A method for suppressing subsynchronous oscillations in a doubly-fed wind farm based on linear active disturbance rejection control

Abstract

The present invention provides a method for suppressing subsynchronous oscillations in a doubly-fed wind farm based on linear active disturbance rejection control (LADRC). The present invention relates to the technical field of safe and stable control of power systems. The present invention performs subsynchronous oscillation suppression in the doubly-fed wind farm by initializing grid parameters and unit parameters; tracking the system with an extended state observer for subsynchronous disturbances; performing feedback control based on state errors; and superimposing the disturbance on the rotor-side current loop. The LESO of the present invention performs real-time estimation and feedback compensation of the system's subsynchronous disturbances. A high-frequency filter is then added to the LESF to suppress low-frequency oscillations. The designed HPF-LADRC does not require complex parameter adjustments and has good anti-interference capabilities against changes in system parameters. Finally, impedance analysis and time-domain simulations verify the effectiveness of the HPF-LADRC in suppressing subsynchronous oscillations.

Description

Technical Field

[0001] The present invention relates to the technical field of power system safety and stability control, and in particular to a method for suppressing subsynchronous oscillations of a doubly-fed wind farm based on linear active disturbance rejection control. Background Art

[0002] Doubly-fed wind farms connected to the grid via series-compensated lines are prone to subsynchronous oscillations (SSO). This type of SSO is primarily caused by the coupling between the doubly-fed wind turbine converter and the series-compensated lines. It does not involve torsional vibration of the turbine shaft and is therefore known as subsynchronous control interaction (SSCI). If left uncontrolled, these oscillations can lead to wind turbine disconnection, equipment damage, and other power quality issues, seriously threatening the safe and stable operation of the power system.

[0003] There are relevant literatures on SSCI. The existing control strategies for suppressing SSCI can be roughly divided into wind turbine side control and wind farm side control. Wind farm side control mainly uses additional damping control of flexible AC transmission equipment to suppress oscillations. Literature - Wang Yuzhi, Wang Liang, Jiang Qirong. SSCI additional damping suppression strategy for wind farms based on STATCOM uses a static VAR compensator based on linear active disturbance rejection control to suppress subsynchronous oscillations in weak AC wind power systems. Literature - Rajaram T, Reddy JM, Xu Y. Kalman filter based detection and mitigation of subsynchronous resonance with SSSC [J]. IEEE Transactions on Power Systems, Based on the filter link, gain link and phase compensation link, a damping controller based on the static synchronous VAR compensator (STATCOM) is designed to enable the STATCOM to provide positive damping in a wide frequency band. References: Moharana A, Varma RK, Seethapathy R. SSRalleviation by STATCOM in induction-generator-based wind farm connected to series compensated line. A Kalman damping controller is used to suppress subsynchronous oscillations in a static synchronous series compensator (SSSC). The design of the Kalman damping controller is based on the magnitude of the damping torque within the torsional modal frequency range, and a genetic algorithm is used to optimize the controller parameters. References: Ling J, Ji H, Yu C, et al. Analysis and Design of UPFC Sub-Synchronous Oscillation Controller. A subsynchronous oscillation damping controller for a unified power flow controller (UPFC) based on modal control is proposed. This controller suppresses subsynchronous oscillations while effectively controlling the power flow. However, flexible AC transmission equipment is expensive, and adding additional flexible AC transmission equipment is not an economical solution for wind farms.

[0004] Wind turbine control mainly suppresses SSO by adjusting parameters or improving control of traditional wind turbine converters, including optimizing converter parameters and improving converter control strategies. SSCI is closely related to the proportional-integral (PI) controller of the rotor-side converter (RSC) current loop. When a disturbance occurs, the stator resonant current induces a corresponding subsynchronous current in the doubly fed generator rotor. The output voltage is then adjusted by the PI controller of the RSC current loop. However, the adjusted output voltage increases the subsynchronous current, amplifies the disturbance, and leads to SSCI. Therefore, some suppression measures have been proposed by tuning the parameters of the PI controller of the doubly fed wind turbine converter. Reference - Wang L, Xie X, Jiang Q, et al. Investigation of SSR in practical DFIG-based wind farms connected to a series-compensated power system proposed a damping method to reduce the proportional gain of the RSC current loop. To suppress SSCI, Chen A, Xie D, Zhang D, et al. (PI parameter tuning of converters for sub-synchronous interactions existing in grid-connected DFIG wind turbines) utilized a non-dominated sorting genetic algorithm to optimize the converter PI parameters. However, adjusting control parameters can degrade the original control performance and has limited suppression capabilities. To overcome these difficulties, a sub-synchronous damping controller based on a bandpass filter was applied to the converter's current loop to suppress SSCI. This suppression signal was introduced into the rotor-side converter's inner control loop or the grid-side converter's power outer control loop, thereby suppressing SSCI on the wind turbine side. However, the oscillation characteristics of SSCI are determined by the wind turbine control system and transmission line parameters, and its oscillation frequency is not fixed. When system operating conditions change, the SSCI oscillation frequency shifts, and the parameters of the bandpass filter and phase shifter must be reset. Furthermore, due to the complexity of design and implementation, the use of complex advanced nonlinear control is not feasible. Summary of the Invention

[0005] This paper introduces a rotor-side circulator to replace the PI controller through linear active disturbance rejection control (LADRC). The linear extended state observer (LESO) in the LADRC is used to estimate and provide feedback on the SSO generated by the wind power system. A high-pass filter is then introduced into the linear state error feedback (LSEF) in the LADRC to suppress low-frequency oscillations. The designed controller, HPF-LADRC, improves the system's oscillation frequency damping while enhancing the system's anti-disturbance capability.

[0006] The present invention provides a method for suppressing subsynchronous oscillations of a doubly-fed wind farm based on linear active disturbance rejection control. The specific scheme is as follows:

[0007] A method for suppressing subsynchronous oscillations of a doubly-fed wind farm based on linear active disturbance rejection control comprises the following steps:

[0008] Step 1: Initialize the grid parameters and unit parameters;

[0009] Step 2: The extended state observer tracks the system and performs subsynchronous perturbations;

[0010] Step 3: Perform feedback control based on state error;

[0011] Step 4: The disturbance is superimposed on the rotor side current loop to suppress the synchronous oscillation of the doubly fed wind farm.

[0012] Preferably, the step 2 is specifically as follows:

[0013] Determine the transfer function of the high-pass filter and express it by the following formula:

[0014]

[0015] Where ξ is the damping coefficient, ω n =2πf c , is the filter angular frequency, andH0 represents the gain;

[0016] Where LESO is shown in formula (2), and LESO is expressed by the following formula:

[0017]

[0018] Where ^ represents the estimated value, h1 and h2 are the gains of LESO;

[0019] Determine the relationship between the observer bandwidth and the observer gain, which can be expressed as follows:

[0020]

[0021] Determine the control rate of the designed controller and express it by the following formula:

[0022]

[0023]

[0024] Among them, ω eso is the observer bandwidth, and u0 is the given reference variable of the system.

[0025] Preferably, the step three is specifically:

[0026] Perform Laplace transform on formula (2)

[0027]

[0028] Substituting formula (6) into formula (5) yields u0:

[0029]

[0030] Substituting formulas (6) and (7) into formula (4), we can obtain the system control variable and determine the relationship between the system reference variable and the system output. The relationship between the system reference variable and the system output can be expressed by the following formula:

[0031]

[0032] Preferably, the step 4 is specifically: performing simplified control according to the controller, and performing simplified control by the following formula:

[0033]

[0034] Determine the closed-loop transfer function of the system and express it by the following formula:

[0035]

[0036] The disturbance is superimposed on the rotor side current loop to suppress the sub-synchronous oscillation of the doubly fed wind farm.

[0037] Beneficial effects:

[0038] This paper provides a method for suppressing subsynchronous oscillations in a doubly-fed wind farm based on linear active disturbance rejection. In this method, the Lesotho (Less Synchronous Response) (LSO) estimates and compensates for subsynchronous disturbances in the system in real time. A high-frequency filter is then added to the Lesotho Filter (Less Synchronous Response) to suppress low-frequency oscillations. The designed HPF-LADRC (High-Power Feedback Filter) eliminates the need for complex parameter adjustments and exhibits excellent immunity to system parameter variations. Finally, impedance analysis and time-domain simulations verify the effectiveness of the HPF-LADRC in suppressing subsynchronous oscillations. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 A flowchart for the invention;

[0040] Figure 2 HPF-LADRC controller to suppress subsynchronous oscillations;

[0041] Figure 3 Simplified block diagram for HPF-LADRC;

[0042] Figure 4 It is a simulation model of a doubly-fed wind farm;

[0043] Figure 5 This is the active power comparison chart;

[0044] Figure 6 This is a system impedance comparison chart. DETAILED DESCRIPTION

[0045] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0046] Combine Figures 1 to 6 As shown, the present invention provides a method for suppressing subsynchronous oscillation of a doubly-fed wind farm based on linear active disturbance rejection control, comprising the following steps:

[0047] Step 1: Initialize the grid parameters and unit parameters;

[0048] Step 2: The extended state observer tracks the system and performs subsynchronous perturbations;

[0049] The step 2 is specifically as follows:

[0050] Determine the transfer function of the high-pass filter and express it by the following formula:

[0051]

[0052] Where ξ is the damping coefficient, ω n =2πf c , is the filter angular frequency, andH0 represents the gain;

[0053] Where LESO is shown in formula (2), and LESO is expressed by the following formula:

[0054]

[0055] Where ^ represents the estimated value, h1 and h2 are the gains of LESO;

[0056] Determine the relationship between the observer bandwidth and the observer gain, which can be expressed as follows:

[0057]

[0058] Determine the control rate of the designed controller and express it by the following formula:

[0059]

[0060]

[0061] Among them, ω eso is the observer bandwidth, and u0 is the given reference variable of the system.

[0062] Step 3: Perform feedback control based on state error;

[0063] The step three is specifically as follows:

[0064] Perform Laplace transform on formula (2)

[0065]

[0066] Substituting formula (6) into formula (5) yields u0:

[0067]

[0068] Substituting formulas (6) and (7) into formula (4), we can obtain the system control variable and determine the relationship between the system reference variable and the system output. The relationship between the system reference variable and the system output can be expressed by the following formula:

[0069]

[0070] Step 4: The disturbance is superimposed on the rotor side current loop to suppress the synchronous oscillation of the doubly fed wind farm.

[0071] The step 4 is specifically as follows: performing simplified control according to the controller, and performing simplified control by the following formula:

[0072]

[0073] Determine the closed-loop transfer function of the system and express it by the following formula:

[0074]

[0075] The disturbance is superimposed on the rotor side current loop to suppress the sub-synchronous oscillation of the doubly fed wind farm. Specific embodiment two:

[0077] The present invention establishes a double-fed wind farm grid-connected system via series compensation lines in MATLAB / Simulink, and verifies the effectiveness of the designed damping controller based on linear active disturbance rejection control through time domain simulation. Figure 4 The DFIG wind farm consists of 66 DFIG wind turbines, each with a rated voltage of 0.69 kV and a capacity of 1.5 MW. The turbines are connected to a transformer and connected to a 220 kV string compensation line for long-distance transmission. The wind farm system simulation parameters are shown in Table 1. The parameters of the designed controller are shown in Table 2.

[0078] Table 1. Main system parameters

[0079]

[0080] Table 2 Main parameters of HPF-LADRC

[0081]

[0082] The present invention further verifies the effectiveness of time domain simulation through impedance analysis. Small-amplitude harmonic currents are injected sequentially at the grid connection points of the simulation model to perform impedance analysis. Frequency sweeps are performed on the power generation side and the grid side of the system within the harmonic current frequency range to establish the impedance model of the system. The equivalent resistance and reactance of the system vary with frequency, and the frequency-impedance curve of the system is used to verify the ability of the system to suppress oscillations. When the equivalent reactance curve of the system intersects the x-axis, the system has an oscillation point. Assume that the equivalent resistance corresponding to the system oscillation frequency is less than zero. In this case, the system will undergo subsynchronous oscillations, and the larger the absolute value of the negative resistance, the easier it is to disperse the oscillation.

[0083] The initial wind speed in the time domain simulation is 11 m / s and the number of doubly fed wind turbines is 66. The system initially operates stably. At t = 3s, the series capacitor is connected to the system with a compensation level of 30%. SSCI occurs when the series capacitor is connected. The simulation results are shown in Figure 2. Figure 5 As shown in (a). It can be seen from the power curves of PI, LADRC and HPF-LADRC that the addition of series capacitors will produce oscillations of varying degrees. As the wind farm operates, the oscillations are suppressed to varying degrees under the action of PI, LADRC and HPF-LADRC. Compared with PI and LADRC, the power curve oscillation amplitude of LADRC is smaller, the convergence speed is faster, and the damping performance is better. Other parameters remain unchanged, and only the compensation degree is changed. The effect of HPF-LADRC in suppressing SSCI is tested at different compensation levels. The simulation starts at 40% and 50% compensation degrees, as shown in Figure 5 As shown in (b) and (c), the oscillations become increasingly severe as the compensation level increases. Simulation results show that at 40% and 50% compensation levels, the PI fails to suppress power oscillations, leading to wind farm oscillations. As the compensation level increases, the LADRC's vibration reduction effect deteriorates, and the oscillation convergence time increases. The power curves demonstrate that the HPF-LADRC has superior damping performance.

[0084] The time domain simulation results are verified based on system impedance analysis. Figure 6 As shown in (a), f1, f2 and f3 are X eq The reactance crosses zero under PI, LADRC and HPF-LADRC. When f1, R eq(f1) is positive, indicating that the wind farm with PI is stable. Similarly, when the equivalent reactance is zero, R eq (f2) and R eq (f3) is positive. The equivalent frequency-impedance curve of the system shows that the equivalent resistance of the HPF-LADRC impedance curve is larger and can better suppress oscillation. Figure 6 As shown in (b) and (c), at f1, R eq ( f1 ) is negative, indicating an unstable state under PI. When the equivalent reactance is zero, R eq (f2) and R eq (f3) is positive. The resistance of the HPF-LADRC is greater than that of the LADRC in the frequency-impedance curve. Analysis based on system impedance shows that the HPF-LADRC has better damping performance than both the LADRC and PI at different compensation levels.

[0085] The above is a detailed introduction to the method for suppressing sub-synchronous oscillations of a doubly-fed wind farm based on linear active disturbance rejection control provided by the present invention. The present invention uses specific examples to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea; at the same time, for general technical personnel in this field, according to the ideas of the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A method for suppressing subsynchronous oscillations of a doubly-fed wind farm based on linear active disturbance rejection control, characterized in that: The following steps are involved: Step 1: Initialize the grid parameters and unit parameters; Step 2: The extended state observer tracks the system and performs subsynchronous perturbations; Step 3: Perform feedback control based on state error; Step 4: The disturbance is added to the rotor-side current loop to suppress the synchronous oscillation of the doubly-fed wind farm. The step 2 is specifically as follows: Determine the transfer function of the high-pass filter and express it by the following formula: Where ξ is the damping coefficient, ω n =2πf c , is the filter corner frequency, and H0 represents the gain; Where LESO is shown in formula (2), and LESO is expressed by the following formula: Where ^ represents the estimated value, h1 and h2 are the gains of LESO, and i q is the q-axis component of the rotor side current, d q is the external disturbance on the q axis, b0 is the control gain constant; Determine the relationship between the observer bandwidth and the observer gain, which can be expressed as follows: Determine the control rate of the designed controller and express it by the following formula: Among them, ω eso is the observer bandwidth, is the reference value of the rotor side current q axis, and u0 is the system given reference variable.

2. The method for suppressing subsynchronous oscillation of a doubly-fed wind farm based on linear active disturbance rejection control according to claim 1, characterized in that: The step three is specifically as follows: Perform Laplace transform on formula (2) Substituting formula (6) into formula (5) yields u0: Substituting formulas (6) and (7) into formula (4), we can obtain the system control variable and determine the relationship between the system reference variable and the system output. The relationship between the system reference variable and the system output can be expressed by the following formula:

3. The method for suppressing subsynchronous oscillation of a doubly-fed wind farm based on linear active disturbance rejection control according to claim 2, characterized in that: The step 4 is specifically as follows: performing simplified control according to the controller, and performing simplified control by the following formula: Determine the closed-loop transfer function of the system and express it by the following formula: The disturbance is superimposed on the rotor side current loop to suppress the sub-synchronous oscillation of the doubly fed wind farm.

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