A low-frequency oscillation suppression method for a marine wind power synchronous machine grid-connected system

Abstract

The present application relates to offshore wind power grid connection technical field, especially to a kind of low-frequency oscillation suppression method for offshore wind power through synchronous machine grid-connected system, including obtaining the inherent parameter of synchronous machine grid-connected system, and the active power reference instruction of each wind turbine of offshore wind farm;According to inherent parameter, the control parameter of preset damping controller is determined, and damping controller is configured based on control parameter;Real-time acquisition actual operating frequency signal of offshore low-frequency power grid, and it is compared with the rated frequency of preset offshore low-frequency power grid, and frequency deviation signal is obtained;Frequency deviation signal is input to configured damping controller, and output damping power instruction;Damping power instruction and active power reference instruction are superimposed, and active power control instruction is generated.By the present application, the low-frequency oscillation of new energy synchronous machine grid-connected system can be effectively inhibited, while avoiding the threat to grid connection safety due to the substantial fluctuation of generator terminal voltage caused by the inhibition measures itself.

Description

Technical Field

[0001] This invention relates to the technical field of offshore wind power grid connection, and in particular to a method for suppressing low-frequency oscillations in offshore wind power grid connection systems via synchronous machines. Background Technology

[0002] As offshore wind power development expands into mid-to-far waters, low-frequency AC transmission (LFAC) schemes, compared to power frequency AC transmission grid connection schemes, can effectively reduce the charging power effect of submarine cables, significantly mitigating the adverse impact of this effect on the grid connection point voltage. Compared to flexible DC transmission schemes, it has lower construction costs and its converter-based control mode is flexible and adjustable. However, the application of converters introduces a large number of nonlinear control elements, which not only complicates the dynamic characteristics of the system but also generates high-order harmonic pollution. To overcome these shortcomings, the frequency conversion grid connection scheme using a new energy synchronous motor (Motor-Generator Pair, MGP) can fundamentally avoid the high-order harmonics and complex control problems introduced by converters.

[0003] The new energy synchronous generator grid connection system consists of two electrically excited synchronous motors connected coaxially via rigid shafts. The electrically excited synchronous motor is used to construct the offshore low-frequency power grid, while the electrically excited synchronous generator is directly connected to the onshore power frequency grid. By reasonably configuring the number of pole pairs of the two synchronous motors, frequency conversion grid connection of offshore wind power can be achieved. However, in actual operation, when disturbances occur in the onshore power grid, the new energy synchronous generator is prone to generating low-frequency oscillations in the 0.2Hz to 2.5Hz frequency band, which seriously affects the stable operation of the system.

[0004] Existing methods for suppressing low-frequency oscillations mainly employ power system stabilizers. These stabilizers detect the oscillation component of the synchronous generator rotor speed and adjust the generator excitation system, thereby injecting an additional damping torque component into the unit's electromagnetic torque to enhance system damping and suppress oscillations. The damping torque of the power system stabilizer is essentially achieved by adjusting the synchronous generator's terminal voltage. During the dynamic process of suppressing rotor oscillations, the generator output voltage will fluctuate accordingly. When grid disturbances are significant and oscillation amplitudes are high, the strong control effect of the power system stabilizer may exacerbate the deviation of the generator terminal voltage from the rated range. If the voltage fluctuation exceeds the safety limits specified at the grid connection point, it will trigger the protection device, forcing the offshore wind farm to disconnect from the grid, thus compromising the operational stability of the entire transmission system.

[0005] Therefore, there is an urgent need for a low-frequency oscillation suppression method that can effectively suppress the low-frequency oscillations of the new energy synchronous machine grid-connected system, while avoiding the threat to grid connection safety caused by large fluctuations in the motor terminal voltage due to the suppression measures themselves. Summary of the Invention

[0006] This invention provides a method for suppressing low-frequency oscillations in offshore wind power grid-connected systems via synchronous generators, which can effectively solve the problems in the background art.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for suppressing low-frequency oscillations in offshore wind power grid-connected systems via synchronous generators includes:

[0009] Obtain the inherent parameters of the synchronous machine grid-connected system, as well as the active power reference command for each wind turbine in the offshore wind farm;

[0010] Based on the inherent parameters, the control parameters of the preset damping controller are determined, and the damping controller is configured based on the control parameters;

[0011] The actual operating frequency signal of the offshore low-frequency power grid is acquired in real time and compared with the preset rated frequency of the offshore low-frequency power grid to obtain the frequency deviation signal.

[0012] The frequency deviation signal is input to the configured damping controller, which outputs a damping power command.

[0013] The damping power command is superimposed with the active power reference command to generate an active power control command, which is used to adjust the active power output of the offshore wind farm and provide additional damping for the synchronous machine grid-connected system to suppress low-frequency oscillations of the system.

[0014] Furthermore, the active power reference command is determined by the local controller of the wind turbine based on wind speed and maximum power point tracking strategy.

[0015] Furthermore, the inherent parameters include the rated capacity of the offshore wind farm, the transient electromotive force inside the synchronous generator, the rated voltage of the onshore power grid, the total equivalent reactance from the generator to the grid connection point, and the inertial time constant of the synchronous generator grid connection system.

[0016] Furthermore, the control parameters include damping torque coefficient, synchronous torque coefficient, filtering time constant, lead time constant, lag time constant, and damping gain coefficient.

[0017] Furthermore, the actual operating frequency of the offshore low-frequency power grid is directly obtained locally by the phase-locked loop at the grid connection point of the offshore wind farm converter, without the need for long-distance communication.

[0018] Furthermore, the damping controller includes a gain adjustment unit, a high-pass filter unit, and a phase compensation unit connected in series.

[0019] Furthermore, the high-pass filter unit is configured with a DC blocking algorithm that has dynamic reference reset capability, which can attenuate DC and ultra-low frequency signal components while allowing signals in the low-frequency oscillation band to pass through.

[0020] The phase compensation unit is configured to perform phase correction on the frequency deviation signal. By distributing the phase compensation to multiple series compensation links and determining the time constant of the compensation links through the principle of frequency center symmetry, the mechanical torque component generated by the output damping power command is out of phase with the rotor speed oscillation component of the synchronous machine grid system, so as to provide a positive damping effect.

[0021] The parameter tuning process of the gain adjustment unit is as follows: construct a characteristic equation reflecting the transmission characteristics of the system from mechanical torque deviation to rotor angle deviation, take the active power capacity of the offshore wind farm as the constraint condition, and take the optimal damping ratio of the closed-loop system as the objective. By analytically establishing the mapping relationship between the damping gain coefficient and the system inertia and torque coefficient, the gain coefficient that satisfies the steady-state constraints and has the optimal damping effect can be calculated.

[0022] Further, the damping power command is superimposed on the active power reference command to generate an active power control command, including:

[0023] Calculate the allocation coefficients for each wind turbine in an offshore wind farm to undertake additional damping tasks;

[0024] Based on the amplitude of the damping power command and the allocation coefficient of each wind turbine, the total damping power demand is proportionally decomposed to generate damping power sub-commands corresponding to each wind turbine.

[0025] The damping power sub-instruction corresponding to each wind turbine is added to its own active power reference instruction to generate the active power control instruction for each wind turbine.

[0026] Furthermore, the allocation coefficient is determined based on the adjustment potential parameter, the system oscillation correlation parameter, and the operating stability coefficient;

[0027] The regulation potential parameters include the remaining regulation capacity of the wind turbine and the maximum regulation rate of the converter.

[0028] The system oscillation correlation parameters include the electrical distance correlation between the wind turbine access node and the system oscillation center and the frequency oscillation amplitude of the node;

[0029] The operational stability coefficient is determined based on the active power fluctuation variance, speed fluctuation variance, and converter module temperature of the fan within a preset period.

[0030] Furthermore, the electrical distance correlation is determined based on the electrical impedance value between the wind turbine access node and the system oscillation center. The smaller the electrical impedance value, the higher the electrical distance correlation. The frequency oscillation amplitude is obtained through a frequency measurement unit deployed at the wind turbine access node.

[0031] Furthermore, when calculating the allocation coefficient, if the remaining regulation capacity of the fan is less than or equal to zero, or the operating stability coefficient is lower than a preset safety threshold, then the allocation coefficient of the fan is set to zero, and it does not participate in the damping task allocation.

[0032] The technical solution of this invention achieves the following technical effects: This method can directly provide damping based on the frequency of the offshore low-frequency power grid to suppress low-frequency oscillations, obtaining the operating status of the MGP without long-distance communication, thus reducing the impact of communication delay on the damping controller's effectiveness; by combining the feature of generating damping power commands based on the offshore power grid frequency deviation signal with the feature of superimposing this command on the active power command of the offshore wind farm, the path for providing damping torque is shifted from the voltage control channel of the synchronous generator to the power control channel of the offshore wind farm; since the wind farm regulates active power through the converter... The process is not directly related to the control of the synchronous generator's terminal voltage. The damping power command precisely matches the oscillation demand, quickly provides additional damping, and suppresses the spread of oscillation. The regulation target is the active power of the wind farm, whose output boundary is constrained by its own rated capacity. It will not cause secondary faults in the wind farm itself or on the generator side due to excessive regulation, while avoiding the secondary risk of voltage exceeding the limit caused by suppression measures. Therefore, this mechanism can effectively suppress the low-frequency oscillation of the MGP rotor while avoiding the impact on the output voltage of the synchronous generator, thereby resolving the contradiction between suppressing oscillation and maintaining grid-connected voltage stability, and ensuring the stability of the grid-connected system.

[0033] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 This is a flowchart illustrating the low-frequency oscillation suppression method for offshore wind power grid-connected systems via synchronous machines in this invention.

[0036] Figure 2 A topology diagram of an offshore wind power low-frequency transmission system connected to the grid via a synchronous machine, in which the present invention is applied;

[0037] Figure 3 This is a small-signal model diagram of the new energy synchronous machine grid-connected system in this invention;

[0038] Figure 4 This figure compares the oscillation suppression effects of power system stabilizers and the damping controller proposed in this invention under onshore power grid disturbances. Detailed Implementation

[0039] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0040] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0041] like Figure 1 As shown, the present invention provides a method for suppressing low-frequency oscillations in offshore wind power grid-connected systems via synchronous generators, specifically comprising the following steps:

[0042] Step S100: Obtain the inherent parameters of the synchronous machine grid-connected system and the active power reference command of each wind turbine in the offshore wind farm;

[0043] Step S200: Determine the control parameters of the preset damping controller based on the inherent parameters, and configure the damping controller based on the control parameters;

[0044] Step S300: Collect the actual operating frequency signal of the offshore low-frequency power grid in real time and compare it with the preset rated frequency of the offshore low-frequency power grid to obtain the frequency deviation signal.

[0045] Step S400: Input the frequency deviation signal to the configured damping controller and output the damping power command;

[0046] Step S500: The damping power command is superimposed with the active power reference command to generate an active power control command, which is used to adjust the active power output of the offshore wind farm and provide additional damping for the synchronous machine grid-connected system to suppress low-frequency oscillations of the system.

[0047] In this embodiment, the feature of generating damping power commands based on offshore grid frequency deviation signals is combined with the feature of superimposing these commands on the active power commands of the offshore wind farm. This shifts the path for providing damping torque from the voltage control channel of the synchronous generator to the power control channel of the offshore wind farm. Since the process of the wind farm adjusting active power through the converter is not directly related to the control of the synchronous generator's terminal voltage, the damping power command precisely matches the oscillation demand, quickly provides additional damping, and suppresses oscillation propagation. The adjustment target is the active power of the wind farm, whose output boundary is constrained by its own rated capacity. Over-adjustment will not cause secondary faults in the wind farm itself or on the generator side, while avoiding the secondary risk of voltage exceeding the limit due to suppression measures. Therefore, this mechanism can effectively suppress the low-frequency oscillation of the MGP rotor while avoiding the impact on the output voltage of the synchronous generator, thereby resolving the contradiction between suppressing oscillation and maintaining grid-connected voltage stability, and ensuring the stability of the grid-connected system.

[0048] It should be noted that the new energy synchronous machine involved in the above embodiments refers to a rotating motor pair consisting of two electrically excited synchronous motors coaxially connected by a rigid mechanical shaft, specifically including:

[0049] Offshore synchronous motor: Located on an offshore platform, its stator windings are connected to a low-frequency AC power grid formed by the collection and preliminary transformation of offshore wind farms;

[0050] Onshore synchronous generator: located near the onshore converter station or grid connection point, its stator winding is directly connected to the onshore power frequency AC main grid;

[0051] Common rotor and excitation system: Two motors share a unified rotor structure consisting of a rigid shaft and are usually equipped with an independent excitation control system that provides adjustable DC excitation current to the rotor windings to control the terminal voltage and reactive power of the synchronous generator.

[0052] When the new energy synchronous machine is running, the electrical energy generated by offshore wind power drives the synchronous motor to rotate, and the motor converts electrical energy into mechanical energy. This mechanical energy is directly transferred to the synchronous generator through a rigid shaft. The synchronous generator then converts the mechanical energy back into electrical energy and injects it into the onshore power frequency grid, thus realizing the transfer and frequency conversion of electrical energy from the offshore low-frequency system to the onshore power frequency system. By configuring different numbers of pole pairs for the synchronous motor and the synchronous generator, their electrical frequencies are naturally decoupled under the premise that the rigid shaft forces the two motors to operate at the same mechanical speed. For example, if the offshore power grid needs to operate at a low frequency, the motor is designed with more pole pairs; the generator uses fewer pole pairs to adapt to the power frequency; frequency conversion is achieved through a purely electromechanical method, without the need for a high-power power electronic converter.

[0053] In a specific implementation, as one example, the inherent parameters include the rated capacity of the offshore wind farm, the transient electromotive force inside the synchronous generator, the rated voltage of the onshore power grid, the total equivalent reactance from the generator to the grid connection point, and the inertial time constant of the synchronous machine grid connection system. By deploying multi-dimensional sensor arrays at key nodes of the synchronous machine grid connection system, corresponding monitoring devices are deployed at the generator terminals, shaft, onshore power grid connection point, and transmission lines from the generator to the grid connection point to collect data related to voltage, current, speed, power, and impedance. The methods for acquiring each parameter are as follows:

[0054] The rated capacity of offshore wind farms is determined by the average of the maximum continuous output power collected by the power sensor at the grid connection point. The data is updated regularly, and abnormal data with instantaneous power fluctuations are removed during the update.

[0055] The transient electromotive force inside the synchronous generator is calculated by combining the instantaneous values ​​of the three-phase voltage collected by the terminal voltage sensor and the instantaneous values ​​of the three-phase current collected by the current sensor with the extended Park equation of the synchronous generator.

[0056] The rated voltage of the onshore power grid is obtained by collecting the effective voltage value from the voltage sensor at the grid connection point. The average value of multiple consecutive calculation cycles is used as the real-time reference value. When the voltage fluctuation exceeds the rated range, a recalculation is triggered.

[0057] The total equivalent reactance from the generator to the grid connection point is obtained by combining the offline design value and the online correction value. The offline design value is the value calculated during the system design stage, and the online correction value is calculated by the line voltage drop to current ratio collected by the impedance monitoring module. The total equivalent reactance is the weighted sum of the two values, and the weighting coefficient is dynamically adjusted according to the line's operating years.

[0058] The inertial time constant of the synchronous machine grid-connected system is calculated by combining the speed change rate collected by the speed sensor and the active power fluctuation collected by the power sensor with the rigid body rotation equation. When the speed change rate or active power fluctuation reaches a set threshold, the inertial time constant is recalculated.

[0059] On the other hand, the active power reference command is determined by the local controller of the wind turbine based on wind speed and maximum power point tracking strategy; the local controller of each wind turbine in the offshore wind farm collects wind speed data at its location in real time, calculates the local active power reference value that maximizes wind energy capture based on the maximum power point tracking strategy, and uses it as the active power reference command for that wind turbine.

[0060] In this embodiment, by deploying a multi-dimensional sensor group at key nodes of the synchronous generator grid-connected system, and combining it with a targeted parameter calculation and update mechanism, the inherent parameters such as the rated capacity of the offshore wind farm and the transient electromotive force inside the synchronous generator are accurately acquired, ensuring that the inherent parameters are consistent with the actual operating state of the system. At the same time, the local controller of each wind turbine in the offshore wind farm collects wind speed data in real time and determines the active power reference command based on the maximum power point tracking strategy, so that the reference command can match the wind energy capture capability and operating characteristics of the wind turbine.

[0061] In a specific implementation, as one example, in order to ensure that the parameter configuration of the damping controller matches the current actual electrical and dynamic characteristics of the new energy synchronous machine grid-connected system, the control parameters of the damping controller are calculated and determined based on the inherent parameters of the synchronous machine grid-connected system that have been acquired in real time; the damping controller includes a gain adjustment unit, a high-pass filter unit, and a phase compensation unit connected in series; the control parameters include the damping torque coefficient, the synchronous torque coefficient, the filter time constant, the lead time constant, the lag time constant, and the damping gain coefficient.

[0062] Specifically, the synchronous torque coefficient is calculated based on the real-time inherent parameters obtained in step S100, and the calculation formula is as follows:

[0063] ;

[0064] Among them, K s It represents the synchronous torque coefficient, which physically means the restoring torque intensity generated by the electromagnetic field when the rotor power angle deviates from a unit angle, attempting to pull it back to synchronization. This represents the transient electromotive force inside the synchronous generator, which is the equivalent electromotive force of the synchronous generator after the transient process. This indicates the rated voltage of the onshore power grid, which is the standard effective value of the onshore power frequency grid voltage; It represents the total equivalent reactance from the generator to the grid connection point, which is the superposition of the generator direct-axis synchronous reactance and the transmission line reactance; express and The phase angle difference at the steady-state operating point is the phase difference between the transient potential and the grid voltage.

[0065] The formula for calculating the synchronous torque coefficient quantifies the ability of a synchronous generator grid-connected system to maintain synchronous operation through a coupled calculation of the synchronous generator's own electrical parameters and transmission line parameters; among which, Used to reflect the power transmission potential of an electrical system The synchronous torque coefficient is used to correct the phase coupling relationship under steady-state operating point, and the final synchronous torque coefficient directly corresponds to the system synchronous stiffness. The above formula uses the real-time inherent parameters of the system to calculate the synchronous torque coefficient, ensuring that the synchronous torque coefficient matches the actual electrical characteristics and transmission characteristics of the system, and avoiding the aggravation of oscillation due to improper synchronous torque adaptation.

[0066] Regarding the determination of the high-pass filter stage and its time constant, the time constant of the high-pass filter stage must ensure sufficient attenuation of DC and ultra-low frequency signals, while allowing low-frequency oscillation signals from 0.2Hz to 2.5Hz to pass effectively. Specifically, the expression for the DC blocking algorithm with dynamic reference reset capability is as follows:

[0067] ;

[0068] in, The high-pass filter stage represents the transfer function; s represents the complex frequency variable, which is a standard complex variable in control system analysis; T represents the filtering time constant, with a value range of 0.5 seconds to 1 second. The transfer function of the high-pass filter stage characterizes the attenuation characteristics of the filter stage for signals of different frequencies. When the signal frequency is much lower than the low-frequency oscillation band (0.2~2.5Hz), the amplitude of the transfer function approaches 0, achieving signal attenuation; when the signal frequency is in the low-frequency oscillation band, the amplitude of the transfer function approaches 1, ensuring that the signal passes through without attenuation. Low-frequency oscillation suppression needs to respond only to dynamic oscillation signals to avoid interfering with the steady-state signal of the system. The above-mentioned high-pass filter stage isolates DC and ultra-low frequency steady-state signals through the reasonable value of the filtering time constant, so that the damping controller only starts working when the system has low-frequency oscillations, without affecting the steady-state operation of the system, while avoiding over-adjustment caused by steady-state signal interference, thereby preventing voltage fluctuations.

[0069] Regarding the determination of relevant parameters for the phase compensation stage, its function is to correct the phase deviation of the control signal, ensuring that the damping torque is out of phase with the speed oscillation, providing a positive damping effect, and thus maximizing the damping effect. Phase compensation is distributed among multiple series-connected compensation stages. Specifically, the expression for the phase compensation stage is as follows:

[0070] ;

[0071] in, The transfer function of the phase compensation stage is represented by T1; the lead time constant is represented by T2; the lag time constant is represented by m; and the order of the phase compensation stage is represented by m. The value at the midpoint of the low-frequency oscillation frequency is also mentioned. At this point, the total phase provided by the phase compensation stage is:

[0072] ;

[0073] T1 and T2 can be determined by the following formula:

[0074] ;

[0075] Among them, the principle of frequency center symmetry is utilized. The transfer function of the aforementioned phase compensation stage, through the cascaded lead and lag correction stages, corrects the phase of the input signal, ensuring that the phase compensation characteristics match the low-frequency oscillation frequency. This ensures that the phase of the compensated control signal is out of phase with the rotor speed oscillation phase. The frequency deviation signal, after filtering, exhibits phase lag. If directly used to generate the damping power command, it may cause the damping torque to be in phase with the speed oscillation, failing to provide effective positive damping and even exacerbating the oscillation. The phase compensation stage corrects the signal phase deviation, ensuring that the damping torque is out of phase with the speed oscillation, maximizing the damping suppression effect.

[0076] The damping torque coefficient is determined in conjunction with the inherent structure of the synchronous generator, such as the damping winding.

[0077] Regarding the determination of the damping gain coefficient, the tuning principle of the damping gain parameter is to stabilize the system and make the system damping ratio close to optimal by changing the damping gain parameter, considering the active power capacity constraint of the offshore wind farm output. Considering the dynamic processes of the high-pass filter and phase compensation stages, a simplified model of the new energy synchronous machine grid-connected system is constructed with mechanical torque deviation as input and rotor angle deviation as output, as follows: Figure 3 As shown, its characteristic equation is:

[0078] ;

[0079] ;

[0080] Where K represents the damping gain coefficient; H represents the inertial time constant of the synchronous machine grid-connected system; K s K represents the synchronous torque coefficient. d T represents the damping torque coefficient; T1 represents the lead time constant; T2 represents the lag time constant; s represents the complex frequency variable, which is a standard complex variable in control system analysis; T represents the filtering time constant.

[0081] When the values ​​of T, T1, and T2 are small, the dynamic processes of the high-pass filter and phase compensation stages can be ignored. By analytically establishing the mapping relationship between the damping gain coefficient and the system inertia and torque coefficient, the expression for the gain coefficient that satisfies the steady-state constraints and has the optimal damping effect can be obtained as follows:

[0082]

[0083] Where K represents the damping gain coefficient; H represents the inertial time constant of the synchronous machine grid-connected system; K sK represents the synchronous torque coefficient. d Indicates the damping torque coefficient; The damping ratio is set to 0.2 in this embodiment. The above method for calculating the damping gain coefficient ensures that the value of the damping gain coefficient meets the requirements of the optimal system damping ratio and does not exceed the active power capacity constraint of the offshore wind farm, thus avoiding voltage fluctuations caused by over-adjustment.

[0084] Since the damping controller adopts a series structure with gain, high-pass filtering, and phase compensation stages, its transfer function model is expressed as:

[0085] ;

[0086] Where K is the damping gain coefficient; This is a high-pass filter stage; For the phase compensation stage, the control parameters calculated above are substituted into the transfer function model of the damping controller and written to the damping controller execution module through the communication link. After configuration, parameter verification is started, and the parameters are substituted into the system characteristic equation to verify stability. If the stability is not qualified, the calculation is readjusted.

[0087] In this embodiment, each formula takes the system's real-time inherent parameters as input, ensuring that the control parameters match the system's actual operating characteristics and dynamic oscillation characteristics. This guarantees that the damping controller can provide effective positive damping and achieve rapid suppression of low-frequency oscillations. The design of the high-pass filter and phase compensation formulas ensures that the controller only responds to low-frequency oscillation signals and does not interfere with the system's steady-state operation, avoiding voltage fluctuations caused by over-adjustment and resolving the risk of tripping due to voltage adjustment in existing power system stabilizer devices. The capacity constraint design of the damping gain coefficient formula ensures that the active power adjustment does not exceed the wind turbine's output capacity.

[0088] In a specific implementation, as one example, in order to collect the frequency spatial distribution differences of the power grid caused by power distribution changes under disturbances, and to provide a frequency signal that can characterize the overall synchronization state of the system for damping control, this embodiment provides a method for obtaining a frequency deviation signal, as detailed below:

[0089] Step S310: Select multiple key measurement points in the offshore low-frequency power grid, including the main collection bus of the offshore wind farm and the grid connection point of the wind farm with a long electrical distance; configure a frequency measurement unit with a high sampling rate at each measurement point. The frequency measurement unit calculates the instantaneous frequency value of the point based on real-time sampling of the three-phase voltage waveform and through a phase-locked loop algorithm; to eliminate the influence of measurement noise and high-order harmonics, each measurement unit has a built-in digital filter. The passband of the filter is designed to cover the low-frequency oscillation band from 0.2Hz to 2.5Hz, while strictly attenuating power frequency harmonics and higher-frequency switching noise.

[0090] Step S320: Transmit the instantaneous frequency signals of multiple measurement points to the central frequency estimator via the communication network. The central frequency estimator does not simply perform an arithmetic average of multiple frequency values, but rather performs a weighted synthesis based on the pre-calculated participation weight of each measurement point in the system power flow calculation or the reciprocal of its electrical distance to the MGP motor. This weighting coefficient reflects the degree of contribution of the frequency dynamics of each node to the overall inertial center frequency of the system. The central frequency estimator outputs the system equivalent inertial frequency signal, which is used to characterize the overall average synchronous speed change of the offshore low-frequency power grid.

[0091] Step S330: The real-time acquired system equivalent inertial frequency signal is algebraically subtracted from the preset rated frequency of the offshore low-frequency power grid to obtain the original frequency deviation signal. To further enhance the signal quality and use it for subsequent control, the original deviation signal needs to undergo a preprocessing stage, including a moving average filter with an extremely short time window to smooth out glitches caused by communication delays or minor measurement asynchrony. At the same time, a low-rate limiter is configured to prevent false frequency deviations far exceeding the oscillation range caused by extreme faults from impacting the controller. Finally, the frequency deviation signal is output.

[0092] In this embodiment, the method directly obtains the frequency of the offshore low-frequency power grid locally through the phase-locked loop at the grid connection point of the offshore wind farm converter, and can obtain the operating status of the MGP without long-distance communication, reducing the impact of communication delay on the effect of the damping controller. The system equivalent inertial frequency signal generated by multi-point measurement and weighted fusion can more comprehensively reflect the overall power imbalance and oscillation mode of the offshore low-frequency power grid, reducing the problems of signal locality, inaccuracy or insensitivity to specific modes that may be caused by the single measurement point location.

[0093] In some embodiments of the present invention, the frequency deviation signal is input to the configured damping controller according to the following steps to generate a damping power command:

[0094] Step S410: Transmit the preprocessed frequency deviation signal to the input interface of the damping controller for per-unit processing; the per-unit coefficient is determined based on the system's rated power, rated frequency, and maximum allowable frequency deviation to match the input signal amplitude with the controller's internal processing range, avoiding processing distortion caused by excessively large or small signal amplitudes; during the adaptation process, the signal continuity is monitored in real time, and if a signal interruption occurs, the valid signal from the previous cycle is automatically used as a temporary substitute to ensure continuous operation of the controller;

[0095] Step S420: The damping controller performs signal processing sequentially according to the preset structure. First, the high-pass filter unit filters out components belonging to the low-frequency oscillation band in the frequency deviation signal, filtering out steady-state deviation and ultra-low frequency interference. The filtering parameters remain consistent with the previous configuration. Then, the signal enters the phase compensation unit, which corrects the signal phase by leading and lagging to cancel the phase lag in the signal transmission and processing process, ensuring that the output signal is out of phase with the system rotor oscillation phase and providing positive damping. Finally, the gain adjustment unit amplifies the signal amplitude according to the damping gain coefficient set in the previous step. The amplified signal matches the equivalent damping requirements of the system.

[0096] Step S430: Based on the output signal of the damping controller, combined with the converter modulation coefficient, system equivalent impedance and synchronous generator transient characteristic parameters, the control signal is converted into a damping power command through the quantization relationship between electromagnetic power and control signal; the conversion process is based on the coupling relationship between electromagnetic torque and power to ensure that the command amplitude corresponds linearly with the amplitude of the controller output signal, and that the phase is consistent with the signal after phase compensation, thus ensuring the effectiveness of the command in suppressing oscillations;

[0097] Step S440: Set the constraint range of the damping power command. The upper limit is determined based on the rated power of the offshore wind farm, the maximum regulating capacity of the converter, and the system's safe operation requirements. The lower limit is set to a negative equal amplitude to avoid execution failure caused by the command exceeding the equipment's regulating capacity. Perform boundary verification on the converted damping power command. If it exceeds the constraint range, automatically clamp the command to the corresponding boundary value and record the over-limit event. At the same time, substitute the command into the system characteristic equation for stability verification to confirm that the command will not trigger a new oscillation mode in the system. If the verification fails, adjust the gain coefficient and recalculate.

[0098] In this embodiment, the per-unit input adaptation ensures the accuracy of signal processing and avoids amplitude distortion; the phase compensation unit accurately corrects the signal phase to ensure that the damping power is in phase with the system oscillation and provides effective positive damping; the dynamic gain adjustment adapts the command amplitude to the system damping requirements and improves the targeting of oscillation suppression; the power boundary constraint prevents the command from exceeding the device's adjustment capability and ensures the safe and stable operation of the device.

[0099] In some embodiments of the present invention, to address the balance issue when multiple wind turbines collaboratively undertake damping tasks, directly applying the total damping power command to the entire wind farm can easily lead to some turbines exceeding their regulation capacity and others failing to fully utilize their regulation potential, thereby affecting the oscillation suppression effect or causing equipment malfunctions. Therefore, by quantifying the actual regulation capacity of each wind turbine, a damping power allocation mechanism is designed to proportionally decompose the total damping power command to each wind turbine, and then superimpose it with the active power baseline command of each wind turbine. This achieves precise adaptation of the additional damping to oscillation requirements without exceeding the equipment operating boundaries. Specific implementation methods are as follows:

[0100] Step S510: Obtain the regulation potential parameters consisting of the remaining regulation capacity of each wind turbine and the maximum regulation rate of the converter. Combine the system oscillation correlation parameters formed by the electrical distance correlation degree and the frequency oscillation amplitude of the access node, and the operating stability coefficient obtained by weighted calculation based on the active power fluctuation variance, speed fluctuation variance and converter module temperature within a near preset period, after normalizing the regulation potential parameters, system oscillation correlation parameters and operating stability coefficient by eliminating dimensions, use the weighted sum of the initial values ​​determined by offline simulation, the online monitoring system oscillation suppression effect and the dynamic correction of the wind turbine operating status, to obtain the comprehensive score value of each wind turbine. The ratio of the comprehensive score value of a single wind turbine to the sum of the comprehensive scores values ​​of all wind turbines is used as the allocation coefficient of the wind turbine, and the sum of the allocation coefficients of all wind turbines is 1. If the remaining regulation capacity of the wind turbine is less than or equal to zero or the operating stability coefficient is lower than the preset safety threshold, its allocation coefficient is set to zero and it does not participate in the damping task allocation.

[0101] Step S520: Obtain the total damping power command. According to the allocation coefficient of each wind turbine, decompose the total damping power command proportionally to obtain the damping power sub-command corresponding to each wind turbine. Perform boundary verification on each damping power sub-command. The verification basis is the remaining adjustment capacity of the corresponding wind turbine to ensure that the absolute value of the damping power sub-command does not exceed the remaining adjustment capacity of the wind turbine. If it exceeds the constraint range, adjust the command to the corresponding boundary value.

[0102] Step S530: Obtain the active power reference command for each wind turbine, superimpose the damping power sub-command corresponding to each wind turbine with its own active power reference command to obtain the final active power control command for each wind turbine; transmit the control command to the converter control module of the corresponding wind turbine, adjust the active power output of the wind turbine through the converter, and the wind turbines work together to provide additional damping for the synchronous machine grid-connected system.

[0103] In this embodiment, the total damping power command is decomposed into sub-commands adapted to the capacity of each wind turbine by an allocation coefficient. The adjustment target is the active power of each wind turbine. The damping path is not directly related to the synchronous generator terminal voltage, rather than applying a uniform damping power command to the entire wind farm. The allocation coefficient is determined based on the adjustment potential parameter, system oscillation correlation parameter, and operating stability coefficient, so that the damping task undertaken by each wind turbine matches its own adjustment capacity, avoiding wind turbine failures caused by overload and other problems. The damping power sub-command is decomposed according to the allocation coefficient to realize the coordinated response of multiple wind turbines to the system oscillation demand, ensuring that the additional damping is accurately adapted to the system demand. Finally, the active power control command is generated by superimposing the base command and the sub-commands. It does not interfere with the wind energy capture function of the wind turbine based on the maximum power point tracking strategy, provides additional damping, and does not affect the stability of the synchronous generator terminal voltage, thus achieving coordinated protection of low-frequency oscillation suppression and grid voltage stability.

[0104] More specifically, the method for determining the allocation coefficients in step S510 is as follows:

[0105] Step S511: For the wind turbine regulation potential parameters, obtain the rated capacity and the current actual output active power of each wind turbine, calculate the difference between the two as the remaining regulation capacity, characterize the basic ability of the wind turbine to undertake the damping task; at the same time, collect the maximum regulation rate of the wind turbine converter. This parameter is determined by the active power regulation slope data fed back in real time by the converter controller, reflecting the speed potential of the wind turbine in response to the damping command.

[0106] For system oscillation correlation parameters, the location of the system oscillation center is determined by the power flow calculation module of the synchronous machine grid-connected system. Based on the electrical impedance value between the wind turbine access node and the oscillation center, the electrical distance correlation is calculated. The smaller the electrical impedance value, the higher the correlation, which indicates that the wind turbine contributes more efficiently to oscillation suppression. At the same time, the frequency oscillation amplitude of the wind turbine access node is collected. This amplitude is obtained by the frequency measurement unit deployed at the node. The larger the oscillation amplitude, the more significant the oscillation influence on the location of the wind turbine. Damping tasks should be prioritized to quickly suppress local oscillations.

[0107] For operational stability parameters, the active power fluctuation variance, speed fluctuation variance, and converter module temperature data of the wind turbine are collected within a preset period. After data normalization, the operational stability coefficient is calculated by weighting. The smaller the fluctuation variance and the closer the temperature is to the rated operating temperature, the higher the stability coefficient, which indicates that the wind turbine has higher operational reliability when undertaking additional damping tasks.

[0108] Step S512: Normalize the above parameters to eliminate the dimensional differences between different parameters; wherein, the remaining regulating capacity, the maximum regulating rate of the converter, the electrical distance correlation, the frequency oscillation amplitude, and the operating stability coefficient are mapped to preset intervals to ensure that each parameter has equal weight in the weighted calculation; during the normalization process, a sliding window is used to update the parameter values ​​in real time, and the window length is dynamically adjusted according to the system oscillation period to ensure that the parameters can reflect the wind turbine operating status and system oscillation characteristics in real time.

[0109] Step S513: Based on the dynamic characteristics of the synchronous machine grid-connected system, the weighting coefficients of each dimension parameter are determined by combining offline simulation and online correction. In the offline stage, a simulation model containing different oscillation scenarios and wind turbine operating conditions is constructed to analyze the sensitivity of each dimension parameter to the oscillation suppression effect and determine the initial weighting coefficients. In the online stage, the system oscillation suppression effect and wind turbine operating status are monitored in real time. If the oscillation decay rate does not reach the preset threshold, or if a wind turbine has abnormal operating parameters, the weighting coefficients of each dimension are automatically adjusted so that the weights are tilted towards the dimensions that contribute more significantly to oscillation suppression and have more stable operation.

[0110] Step S514: For the normalized three-dimensional basic parameters, perform weighted summation according to the determined weighting coefficients to obtain the comprehensive score value of each wind turbine; calculate the sum of the comprehensive scores of all wind turbines, and use the ratio of the comprehensive score value of a single wind turbine to the total comprehensive score value as the final allocation coefficient of that wind turbine, and the sum of the allocation coefficients of all wind turbines is 1; if the remaining regulation capacity of a certain wind turbine is zero or negative, or the operating stability coefficient is lower than the preset safety threshold, its allocation coefficient is directly set to zero and it does not participate in the damping task allocation.

[0111] In this embodiment, the above parameters cover the wind turbine's own capabilities, system requirements, and operational safety, making the allocation coefficient more closely match the actual application scenario and improving the accuracy of damping allocation. The introduction of the correlation between the converter's maximum adjustment rate and electrical distance ensures the timeliness of damping response and suppression efficiency, enabling the additional damping to act quickly on the critical oscillation area. The introduction of the operating stability coefficient avoids the wind turbine from malfunctioning due to its own unstable operating conditions when undertaking damping tasks. The combination of offline preset and online correction of the weighting coefficients enables the allocation mechanism to adapt to different oscillation scenarios and changes in wind turbine operating status, ensuring the stability of the oscillation suppression effect.

[0112] To verify the effectiveness of the low-frequency oscillation suppression method proposed in this invention in suppressing low-frequency oscillations in offshore wind power grid-connected systems via synchronous machines, this embodiment uses PSCAD / EMTDC simulation software to construct an electromagnetic transient simulation model and completes the verification through simulation experiments.

[0113] First, execute step S100 of the method of this invention: acquire the inherent parameters of the synchronous machine grid-connected system, and then execute step S200 based on these inherent parameters: complete the parameter initialization of the damping controller; in the PSCAD / EMTDC simulation software, build as follows Figure 2 The electromagnetic transient simulation model of the offshore wind power grid-connected system via a synchronous generator is shown below. The simulation parameters of the system are configured as follows:

[0114]

[0115] In the above simulation model, a phase A ground fault lasting 0.18s occurs in the shore-connected power grid at time 5s to simulate the disturbance scenario experienced by the system. The oscillation response characteristics of the system frequency and the rotor speed of the synchronous machine grid-connected system are observed. The simulation results are as follows: Figure 4As shown: A phase-A ground fault caused the onshore grid frequency to oscillate at 5.18s. Both existing power system stabilizers and the low-frequency oscillation suppression method proposed in this invention can suppress this oscillation, making the new energy synchronous machine grid-connected system more stable. However, the low-frequency oscillation suppression method of this invention performs better than the power system stabilizer in terms of oscillation suppression speed and reduction of frequency oscillation amplitude. The simulation results show that the low-frequency oscillation suppression method of this invention can effectively suppress the rotor speed oscillation of the synchronous machine grid-connected system, thereby improving the stability of offshore wind power low-frequency transmission through the synchronous machine grid-connected system.

[0116] Although this application has been described in conjunction with specific features and embodiments, it is obvious that various modifications and combinations can be made thereto without departing from the spirit and scope of this application. Accordingly, this specification and drawings are merely exemplary illustrations of the application as defined herein, and are to be considered as covering any and all modifications, variations, combinations, or equivalents within the scope of this application. Clearly, those skilled in the art can make various alterations and modifications to this application without departing from its scope. Thus, if such modifications and modifications fall within the scope of this application and its equivalents, this application intends to include such modifications and modifications.

Claims

1. A method for suppressing low-frequency oscillations in offshore wind power grid-connected systems via synchronous generators, characterized in that, include: Obtain the inherent parameters of the synchronous machine grid-connected system, as well as the active power reference command for each wind turbine in the offshore wind farm; Based on the inherent parameters, control parameters for the preset damping controller are determined, and the damping controller is configured based on the control parameters; the damping controller includes a gain adjustment unit, a high-pass filter unit, and a phase compensation unit connected in series; wherein, the high-pass filter unit is configured with a DC blocking algorithm having dynamic reference reset capability, the expression of which is: ； in, The transfer function of the high-pass filter is denoted as s; the complex frequency variable is denoted as s, which is a standard complex variable in control system analysis; T represents the filtering time constant, with a value range of 0.5 seconds to 1 second, used to attenuate DC and ultra-low frequency signals below 0.2Hz, while allowing low-frequency oscillation signals from 0.2Hz to 2.5Hz to pass through; the phase compensation unit is configured to perform phase correction on the frequency deviation signal, and its transfer function is: ； in, The transfer function of the phase compensation stage is represented by T1; the lead time constant is represented by T2; the lag time constant is represented by m; and the order of the phase compensation stage is represented by m. The value at the midpoint of the low-frequency oscillation frequency is also mentioned. At this point, the total phase provided by the phase compensation stage is: ； in, Calculated using the principle of frequency central symmetry. ; The parameter tuning process of the gain adjustment unit is as follows: A characteristic equation reflecting the transmission characteristics of the system from mechanical torque deviation to rotor angular deviation is constructed. Using the active power capacity of the offshore wind farm as a constraint and the optimal damping ratio of the closed-loop system as the objective, the mapping relationship between the damping gain coefficient and the system inertia and torque coefficient is established analytically. The gain coefficient that satisfies the steady-state constraints and has the optimal damping effect is calculated. The characteristic equation is: ； ； Where K represents the damping gain coefficient; H represents the inertial time constant of the synchronous machine grid-connected system; K s K represents the synchronous torque coefficient. d Indicates the damping torque coefficient; The expression for the gain coefficient is: ； Where K represents the damping gain coefficient; H represents the inertial time constant of the synchronous machine grid-connected system; The damping ratio; The actual operating frequency signal of the offshore low-frequency power grid is acquired in real time and compared with the preset rated frequency of the offshore low-frequency power grid to obtain the frequency deviation signal. The frequency deviation signal is input to the configured damping controller, which outputs a damping power command. The damping power command is superimposed with the active power reference command to generate an active power control command, which is used to adjust the active power output of the offshore wind farm and provide additional damping for the synchronous machine grid-connected system to suppress low-frequency oscillations of the system. This includes: calculating the allocation coefficient for each wind turbine in the offshore wind farm to undertake the additional damping task; decomposing the total damping power demand proportionally according to the amplitude of the damping power command and the allocation coefficient of each wind turbine to generate a damping power sub-command corresponding to each wind turbine; and adding the damping power sub-command corresponding to each wind turbine to its own active power reference command to generate an active power control command for each wind turbine.

2. The method for suppressing low-frequency oscillations in an offshore wind power grid-connected system via a synchronous generator, as described in claim 1, is characterized in that... The active power reference command is determined by the local controller of the wind turbine based on wind speed and maximum power point tracking strategy.

3. The method for suppressing low-frequency oscillations in an offshore wind power grid-connected system via a synchronous generator, as described in claim 2, is characterized in that... The inherent parameters include the rated capacity of the offshore wind farm, the transient electromotive force inside the synchronous generator, the rated voltage of the onshore power grid, the total equivalent reactance from the generator to the grid connection point, and the inertial time constant of the synchronous generator grid connection system.

4. The method for suppressing low-frequency oscillations in an offshore wind power grid-connected system via a synchronous generator, as described in claim 3, is characterized in that... The control parameters include damping torque coefficient, synchronous torque coefficient, filter time constant, lead time constant, lag time constant, and damping gain coefficient.

5. A method for suppressing low-frequency oscillations in an offshore wind power grid-connected system via a synchronous generator, as described in claim 4, is characterized in that... The actual operating frequency signal of the offshore low-frequency power grid is directly obtained locally by the offshore wind farm through the phase-locked loop at the converter grid connection point using the inherent magnetic field synchronization characteristics of the synchronous machine.

6. The method for suppressing low-frequency oscillations in an offshore wind power grid-connected system via a synchronous generator, as described in claim 1, is characterized in that... The allocation coefficient is determined based on the adjustment potential parameter, the system oscillation correlation parameter, and the operating stability coefficient. The regulation potential parameters include the remaining regulation capacity of the wind turbine and the maximum regulation rate of the converter. The system oscillation correlation parameters include the electrical distance correlation between the wind turbine access node and the system oscillation center and the frequency oscillation amplitude of the node; The operational stability coefficient is determined based on the active power fluctuation variance, speed fluctuation variance, and converter module temperature of the fan within a preset period.

7. A method for suppressing low-frequency oscillations in an offshore wind power grid-connected system via a synchronous generator, as described in claim 6, is characterized in that... The electrical distance correlation is determined based on the electrical impedance value between the wind turbine access node and the system oscillation center. The smaller the electrical impedance value, the higher the electrical distance correlation. The frequency oscillation amplitude is obtained by a frequency measurement unit deployed at the wind turbine access node.

8. A method for suppressing low-frequency oscillations in an offshore wind power grid-connected system via a synchronous generator, as described in claim 7, is characterized in that... When calculating the allocation coefficient, if the remaining regulation capacity of the fan is less than or equal to zero, or the operating stability coefficient is lower than the preset safety threshold, the allocation coefficient of the fan is set to zero and it does not participate in the damping task allocation.

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