True Bipolar MMC Sliding Mode Variable Structure Control Method with Inertia Support Capability

Abstract

The present invention discloses a true bipolar MMC sliding mode variable structure control method with inertia support ability. This method is based on the true bipolar MMC system. Aiming at the problem that the existing sliding mode variable structure control strategy cannot provide inertia support for the power grid during the power grid fault stage and does not consider the coordinated control between the positive and negative poles, by simulating the mathematical model of the traditional synchronous machine, a new sliding mode variable structure control strategy applicable to the true bipolar MMC system is proposed, which can provide inertia support for the AC power grid. Compared with the existing methods, this method proposes a true bipolar MMC sliding mode variable structure control strategy and its virtual inertia command configuration method, which can enhance the inertia stability of the system when the power grid fault occurs and is more friendly to the new power system containing the grid connection of large-scale power electronic systems.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and particularly relates to a true bipolar MMC sliding mode variable structure control method with inertia support capability. Background Art

[0002] The flexible DC transmission technology based on the modular multilevel converter (MMC) topology has the advantages of low manufacturing difficulty, low system loss, high waveform quality, etc., and has very good application prospects in long-distance electric energy collection and transmission. 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] Due to the large-scale access of new energy power generation systems based on power electronic devices such as wind power and photovoltaic power, the inertia stability of the AC power grid has been significantly reduced, the grid volatility and uncertainty have increased, and ultimately the safety risk of grid operation has increased significantly. Therefore, it is urgent to study the MMC control strategy with inertia support capability, so as to improve the inertia support capability of MMC for the power grid and improve the grid stability.

[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 is mainly for the single-pole MMC system. This control strategy has good control performance under steady-state operating conditions, but when a grid fault occurs, it does not have the ability to provide inertia support for the power grid like a traditional synchronous generator set, which is likely to cause system instability, and the traditional sliding mode variable structure control strategy does not consider the coordinated control problem between the positive and negative poles of the true bipolar MMC system.

[0005] For the true bipolar MMC system, it is necessary to provide inertia support for the power grid under the condition of ensuring no overcurrent, and also consider the coordination between the positive and negative poles. At present, there is little research on the application of the sliding mode variable structure control strategy in the true bipolar MMC system, especially the research on the sliding mode variable structure control strategy with inertia support capability is still a technical blank. Therefore, it is urgent to propose a true bipolar MMC sliding mode variable structure control method with inertia support capability, so as to provide inertia support for the AC power grid and ensure the safe and stable operation of a new power system including the grid connection of a large-scale power electronic system. Summary of the Invention

[0006] The object of the present invention is to overcome the deficiencies that the existing sliding mode variable structure control strategy cannot provide inertia support for the power grid during the power grid fault stage and does not consider the coordinated control problem between the positive and negative poles. By simulating the operating characteristics of traditional synchronous machines, a new sliding mode variable structure control strategy applicable to a true bipolar MMC system is proposed, which can provide inertia support for the AC power grid when a power grid fault occurs.

[0007] In order to achieve the above object of the invention, the following technical solutions are adopted:

[0008] A true bipolar MMC sliding mode variable structure control method with inertia support ability, characterized in that the control system adopted to implement the method includes: a virtual mechanical module, a virtual excitation module, a reference voltage calculation module, 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 positive pole power calculation module and a negative pole power calculation module, a positive pole voltage controller and a negative pole voltage controller, a positive pole sliding mode variable structure controller and a negative pole sliding mode variable structure controller, a positive pole internal circulating current controller and a negative pole internal circulating current controller, a positive pole arm voltage calculation module and a negative pole arm voltage calculation module, a positive pole modulation module and a negative pole modulation module;

[0009] The virtual mechanical module controls the total active power of the positive and negative poles by simulating the mechanical equation of a synchronous generator, and the output of the virtual mechanical module serves as the reference voltage phase θ of the true bipolar MMC system r ;

[0010] The virtual excitation module controls the total reactive power of the positive and negative poles by simulating the excitation equation of a synchronous generator, and the output of the virtual excitation module serves as the reference voltage amplitude |U gr |;

[0011] The voltage reference value calculation module calculates the voltage vector reference value U in the two-phase stationary coordinate system according to the reference voltage phase θ r and the amplitude |U gr |; gαβref ;

[0012] 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;

[0013] The positive pole voltage sampling module and the negative pole voltage sampling module sample the three-phase voltage U of the AC power grid gabc ;

[0014] The positive pole current sampling module and the negative pole current sampling module respectively sample the three-phase currents I of the positive pole and negative pole MMC AC power grids gabc1 and I gabc2, the internal circulating current I of the positive and negative MMCs cabc1 and I cabc2 are sampled;

[0015] 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 module and a negative - pole Clark inverse transformation module;

[0016] The positive - pole Clark transformation module and the negative - pole Clark transformation module perform Clark transformation on the three - phase voltages U of the AC power grid gabc and the three - phase currents I of the positive and negative MMCs in the AC power grid gabc1 and I gabc2 to obtain the corresponding voltage vectors U in the two - phase stationary coordinate system gαβ and the corresponding current vectors I in the two - phase stationary coordinate system gαβ1 and I gαβ2 ;

[0017] The positive - pole Clark inverse transformation module and the negative - pole Clark inverse transformation module respectively perform Clark inverse transformation on the positive - pole and negative - pole reference differential - mode voltages U difαβ1 and U difαβ2 to obtain the reference differential - mode voltages U in the positive - pole and negative - pole stationary three - phase coordinate systems difabc1 and U difabc2 ;

[0018] The positive - pole power calculation module and the negative - pole power calculation module respectively calculate the positive - pole and negative - pole active powers P gαβ and the positive - pole current vector, negative - pole current vectors I gαβ1 、I gαβ2 to obtain the positive - pole and negative - pole reactive powers Q g1 and P g2 and the positive - pole and negative - pole reactive powers Q g1 and Q g2 ;

[0019] The positive - pole voltage controller and the negative - pole voltage controller control the α, β - axis voltages U gαβ through a resonant controller to make them follow the reference values. The outputs of the positive - pole voltage controller and the negative - pole voltage controller are respectively used as the reference values I of the positive - pole and negative - pole α, β - axis currents gαβref1 and I gαβref2 ; where, U gαβ is the vector expression form of the α, β - axis voltage components u gα and u gβ , I gαβref1 is the vector expression form of the α, β - axis current components i gαref1 and i gβref1 , I gαβref2 is the α, β - axis current component igαref2 and i gβref2 vector expression form;

[0020] The positive - terminal sliding - mode variable - structure controller and the negative - terminal sliding - mode variable - structure controller respectively control the positive - terminal and negative - terminal α, β - axis currents I gαβ1 and I gαβ2 to make them respectively follow the reference values I gαβref1 、I gαβref2 of the positive - terminal and negative - terminal α, β - axis currents. The outputs of the positive - terminal sliding - mode variable - structure controller and the negative - terminal sliding - mode variable - structure controller are respectively used as the α, β - axis components U difαβ1 and U difαβ2 of the reference differential - mode voltages of the positive - terminal and negative - terminal MMCs;

[0021] The positive - terminal internal - circulating - current controller and the negative - terminal internal - circulating - current controller respectively control the positive - terminal and negative - terminal MMC internal - circulating currents I cabc1 and I cabc2 by using resonant controllers, which can be realized in the stationary coordinate system. The outputs of the positive - terminal internal - circulating - current controller and the negative - terminal internal - circulating - current controller are respectively used as the positive - terminal and negative - terminal MMC reference common - mode voltages U comabc1 and U comabc2 ;

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

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

[0024] Furthermore: In the virtual mechanical module, calculate the reference phase θ r according to the following method:

[0025] θ r (k + 1)=∫ω r (k + 1)dt

[0026]

[0027] Among them, θ r (k + 1) is the reference phase of the next sampling period, ω r (k + 1) is the angular frequency of the next sampling period, ω r (k) is the angular frequency of this sampling period, ω n is the rated angular frequency, P ref is the reference value of active power, P g1 (k) and P g2 (k) are the positive and negative active powers of this sampling period, J is the virtual rotor inertia, D p is the active damping coefficient.

[0028] Furthermore: In the virtual excitation module, calculate the reference voltage amplitude |U gr | according to the following method:

[0029]

[0030] Among them, |U gr (k)| and |U gr (k + 1)| are the reference voltage amplitudes of |U gr | in this sampling period and the next sampling period respectively, U ref is the preset voltage reference value, Q ref is the reference value of reactive power, Q g1 (k) and Q g2 (k) are the positive and negative reactive powers of this sampling period, K is the virtual excitation coefficient, D q is the reactive damping coefficient.

[0031] Furthermore: In the voltage reference value calculation module, calculate the voltage vector reference value U gαβref in the two-phase stationary coordinate system according to the following method;

[0032]

[0033] Among them, u gαref and u gβref are the α and β axis components of U gαβref respectively.

[0034] Furthermore: In the positive and negative sliding mode variable structure controllers, use the following method to control the positive and negative α and β axis currents I gαβ1 and I gαβ2 , and the outputs of the positive and negative sliding mode variable structure controllers are used as the positive and negative differential mode voltage reference values U difαβ1 and U difαβ2 respectively:

[0035]

[0036] Among them, K α1 and K β1 are the integral coefficients of the positive - pole α - axis and β - axis sliding mode surfaces respectively, and K α2 and K β2 are the integral coefficients of the negative - pole α - axis and β - axis sliding mode surfaces respectively. S α1 and S β1 are the positive - pole α - axis and β - axis sliding mode surfaces respectively, and S α2 and S β2 are the negative - pole α - axis and β - axis sliding mode surfaces respectively. Their expressions are as follows:

[0037]

[0038] K αs1 and K βs1 are the coefficients of the improved sign functions of the positive - pole α - axis and β - axis respectively, and K αs2 and K βs2 are the coefficients of the improved sign functions of the negative - pole α - axis and β - axis respectively. rsgn(S α1 ) and rsgn(S β1 ) are the improved sign functions. Their expressions are as follows:

[0039]

[0040] Among them, λ j is the buffer critical value of the improved sign function.

[0041] The beneficial effects of the present invention are as follows:

[0042] Due to the adoption of the technical solution of the present invention, by simulating the inertia characteristics of the synchronous machine, rationally configuring the positive - and negative - pole reference command values, and adopting the sliding - mode variable - structure control strategy, the coordinated control between the positive and negative poles of the true - bipolar MMC is realized, which can provide inertia support for the AC power grid, improve the stability of the power grid, and is more friendly to the new power system containing the grid - connection of large - scale power - electronic systems. Brief Description of the Drawings

[0043] Figure 1 It is a specific example structure diagram of the true - bipolar MMC transmission system. Among them, u ga , u gb , u gc are the grid voltages; i ga , i gb , i gc are the grid currents; u pa , u pb , u pc are the upper - arm voltages of the MMC; u na , u nb, u nc is the lower arm voltage 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.

[0044] Figure 2 This is a schematic diagram of a specific example system of the control method of the present invention. Among them:

[0045] 1 - Virtual mechanical module, 2 - Virtual excitation module, 3 - Voltage reference value calculation module, 4 - Positive voltage sampling module, 5 - Positive current sampling module, 6 - Positive Clark transformation module, 7 - Positive power calculation module, 8 - Positive voltage controller, 9 - Positive sliding mode variable structure controller, 10 - Positive Clark inverse transformation module, 11 - Positive internal circulating current controller, 12 - Positive arm voltage calculation module, 13 - Positive modulation module, 14 - Negative voltage sampling module, 15 - Negative current sampling module, 16 - Negative Clark transformation module, 17 - Negative power calculation module, 18 - Negative voltage controller, 19 - Negative sliding mode variable structure controller, 20 - Negative Clark inverse transformation module, 21 - Negative internal circulating current controller, 22 - Negative arm voltage calculation module, 23 - Negative modulation module.

[0046] Figure 3 This is the specific control schematic diagram of the virtual mechanical module; among them: θ r is the system reference phase, ω r is the angular frequency, ω n is the rated angular frequency, P ref is the reference value of active power, P g1 and P g2 are the positive and negative active powers, J is the virtual rotor moment of inertia, D p is the active damping coefficient.

[0047] Figure 4 This is the specific control schematic diagram of the virtual flux linkage module; among them: |U gref | is the reference voltage amplitude, U g is the voltage amplitude, U ref is the preset voltage reference value, Q ref is the reference value of reactive power, Q g1 and Q g2 are the positive and negative reactive powers, K is the virtual excitation coefficient, Dq is the reactive damping coefficient. Detailed implementation manners

[0048] 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.

[0049] The system implementation of the true bipolar MMC sliding mode variable structure control method with inertia support ability in the present invention is as Figure 2 shown, and includes a virtual mechanical module 1, a virtual excitation module 2, a voltage reference value calculation module 3, a positive voltage sampling module 4, a positive current sampling module 5, a positive Clark transformation module 6, a positive power calculation module 7, a positive voltage controller 8, a positive sliding mode variable structure controller 9, a positive Clark inverse transformation module 10, a positive internal circulating current controller 11, a positive arm voltage calculation module 12, a positive modulation module 13, a negative voltage sampling module 14, a negative current sampling module 15, a negative Clark transformation module 16, a negative power calculation module 17, a negative voltage controller 18, a negative sliding mode variable structure controller 19, a negative Clark inverse transformation module 20, a negative internal circulating current controller 21, a negative arm voltage calculation module 22, and a negative modulation module 23.

[0050] As Figure 2 shown, the true bipolar MMC sliding mode variable structure control method with inertia support ability in the present invention includes the following steps:

[0051] Collect the three-phase voltage U of the AC power grid through the positive (negative) voltage sampling module 4 (14), gabc and collect the three-phase current I of the positive (negative) MMC AC power grid side through the positive (negative) current sampling module 5 (15) gabc1 (I gabc2 ), and the internal circulating current I cabc1 (I cabc2 ).

[0052] Utilize the positive (negative) power calculation module 7 (17) to calculate the positive (negative) active and reactive powers P gabc and Q gabc1 (I gabc2 ) according to the three-phase voltage U of the AC power grid and the three-phase current I of the positive (negative) MMC AC power grid side; g1 and Q g1 (P g2 and Q g2 );

[0053] Utilize the virtual mechanical module 1 to calculate the reference phase θ r according to the following method.

[0054] θ r (k + 1) = ∫ωr (k + 1)dt

[0055]

[0056] where θ r (k + 1) is the reference phase of the next sampling period, ω r (k + 1) is the angular frequency of the next sampling period, ω r (k) is the angular frequency of this sampling period, ω n is the rated angular frequency, P ref is the reference value of active power, P g1 (k) and P g2 (k) are the positive and negative active powers of this sampling period, J is the virtual rotor inertia, D p is the active damping coefficient.

[0057] Using the virtual excitation module 2, calculate the reference voltage amplitude |U gr | according to the following method.

[0058]

[0059] where |U gr (k)| and |U gr (k + 1)| are the reference voltage amplitudes of |U gr | in this sampling period and the next sampling period respectively, U ref is the preset voltage reference value, Q ref is the reference value of reactive power, Q g1 (k) and Q g2 (k) are the positive and negative reactive powers of this sampling period, K is the virtual excitation coefficient, D q is the reactive damping coefficient.

[0060] Using the positive (negative) pole Clark transformation module 6(16), transform the three-phase voltages U gabc of the AC power grid and the three-phase currents I gabc1 (I gabc2 ) of the positive (negative) pole MMC AC power grid by Clark transformation to obtain the corresponding voltage vector U gαβ and current vector I gαβ1 (I gαβ2 ) in the two-phase stationary coordinate system.

[0061] In the voltage reference value calculation module 3, calculate the voltage vector reference value U gαβref in the two-phase stationary coordinate system according to the following method;

[0062]

[0063] where u gαrefand u gβref are the α and β axis components of U gαβref respectively.

[0064] The positive (negative) pole voltage controller 8 (18) is used to control the α and β axis voltages U gαβ through the resonant controller, so that it follows the given voltage vector reference value U gαβref . The outputs of the positive (negative) pole voltage controller are respectively used as the reference values I gαβref1 (I gαβref2 ) of the positive (negative) pole α and β axis currents. The implementation method of the positive pole voltage controller 8 is as follows:

[0065]

[0066] Among them, F 1R50 (s) is the transfer function of the resonant controller with a resonant frequency of ±50Hz, k r1 is the gain coefficient of the resonant controller, and ω cr1 is the cut-off frequency.

[0067] The implementation method of the negative pole voltage controller 18 is as follows:

[0068]

[0069] Among them, F 2R50 (s) is the transfer function of the resonant controller with a resonant frequency of ±50Hz, k r2 is the gain coefficient of the resonant controller, and ω cr2 is the cut-off frequency.

[0070] The positive (negative) pole sliding mode variable structure controller 9 (19) is used to control the positive (negative) pole α and β axis currents I gαβ1 (I gαβ2 ) by the following method. The output of the positive (negative) pole sliding mode variable structure controller is used as the positive (negative) pole differential mode voltage reference value U difαβ1 (U difαβ2 ):

[0071]

[0072] Among them, K α1 and K β1 (K α2 and K β2 ) are respectively the integral coefficients of the positive (negative) pole α and β axis sliding mode surfaces, S α1 and S β1 (S α2 and S β2 ) are respectively the positive (negative) pole α and β axis sliding mode surfaces, and their expressions are as follows:

[0073]

[0074] K αs1 and K βs1 (K αs2 and K βs2 ) are the coefficients of the improved sign functions of the positive (negative) pole α and β axes, rsgn(S α1 ) and rsgn(S β1 ) are the improved sign functions, and their expressions are as follows:

[0075]

[0076] where λ j is the buffer critical value of the improved sign function.

[0077] Using the positive (negative) pole Clark inverse transformation module 10 (20), perform Clark inverse transformation on the positive (negative) pole reference differential mode voltage U difαβ1 (U difαβ2 ) to obtain the reference differential mode voltage U difabc1 (U difabc2 ) in the stationary three-phase coordinate system;

[0078] Using the positive (negative) pole internal circulating current controller 11 (21) to control the positive (negative) pole MMC internal circulating current I cabc1 (I cabc2 ) with a resonant controller, which can be achieved in the stationary coordinate system; the output of the positive (negative) pole internal circulating current controller serves as the positive (negative) pole MMC reference common mode voltage U comabc1 (U comabc2 );

[0079] Using the positive (negative) pole bridge arm voltage calculation module 12 (22), through the positive (negative) pole reference differential mode voltage U difabc1 (U difabc2 ) and the reference common mode voltage U comabc1 (U comabc2 ), calculate the reference voltages U prefabc1 and U nrefabc1 (U prefabc2 and U nrefabc2 ) of the upper and lower bridge arms of the positive (negative) pole MMC;

[0080] Using the positive (negative) pole modulation module 13 (23), according to the reference voltages U prefabc1 and U nrefabc1 (U prefabc2 and U nrefabc2 ) of the upper and lower bridge arms of the positive (negative) pole MMC, realize the control of the positive (negative) pole MMC.

[0081] The above description of the embodiments is provided to enable 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 all improvements and modifications made by those skilled in the art based on the disclosure of the present invention should fall within the protection scope of the present invention.

Claims

1. A true bipolar MMC sliding mode variable structure control method with inertia support ability, characterized in that, The control system adopted to implement the described method includes: a virtual mechanical module, a virtual excitation module, a voltage reference value calculation module, 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 positive electrode power calculation module and a negative electrode power calculation module, a positive electrode voltage controller and a negative electrode voltage controller, a positive electrode sliding mode variable structure controller and a negative electrode 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 virtual mechanical module controls the active power of the positive and negative poles by simulating the mechanical equation of the synchronous generator, and the output of the virtual mechanical module is used as the reference voltage phase θ of the true bipolar MMC system r ; The virtual excitation module controls the total positive and negative reactive powers by simulating the excitation equation of a synchronous generator. The output of the virtual excitation module serves as the reference voltage amplitude |U gr | of the true bipolar MMC system; The voltage reference value calculation module calculates the voltage vector reference value U in the two-phase stationary coordinate system according to the reference voltage phase θ r and amplitude |U gr | gαβref ; 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 voltages 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 ; 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 module and a negative electrode Clark inverse transformation module; The positive - pole Clark transformation module and the negative - pole Clark transformation module perform Clark transformation on the three - phase voltages U of the AC power grid gabc and the three - phase currents I of the positive - pole and negative - pole MMCs in the AC power grid gabc1 and I gabc2 to obtain the corresponding voltage vectors U in the two - phase stationary coordinate system gαβ and the corresponding current vectors I in the two - phase stationary coordinate system gαβ1 and 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 positive - electrode power calculation module and the negative - electrode power calculation module respectively calculate, based on the voltage vector U gαβ and the positive - electrode current vector and the negative - electrode current vectors I gαβ1 、I gαβ2 , the active powers P g1 and P g2 of the positive electrode and the negative electrode and the reactive powers Q g1 and Q g2 ; The positive - voltage controller and the negative - voltage controller control the α - and β - axis voltages U gαβ through a resonant controller to make them follow the reference values. The outputs of the positive - voltage controller and the negative - voltage controller are respectively used as the reference values I gαβref1 and I gαβref2 of the α - and β - axis currents of the positive and negative electrodes; where U gαβ is the vector representation of the α - and β - axis voltage components u gα and u gβ , I gαβref1 is the vector representation of the α - and β - axis current components i gαref1 and i gβref1 , and I gαβref2 is the vector representation of the α - and β - axis current components i gαref2 and i gβref2 . The positive - terminal sliding - mode variable - structure controller and the negative - terminal sliding - mode variable - structure controller respectively control the positive - terminal and negative - terminal α,β - axis currents I gαβ1 and I gαβ2 to make them respectively follow the reference values I gαβref1 and I gαβref2 of the positive - terminal and negative - terminal α,β - axis currents. The outputs of the positive - terminal sliding - mode variable - structure controller and the negative - terminal sliding - mode variable - structure controller are respectively used as the α,β - axis components U difαβ1 and U difαβ2 of the reference differential - mode voltages of the positive - terminal and negative - terminal MMCs; 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 utilize 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 implement the control of the positive electrode and negative electrode MMCs according to the reference voltages U prefabc1 and U prefabc2 of the upper bridge arms of the positive electrode and negative electrode MMCs, and the reference voltages U nrefabc1 and U nrefabc2 of the lower bridge arms of the positive electrode and negative electrode MMCs.

2. The true bipolar MMC sliding mode variable structure control method with inertia support ability according to claim 1, characterized in that: In the virtual mechanical module, the reference phase θ is calculated according to the following method r :[[]]END]] θ r (k + 1) = ∫ω r (k + 1)dt where θ r (k + 1) is the reference phase for the next sampling period, ω r (k + 1) is the angular frequency for the next sampling period, ω r (k) is the angular frequency for this sampling period, ω n is the rated angular frequency, P ref is the reference value of active power, P g1 (k) and P g2 (k) are the positive and negative active powers for this sampling period, J is the virtual rotor inertia, D p is the active damping coefficient.

3. The true bipolar MMC sliding mode variable structure control method with inertia support ability according to claim 1, characterized in that: In the virtual excitation module, the reference voltage amplitude |U gr | is calculated according to the following method: Among them, |U gr (k)| and |U gr (k + 1)| are the reference voltage amplitudes of |U gr | in this sampling period and the next sampling period respectively. U ref is the preset voltage reference value, Q ref is the reactive power reference value, Q g1 (k) and Q g2 (k) are the positive and negative reactive powers in this sampling period. K is the virtual excitation coefficient, D q is the reactive power damping coefficient.

4. The true bipolar MMC sliding mode variable structure control method with inertia support ability according to claim 1, characterized in that: In the voltage reference value calculation module, the voltage vector reference value U in the two-phase stationary coordinate system is calculated according to the following method gαβref , where u gαref and u gβref are the α-axis and β-axis components of U gαβref respectively, 5. The true bipolar MMC sliding mode variable structure control method with inertia support ability according to claim 1, characterized in that: In the positive - terminal sliding - mode variable - structure controller and the negative - terminal sliding - mode variable - structure controller, the following method is used to control the positive - terminal and negative - terminal α and β - axis currents I gαβ1 and I gαβ2 . The outputs of the positive - terminal sliding - mode variable - structure controller and the negative - terminal sliding - mode variable - structure controller are respectively used as the positive - terminal and negative - terminal differential - mode voltage reference values U difαβ1 and U difαβ2 : where K α1 and K β1 are the integral coefficients of the sliding mode surfaces on the α and β axes of the positive electrode respectively, and K α2 and K β2 are the integral coefficients of the sliding mode surfaces on the α and β axes of the negative electrode respectively. S α1 and S β1 are the sliding mode surfaces on the α and β axes of the positive electrode respectively, and S α2 and S β2 are the sliding mode surfaces on the α and β axes of the negative electrode respectively. Their expressions are as follows: K αs1 and K βs1 are the coefficients of the improved sign functions for the α and β axes of the positive electrode, respectively. K αs2 and K βs2 are the coefficients of the improved sign functions for the α and β axes of the negative electrode, respectively. rsgn(S α1 ) and rsgn(S β1 ) are the improved sign functions, and their expressions are as follows: where λ j is the buffer critical value for improving the sign function.

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