Subsynchronous oscillation suppression method and system for wind power plant through flexible direct current grid connection

Abstract

The invention relates to a subsynchronous oscillation suppression method and system for a wind power plant through flexible direct current grid connection, and relates to the technical field of wind power generation, and the method comprises the steps: building an impedance model of a direct-driven wind power plant through a modularized multi-level converter type flexible direct current power transmission system, verifying the accuracy of the impedance model, and analyzing the influence of a delay factor on the direct current power transmission system; a linear active-disturbance-rejection control strategy is adopted to operate the direct-current power transmission system, and a delay compensation link is added to the linear active-disturbance-rejection control strategy; a linear active-disturbance-rejection control strategy added with a delay compensation link is adopted to estimate disturbance of the direct-current power transmission system, and a voltage compensation value is obtained through calculation and provided for a modulating wave. Therefore, by adding the delay compensation link, disturbance can be rapidly and accurately judged, error compensation is provided, and the system stability is effectively improved.

Description

Technical Field

[0001] The present disclosure relates to the technical field of wind power generation, and in particular to a method and system for suppressing subsynchronous oscillation of a wind farm connected to a grid via flexible direct current. Background Art

[0002] With the annual increase in wind power installed capacity, the problem of power transmission in large-capacity wind farms has become increasingly prominent. Flexible DC transmission systems have emerged as a key technology to solve this problem. However, due to environmental restrictions in some wind farms, wind farms need to be directly connected to flexible DC transmission systems. This interconnection method leads to complex interactions between wind farms and transmission systems, which can easily cause subsynchronous oscillations in the system, leading to wind turbines being disconnected from the grid. For example, when Nanhui Wind Farm was connected to the grid through modular multilevel converter high-voltage DC transmission, subsynchronous oscillations occurred, resulting in grid connection failure; a wind farm in Nan'ao experienced 20-30Hz oscillations when it was sent out through a flexible DC transmission system, forcing the system to shut down; Zhangbei's flexible DC transmission system also experienced oscillations many times during operation. The occurrence of subsynchronous oscillation accidents not only brings serious losses to wind farms, but also poses a threat to power grid security.

[0003] Large-scale renewable energy generation is transmitted through high-voltage direct current transmission systems. These power electronic devices pose challenges to the stability of the power system due to their nonlinear, periodic time-varying and frequency-coupled properties. Therefore, establishing an accurate mathematical model is a prerequisite for analyzing the stability of wind power flexible direct current grid-connected systems. At present, the impedance model method and the state space method are the main analysis methods for studying the stability of wind farm access to flexible direct current transmission. Some domestic and foreign scholars have studied the harmonic state space theory based on the state space theory, which linearizes the time-varying periodic system in the frequency domain and theoretically considers all harmonic orders. Compared with traditional state space models and impedance modeling, HSS (Harmonic State Space) can effectively handle multi-frequency coupling and is suitable for dynamic characteristics and stability analysis of multi-frequency systems. Some researchers have established a capacitor current feedback active damping LCL (Inductance-Capacitance-Inductance) grid-connected inverter model based on HSS to analyze the harmonic interaction between the grid-connected inverter and the power grid; some researchers have proposed a truncation number selection method for the HSS model based on the Floquet characteristic index theory of the linear time-periodic model, which has direct physical significance; some researchers have proposed a discrete harmonic state space modeling method for analyzing the small signal stability of a digitally controlled single-phase current source converter with an active power decoupling circuit. The model retains the switching details and has the potential to accurately describe high-frequency dynamics; some researchers have extended the original HSS framework based on ordinary differential equations to delayed differential equations, while considering MMC (Modular Multilevel Converter, modular multilevel converter), further analyzed the equation eigenvalues ​​and participation factors, and established an accurate frequency response model in a wide frequency range; some researchers established the state equation of MMC in the time domain considering the AC and DC side coupling, converted it to the harmonic domain through the harmonic state space method, and derived the step response analytical expression under its open-loop control, while being able to independently provide transient waveforms of each order of harmonics, and achieved large step simulation. Accurate models are conducive to finding out the key factors affecting system stability. Some researchers used the HSS model to explore the oscillation mechanism of wind turbines connected to the grid through MMC-type flexible direct current, and pointed out that both control delay and MMC control parameters will have a certain impact on the occurrence of oscillation phenomena.

[0004] Experts and scholars have studied the oscillation suppression of wind farms connected to the grid through MMC-type flexible direct current. Relevant literature has proposed methods to improve system stability from the perspectives of optimizing the control system architecture and parameters and proposing damping control. Modeling and analysis of the phase-locked loop and the current inner loop are the key to the oscillation between the doubly fed wind farm and the sending-end flexible direct current. Therefore, a damping control loop is added to the phase-locked loop of the wind turbine to achieve oscillation suppression; some researchers feed the current compensation signal into the modulation circuit, improve the control signal and suppress the impedance interaction between the MMC and the power line. At the same time, the analysis and verification of the proposed method effectively reduce the negative damping area in the frequency band where the SSO (Subsynchronous Oscillatio) is located; some researchers have proposed voltage feedforward additional damping controllers based on low-pass filters, band-stop filters and nonlinear filters, which improve the impedance characteristics of the MMC and have the effect of harmonic suppression. However, the above studies rarely consider the impact of control delay on system stability, and most existing studies do not take delay compensation as the main research direction for enhancing system stability. Summary of the invention

[0005] In order to solve the above technical problems, the present disclosure provides a method and system for suppressing subsynchronous oscillation of wind farms connected to the grid via flexible direct current (DC) circuits, which are used to suppress subsynchronous oscillation of wind farms connected to the grid via MMC flexible direct current (DC) circuits, while reducing the impact of control delay on the wind power system.

[0006] In a first aspect, the present disclosure provides a method for suppressing subsynchronous oscillations of a wind farm via a flexible direct current grid-connected to a power grid, comprising: establishing an impedance model of a direct-drive wind farm via a modular multilevel converter-type flexible direct current transmission system, verifying the accuracy of the impedance model, and analyzing the influence of delay factors on the direct current transmission system based on the impedance model; operating the direct current transmission system using a linear active disturbance rejection control strategy, and adding a delay compensation link to the linear active disturbance rejection control strategy; estimating the disturbance of the direct current transmission system using the linear active disturbance rejection control strategy with the delay compensation link added, calculating a voltage compensation value and providing it to a modulation wave.

[0007] Optionally, the establishing of the impedance model of the direct-drive wind farm through the modular multilevel converter type flexible DC transmission system includes: establishing a wind farm side receiving end converter port impedance model based on harmonic state space theory, and establishing a virtual synchronous type direct-drive wind turbine impedance model.

[0008] Optionally, the port impedance of the receiving-end converter on the wind farm side is:

[0009]

[0010] Among them, ω p is the disturbance frequency, U Sis the AC voltage at the converter port at the wind farm side, I S It is the AC power supply for the converter port at the receiving end on the wind farm side.

[0011] Optionally, the AC port impedance of the grid-side inverter corresponding to the virtual synchronous direct-drive wind turbine group is:

[0012]

[0013] Among them, U g is the AC voltage of the AC port of the grid-side inverter, I g is the AC power supply of the AC port of the grid-side inverter, ω p is the disturbance frequency.

[0014] Optionally, operating the DC power transmission system using a linear active disturbance rejection control strategy includes: implementing the linear active disturbance rejection control strategy on a current inner loop of the modular multilevel converter.

[0015] Optionally, the linear active disturbance rejection control strategy includes: comparing a tracking signal output by a tracking differentiator with an output signal of a linear extended state observer to generate an error signal, and performing disturbance estimation using the linear extended state observer;

[0016] The error signal is processed by a linear error feedback controller, and the disturbance estimated by the linear extended state observer is used to compensate the error signal, so as to finally generate a control signal acting on the controlled object.

[0017] Optionally, adding a delay compensation link to the linear active disturbance rejection control strategy includes: introducing the delay compensation link before the control signal is fed back to the linear extended state observer.

[0018] Optionally, after adding the delay compensation link to the linear active disturbance rejection control strategy, the designed linear extended state observer is:

[0019]

[0020] Among them, Z n (n=1, 2, 3) is the observation value of the observer, β n is the observer gain, E ue is the voltage compensation value calculated by the controller according to the disturbance, α is the compensation coefficient, T d is the system delay, y is the output signal, u is the input signal, and b0 is the gain parameter.

[0021] Optionally, the compensation coefficient α is 3.

[0022] In a second aspect, the present disclosure provides a subsynchronous oscillation suppression system for a wind farm connected to the grid via a flexible direct current connection, which operates using the subsynchronous oscillation suppression method for a wind farm connected to the grid via a flexible direct current connection as described in the first aspect.

[0023] The technical solution provided by the embodiments of the present disclosure has the following advantages over the prior art: when a direct-drive wind farm is connected to the grid via a flexible direct current transmission system, the interaction between the two is likely to cause subsynchronous oscillations, and the control delay will aggravate the oscillation situation and lead to serious power grid accidents. The present disclosure provides a method and system for suppressing subsynchronous oscillations of a wind farm connected to the grid via a flexible direct current. The disturbance value generated by the interconnected system is regarded as the disturbance of the system, and the disturbance is observed using a linear extended state observer, and the voltage compensation value of the system is calculated to reduce the impact of the disturbance on the stability of the system. At the same time, time delay compensation is added to further eliminate the impact caused by the control delay and improve the robustness of the system. A simulation model is established on the MATLAB / Simulink platform, and the simulation verifies that the proposed control method effectively suppresses subsynchronous oscillations and improves the dynamic performance and anti-interference ability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present disclosure and, together with the description, serve to explain the principles of the present disclosure.

[0025] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0026] Figure 1 The figure shows a topological structure and control strategy diagram of a direct-drive wind farm transmission system via MMC-HVDC provided by an embodiment of the present disclosure;

[0027] Figure 2 The figure is a flowchart of the steps of a method for suppressing subsynchronous oscillation of a wind farm via flexible direct current grid connection provided by an embodiment of the present disclosure;

[0028] Figure 3 The figure shows a schematic diagram of a topological structure of an MMC receiving-end converter station provided by an embodiment of the present disclosure;

[0029] Figure 4 The figure shows a schematic diagram of frequency sweep verification of an MMC impedance model provided by an embodiment of the present disclosure;

[0030] Figure 5 Shown is a schematic diagram of a GSC equivalent impedance simulation verification provided by an embodiment of the present disclosure;

[0031] Figure 6 Another GSC equivalent impedance simulation verification schematic diagram provided by an embodiment of the present disclosure is shown;

[0032] Figure 7 Shown is a schematic diagram of an improved MMC converter station control strategy provided by an embodiment of the present disclosure;

[0033] Figure 8 FIG. 1 is a schematic diagram of LADRC additional damping control considering delay compensation provided by an embodiment of the present disclosure;

[0034] Fig. 9 It is a schematic diagram showing the influence of a compensation coefficient on system stability provided by an embodiment of the present disclosure;

[0035] Fig.10 Shown is a schematic diagram of AC side voltage and current waveforms under a subsynchronous oscillation condition provided by an embodiment of the present disclosure;

[0036] Fig.11 Shown is a schematic diagram of an active power oscillation waveform with / without delay provided by an embodiment of the present disclosure;

[0037] Fig.12 Shown is an active power harmonic analysis diagram with and without delay provided by an embodiment of the present disclosure;

[0038] Fig.13 Shown is a schematic diagram of a power response curve before and after an improvement of a LADRC control strategy provided by an embodiment of the present disclosure;

[0039] Fig.14 Shown is a schematic diagram of a power waveform during an active power step provided by an embodiment of the present disclosure. DETAILED DESCRIPTION

[0040] In order to more clearly understand the above-mentioned objectives, features and advantages of the present disclosure, the scheme of the present disclosure will be further described below. It should be noted that the embodiments of the present disclosure and the features in the embodiments can be combined with each other without conflict.

[0041] In the following description, many specific details are set forth to facilitate a full understanding of the present disclosure, but the present disclosure may also be implemented in other ways different from those described herein; it is obvious that the embodiments in the specification are only part of the embodiments of the present disclosure, rather than all of the embodiments.

[0042] The direct-drive wind farm MMC-type flexible direct current transmission system consists of three parts: direct-drive wind farm, MMC-HVDC (Modular Multilevel Converter-High Voltage Direct Current, a high-voltage direct current system based on modular multilevel converters) and the interconnection system between the two. The system topology and its control strategy are as follows: Figure 1 shown.

[0043] The direct-drive wind turbine is composed of permanent magnet synchronous generator, machine-side converter (MSC), grid-side converter (GSC) and DC bus capacitor. Existing studies have shown that when subsynchronous oscillation occurs in the wind power grid-connected system, the wind farm as a whole dynamically interacts with other units in the system. Therefore, in order to simplify the analysis and calculation process, the embodiment of the present disclosure adopts a single-machine aggregation equivalent model to represent the direct-drive wind farm.

[0044] Figure 1 The figure shows a topological structure and control strategy diagram of a direct-drive wind farm transmission system via MMC-HVDC provided by an embodiment of the present disclosure. Please refer to Figure 1 , Figure 1 Some system variables in PMSG control are explained as follows: dc is the DC bus voltage, i gd 、i gq 、u cd 、u cq 、i cd 、i Lq 、i Ld 、i cq They are the dq axis components of the motor output current, GSC AC side voltage, current and filter inductor current respectively. For economic and functional reasons, the interconnection between the wind farm and the MMC flexible direct current transmission system adopts a 66kV voltage level collector line. After the large-capacity wind turbine is boosted to 66kV by the transformer, it will be collected on the collector line, and then boosted to 220kV by the transformer and connected to the MMC receiving-end converter station through the AC transmission line. MMC-HVDC is divided into the wind farm side receiving-end converter station (WFMMC) and the grid side sending-end converter station (GSMMC). Because it is necessary to provide voltage support for the wind farm, the receiving-end converter station adopts a V / f control strategy. Figure 1 Some variables in PMSG (Directly Driven Permanent Magnetic Synchronous Generator) control are explained as follows: xu ,I xp(x=a,b,c) are the three-phase upper and lower bridge arm circulation flow respectively, U sd , U sq ,I sd ,I sq are the dq axis components of the AC side voltage and current, U sdref is the AC voltage amplitude, U sqref =0, θ=ω1*t, ω1 is the fundamental angular frequency, and h is the harmonic order.

[0045] When a direct-drive wind farm is connected to the grid via a flexible DC transmission system, the interaction between the two can easily lead to subsynchronous oscillations, and control delays will aggravate the oscillations and cause serious grid accidents. Figure 2 The figure is a flowchart of a method for suppressing subsynchronous oscillation of a wind farm connected to the grid via flexible direct current provided by an embodiment of the present disclosure. Please refer to Figure 2 The present disclosure provides a method for suppressing subsynchronous oscillation of a wind farm connected to a flexible direct current grid, which is applied to the direct-drive wind farm via the MMC type flexible direct current transmission system as described above, including: Step S1: Establishing an impedance model of the direct-drive wind farm via the modular multilevel converter type flexible direct current transmission system, verifying the accuracy of the impedance model, and analyzing the influence of the delay factor on the direct current transmission system according to the impedance model; Optionally, the verification method includes but is not limited to sweep frequency verification, which can be used to verify the consistency of the above impedance model with the theory. The present disclosure does not specifically limit the verification method, which shall be subject to actual needs. Step S2: Adopting a linear active disturbance rejection control strategy to operate the direct current transmission system, and adding a delay compensation link to the linear active disturbance rejection control strategy. Optionally, adding a delay compensation link to the linear active disturbance rejection control (LADRC) strategy, and further analyzing the influence of the compensation link on the system stability. Step S3: Adopting a linear active disturbance rejection control strategy with a delay compensation link to estimate the disturbance of the direct current transmission system, calculating a voltage compensation value and providing it to the modulation wave. Optionally, when oscillation occurs, a linear anti-disturbance control strategy with a delay compensation link is used to estimate the system disturbance, calculate the voltage compensation value and provide it to the modulation wave to achieve the purpose of suppressing subsynchronous oscillation. In this way, by establishing an impedance model and adding a delay compensation link to the linear anti-disturbance control strategy, the subsynchronous oscillation phenomenon of the system can be effectively suppressed, and the system can be quickly restored to a stable operating state. The improved control suppression effect of adding a delay compensation link is better, and the system can be compensated more accurately and the power fluctuation amplitude can be smaller.

[0046] Please continue to refer to Figure 2In an optional embodiment provided in the present disclosure, step S1: establishing an impedance model of a direct-drive wind farm through a modular multi-level converter-type flexible direct current transmission system includes: establishing an impedance model of the converter port on the receiving end of the wind farm side based on the harmonic state space theory; and establishing an impedance model of a virtual synchronous direct-drive wind turbine group.

[0047] Figure 3 The figure shows a schematic diagram of the topological structure of an MMC receiving-end converter station provided by an embodiment of the present disclosure. Figure 3 In an optional embodiment provided in the present disclosure, the modular multilevel converter includes a wind farm side receiving end converter.

[0048] Specifically, the typical MMC type flexible DC transmission system converter station topology is as follows: Figure 3 As shown, each bridge arm contains N submodules, the submodule structure is a half-bridge type, L0 is the bridge arm inductance, and R0 is the bridge arm equivalent resistance. The embodiment of the present disclosure establishes an impedance model taking into account the internal dynamics of the MMC based on the harmonic state space method. Since the three-phase impedance of the MMC is equal, taking phase a as an example, the main circuit linear model of the MMC can be expressed as:

[0049]

[0050] Where: Δi c , Δi s , Δv cu , Δv cl are the sum of the MMC circulating current, the AC voltage and current, and the upper and lower bridge arm submodule capacitor voltages. The subscripts with s are all steady-state operating points.

[0051] The small perturbation equation of the modulation signal is:

[0052]

[0053] Where Δm f is the three-phase fundamental frequency modulation voltage, Δm 2f It is a 2-fold frequency modulation voltage.

[0054] The periodic time-varying signal is expanded by Fourier series as shown in formula (3):

[0055]

[0056] Where, X k is the Fourier coefficient of the kth harmonic.

[0057] Combining equation (3) with the linear state space equation of MMC, we can obtain:

[0058]

[0059] After Laplace transforming the above equation, the harmonic state space model can be obtained as follows:

[0060] X M =-(A M -N M ) -1 B M ·U M (5)

[0061] Where A M , B M is a Toeplitz matrix, such as A M The expression is:

[0062]

[0063] A M , B M is the internal variable matrix.

[0064] N M It is defined as:

[0065] N M =diag[j(ω p -hω1)I,…,O…,j(ω p +hω1)I] (7)

[0066] Where O is the zero matrix and I is the identity matrix. p is the disturbance frequency.

[0067] We can get ω p The port impedance of the converter at the receiving end of the wind farm side at the frequency:

[0068]

[0069] Among them, ω p is the disturbance frequency, U S is the AC voltage at the converter port at the wind farm side, I S It is the AC power supply for the converter port at the receiving end on the wind farm side.

[0070] Figure 4 The figure shows a schematic diagram of a frequency sweep verification of an MMC impedance model provided by an embodiment of the present disclosure. Please refer to Figure 4 To verify the accuracy of the MMC model, a detailed MMC model was built in the simulation software, and the simulation parameters are shown in Table 1.

[0071] Table 1 System parameters

[0072] Table 1

[0073]

[0074] The model is verified by frequency sweep. The frequency sweep result is used as a benchmark and compared with the calculated MMC model impedance characteristics. The comparison results are as follows: Figure 4 As shown in the figure, the impedance characteristics of the MMC model are basically consistent with the swept frequency impedance characteristics in the software modeling, which verifies the accuracy of the model.

[0075] In an optional embodiment provided by the present disclosure, a harmonic state space model is also established for the wind turbine set, and the state variables and input variables of the grid-side converter of the VSG type wind turbine are:

[0076]

[0077] Where: E is the VSG potential, δ is the VSG power angle, ω g is the VSG angular frequency, e d 、e q , S d , S q They are the dq axis components of the converter modulation wave and the switching function respectively.

[0078] The state space equation of the HSS model is:

[0079] X G =(A G -N G )X G +B G U(10)

[0080] The theoretical basis is the same as that of MMC modeling and will not be elaborated here.

[0081] Similarly, the AC port impedance of the grid-side inverter corresponding to the virtual synchronous direct-drive wind turbine group can be obtained as:

[0082]

[0083] Among them, U g is the AC voltage of the AC port of the grid-side inverter, I g is the AC power supply of the AC port of the grid-side inverter, ω p is the disturbance frequency.

[0084] Figure 5 The figure shows a GSC equivalent impedance simulation verification schematic diagram provided by the embodiment of the present disclosure. Please refer to Figure 5 To verify the accuracy of the wind turbine impedance model, a detailed wind turbine impedance model was built in the simulation software. The simulation parameters are shown in Table 1. The model was verified by frequency sweep. The frequency sweep results were used as a benchmark to compare the impedance characteristics of the wind turbine impedance model calculated. The comparison results are shown in Table 1. Figure 5As shown in the figure, the impedance characteristics of the wind turbine impedance model are basically consistent with the swept frequency impedance characteristics in the software modeling, which verifies the accuracy of the model.

[0085] In this way, by establishing a wind farm side receiving end converter port impedance model based on the harmonic state space theory, it can be used to simulate the wind farm side receiving end converter port impedance characteristics in the system; by establishing a virtual synchronous direct-drive wind turbine impedance model, it can be used to simulate the AC port impedance characteristics of the VGS type wind turbine grid-side converter; the impact of subsynchronous oscillations on the two can be analyzed from a mathematical theory perspective, and the physical model built on the simulation platform can be verified to be consistent with the theory through frequency sweeping, proving the accuracy of model construction, and providing a theoretical basis for further analysis of the impact of delay changes on system stability.

[0086] Figure 6 FIG. 1 is another schematic diagram of GSC equivalent impedance simulation verification provided by an embodiment of the present disclosure. Please refer to FIG. Figure 6 Due to the characteristics of the flexible DC transmission system, the MMC flexible DC transmission system has a large delay, which is about 200 to 800 microseconds in actual engineering projects. Therefore, it is necessary to explore the impact of delay parameter changes on system stability. Figure 5 From the analysis results, it can be observed that as the system delay parameter increases, the system steady-state error increases, resulting in a decrease in system stability. This shows that the delay of the MMC system has a certain impact on the system stability, and the risk of system instability oscillation increases with the increase of the delay parameter. Therefore, reducing the system delay will help improve the stability of the system.

[0087] Figure 7 FIG. 1 is a schematic diagram of an improved MMC converter station control strategy provided by an embodiment of the present disclosure. Please refer to FIG. Figure 7 In an optional embodiment provided in the present disclosure, a linear active disturbance rejection control strategy is adopted to operate a DC transmission system, including: implementing a linear active disturbance rejection control strategy on a current inner loop of a modular multilevel converter.

[0088] Specifically, based on the above analysis, a delay compensation control strategy is proposed to solve the problem that the MMC control parameters affect the subsynchronous oscillation: an additional damping controller based on the improved LADRC is added to the current inner loop of the MMC control strategy. In this way, by adding the delay compensation link, LADRC can compensate the system more accurately and suppress the oscillation.

[0089] Figure 8 FIG. 1 is a schematic diagram of LADRC additional damping control considering delay compensation provided by an embodiment of the present disclosure. Please refer to FIG. Figure 8 In an optional embodiment provided in the present disclosure, the linear active disturbance rejection LADRC control strategy includes:

[0090] Comparing the tracking signal output by the tracking differentiator TD with the output signal of the linear extended state observer LESO to generate an error signal, and using the linear extended state observer LESO to perform disturbance estimation;

[0091] The error signal is processed by a linear error feedback controller LSEF, and the disturbance estimated by the linear extended state observer LESO is used to compensate the error signal, and finally a control signal acting on the controlled object is generated. Optionally, the tracking differentiator TD also outputs the differential of the tracking signal, and the differential refers to the rate of change of the tracking signal output by the tracking differentiator TD, that is, the speed of change of the signal over time, which can help the system respond to changes faster.

[0092] Specifically, the advantage of LADRC is that even without precise mathematical modeling, it can estimate the system disturbance in real time based on its own characteristics and feed back a compensation value calculated by the disturbance to the system. The LADRC control strategy consists of three parts: tracking differentiator TD, linear extended state observer LESO, and linear error feedback LSEF.

[0093] The second-order controlled object can be expressed as:

[0094]

[0095] f(t) is the total disturbance of the system. b0 is the control parameter. u is the system control quantity. Convert the above formula into state space form:

[0096]

[0097] Therefore, LESO can be derived as:

[0098]

[0099] In the formula, Z n (n=1, 2, 3) is the observation value of the observer, β n is the observer gain. After selecting the appropriate observer gain, the observed value gradually converges to the actual value, and the purpose of real-time tracking of each state variable can be achieved. The system is compensated by the disturbance compensation link. The designed control rate is:

[0100]

[0101] LSEF is designed to:

[0102] u0=k1(r1-z1)-k2z2(16)

[0103] The LADRC control parameters are:

[0104]

[0105] ω o and ω F are the bandwidths of LESO and LSEF respectively, and k1 and k2 are the parameter gains in LSEF.

[0106] Therefore, it can be concluded that the LADRC structure is relatively simple and only needs to set three parameters: b0, ω o and ω F b0 can reflect the target characteristics, which is determined by the characteristics of the control system and can be obtained by analyzing the behavior of the system in the step response. o Affects the frequency range of LESO and increases ω o It helps to observe disturbances more accurately, thus improving control performance. F Proportional to the system reaction speed, ω F The larger it is, the faster the dynamic process is.

[0107] Due to the operating characteristics of MMC, the overall system delay needs to be considered in the sampling, calculation and transmission of MMC signals. d It is approximately equivalent to a first-order inertia link, that is:

[0108]

[0109] In order to improve the control accuracy of LADRC, in an optional embodiment provided by the present disclosure, adding a delay compensation link in the linear active disturbance rejection control includes: introducing a delay compensation link before the control signal u is fed back to the linear extended state observer LESO. This ensures that the output signal y is consistent with the input signal u in the time dimension, thereby optimizing the performance of the LADRC controller in the system and reducing the impact of the delay on the stability of the system.

[0110] Please continue to refer to Figure 8 , add a delay compensation link at the point where the signal is fed back to LSEO to improve the control accuracy of the LADRC control strategy in the time-delay system. α is the delay compensation coefficient. In an optional embodiment provided by the present disclosure, after adding the delay compensation link to the linear active disturbance rejection control strategy, the designed linear extended state observer LESO is:

[0111]

[0112] Among them, Z n (n=1, 2, 3) are the observed values ​​of the linear extended state observer LESO, β n is the observer gain, E ue is the voltage compensation calculated by the controller according to the disturbance, α is the compensation coefficient, T dis the system delay, y is the output signal, u is the input signal, and b0 is the gain parameter.

[0113] The improved LADRC transfer function can be expressed as:

[0114]

[0115] Fig. 9 FIG. 1 is a schematic diagram showing the effect of a compensation coefficient on system stability provided by an embodiment of the present disclosure. Please refer to FIG. Fig. 9 In an optional embodiment provided in the present disclosure, the compensation coefficient α is 3.

[0116] Specifically, in order to verify the impact of delay compensation on system stability, a quantitative analysis is performed on the compensation coefficient α. When the value of α is larger, the system phase margin decreases, which will have a certain impact on system stability. When α=1, it is equivalent to not adding delay compensation. Therefore, it is necessary to select the compensation coefficient within a reasonable range. In the embodiment of the present disclosure, α=3 is selected in combination with the stability analysis of the control strategy.

[0117] The present disclosure also provides a subsynchronous oscillation suppression system for a wind farm connected to the grid via flexible direct current. The system operates using the subsynchronous oscillation suppression method for a wind farm connected to the grid via flexible direct current as described above.

[0118] Simulation Verification

[0119] The simulation waveform is verified under the oscillation condition. On the MATLAB / Simulink platform, a direct-drive wind farm grid-connected model proposed in the embodiment of the present disclosure is built through the MMC-type flexible direct current transmission system. The model operates using the subsynchronous oscillation suppression method of the wind farm through the flexible direct current grid-connected proposed in the present disclosure. The rated capacity of the wind farm is 650MW. Existing accident analysis and research have shown that subsynchronous oscillations are prone to occur in wind farms when the power rises in the initial stage of grid connection. Fig.10 FIG. 1 is a schematic diagram of the voltage and current waveforms of the AC side under a subsynchronous oscillation condition provided by an embodiment of the present disclosure. Please refer to FIG. Fig.10 , the wind speed of the wind farm is set to 7m / s at the initial moment, and the wind speed gradually increases from 0.8s to 12m / s, and the series compensation capacitor is put into use at 1.5s to produce a sub-synchronous oscillation condition. At this time, the voltage and current changes on the AC side of the MMC are as follows Fig. 9 shown.

[0120] Fig.11 FIG. 1 is a schematic diagram of an active power oscillation waveform with or without delay provided in an embodiment of the present disclosure. Please refer to FIG. Fig.11 To further verify the effect of delay on system stability, the MMC control delay T is set. d =400μs, compare the active power waveforms when subsynchronous oscillation occurs with and without delay.

[0121] according to Fig.11 The active power waveforms with and without delay are obtained. When the oscillation occurs, the delay has little effect on the transient state when the oscillation just occurs. However, when the oscillation gradually diverges, it can be clearly seen that the power oscillation amplitude after adding the control delay is much larger than the power oscillation without adding the delay. Fig.12 The figure shows an active power harmonic analysis diagram with and without delay provided by an embodiment of the present disclosure. Please refer to Fig.12 , it can be further analyzed that, taking the power waveform in the same band for harmonic analysis, the harmonic content of the subsynchronous oscillation frequency after adding the control delay is greater than the harmonic content without delay. The simulation further verifies that the control delay will affect the stability of the system.

[0122] Suppress validity verification. Fig.13 FIG. 1 is a schematic diagram of a power response curve before and after the improvement of a LADRC control strategy provided by an embodiment of the present disclosure. Please refer to FIG. Fig.13 , comparing the two oscillation suppression strategies of LADRC without delay compensation and LADRC with delay compensation, Fig.13 It can be clearly seen that both oscillation suppression strategies can effectively suppress the subsynchronous oscillation phenomenon of the system, so that the system can quickly recover to a stable operating state. The improved control suppression effect with the delay compensation link is significantly better than the LADRC control, with a smaller power fluctuation amplitude and a smoother waveform.

[0123] In order to further verify the effectiveness of the strategy, Fig.14 FIG. 1 is a schematic diagram of a power waveform of an active power step provided by an embodiment of the present disclosure. Please refer to FIG. Fig.14 The embodiment of the present disclosure sets the second working condition, that is, the wind farm output active power is stable at 400MW in the early stage, and then suddenly increases to the rated power of 650MW at 1.5s, and other conditions remain unchanged. At this time, the system simulation waveform is as follows: Fig.14 As shown. Fig.14 It can be seen that without adding the suppression strategy, Fig.10 Compared with the power oscillation waveform in the example, the oscillation degree of the waveform during the power step will be further aggravated, and the oscillation amplitude will increase. When the suppression strategy is added, the system can also quickly recover to a stable operating state, and the suppression effect of the improved LADRC is still optimal. The simulation results show that the control strategy proposed in the embodiment of the present disclosure significantly enhances the robustness of the system.

[0124] Thus, compared with LADRC, the improved control strategy considering delay compensation can provide more accurate compensation for disturbances. The simulation waveform after the improved LADRC control shows that the amplitude of power oscillation during system operation is significantly reduced, and the system convergence speed is also improved. In addition, the improved control strategy does not require strategy switching, making the entire control process smoother and more stable. Under the stable operation state of the system, the improved control strategy maintains basic consistency in power characteristics compared with the traditional control strategy, ensuring efficient and reliable operation of the system.

[0125] In summary, the present disclosure provides a method and system for suppressing subsynchronous oscillations of a wind farm connected to the grid via flexible direct current, which effectively improves system stability and reduces the risk of system oscillations. By establishing an impedance model of a direct-drive wind farm via an MMC-type flexible direct current transmission system, the accuracy of the model is verified by frequency sweeping, and stability analysis is used to determine that the MMC control delay is one of the factors affecting system stability. An additional damping controller based on an improved LADRC is added to the current inner loop of the MMC control strategy. The addition of a delay compensation link allows LADRC to compensate the system more accurately to suppress oscillations. Simulation tests were carried out on the MATLAB / Simulink platform, and the results verified the effectiveness and feasibility of the proposed control strategy, and were able to quickly and accurately identify disturbances and provide error compensation, so that the system can quickly return to a stable state, and the system has good robustness.

[0126] The above description is only a specific embodiment of the present disclosure, so that those skilled in the art can understand or implement the present disclosure. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined in the embodiments of the present disclosure can be implemented in other embodiments without departing from the spirit or scope of the present disclosure. Therefore, the present disclosure will not be limited to these embodiments described in the embodiments of the present disclosure, but will conform to the widest range consistent with the principles and novel features disclosed in the embodiments of the present disclosure.

Claims

1. A method for suppressing subsynchronous oscillation of a wind farm connected to the grid via flexible direct current, characterized in that: include: Establish an impedance model of a direct-drive wind farm through a modular multi-level converter type flexible direct current transmission system, verify the accuracy of the impedance model, and analyze the impact of delay factors on the direct current transmission system based on the impedance model; Adopting a linear active disturbance rejection control strategy to operate the DC power transmission system, and adding a delay compensation link to the linear active disturbance rejection control strategy; The linear anti-disturbance control strategy with the delay compensation link is adopted to estimate the disturbance of the DC power transmission system, and the voltage compensation value is calculated and provided to the modulation wave.

2. The method for suppressing subsynchronous oscillation of a wind farm via flexible direct current grid connection according to claim 1, characterized in that: The impedance model of the direct-drive wind farm through the modular multi-level converter type flexible direct current transmission system is established, which includes: A converter port impedance model for the receiving end of the wind farm is established based on the harmonic state space theory, and an impedance model of a virtual synchronous direct-drive wind turbine is established.

3. The method for suppressing subsynchronous oscillation of a wind farm via flexible direct current grid connection according to claim 2, characterized in that: The port impedance of the receiving converter on the wind farm side is: Among them, ω p is the disturbance frequency, U S is the AC voltage at the converter port at the wind farm side, I S It is the AC power supply for the converter port at the receiving end on the wind farm side.

4. The method for suppressing subsynchronous oscillation of a wind farm via flexible direct current grid connection according to claim 2, characterized in that: The AC port impedance of the grid-side inverter corresponding to the virtual synchronous direct-drive wind turbine set is: Among them, U g is the AC voltage of the AC port of the grid-side inverter, I g is the AC power supply of the AC port of the grid-side inverter, ω p is the disturbance frequency.

5. The method for suppressing subsynchronous oscillation of a wind farm via flexible direct current grid connection according to claim 1, characterized in that: The adopting of a linear active disturbance rejection control strategy to operate the DC power transmission system comprises: implementing the linear active disturbance rejection control strategy on a current inner loop of the modular multilevel converter.

6. The method for suppressing subsynchronous oscillation of a wind farm via flexible direct current grid connection according to claim 1, characterized in that: The linear active disturbance rejection control strategy includes: Comparing the tracking signal output by the tracking differentiator with the output signal of the linear extended state observer to generate an error signal, and using the linear extended state observer to perform disturbance estimation; The error signal is processed by a linear error feedback controller, and the disturbance estimated by the linear extended state observer is used to compensate the error signal, so as to finally generate a control signal acting on the controlled object.

7. The method for suppressing subsynchronous oscillation of a wind farm via flexible direct current grid connection according to claim 1, characterized in that: The adding of a delay compensation link into the linear active disturbance rejection control strategy comprises: introducing the delay compensation link before the control signal is fed back to the linear extended state observer.

8. The method for suppressing subsynchronous oscillation of a wind farm via flexible direct current grid connection according to claim 7, characterized in that: After adding the delay compensation link into the linear active disturbance rejection control strategy, the designed linear extended state observer is: Among them, Z n (n=1, 2, 3) is the observation value of the observer, β n is the observer gain, E ue is the voltage compensation value calculated by the controller according to the disturbance, α is the compensation coefficient, T d is the system delay, y is the output signal, u is the input signal, and b0 is the gain parameter.

9. The method for suppressing subsynchronous oscillation of a wind farm via flexible direct current grid connection according to claim 8, characterized in that: The compensation coefficient α is 3.

10. A subsynchronous oscillation suppression system for a wind farm connected to the grid via flexible direct current, characterized in that: The wind farm is operated by adopting the subsynchronous oscillation suppression method of flexible direct current grid connection as described in any one of claims 1 to 9.

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