, and then the V / f control of M3C is realized by modulation.

[0011] Furthermore, the droop control expression in the M3C control strategy is as follows: in: The rated angular frequency of the offshore low-frequency power grid. k dr1 The frequency droop coefficient and k dr1 <0.

[0012] Furthermore, the reconstructed wind turbine grid-side converter control strategy in step (2) is as follows: First, the PCC voltage at the wind turbine end side is input to the low-frequency side SRF-PLL to generate the wind turbine end-side angular frequency. oh 3. oh 3. The frequency deviation between the wind turbine speed and the rated angular frequency of the offshore low-frequency power grid is used to obtain the reference value of the wind turbine speed through droop control. At the same time, oh 3. Integrate to obtain the end-side power angle of the wind turbine. i 3; Output active power of wind turbine units P wf The fan speed reference value is obtained through MPPT (Maximum Power Point Tracking) control. Under normal system conditions, the following is adopted: As a reference value for fan speed In the case of frequency drop in land power grid, adopt As a reference value for fan speed Then make The stator current control command is obtained through PI (Proportional-Integral) control, and the dq-axis stator voltage control command is obtained after current control of the stator current control command. Finally, the power angle is used to determine the voltage control command. i 3. The coordinate transformation of the dq axis stator voltage control command is used to obtain the three-phase stator voltage control command, and then the grid-side converter is controlled by modulation to achieve grid-following control.

[0013] Furthermore, the droop control expression in the wind turbine grid-side converter control strategy is as follows: in: The rated angular frequency of the offshore low-frequency power grid. k dr2 The frequency droop coefficient and kdr2 <0.

[0014] 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 above-described active support method for a low-frequency sea breeze transmission system based on reverse frequency transfer.

[0015] A computer-readable storage medium storing a computer program, which, when executed by a processor, implements the above-described active support method for a low-frequency sea breeze transmission system based on reverse frequency transmission.

[0016] The core purpose and breakthrough of this invention lies in completely abandoning the empirical inertia of "a decrease in main grid frequency inevitably leads to a decrease in microgrid frequency," and deeply exploring the evolution law of transient reactive power and the physical mechanism of voltage support. By reconstructing the frequency command transmission network on both sides of the M3C system, the originally harmful dynamic delay of the weak grid phase-locked loop is cleverly transformed into a voltage boosting source with capacitor series compensation effect, thereby eradicating the transient power mismatch on both sides of the AC-AC conversion from the source. This invention greatly improves the transient stability margin of the lightweight M3C system while absolutely ensuring the lossless active support capacity of the onshore power grid.

[0017] First, this invention deeply analyzes and establishes a generalized large-signal transient energy evolution model for offshore wind power transmitted to the onshore power grid via the M3C converter. In the traditional control architecture, frequency drop events in the onshore power grid are captured by the M3C power frequency side and mapped to frequency drop commands on the low-frequency side (e.g., the 20Hz side) using constant droop control, thereby prompting the wind farm to generate more active power.

[0018] Secondly, this invention, for the first time, precisely quantifies the adverse effects of second-order phase-locked loop (PLL) delay under low-frequency weak power grid conditions at the mathematical model and electromechanical transient physics levels. Due to the high impedance characteristics of submarine cables, the wind farm controller is forced to use a PLL with an extremely low natural resonant frequency. When the system receives a step-like or ramp-like frequency drop signal, the second-order damped PLL system inevitably generates a dynamic phase deviation with a considerable peak value that lasts for hundreds of milliseconds. i err ( t This phase error, acting as an angle deflection factor, directly distorts the inverter's synchronous rotating current vector, generating a large amount of lagging inductive reactive current injection under traditional control strategies. This causes extremely severe voltage drop (ΔΔ) at the end of the high-inductive reactance submarine line. V wf (Decrease), and based on the constant power limiting law, the cascading effect leads to a decrease in the actual active power output of the generator sets. P wf A sharp drop occurred, contrary to expectations of support.

[0019] To completely overcome this "Achilles' heel" hindering low-frequency power transmission in deep-sea areas, this invention creatively proposes a multi-degree-of-freedom control architecture for reverse frequency transmission. Breaking away from traditional rigid thinking, it artificially forces the negative-to-frequency droop coefficient to be configured in the valve-level control and frequency down-transmission loop of the M3C. k dr1 <0), meaning that when the onshore main power grid faces a shortage of active power leading to a frequency drop (urgently requiring inertial active power support), the M3C counterintuitively chooses to raise rather than lower the interaction frequency command of the offshore low-frequency network. Through this seemingly insignificant yet far-reaching polarity reversal, the phase-locked loop, forced to track this abnormal rising frequency ramp, experiences a mirror reversal of the polarity of the phase error calculated in the complex plane. This means that the highly destructive inductive reactive current originally forced into the wind turbine and grid converter is physically reversed into a highly constructive capacitive reactive current (i.e., phase lead current). When this capacitive lead current flows through a long-distance submarine cable with strong inductive reactance, a remarkable impedance cancellation effect occurs, not only preventing a voltage drop at the end of the line but also generating a voltage rise effect (Δ) similar to the series capacitor compensation of flexible AC transmission equipment in a power system due to the reactive power reversal. V wf >0). Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the active support method for the low-frequency sea breeze transmission system based on reverse frequency transmission according to the present invention.

[0021] Figure 2 This is a schematic diagram of the detailed electrical topology inside a low-frequency converter station based on M3C.

[0022] Figure 3 This is a block diagram of the composite logic structure of the M3C low-frequency side and the core control system of the offshore wind turbine group in an embodiment of the present invention.

[0023] Figure 4 This is a schematic diagram of the dynamic state causal process of the malignant evolution mechanism of transient power mismatch in M3C-OWF under traditional control.

[0024] Figure 5 To follow the traditional positive feedforward droop factor ( k dr1 >0) A schematic diagram comparing the measured transient response waveforms of the actual power injected from the M3C on land to the main grid and the power drawn from the wind power at sea during the moment of a sudden fault in the main grid frequency drop. In the figure, (a) corresponds to the active power on land and (b) corresponds to the active power at sea.

[0025] Figure 6This is a schematic diagram of the topology of the active support global control strategy based on reverse frequency transmission of the present invention.

[0026] Figure 7 To improve the control method of this invention (enable) k dr1 The diagram shows the improved transient response waveform of active power transmission at the core AC / DC nodes of the system after the <0 mechanism. In the diagram, (a) corresponds to the active power on land and (b) corresponds to the active power at sea. Detailed Implementation

[0027] 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.

[0028] like Figure 1 As shown, this embodiment provides an active support method for a low-frequency sea wind transmission system based on reverse frequency transfer. The specific implementation process includes the following steps: Step S1: Based on the topology and energy transmission physical characteristics of the M3C low-frequency AC transmission system, construct a frequency coupling control and large-signal model between the offshore low-frequency power grid and the onshore power frequency power grid to quantify the impact of onshore power grid frequency drops on the transient support power of offshore wind farms.

[0029] S11: Determine the topology of the M3C-LFAC system.

[0030] The active power output of an offshore wind farm (OWF) via grid-connected control, the input power from the M3C low-frequency side (offshore side), and the output power injected into the main grid from the M3C power frequency side (onshore side) are respectively represented by... P wf , P offshore and P onshore express.

[0031] Define at each critical common coupling point (PCC) of the system. U i ∠ i i and oh i (in i =1,2,3) represents the voltage phasor and angular frequency. i =1 corresponds to the PCC on the onshore power frequency (PF) grid side. i =2 corresponds to the PCC on the low-frequency (LP) side of the M3C. i =3 corresponds to the PCC on the end side of the offshore wind turbine.

[0032] S12: Determine the control method for M3C.

[0033] like Figure 2 As shown, the M3C converter consists of nine matrix arms, each employing an H-bridge structure with multiple sub-modules (SMs) connected in series. The M3C's valve-level control generates voltage reference values ​​for each arm by decoupling and integrating onshore power frequency voltage control commands with offshore low-frequency voltage control commands. v ij Its control law expression is: In the formula: v i,ref and v j,ref These represent the three-phase voltage differential mode control reference signals generated after αβ transformation and dq transformation, respectively, for onshore power frequency grids and offshore low-frequency grids.

[0034] Under traditional active support control based on droop characteristics, when a frequency drop occurs in the onshore power grid due to active power loss, the amplitude of this frequency deviation Δ oh fault The angular frequency control command is sensed via a 50Hz synchronous rotating coordinate system phase-locked loop (SRF-PLL) on the M3C's power frequency side and linearly superimposed onto the offshore low-frequency AC network using a droop control strategy. The angular frequency control command output from the M3C's low-frequency side... oh 2 is: In the formula: The reference value for the rated angular frequency of the low-frequency power grid is 20Hz. k dr1 The active-frequency droop factor for forward frequency transmission. u ( t ) is the unit step function that characterizes the sudden onset of a fault.

[0035] Step S2: Based on the operating characteristics of offshore wind farms in low-frequency weak grid environments, a grid-connected (GFL) wind turbine model with low-bandwidth phase-locked loop dynamic response is constructed. The closed-loop transfer function of the phase-locked loop phase error and the resulting dynamic reactive current injection equation on the stator side are derived when the system is subjected to sudden frequency disturbances.

[0036] S21: Calculate the phase angle integral perturbation of the grid voltage at the low-frequency PCC point based on the frequency drop over the seabed and on land.

[0037] like Figure 3 As shown, the M3C employs a low-frequency V / f control strategy: firstly, the PCC voltage on the low-frequency side of the M3C is... U 1. Input to the 50Hz power frequency side SRF-PLL to generate the power frequency side angular frequency. oh 1. oh The frequency deviation between 1 and the power frequency is used to obtain the low-frequency side angular frequency control command through droop control. oh 2, and then to oh 2. Integrate to obtain the low-frequency side power angle. i 2; then the low-frequency side voltage reference value The low-frequency dq-axis voltage control command is obtained through dual closed-loop control of voltage and current, and finally based on the power angle... i 2. Perform coordinate transformation on the low-frequency side dq-axis voltage control command to obtain the low-frequency side three-phase voltage control command. V j,ref This allows for the control of the M3C through modulation.

[0038] The grid-side converter of wind turbine generators (WTGs) adopts the GFL control strategy: firstly, the PCC voltage at the wind turbine end side is... U 3. Input to the 20Hz low-frequency side SRF-PLL to generate the end-side angular frequency of the wind turbine. oh 3. oh 3. The frequency deviation between the wind turbine speed and the rated angular frequency of the offshore low-frequency power grid is used to obtain the reference value of the wind turbine speed through droop control. At the same time, oh 3. Integrate to obtain the end-side power angle of the wind turbine. i 3; Output active power of wind turbine units P wf The fan speed reference value is obtained through MPPT control. Under normal system conditions, the following is adopted: As a reference value for fan speed In the case of frequency drop in land power grid, adopt As a reference value for fan speed Then make The stator current control command is obtained through PI control, and the stator voltage control command for the dq axis is obtained after current control of the stator current control command. Finally, the power angle is used to determine the stator voltage control command. i 3. Perform coordinate transformation on the dq-axis stator voltage control commands to obtain the three-phase stator voltage control commands. V wf,ref This allows for the control of the grid-side converter through modulation.

[0039] Given the parasitic inductance effect of long-distance submarine cables (in terms of equivalent inductance) L cable (Characteristics) Offshore wind turbines operate in a typical low-frequency, weak grid environment. To suppress high-frequency resonance and ensure stable system operation under extremely low short-circuit ratio (SCR) conditions, the natural resonant frequency of the low-frequency phase-locked loop configured with the wind turbine and grid control is... oh n2 It was tuned to a frequency significantly lower than the natural resonant frequency of the power frequency phase-locked loop of the onshore power grid. oh n1Define the disturbance angular frequency signal at the low-frequency PCC point as Δ. oh 2= k dr1 ·Δ oh fault · u ( t Since frequency is the derivative of phase, the low-frequency PCC point voltage phase angle integral perturbation caused by this disturbance... i The time-domain expression for 2 is: S22: Derive the phase error closed-loop transfer function of the phase-locked loop when the system is subjected to sudden frequency disturbances and the resulting dynamic reactive current injection equation on the stator side.

[0040] The slope-shaped input phase perturbation i in ( t )=Δ oh 2. t Performing the Laplace transform yields the complex frequency domain representation Θ. in ( s )=Δ oh 2 / s 2 Define the output tracking phase of the phase-locked loop (PLL) for wind turbines in a weak power grid environment. i pll,3 Phase with actual power grid evolution i The dynamic deviation between 3 is the phase error. i err The closed-loop transfer function of this error system E ( s This can be expressed as a standard second-order oscillatory system: In the formula: g 2 and oh n2 These represent the damping ratio and natural resonant frequency of the 20Hz phase-locked loop on the low-frequency side of the wind turbine generator.

[0041] Perturb the phase input Θ in ( s ) and transfer function E ( s Convolution, and apply the phase error signal Θ err ( s )=Θ in ( s )· E ( s Performing the inverse Laplace transform yields the analytical equation for the transient phase error in the time domain, which characterizes the degree of phase lag: This phase error i err ( t The presence of this component directly caused a momentary shift in the orientation of the synchronous rotation coordinate system of the grid-side converter and the grid-type control. Based on the principle of vector projection, this orientation deviation caused the portion of the active current originally planned to be fully output to be incorrectly projected onto the reactive axis, inducing an unexpected reactive current injection. I q,3 According to the conventions for generator set current, its expression is: Step S3: Based on the impedance distribution characteristics of long-distance submarine cables and the above reactive current injection model, construct the dynamic correlation equation between the voltage drop at the wind turbine terminals and the power transmitted between the offshore wind farm and the M3C, and reveal the transient power mismatch mechanism between the decrease in wind farm output power caused by the phase-locked loop delay and the increase in M3C output power demand.

[0042] S31: Calculate and derive the effect of low-frequency system frequency transformation on OWF voltage.

[0043] Based on the small-angle approximation theory of AC power flow in power systems, this is used to address the injection of unexpected reactive current. I q,3 High-inductive flux forced through submarine cable X cable The resulting longitudinal voltage drop Δ V wf Expressed as: S32: Calculate and derive the impact of voltage drop on the output power of the grid-connected fan.

[0044] During the transient period of frequency drops in the onshore power grid, the inner loop current of the converter in grid-connected wind turbines operating in constant power output mode is limited by hardware overcurrent protection and cannot increase indefinitely. Therefore, the transient change in the terminal voltage will directly determine the actual active power output variation Δ of the wind turbine. P wf It satisfies the following instantaneous power dynamic constraint equation: In the traditional strategy, a positive feedforward droop coefficient is used ( k dr1 >0), the land frequency drops (Δ oh fault <0) triggers negative phase angle disturbance and positive phase-locked loop tracking delay, inducing I q,3 It exhibits hysteretic inductive behavior; this inductive reactive current flowing through the submarine cable causes a severe voltage drop at the generator terminals (Δ). Vwf <0), which in turn forces the output power of offshore wind farms to drop sharply (Δ P wf <0). Simultaneously, as a hub for active grid support, the M3C must instantly increase its power frequency active power transmitted to the onshore grid (i.e., P onshore The surge in offshore output, coupled with the sharp increase in onshore demand, creates an extreme M3C-OWF transient power mismatch effect.

[0045] Figure 4 The dynamic state causal flow of the malignant evolution mechanism of transient power mismatch in M3C-OWF under traditional control is demonstrated. However, traditional control methods have shortcomings in transient stability. Figure 5 As can be seen, after a drop in power frequency, the traditional control method exhibits a larger transient overshoot and a longer steady-state time.

[0046] Step S4: Based on the above power mismatch mechanism, the frequency transmission channel of the M3C low-frequency converter station is reconstructed, and an active support control strategy based on reverse frequency transmission is proposed. By configuring a negative frequency droop coefficient, the polarity of the reactive current induced by the phase-locked loop delay is reversed to completely eliminate the power mismatch between M3C and offshore wind farm and improve the transient stability of the system.

[0047] S41: Propose an active support control strategy based on reverse frequency transmission.

[0048] Figure 6 This is a topology structure based on an active support global control strategy using reverse frequency transmission. This control structure breaks away from the traditional in-direction frequency mapping logic, reconstructs the frequency transmission droop control loop of the M3C low-frequency converter station, and sets a system-level frequency-voltage droop control coefficient with negative polarity (i.e., k dr1 <0). When the above negative droop coefficient is used, for frequency drop events in the onshore power grid, the M3C low-frequency converter station no longer sends a frequency-down signal to the offshore low-frequency power grid, but rather a reversed frequency-up signal (Δ). oh 2 > 0).

[0049] S42: Propose an active support control strategy based on reverse frequency transmission.

[0050] Figure 7 This invention demonstrates an improved control method (enabled). k dr1<0) After the mechanism, the transient response waveform of active power transmission at the core AC / DC node of the system is improved. This reverse step signal causes the polarity reversal of the transient phase delay generated by the wind turbine phase-locked loop, thereby inducing a phase-leading capacitive reactive current. When this capacitive reactive current is injected and flows through the inductive submarine cable, it not only does not cause voltage loss, but also produces a line voltage rise effect similar to series capacitor compensation, causing the wind turbine terminal voltage to rise significantly (Δ V wf >0). The aforementioned positive voltage compensation directly drives the surge in the instantaneous output active power of the wind turbine (Δ). P wf >0), this spontaneously increased power flow perfectly fills the extra energy gap on the M3C power frequency side caused by providing synthetic inertia support at the physical level, thereby fundamentally eliminating the transient power mismatch phenomenon inside the M3C-OWF system.

[0051] To prevent wind turbines from erroneously reducing output based on the original control law due to detected grid frequency increases, this invention simultaneously reverses the polarity of the active power-frequency droop control coefficients used within the wind turbine to respond to local frequency changes, i.e., reconfigures the local coefficients. k dr2 <0; By implementing the polarity mirror reversal of the two-end parameters, not only is the power inversion caused by the phase-locked loop delay eliminated, but the long-term steady-state active power support responsibility of the wind turbine is also maintained, realizing the lossless maintenance of transient system stability and overall active support capacity.

[0052] 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. An active support method for a low-frequency sea wind transmission system based on reverse frequency transfer, characterized in that, Includes the following steps: (1) Based on the topology and energy transmission physical characteristics of the offshore wind low-frequency transmission system, the transient power mismatch mechanism between the decrease in offshore wind farm output power and the increase in M3C output power demand caused by the delay of phase-locked loop is revealed through theoretical analysis; (2) Based on the transient power mismatch mechanism, the control strategy of M3C and wind turbine grid-side converter is reconstructed. By configuring a negative frequency droop coefficient, the polarity of reactive current induced by phase-locked loop delay is reversed to completely eliminate the power mismatch between M3C and offshore wind farm and improve the transient stability of the system. The reconfigured M3C control strategy is as follows: First, the PCC voltage on the low-frequency side of the M3C is input to the power frequency side SRF-PLL to generate the power frequency side angular frequency. ω 1. ω The frequency deviation between 1 and the power frequency is used to obtain the low-frequency side angular frequency control command through droop control. ω 2, and then to ω 2. Integrate to obtain the low-frequency side power angle. θ 2; then the low-frequency side voltage reference value The low-frequency dq-axis voltage control command is obtained through dual closed-loop control of voltage and current, and finally based on the power angle... θ 2. The coordinate transformation of the low-frequency side dq axis voltage control command is performed to obtain the low-frequency side three-phase voltage control command, and then the V / f control of M3C is realized by modulation. The reconfigured grid-side converter control strategy for wind turbines is as follows: First, the PCC voltage at the wind turbine end side is input to the low-frequency SRF-PLL to generate the wind turbine end-side angular frequency. ω 3. ω 3. The frequency deviation between the wind turbine speed and the rated angular frequency of the offshore low-frequency power grid is used to obtain the reference value of the wind turbine speed through droop control. At the same time, ω 3. Integrate to obtain the end-side power angle of the wind turbine. θ 3; Output active power of wind turbine units P wf The fan speed reference value is obtained through MPPT control. Under normal system conditions, the following is adopted: As a reference value for fan speed In the case of frequency drop in land power grid, adopt As a reference value for fan speed Then make The stator current control command is obtained through PI control, and the stator voltage control command for the dq axis is obtained after current control of the stator current control command. Finally, the power angle is used to determine the stator voltage control command. θ 3. The coordinate transformation of the dq axis stator voltage control command is used to obtain the three-phase stator voltage control command, and then the grid-side converter is controlled by modulation to achieve grid-following control.

2. The active support method for a low-frequency sea breeze transmission system based on reverse frequency transmission according to claim 1, characterized in that: The offshore wind low-frequency transmission system includes an offshore wind farm, a submarine AC cable, a low-frequency converter station based on M3C, and an onshore power grid. The wind turbines in the offshore wind farm are connected to the AC bus via grid-side converters. One end of the submarine AC cable is connected to the AC bus, and the other end is connected to the low-frequency side of the M3C. The power frequency side of the M3C is connected to the onshore power grid. The M3C adopts a V / f control strategy, and the grid-side converter adopts a grid-following control strategy.

3. The active support method for a low-frequency sea breeze transmission system based on reverse frequency transmission according to claim 1, characterized in that, The specific implementation of step (1) is as follows: First, based on the topology and energy transmission physical characteristics of the system, a frequency coupling control and large-signal model between the offshore low-frequency power grid and the onshore power frequency power grid is constructed to quantify the impact of the frequency drop of the onshore power frequency power grid on the transient support power of the offshore wind farm; then, based on the operating characteristics of the offshore wind farm in the low-frequency weak power grid environment, a grid-following wind turbine model including the dynamic response of the low-bandwidth phase-locked loop is constructed, and the phase error closed-loop transfer function of the phase-locked loop and the dynamic reactive current injection equation of the stator side caused by the sudden frequency disturbance are derived; then, based on the impedance distribution characteristics of the long-distance submarine AC cable and the dynamic reactive current injection equation of the stator side, a dynamic correlation equation between the voltage drop of the wind turbine end side and the transmission power of the offshore wind farm-M3C is constructed, thereby revealing the transient power mismatch mechanism between the decrease in the output power of the offshore wind farm and the increase in the output power demand of the M3C caused by the delay of the phase-locked loop.

4. The active support method for a low-frequency sea breeze transmission system based on reverse frequency transmission according to claim 1, characterized in that, The droop control expression in the M3C control strategy is as follows: in: The rated angular frequency of the offshore low-frequency power grid. k dr1 The frequency droop coefficient and k dr1 <0.

5. The active support method for a low-frequency sea breeze transmission system based on reverse frequency transmission according to claim 1, characterized in that, The droop control expression in the control strategy of the grid-side converter of the wind turbine is as follows: in: The rated angular frequency of the offshore low-frequency power grid. k dr2 The frequency droop coefficient and k dr2 <0.

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 active support method for the low-frequency sea breeze transmission system based on reverse frequency transmission 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 active support method for the low-frequency sea breeze transmission system based on reverse frequency transmission as described in any one of claims 1 to 5.

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