A transient synchronization optimization method and device for a flexible low-frequency alternating current transmission system

Abstract

The application discloses a transient synchronization optimization method and device for a flexible low-frequency AC sending-out system, and the method comprises the following steps: constructing a pre-complex vector filter based on three-phase voltage signals of detection points; constructing a phase closed-loop transfer model of a post-positioned phase-locked loop according to the pre-complex vector filter, so as to design a refined linear model of a phase-locked loop controller; calculating optimal parameters according to the refined linear model of the phase-locked loop controller; and realizing transient synchronization optimization for the flexible low-frequency AC sending-out system, so as to improve the dynamic performance of frequency and phase detection, reduce oscillation amplitude and instantaneous error, and improve the transient stability of the system.

Description

Technical Field

[0001] This invention relates to the field of new energy power generation control technology, and in particular to a transient synchronization optimization method and device for a flexible low-frequency AC transmission system. Background Technology

[0002] Currently, driven by the "dual carbon" target, the development speed of new energy power generation such as solar and wind power in my country is accelerating year by year. Offshore wind power has achieved significant development due to its advantages of high stability, abundant wind energy resources, and no land occupation. Most of the wind power projects that have been put into operation and are under construction are concentrated in near-shore wind power within 50km of the coast. With the saturation of near-shore wind energy development, the future development of offshore wind power will inevitably move towards large-scale farms, larger single-unit capacity, and deep-sea development. High-voltage AC transmission (HVAC) uses 50Hz power frequency AC transmission, which does not require converter stations at sea or on land, and is the most mature technology. However, the high operating frequency of HVAC transmission lines and the significant capacitance effect of submarine cables generate a large amount of charging current and reactive power consumption, which limits the power transmission capacity of the cables. High-voltage direct current (HVDC) transmission converts AC power from wind farms into DC power via offshore converter stations and then transmits it to the onshore power grid via onshore converter stations. This overcomes the shortcomings of HVAC systems and is suitable for ultra-long-distance offshore wind power transmission. However, HVDC requires costly offshore converter stations for construction and maintenance, and low-cost, high-reliability HVDC circuit breaker technology still needs further research, while multi-terminal networking technology is relatively immature. Low-frequency transmission, on the other hand, combines the advantages of both. Compared to HVAC, lowering the transmission frequency reduces line reactance, increases transmission capacity, and reduces capacitive current in submarine cables. Compared to HVDC, low-frequency transmission systems do not require offshore converter stations, saving on the construction and maintenance costs of offshore platforms. Furthermore, the AC transmission system avoids the broadband oscillation problem of flexible DC transmission systems, making it widely applicable in medium- to long-distance (more than 70km offshore) offshore wind power collection and transmission scenarios.

[0003] Existing offshore wind power transmission systems via flexible low-frequency AC transmission utilize a synchronization mechanism consisting of filters and phase-locked loops (PLLs). This synchronization mechanism, by detecting the real-time phase and frequency of the offshore low-frequency collection bus and the onshore power frequency grid connection point, is crucial for achieving transient synchronization between the renewable energy power plant and the equivalent grid. However, the long transmission distance of the system limits the detection performance and synchronization capability of the PLL. PI controller-based PLLs exhibit poor dynamic performance in phase and frequency detection under transient disturbances, with large overshoot and long settling times. This makes it impossible to achieve high-performance transient synchronization between the converter and the grid, easily leading to system transient instability and reducing the transmission capacity of the offshore wind power transmission system via flexible low-frequency AC transmission. Summary of the Invention

[0004] This invention provides a transient synchronization optimization method and apparatus for a flexible low-frequency AC transmission system, in order to solve the problem that existing offshore wind power transmission systems via flexible low-frequency AC transmission cannot achieve high-performance transient synchronization between the converter and the power grid, which easily leads to transient instability of the system.

[0005] To address the aforementioned technical problems, embodiments of the present invention provide a transient synchronization optimization method for a flexible low-frequency AC transmission system, comprising:

[0006] The three-phase voltage signals of the offshore low-frequency collection bus and the onshore power frequency grid connection point are obtained, and positive and negative sequence component models are constructed based on the three-phase voltage signals of the offshore low-frequency collection bus and the onshore power frequency grid connection point.

[0007] A pre-processing complex vector filter is constructed based on the positive and negative sequence component model, and the positive and negative sequence components of the voltage signal at the detection point are obtained based on the pre-processing complex vector filter and the fundamental frequency of the voltage signal at the detection point.

[0008] A small-signal linear model is constructed based on the pre-processor complex vector filter, and a phase closed-loop transfer model of the post-processor phase-locked loop is constructed based on the small-signal linear model. The phase closed-loop transfer model is then optimized.

[0009] A constraint model is constructed based on the optimized phase closed-loop transfer model, and a refined linear model of the phase-locked loop controller is constructed based on the constraint model and the phase closed-loop transfer model.

[0010] The optimal parameters are calculated based on the refined linear model of the phase loop controller, and the flexible low-frequency AC transmission system is controlled according to the optimal parameters to achieve transient synchronization optimization; the optimal parameters include the frequency deviation reference value and the detected phase angle.

[0011] Furthermore, the construction of the positive and negative sequence component model based on the three-phase voltage signals of the offshore low-frequency collection bus and the onshore power frequency grid connection point specifically involves:

[0012] The three-phase voltage signals of the offshore low-frequency bus and the onshore power frequency grid connection point are detected, and the positive sequence component model and negative sequence component model under the three-phase stationary reference frame are calculated based on the three-phase voltage signals of the offshore low-frequency bus and the onshore power frequency grid connection point.

[0013] The detection voltage under a two-phase stationary reference frame is obtained, and a positive-sequence component model and a negative-sequence component model under the two-phase stationary reference frame are constructed based on the detection voltage under the two-phase stationary reference frame.

[0014] Furthermore, the construction of the pre-processed complex vector filter based on the positive and negative order component model specifically involves:

[0015] A pre-processed complex vector filter is constructed based on the positive-sequence component model and the negative-sequence component model under the two-phase stationary reference frame. The pre-processed complex vector filter includes a filter transfer function model and a time-domain expression.

[0016] The filter transfer function model is constructed based on the positive-sequence component model and the negative-sequence component model, the damping coefficient, the filtering frequency, and the Laplace operator.

[0017] The time-domain expression is constructed based on the Laplace transform of the output signal of the filter transfer function model.

[0018] Furthermore, the step of constructing a small-signal linear model based on the pre-filter and then constructing a phase-locked loop transfer model based on the small-signal linear model specifically involves:

[0019] The fundamental positive sequence phase angle and fundamental positive sequence amplitude of the voltage detection point are obtained based on the pre-processed complex vector filter.

[0020] Based on the filter transfer function model, the rectangular coordinates of the complex vector voltage signal are converted to polar coordinates;

[0021] A first small-signal linear model is constructed based on the polar coordinates, actual phase angle, fundamental positive-sequence phase angle, actual amplitude, and fundamental positive-sequence amplitude of the complex vector voltage signal.

[0022] The phase closed-loop transfer model of the post-phase-locked loop is constructed based on the first small-signal linear model.

[0023] Furthermore, the step of constructing a constraint model based on the optimized phase closed-loop transfer model, and then constructing a refined linear model of the phase-locked loop controller based on the constraint model and the phase closed-loop transfer model, specifically involves:

[0024] Obtain the constraints of the phase closed-loop transfer model, and construct a constraint model based on the constraints. The constraint model includes an order constraint model and a mapping relationship constraint model.

[0025] A first-order low-pass filter model is constructed based on the constraint model, and a refined linear model of the phase-locked loop controller is constructed based on the first-order low-pass filter model and the phase closed-loop transfer model.

[0026] In a second aspect, the present invention provides a transient synchronization optimization device for a flexible low-frequency AC transmission system, comprising: a positive and negative sequence component model construction module, a filter construction module, a transfer model construction module, a refined linear model construction module, and a control module;

[0027] The positive and negative sequence component model construction module is used to acquire the three-phase voltage signals of the offshore low-frequency collection bus and the onshore power frequency grid connection point, and construct a positive and negative sequence component model based on the three-phase voltage signals of the offshore low-frequency collection bus and the onshore power frequency grid connection point.

[0028] The filter construction module is used to construct a pre-processor complex vector filter based on the positive and negative sequence component model, and to obtain the positive and negative sequence components of the voltage signal at the detection point based on the pre-processor complex vector filter and the fundamental frequency of the voltage signal at the detection point.

[0029] The transfer model construction module is used to construct a small-signal linear model based on the pre-processor complex vector filter, construct a phase closed-loop transfer model of the post-processor phase-locked loop based on the small-signal linear model, and optimize the phase closed-loop transfer model.

[0030] The refined linear model construction module is used to construct a constraint model based on the optimized phase closed-loop transfer model, and to construct a refined linear model of the phase-locked loop controller based on the constraint model and the phase closed-loop transfer model.

[0031] The control module is used to calculate the optimal parameters based on the refined linear model of the phase loop controller, and control the flexible low-frequency AC transmission system according to the optimal parameters to achieve transient synchronization optimization; the optimal parameters include frequency deviation reference value and detection phase angle.

[0032] Furthermore, the positive and negative order component model construction module is specifically used for:

[0033] The three-phase voltage signals of the offshore low-frequency bus and the onshore power frequency grid connection point are detected, and the positive sequence component model and negative sequence component model under the three-phase stationary reference frame are calculated based on the three-phase voltage signals of the offshore low-frequency bus and the onshore power frequency grid connection point.

[0034] The detection voltage under a two-phase stationary reference frame is obtained, and a positive-sequence component model and a negative-sequence component model under the two-phase stationary reference frame are constructed based on the detection voltage under the two-phase stationary reference frame.

[0035] Furthermore, the filter construction module is specifically used for:

[0036] A pre-processed complex vector filter is constructed based on the positive-sequence component model and the negative-sequence component model under the two-phase stationary reference frame. The pre-processed complex vector filter includes a filter transfer function model and a time-domain expression.

[0037] The filter transfer function model is constructed based on the positive-sequence component model and the negative-sequence component model, the damping coefficient, the filtering frequency, and the Laplace operator.

[0038] The time-domain expression is constructed based on the Laplace transform of the output signal of the filter transfer function model.

[0039] Furthermore, the transfer model construction module is specifically used for:

[0040] The fundamental positive sequence phase angle and fundamental positive sequence amplitude of the voltage detection point are obtained based on the pre-processed complex vector filter.

[0041] Based on the filter transfer function model, the rectangular coordinates of the complex vector voltage signal are converted to polar coordinates;

[0042] A first small-signal linear model is constructed based on the polar coordinates, actual phase angle, fundamental positive-sequence phase angle, actual amplitude, and fundamental positive-sequence amplitude of the complex vector voltage signal.

[0043] The phase closed-loop transfer model of the post-phase-locked loop is constructed based on the first small-signal linear model.

[0044] Furthermore, the refined linear model construction module is specifically used for:

[0045] Obtain the constraints of the phase closed-loop transfer model, and construct a constraint model based on the constraints. The constraint model includes an order constraint model and a mapping relationship constraint model.

[0046] A first-order low-pass filter model is constructed based on the constraint model, and a refined linear model of the phase-locked loop controller is constructed based on the first-order low-pass filter model and the phase closed-loop transfer model. Attached Figure Description

[0047] Figure 1 A flowchart illustrating a transient synchronization optimization method for a flexible low-frequency AC transmission system provided in an embodiment of the present invention;

[0048] Figure 2 This is a schematic diagram of a flexible low-frequency AC transmission system for offshore wind farms, provided as an embodiment of the present invention.

[0049] Figure 3 This is a schematic diagram of the structure of a pre-processed complex vector filter provided in an embodiment of the present invention;

[0050] Figure 4 A waveform diagram of observation frequency and phase angle error during phase perturbation is provided for an embodiment of the present invention;

[0051] Figure 5 A waveform diagram showing the observation frequency and phase angle error during amplitude perturbation in an embodiment of the present invention;

[0052] Figure 6A waveform diagram showing the observation frequency and phase angle error during voltage imbalance disturbance provided in an embodiment of the present invention;

[0053] Figure 7 The diagram illustrates the observation frequency and phase angle error waveforms during harmonic voltage disturbances, as provided in an embodiment of the present invention. Detailed Implementation

[0054] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0055] Example 1

[0056] Please refer to Figure 1 , Figure 1 A flowchart illustrating a transient synchronization optimization method for a flexible low-frequency AC transmission system provided in an embodiment of the present invention includes steps 101 to 105, as detailed below:

[0057] Step 101: Obtain the three-phase voltage signals of the offshore low-frequency collection bus and the onshore power frequency grid connection point, and construct a positive and negative sequence component model based on the three-phase voltage signals of the offshore low-frequency collection bus and the onshore power frequency grid connection point;

[0058] Please refer to Figure 2 , Figure 2 This is a schematic diagram of a flexible low-frequency AC transmission system for offshore wind farms, provided as an embodiment of the present invention.

[0059] In this embodiment, the three-phase voltage signal u between the offshore low-frequency collection bus of the flexible low-frequency AC transmission system and the onshore power frequency grid connection point is acquired. pcc1 u pcc2 .

[0060] In this embodiment, the construction of the positive and negative sequence component model based on the three-phase voltage signals of the offshore low-frequency collection bus and the onshore power frequency grid connection point specifically involves:

[0061] The three-phase voltage signals of the offshore low-frequency bus and the onshore power frequency grid connection point are detected, and the positive sequence component model and negative sequence component model under the three-phase stationary reference frame are calculated based on the three-phase voltage signals of the offshore low-frequency bus and the onshore power frequency grid connection point.

[0062] The detection voltage under a two-phase stationary reference frame is obtained, and a positive-sequence component model and a negative-sequence component model under the two-phase stationary reference frame are constructed based on the detection voltage under the two-phase stationary reference frame.

[0063] In this embodiment, the three-phase voltage signals u at two detection points—the offshore low-frequency collection bus and the onshore power frequency grid connection point—are detected. pcc1 u pcc2 Therefore, a positive and negative sequence component model is constructed based on the three-phase voltage signals from the two detection points. The specific three-phase voltage signals from the two detection points are as follows:

[0064] u pcc1 =[u A1 u B1 u C1 ] T (1)

[0065] u pcc2 =[u A2 u B2 u C2 ] T (2)

[0066] Among them, u Xy (X=A,B;y=1,2,3) represents the instantaneous values ​​of the detected phase voltage.

[0067] In this embodiment, a three-phase stationary system positive and negative sequence component model is constructed based on the three-phase voltage signal. The positive and negative sequence component model includes a positive sequence component model and a negative sequence component model, specifically:

[0068]

[0069]

[0070] Among them, u P For the positive sequence component model in a three-phase stationary reference frame, u N For the negative sequence component model in a three-phase stationary reference frame, T P T N These are the positive-order transformation matrix and the negative-order transformation matrix, respectively.

[0071] In this embodiment, based on the transformation relationship between the three-phase stationary reference frame and the two-phase stationary reference frame, and based on the positive-sequence component model and negative-sequence component model under the three-phase stationary reference frame, the detection voltage under the two-phase stationary reference frame is constructed using the following methods:

[0072]

[0073]

[0074] in, For the positive sequence component model in a two-phase stationary reference frame, For the negative-order component model in a two-phase stationary reference frame, uα u β The voltage detected is in a two-phase stationary reference frame.

[0075] Step 102: Construct a pre-processing complex vector filter based on the positive and negative sequence component model, and obtain the positive and negative sequence components of the voltage signal at the detection point based on the pre-processing complex vector filter and the fundamental frequency of the voltage signal at the detection point;

[0076] In this embodiment, the construction of the pre-processed complex vector filter based on the positive and negative order component model specifically involves:

[0077] A pre-processed complex vector filter is constructed based on the positive-sequence component model and the negative-sequence component model under the two-phase stationary reference frame. The pre-processed complex vector filter includes a filter transfer function model and a time-domain expression.

[0078] The filter transfer function model is constructed based on the positive-sequence component model and the negative-sequence component model, the damping coefficient, the filtering frequency, and the Laplace operator.

[0079] The time-domain expression is constructed based on the Laplace transform of the output signal of the filter transfer function model.

[0080] Please refer to Figure 3 , Figure 3 This is a schematic diagram of the structure of a pre-processed complex vector filter provided in an embodiment of the present invention.

[0081] In this embodiment, the pre-filter is a single-input dual-output structure, and the pre-filter includes a filter transfer function model and a time-domain expression. The voltage signal u in a two-phase stationary reference frame is used as an example. α or u β Using the filtered signals y and q as inputs, and the positive-sequence and negative-sequence component models, damping coefficients, filtering frequency, and Laplace operator as outputs, a filter transfer function model is constructed. Specifically, the filter transfer function model of the pre-processor complex vector filter is as follows:

[0082]

[0083] Where U(s) is the Laplace transform of the input signal of the pre-filter complex vector filter, Y(s) and Q(s) are the Laplace transforms of the output signals, and k s ω is the damping coefficient. SOGI denoted as the filtering frequency, and s as the Laplace operator.

[0084] In this embodiment, the damping coefficient k s The value range is generally 0.5 to 2, and the filtering frequency can be the phase-locked loop observation frequency or the rated frequency.

[0085] As a specific example of an embodiment of the present invention, the damping coefficient is taken as 0.707; the filtering frequency is taken as the rated frequency of the detected node, where the low-frequency side is 20Hz and the power frequency side is 50Hz, that is:

[0086]

[0087] In this embodiment, the output signal of the filter transfer function model is obtained, and the output signal is subjected to Laplace transform processing to construct a time-domain expression. Specifically, the time-domain expression is as follows:

[0088]

[0089] in, This represents the inverse Laplace transform operator.

[0090] In this embodiment, a pre-filter is constructed based on the α-axis and β-axis in a two-phase stationary reference frame, and the fundamental positive and negative sequence components of the voltage signal at the detection point in the two-phase stationary reference frame are calculated based on the pre-filter. Specifically:

[0091]

[0092] in, and It is the fundamental positive-sequence component after filtering in a two-phase stationary reference frame. and It is the fundamental negative sequence component after filtering in a two-phase stationary reference frame.

[0093] Step 103: Construct a small-signal linear model based on the pre-processor complex vector filter, construct a phase closed-loop transfer model for the post-processor phase-locked loop based on the small-signal linear model, and optimize the phase closed-loop transfer model.

[0094] In this embodiment, the step of constructing a small-signal linear model based on the pre-filter and then constructing a phase-locked loop transfer model based on the small-signal linear model specifically involves:

[0095] The fundamental positive sequence phase angle and fundamental positive sequence amplitude of the voltage detection point are obtained based on the pre-processed complex vector filter.

[0096] Based on the filter transfer function model, the rectangular coordinates of the complex vector voltage signal are converted to polar coordinates;

[0097] A first small-signal linear model is constructed based on the polar coordinates, actual phase angle, fundamental positive-sequence phase angle, actual amplitude, and fundamental positive-sequence amplitude of the complex vector voltage signal.

[0098] The phase closed-loop transfer model of the post-phase-locked loop is constructed based on the first small-signal linear model.

[0099] In this embodiment, the fundamental positive-sequence phase angle and fundamental positive-sequence amplitude of the voltage detection point are obtained based on the pre-filter, specifically as follows:

[0100]

[0101]

[0102] Where, θ s θ sP U represents the actual phase angle and the fundamental positive sequence phase angle at the voltage detection point, respectively. m U Pm These represent the actual amplitude and the fundamental positive sequence amplitude, respectively.

[0103] In this embodiment, the filter transfer function model is specifically as follows:

[0104]

[0105] In this embodiment, based on the filter transfer function model, the rectangular coordinate expression of the complex vector voltage signal is converted into polar coordinate form, and a first small-signal linear model of the complex vector pre-filter, represented by phase and amplitude, is constructed, specifically:

[0106]

[0107] Where Δ represents the small signal quantity corresponding to the signal, U pccm0 U pccPm0 G is the steady-state amplitude of the voltage. DSOGI (s) is the phase magnitude transfer function matrix.

[0108] In this embodiment, the magnitude transfer function matrix is ​​specifically:

[0109]

[0110] In this embodiment, G DSOGI Since (s) is a non-diagonal matrix, the filter system has an inherent phase and amplitude coupling effect. When the phase or amplitude of the input voltage is disturbed, the phase and amplitude of the filter output voltage will change simultaneously.

[0111] In this embodiment, the phase closed-loop transfer model of the post-phase-locked loop is constructed based on the first small-signal linear model, the transfer function of the phase-locked loop controller, the actual phase, and the observed phase angle.

[0112] In this embodiment, the post-phase-locked loop comprises a coordinate transformation module, an amplitude calculation module, a control module, and an integration module. Its input is the three-phase voltage signal at the detection point, and its output is the observed phase. The expressions for the actual phase and the observed phase at the detection point are as follows:

[0113] θ PLL =∫ω PLL dt=∫(ω n +Δω PLL )dt=θ n +Δθ PLL (15)

[0114] θ s =∫ω s dt=∫(ω n +Δω s )dt=θ n +Δθ s (16)

[0115] Where, ω PLL ω is the angular frequency observed by the phase-locked loop. n The rated angular frequency of the power grid, Δω PLL For the observed angular frequency perturbation, θ n The rated phase angle is Δθ. PLL The observed phase angle perturbation; ω s The actual angular frequency at point PCC, Δω s For the actual angular frequency perturbation, Δθ s This represents the actual phase angle disturbance.

[0116] In this embodiment, the phase closed-loop transfer model of the post-phase-locked loop is specifically as follows:

[0117]

[0118] In the formula, G θ (s) is the transfer function of the phase-locked loop controller.

[0119] In this embodiment, the phase closed-loop transfer model is optimized. Specifically, the pre-processor complex vector filter and the post-processor phase-locked loop are cascaded to optimize the post-processor phase-locked loop. The second small-signal linear model of the optimized post-processor phase-locked loop is obtained, and the optimized phase closed-loop transfer model is constructed based on the second small-signal linear model.

[0120] In this embodiment, a pre-processor complex vector filter is cascaded with a post-processor phase-locked loop to form an improved phase-locked loop, and the second small-signal linearization transfer model of the improved phase-locked loop after cascading is obtained, specifically:

[0121]

[0122] Among them, G DP (s) is the transfer function matrix of the improved phase-locked loop in polar coordinates, specifically:

[0123]

[0124] In this embodiment, the phase closed-loop transfer model is obtained based on factorization, specifically as follows:

[0125]

[0126] Among them, a i b i denoted as i-th order coefficients of the numerator and denominator of the phase closed-loop transfer function L(s) of the improved phase-locked loop, respectively, and m and n are the orders of the numerator and denominator, respectively.

[0127] Step 104: Construct a constraint model based on the optimized phase closed-loop transfer model, and construct a refined linear model of the phase-locked loop controller based on the constraint model and the phase closed-loop transfer model;

[0128] In this embodiment, the step of constructing a constraint model based on the optimized phase closed-loop transfer model, and constructing a refined linear model of the phase-locked loop controller based on the constraint model and the phase closed-loop transfer model, specifically involves:

[0129] Obtain the constraints of the phase closed-loop transfer model, and construct a constraint model based on the constraints. The constraint model includes an order constraint model and a mapping relationship constraint model.

[0130] A first-order low-pass filter model is constructed based on the constraint model, and a refined linear model of the phase-locked loop controller is constructed based on the first-order low-pass filter model and the phase closed-loop transfer model.

[0131] In this embodiment, the constraints of the phase closed-loop transfer model include G. θ (s) The denominator order is not less than the numerator order, and the steady-state gain of the closed-loop system is 1. Based on the above constraints, the feasibility conditions for the improved phase-locked loop phase-locked loop transfer model are: the denominator is at least two orders higher than the numerator; the constant terms of the numerator and denominator are equal. Based on the above constraints, an order constraint model is constructed, specifically:

[0132]

[0133] In this embodiment, due to the phase-locked loop controller transfer function G θ There is a one-to-one mapping relationship between L(s) and the phase transfer function L(s). Based on the order constraint model, a mapping relationship constraint model is constructed:

[0134]

[0135] In this embodiment, n first-order low-pass filter models are constructed based on the constraint model, and a refined linear model of the phase-locked loop controller is constructed based on the first-order low-pass filter models and the phase closed-loop transfer model. Specifically, the first-order low-pass filter model is as follows:

[0136]

[0137] Among them, T Hi The time constant of the i-th first-order low-pass filter can be adjusted by changing T. Hi And n improves the dynamic performance of the synchronous detection system.

[0138] As a specific example of an embodiment of the present invention, the parameters of the first-order low-pass filter are set as follows: n = 4, T H1 =T H2 =0.002,T H3 =T H4 =0.003.

[0139] In this embodiment, the mapping constraint model is combined with the phase closed-loop transfer model to obtain a refined linear model of the phase-locked loop controller, specifically:

[0140]

[0141] As a specific example of an embodiment of the present invention, after substituting the parameters, the phase-locked loop controller transfer model for the detection point of the offshore low-frequency gathering bus is as follows:

[0142]

[0143] As a specific example of an embodiment of the present invention, after substituting the parameters, the phase-locked loop controller transfer model for the onshore power frequency grid-connected detection point is as follows:

[0144]

[0145] Step 105: Calculate the optimal parameters based on the refined linear model of the phase loop controller, and control the flexible low-frequency AC transmission system according to the optimal parameters to achieve transient synchronization optimization; the optimal parameters include the frequency deviation reference value and the detection phase angle.

[0146] In this embodiment, the refined linear model of the phase-locked loop controller is applied to the phase-locked loop detection system. The normalized q-axis voltage generates a frequency deviation reference value through the controller, which is then superimposed with the rated frequency and integrated to output the optimized detection phase angle, so as to achieve transient synchronization optimization of offshore wind power through the flexible low-frequency AC transmission system.

[0147] Please refer to Figure 4 , Figure 4 The waveform diagram of observation frequency and phase angle error during phase perturbation is provided for an embodiment of the present invention.

[0148] In this embodiment, the input voltage phase suddenly increases by 0.1 rad, such as Figure 4 As shown, when the phase is disturbed, the observed frequency has an error. Before optimization, the error first decreases and then increases in the opposite direction, and the recovery time is relatively long. After optimization by the refined linear model of the phase loop controller, the error can be quickly reduced to 0, which verifies that the refined linear model of the phase loop controller can improve the dynamic performance of frequency and phase detection.

[0149] Please refer to Figure 5 , Figure 5 The waveform diagram of observation frequency and phase angle error during amplitude perturbation is provided in an embodiment of the present invention.

[0150] In this embodiment, the input voltage amplitude suddenly drops by 0.1 pu, such as Figure 5 As shown, before optimization, the oscillation amplitude and instantaneous error of the observed frequency and phase were relatively large. After optimization by the refined linear model of the phase loop controller, the oscillation amplitude and instantaneous error were significantly reduced.

[0151] Please refer to Figure 6 , Figure 6 The waveform diagram of observation frequency and phase angle error during voltage imbalance disturbance is provided in an embodiment of the present invention.

[0152] In this embodiment, a negative sequence component with an amplitude of 0.1 pu is suddenly added to the input voltage, such as... Figure 6 As shown, after optimization by the refined linear model of the phase loop controller, the influence of unbalanced components in the input voltage can be eliminated, thereby improving the dynamic performance of frequency and phase detection.

[0153] Please refer to Figure 7 , Figure 7 The waveform diagram of observation frequency and phase angle error during harmonic voltage disturbance is provided for an embodiment of the present invention.

[0154] In this embodiment, the input voltage is suddenly increased by a fifth harmonic with an amplitude of 0.1 pu and a seventh harmonic with an amplitude of 0.1 pu, such as... Figure 7 As shown, compared with the original model, the refined linear model of the phase loop controller can filter out most harmonics and significantly reduce the ripple of the observation frequency and phase angle.

[0155] In this embodiment, the present invention constructs a pre-processor complex vector filter based on the three-phase voltage signal of the detection point, and then constructs a phase closed-loop transfer model of the post-processor phase-locked loop based on the pre-processor complex vector filter to design a refined linear model of the phase-locked loop controller. The optimal parameters are calculated based on the refined linear model of the phase-locked loop controller to achieve transient synchronization optimization of the flexible low-frequency AC transmission system, thereby improving the dynamic performance of frequency and phase detection, reducing oscillation amplitude and instantaneous error, and improving the transient stability of the system.

[0156] This invention also provides a transient synchronization optimization device for a flexible low-frequency AC transmission system, comprising: a positive and negative sequence component model construction module, a filter construction module, a transfer model construction module, a refined linear model construction module, and a control module;

[0157] The positive and negative sequence component model construction module is used to acquire the three-phase voltage signals of the offshore low-frequency collection bus and the onshore power frequency grid connection point, and construct a positive and negative sequence component model based on the three-phase voltage signals of the offshore low-frequency collection bus and the onshore power frequency grid connection point.

[0158] The filter construction module is used to construct a pre-processor complex vector filter based on the positive and negative sequence component model, and to obtain the positive and negative sequence components of the voltage signal at the detection point based on the pre-processor complex vector filter and the fundamental frequency of the voltage signal at the detection point.

[0159] The transfer model construction module is used to construct a small-signal linear model based on the pre-processor complex vector filter, construct a phase closed-loop transfer model of the post-processor phase-locked loop based on the small-signal linear model, and optimize the phase closed-loop transfer model.

[0160] The refined linear model construction module is used to construct a constraint model based on the optimized phase closed-loop transfer model, and to construct a refined linear model of the phase-locked loop controller based on the constraint model and the phase closed-loop transfer model.

[0161] The control module is used to calculate the optimal parameters based on the refined linear model of the phase loop controller, and control the flexible low-frequency AC transmission system according to the optimal parameters to achieve transient synchronization optimization; the optimal parameters include frequency deviation reference value and detection phase angle.

[0162] In this embodiment, the positive and negative order component model construction module is specifically used for:

[0163] The three-phase voltage signals of the offshore low-frequency bus and the onshore power frequency grid connection point are detected, and the positive sequence component model and negative sequence component model under the three-phase stationary reference frame are calculated based on the three-phase voltage signals of the offshore low-frequency bus and the onshore power frequency grid connection point.

[0164] The detection voltage under a two-phase stationary reference frame is obtained, and a positive-sequence component model and a negative-sequence component model under the two-phase stationary reference frame are constructed based on the detection voltage under the two-phase stationary reference frame.

[0165] In this embodiment, the filter construction module is specifically used for:

[0166] A pre-processed complex vector filter is constructed based on the positive-sequence component model and the negative-sequence component model under the two-phase stationary reference frame. The pre-processed complex vector filter includes a filter transfer function model and a time-domain expression.

[0167] The filter transfer function model is constructed based on the positive-sequence component model and the negative-sequence component model, the damping coefficient, the filtering frequency, and the Laplace operator.

[0168] The time-domain expression is constructed based on the Laplace transform of the output signal of the filter transfer function model.

[0169] In this embodiment, the transfer model construction module is specifically used for:

[0170] The fundamental positive sequence phase angle and fundamental positive sequence amplitude of the voltage detection point are obtained based on the pre-processed complex vector filter.

[0171] Based on the filter transfer function model, the rectangular coordinates of the complex vector voltage signal are converted to polar coordinates;

[0172] A first small-signal linear model is constructed based on the polar coordinates, actual phase angle, fundamental positive-sequence phase angle, actual amplitude, and fundamental positive-sequence amplitude of the complex vector voltage signal.

[0173] The phase closed-loop transfer model of the post-phase-locked loop is constructed based on the first small-signal linear model.

[0174] In this embodiment, the refined linear model construction module is specifically used for:

[0175] Obtain the constraints of the phase closed-loop transfer model, and construct a constraint model based on the constraints. The constraint model includes an order constraint model and a mapping relationship constraint model.

[0176] A first-order low-pass filter model is constructed based on the constraint model, and a refined linear model of the phase-locked loop controller is constructed based on the first-order low-pass filter model and the phase closed-loop transfer model.

[0177] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. In particular, it should be noted that any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention for those skilled in the art.

Claims

1. A transient synchronization optimization method for a flexible low-frequency AC transmission system, characterized in that, include: The three-phase voltage signals of the offshore low-frequency collection bus and the onshore power frequency grid connection point are obtained, and positive and negative sequence component models are constructed based on the three-phase voltage signals of the offshore low-frequency collection bus and the onshore power frequency grid connection point. A pre-processing complex vector filter is constructed based on the positive and negative sequence component model, and the positive and negative sequence components of the voltage signal at the detection point are obtained based on the pre-processing complex vector filter and the fundamental frequency of the voltage signal at the detection point. A small-signal linear model is constructed based on the pre-processor complex vector filter, and a phase closed-loop transfer model for the subsequent phase-locked loop (PLL) is constructed based on the small-signal linear model. Specifically: the fundamental positive-sequence phase angle and fundamental positive-sequence amplitude of the voltage detection point are obtained based on the pre-processor complex vector filter; the rectangular coordinates of the complex vector voltage signal are converted to polar coordinates based on the filter transfer function model; a first small-signal linear model is constructed based on the polar coordinates, actual phase angle, fundamental positive-sequence phase angle, actual amplitude, and fundamental positive-sequence amplitude of the complex vector voltage signal; a phase closed-loop transfer model for the subsequent PLL is constructed based on the first small-signal linear model; and the phase closed-loop transfer model is optimized. Specifically: the pre-processor complex vector filter and the subsequent PLL are cascaded to optimize the subsequent PLL; a second small-signal linear model of the optimized subsequent PLL is obtained, and an optimized phase closed-loop transfer model is constructed based on the second small-signal linear model. A constraint model is constructed based on the optimized phase closed-loop transfer model, and a refined linear model of the phase-locked loop controller is constructed based on the constraint model and the optimized phase closed-loop transfer model. The optimal parameters are calculated based on the refined linear model of the phase-locked loop controller, and the flexible low-frequency AC transmission system is controlled according to the optimal parameters to achieve transient synchronization optimization; the optimal parameters include the frequency deviation reference value and the detection phase angle.

2. The transient synchronization optimization method for a flexible low-frequency AC transmission system as described in claim 1, characterized in that, The construction of the positive and negative sequence component model based on the three-phase voltage signals of the offshore low-frequency collection bus and the onshore power frequency grid connection point is specifically as follows: The three-phase voltage signals of the offshore low-frequency bus and the onshore power frequency grid connection point are detected, and the positive sequence component model and negative sequence component model under the three-phase stationary reference frame are calculated based on the three-phase voltage signals of the offshore low-frequency bus and the onshore power frequency grid connection point. The detection voltage under a two-phase stationary reference frame is obtained, and a positive-sequence component model and a negative-sequence component model under the two-phase stationary reference frame are constructed based on the detection voltage under the two-phase stationary reference frame.

3. The transient synchronization optimization method for a flexible low-frequency AC transmission system as described in claim 2, characterized in that, The construction of the pre-processed complex vector filter based on the positive and negative order component model is specifically as follows: A pre-processed complex vector filter is constructed based on the positive-sequence component model and the negative-sequence component model under the two-phase stationary reference frame. The pre-processed complex vector filter includes a filter transfer function model and a time-domain expression. The filter transfer function model is constructed based on the positive-sequence component model and the negative-sequence component model, the damping coefficient, the filtering frequency, and the Laplace operator. The time-domain expression is constructed based on the Laplace transform of the output signal of the filter transfer function model.

4. The transient synchronization optimization method for a flexible low-frequency AC transmission system as described in claim 1, characterized in that, The process involves constructing a constraint model based on the optimized phase closed-loop transfer model, and then constructing a refined linear model of the phase-locked loop controller based on the constraint model and the phase closed-loop transfer model. Specifically: Obtain the constraints of the phase closed-loop transfer model, and construct a constraint model based on the constraints. The constraint model includes an order constraint model and a mapping relationship constraint model. A first-order low-pass filter model is constructed based on the constraint model, and a refined linear model of the phase-locked loop controller is constructed based on the first-order low-pass filter model and the phase closed-loop transfer model.

5. A transient synchronization optimization device for a flexible low-frequency AC transmission system, characterized in that, include: The module includes a positive and negative order component model building module, a filter building module, a transfer model building module, a refined linear model building module, and a control module. The positive and negative sequence component model construction module is used to acquire the three-phase voltage signals of the offshore low-frequency collection bus and the onshore power frequency grid connection point, and construct a positive and negative sequence component model based on the three-phase voltage signals of the offshore low-frequency collection bus and the onshore power frequency grid connection point. The filter construction module is used to construct a pre-processor complex vector filter based on the positive and negative sequence component model, and to obtain the positive and negative sequence components of the voltage signal at the detection point based on the pre-processor complex vector filter and the fundamental frequency of the voltage signal at the detection point. The transfer model construction module is used to construct a small-signal linear model based on the pre-processor complex vector filter, and to construct a phase closed-loop transfer model of the post-processor phase-locked loop based on the small-signal linear model. Specifically: it obtains the fundamental positive-sequence phase angle and fundamental positive-sequence amplitude of the voltage detection point based on the pre-processor complex vector filter; it converts the rectangular coordinates of the complex vector voltage signal to polar coordinates based on the filter transfer function model; it constructs a first small-signal linear model based on the polar coordinates, actual phase angle, fundamental positive-sequence phase angle, actual amplitude, and fundamental positive-sequence amplitude of the complex vector voltage signal; it constructs a phase closed-loop transfer model of the post-processor phase-locked loop based on the first small-signal linear model; and it optimizes the phase closed-loop transfer model. Specifically: it cascades the pre-processor complex vector filter and the post-processor phase-locked loop to optimize the post-processor phase-locked loop; it obtains a second small-signal linear model of the optimized post-processor phase-locked loop, and constructs an optimized phase closed-loop transfer model based on the second small-signal linear model. The refined linear model construction module is used to construct a constraint model based on the optimized phase closed-loop transfer model, and to construct a refined linear model of the phase-locked loop controller based on the constraint model and the optimized phase closed-loop transfer model. The control module is used to calculate optimal parameters based on the refined linear model of the phase-locked loop controller, and control the flexible low-frequency AC transmission system according to the optimal parameters to achieve transient synchronization optimization; the optimal parameters include frequency deviation reference value and detection phase angle.

6. The transient synchronization optimization device for a flexible low-frequency AC transmission system as described in claim 5, characterized in that, The positive and negative order component model construction module is specifically used for: The three-phase voltage signals of the offshore low-frequency bus and the onshore power frequency grid connection point are detected, and the positive sequence component model and negative sequence component model under the three-phase stationary reference frame are calculated based on the three-phase voltage signals of the offshore low-frequency bus and the onshore power frequency grid connection point. The detection voltage under a two-phase stationary reference frame is obtained, and a positive-sequence component model and a negative-sequence component model under the two-phase stationary reference frame are constructed based on the detection voltage under the two-phase stationary reference frame.

7. The transient synchronization optimization device for a flexible low-frequency AC transmission system as described in claim 6, characterized in that, The filter construction module is specifically used for: A pre-processed complex vector filter is constructed based on the positive-sequence component model and the negative-sequence component model under the two-phase stationary reference frame. The pre-processed complex vector filter includes a filter transfer function model and a time-domain expression. The filter transfer function model is constructed based on the positive-sequence component model and the negative-sequence component model, the damping coefficient, the filtering frequency, and the Laplace operator. The time-domain expression is constructed based on the Laplace transform of the output signal of the filter transfer function model.

8. The transient synchronization optimization device for a flexible low-frequency AC transmission system as described in claim 5, characterized in that, The refined linear model construction module is specifically used for: Obtain the constraints of the phase closed-loop transfer model, and construct a constraint model based on the constraints. The constraint model includes an order constraint model and a mapping relationship constraint model. A first-order low-pass filter model is constructed based on the constraint model, and a refined linear model of the phase-locked loop controller is constructed based on the first-order low-pass filter model and the phase closed-loop transfer model.

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