Sliding Mode Variable Structure Control Method for Wind Farm - Bipolar Flexible DC System Under Asymmetrical Faults

Abstract

The present invention discloses a sliding mode variable structure control method for a wind farm - bipolar flexible DC system under asymmetrical faults. Aiming at the deficiencies of the existing control strategies that cannot achieve multi - objective flexible control under grid asymmetrical faults, do not consider the coordinated control between the positive and negative poles, and have a complex control system, a sliding mode variable structure control strategy for a wind farm - bipolar flexible DC system under asymmetrical faults is proposed by adopting a sliding mode variable structure power controller in the negative - pole MMC and optimizing the design method of power commands. This method can achieve coordinated operation between the bipolar MMCs under asymmetrical grid faults and can achieve different optimal control objectives, including suppressing the negative - sequence current or active - power pulsation of the bipolar MMCs.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronics, and particularly relates to a sliding mode variable structure control method for a wind farm - bipolar flexible DC system under asymmetric faults. Background Technique

[0002] At present, the construction of offshore wind farms is gradually developing towards deep - sea and large - scale directions from short - distance and small - capacity. The flexible DC transmission technology based on the modular multilevel converter (MMC) topology has the advantages of low manufacturing difficulty, low switching loss, high waveform quality, etc., and has very good application prospects in the collection and transmission of long - distance offshore wind power. With the gradual increase of the voltage level and transmission capacity of the flexible DC transmission system, the true bipolar MMC system has received more and more attention due to its high flexibility and reliability.

[0003] When an asymmetric fault occurs in the sending - end AC power grid, a large amount of negative - sequence current and power pulsation will be generated in the bipolar MMC, which will seriously affect the operation performance of the system. In addition, the bipolar MMC system also needs to consider the coordinated control between the two poles, which not only needs to provide a stable operating voltage for the wind farm, but also needs to maintain the power balance between the two poles. Therefore, if the traditional control strategy based on the linear PI controller is still adopted, the control strategy of the wind farm - bipolar flexible DC system under asymmetric grid faults will be very complex.

[0004] As a non - linear control strategy, the sliding mode variable structure control strategy has good control performance and parameter robustness, and has been widely used in the field of power electronic converter control. The existing research on the sliding mode variable structure control strategy for MMC mainly focuses on the single - pole MMC system, without considering the coordinated control problem between the positive and negative poles of the bipolar MMC system; in addition, when an asymmetric grid fault occurs, the traditional sliding mode variable structure control strategy cannot achieve different control objectives, such as effectively suppressing the negative - sequence current or active power pulsation of the bipolar MMC. Therefore, it is urgent to propose a sliding mode variable structure control method for the wind farm - bipolar flexible DC system to realize the coordinated operation of the bipolar flexible DC system under asymmetric grid faults and be able to achieve different optimal control objectives according to the control instructions of the system. Summary of the Invention

[0005] The purpose of the invention is to overcome the deficiencies of the existing control strategy that cannot achieve multi - objective flexible control under asymmetric grid faults, does not consider the coordinated control between the positive and negative poles, and has a complex control system. By adopting a sliding mode variable structure power controller in the negative - pole MMC and optimizing the design method of the power command, a sliding mode variable structure control method for the wind farm - bipolar flexible DC system under asymmetric faults is proposed, which can achieve different optimal control objectives under asymmetric grid faults.

[0006] To achieve the above-mentioned invention object, the present method adopts the following technical solutions:

[0007] A sliding mode variable structure control method for a wind farm - bipolar flexible DC system under asymmetric faults, comprising:

[0008] A positive - pole sampling module and a negative - pole sampling module, a positive - pole coordinate transformation module and a negative - pole coordinate transformation module, a negative - pole power calculation module, a positive - pole voltage controller, a positive - pole current sliding mode variable structure controller, a negative - pole power reference value calculation module, a negative - pole power sliding mode variable structure controller, a positive - pole internal circulating current controller and a negative - pole internal circulating current controller, a positive - pole bridge - arm voltage calculation module and a negative - pole bridge - arm voltage calculation module, a positive - pole modulation module and a negative - pole modulation module;

[0009] The positive - pole sampling module includes a positive - pole voltage sampling module and a positive - pole current sampling module, and the negative - pole sampling module includes a negative - pole voltage sampling module and a negative - pole current sampling module;

[0010] The positive - pole voltage sampling module and the negative - pole voltage sampling module sample the three - phase voltage U gabc of the AC power grid;

[0011] The positive - pole current sampling module and the negative - pole current sampling module respectively sample the three - phase currents I gabc1 and I gabc2 of the positive - pole and negative - pole MMCs with respect to the AC power grid, and the internal circulating currents I cabc1 and I cabc2 of the positive - pole and negative - pole MMCs;

[0012] The positive - pole coordinate transformation module includes a positive - pole Clark transformation module and a positive - pole Clark inverse transformation module, and the negative - pole coordinate transformation module includes a negative - pole Clark transformation and positive - negative sequence separation module and a negative - pole Clark inverse transformation module;

[0013] The positive - pole Clark transformation module performs Clark transformation on the three - phase voltage U gabc of the AC power grid and the three - phase current I gabc1 of the positive - pole MMC with respect to the AC power grid to obtain the corresponding voltage vector U gαβ in the two - phase stationary coordinate system and the corresponding current vector I gαβ1 in the two - phase stationary coordinate system;

[0014] The negative - pole Clark transformation and positive - negative sequence separation module performs Clark transformation on the three - phase voltage U gabc of the AC power grid and the three - phase current I gabc2 of the negative - pole MMC with respect to the AC power grid, and performs positive - negative sequence separation to obtain the positive - sequence voltage vector U gαβ+ , negative - sequence voltage vector U gαβ- in the two - phase stationary coordinate system, and the positive - sequence current vector I gαβ2+, negative sequence current vector I gαβ2- ;

[0015] The positive Clark inverse transformation module and the negative Clark inverse transformation module respectively perform Clark inverse transformation on the positive and negative reference differential mode voltages U difαβ1 and U difαβ2 to obtain the reference differential mode voltages U difabc1 and U difabc2 in the positive and negative stationary three-phase coordinate systems;

[0016] The negative power calculation module calculates the negative active power P gαβ+ , negative sequence voltage vector U gαβ- , positive sequence current vector I gαβ2+ , negative sequence current vector I gαβ2- to obtain the negative active power P g2ref and reactive power Q g2ref ;

[0017] The positive voltage controller controls the α and β axis voltages U gαβ through a resonant controller to make it follow the reference value, and the output of the positive voltage controller serves as the reference value I gαβref1 of the positive α and β axis currents;

[0018] The positive current sliding mode variable structure controller controls the α and β axis currents I gαβ1 to make it follow the positive current reference value I gαβref1 , and the output of the positive current sliding mode variable structure controller serves as the α and β axis components U difαβ1 of the positive MMC reference differential mode voltage;

[0019] The negative power reference value calculation module calculates the negative power reference values P gαβ+ , negative sequence voltage vector U gαβ- , positive sequence current vector I gαβ2+ , negative sequence current vector I gαβ2- to obtain the negative power reference values P g2ref and Q g2ref ;

[0020] The negative power sliding mode variable structure controller controls the active power P g2 and reactive power Q g2 to make them follow the negative power reference values P g2ref and Q g2ref , and the output of the negative power sliding mode variable structure controller serves as the α and β axis components U difαβ2 of the negative MMC reference differential mode voltage;

[0021] The positive - side internal - circulation controller and the negative - side internal - circulation controller respectively control the internal circulations I cabc1 and I cabc2 of the positive - side and negative - side MMCs by using a resonant controller, which can be achieved in the stationary coordinate system. The outputs of the positive - side internal - circulation controller and the negative - side internal - circulation controller are respectively used as the reference common - mode voltages U comabc1 and U comabc2 of the positive - side and negative - side MMCs;

[0022] The positive - side arm - voltage calculation module and the negative - side arm - voltage calculation module respectively use the positive - side and negative - side reference differential - mode voltages U difabc1 and U difabc2 and the positive - side and negative - side reference common - mode voltages U comabc1 and U comabc2 to calculate the reference voltages U prefabc1 and U prefabc2 of the upper arms of the positive - side and negative - side MMCs and the reference voltages U nrefabc1 and U nrefabc2 of the lower arms of the positive - side and negative - side MMCs;

[0023] The positive - side modulation module and the negative - side modulation module respectively control the positive - side and negative - side MMCs according to the reference voltages U prefabc1 and U prefabc2 of the upper arms of the positive - side and negative - side MMCs and the reference voltages U nrefabc1 and U nrefabc2 of the lower arms of the positive - side and negative - side MMCs.

[0024] Furthermore: In the negative - side power sliding - mode variable - structure controller, the following method is used to control the negative - side active power and reactive power. The output of the negative - side power sliding - mode variable - structure controller is used as the reference value U difαβ2 of the negative - side differential - mode voltage:

[0025]

[0026] where u difα2 and u difβ2 are the α - and β - axis components of the voltage vector U difαβ2 , u gα and u gβ are the α - and β - axis components of the voltage vector U gαβ , L is the equivalent reactance of the arm, ω1 is the rated angular frequency of the system, K p2 and K q2 are respectively the integral coefficients of the sliding - mode surfaces of the negative - side active power and reactive power, and S p2 and S q2 are respectively the sliding - mode surfaces of the negative - side active power and reactive power, and their expressions are as follows:

[0027]

[0028] K ps2 and K qs2 are the coefficients of the improved sign functions of the negative - active power and reactive power respectively. rsgn(S p2 ) and rsgn(S q2 ) are the improved sign functions of the negative - active power and reactive power respectively, and their expressions are as follows:

[0029]

[0030] where λ j is the buffer critical value of the improved sign function.

[0031] The beneficial effects of the present invention are:

[0032] Due to the adoption of the technical solution of the present invention, it is possible to achieve the coordinated operation between the bipolar of the wind farm - bipolar VSC - HVDC system under asymmetric grid faults, and to achieve different optimal control objectives according to the control instructions of the system, including effectively suppressing the negative - sequence current or active - power pulsation of the bipolar MMC; in addition, compared with the traditional control strategy, this method has a simpler control structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is a typical topology diagram of an islanded wind farm - bipolar VSC - HVDC transmission system.

[0034] Figure 2 is a specific example structure diagram of a single - pole MMC in a bipolar VSC - HVDC transmission system. Among them, u ga , u gb , u gc are grid voltages; i ga , i gb , i gc are grid currents; u pa , u pb , u pc are the upper - arm voltages of the MMC; u na , u nb , u nc are the lower - arm voltages of the MMC; i pa , i pb , i pc are the upper - arm currents of the MMC; i na , i nb , i nc are the lower - arm currents of the MMC; U dc is the DC - bus voltage, i dc is the DC - bus current, L0 is the arm inductor, SM (N) is the sub - module in the MMC; N is the sub - module serial number.

[0035] Figure 3 This is a schematic diagram of a specific example system for the control method of the present invention. The names of each module are as follows:

[0036] 1 - Positive - pole voltage sampling module, 2 - Positive - pole current sampling module, 3 - Positive - pole Clark transformation module, 4 - Positive - pole voltage controller, 5 - Positive - pole current sliding - mode variable - structure controller, 6 - Positive - pole Clark inverse transformation module, 7 - Positive - pole internal circulating - current controller, 8 - Positive - pole arm - voltage calculation module, 9 - Positive - pole modulation module, 10 - Negative - pole voltage sampling module, 11 - Negative - pole current sampling module, 12 - Negative - pole Clark transformation and positive - negative sequence separation module, 13 - Negative - pole power calculation module, 14 - Negative - pole power reference - value calculation module, 15 - Negative - pole power sliding - mode variable - structure controller, 16 - Negative - pole Clark inverse transformation module, 17 - Negative - pole internal circulating - current controller, 18 - Negative - pole arm - voltage calculation module, 19 - Negative - pole modulation module. Detailed implementation manners

[0037] To describe the present invention more specifically, the technical solutions of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0038] The system implementation of the sliding - mode variable - structure control method for a wind farm - bipolar VSC system under asymmetrical faults in the present invention is as Figure 3 shown, and includes a positive - pole voltage sampling module 1, a positive - pole current sampling module 2, a positive - pole Clark transformation module 3, a positive - pole voltage controller 4, a positive - pole current sliding - mode variable - structure controller 5, a positive - pole Clark inverse transformation module 6, a positive - pole internal circulating - current controller 7, a positive - pole arm - voltage calculation module 8, a positive - pole modulation module 9, a negative - pole voltage sampling module 10, a negative - pole current sampling module 11, a negative - pole Clark transformation and positive - negative sequence separation module 12, a negative - pole power calculation module 13, a negative - pole power reference - value calculation module 14, a negative - pole power sliding - mode variable - structure controller 15, a negative - pole Clark inverse transformation module 16, a negative - pole internal circulating - current controller 17, a negative - pole arm - voltage calculation module 18, and a negative - pole modulation module 19.

[0039] As Figure 3 shown, the sliding - mode variable - structure control method for a wind farm - bipolar VSC system under asymmetrical faults in the present invention includes the following steps:

[0040] Collect the three - phase voltage U of the MMC AC grid - side through the positive - pole voltage sampling module 1 gabc , and collect the three - phase current I of the positive - pole MMC AC grid - side through the positive - pole current sampling module 2 gabc1 , as well as the internal circulating current I cabc1 .

[0041] Utilize the positive - pole Clark transformation module 3 to perform Clark transformation on the three - phase voltage U of the AC grid gabcand the three-phase current I of the positive electrode MMC AC power grid gabc1 Perform Clark transformation to obtain the corresponding voltage vector U in the two-phase stationary coordinate system gαβ and the corresponding current vector I in the two-phase stationary coordinate system gαβ1 .

[0042] In the positive electrode voltage controller 4, the control of the α and β axis voltages U is achieved according to the following method gαβ . The outputs of the two α and β axis resonant controllers are respectively used as the reference values I of the positive electrode α and β axis currents gαβref1 .

[0043]

[0044] Among them, F R50 (s) is the transfer function of the resonant controller with a resonant frequency of ±50Hz, k r is the gain coefficient of the resonant controller, ω cr is the cut-off frequency; u gα and u gβ are the α and β axis components of the voltage vector U gαβ , i gαref1 and i gβref1 are the α and β axis components of the current vector I gαβref1 , u gαref and u gβref are the α and β axis components of the voltage vector U gαβref , and their given values are as follows

[0045]

[0046] Among them, |U g | is the reference voltage amplitude, and θ r is the reference voltage phase

[0047] Utilize the positive electrode current sliding mode variable structure controller 5 to control the positive electrode α and β axis currents I gαβ1 by the following method. The output of the positive electrode sliding mode variable structure controller is used as the reference value U of the positive electrode differential mode voltage difαβ1 :

[0048]

[0049] Among them, K α1 and K β1 are respectively the integral coefficients of the positive electrode α and β axis sliding surfaces, S α1 and S β1 are respectively the positive electrode α and β axis sliding surfaces, and their expressions are as follows

[0050]

[0051] K αs1 and K βs1 are the coefficients of the improved sign functions for the positive - electrode α and β axes respectively. rsgn(S α1 ) and rsgn(S β1 ) are the improved sign functions, and their expressions are as follows:

[0052]

[0053] where λ j is the buffer critical value of the improved sign function.

[0054] Using the positive - electrode Clark inverse transformation module 6, perform Clark inverse transformation on the positive - electrode reference differential - mode voltage U difαβ1 to obtain the reference differential - mode voltage U difabc1 in the stationary three - phase coordinate system.

[0055] Using the positive - electrode internal circulating - current controller 7 to control the positive - electrode MMC internal circulating current I cabc1 with a resonant controller, which can be achieved in the stationary coordinate system; the output of the positive - electrode internal circulating - current controller serves as the positive - electrode MMC reference common - mode voltage U comabc1 .

[0056] Using the positive - electrode bridge - arm voltage calculation module 8, calculate the reference voltages U difabc1 and U comabc1 of the upper and lower bridge arms of the positive - electrode MMC through the positive - electrode reference differential - mode voltage U prefabc1 and the reference common - mode voltage U nrefabc1 .

[0057] Using the positive - electrode modulation module 9, according to the reference voltages U prefabc1 and U nrefabc1 of the upper and lower bridge arms of the positive - electrode MMC, realize the control of the positive - electrode MMC.

[0058] Using the negative - electrode voltage sampling module 10 to collect the three - phase voltages U gabc of the MMC AC grid side, and using the negative - electrode current sampling module 11 to collect the three - phase currents I gabc2 of the negative - electrode MMC AC grid side, as well as the internal circulating current I cabc2 .

[0059] Using the negative - electrode Clark transformation and positive - negative sequence separation module 12, perform Clark transformation on the three - phase voltages U gabc of the AC grid and the three - phase currents I gabc2 of the negative - electrode MMC AC grid side, and perform positive - negative sequence separation to obtain the positive - sequence voltage vector U gαβ+ , the negative - sequence voltage vector U gαβ- , and the positive - sequence current vector I gαβ2+ in the two - phase stationary coordinate system., negative sequence current vector I gαβ2- .

[0060] Using the negative - pole power calculation module 13, based on the positive - sequence voltage vector U gαβ+ , negative - sequence voltage vector U gαβ- , positive - sequence current vector I gαβ2+ , negative - sequence current vector I gαβ2- , calculate the negative - pole active power P g2 and reactive power Q g2 , the calculation method is as follows:

[0061]

[0062] where, u gα+ and u gβ+ are the α, β - axis components of the positive - sequence voltage vector U gαβ+ , u gα- and u gβ- are the α, β - axis components of the negative - sequence voltage vector U gαβ- , i gα2+ and i gβ2+ are the α, β - axis components of the positive - sequence current vector I gαβ2+ , i gα2- and i gβ2- are the α, β - axis components of the negative - sequence current vector I gαβ2- .

[0063] Using the negative - pole power reference value calculation module 14, according to the system control objective and the positive - sequence voltage vector U gαβ+ , negative - sequence voltage vector U gαβ- , positive - sequence current vector I gαβ2+ , negative - sequence current vector I gαβ2- , calculate the negative - pole power reference values P g2ref and Q g2ref , the calculation method is as follows:

[0064] When the control objective is to suppress negative - sequence current:

[0065]

[0066] When the control objective is to suppress active - power pulsation:

[0067]

[0068] where, P g2ref and Q g2ref are the DC - component commands of active power and reactive power.

[0069] Using the negative - pole power sliding - mode variable - structure controller 15, for the active power P g2and reactive power Q g2 for control to make it follow the negative - power reference value P g2ref and Q g2ref The output of the negative - power sliding - mode variable - structure controller serves as the α - and β - axis components U difαβ2 of the negative - terminal MMC reference differential - mode voltage. The implementation method of the controller is as follows:

[0070]

[0071] where u difα2 and u difβ2 are the α - and β - axis components of the voltage vector U difαβ2 , u gα and u gβ are the α - and β - axis components of the voltage vector U gαβ , L is the equivalent reactance of the arm, ω1 is the rated angular frequency of the system, K p2 and K q2 are the integral coefficients of the sliding - mode surfaces of the negative - terminal active power and reactive power respectively, S p2 and S q2 are the sliding - mode surfaces of the negative - terminal active power and reactive power respectively, and their expressions are as follows:

[0072]

[0073] K ps2 and K qs2 are the coefficients of the improved sign functions of the negative - terminal active power and reactive power respectively, rsgn(S p2 ) and rsgn(S q2 ) are the improved sign functions of the negative - terminal active power and reactive power respectively, and their expressions are as follows:

[0074]

[0075] where λ j is the buffer critical value of the improved sign function.

[0076] Using the negative - terminal Clark inverse - transformation module 16, perform Clark inverse - transformation on the negative - terminal reference differential - mode voltage U difαβ2 to obtain the reference differential - mode voltage U difabc2 in the stationary three - phase coordinate system.

[0077] Using the negative - terminal internal - circulating - current controller 17 to control the negative - terminal MMC internal circulating current I cabc2 with a resonant controller, which can be achieved in the stationary coordinate system; the output of the negative - terminal internal - circulating - current controller serves as the negative - terminal MMC reference common - mode voltage U comabc2 .

[0078] Using the negative-arm voltage calculation module 18, through the negative reference differential-mode voltage U difabc2 and the reference common-mode voltage U comabc2 , the reference voltages U prefabc2 and U nrefabc2 of the upper and lower arms of the negative MMC are calculated and obtained.

[0079] Using the negative modulation module 19, according to the reference voltages U prefabc2 and U nrefabc2 of the upper and lower arms of the negative MMC, the control of the negative MMC is realized.

[0080] The above description of the embodiments is for the convenience of those of ordinary skill in the art to understand and apply the present invention. It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative efforts. Therefore, the present invention is not limited to the above embodiments, and the improvements and modifications made by those skilled in the art to the present invention according to the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A sliding mode variable structure control method for a wind farm - bipolar flexible DC system under asymmetric faults, characterized in that, The control system adopted to implement the above method includes: a positive electrode sampling module and a negative electrode sampling module, a positive electrode coordinate transformation module and a negative electrode coordinate transformation module, a negative electrode power calculation module, a positive electrode voltage controller, a positive electrode current sliding mode variable structure controller, a negative electrode power reference value calculation module, a negative electrode power sliding mode variable structure controller, a positive electrode internal circulating current controller and a negative electrode internal circulating current controller, a positive electrode bridge arm voltage calculation module and a negative electrode bridge arm voltage calculation module, a positive electrode modulation module and a negative electrode modulation module; The positive electrode sampling module includes a positive electrode voltage sampling module and a positive electrode current sampling module, and the negative electrode sampling module includes a negative electrode voltage sampling module and a negative electrode current sampling module; The positive voltage sampling module and the negative voltage sampling module sample the three-phase voltage U of the AC power grid gabc for sampling; The positive current sampling module and the negative current sampling module respectively sample the three-phase currents I gabc1 and I gabc2 of the positive and negative MMC AC power grids, as well as the internal circulating currents I cabc1 and I cabc2 of the positive and negative MMCs; The positive electrode coordinate transformation module includes a positive electrode Clark transformation module and a positive electrode Clark inverse transformation module, and the negative electrode coordinate transformation module includes a negative electrode Clark transformation and positive and negative sequence separation module and a negative electrode Clark inverse transformation module; The positive - pole Clark transformation module performs Clark transformation on the three - phase voltages U of the AC power grid gabc and the three - phase currents I of the positive - pole MMC AC power grid gabc1 to obtain the corresponding voltage vector U in the two - phase stationary coordinate system gαβ and the corresponding current vector I in the two - phase stationary coordinate system gαβ1 ; Negative Clark transformation and positive and negative sequence separation module, which performs Clark transformation on the three-phase voltages U of the AC power grid gabc and the three-phase currents I of the negative MMC AC power grid gabc2 and separates the positive and negative sequences to obtain the positive sequence voltage vector U in the two-phase stationary coordinate system gαβ+ , negative sequence voltage vector U gαβ- , and the positive sequence current vector I in the two-phase stationary coordinate system gαβ2+ , negative sequence current vector I gαβ2- ; The positive Clark inverse transformation module and the negative Clark inverse transformation module respectively perform Clark inverse transformation on the positive and negative reference differential-mode voltages U difαβ1 and U difαβ2 to obtain the reference differential-mode voltages U difabc1 and U difabc2 in the positive and negative stationary three-phase coordinate systems; The negative - terminal power calculation module calculates the negative - terminal active power P gαβ+ and the negative - terminal reactive power Q gαβ- based on the positive - sequence voltage vector U gαβ2+ , the negative - sequence voltage vector U gαβ2- , the positive - sequence current vector I g2 and the negative - sequence current vector I g2 ; The positive voltage controller controls the α and β axis voltages U gαβ through a resonant controller to make them follow the reference values. The output of the positive voltage controller serves as the reference value I of the positive α and β axis currents gαβref1 ; The positive current sliding mode variable structure controller controls the α and β axis currents I gαβ1 to make them follow the positive current reference value I gαβref1 . The output of the positive current sliding mode variable structure controller serves as the α and β axis components U difαβ1 of the positive MMC reference differential mode voltage; The negative - terminal power reference value calculation module calculates the negative - terminal power reference values \(P\) gαβ+ and \(Q\) gαβ- based on the system control objective and the positive - sequence voltage vector \(U\) gαβ2+ , negative - sequence voltage vector \(U\) gαβ2- , positive - sequence current vector \(I\) g2ref , and negative - sequence current vector \(I\) g2ref ; The negative - terminal power sliding - mode variable - structure controller controls the active power P g2 and the reactive power Q g2 to make them follow the negative - terminal power reference values P g2ref and Q g2ref . The output of the negative - terminal power sliding - mode variable - structure controller serves as the α - axis and β - axis components U difαβ2 of the reference differential - mode voltage of the negative - terminal MMC; The positive internal circulating current controller and the negative internal circulating current controller respectively control the internal circulating currents I cabc1 and I cabc2 of the positive and negative MMCs by using a resonant controller, which can be achieved in the stationary coordinate system; the outputs of the positive internal circulating current controller and the negative internal circulating current controller are respectively used as the reference common-mode voltages U comabc1 and U comabc2 of the positive and negative MMCs; The positive bridge arm voltage calculation module and the negative bridge arm voltage calculation module respectively use the positive and negative reference differential mode voltages U difabc1 and U difabc2 and the positive and negative reference common mode voltages U comabc1 and U comabc2 , and through calculation, obtain the reference voltages U prefabc1 and U prefabc2 of the upper bridge arms of the positive and negative MMCs and the reference voltages U nrefabc1 and U nrefabc2 of the lower bridge arms of the positive and negative MMCs; The positive electrode modulation module and the negative electrode modulation module respectively control the positive electrode and negative electrode MMCs according to the reference voltages U prefabc1 and U prefabc2 of the upper arms of the positive electrode and negative electrode MMCs, and the reference voltages U nrefabc1 and U nrefabc2 of the lower arms of the positive electrode and negative electrode MMCs.

2. The sliding mode variable structure control method for a wind farm - bipolar flexible DC system under asymmetric faults according to claim 1, characterized in that: In the negative - terminal power sliding - mode variable - structure controller, the following method is used to control the negative - terminal active power and reactive power. The output of the negative - terminal power sliding - mode variable - structure controller serves as the reference value \(U\) of the negative - terminal differential - mode voltage difαβ2 : where, u difα2 and u difβ2 are the α and β axis components of the voltage vector U difαβ2 , u gα and u gβ are the α and β axis components of the voltage vector U gαβ , L is the equivalent reactance of the bridge arm, ω1 is the rated angular frequency of the system, K p2 and K q2 are the integral coefficients of the sliding mode surfaces of the negative active power and reactive power respectively, S p2 and S q2 are the sliding mode surfaces of the negative active power and reactive power respectively, and their expressions are as follows: K ps2 and K qs2 are the coefficients of the improved sign functions of the negative - terminal active power and reactive power respectively. rsgn(S p2 ) and rsgn(S q2 ) are the improved sign functions of the negative - terminal active power and reactive power respectively, and their expressions are as follows: Among them, λ j is the buffer critical value for improving the sign function.

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