Method and device for establishing state space model of offshore boost converter

Abstract

The invention provides a method and device for establishing a state space model of an offshore boost converter, and relates to the field of offshore wind power MMC modeling, and the method comprises the steps: obtaining an original model of the offshore wind power MMC, and carrying out the dynamic separation of the AC and DC of the MMC according to a difference-common mode conversion method; an LTP modeling method is adopted to carry out linear processing on the MMC original model after difference-common mode conversion on a steady-state time-varying periodic trajectory of the MMC original model, and a small-signal state space model of the MMC is generated; converting the small signal state space model into a complex vector form through a preset coordinate transformation matrix, and obtaining an open-loop MMC-HSS model in a frequency domain based on an HSS theory; and a closed-loop MMC-HSS model considering the control system is established. For offshore wind power MMC modeling, an MMC small signal state space model is converted into a complex vector form, so that a zero-sequence component is reflected, research of a sequence circulation controller on the interference stability of a grid-connected system is facilitated, and the defects that existing coordinate conversion based on real number variable modeling is complex, the workload is large and the like are overcome.

Description

Technical Field

[0001] The present application relates to the field of offshore wind power MMC modeling, and in particular to a method and device for establishing a state space model of an offshore boost converter. Background Art

[0002] The refined impedance modeling process of each equipment in the offshore wind power MMC-HVDC grid-connected system is particularly important. The complex topology and control system of MMC pose great challenges to the establishment of the model. Compared with the traditional two-level converter, the complex internal dynamic characteristics of MMC also put forward higher requirements for the impedance modeling of MMC.

[0003] Since the traditional impedance modeling method based on harmonic linearization is difficult to reflect the dynamic multi-harmonic coupling characteristics inside the MMC, its application in MMC equipment is limited. Most of the latest studies use multi-harmonic linearization to model the impedance of MMC, but multi-harmonic linearization has the disadvantages of complex coordinate transformation and large workload. Summary of the invention

[0004] In view of the above problems, a method and device for establishing a state space model of an offshore boost converter are proposed to solve the shortcomings of existing modeling, such as complex coordinate transformation and heavy workload.

[0005] In a first aspect, the present application proposes a method for establishing a harmonic state space model of an offshore wind power MMC, comprising:

[0006] Obtain the original model of the offshore wind power MMC and dynamically separate the AC and DC of the MMC based on the differential common mode conversion method;

[0007] The LTP modeling method is used to perform linear processing on the MMC original model after the differential common mode conversion on its steady-state time-varying periodic trajectory to generate a small signal state space model of the MMC;

[0008] The small signal state space model is converted into a complex vector form by a preset coordinate transformation matrix, and an open-loop MMC-HSS model in the frequency domain is obtained based on the HSS theory;

[0009] A closed-loop MMC-HSS model considering the control system is established.

[0010] Optionally, obtaining the offshore wind power MMC original model includes:

[0011] When the preset conditions are met, the voltage equations are written for the upper and lower bridge arms of the kth phase of the MMC. The expressions are as follows:

[0012]

[0013] In the formula, v sk With v dcare the AC system voltage and the DC system voltage, v NO is the voltage difference between the neutral point N on the AC side of the MMC and the neutral point O on the DC side, v NO The expression is:

[0014]

[0015] Among them, the preset condition is that the capacitor voltages of all submodules in the same bridge arm remain the same at all times and the voltage balancing algorithm ensures that the voltage fluctuations of all submodules are consistent and the flow system voltage remains constant.

[0016] Optionally, dynamically separating the MMC AC and DC according to the differential common mode conversion method includes:

[0017] The differential and common mode components in the MMC original model are obtained as shown in formulas (2), (3) and (4), and the expressions are as follows:

[0018]

[0019]

[0020]

[0021] In the formula, the subscript c of the variable represents the common mode component, and the subscript d represents the differential mode component;

[0022] The MMC original model after transformation is represented according to the difference and common mode components, as shown in formulas (5), (6) and (7), and the expression is as follows:

[0023]

[0024]

[0025]

[0026] According to Kirchhoff's voltage theorem, the voltage on the AC side of the MMC is v sk The form after differential common mode conversion is shown in formula (8), and the expression is as follows:

[0027]

[0028] Optionally, the LTP modeling method is used to perform linear processing on the MMC original model after the differential common mode conversion on its steady-state time-varying periodic trajectory to generate a small signal state space model of the MMC, including:

[0029]

[0030]

[0031]

[0032] Among them, the expressions of the selected Δx(t) state variable and the input variable Δu(t) are:

[0033]

[0034] In the formula, a, b, and c in the variable subscripts represent the three-phase components of the corresponding electrical quantities.

[0035] Optionally, converting the small signal state space model into a complex vector form by using a preset coordinate transformation matrix includes:

[0036] For any three-phase electrical component x abc According to the coordinate transformation theory, we can get the expression as shown in formula (13):

[0037]

[0038] In the formula, is the positive sequence component of the variable in the frequency range of (0, +∞), is the negative sequence component of the variable in the frequency range of (-∞∞, 0), is the zero sequence component, T dq / +-0 With T abc / dq is the transformation matrix, where the transformation matrix T abc / dq Obtained by multiplying the Clack transform and the Park transform;

[0039] For the Clack transform, the linearization equation is:

[0040]

[0041] For the Park transform, the linearized equation is:

[0042]

[0043] According to the transformation matrix T abc / dq A preset transformation matrix (14) is obtained, and the small signal state space model shown in formula (11) is converted into a complex vector form (15) through the preset transformation matrix (14), wherein the expressions of formulas (14) and (15) are as follows:

[0044] T=diag[T2T1 T2 T1 T2T1 T2T1] (14)

[0045]

[0046] In the formula, The expression is:

[0047]

[0048] Optionally, obtaining an open-loop MMC-HSS model in the frequency domain based on the HSS theory includes:

[0049] Expanding the converted MMC electrical component of the small signal state space model into a form of superposition of multiple frequency components;

[0050] The small signal state space model expanded by the harmonic balance principle is converted into an MMC-HSS model in the frequency domain. The expression of the MMC-HSS model is:

[0051]

[0052] Optionally, the establishing of a closed-loop MMC-HSS model considering the control system includes:

[0053] Obtain controller models of an AC voltage controller, a base frequency current controller, and a circulating current controller respectively, and obtain modulation functions of the AC voltage controller, the base frequency current controller, and the circulating current controller according to the controller models and control logic;

[0054] The modulation functions of the AC voltage controller, the base frequency current controller and the circulating current controller are introduced into the open-loop MMC-HSS model to obtain the closed-loop MMC-HSS model of the corresponding control system.

[0055] Optionally, the AC voltage controller includes a direct-axis-quadrature-axis dq voltage proportional-integral PI controller in a positive-sequence fundamental frequency synchronous rotating coordinate system, and the controller model of the AC voltage controller is:

[0056]

[0057] The base frequency current controller includes a dq current PI controller, a dq current cross-decoupling controller and a voltage feedforward controller in a positive sequence base frequency synchronous rotating coordinate system. The controller model of the base frequency current controller is:

[0058]

[0059] The circulating current controller includes a negative-sequence double frequency circulating current controller and a zero-sequence circulating current controller. The negative-sequence double frequency circulating current controller includes a dq current PI controller and a dq current cross-decoupling controller in a negative-sequence double frequency rotating coordinate system. The controller model of the negative-sequence double frequency circulating current controller is:

[0060]

[0061] In the formula, kcp With k ci are the proportional and integral coefficients of the circulation controller respectively;

[0062] The controllability model of the zero-sequence circulating current controller is:

[0063] ΔV c0 (S) = F c0 (s)i c0

[0064] Among them, F c0 (s) is the transfer function of the damping controller.

[0065] Optionally, obtaining the modulation functions of the AC voltage controller, the base frequency current controller and the circulating current controller according to the controller model and the control logic includes:

[0066] For the base frequency current controller, the expression of the modulation function is:

[0067]

[0068] For the circulating flow controller, the expression of the modulation function is:

[0069]

[0070] The second aspect of the present application proposes a device for establishing a state space model of an offshore boost converter, comprising:

[0071] The differential and common mode conversion module is used to obtain the original model of the offshore wind power MMC and dynamically separate the AC and DC of the MMC according to the differential and common mode conversion method;

[0072] An LTP modeling module is used to perform linearization processing on the MMC original model after differential common mode conversion on its steady-state time-varying periodic trajectory using an LTP modeling method to generate a small signal state space model of the MMC;

[0073] A coordinate transformation module, used for converting the small signal state space model into a complex vector form through a preset coordinate transformation matrix, and obtaining an open-loop MMC-HSS model in the frequency domain based on the HSS theory;

[0074] Output module, used to establish a closed-loop MMC-HSS model considering the control system.

[0075] The technical solution provided by the embodiments of the present application brings at least the following beneficial effects:

[0076] The idea of ​​differential common mode transformation is used to separate the AC and DC dynamics of offshore wind power MMC, which improves the accuracy of the analysis process of MMC AC and DC dynamics. By converting the MMC small signal state space model into a complex vector form, the zero-sequence component is reflected, which is beneficial to the study of the stability of the grid-connected system interference by the sequence circulating current controller, and solves the shortcomings of the existing modeling based on real variables, such as complex coordinate transformation and large workload. Based on the control system, a closed-loop MMC-HSS model is obtained, and the modeling model is further improved.

[0077] Additional aspects and advantages of the present application will be given in part in the description below, and in part will become apparent from the description below, or will be learned through the practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the following description of the embodiments in conjunction with the accompanying drawings, in which:

[0079] Figure 1 is a flow chart of a method for establishing a state space model of an offshore boost converter according to an embodiment of the present application;

[0080] Figure 2 is a control block diagram of a controller according to an embodiment of the present application;

[0081] Figure 3 It is a block diagram of a device for establishing a state space model of an offshore boost converter according to an embodiment of the present application. DETAILED DESCRIPTION

[0082] The embodiments of the present application are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present application, and should not be construed as limiting the present application.

[0083] Figure 1 1 is a flow chart of a method for establishing a state space model of an offshore boost converter according to an embodiment of the present application, comprising:

[0084] Step 101, obtaining the original model of the offshore wind power MMC, and dynamically separating the AC and DC of the MMC according to the differential common mode conversion method.

[0085] In the embodiment of the present application, without affecting the theoretical analysis, preset conditions are set, and the following assumptions and simplifications are made for the offshore wind power MMC;

[0086] (1) The capacitor voltages of all submodules in the same bridge arm are kept the same at all times, and the MMC bridge arm adopts the average value model equivalent;

[0087] (2) Assume that the voltage balancing algorithm can ensure that the voltage fluctuations of all sub-modules are consistent, ignoring the impact of the modulation link:

[0088] (3) It is assumed that the DC system voltage remains constant, that is, the grid-side MMC converter station does not affect the offshore wind power MMC-HVDC grid-connected system.

[0089] That is, when the preset conditions are met, the voltage equations are written for the upper and lower bridge arms of the kth phase of the MMC, and the expressions are as follows:

[0090]

[0091] In the formula, v sk With v dc are the AC system voltage and the DC system voltage, v NO is the voltage difference between the neutral point N on the AC side of the MMC and the neutral point 0 on the DC side, v NO The expression is:

[0092]

[0093] Thus, an original mathematical model including the electrical components of MMC was established.

[0094] In the embodiment of the present application, according to expression (1), the dynamics of the MMC AC side is mainly affected by the difference between the electrical quantities of the upper and lower bridge arms, while the dynamics of the DC side is mainly affected by the sum of the electrical quantities of the upper and lower bridge arms. However, the MMC AC and DC dynamics are coupled in the original model, which is not conducive to analyzing its dynamic process.

[0095] Therefore, this application uses the idea of ​​differential common mode conversion to dynamically separate the MMC AC and DC. The specific process is:

[0096] The differential and common mode components in the original MMC model are obtained as shown in formulas (2), (3) and (4). The expressions are as follows:

[0097]

[0098]

[0099]

[0100] In the formula, the subscript c of the variable represents the common mode component, and the subscript d represents the differential mode component;

[0101] The transformed MMC original model is represented by the differential and common mode components, as shown in formulas (5), (6) and (7). The expression is as follows:

[0102]

[0103]

[0104]

[0105] According to Kirchhoff's voltage theorem, the voltage on the AC side of the MMC is v sk The form after differential common mode conversion is shown in formula (8), and the expression is as follows:

[0106]

[0107] Step 102 , using the LTP modeling method to perform linearization processing on the MMC original model after the differential common mode conversion on its steady-state time-varying periodic trajectory, to generate a small signal state space model of the MMC.

[0108] In the embodiment of the present application, the original MMC model after differential common mode conversion is linearized on its steady-state time-varying periodic trajectory by formulas (9), (10) and (11), and a small signal state space model of the MMC is generated, which is expressed as follows:

[0109]

[0110]

[0111]

[0112] Among them, the expressions of the selected Δx(t) state variable and the input variable Δu(t) are:

[0113]

[0114] In the formula, since the MMC three-phase main circuit model is established, the a, b and c in the variable subscripts represent the three-phase components of the corresponding electrical quantities.

[0115] Step 103: Convert the small signal state space model into a complex vector form through a preset coordinate transformation matrix, and obtain an open-loop MMC-HSS model in the frequency domain based on the HSS theory.

[0116] In the embodiment of the present application, since the traditional modeling method based on real variables cannot reflect the sequence component, the present application converts the model into a complex vector form through coordinate transformation, so that the zero-sequence component can be reflected in the MMC small signal state space model, specifically:

[0117] For any three-phase electrical component x abc According to the coordinate transformation theory, we can get the expression as shown in formula (13):

[0118]

[0119] In the formula, is the positive sequence component of the variable in the frequency range of (0, +∞∞), is the negative sequence component of the variable in the frequency range of (-∞∞, 0), is the zero sequence component, T dq / +-0 With T abc / dq is the transformation matrix, where the transformation matrix T abc / dq Obtained by multiplying the Clack transform and the Park transform.

[0120] It should be noted that T dq / +-0 With T abc / dq The matrix is:

[0121]

[0122]

[0123] In addition, the relationship established by the Clack transformation is a purely algebraic linear relationship, and its linearized equation is:

[0124]

[0125] Similarly, the relationship established by the Park transformation is a nonlinear relationship involving trigonometric functions, and its linearized equation is:

[0126]

[0127] It is worth noting that, since the physical quantities controlled by the MMC converter station-level controller in the scenario studied in this application are the AC system voltage and frequency (island control mode), the AC side frequency can be considered as a given value, and the angle of coordinate transformation is completely determined. Since the state variable Δx(t) and input variable Δu(t) selected in formula (12) both contain four sets of three-phase variables, in order to convert them into complex vector form, the preset transformation matrix T is defined as:

[0128] T=diag[T2T1 T2T1 T2T1 T2T1] (14)

[0129] Then, the small signal state space model shown in formula (11) is converted into a complex vector form (15) by a preset transformation matrix (14), where the expression of (15) is as follows:

[0130]

[0131] In the formula, The expression is:

[0132]

[0133] Based on the above process, the original MMC state space equation is converted into a complex vector form, and then the MMC electrical component of the converted small signal state space model is expanded into a multi-frequency component superposition form. The small signal state space model after expansion is converted into an MMC-HSS model in the frequency domain through the harmonic balance principle. The expression of the MMC-HSS model is:

[0134]

[0135] Step 104: Establish a closed-loop MMC-HSS model considering the control system.

[0136] In the embodiment of the present application, in order to establish a closed-loop MMC-HSS model, its control system needs to be considered. The goal of the offshore MMC converter station-level controller is to provide stable voltage and frequency for the offshore wind farm. Therefore, the typical control link includes an AC voltage controller, a base frequency current controller, and a circulating current controller. The control block diagram is shown in FIG. Figure 2 shown.

[0137] For the AC voltage controller, the AC voltage controller includes a dq voltage PI controller in a positive sequence fundamental frequency synchronous rotating coordinate system, and the controller model of the AC voltage controller is:

[0138]

[0139] For the fundamental frequency current controller, the fundamental frequency current controller includes a dq current PI controller, a dq current cross-decoupling controller and a voltage feedforward controller in a positive sequence fundamental frequency synchronous rotating coordinate system. The controller model of the fundamental frequency current controller is:

[0140]

[0141] It should be noted that since the AC side of the MMC is a three-phase balanced system, there is no zero-sequence component in the electrical quantities on its AC side, so it does not need to be considered in the modeling of the AC voltage controller and the basefrequency current controller.

[0142] For the circulating current controller, the modeling process of the circulating current controller includes the modeling of the negative-sequence double frequency circulating current controller and the modeling of the zero-sequence circulating current controller. Specifically, the negative-sequence double frequency circulating current controller includes a dq current PI controller and a dq current cross-decoupling controller in a negative-sequence double frequency rotating coordinate system. The controller model of the negative-sequence double frequency circulating current controller is:

[0143]

[0144] In the formula, k cp With k ci are the proportional and integral coefficients of the circulating flow controller respectively;

[0145] The controllability model of the zero-sequence circulating current controller is:

[0146] Δv c0 (s) = F c0 (s)i c0

[0147] Among them, F c0 (s) is the transfer function of the damping controller.

[0148] In the embodiment of the present application, the dq transformation projects the a, b, and c three-phase currents of the stator onto the direct axis d axis, the quadrature axis q axis, and the zero axis (0 axis) perpendicular to the dq plane that rotates with the rotor, thereby achieving the diagonalization of the stator inductance matrix and simplifying the operation analysis of the synchronous motor, that is, transforming the abc coordinate system into the dq coordinate system.

[0149] Since the controller model is established in the frequency domain, the transfer function relationship between the input variable and the output variable at different frequencies can be obtained by frequency shifting the model. Secondly, by expressing it in matrix form, the harmonic state space model of the linear part of the control system can be obtained.

[0150] Then, according to the control model of each controller and Figure 2 The control logic in the control block diagram shown in FIG. 1 obtains the modulation function of each control link. Specifically, for the baseband current controller, the expression of the modulation function is:

[0151]

[0152] For the circulating flow controller, the modulation function is expressed as:

[0153]

[0154] Subsequently, the above modulation function is input into the main loop equation of the open-loop MMC-HSS model, and the corresponding closed-loop MMC-HSS model whose controller includes the MMC control system can be obtained.

[0155] The embodiment of the present application utilizes the idea of ​​differential common mode conversion to separate the AC and DC dynamics of offshore wind power MMC, thereby improving the accuracy of the analysis process of the AC and DC dynamics of MMC. By converting the MMC small signal state space model into a complex vector form, the zero-sequence component is reflected, which is beneficial to the study of the stability of the grid-connected system interference by the sequence circulation controller, and solves the shortcomings of the existing modeling based on real number variables, such as complex coordinate transformation and large workload.

[0156] Figure 3 1 is a block diagram of a device for establishing a state space model of an offshore boost converter according to an embodiment of the present application, comprising:

[0157] The differential common mode conversion module 310 is used to obtain the original model of the offshore wind power MMC and dynamically separate the AC and DC of the MMC according to the differential common mode conversion method;

[0158] The LTP modeling module 320 is used to perform linearization processing on the original MMC model after differential common mode conversion on its steady-state time-varying periodic trajectory using the LTP modeling method to generate a small signal state space model of the MMC;

[0159] A coordinate transformation module 330 is used to transform the small signal state space model into a complex vector form through a preset coordinate transformation matrix, and obtain an open-loop MMC-HSS model in the frequency domain based on the HSS theory;

[0160] The output module 340 is used to establish a closed-loop MMC-HSS model considering the control system.

[0161] Regarding the device in the above embodiment, the specific manner in which each module performs operations has been described in detail in the embodiment of the method, and will not be elaborated here.

[0162] It should be understood that the various forms of processes shown above can be used to reorder, add or delete steps. For example, the steps recorded in this disclosure can be executed in parallel, sequentially or in different orders, as long as the desired results of the technical solution of this disclosure can be achieved, and this document is not limited here.

[0163] The above specific implementations do not constitute a limitation on the protection scope of the present disclosure. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent substitution and improvement made within the spirit and principle of the present disclosure shall be included in the protection scope of the present disclosure.

Claims

1. A method for establishing a state space model of an offshore boost converter, characterized in that: include: Obtain the original model of the offshore wind power MMC and dynamically separate the AC and DC of the MMC based on the differential common mode conversion method; The LTP modeling method is used to perform linear processing on the MMC original model after the differential common mode conversion on its steady-state time-varying periodic trajectory to generate a small signal state space model of the MMC; The small signal state space model is converted into a complex vector form by a preset coordinate transformation matrix, and an open-loop MMC-HSS harmonic state space model in the frequency domain is obtained based on the HSS theory; A closed-loop MMC-HSS model considering the control system is established.

2. The method according to claim 1, characterized in that The obtaining of the offshore wind power MMC original model includes: When the preset conditions are met, the voltage equations are written for the upper and lower bridge arms of the kth phase of the MMC. The expressions are as follows: In the formula, v sk With v dc are the AC system voltage and the DC system voltage, v NO is the voltage difference between the neutral point N on the AC side of the MMC and the neutral point O on the DC side, v NO The expression is: Among them, the preset condition is that the capacitor voltages of all submodules in the same bridge arm remain the same at all times and the voltage balancing algorithm ensures that the voltage fluctuations of all submodules are consistent and the flow system voltage remains constant.

3. The method according to claim 1, characterized in that: The method of dynamically separating the MMC AC and DC according to the differential common mode conversion method includes: The differential and common mode components in the MMC original model are obtained as shown in formulas (2), (3) and (4), and the expressions are as follows: In the formula, the subscript c of the variable represents the common mode component, and the subscript d represents the differential mode component; The MMC original model after transformation is represented according to the difference and common mode components, as shown in formulas (5), (6) and (7), and the expression is as follows: According to Kirchhoff's voltage theorem, the voltage on the AC side of the MMC is v sk The form after differential common mode conversion is shown in formula (8), and the expression is as follows:

4. The method according to claim 3, characterized in that: The LTP modeling method is used to perform linear processing on the MMC original model after the differential common mode conversion on its steady-state time-varying periodic trajectory to generate a small signal state space model of the MMC, including: Among them, the expressions of the selected Δx(t) state variable and the input variable Δu(t) are: In the formula, a, b, and c in the variable subscripts represent the three-phase components of the corresponding electrical quantities.

5. The method according to claim 4, characterized in that The converting the small signal state space model into a complex vector form by a preset coordinate transformation matrix includes: For any three-phase electrical component x abc According to the coordinate transformation theory, we can get the expression as shown in formula (13): In the formula, is the positive sequence component of the variable in the frequency range of (0, +∞∞), is the negative sequence component of the variable in the frequency range of (-∞∞, 0), is the zero sequence component, T dq / +-0 With T abc / dq is the transformation matrix, where the transformation matrix T abc / dq Obtained by multiplying the Clack transform and the Park transform; For the Clack transform, the linearization equation is: For the Park transform, the linearized equation is: According to the transformation matrix T abc / dq A preset transformation matrix (14) is obtained, and the small signal state space model shown in formula (11) is converted into a complex vector form (15) through the preset transformation matrix (14), wherein the expressions of formulas (14) and (15) are as follows: T=diag[T2T1 T2T1 T2T1 T2T1] (14) In the formula, The expression is:

6. The method according to claim 1, characterized in that The open-loop MMC-HSS model in the frequency domain obtained based on the HSS theory includes: Expanding the converted MMC electrical component of the small signal state space model into a form of superposition of multiple frequency components; The small signal state space model expanded by the harmonic balance principle is converted into an MMC-HSS model in the frequency domain. The expression of the MMC-HSS model is:

7. The method according to claim 1, characterized in that The closed-loop MMC-HSS model considering the control system is established, including: Obtain controller models of an AC voltage controller, a base frequency current controller, and a circulating current controller respectively, and obtain modulation functions of the AC voltage controller, the base frequency current controller, and the circulating current controller according to the controller models and control logic; The modulation functions of the AC voltage controller, the base frequency current controller and the circulating current controller are introduced into the open-loop MMC-HSS model to obtain the closed-loop MMC-HSS model of the corresponding control system.

8. The method according to claim 7, characterized in that The AC voltage controller includes a direct-axis-quadrature-axis dq voltage proportional-integral PI controller in a positive-sequence fundamental frequency synchronous rotating coordinate system. The controller model of the AC voltage controller is: The base frequency current controller includes a dq current PI controller, a dq current cross-decoupling controller and a voltage feedforward controller in a positive sequence base frequency synchronous rotating coordinate system. The controller model of the base frequency current controller is: The circulating current controller includes a negative-sequence double frequency circulating current controller and a zero-sequence circulating current controller. The negative-sequence double frequency circulating current controller includes a dq current PI controller and a dq current cross-decoupling controller in a negative-sequence double frequency rotating coordinate system. The controller model of the negative-sequence double frequency circulating current controller is: In the formula, k cp With k ci are the proportional and integral coefficients of the circulation controller respectively; The controllability model of the zero-sequence circulating current controller is: Δv c0 (s)=F c0 (s)i c0 Among them, F c0 (s) is the transfer function of the damping controller.

9. The method according to claim 8, characterized in that The modulation functions of the AC voltage controller, the base frequency current controller and the circulating current controller are obtained according to the controller model and the control logic, including: For the base frequency current controller, the expression of the modulation function is: For the circulating flow controller, the expression of the modulation function is:

10. A device for establishing a state space model of an offshore boost converter, characterized in that: include: The differential and common mode conversion module is used to obtain the original model of the offshore wind power MMC and dynamically separate the AC and DC of the MMC according to the differential and common mode conversion method; An LTP modeling module is used to perform linearization processing on the MMC original model after differential common mode conversion on its steady-state time-varying periodic trajectory using an LTP modeling method to generate a small signal state space model of the MMC; A coordinate transformation module, used to transform the small signal state space model into a complex vector form through a preset coordinate transformation matrix, and obtain an open-loop MMC-HSS harmonic state space model in the frequency domain based on the HSS theory; Output module, used to establish a closed-loop MMC-HSS model considering the control system.

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