Oscillation self-stabilization control method of non-linear control flexible direct current power transmission system

Abstract

The invention relates to the technical field of power systems, in particular to a nonlinear control flexible direct current power transmission system oscillation self-stabilization control method, which comprises the following steps of S10, selecting a control signal containing a significant oscillation component when a flexible direct current power transmission system is unstable as an input signal of nonlinear additional control; s20, the selected input signals are collected and preprocessed; s30, generating a nonlinear self-stabilization controller additional signal from the signal obtained in the step S20 through a nonlinear self-stabilization control link; s40, optimizing parameters of a nonlinear self-stabilization control link by adopting a particle swarm algorithm; s50, feedforward compensation is carried out on the non-linear self-stabilization controller additional signal generated in the step S30 to a flexible DC converter control link; according to the method, self-stabilization control is realized through a nonlinear control link, and parameters of the nonlinear control link are optimized through the particle swarm optimization, so that the oscillation phenomenon in the flexible direct-current power transmission system is effectively suppressed.

Description

Technical Field

[0001] The present invention relates to the technical field of electric power systems, and in particular to an oscillation self-stabilizing control method for a nonlinearly controlled flexible direct current power transmission system. Background Art

[0002] Flexible DC transmission technology has been widely used in the fields of new energy grid connection and large power grid interconnection due to its advantages such as good controllability, low low-order harmonic content, convenient boosting and expansion, and adaptability to offshore wind power access. However, the large-scale commissioning of flexible DC transmission projects has led to an increase in the penetration rate of power electronic equipment in the power system, and the high-frequency nonlinear links introduced by power electronic control will interact with multiple heterogeneous power electronic equipment such as new energy converters and energy storage equipment and between power grids, bringing many stability problems to the dynamic process of the power grid, among which the oscillation instability problem is particularly prominent.

[0003] In the related technologies, the main suppression measures for the oscillation problem of flexible direct current transmission systems currently include two major methods: passive filtering and active damping suppression. The passive filtering strategy constructs an oscillation frequency band signal damping channel by connecting a series blocking filter or a parallel bypass damping filter to suppress the oscillation phenomenon in the system, but the parameters are difficult to adjust, it is easy to detune, and it is large in size, high in cost, and difficult to maintain. In the active damping control strategy, the additional damping controller has received widespread attention and application due to its advantages such as low investment cost and low control difficulty. It enhances the stability of the system by providing positive damping to the system and achieves oscillation suppression. However, although the traditional additional damping strategy has a flexible structure, the control performance is limited by parameters, and it is difficult to effectively suppress the oscillation frequency outside the control bandwidth, and it may even affect the temporary stability performance of the system.

[0004] Therefore, there is an urgent need for an oscillation self-stabilizing control method for a flexible DC transmission system to suppress the oscillation phenomenon in the flexible DC transmission system. Summary of the invention

[0005] In view of at least one of the above technical problems, the present invention provides an oscillation self-stabilizing control method for a flexible direct current transmission system with nonlinear control, which optimizes the parameters of the nonlinear control link through a particle swarm algorithm to suppress the oscillation phenomenon in the flexible direct current transmission system.

[0006] According to a first aspect of the present invention, there is provided a method for oscillation self-stabilization control of a nonlinearly controlled flexible direct current transmission system, comprising the following steps:

[0007] S10: selecting a control signal containing a significant oscillation component when the flexible DC transmission system is unstable as an input signal for nonlinear additional control;

[0008] S20: collecting the selected input signal and performing preprocessing;

[0009] S30: generating a nonlinear self-stabilizing controller additional signal by passing the signal obtained in S20 through a nonlinear self-stabilizing control link;

[0010] S40: Particle swarm algorithm is used to optimize the parameters of nonlinear self-stabilizing control link;

[0011] S50: feed-forward compensation of the additional signal of the nonlinear self-stabilizing controller generated by S30 to the flexible DC converter control link.

[0012] In some embodiments of the present invention, the input signal in S10 selects a control signal containing a significant oscillation component when the flexible DC power transmission system is unstable as the input signal of the nonlinear additional control.

[0013] In some embodiments of the present invention, the preprocessing in S20 includes sampling filtering processing.

[0014] In some embodiments of the present invention, the preprocessing in S20 further includes outlier data processing.

[0015] In some embodiments of the present invention, the nonlinear self-stabilizing control link in S30 adopts a nonlinear control law N(A) designed based on a describing function method.

[0016] In some embodiments of the present invention, the nonlinear control rate N(A) is an odd function.

[0017] In some embodiments of the present invention, in S40, the nonlinear self-stabilizing control link is in the form of k l N l (A)+k n N n (A), where the coefficient k l and k n They are the linear link action coefficient and the nonlinear link action coefficient respectively. The parameter optimization takes the product integral of the system time and the absolute error as the objective function, and the particle swarm algorithm is used to optimize the parameters of the nonlinear self-stabilizing controller.

[0018] In some embodiments of the present invention, the nonlinear self-stabilizing controller parameter optimization comprises the following steps:

[0019] S41: Establish the product integral of time and absolute error as the objective function;

[0020] S42: Set the initial value of the nonlinear self-stabilizing controller parameter to [k l ,k n ];

[0021] S43: Set the parameter range of the particle swarm and nonlinear self-stabilizing controller and the maximum number of iterations of the nonlinear self-stabilizing controller, and use the particle swarm algorithm to optimize k land k n ;

[0022] S44: using a simulation program to calculate the objective function value;

[0023] S45: Determine whether the maximum number of iterations has been reached. If so, output a set of solutions [k l ,k n ] as the optimal parameters of the nonlinear self-stabilizing controller; if not, go to S44 for the next iteration.

[0024] In some embodiments of the present invention, k l The larger the value, the closer the system dynamic performance is to normal operation; n The larger it is, the faster the system converges and the more obvious the control effect is.

[0025] In some embodiments of the present invention, the additional signal is fed forward to compensate for the lack of damping in the flexible DC converter control channel.

[0026] The beneficial effects of the present invention are as follows: the present invention realizes self-stabilizing control through a nonlinear control link, and optimizes the parameters of the nonlinear control link through a particle swarm algorithm; compared with the prior art, the oscillation phenomenon in the flexible direct current transmission system is effectively suppressed, and at the same time, there is no need to change the operation mode of the flexible direct current transmission system, and the operation cost is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] 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, the drawings described below are only some embodiments recorded in the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0028] Figure 1 It is a schematic flow chart of an oscillation self-stabilizing control method of a flexible direct current transmission system with nonlinear control in an embodiment of the present invention;

[0029] Figure 2 A schematic diagram of the structure of a nonlinear self-stabilizing controller according to an embodiment of the present invention;

[0030] Figure 3 A schematic diagram of a flow chart of nonlinear control parameter optimization in an embodiment of the present invention;

[0031] Figure 4 A schematic diagram of the installation position of a nonlinear self-stabilizing controller in a flexible DC power transmission system according to an embodiment of the present invention;

[0032] Figure 5Schematic diagram of the oscillation suppression effect of the nonlinear self-stabilizing control method under small disturbances in an embodiment of the present invention;

[0033] Figure 6 It is a schematic diagram of the oscillation suppression effect of the nonlinear self-stabilizing control method under a large disturbance in an embodiment of the present invention. DETAILED DESCRIPTION

[0034] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0035] It should be noted that when an element is referred to as being "fixed to" another element, it may be directly on the other element or there may be a central element. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be a central element at the same time. The terms "vertical", "horizontal", "left", "right" and similar expressions used herein are for illustrative purposes only and do not represent the only implementation method.

[0036] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0037] like Figures 1 to 6 The nonlinear control flexible direct current transmission system oscillation self-stabilization control method shown in the figure comprises the following steps: Figure 1 As shown:

[0038] S10: selecting a control signal containing a significant oscillation component when the flexible DC transmission system is unstable as an input signal for nonlinear additional control;

[0039] S20: collecting the selected input signal and performing preprocessing;

[0040] S30: generating a nonlinear self-stabilizing controller additional signal by passing the signal obtained in S20 through a nonlinear self-stabilizing control link;

[0041] S40: Particle swarm algorithm is used to optimize the parameters of nonlinear self-stabilizing control link;

[0042] S50: feed-forward compensation of the additional signal of the nonlinear self-stabilizing controller generated by S30 to the flexible DC converter control link.

[0043] In the above embodiment, the present invention realizes self-stabilizing control through a nonlinear control link, and optimizes the parameters of the nonlinear control link through a particle swarm algorithm; compared with the prior art, the oscillation phenomenon in the flexible direct current transmission system is effectively suppressed, and at the same time, there is no need to change the operation mode of the flexible direct current transmission system, and the operation cost is low.

[0044] In an embodiment of the present invention, the input signal in S10 selects a control signal containing a significant oscillation component when the flexible DC power transmission system is unstable as the input signal of the nonlinear additional control.

[0045] In the embodiment of the present invention, the preprocessing in S20 includes sampling filtering processing.

[0046] In the embodiment of the present invention, the preprocessing in S20 also includes outlier data processing.

[0047] First, the collected measurement data is input into a low-pass filter to filter out the high-frequency noise in the original signal. On this basis, the abnormal point data is further processed to minimize the interference introduced by the sampling link and avoid affecting the nonlinear self-stabilization strategy. The specific steps are:

[0048] Step 1: Select an appropriate low-pass filter cutoff frequency. The low-pass filter must ensure that the signals of the base frequency and oscillation frequency band are retained while filtering out high-frequency noise. For the oscillation problem of the flexible DC transmission system, the oscillation frequency is generally within 10kHz, so the low-pass filter cutoff frequency can be set to 10kHz.

[0049] Step 2: Detect abnormal data points. For the collected data points, it is necessary to determine whether there are outliers. The present invention uses the improved Z-score method to detect outliers: for a data point x in the collected data, calculate the local average value x of the window consisting of the point and the six surrounding data points. m and the local standard deviation x σ , take three each. Then x is considered to be an outlier.

[0050] Step 3: Processing abnormal data points. Common methods for processing outliers include deletion, correction, and replacement. Based on the abnormal data judgment rule of the improved Z-score method, the abnormal point data x is replaced with the window local average x. m .

[0051] In an embodiment of the present invention, the nonlinear self-stabilizing control link in S30 adopts a nonlinear control law N(A) designed based on the describing function method, so that it satisfies that the larger the input signal, the greater the damping provided by the nonlinear self-stabilizing controller for the system and the stronger the control effect.

[0052] In the embodiment of the present invention, the nonlinear control rate N(A) is an odd function to ensure the consistency of the control direction.

[0053] In the embodiment of the present invention, in S40, the nonlinear self-stabilizing control link is in the form of k l N l (A)+k n N n (A), where the coefficient k l and k n They are the linear link action coefficient and the nonlinear link action coefficient respectively. The parameter optimization takes the product integral of the system time and the absolute error as the objective function, and the particle swarm algorithm is used to optimize the parameters of the nonlinear self-stabilizing controller.

[0054] The specific steps are:

[0055] Step 1: Assume that the approximate equivalent frequency characteristic of the nonlinear control law to be established is N(A).

[0056] Among them, in order to meet the application conditions of the describing function method, the nonlinear control link should meet the following requirements:

[0057] 1. The nonlinear link cannot be too complicated, and it can ensure that the flexible DC transmission system after the application of the nonlinear control link can be simplified to a typical structure with a nonlinear part N(A) and a linear part G(S) in series.

[0058] 2. The nonlinear part N(A) satisfies odd symmetry, that is, y(x) = -y(-x), to ensure that its sinusoidal response does not contain a DC component and that the control direction is consistent.

[0059] Step 2: Analyze the stability of the nonlinear control link based on the Nyquist criterion. The system characteristic equation is:

[0060] 1+N(A)G(jω)=0

[0061] Draw separately The stability of the system is analyzed by the amplitude-phase characteristic curve of G(Jω):

[0062] 1. If the characteristic curve of G(Jω) does not enclose characteristic curve, the nonlinear control link converges and the system is stable.

[0063] 2. If the characteristic curve of G(Jω) encloses characteristic curve, the nonlinear control link diverges and the system is unstable.

[0064] 3. If the characteristic curve of G(Jω) is If the characteristic curve intersects, the system will produce self-oscillation.

[0065] Step 3: Considering the initial structure of the current control link of the flexible DC transmission system, combined with the actual transient stability characteristics of the system, determine the transfer function expression of the nonlinear additional control link so that The characteristic curve has no intersection with the area surrounded by the G(Jω) characteristic curve, which ensures the convergence of the system and realizes nonlinear self-stabilizing control.

[0066] Step 4: In order to improve the flexibility of the nonlinear self-stabilizing control link and maximize the self-stabilizing control performance, the linear link action coefficient k is introduced l and the nonlinear link action coefficient k n , the value can be adjusted according to the specific operating conditions to achieve oscillation suppression as quickly as possible. Combining the above steps, the expression of the nonlinear control link y(t) = k n ext 3 +k l e(t), as Figure 2 shown.

[0067] In some embodiments of the present invention, the nonlinear self-stabilizing controller parameter optimization includes the following steps: Figure 3 As shown:

[0068] S41: Establish the product integral of time and absolute error as the objective function;

[0069] for:

[0070] Where, t f represents the rise time of the nonlinear self-stabilizing controller; e(t) represents the static error e(t) = i ref -i(t).

[0071] S42: Set the initial value of the nonlinear self-stabilizing controller parameter to [k l ,k n ];

[0072] S43: Set the parameter range of the particle swarm and nonlinear self-stabilizing controller and the maximum number of iterations of the nonlinear self-stabilizing controller, and use the particle swarm algorithm to optimize k l and k n ;

[0073] The particle swarm size is set to 20, the maximum number of iterations is set to 200, and k l The range of k is set to [0.3, 1], n The range of is set to [0, 1.5].

[0074] S44: using a simulation program to calculate the objective function value;

[0075] S45: Determine whether the maximum number of iterations has been reached. If so, output a set of solutions [kl ,k n ] as the optimal parameters of the nonlinear self-stabilizing controller; if not, go to S44 for the next iteration.

[0076] In the embodiment of the present invention, k l The larger the value, the closer the system dynamic performance is to normal operation and the smaller the static error is. n The larger it is, the faster the system converges and the more obvious the control effect is.

[0077] In an embodiment of the present invention, an additional signal feed-forward compensation is added to the control channel of the flexible DC converter to compensate for the lack of damping. Figure 2 As shown in the figure, taking the unstable oscillation caused by the lack of current loop damping as an example, when the flexible DC transmission system oscillates, the oscillation energy circulates and is continuously amplified in the channel composed of the AC system-flexible DC converter current control link-external system. Since the damping of the current control link is small, the system oscillation is amplified more significantly, which easily leads to the rapid divergence of system oscillation. Therefore, the additional signal should be fed forward to the current control link to provide dynamic damping for the system and achieve oscillation suppression.

[0078] The verification example used in the present invention is a flexible DC transmission system including an AC system and an AC filter. The system structure diagram after adding the nonlinear self-stabilizing controller proposed in the present invention is as follows: Figure 4 As shown. The flexible DC transmission system adopts a single-machine equivalent model and is connected to the 500kV AC system through 500 / 220kV and 220 / 110kV two-stage transformers. An AC filter is installed at the PCC point where the flexible DC transmission system is connected to the AC system. In this example, the signal input to the nonlinear self-stabilizing controller is the current deviation signal Δi between the d-axis and q-axis in the current control link of the flexible DC converter. d , Δi q .according to Figure 3 The flowchart of the present invention optimizes the nonlinear control parameters, setting the initial value to k l =1, k n =0.5.

[0079] Embodiment 1:

[0080] Built on MATLAB / Simulink platform Figure 4 The time domain simulation model of the flexible DC transmission system is shown in Figure 1. The operating conditions are set as follows: when t = 1 to 1.01s, a small disturbance signal with an amplitude of 1% is introduced into the current control link of the flexible DC transmission system; when t = 1.2s, the nonlinear self-stabilizing control strategy is put into use. The power waveform at the PCC point where the flexible DC system is connected to the AC system under the action of small disturbances is shown in Figure 1. Figure 5 shown.

[0081] It can be found that when the system does not add the self-stabilizing control strategy, the PCC point power gradually diverges under the influence of small disturbances, and the system gradually becomes unstable. If the nonlinear self-stabilizing control strategy proposed in the present invention is put into use at t=1.2s, it can be found that the system power oscillation is effectively suppressed within 0.5s, and the system recovers stability, which verifies the suppression effect of the proposed self-stabilizing control strategy on subsynchronous oscillations under small disturbances.

[0082] Embodiment 2:

[0083] In order to verify the oscillation suppression effect of the nonlinear self-stabilizing control strategy proposed in this invention under large disturbances, the operating conditions are set as follows: when t = 1 to 1.01s, a disturbance signal with an amplitude of 10% is introduced into the current control link of the flexible DC transmission system; when t = 1.2s, the nonlinear self-stabilizing control strategy is put into use. The power waveform at the PCC point where the flexible DC system is connected to the AC system under disturbance is as follows: Figure 6 shown.

[0084] It can be found that when the system does not add the self-stabilizing control strategy, the power of the PCC point gradually diverges under the influence of the disturbance, and the system gradually becomes unstable. If the nonlinear self-stabilizing control strategy proposed in the present invention is put into use at t=1.2s, it can be found that the system power has a more obvious overshoot phenomenon at the moment the strategy is put into use, but the oscillation is effectively suppressed within 0.1s. Compared with when the disturbance is smaller, the system recovers stability in a shorter time. It shows that the nonlinear control flexible direct current transmission system oscillation self-stabilizing control method proposed in the present invention still has a superior suppression effect for larger disturbances, and the larger the input control signal, the greater the dynamic damping provided by the strategy for the system, and the more significant the control effect on the system.

[0085] Those skilled in the art should understand that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, and these changes and improvements fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A nonlinear controlled flexible direct current transmission system oscillation self-stabilization control method, characterized in that: The following steps are involved: S10: selecting a control signal containing a significant oscillation component when the flexible DC transmission system is unstable as an input signal for nonlinear additional control; S20: collecting the selected input signal and performing preprocessing; S30: generating a nonlinear self-stabilizing controller additional signal by passing the signal obtained in S20 through a nonlinear self-stabilizing control link; S40: Particle swarm algorithm is used to optimize the parameters of nonlinear self-stabilizing control link; S50: feed-forward compensation of the additional signal of the nonlinear self-stabilizing controller generated by S30 to the flexible DC converter control link.

2. The nonlinear controlled flexible direct current transmission system oscillation self-stabilization control method according to claim 1, characterized in that: The input signal in S10 selects a control signal containing a significant oscillation component when the flexible DC power transmission system is unstable as the input signal of the nonlinear additional control.

3. The nonlinear controlled flexible direct current transmission system oscillation self-stabilization control method according to claim 2 is characterized in that: The preprocessing in S20 includes sampling filtering processing.

4. The nonlinear controlled flexible direct current transmission system oscillation self-stabilization control method according to claim 3 is characterized in that: The preprocessing in S20 also includes outlier data processing.

5. The nonlinear controlled flexible direct current transmission system oscillation self-stabilization control method according to claim 1, characterized in that: The nonlinear self-stabilizing control link in S30 adopts a nonlinear control law N(A) designed based on the describing function method.

6. The nonlinear controlled flexible direct current transmission system oscillation self-stabilization control method according to claim 5, characterized in that: The nonlinear control rate N(A) is an odd function.

7. The nonlinear controlled flexible direct current transmission system oscillation self-stabilization control method according to claim 6, characterized in that: In S40, the nonlinear self-stabilizing control link is in the form of k l N l (A)+k n N n (A), where the coefficient k l and k n They are the linear link action coefficient and the nonlinear link action coefficient respectively. The parameter optimization takes the product integral of the system time and the absolute error as the objective function, and the particle swarm algorithm is used to optimize the parameters of the nonlinear self-stabilizing controller.

8. The nonlinear controlled flexible direct current transmission system oscillation self-stabilization control method according to claim 7, characterized in that: The nonlinear self-stabilizing controller parameter optimization comprises the following steps: S41: Establish the product integral of time and absolute error as the objective function; S42: Set the initial value of the nonlinear self-stabilizing controller parameter to [k l ,k n ]; S43: Set the parameter range of the particle swarm and nonlinear self-stabilizing controller and the maximum number of iterations of the nonlinear self-stabilizing controller, and use the particle swarm algorithm to optimize k l and k n ; S44: using a simulation program to calculate the objective function value; S45: Determine whether the maximum number of iterations has been reached. If so, output a set of solutions [k l ,k n ] as the optimal parameters of the nonlinear self-stabilizing controller; if not, go to S44 for the next iteration.

9. The nonlinear controlled flexible direct current transmission system oscillation self-stabilization control method according to claim 8, characterized in that: k l The larger the value, the closer the system dynamic performance is to normal operation; n The larger it is, the faster the system converges and the more obvious the control effect is.

10. The nonlinear controlled flexible direct current transmission system oscillation self-stabilization control method according to claim 1, characterized in that: The additional signal is fed forward to compensate for the lack of damping in the flexible DC converter control channel.

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