MMC-HVDC control system and method

Abstract

The present invention provides an MMC-HVDC control system and method, relating to the technical field of direct current transmission of power systems. The control system of the present invention adopts a field programmable gate array (FPGA) chipset with a parallel architecture to carry out complex calculations such as MMC operation and control. In terms of communication, remote communication is carried out through a 5G channel, which is fast and flexible, and convenient for decentralized and remote data acquisition. At the same time, a remote cloud computing platform is used for large data storage, complex calculations and remote monitoring. Compared with the traditional PI control strategy, the interval sliding mode control method designed by the present invention has the advantages of high control accuracy, strong dynamic response capability, good stability, easy parameter debugging, strong robustness, etc. when parameter perturbations occur in MMC. It is suitable for various practical projects of MMC-HVDC and has important application significance for improving the robustness and control accuracy of flexible direct current transmission systems in actual production and promoting the development of related industries.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current transmission of power systems, and in particular to an MMC-HVDC control system and method. Background Art

[0002] Modular multilevel converters (MMCs) offer significant advantages in waveform quality, switching frequency, switching losses, and fault handling, and have garnered extensive research and attention in the field of high-voltage direct current transmission (HVDC). With the continuous development of flexible HVDC technology in China, there is a significant demand for control systems and control methods supporting MMCs. Currently, MMC control systems suffer from inconsistent design standards, a lack of computational power for system modeling and complex control algorithms, and a predominantly local control approach. These inflexible communication channels hinder decentralized and remote data collection, and the lack of large-scale data storage and remote monitoring capabilities. The current mainstream MMC control method is vector control based on the PI control algorithm. While PI controllers offer good tracking performance for traditional MMC steady-state conditions, their robustness is poor. Especially when the MMC model parameters are subject to random perturbations, PI controllers are prone to significant overshoot, large steady-state errors, and even control system instability, hindering the safe and stable operation of MMC-HVDC systems. Summary of the Invention

[0003] Aiming at the shortcomings of the existing technology, an MMC-HVDC control system and method are proposed to address the problems of poor versatility, limited computing power, inflexible communication, and poor robustness in conventional MMC control systems and control methods.

[0004] In order to achieve the above object, the technical solution of the present invention is:

[0005] In one aspect, an MMC-HVDC control system includes a control module, a 5G communication module, a cloud computing module, and an execution module;

[0006] The control module includes an FPGA control mainboard and a touch screen; the FPGA control mainboard includes an FPGA core module and a USB communication module; one end of the USB communication module is connected to the FPGA core module, and the other end is connected to the touch screen;

[0007] The 5G communication module includes a 5G sending module and a 5G receiving module; the 5G sending module and the 5G receiving module send and receive information through the 5G network.

[0008] The cloud computing module includes a cloud computing platform module and a monitoring terminal module. The cloud computing platform module uses the public cloud as a computing platform and exchanges data with the FPGA control motherboard via the 5G communication module. It contains an application software module and a database module. The application software module has system model identification, controller parameter optimization, system fault diagnosis, and system monitoring functions. The database module is responsible for the distributed storage and management of system data. The monitoring terminal module uses a browser / server (B / S) model to exchange data with the cloud computing platform via the public Internet and realizes remote monitoring of the system by calling the system monitoring function of the application software in the cloud computing platform.

[0009] The execution module includes an analog-to-digital conversion A / D module, a digital-to-analog conversion D / A module, a current feedback module, a voltage feedback module, an MMC circuit module, a nearest level modulation module, and a sub-module capacitor voltage modulation module; the output end of the analog-to-digital conversion A / D module is connected to the FPGA core module through the 5G communication module, and the input end is respectively connected to the voltage feedback module and the current feedback module, and the voltage feedback module and the current feedback module are respectively composed of a voltage transformer and a current transformer, and their input ends are respectively connected to the output end of the MMC circuit module; the input end of the digital-to-analog conversion D / A module is connected to the FPGA core module through the 5G communication module, and the output end is connected to the input end of the nearest level modulation module, the input end of the sub-module capacitor voltage modulation module is connected to the output end of the nearest level modulation module, and the output end of the sub-module capacitor voltage modulation module is connected to the input end of the MMC circuit module.

[0010] In another aspect, an MMC-HVDC control method is implemented by the aforementioned MMC-HVDC control system, comprising the following steps:

[0011] Step 1: After the MMC-HVDC system is running and has been initialized and self-tested, select the MMC control strategy and set the control parameters through the touch screen according to the operating mode of the MMC converter;

[0012] The MMC control strategy includes a DC voltage and reactive power control strategy, and an active power and reactive power control strategy, and the set values ​​of DC voltage, active power and reactive power are given according to the selected control strategy;

[0013] Step 2: The DC voltage and reactive power control strategy adopts a dual-loop control of inner and outer loops, which is implemented by designing an outer loop controller and an interval sliding mode inner loop current controller; wherein the outer loop controller includes an outer loop DC voltage controller and an outer loop reactive power controller, and a reference value of the inner loop current is calculated by the outer loop DC voltage controller and the outer loop reactive power controller;

[0014] The outer-loop DC voltage controller and outer-loop reactive power controller are: the MMC outer-loop DC voltage controller outputs an inner-loop current reference value to maintain the DC side voltage stability and AC side reactive power balance of the MMC. Its internal structure adopts a PI controller for the purpose of controlling DC voltage and reactive power, namely an outer-loop DC voltage PI controller and an outer-loop reactive power PI controller. The PI controller outputs the corresponding inner-loop current reference value, thereby stabilizing the DC system voltage and controlling the reactive power flow;

[0015] The expressions of the outer loop DC voltage PI controller and the outer loop reactive power PI controller are:

[0016]

[0017] Where k p1 、k p2 are the proportional coefficients of the outer loop DC voltage PI controller and the outer loop reactive power PI controller respectively; k I1 、k I2 are the integral coefficients of the outer loop DC voltage PI controller and the outer loop reactive power PI controller respectively; i vdref 、i vqref are the reference values ​​of the inner loop current controller, and are also the output values ​​of the outer loop DC voltage PI controller and the outer loop reactive power PI controller; U dcref 、U dc are the reference value of DC voltage and the voltage of DC circuit respectively; Q ref and Q are the reference value of reactive power and the reactive power output by the MMC, respectively.

[0018] The active power and reactive power control strategy adopts a dual-loop control of the inner and outer loop types, which is implemented by designing an outer loop controller and an interval sliding mode inner loop current controller; the outer loop controller includes an outer loop active power controller and an outer loop reactive power controller, which output inner loop current reference values ​​for given active power and reactive power, and respectively adopts PI controllers for the purpose of controlling active power and reactive power, namely, an outer loop active power PI controller and an outer loop reactive power PI controller, and outputs corresponding inner loop current reference values ​​through the PI controllers, thereby stabilizing the DC system voltage and controlling the reactive power flow.

[0019] The outer loop active power controller and the outer loop reactive power controller are as follows: the MMC outer loop power controller outputs an inner loop current reference value that maintains the MMC output of a given active power and reactive power. Its internal structure adopts a PI controller for the purpose of controlling active power and reactive power, that is, an outer loop reactive PI controller. The PI controller outputs the corresponding inner loop current reference value, thereby realizing that the MMC outputs constant active power and reactive power, ensuring that the DC system power flow operates according to the set value.

[0020] The expressions of the outer loop active power PI controller and the outer loop reactive power PI controller are:

[0021]

[0022] Where k p3 、k p4 are the proportional coefficients of the outer loop active power PI controller and the outer loop reactive power PI controller respectively; k I3 、k I4 are the integral coefficients of the outer loop active power PI controller and the outer loop reactive power PI controller respectively; P ref and P are the reference value of active power and the active power output by the MMC, respectively.

[0023] The design methods of the interval sliding mode inner loop controllers of the MMC DC voltage and reactive power control strategy and the active power and reactive power control strategy are both equation (32), and the calculation method of the final control quantity is both equation (47).

[0024] Step 3: During the MMC operation, the inner loop current controller and the circulating current suppression controller are designed using the interval sliding mode control algorithm, and the sliding mode control variable for the nearest level modulation is calculated;

[0025] The interval sliding mode control algorithm design method is as follows:

[0026] Step 3.1: Based on the working principle of MMC-HVDC, establish the fundamental frequency dynamic mathematical model of the AC circuit of MMC in a synchronous rotating coordinate system;

[0027] The reference direction of the current at the output of the MMC converter circuit is from the DC side to the AC side. The neutral point of the DC side is represented by point O, and the neutral point of the AC side is represented by point O'. The resistance R0 is the equivalent loss of the entire bridge arm, and L0 is the bridge arm reactor; R ac and L ac They are the AC output v of the converter respectively k The equivalent resistance and inductance between the equivalent potential of the AC system; the six bridge arms of the MMC are composed of N sub-modules SM; u pk and u nk are the bridge arm voltages of all submodules of the upper and lower bridge arms, respectively, where k = a, b, c; i pk and i nk are the currents flowing through the upper and lower bridge arms respectively; U sk is the AC grid voltage; U dc is the DC voltage; i vkis the three-phase current output by the valve-side converter outlet; since R0 is the sum of the losses of all submodules on a single bridge arm, the loss of the submodule is obtained through its Thevenin equivalent circuit. The submodule SM is composed of two IGBTs T1 and T2 and two diodes D1 and D2, respectively. T1, D1 and T2, D2 can be regarded as a variable resistor R1, R2 controlled by a switch instruction, respectively. When T1 is turned on, R1 takes a minimum value; when T1 is turned off, R1 takes a maximum value; when T2 is turned on, R2 takes a minimum value; when T2 is turned off, R2 takes a maximum value, u ceq is the equivalent voltage of the submodule capacitor;

[0028] For a single submodule, the loss R eq The calculation using Thevenin's theorem is as follows:

[0029]

[0030] The loss of the entire bridge arm, that is, the resistance R0, is:

[0031]

[0032] Where N is the number of submodules in each bridge arm.

[0033] For phase k, according to Kirchhoff's voltage law and Kirchhoff's current law, the KVL and KCL equations are written for the upper and lower bridge arms respectively:

[0034]

[0035]

[0036] i vk =i pk -i nk (7)

[0037] Among them, the definition is:

[0038]

[0039] Where U diffk is the differential mode voltage of the upper and lower bridge arms of the k-phase MMC, also known as the internal virtual electromotive force; R is the equivalent resistance of the AC and DC sides of the MMC; L is the equivalent inductance of the AC and DC sides of the MMC;

[0040] By adding equations (5) and (6) and combining them with the definition in equation (8), we can obtain the fundamental frequency dynamic mathematical model of the MMC AC circuit in the three-phase stationary coordinate system:

[0041]

[0042] Perform coordinate transformation on equation (9) and transform the three-phase stationary coordinate system intoabc The sinusoidal AC quantity in the coordinate system is transformed into the DC quantity in the two-axis synchronous rotating coordinate system dq coordinate system;

[0043] The coordinate transformation adopts the classic Park transformation, as shown in the following formula:

[0044] f dq (t) = T 3s-dq (θ)f abc (t) (10)

[0045] f abc (t) = T dq-3s (θ)f dq (t) (11)

[0046]

[0047]

[0048] Where f dq () is the expression in dq coordinate system; f abc () is the expression in the abc coordinate system; θ is the angle used in the Park transform, which is obtained by the phase-locked loop PLL. When the PLL achieves phase-locked synchronization, θ is equal to the A-phase AC voltage u sa The phase angle of T 3s-dq (θ) is the transformation matrix from the abc three-phase stationary coordinate system to the dq rotating coordinate system, 3s refers to the three-phase stationary coordinate system; T dq-3s (θ) is the transformation matrix from the dq rotating coordinate system to the abc three-phase stationary coordinate system;

[0049] Applying the coordinate transformation of equation (12) to equation (9) yields the mathematical model of the AC circuit of MMC in the dq coordinate system:

[0050]

[0051] Where U sd 、U sq and i vd 、i vq are the d-axis and q-axis components of the grid voltage and current respectively; U diffd 、U diffq are the d-axis and q-axis voltage components of the differential mode voltage respectively; ω is the rated angular frequency of the power grid;

[0052] Step 3.2: Based on the working principle of MMC-HVDC, establish the mathematical model of the DC circuit of MMC in the synchronous rotating coordinate system;

[0053] The common mode voltage of the upper and lower bridge arms of the K-phase of the MMC is defined as:

[0054]

[0055] Subtracting equation (5) from equation (6) yields the DC circuit mathematical model, as shown below:

[0056]

[0057] Where i diffk is the unbalanced current of phase k inside the MMC, defined as:

[0058]

[0059] Where, I dck is the unbalanced current i diffk The DC component, i.e. the DC current flowing through the k-phase unit; cirk is the unbalanced current i diffk The AC component is the circulating current;

[0060] The mathematical model of the DC circuit of MMC in the abc coordinate system (15) is d -2 q -2 The mathematical model of the DC circuit of MMC in the dq coordinate system is obtained by coordinate transformation:

[0061]

[0062] Where i diffd 、i diffq are the components of the unbalanced current inside the MMC on the d-axis and q-axis respectively; U comd 、U comq are the components of the MMC common mode voltage on the d-axis and q-axis respectively;

[0063] Step 3.3: Design an inner loop current controller based on interval sliding mode variable structure control based on the mathematical model of the MMC AC circuit;

[0064] For Equation (14), it can be written as the following state equation:

[0065]

[0066] Where i vd 、i vq is the state variable; i vdref 、i vqref is the input reference value; U diffd 、U diffq is the control quantity; d1 and d2 are the grid disturbances, and |d1|≤D1, |d2|≤D2; D1 and D2 are the upper limits of the absolute value of the grid disturbance, respectively. The control target is: i vd →i vdref ,i vq →i vqref ;

[0067] In order to facilitate the expression of the formula, set the parameters x1, x2, x1 * 、x2 * , u1, u2, e1, e2, their specific meanings are shown in the following formula:

[0068]

[0069] Therefore, formula (19) can be written as:

[0070]

[0071] For equation (21), interval sliding mode control is used to design the controller, and the integral sliding mode surface is used, which is in the form of:

[0072]

[0073] Where s1 and s2 are the switching functions of the two control targets in equation (19); t is time; k s1 、k s2 is the integration constant; e1 and e2 are the input deviations of x1 and x2 respectively;

[0074] Select the Lyapunov function V1 as:

[0075]

[0076] Therefore, the derivative of V1 is expressed as:

[0077]

[0078] Where v1 and v2 are the two components of the Lyapunov function V1;

[0079] for have:

[0080]

[0081] In order to facilitate the expression of the formula, set the variable η1

[0082]

[0083] get:

[0084]

[0085] Where |η1| max is the maximum absolute value of η1;

[0086] Therefore, when When satisfied Where ε1 is a positive number greater than 0;

[0087] for Know:

[0088]

[0089] In order to facilitate the expression of the formula, set the variable η2

[0090]

[0091] get:

[0092]

[0093] Where |η2| max is the maximum absolute value of η2;

[0094] Therefore, when When satisfied Where ε2 is a positive number greater than 0;

[0095] Therefore, the expression of the inner loop current controller is obtained as:

[0096]

[0097] In order to obtain a more stable sliding mode, the saturation function sat(s) is used to replace the sign function sgn(s), and then Equation (32) is the final inner loop current controller expression as shown below:

[0098]

[0099] Step 3.4: Design a circulating current suppression controller based on interval sliding mode variable structure control in combination with the mathematical model of the MMC DC circuit;

[0100] For Equation (18), rewrite it into the form of the following state equation:

[0101]

[0102] Where i diffd 、i diffq is the state variable; U comd 、U comq is the controlled quantity; the control target is: i diffd →i diffdref 、i diffq →i diffqref ;

[0103] In order to facilitate the expression of the formula, set the parameters x3, x4, x3 * 、x4 * , u3, u4, e3, e4, their specific meanings are shown in the following formula:

[0104]

[0105] Where i diffdref 、i diffqref i diffd 、i diffq reference value.

[0106] Therefore, formula (33) can be written as:

[0107]

[0108] For equation (35), interval sliding mode control is used to design the controller, and the integral sliding mode surface is used, which is in the form of:

[0109]

[0110] Where s3 and s4 are the switching functions of the two control objectives in equation (33); k s3 、k s4 is the integration constant; e3 and e4 are the input deviations of x3 and x4 respectively;

[0111] Select the Lyapunov function V2 as:

[0112]

[0113] Therefore, the derivative of V2 is expressed as:

[0114]

[0115] Where v3 and v4 are the two components of the Lyapunov function V2;

[0116] for Know:

[0117]

[0118] In order to facilitate the expression of the formula, set the variable η3

[0119]

[0120] Know:

[0121]

[0122] Where |η3| max is the maximum absolute value of η3;

[0123] Therefore, when When satisfied Where ε3 is a positive number greater than 0;

[0124] for Know:

[0125]

[0126] In order to facilitate the expression of the formula, set the variable η4

[0127]

[0128] Know:

[0129]

[0130] Where |η4| max is the maximum absolute value of η4;

[0131] Therefore, when When satisfied Where ε4 is a positive number greater than 0;

[0132] Therefore, the expression of the circulating current suppression controller is obtained as follows:

[0133]

[0134] The saturation function sat(s) is used to replace the sign function sgn(s), and the expression of the final circulating current suppression controller is:

[0135]

[0136] Step 3.5: Based on the control variables output by the invented inner-loop current controller based on interval sliding mode variable structure control and the circulating current suppression controller based on interval sliding mode variable structure control, the reference voltages of the upper and lower bridge arms of the MMC three-phase are finally obtained, and the trigger signals of the IGBTs in each MMC submodule are obtained using the nearest level modulation strategy;

[0137] The u1 and u2 obtained by the inner loop current controller (32) are subjected to dq inverse transformation through formula (13) to obtain the reference value U of the MMC differential mode voltage diffk_ref Similarly, u3 and u4 obtained by the circulating current suppression controller (46) are inversely transformed by formula (13) (replacing θ with -2θ) to obtain the reference value U of the MMC common mode voltage comk_ref Finally, the MMC three-phase upper and lower bridge arm voltage command values ​​U are calculated. pk_ref 、U nk_ref The final control quantity output by the FPGA control mainboard is:

[0138]

[0139] The final control quantity output by the FPGA control mainboard is input into the nearest level modulation module and the sub-module capacitance voltage modulation module, and then the trigger signal of the IGBT in each sub-module of the MMC is calculated to complete the control of the MMC.

[0140] Step 4: All MMC-related data in the controller, including differential-mode voltage, common-mode voltage, AC side current, AC side voltage, DC voltage, inner loop current reference value, DC voltage reference value, active power, reactive power, active power reference value, reactive power reference value, inner circulating current, PI controller parameters, interval sliding mode controller parameters, and MMC circuit parameters, are transmitted to the cloud computing platform to display the operating status of the MMC and continuously control the MMC, thereby optimizing the control of the entire MMC-HVDC.

[0141] Beneficial effects of the present invention:

[0142] The present invention provides an MMC-HVDC control system and method, which has the following beneficial effects:

[0143] (1) The MMC interval sliding mode control system suitable for parameter perturbation in the present invention applies conventional outer loop voltage and power control technology based on PI control algorithm, anti-interference inner loop current control technology based on interval sliding mode variable structure algorithm, FPGA control technology, 5G communication technology, and cloud computing technology to the design of MMC control system, thereby improving the flexibility of system structure and data acquisition, enhancing data processing capability, achieving high-quality control of MMC inner loop current, and improving the robustness of MMC control;

[0144] (2) The MMC interval sliding mode control method suitable for parameter perturbations in the present invention adopts the integral sliding surface and interval sliding mode variable structure control method, and is designed for various parameter disturbances faced by MMC, so that MMC has good robustness under various parameter perturbation conditions.

[0145] (3) The present invention also has remote modeling, optimization, diagnosis, data storage and monitoring mechanisms, which greatly expands the control function of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0146] Figure 1 This is an overall schematic diagram of an MMC-HVDC control system provided by the present invention;

[0147] Figure 2 This is the MMC control flow chart provided by the present invention;

[0148] Figure 3 The present invention provides an MMC DC voltage and reactive power control dual-loop controller;

[0149] Figure 4This is a circuit diagram of the MMC converter provided by the present invention;

[0150] Figure 5 This is a loss equivalent circuit diagram of a single bridge arm provided by the present invention;

[0151] Figure 6 The MMC circulating current suppression controller provided by the present invention;

[0152] Figure 7 The invention provides an MMC active power and reactive power control dual-loop controller. DETAILED DESCRIPTION

[0153] The specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0154] On the one hand, an MMC-HVDC control system uses an FPGA as a core controller and implements the following main functions: before the MMC is operated, the control mode is selected through the settings of the touch screen, and the control of different electrical quantities, namely DC voltage, active power, and reactive power, is achieved, thereby improving the versatility of the MMC controller; during the operation of the MMC, a conventional negative feedback control method based on PI control is adopted for the outer loop DC voltage and reactive power controller, and the outer loop active power and reactive power controller, and interval sliding mode control is adopted to replace traditional PI control for the inner loop current controller, and interval sliding mode control is adopted to replace traditional PI control for the circulating current suppression controller, thereby comprehensively improving the robustness of the MMC operation under parameter perturbations and improving the MMC control performance; in terms of communication, 5G communication technology is used to realize remote transmission of control quantities, thereby improving the flexibility of communication; a public cloud computing platform is used for dynamic modeling, optimization, diagnosis, data storage, and remote monitoring; in terms of human-computer interaction, a touch screen is selected to realize communication with the FPGA to complete the selection of the control mode and display the system status.

[0155] like Figure 1 As shown, it includes a control module, a 5G communication module, a cloud computing module, and an execution module;

[0156] The control module includes an FPGA control mainboard and a touch screen;

[0157] The FPGA control mainboard includes an FPGA core module and a USB communication module. The FPGA core module is responsible for real-time calculation of the control algorithm and for exchanging data with the touch screen, execution module, and cloud computing module. The USB communication module is connected to the FPGA core module at one end and the touch screen at the other end, responsible for communication between the FPGA control mainboard and the touch screen. The module inputs the reference values ​​of voltage, active power, and reactive power set in the touch screen into the FPGA control mainboard through the module. The touch screen module is primarily used for human-computer interaction between the user and the MMC-HVDC control system, facilitating the user's selection of the appropriate control mode and real-time understanding of the current operating status. The FPGA core module is the FPGA core processor and the data acquisition and processing center of the FPGA control mainboard. The FPGA core module performs system self-tests based on the current parameter settings to detect whether the interface is successfully matched.

[0158] The 5G communication module includes a 5G sending module and a 5G receiving module; the 5G sending module sends data through the 5G network and is responsible for three tasks: (1) sending the scattered collected data in the execution module to the FPGA control mainboard, (2) sending the control command of the FPGA control mainboard to the execution module; (3) sending the monitoring data in the FPGA mainboard to the cloud computing module;

[0159] The 5G receiving module receives data through the 5G network and is responsible for two tasks: (1) receiving the distributed collection data sent from the execution module; (2) receiving the calculation data sent from the cloud computing module. The cloud computing module has its own 5G sending module and 5G receiving module;

[0160] The cloud computing module includes a cloud computing platform module and a monitoring terminal module;

[0161] The cloud computing platform module uses a public cloud as a computing platform (such as Tencent Cloud, Alibaba Cloud, etc.), exchanges data with the FPGA control motherboard through a 5G communication module, and contains an application software module and a database module. The application software module has system model identification, controller parameter optimization, system fault diagnosis, and system monitoring functions; the database module is responsible for the distributed storage and management of system data; the monitoring terminal module uses a browser / server, that is, a B / S mode, to exchange data with the cloud computing platform through the public Internet, and realizes remote monitoring of the system by calling the system monitoring function of the application software in the cloud computing platform.

[0162] The execution module includes an analog-to-digital conversion A / D module, a digital-to-analog conversion D / A module, a current feedback module, a voltage feedback module, an MMC circuit module, a nearest level modulation module, and a submodule capacitance voltage modulation module;

[0163] The output end of the analog-to-digital conversion A / D module is connected to the FPGA core module through the 5G communication module, and the input end is respectively connected to the voltage feedback module and the current feedback module. These modules are responsible for converting the collected analog quantities such as voltage and current into digital quantities and transmitting them to the FPGA control mainboard through 5G communication; the voltage feedback module and the current feedback module are respectively composed of a voltage transformer and a current transformer, and their input ends are respectively connected to the output end of the MMC circuit module. The voltage and current on the DC side and AC side of the MMC are collected in real time through the voltage transformer and the current transformer and converted into voltage and current that are suitable for the magnitude of the analog-to-digital conversion module;

[0164] The input end of the digital-to-analog conversion D / A module is connected to the FPGA core module through the 5G communication module, and the output end is connected to the input end of the nearest level modulation module. It is responsible for converting the digital control signal received from the 5G communication module into an analog quantity and sending it to the nearest level modulation module. The nearest level modulation module calculates the real-time input quantity of the sub-modules included in the 6 bridge arms in the MMC by receiving the control signal output by the digital-to-analog conversion D / A module; the input end of the sub-module capacitance voltage modulation module is connected to the output end of the nearest level modulation module, and the output end of the sub-module capacitance voltage modulation module is connected to the input end of the MMC circuit module; by receiving the real-time input quantity of each MMC bridge arm sub-module output by the nearest level modulation module, the trigger signals of all IGBTs in the sub-modules included in the 6 bridge arms in the MMC are calculated and sent to the MMC circuit module to complete the control process of the MMC.

[0165] The MMC circuit module is as follows Figure 4 As shown, it consists of an AC side power supply, an MMC converter, a DC circuit, and a bridge arm reactor. The MMC converter consists of six upper and lower bridge arm sub-modules. By receiving the IGBT signal output by the sub-module capacitor voltage modulation module, the on-off sequence of the IGBT in each MMC sub-module is controlled, so that the MMC outputs a certain amount of inner loop current according to the control target of the outer loop controller, thereby completing the control target of the MMC corresponding to the outer loop controller.

[0166] On the other hand, an MMC-HVDC control method, such as Figure 2 As shown, it is realized by the aforementioned MMC-HVDC control system, including the following steps:

[0167] Step 1: After the MMC-HVDC system is running and has been initialized and self-tested, select the MMC control strategy and set the control parameters through the touch screen according to the operating mode of the MMC converter;

[0168] The MMC control strategy includes a DC voltage and reactive power control strategy, and an active power and reactive power control strategy, and the set values ​​of DC voltage, active power and reactive power are given according to the selected control strategy;

[0169] Step 2: If Figure 3 As shown, the DC voltage and reactive power control strategy adopts a dual-loop control of inner and outer loop types, which is implemented by designing an outer loop controller and an interval sliding mode inner loop current controller; wherein the outer loop controller includes an outer loop DC voltage controller and an outer loop reactive power controller, and the reference value of the inner loop current is calculated by the outer loop DC voltage controller and the outer loop reactive power controller;

[0170] The outer-loop DC voltage controller and outer-loop reactive power controller are: the MMC outer-loop DC voltage controller outputs an inner-loop current reference value to maintain the DC side voltage stability and AC side reactive power balance of the MMC. Its internal structure adopts a PI controller for the purpose of controlling DC voltage and reactive power, namely an outer-loop DC voltage PI controller and an outer-loop reactive power PI controller. The PI controller outputs the corresponding inner-loop current reference value, thereby stabilizing the DC system voltage and controlling the reactive power flow;

[0171] The expressions of the outer loop DC voltage PI controller and the outer loop reactive power PI controller are:

[0172]

[0173] Where k p1 、k p2 are the proportional coefficients of the outer loop DC voltage PI controller and the outer loop reactive power PI controller respectively; k I1 、k I2 are the integral coefficients of the outer loop DC voltage PI controller and the outer loop reactive power PI controller respectively; i vdref 、i vqref are the reference values ​​of the inner loop current controller, and are also the output values ​​of the outer loop DC voltage PI controller and the outer loop reactive power PI controller; U dcref 、U dc are the reference value of DC voltage and the voltage of DC circuit respectively; Q ref and Q are the reference value of reactive power and the reactive power output by the MMC, respectively.

[0174] like Figure 7 As shown, the active power and reactive power control strategy adopts a dual-loop control of inner and outer loop types, which is implemented by designing an outer loop controller and an interval sliding mode inner loop current controller; wherein the outer loop controller includes an outer loop active power controller and an outer loop reactive power controller, and the reference value of the inner loop current is calculated by the outer loop active power controller and the outer loop reactive power controller;

[0175] The outer loop active power controller and the outer loop reactive power controller are as follows: the MMC outer loop power controller outputs an inner loop current reference value that maintains the MMC output of a given active power and reactive power. Its internal structure adopts a PI controller for the purpose of controlling active power and reactive power, that is, an outer loop reactive PI controller. The PI controller outputs the corresponding inner loop current reference value, thereby realizing that the MMC outputs constant active power and reactive power, ensuring that the DC system power flow operates according to the set value.

[0176] The expressions of the outer loop active power PI controller and the outer loop reactive power PI controller are:

[0177]

[0178] Where k p3 、k p4 are the proportional coefficients of the outer loop active power PI controller and the outer loop reactive power PI controller respectively; k I3 、k I4 are the integral coefficients of the outer loop active power PI controller and the outer loop reactive power PI controller respectively; P ref and P are the reference value of active power and the active power output by the MMC, respectively.

[0179] The design methods of the interval sliding mode inner loop controller and the interval sliding mode circulating current suppression controller of the MMC active power and reactive power control strategy are the same as those of Equation (37) and Equation (55), respectively, and the calculation method of the final control quantity is the same as that of Equation (56).

[0180] Step 3: During the MMC operation, the inner loop current controller is designed using the interval sliding mode control algorithm to calculate the sliding mode control variable for the nearest level modulation;

[0181] The inner loop current controller is designed based on the mathematical model of the MMC AC and DC loops in a synchronously rotating coordinate system. Its function is to control the inner loop current, track the inner loop current reference value output by the outer loop controller, and output the reference value of the differential mode voltage. In an actual flexible DC system, the resistance and inductance values ​​of the MMC AC and DC loops are not necessarily fixed values, but will change with changes in the external environment such as temperature and humidity. Therefore, it is more reasonable to express them in the form of intervals, that is, R is the equivalent resistance of the MMC AC side and DC side (see formula (8)), L is the equivalent inductance of the MMC AC side and DC side (see formula (8)), R, are the minimum and maximum values ​​of the fluctuation of the equivalent resistance, L and are the minimum and maximum values ​​of the fluctuation of the equivalent inductance respectively; R0 is the equivalent loss of the entire bridge arm of MMC, L0 is the bridge arm reactor, R 0 、 are the minimum and maximum values ​​of the fluctuation of the equivalent resistance, L 0 and are the minimum and maximum values ​​of the fluctuation of the equivalent inductance respectively. Under the above parameter perturbation conditions, in view of the poor anti-interference ability of traditional PI control, an inner loop current controller based on interval sliding mode control with strong anti-interference ability is designed. Figure 3 As shown in Figure 3, by adopting decentralized feedback sliding mode control, a more robust controller can be obtained, which improves the output effect of the actual system.

[0182] The interval sliding mode control algorithm design method is as follows:

[0183] Step 3.1: Based on the working principle of MMC-HVDC, establish the fundamental frequency dynamic mathematical model of the AC circuit of MMC in a synchronous rotating coordinate system;

[0184] like Figure 4 As shown in FIG, the MMC converter circuit module is shown. The reference direction of the current at the output of the MMC converter circuit is from the DC side to the AC side. The neutral point of the DC side is represented by point O, and the neutral point of the AC side is represented by point O'. The resistance R0 is the equivalent loss of the entire bridge arm (the loss of each bridge arm is considered to be approximately the same), and L0 is the bridge arm reactor; R ac and L ac They are the AC output v of the converter respectively k The equivalent resistance and inductance between the equivalent potential of the AC system; the six bridge arms of the MMC are composed of N sub-modules SM; u pk and u nk are the bridge arm voltages of all submodules of the upper and lower bridge arms, respectively, where k = a, b, c; i pk and i nk are the currents flowing through the upper and lower bridge arms respectively; U sk is the AC grid voltage; U dc is the DC voltage; i vk is the three-phase current output from the valve-side converter outlet; since R0 is the sum of the losses of all submodules on a single bridge arm, the loss of the submodule is obtained through its Thevenin equivalent circuit. The time-varying Thevenin equivalent circuit of a single submodule is as follows: Figure 4 As shown. Under normal controlled conditions of the submodule, Figure 4 Each submodule SM of the MMC consists of two IGBTs T1 and T2 and two diodes D1 and D2, where T1, D1 and T2, D2 can be regarded as a variable resistor R1, R2 controlled by a switch command, respectively. Figure 5As shown; when T1 is turned on, R1 takes the minimum value; when T1 is turned off, R1 takes the maximum value; when T2 is turned on, R2 takes the minimum value; when T2 is turned off, R2 takes the maximum value, u ceq is the equivalent voltage of the submodule capacitor.

[0185] like Figure 5 As shown, for a single submodule the loss R eq The calculation using Thevenin's theorem is as follows:

[0186]

[0187] The loss of the entire bridge arm, that is, the resistance R0, is:

[0188]

[0189] Where N is the number of submodules in each bridge arm.

[0190] Depend on Figure 4 It can be seen that for phase k, according to Kirchhoff's Voltage Law (KVL) and Kirchhoff's Current Law (KCL), the KVL and KCL equations for the upper and lower bridge arms are written respectively:

[0191]

[0192]

[0193] i vk =i pk -i nk (7)

[0194] Among them, the definition is:

[0195]

[0196] Where U diffk is the differential mode voltage between the upper and lower bridge arms of the MMC's k-phase, also known as the internal virtual electromotive force; R is the equivalent resistance of the MMC's AC and DC sides; and L is the equivalent inductance of the MMC's AC and DC sides.

[0197] By adding equations (5) and (6) and combining them with the definition in equation (8), we can obtain the fundamental frequency dynamic mathematical model of the MMC AC circuit in the three-phase stationary coordinate system:

[0198]

[0199] Since the voltage and current of the MMC are both sinusoidal AC quantities during steady-state operation, this is not conducive to the design of subsequent controllers. To obtain a DC quantity that is easy to control, a coordinate transformation is performed on Equation (9), transforming the sinusoidal AC quantity in the three-phase stationary coordinate system abc into a DC quantity in the two-axis synchronous rotating coordinate system dq.

[0200] The coordinate transformation adopts the classic Park transformation, as shown in the following formula:

[0201] f dq (t) = T 3s-dq (θ)f abc (t) (10)

[0202] f abc (t) = T dq-3s (θ)f dq (t) (11)

[0203]

[0204]

[0205] Where, f dq () is the expression in dq coordinate system; f abc () is the expression in the abc coordinate system; θ is the angle used in the Park transform, which is obtained by the phase-locked loop PLL. When the PLL achieves phase-locked synchronization, θ is equal to the A-phase AC voltage u sa The phase angle of T 3s-dq (θ) is the transformation matrix from the abc three-phase stationary coordinate system to the dq rotating coordinate system, 3s refers to the three-phase stationary coordinate system; T dq-3s (θ) is the transformation matrix from the dq rotating coordinate system to the abc three-phase stationary coordinate system.

[0206] Applying the coordinate transformation of equation (12) to equation (9) yields the mathematical model of the AC circuit of MMC in the dq coordinate system:

[0207]

[0208] Among them, U sd 、U sq and i vd 、i vq are the d-axis and q-axis components of the grid voltage and current respectively; U diffd 、U diffq are the d-axis and q-axis voltage components of the differential mode voltage respectively; ω is the rated angular frequency of the power grid.

[0209] Step 3.2: Based on the working principle of MMC-HVDC, establish the mathematical model of the DC circuit of MMC in the synchronous rotating coordinate system;

[0210] The common mode voltage of the upper and lower bridge arms of the K-phase of the MMC is defined as:

[0211]

[0212] Subtracting equation (5) from equation (6) yields the DC circuit mathematical model, as shown below:

[0213]

[0214] Where i diffk is the unbalanced current of phase k inside the MMC, defined as:

[0215]

[0216] Where, I dck is the unbalanced current i diffk The DC component, i.e. the DC current flowing through the k-phase unit; cirk is the unbalanced current i diffk The AC component is the circulating current, which is generally dominated by the second harmonic.

[0217] Formula (16) is the mathematical model of the DC circuit of MMC in the abc coordinate system. Since the phase sequence of the second harmonic in the internal circulating current of MMC is negative, in order to obtain the circulating current component in the form of DC, it is necessary to transform Formula (16) from the abc three-phase stationary coordinate system to the d -2 q -2 The rotation coordinate system uses the transformation matrix T 3s-dq (-2θ).

[0218] The mathematical model (16) of the DC circuit of MMC in the abc coordinate system is d -2 q -2 The mathematical model of the DC circuit of MMC in the dq coordinate system is obtained by coordinate transformation:

[0219]

[0220] Where i diffd 、i diffq are the components of the unbalanced current inside the MMC on the d-axis and q-axis respectively; U comd 、U comq are the components of the MMC common mode voltage on the d-axis and q-axis respectively.

[0221] Step 3.3: Design an inner-loop current controller based on interval sliding-mode variable structure control, combining the mathematical model of the MMC AC circuit. This inner-loop current controller consists of an integral sliding-mode surface and an interval sliding-mode control ratio. The interval sliding-mode control ratio is derived using Lyapunov's theorem based on the mathematical model of the MMC AC circuit in a synchronously rotating coordinate system. This inner-loop current controller can accurately control the inner-loop current to track its reference value even when the MMC parameters are subject to various disturbances, thereby achieving the control objectives of the corresponding outer-loop controller and ensuring stable operation of the MMC system.

[0222] For Equation (14), it can be written as the following state equation:

[0223]

[0224] Where i vd 、i vq is the state variable; i vdref 、i vqref is the input reference value; U diffd 、U diffq is the control quantity; d1 and d2 are the grid disturbances, and |d1|≤D1, |d2|≤D2; D1 and D2 are the upper limits of the absolute value of the grid disturbance, respectively. The control target is: i vd →i vdref ,i vq →i vqref .

[0225] In order to facilitate the expression of the formula, set the parameters x1, x2, x1 * 、x2 * , u1, u2, e1, e2, their specific meanings are shown in the following formula:

[0226]

[0227] Therefore, formula (19) can be written as:

[0228]

[0229] For equation (21), interval sliding mode control is used to design the controller, and the integral sliding mode surface is used, which is in the form of:

[0230]

[0231] Where s1 and s2 are the switching functions of the two control targets in equation (19); t is time; k s1 、k s2 is the integration constant; e1 and e2 are the input deviations of x1 and x2 respectively.

[0232] Select the Lyapunov function V1 as:

[0233]

[0234] Therefore, the derivative of V1 is expressed as:

[0235]

[0236] Where v1 and v2 are the two components of the Lyapunov function V1.

[0237] for have:

[0238]

[0239] In order to facilitate the expression of the formula, set the variable η1

[0240]

[0241] get:

[0242]

[0243] Where |η1| max is the maximum absolute value of η1.

[0244] Therefore, when When satisfied Where ε1 is a positive number greater than 0, and the proof is as follows:

[0245] When s1>0, substituting u1 into formula (25) yields:

[0246]

[0247] because Therefore, Where △≥0, we know:

[0248]

[0249] When s1<0, substituting u1 into formula (25) yields:

[0250]

[0251] According to the above formula, no matter s1>0 or s1<0, All of them are established, satisfying the arrival condition and stability condition of sliding mode variable structure control, that is, when there is uncertainty in system parameters, the sliding mode controller achieves a stable desired output.

[0252] for Know:

[0253]

[0254] In order to facilitate the expression of the formula, set the variable η2

[0255]

[0256] get:

[0257]

[0258] Where |η2| max is the maximum absolute value of η2.

[0259] Therefore, when When satisfied Where ε2 is a positive number greater than 0, and the proof is as follows:

[0260] When s2>0, substituting u2 into formula (28) yields:

[0261]

[0262] When s2<0, substituting u2 into formula (28) yields:

[0263]

[0264] According to the above formula, no matter s2>0 or s2<0, All of them are established, satisfying the arrival condition and stability condition of sliding mode variable structure control, that is, when there is uncertainty in system parameters, the sliding mode controller can achieve stable desired output.

[0265] Therefore, the expression of the inner loop current controller is obtained as:

[0266]

[0267] In order to obtain a more stable sliding mode, the saturation function sat(s) is used to replace the sign function sgn(s), and then Equation (32) is the final inner loop current controller expression as shown below:

[0268]

[0269] Step 3.4: Design a circulating current suppression controller based on interval sliding mode variable structure control in combination with the mathematical model of the MMC DC loop: The circulating current suppression controller based on interval sliding mode variable structure control consists of an integral sliding mode surface and an interval sliding mode control rate. The interval sliding mode control rate is derived through Lyapunov's theorem based on the mathematical model of the DC loop of the MMC in the synchronous rotating coordinate system. Through this circulating current suppression controller, the second-harmonic frequency component of the inner loop circulating current of the MMC can be accurately controlled to remain at 0 even when the MMC parameters are subject to various disturbances, thereby maintaining the safe and stable operation of the MMC-HVDC system under parameter perturbations.

[0270] like Figure 6 The figure shows an interval sliding mode circulating current suppression controller, which is used to suppress the double frequency component of the internal circulating current of the MMC to zero and output a common-mode voltage reference value. When the resistance and inductance parameters of the MMC are disturbed, the traditional PI-controlled circulating current suppression controller cannot accurately suppress the double frequency component of the internal circulating current to zero. Therefore, the present invention designs a new MMC circulating current suppression controller based on interval sliding mode control. The specific design method is as follows:

[0271] For Equation (16), rewrite it into the following state equation:

[0272]

[0273] Where i diffd 、i diffq is the state variable; U comd 、U comq is the controlled quantity; the control target is: i diffd →i diffdref 、i diffq →i diffqref .

[0274] In order to facilitate the expression of the formula, set the parameters x3, x4, x3 * 、x4 * , u3, u4, e3, e4, their specific meanings are shown in the following formula:

[0275]

[0276] Where i diffdref 、i diffqref i diffd 、i diffq reference value.

[0277] Therefore, formula (33) can be written as:

[0278]

[0279] For equation (35), interval sliding mode control is used to design the controller, and the integral sliding mode surface is used, which is in the form of:

[0280]

[0281] Where s3 and s4 are the switching functions of the two control objectives in equation (33); k s3 、k s4 is the integration constant; e3 and e4 are the input deviations of x3 and x4 respectively.

[0282] Select the Lyapunov function V2 as:

[0283]

[0284] Therefore, the derivative of V2 is expressed as:

[0285]

[0286] Where v3 and v4 are the two components of the Lyapunov function V2.

[0287] for Know:

[0288]

[0289] In order to facilitate the expression of the formula, set the variable η3

[0290]

[0291] Know

[0292]

[0293] Where |η3| max is the maximum absolute value of η3.

[0294] Therefore, when When satisfied Where ε3 is a positive number greater than 0, and the proof is as follows:

[0295] When s3>0, substituting u3 into formula (39) yields:

[0296]

[0297] When s3<0, substituting u3 into formula (39) yields:

[0298]

[0299] According to the above formula, no matter s3>0 or s3<0, All of them are established, satisfying the arrival condition and stability condition of sliding mode variable structure control, that is, when there is uncertainty in system parameters, the sliding mode controller can achieve stable desired output.

[0300] for Know:

[0301]

[0302] In order to facilitate the expression of the formula, set the variable η4

[0303]

[0304] Easy to know:

[0305]

[0306] Where |η4| max is the maximum absolute value of η4.

[0307] Therefore, when When satisfied Where ε4 is a positive number greater than 0, and the proof is as follows:

[0308] When s4>0, substituting u4 into formula (42) yields:

[0309]

[0310] When s4<0, substituting u4 into formula (42) yields:

[0311]

[0312] According to the above formula, no matter s4>0 or s4<0, All of them are established, satisfying the arrival condition and stability condition of sliding mode variable structure control, that is, when there is uncertainty in system parameters, the sliding mode controller can achieve stable desired output.

[0313] Therefore, the expression of the circulating current suppression controller is obtained as follows:

[0314]

[0315] In order to obtain a more stable sliding mode, realize a “quasi-sliding mode”, and reduce chattering, the saturation function sat(s) is used to replace the sign function sgn(s). The final expression of the circulating current suppression controller is:

[0316]

[0317] Step 3.5: Based on the invented inner-loop current controller based on interval sliding mode variable structure control and the control variables output by the inner-loop current controller based on interval sliding mode variable structure control, the reference voltages of the upper and lower bridge arms of the MMC three-phase are finally obtained, and the trigger signals of the IGBTs in each MMC submodule are obtained using the nearest level modulation strategy;

[0318] The inner loop current controller (32) obtains u1 and u2, which are then transformed inversely by dq using equation (13) to obtain the reference value U of the MMC differential mode voltage. diffk_ref Similarly, u3 and u4 obtained by the circulating current suppression controller (46) are inversely transformed by formula (13), and θ is replaced by -2θ to obtain the reference value U of the MMC common mode voltage. comk_ref Finally, the voltage command value U of the upper and lower bridge arms of the three-phase MMC is calculated. pk_ref 、U nk_ref The final control quantity output by the FPGA control mainboard is:

[0319]

[0320] The final control quantity output by the FPGA control mainboard is input into the nearest level modulation module and the submodule capacitance voltage modulation module, and then the trigger signal of the IGBT in each submodule of the MMC is calculated to complete the control of the MMC;

[0321] Step 4: Transmit all MMC-related data in the controller, including differential-mode voltage, common-mode voltage, AC side current, AC side voltage, DC voltage, inner loop current reference value, DC voltage reference value, active power, reactive power, active power reference value, reactive power reference value, inner circulating current, PI controller parameters, interval sliding mode controller parameters, and MMC circuit parameters, to the cloud computing platform to display the operating status of the MMC, continuously control the MMC, and complete the tasks of system modeling, optimization, diagnosis, data storage, and remote monitoring, thereby optimizing the entire MMC-HVDC control.

[0322] The control method based on the above-mentioned MMC interval sliding mode control system effectively combines the outer loop controller based on the conventional PI control algorithm, the inner loop controller based on the interval sliding mode variable structure control algorithm, and the circulation suppression controller based on the interval sliding mode variable structure control algorithm, and applies it to the advanced control method of the MMC interval sliding mode control system to achieve precise control of the MMC under parameter perturbation conditions.

[0323] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or make equivalent replacements for some or all of the technical features therein. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope defined by the claims of the present invention.

Claims

1. A MMC-HVDC control method, characterized in that: The following steps are involved: Step 1: After the MMC-HVDC system is running and has been initialized and self-tested, select the MMC control strategy and set the control parameters through the touch screen according to the operating mode of the MMC converter; The MMC control mode includes DC voltage and reactive power control strategy, as well as active power and reactive power control strategy, and the set values ​​of DC voltage, active power and reactive power are given according to the selected control strategy; Step 2: The DC voltage and reactive power control strategy adopts a dual-loop control of inner and outer loops, which is achieved by designing an outer loop controller and an interval sliding mode inner loop current controller; The outer loop controller includes an outer loop DC voltage controller and an outer loop reactive power controller, and the reference value of the inner loop current is calculated by the outer loop DC voltage controller and the outer loop reactive power controller; The active power and reactive power control strategy adopts a dual-loop control of inner and outer loop types, which is achieved by designing an outer loop controller and an interval sliding mode inner loop current controller; The outer loop controller includes an outer loop active power controller and an outer loop reactive power controller, which output inner loop current reference values ​​for given active power and reactive power. PI controllers are used for controlling active power and reactive power, respectively, namely, an outer loop active power PI controller and an outer loop reactive power PI controller. The PI controllers output corresponding inner loop current reference values, thereby stabilizing the DC system voltage and controlling the reactive power flow. Step 3: During the MMC operation, the inner loop current controller is designed using the interval sliding mode control algorithm to calculate the sliding mode control variable for the nearest level modulation; The step 3 specifically includes the following steps: Step 3.1: Based on the working principle of MMC-HVDC, establish the fundamental frequency dynamic mathematical model of the AC circuit of MMC in a synchronous rotating coordinate system; Step 3.2: Based on the working principle of MMC-HVDC, establish the mathematical model of the DC circuit of MMC in the synchronous rotating coordinate system; Step 3.3: Design an inner loop current controller based on interval sliding mode variable structure control based on the mathematical model of the MMC AC circuit; Establish the state equation: （19） Where i vd 、i vq is the state variable; i vdref 、i vqref is the input reference value; U diffd 、U diffq is the control variable; d1 and d2 are the grid disturbances, and , ; D1 and D2 are the upper limits of the absolute value of the power grid disturbance, and the control target is: i vd →i vdref ,i vq →i vqref ; R is the equivalent resistance of the MMC AC side and DC side; L is the equivalent inductance of the MMC AC side and DC side, U sd 、U sq and i vd 、i vq are the d-axis and q-axis components of the grid voltage and current respectively; ω is the rated angular frequency of the grid; In order to facilitate the expression of the formula, set the parameters x1, x2, x1 * 、x2 * , u1, u2, e1, e2, their specific meanings are shown in the following formula: （20） Therefore, formula (19) can be written as: （21） For Equation (21), interval sliding mode control is used to design the controller, and the integral sliding mode surface is used, which is in the form of: （22） Where s1 and s2 are the switching functions of the two control targets in equation (19); t is time; k s1 、k s2 is the integration constant; e1 and e2 are the input deviations of x1 and x2 respectively; Select the Lyapunov function V1 as: （23） Therefore, the derivative of V1 is expressed as: （24） Where v1 and v2 are the two components of the Lyapunov function V1; for have: （25） In order to facilitate the expression of the formula, set the variable η1 （26） get: （27） Where |η1| max is the maximum absolute value of η1; Therefore, when When satisfied ,in is a positive number greater than 0; for Know: （28） In order to facilitate the expression of the formula, set the variable η2 （29） get: （30） Where |η2| max is the maximum absolute value of η2; Therefore, when When satisfied ,in is a positive number greater than 0; Therefore, the expression of the inner loop current controller is obtained as: （31） In order to obtain a more stable sliding mode, the saturation function sat(s) is used to replace the sign function sgn(s), and then Equation (32) is the final inner loop current controller expression as shown below: （32） Step 3.4: Design a circulating current suppression controller based on interval sliding mode variable structure control in combination with the mathematical model of the MMC DC circuit; Establish the state equation: （33） Where i diffd 、i diffq is the state variable; i diffdref 、i diffqref is the input reference value; U comd 、U comq is the controlled quantity; the control target is: i diffd →i diffdref 、i diffq →i diffqref ; R0 is the equivalent loss of the entire bridge arm of MMC, L0 is the bridge arm reactor; In order to facilitate the expression of the formula, set the parameters x3, x4, x3 * 、x4 * , u3, u4, e3, e4, their specific meanings are shown in the following formula: （34） Therefore, formula (33) can be written as: （35） For Equation (35), interval sliding mode control is used to design the controller, and the integral sliding mode surface is used, which is in the form of: （36） Where s3 and s4 are the switching functions of the two control objectives in equation (33); k s3 、k s4 is the integration constant; e3 and e4 are the input deviations of x3 and x4 respectively; Select the Lyapunov function V2 as: （37） Therefore, the derivative of V2 is expressed as: （38） Where v3 and v4 are the two components of the Lyapunov function V2; for Know: （39） In order to facilitate the expression of the formula, set the variable η3 （40） Easy to know: （41） Where |η3| max is the maximum absolute value of η3; Therefore, when When satisfied ,in is a positive number greater than 0; for Know: （42） In order to facilitate the expression of the formula, set the variable η4 （43） Know: （44） Where |η4| max is the maximum absolute value of η4; Therefore, when When satisfied ,in is a positive number greater than 0; Therefore, the expression of the circulating current suppression controller is obtained as follows: （45） The saturation function sat(s) is used to replace the sign function sgn(s), and the expression of the final circulating current suppression controller is: （46） Step 3.5: Output the final control variable according to the inner loop current controller based on interval sliding mode variable structure control and the circulating current suppression controller based on interval sliding mode variable structure control, that is, the MMC three-phase upper and lower bridge arm voltage command values ​​U pk_ref 、U nk_ref The value of The final control quantity output is: （47） Among them U diffk_ref is the reference value of MMC differential mode voltage, U comk_ref As the reference value of the common mode voltage of the MMC, the final control quantity output by the FPGA control mainboard is input into the nearest level modulation module and the submodule capacitor voltage modulation module, and then the trigger signal of the IGBT in each submodule of the MMC is calculated to complete the control of the MMC; Step 4: All MMC-related data in the controller are transmitted to the cloud computing platform, the operating status of the MMC is displayed, and the MMC is continuously controlled, thereby completing the entire MMC-HVDC control.

2. The MMC-HVDC control method according to claim 1, characterized in that: The outer loop DC voltage and reactive power controller in step 2 is: the MMC outer loop DC voltage controller outputs an inner loop current reference value to maintain the DC side voltage stability and AC side reactive power balance of the MMC. Its internal structure adopts a PI controller for the purpose of controlling DC voltage and reactive power, namely an outer loop DC voltage PI controller and an outer loop reactive power rate PI controller. The PI controller outputs the corresponding inner loop current reference value, thereby stabilizing the DC system voltage and controlling the reactive power flow. The expressions of the outer loop DC voltage PI controller and the outer loop reactive power rate PI controller in the DC voltage and reactive power control strategy are: （1） Where k p1 、k p2 are the proportional coefficients of the outer loop voltage PI controller and the outer loop reactive PI controller respectively; k I1 、k I2 are the integral coefficients of the outer loop voltage PI controller and the outer loop reactive PI controller respectively; i vdref 、i vqref are the reference values ​​of the inner loop current controller, and are also the output values ​​of the outer loop DC voltage PI controller and the outer loop reactive power PI controller; U dcref 、U dc are the reference value of DC voltage and the voltage of DC circuit respectively; Q ref and Q are the reference value of reactive power and the reactive power output by the MMC, respectively.

3. The MMC-HVDC control method according to claim 1, characterized in that: The outer loop active power and reactive power controller described in step 2 is as follows: the MMC outer loop power controller outputs an inner loop current reference value that maintains the MMC output of a given active power and reactive power. Its internal structure adopts a PI controller for the purpose of controlling active power and reactive power, that is, an outer loop reactive PI controller. The PI controller outputs the corresponding inner loop current reference value, thereby achieving constant active power and reactive power output by the MMC, ensuring that the DC system power flow operates according to the set value; The expressions of the outer loop active power PI controller and the outer loop reactive power PI controller in the active power and reactive power control strategy are: （2） Where k p3 、k p4 are the proportional coefficients of the outer loop active power PI controller and the outer loop reactive power PI controller respectively; k I3 、k I4 are the integral coefficients of the outer loop active power PI controller and the outer loop reactive power PI controller respectively; P ref and P are the reference value of active power and the active power output by the MMC, respectively.

4. The MMC-HVDC control method according to claim 1, characterized in that: The relevant data described in step 4 include differential mode voltage, common mode voltage, AC side current, AC side voltage, DC voltage, inner loop current reference value, DC voltage reference value, active power, reactive power, active power reference value, reactive power reference value, inner circulating current, PI controller parameters, interval sliding mode controller parameters, and MMC circuit parameters.

5. An MMC-HVDC control system, used to implement the control method according to claim 1, characterized in that: Including control module, 5G communication module, cloud computing module, and execution module; The control module includes an FPGA control mainboard and a touch screen; the FPGA control mainboard includes an FPGA core module and a USB communication module; one end of the USB communication module is connected to the FPGA core module, and the other end is connected to the touch screen; The cloud computing module includes a cloud computing platform module and a monitoring terminal module. The cloud computing platform module uses the public cloud as a computing platform and exchanges data with the FPGA control motherboard via the 5G communication module. It contains an application software module and a database module. The application software module has system model identification, controller parameter optimization, system fault diagnosis, and system monitoring functions. The database module is responsible for the distributed storage and management of system data. The monitoring terminal module uses a browser / server (B / S) model to exchange data with the cloud computing platform via the public Internet and realizes remote monitoring of the system by calling the system monitoring function of the application software in the cloud computing platform. The execution module includes an analog-to-digital conversion A / D module, a digital-to-analog conversion D / A module, a current feedback module, a voltage feedback module, an MMC circuit module, a nearest level modulation module, and a sub-module capacitor voltage modulation module; the output end of the analog-to-digital conversion A / D module is connected to the FPGA core module through the 5G communication module, and the input end is respectively connected to the voltage feedback module and the current feedback module, and the voltage feedback module and the current feedback module are respectively composed of a voltage transformer and a current transformer, and their input ends are respectively connected to the output end of the MMC circuit module; the input end of the digital-to-analog conversion D / A module is connected to the FPGA core module through the 5G communication module, and the output end is connected to the input end of the nearest level modulation module, the input end of the sub-module capacitor voltage modulation module is connected to the output end of the nearest level modulation module, and the output end of the sub-module capacitor voltage modulation module is connected to the input end of the MMC circuit module.

6. The MMC-HVDC control system according to claim 5, characterized in that: The 5G communication module includes a 5G sending module and a 5G receiving module; the 5G sending module and the 5G receiving module send and receive information through the 5G network.

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