Model predictive control method for flexible low-frequency power transmission system with frequency support capability

Abstract

The application discloses a flexible low-frequency power transmission system model prediction control method with frequency support capability, and aims at the problem that a traditional low-frequency power transmission system M3C converter control strategy cannot provide active support for the frequency of a power frequency alternating current grid, formulates a prediction calculation method for the frequency of the power frequency grid, the low-frequency side current and the power frequency side current in the M3C converter based on a model prediction control algorithm, designs a target function based on the frequency of the power frequency grid, the low-frequency side current and the power frequency side current, compares the target function values in different modes through all sub-module investment methods, and obtains the optimal control mode, which can actively support the grid frequency when the frequency of the power frequency grid fluctuates, and has a good application prospect.

Description

Technical Field

[0001] This invention belongs to the field of power electronics technology, specifically relating to a model predictive control method for a flexible low-frequency power transmission system with frequency support capability. Background Technology

[0002] Currently, the construction of offshore wind farms is gradually shifting from short-distance, small-capacity operations to large-scale, deep-sea deployments. When offshore wind farms are located beyond a certain distance from shore, traditional power frequency AC transmission suffers from excessive reactive power consumption due to the equivalent capacitance of the submarine cable. Flexible low-frequency (LHF) transmission, with its significantly lower transmission frequency than power frequency transmission, requires less charging power for the LHF AC submarine cable, thus increasing its transmission capacity. Furthermore, compared to flexible DC transmission, LHF transmission offers advantages such as zero-crossing current interruption and ease of grid connection, allowing wind turbines to directly output low-frequency energy without the need for an offshore converter platform. Therefore, offshore wind power flexible LHF AC transmission combines the advantages of both power frequency AC and flexible DC transmission, making it advantageous for medium- to long-distance offshore wind power transmission.

[0003] With the increasing proportion of renewable energy grid connection, the demand for active frequency support for renewable energy power generation systems is becoming increasingly prominent. Offshore wind power low-frequency transmission systems, as a renewable energy grid connection solution, also need to consider how to provide frequency support to the grid. Using M3C converters as power frequency and low-frequency conversion devices is currently the mainstream technical solution for offshore wind power low-frequency transmission. Existing M3C converter control strategies employ vector control strategies based on PI controllers. The control objectives on the power frequency side are generally set as the average value of the control capacitor voltage and the reactive power on the power frequency side, without considering active frequency support for the power frequency grid, thus failing to meet the system stability requirements under scenarios with a high proportion of renewable energy grid connection. Summary of the Invention

[0004] The purpose of this invention is to provide a model predictive control method for flexible low-frequency power transmission systems with frequency support capabilities, solving the problem that traditional M3C converter control strategies for low-frequency power transmission systems cannot provide active frequency support for the industrial frequency AC power grid.

[0005] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0006] A model predictive control method for a flexible low-frequency power transmission system with frequency support capability is characterized by the following: the control method is based on a model predictive control algorithm, which formulates a predictive calculation method for the power grid frequency, low-frequency side current, and power frequency side current in the M3C converter, and designs an objective function based on the power grid frequency, low-frequency side current, and power frequency side current. By traversing all full-bridge submodule input methods, the objective function values ​​under different methods are compared to obtain the optimal control method, which can actively support the power grid frequency when the power grid frequency fluctuates. The control system implementing the method includes: a coordinate transformation module, a low-frequency side current reference value calculation module, a power frequency side current reference value calculation module, a low-frequency side current prediction module, a power frequency side current prediction module, a power grid frequency prediction module, a low-frequency side current objective function calculation module, a power frequency side current objective function calculation module, a power grid frequency objective function calculation module, a final objective function calculation module, and an objective function comparison and control command output module.

[0007] The coordinate transformation module performs Clark transformation on the low-frequency grid voltage, power frequency grid voltage, low-frequency side current, and power frequency side current to obtain the corresponding voltage and current in the two-phase stationary coordinate system.

[0008] The low-frequency side current reference value calculation module calculates the low-frequency side current reference value according to the system's low-frequency side control target.

[0009] The power frequency side current reference value calculation module calculates the power frequency side current reference value according to the system power frequency side control target;

[0010] The low-frequency side current prediction module, based on the low-frequency grid voltage U obtained in this sampling period... iαβ Low-frequency side current I wαβ Calculate the low-frequency side current I in the next sampling period when different sub-module activation methods are used in the current sampling period. wαβ(next) In this sampling period, there are M seed module input methods, requiring a total of M calculations of I. wαβ(next) , to obtain I wαβ(next) (m), m=1,2,...M;

[0011] The power frequency side current prediction module, based on the power frequency grid voltage U obtained in this sampling period... oαβ Low-frequency side current I vαβ Calculate the power frequency side current I in the next sampling period when different sub-module activation methods are used in the current sampling period. vαβ(next) In this sampling period, there are M seed module input methods, requiring a total of M calculations of I. vαβ(next) , to obtain I vαβ(next) (m);

[0012] The power grid frequency prediction module calculates the power grid frequency f1 for the next sampling period based on the power grid frequency f1 obtained in the current sampling period, when different sub-module input methods are used in the current sampling period. 1(next) In this sampling period, there are M seed module input methods, requiring a total of M calculations of f. 1(next) , to obtain f 1(next) (m);

[0013] The low-frequency side current objective function calculation module calculates the low-frequency side current I based on the next sampling period. wαβ(next) (m) and low-frequency side current reference value I wαβref Calculate the objective function J of the low-frequency side current. 1m ;

[0014] The power frequency side current objective function calculation module calculates the power frequency side current I based on the next sampling period. vαβ(next) (m) and reference value of power frequency side current I vαβref Calculate the objective function J of the power frequency side current. 2m ;

[0015] The power grid frequency objective function calculation module calculates the power grid frequency f based on the next sampling period. 1(next) (m) and the reference value of the power grid frequency f ref Calculate the frequency objective function J of the power frequency grid 3m ;

[0016] The final objective function calculation module calculates the objective function J based on the low-frequency side current objective function. 1m Objective function of power frequency side current J 2m and the objective function of power grid frequency J 3m The final objective function J is calculated. m (m=1,2,...M);

[0017] The objective function comparison and control command output module compares the final objective function J using the M-seed module input method. m (m=1,2,...M), the submodule with the smallest objective function is selected as the control command for this sampling period to control the M3C converter.

[0018] The full-bridge submodule consists of four IGBTs with anti-parallel diodes and one capacitor.

[0019] Furthermore, the low-frequency side current prediction module calculates the low-frequency side current I for the next sampling period according to the following method. wαβ(next) (m) (m=1,2,...M):

[0020]

[0021] Where L0 is the inductance of the bridge arm reactor, L is T is the equivalent inductance of the transformer connected to the low-frequency side. s For the sampling period, u iα and u iβ These are the low-frequency grid voltages U iαβ α-axis, β-axis components, i wα and i wβ These are the low-frequency side currents I and I. wαβ α-axis, β-axis components, I wα(next) (m) and I wβ(next) (m) represents the low-frequency side current I in the next sampling period. wαβ(next) (m) α-axis and β-axis components; u sumα (m), u sumβ (m) represents the low-frequency output voltage U using the m-th seed module input method. sumαβ The α-axis and β-axis components of (m) are calculated as follows:

[0022]

[0023] Among them, u Aa u Ab u Ac u Ba u Bb u Bc u Ca u Cb u Cc The arm voltages of the nine arms of the M3C converter when the m-th seed module is activated.

[0024] Furthermore, the power frequency side current prediction module calculates the power frequency side current I for the next sampling period according to the following method. vαβ(next) (m) (m=1,2,...M):

[0025]

[0026] Where L0 is the inductance of the bridge arm reactor, L os T is the equivalent inductance of the transformer connected to the power frequency side. s For the sampling period, u oα and u oβ These are the power frequency grid voltages U oαβ α-axis, β-axis components, i vα and i vβ These are the power frequency side currents I vαβ α-axis, β-axis components, I vα(next) (m) and I vβ(next) (m) represents the power frequency side current I in the next sampling period.vαβ(next) (m) α-axis and β-axis components; u comα (m), u comβ (m) represents the power frequency output voltage U using the m-th seed module input method. comαβ The α-axis and β-axis components of (m) are calculated as follows:

[0027]

[0028] Among them, u Aa u Ab u Ac u Ba u Bb u Bc u Ca u Cb u Cc The arm voltages of the nine arms of the M3C converter when the m-th seed module is activated.

[0029] Furthermore, the power grid frequency prediction module calculates the power grid frequency f for the next sampling period according to the following method. 1(next) (m) (m=1,2,...M):

[0030]

[0031]

[0032] Among them, T s For the sampling period, P ref P is the reference value for active power on the power frequency side. v(next) (m) represents the active power on the power frequency side in the next sampling period, J represents the equivalent rotational inertia of the system, and D represents the active power on the power frequency side in the next sampling period. p is the system's equivalent damping coefficient.

[0033] Furthermore, the low-frequency side current objective function calculation module calculates the low-frequency side current objective function J according to the following method. 1m (m=1,2,...M):

[0034]

[0035] Among them, I wαref and I wβref These are the low-frequency side current reference values ​​I. wαβref α-axis and β-axis components, J 1m Let m be the objective function of the low-frequency side current when using the m-th seed module input method.

[0036] Furthermore, the power frequency side current objective function calculation module calculates the power frequency side current objective function J according to the following method. 2m(m=1,2,...M):

[0037]

[0038] Among them, I vαref and I vβref These are the reference values ​​for the power frequency side current I. vαβref α-axis and β-axis components, J 2m Let m be the objective function of the power frequency side current when using the m-th seed module input method.

[0039] Furthermore, the power grid frequency objective function calculation module calculates the power grid frequency objective function J according to the following method. 3m (m=1,2,...M):

[0040]

[0041] Furthermore, the final objective function calculation module calculates the final objective function J according to the following method. m (m=1,2,...M):

[0042]

[0043] Where p1 is the low-frequency side current weighting factor, p2 is the power frequency side current weighting factor, and p3 is the power frequency grid frequency weighting factor.

[0044] The beneficial effects of this invention are:

[0045] By adopting the technical solution of this invention, a predictive calculation method for the power grid frequency, low-frequency side current, and power frequency side current in the M3C converter is formulated based on the model predictive control algorithm. Based on the power grid frequency, low-frequency side current, and power frequency side current, an objective function is designed. By traversing all sub-module input methods and comparing the objective function values ​​under different methods, the optimal control method is obtained. This method can actively support the power grid frequency when the power grid frequency fluctuates, and has good application prospects. Attached Figure Description

[0046] Figure 1 This is a typical topology diagram of the M3C converter in this invention.

[0047] Figure 2 This is a schematic diagram of a specific example of the control system of the frequency active support method based on model predictive control in this invention.

[0048] Figure 3 This is a typical topology diagram of the full-bridge submodule in the M3C converter of this invention. Detailed Implementation

[0049] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0050] like Figure 1 As shown in the embodiment of the present invention, the M3C converter consists of three converter modules, each converter module includes three bridge arm branches, and the entire M3C converter consists of nine bridge arm branches. Each bridge arm branch is composed of multiple cascaded full-bridge sub-modules and bridge arm inductors connected in series. The three-phase AC systems on both sides of the M3C converter are connected through the bridge arm branches, and each phase of the system on one side is connected to the three phases of the system on the other side through three bridge arm branches.

[0051] like Figure 3 As shown, the full-bridge submodule consists of four IGBTs with anti-parallel diodes and one capacitor.

[0052] like Figure 2 As shown, in this embodiment of the invention, the M3C converter control system implementing the frequency active support method based on model predictive control includes: a coordinate transformation module 101, a low-frequency side current reference value calculation module 102, a power frequency side current reference value calculation module 103, a low-frequency side current prediction module 104, a power frequency side current prediction module 105, a power frequency grid frequency prediction module 106, a low-frequency side current objective function calculation module 107, a power frequency side current objective function calculation module 108, a power frequency grid frequency objective function calculation module 109, a final objective function calculation module 110, and an objective function comparison and control command output module 111.

[0053] In coordinate transformation module 101, Clark transformation is performed on low-frequency grid voltage, power frequency grid voltage, low-frequency side current, and power frequency side current to obtain the corresponding voltage and current in a two-phase stationary coordinate system. The specific implementation method is as follows:

[0054]

[0055]

[0056]

[0057]

[0058] In the two-phase stationary coordinate system, u iα and u iβ These are the low-frequency grid voltages U iαβ α-axis, β-axis components, u oα and u oβ These are the power frequency grid voltages U oαβ α-axis, β-axis components, i wα and i wβThese are the low-frequency side currents I and I. wαβ α-axis, β-axis components, i vα and i vβ These are the power frequency side currents I vαβ The α-axis and β-axis components; in the three-phase coordinate system, u ia u ib and u ic These are the low-frequency grid voltages U iabc The three-phase components a, b, and c, u oa u ob and u oc These are the power frequency grid voltages U oabc The three-phase components a, b, and c, i wa i wb and i wc These are the low-frequency side currents I and I. wabc The three-phase components a, b, and c, i va i vb and i vc These are the power frequency side currents I vabc The three-phase components a, b, and c.

[0059] In the low-frequency side current reference value calculation module 102, the low-frequency side current reference value is calculated according to the system's low-frequency side control target. In this embodiment of the invention, combined with the actual needs of the offshore wind power low-frequency transmission application scenario, the low-frequency side control target is set to control the low-frequency grid voltage and frequency.

[0060] In the power frequency side current reference value calculation module 103, the power frequency side current reference value is calculated according to the system power frequency side control target. In this embodiment of the invention, combined with the actual needs of the offshore wind power low frequency transmission application scenario, the power frequency side control target is set as the average voltage of the control submodule and the power frequency side reactive power.

[0061] In the low-frequency side current prediction module 104, the low-frequency grid voltage U obtained in this sampling period is used as a basis. iαβ Low-frequency side current I wαβ Calculate the low-frequency side current I in the next sampling period when different sub-module activation methods are used in the current sampling period. wαβ(next) In this sampling period, there are M seed module input methods, requiring a total of M calculations of I. wαβ(next) , to obtain I wαβ(next) (m), m=1,2,...M, the specific implementation is as follows:

[0062]

[0063] Where L0 is the inductance of the bridge arm reactor, L is T is the equivalent inductance of the transformer connected to the low-frequency side. sFor the sampling period, u iα and u iβ These are the low-frequency grid voltages U iαβ α-axis, β-axis components, i wα and i wβ These are the low-frequency side currents I and I. wαβ α-axis, β-axis components, I wα(next) (m) and I wβ(next) (m) represents the low-frequency side current I in the next sampling period. wαβ(next) (m) α-axis and β-axis components; u sumα (m), u sumβ (m) represents the low-frequency output voltage U using the m-th seed module input method. sumαβ The α-axis and β-axis components of (m) are calculated as follows:

[0064]

[0065] Among them, u Aa u Ab u Ac u Ba u Bb u Bc u Ca u Cb u Cc The arm voltages of the nine arms of the M3C converter when the m-th seed module is activated.

[0066] In the power frequency side current prediction module 105, the power frequency grid voltage U obtained in this sampling period is used as a basis. oαβ Low-frequency side current I vαβ Calculate the power frequency side current I in the next sampling period when different sub-module activation methods are used in the current sampling period. vαβ(next) In this sampling period, there are M seed module input methods, requiring a total of M calculations of I. vαβ(next) , to obtain I vαβ(next) (m), the specific implementation method is as follows:

[0067]

[0068] Where L0 is the inductance of the bridge arm reactor, L os T is the equivalent inductance of the transformer connected to the power frequency side. s For the sampling period, u oα and u oβ These are the power frequency grid voltages U oαβ α-axis, β-axis components, i vα and i vβ These are the power frequency side currents I vαβ α-axis, β-axis components, I vα(next)(m) and I vβ(next) (m) represents the power frequency side current I in the next sampling period. vαβ(next) (m) α-axis and β-axis components; u comα (m), u comβ (m) represents the power frequency output voltage U using the m-th seed module input method. comαβ The α-axis and β-axis components of (m) are calculated as follows:

[0069] .

[0070] In the power grid frequency prediction module 106, based on the power grid frequency f1 obtained in the current sampling period, the power grid frequency f1 in the next sampling period is calculated when different sub-module input methods are used in the current sampling period. 1(next) In this sampling period, there are M seed module input methods, requiring a total of M calculations of f. 1(next) , to obtain f 1(next) (m), the specific implementation method is as follows:

[0071]

[0072]

[0073] Among them, T s For the sampling period, P ref P is the reference value for active power on the power frequency side. v(next) (m) represents the active power on the power frequency side in the next sampling period, J represents the equivalent rotational inertia of the system, and D represents the active power on the power frequency side in the next sampling period. p is the system's equivalent damping coefficient.

[0074] In the low-frequency side current objective function calculation module 107, based on the low-frequency side current I in the next sampling period wαβ(next) (m) and low-frequency side current reference value I wαβref Calculate the objective function J of the low-frequency side current. 1m The specific implementation method is as follows:

[0075]

[0076] Among them, I wαref and I wβref These are the low-frequency side current reference values ​​I. wαβref α-axis and β-axis components, J 1m Let m be the objective function of the low-frequency side current when using the m-th seed module input method.

[0077] In the power frequency side current objective function calculation module 108, based on the power frequency side current I in the next sampling period... vαβ(next) (m) and reference value of power frequency side current Ivαβref Calculate the objective function J of the power frequency side current. 2m The specific implementation method is as follows:

[0078]

[0079] Among them, I vαref and I vβref These are the reference values ​​I for the power frequency side current. vαβref α-axis and β-axis components, J 2m Let m be the objective function of the power frequency side current when using the m-th seed module input method.

[0080] In the power grid frequency objective function calculation module 109, based on the power grid frequency f of the next sampling period... 1(next) (m) and the reference value of the power grid frequency f ref Calculate the frequency objective function J of the power frequency grid 3m , .

[0081]

[0082] In the final objective function calculation module 110, based on the low-frequency side current objective function J... 1m Objective function of power frequency side current J 2m and the objective function of power grid frequency J 3m The final objective function J is calculated. m (m=1,2,...M), the specific implementation method is as follows:

[0083]

[0084] Where p1 is the low-frequency side current weighting factor, p2 is the power frequency side current weighting factor, and p3 is the power frequency grid frequency weighting factor.

[0085] In the objective function comparison and control command output module 111, the final objective function J is compared using the M-seed module input method. m The submodule with the smallest objective function is selected as the control command for this sampling period to control the M3C converter.

[0086] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. It will be apparent to those skilled in the art that various modifications can be made to the above embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A model predictive control method for a flexible low-frequency transmission system with frequency support capability, characterized in that, The control method is based on the model predictive control algorithm. It formulates a predictive calculation method for the power grid frequency, low-frequency side current, and power frequency side current in the M3C converter. Based on the power grid frequency, low-frequency side current, and power frequency side current, it designs an objective function. By traversing all full-bridge submodule input methods, it compares the objective function values ​​under different methods to obtain the optimal control method, which can actively support the power grid frequency when the power grid frequency fluctuates. The control system for implementing the method includes: a coordinate transformation module, a low-frequency side current reference value calculation module, a power frequency side current reference value calculation module, a low-frequency side current prediction module, a power frequency side current prediction module, a power frequency grid frequency prediction module, a low-frequency side current objective function calculation module, a power frequency side current objective function calculation module, a power frequency grid frequency objective function calculation module, a final objective function calculation module, and an objective function comparison and control command output module. The coordinate transformation module performs Clark transformation on the low-frequency grid voltage, power frequency grid voltage, low-frequency side current, and power frequency side current to obtain the corresponding voltage and current in the two-phase stationary coordinate system. The low-frequency side current reference value calculation module calculates the low-frequency side current reference value according to the system's low-frequency side control target. The power frequency side current reference value calculation module calculates the power frequency side current reference value according to the system power frequency side control target; The low-frequency side current prediction module, based on the low-frequency grid voltage U obtained in this sampling period... iαβ Low-frequency side current I wαβ Calculate the low-frequency side current I in the next sampling period when different sub-module activation methods are used in the current sampling period. wαβ(next) In this sampling period, there are M seed module input methods, requiring a total of M calculations of I. wαβ(next) , to obtain I wαβ(next) (m), m=1,2,...M; The power frequency side current prediction module, based on the power frequency grid voltage U obtained in this sampling period... oαβ Low-frequency side current I vαβ Calculate the power frequency side current I in the next sampling period when different sub-module activation methods are used in the current sampling period. vαβ(next) In this sampling period, there are M seed module input methods, requiring a total of M calculations of I. vαβ(next) , to obtain I vαβ(next) (m); The power grid frequency prediction module calculates the power grid frequency f1 for the next sampling period based on the power grid frequency f1 obtained in the current sampling period, when different sub-module input methods are used in the current sampling period. 1(next) In this sampling period, there are M seed module input methods, requiring a total of M calculations of f. 1(next) , to obtain f 1(next) (m); The low-frequency side current objective function calculation module calculates the low-frequency side current I based on the next sampling period. wαβ(next) (m) and low-frequency side current reference value I wαβref Calculate the objective function J of the low-frequency side current. 1m ; The power frequency side current objective function calculation module calculates the power frequency side current I based on the next sampling period. vαβ(next) (m) and reference value of power frequency side current I vαβref Calculate the objective function J of the power frequency side current. 2m ; The power grid frequency objective function calculation module calculates the power grid frequency f based on the next sampling period. 1(next) (m) and the reference value of the power grid frequency f ref Calculate the frequency objective function J of the power frequency grid 3m ; The final objective function calculation module calculates the objective function J based on the low-frequency side current objective function. 1m Objective function of power frequency side current J 2m The objective function of the power grid frequency J 3m The final objective function J is calculated. m ; The objective function comparison and control command output module compares the final objective function J using the M-seed module input method. m The submodule with the smallest objective function is selected as the control command for this sampling period to control the M3C converter.

2. The model predictive control method for a flexible low-frequency transmission system with frequency support capability according to claim 1, characterized in that: The low-frequency side current prediction module calculates the low-frequency side current I for the next sampling period according to the following method. wαβ(next) (m): Where L0 is the inductance of the bridge arm reactor, L is For the equivalent inductance of the transformer connected to the low-frequency side, T s For the sampling period, u iα and u iβ These are the low-frequency grid voltages U iαβ α-axis, β-axis components, i wα and i wβ These are the low-frequency side currents I and I. wαβ α-axis, β-axis components, I wα(next) (m) and I wβ(next) (m) represents the low-frequency side current I in the next sampling period. wαβ(next) (m) α-axis and β-axis components; u sumα (m), u sumβ (m) represents the low-frequency output voltage U using the m-th seed module input method. sumαβ The α-axis and β-axis components of (m) are calculated as follows: Among them, u Aa u Ab u Ac u Ba u Bb u Bc u Ca u Cb u Cc The arm voltages of the nine arms of the M3C converter when the m-th seed module is activated.

3. The model predictive control method for a flexible low-frequency transmission system with frequency support capability according to claim 1, characterized in that: The power frequency side current prediction module calculates the power frequency side current I for the next sampling period according to the following method. vαβ(next) (m): Where L0 is the inductance of the bridge arm reactor, L os T is the equivalent inductance of the transformer connected to the power frequency side. s For the sampling period, u oα and u oβ These are the power frequency grid voltages U oαβ α-axis, β-axis components, i vα and i vβ These are the power frequency side currents I vαβ α-axis, β-axis components, I vα(next) (m) and I vβ(next) (m) represents the power frequency side current I in the next sampling period. vαβ(next) (m) α-axis and β-axis components; u comα (m), u comβ (m) represents the power frequency output voltage U using the m-th seed module input method. comαβ The α-axis and β-axis components of (m) are calculated as follows:

4. The model predictive control method for a flexible low-frequency transmission system with frequency support capability according to claim 1, characterized in that: The power grid frequency prediction module calculates the power grid frequency f for the next sampling period according to the following method. 1(next) (m): Among them, T s For the sampling period, P ref P is the reference value for active power on the power frequency side. v(next) (m) represents the active power on the power frequency side in the next sampling period, J represents the equivalent rotational inertia of the system, and D represents the active power on the power frequency side in the next sampling period. p is the system's equivalent damping coefficient.

5. The model predictive control method for a flexible low-frequency transmission system with frequency support capability according to claim 1, characterized in that: The low-frequency side current objective function calculation module calculates the low-frequency side current objective function J according to the following method. 1m : Among them, I wαref and I wβref These are the low-frequency side current reference values ​​I. wαβref α-axis and β-axis components, J 1m Let m be the objective function of the low-frequency side current when using the m-th seed module input method.

6. The model predictive control method for a flexible low-frequency transmission system with frequency support capability according to claim 1, characterized in that: The power frequency side current objective function calculation module calculates the power frequency side current objective function J according to the following method. 2m : Among them, I vαref and I vβref These are the reference values ​​I for the power frequency side current. vαβref α-axis and β-axis components, J 2m Let m be the objective function of the power frequency side current when using the m-th seed module input method.

7. The model predictive control method for a flexible low-frequency transmission system with frequency support capability according to claim 1, characterized in that: The power grid frequency objective function calculation module calculates the power grid frequency objective function J according to the following method. 3m :

8. The model predictive control method for a flexible low-frequency transmission system with frequency support capability according to claim 1, characterized in that: The final objective function calculation module calculates the final objective function J according to the following method. m : Where p1 is the low-frequency side current weighting factor, p2 is the power frequency side current weighting factor, and p3 is the power frequency grid frequency weighting factor.

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