Modular multilevel converter full-band four-quadrant admittance correction method and system

Abstract

This invention discloses a method and system for full-band four-quadrant admittance correction of modular multilevel converters (MMCs). The method includes: modeling the inner-loop control process of the MMC to obtain the equivalent admittance-frequency characteristic relationship of the MMC; establishing a full-band admittance correction criterion for stable operation of the power grid system after the MMC is connected to the grid; and performing full-band four-quadrant admittance correction on the MMC using a virtual damping correction method, based on the equivalent admittance-frequency characteristic relationship and with the goal of satisfying the full-band admittance correction criterion. This invention combines MMC current inner-loop control with virtual admittance correction, and achieves full-band positive damping characteristics of the MMC based on the full-band admittance correction criterion that meets grid connection requirements, enabling the MMC to be safely and stably connected to the AC power grid.

Description

Technical Field

[0001] This invention belongs to the field of MMC admittance correction technology, and relates to a method and system for full-band four-quadrant admittance correction of modular multilevel converters. Background Technology

[0002] A Modular Multilevel Converter (MMC) is composed of multiple cascaded sub-modules (SMs) with identical structures. The sub-modules can be classified into three types: half-H-bridge, full-H-bridge, and double-clamped. MMCs have extremely important engineering application prospects and are fundamental equipment for building DC distribution networks.

[0003] With the development of new power distribution systems, a power distribution topology with high integration of new energy sources and a high proportion of power electronics is an important characteristic of future urban power distribution systems. While MMC (Multi-Converter Modulator) is used in traditional AC generator projects, offering fast response, flexible control, and rich functionality, its digital control method leads to nonlinear impedance characteristics in multiple converters, which can cause broadband oscillations during network operation.

[0004] Many scholars have conducted research on improving the MMC control process to enhance MMC stability and have proposed numerous methods for feedback control of various electrical quantities of the MMC. However, due to the insufficient understanding of the resonance mechanism of the MMC grid-connected system, these methods generally take the zero-pole distribution of the grid-connected system and the Nyquist stability criterion as control targets, and are aimed at some combination of characteristics of the power grid and the MMC. The conclusions or parameters obtained are not universally applicable. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method and system for full-band four-quadrant admittance correction of modular multilevel converters (MMCs). It performs full-band four-quadrant AC admittance correction for MMCs used in distribution networks. Through current inner-loop control and admittance correction criteria, it achieves full-band positive damping characteristics of the MMC. This method can serve as the basis for grid-connected operation of multiple AC converters and support multi-terminal grid operation in all scenarios.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for full-band four-quadrant admittance correction of a modular multilevel converter, the method comprising the following steps:

[0008] Step 1: Model the inner loop control process of the modular multilevel converter (MMC) to obtain the equivalent admittance-frequency characteristic relationship of the MMC;

[0009] Step 2: Establish a full-band admittance correction criterion for stable operation of the power grid system after MMC grid connection;

[0010] Step 3: Using a virtual damping correction method, based on the equivalent admittance-frequency characteristic relationship, and with the goal of satisfying the full-band admittance correction criterion, perform full-band four-quadrant admittance correction on the MMC.

[0011] The present invention further includes the following preferred embodiments:

[0012] Preferably, in step 1, the inner-loop control process of the MMC is modeled to obtain...

[0013] i = G r0 ·i r +Y0·e=i inv +Y0·e;

[0014] Where e and i are the voltage and output current of the MMC equivalent two-port network, respectively;

[0015] i inv It is an equivalent current source;

[0016] G r0 The command current i obtained from the outer loop control r The transfer function;

[0017] Y0 is the MMC equivalent admittance.

[0018] Preferably, in step 1, by substituting s = jω = j2πf into Y0, the real part G and the imaginary part B of the admittance at frequency f can be obtained. Then, at frequency f, the MMC equivalent admittance can be expressed as Y0 = G + jB.

[0019] Where s is the Laplace operator, and ω and f are the angular frequency and frequency, respectively.

[0020] Preferably, the frequency f ranges across the entire frequency band, from the fundamental frequency to the Nyquist frequency.

[0021] Preferably, the full-band admittance correction criterion for stable operation of the power grid system after MMC grid connection established in step 2 is as follows:

[0022] The MMC equivalent admittance-frequency characteristic satisfies:

[0023] There is no negative damping characteristic across the entire frequency band, meaning that the MMC conductance is positive across the entire frequency band.

[0024] Preferably, the virtual damping correction method described in step 3 refers to assuming that a virtual series impedance Z exists between the physical connection point of the MMC and the power grid. v and virtual parallel admittance Y v The output current i of the MMC controller flows into the power grid through a virtual impedance network, and the grid voltage is applied to the MMC controller through the virtual impedance network. The virtual series impedance Z v and virtual parallel admittance Yv A virtual impedance network is formed, and the virtual impedance network, together with the equivalent admittance of the MMC itself, constitutes the MMC admittance after virtual damping correction.

[0025] Preferably, in step 3, 1) let Z in the virtual impedance network v =R v +L v s, Y v =G v +C v If s, then by applying the voltage e and output current i of the MMC two-port network to the virtual impedance network, we can obtain i v =i+e·Y v e v =i·Z v +e·Z v ·Y v +e;

[0026] Among them, R v L is the series resistance correction factor. v G is the series reactance correction factor. v C is the parallel conductance correction factor. v This is the parallel susceptance correction factor;

[0027] e v i v The two-port voltage and output current of the MMC inner loop control after virtual damping correction;

[0028] 2) i is obtained based on the inner loop control. v =G r0 ·i r +Y0·e v ;

[0029] Among them, G r0 The command current i obtained from the outer loop control r The transfer function is Y0, which is the MMC equivalent admittance, Y0 = G + jB;

[0030] 3) Change i v =i+e·Y v e v =i·Z v +e·Z v ·Y v +e brings in i v =G r0 ·i r +Y0·e v Combining the equivalent admittance-frequency characteristic relationship Y 0v =G vinv +jB vinv The derivation yields

[0031] Among them, G r0v Y 0v i inv These are the command currents i after virtual damping correction. r The transfer function, MMC equivalent admittance, and equivalent current source;

[0032] G vinv B vinv The equivalent admittance Y of MMC after virtual damping correction 0v The real and imaginary parts of the admittance at frequency f.

[0033] Preferably, in step 3, Z is adjusted based on equation (1). v and Y v The equivalent admittance of MMC can be corrected, and the correction results are as follows:

[0034] 1) When only Z is increased v Y v When = 0, the MMC equivalent admittance is determined by Y. 0v Become

[0035] 2) When only Y is increased v Z v When = 0, the MMC equivalent admittance is determined by Y. 0v Become

[0036] 3) When both corrections are used, i.e., Z is increased v and Y v At that time, the MMC equivalent admittance is determined by Y. 0v It becomes:

[0037]

[0038] Where Z1 is the corrected MMC equivalent impedance.

[0039] Preferably, in step 3, based on formula Adjust Z v =R v +L v s, Y v =G v +C v The equivalent admittance of MMC is corrected by s. Based on equation (1) and the correction result, the dynamic equivalent admittance-frequency characteristic relationship curve is obtained. When the equivalent admittance-frequency characteristic relationship curve satisfies the full-band admittance correction criterion, that is, G vinv When the entire frequency band is positive, the full-band four-quadrant equivalent admittance correction of the MMC is completed.

[0040] This invention also provides a modular multilevel converter full-band four-quadrant admittance correction system, comprising:

[0041] The MMC inner loop control modeling module is used to model the MMC inner loop control process of the modular multilevel converter to obtain the MMC equivalent admittance-frequency characteristic relationship.

[0042] The full-band admittance correction criterion construction module is used to establish a full-band admittance correction criterion for the stable operation of the power grid system after MMC is connected to the grid;

[0043] The full-band four-quadrant admittance correction module is used to perform full-band four-quadrant admittance correction on MMC by adopting a virtual damping correction method and based on the equivalent admittance-frequency characteristic relationship, with the goal of satisfying the full-band admittance correction criterion.

[0044] The beneficial effects of this invention are compared with those of the prior art:

[0045] This invention combines MMC current inner loop control with virtual damping correction, and achieves full-band positive damping characteristics of MMC based on the full-band admittance correction criterion that meets grid connection requirements, enabling MMC to be safely and stably connected to the AC power grid.

[0046] This invention studies a virtual damping correction method for MMC. By introducing sampling of the grid-connected voltage and current of MMC, the equivalent impedance of MMC is corrected. Without adding new sampling channels or hardware, the impedance characteristics of MMC are corrected and the damping of MMC is made positive by using the four quadrants of series resistance, series reactance, parallel conductance and parallel susceptance in the virtual impedance network. Attached Figure Description

[0047] Figure 1 This is a flowchart of the method of the present invention;

[0048] Figure 2 This is a block diagram of the MMC inner loop control.

[0049] Figure 3 This is a schematic diagram of the MMC inner loop control.

[0050] Figure 4 This is the equivalent two-port network for MMC;

[0051] Figure 5 This is a schematic diagram of the full-band four-quadrant admittance correction principle. Detailed Implementation

[0052] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. The embodiments described in this application are merely some embodiments of this invention, and not all embodiments. Based on the spirit of this invention, other embodiments obtained by those skilled in the art without creative effort are all within the protection scope of this invention.

[0053] like Figure 1 As shown, Embodiment 1 of the present invention provides a full-band four-quadrant admittance correction method for modular multilevel converters. In a preferred but non-limiting embodiment of the present invention, the method includes the following steps 1-3:

[0054] like Figure 1 As shown, Embodiment 1 of the present invention provides a full-band four-quadrant admittance correction method for modular multilevel converters. In a preferred but non-limiting embodiment of the present invention, the method includes the following steps 1-3:

[0055] Step 1: Model the inner loop control process of the modular multilevel converter (MMC) to obtain the equivalent admittance-frequency characteristic relationship of the MMC;

[0056] More preferably, the MMC employs a deadbeat current inner loop control method, such as... Figure 2 As shown, i r The command current is obtained from the outer loop control, e is the PCC point voltage, i is the grid-connected current, L is the MMC connection inductance, and T is the grid connection current. S To control the cycle.

[0057] In the open-loop path, D is the inner-loop transfer function, representing the gain of the current loop control, and G is the system model, representing the current generation process connected to the reactor. sam G represents the sampling process. delay This represents the delay process of the control cycle, and the delay time of the control cycle is: G delay =e -s·Ts The sampling time for the sampling process is:

[0058]

[0059] Where s is the Laplace operator;

[0060] The meanings of the outer loop, inner loop, open loop, and current loop mentioned above are briefly explained below:

[0061] Outer loop: the process of obtaining the command current; inner loop: the process of realizing the command current; open loop: an automatic control term; current loop: current-based inner loop control.

[0062] Will Figure 2 It can be simplified to Figure 3 Therefore, we can obtain i = G r0 ·i r +Y0·e=i inv +Y0·e

[0063] Among them, G r0 =G i0 G D0 G L0 / (1+G i0 G D0 G L0 G H0 ), where i is the command current. r The transfer function (hereinafter referred to as the current transfer function), Y0=(G e0 G D0 -1)G L0 / (1+G i0 G D0 G L0 G H0 ), which is the transfer function of the voltage e at the connection point (hereinafter referred to as the voltage transfer function).

[0064] In the formula G i0 G D0 G L0 G H0 G e0 Represent Figure 3 Transfer functions for each component in the block diagram.

[0065]

[0066] G D0 =e -sTs

[0067]

[0068] G H0 =e -0.5sTs

[0069] G e0 =e -0.5sTs

[0070] k dead This is the current control coefficient.

[0071] Therefore, the MMC output current i is generated by the shunting of the MMC equivalent current source across the MMC impedance and the power system impedance, and by the current generated by the power system voltage acting on the series circuit of the MMC impedance and the power system impedance. The transfer functions of the two control processes are G and G, respectively. r0 And Y0. Therefore, MMC can be further equivalently represented as a two-port network, such as Figure 4As shown, grid-connected MMC can be equivalent to current source i inv (G r0 ·i r The Norton equivalent form, which is connected in parallel with the equivalent admittance Y0=G+jB=1 / Z0, transforms the control process of the grid-connected MMC into a circuit problem.

[0072] Substituting s = jω = j2πf into Y0, we can obtain the real part G and the imaginary part B of the admittance at frequency f. Then, at frequency f, the MMC equivalent admittance can be expressed as Y0 = G + jB.

[0073] Let s = jω, ω = 2πf, and substitute i = G. r0 ·i r +Y0·e=i inv +Y0·e=i inv +(G+jB)·e yields the numerical model of the equivalent admittance, which in turn yields the accurate equivalent admittance-frequency characteristic curve of the MMC, where ω and f are the angular frequency and frequency, respectively, and the voltage and output current of the MMC equivalent two-port network are e and i, respectively, which are the voltage and current at the MMC grid connection point.

[0074] Where s is the Laplace operator, and ω and f are the angular frequency and frequency, respectively.

[0075] The frequency f ranges across the entire frequency band, from the fundamental frequency to the Nyquist frequency.

[0076] Step 2: Establish a full-band admittance correction criterion for stable operation of the power grid system after MMC grid connection;

[0077] The negative damping characteristic of the MMC is the fundamental reason why grid-connected MMC systems are prone to resonance. Considering the parallel network case, since the total susceptance of the grid-connected system is negative, its self-excited oscillation frequency depends on the zero-crossing point of the total susceptance curve. Parallel inductive and capacitive loads and other inverters in the power system can shift the total susceptance curve left and right. When the total admittance characteristic of the system meets the grid resonance condition, parallel resonance will occur. If it is required that the MMC does not self-excited oscillation after being connected to the grid, the MMC's accurate equivalent admittance-frequency characteristic should be consistently positive across the entire frequency band.

[0078] Therefore, taking admittance characteristics as an example, in order to avoid grid-connected system resonance to the greatest extent, the equivalent admittance-frequency characteristics of grid-connected MMC operating in grid-connected systems should meet the following criteria:

[0079] MMC does not exhibit negative damping characteristics across the entire frequency band, meaning the MMC conductance is positive across the entire frequency band, i.e., i.e., i.e. inv In +(G+jB)·e, G takes a positive value across the entire frequency band. If the susceptance curve is approximately zero at the same time, the MMC equivalent impedance has a purely resistive characteristic, which is the most ideal MMC admittance characteristic.

[0080] Step 3: Using a virtual damping correction method, based on the equivalent admittance-frequency characteristic relationship, and with the full-band admittance correction criterion as the target, perform full-band four-quadrant equivalent admittance correction on the MMC.

[0081] This invention studies a virtual damping correction method for MMC. By introducing the sampling of the grid connection point voltage e and current i of MMC, the equivalent impedance of MMC is corrected. Without adding new sampling channels or hardware, the impedance characteristics of MMC are corrected and the damping of MMC is made positive.

[0082] The damping correction algorithm proposed in this invention is as follows: Figure 4 As shown, e and i represent the voltage and output current of the MMC equivalent two-port network, which are the actual electrical quantities of the system. v i v To adjust the two-port voltage and output current of the MMC inner loop control after adding damping correction, Z v For series damping correction, Y v For parallel admittance correction, its expression is shown in the following formula:

[0083] Z v =R v +L v s

[0084] Y v =G v +C v s

[0085] Among them, R v L is the series resistance correction factor. v G is the series reactance correction factor. v C is the parallel conductance correction factor. v This is the parallel susceptance correction factor;

[0086] Z v Y v This is the setting value used for admittance correction.

[0087] As can be seen, the virtual damping correction method proposed in this invention is essentially feedback control of the voltage and current sampling at the MMC grid connection point. However, this invention gives the feedback control a certain physical meaning, namely, it assumes that there is a virtual series impedance Z between the physical connection point of the MMC and the power grid. v and virtual parallel admittance Y v The output current of the MMC controller flows into the power grid through the virtual impedance network, and the system voltage is also applied to the MMC controller through the virtual impedance network. The virtual impedance network and the equivalent admittance of the MMC itself together constitute the damped corrected MMC admittance.

[0088] Depend on Figure 5 It can be seen that, let the virtual impedance be Z v =R v +L v s, Y v =G v +C v s, i is obtained by applying the voltage e and output current i of the MMC two-port network to a virtual impedance. v =i+e·Y v e v =i·Z v +e·Z v ·Y v +e;

[0089] Depend on Figure 5 It can be seen that the inner ring remains unchanged, i. v =G r0 ·i r +Y0·e v ;

[0090] will i v =i+e·Y v e v =i·Z v +e·Z v ·Y v +e brings in i v =G r0 ·i r +Y0·e v The derivation yields

[0091]

[0092] Among them, G r0v Y 0v i inv These are the command currents i after virtual damping correction. r The transfer function, MMC equivalent admittance, and equivalent current source;

[0093] G vinv B vinv The equivalent admittance Y of MMC after virtual damping correction 0v The real and imaginary parts of the admittance at frequency f.

[0094] e and i represent the voltage and output current of the MMC equivalent two-port network, which are the actual electrical quantities of the system.

[0095] The above is the principle of the damping correction algorithm proposed in this invention. It can be seen that Z... v and Y v The MMC impedance and admittance were corrected separately, and the correction results are as follows:

[0096] 1) When only Z is increased v Y v When = 0, the MMC equivalent admittance is determined by Y. 0v Become

[0097] 2) When only Y is increased v Z v When = 0, the MMC equivalent admittance is determined by Y. 0v Become

[0098] 3) When both corrections are used, i.e., Z is increased v and Y v At that time, the MMC equivalent admittance is determined by Y. 0v It becomes:

[0099]

[0100] Where Z1 is the corrected MMC equivalent impedance.

[0101] That is, adjust Z according to equation (1). v =R v +L v s, Y v =G v +C v s, then the MMC equivalent admittance correction is The change in this value yields the dynamic equivalent admittance-frequency response curve, i.e., G at frequency f. vinv +jB vinv When the equivalent admittance-frequency characteristic curve satisfies the full-band admittance correction criterion, i.e., G vinv When the entire frequency band is positive, the full-band four-quadrant equivalent admittance correction of the MMC is completed.

[0102] As can be seen, theoretically, the addition of the damping correction stage can correct the equivalent impedance or admittance of the MMC according to the design requirements. Based on the equivalent admittance-frequency characteristic curve, the MMC can be connected to the power grid after meeting the grid access criteria proposed in this invention (the MMC does not have negative damping characteristics in the entire frequency band, that is, the MMC conductance takes a positive value in the entire frequency band).

[0103] This invention also provides a modular multilevel converter full-band four-quadrant admittance correction system, the system comprising:

[0104] The MMC inner loop control modeling module is used to model the MMC inner loop control process of the modular multilevel converter to obtain the MMC equivalent admittance-frequency characteristic relationship.

[0105] The full-band admittance correction criterion construction module is used to establish a full-band admittance correction criterion for the stable operation of the power grid system after MMC is connected to the grid;

[0106] The full-band four-quadrant admittance correction module is used to perform full-band four-quadrant admittance correction on MMC by adopting a virtual damping correction method and based on the equivalent admittance-frequency characteristic relationship, with the goal of satisfying the full-band admittance correction criterion.

[0107] The beneficial effects of this invention are compared with those of the prior art:

[0108] This invention combines MMC current inner loop control with virtual admittance correction, and achieves full-band positive damping characteristics of MMC based on the full-band admittance correction criterion that meets grid connection requirements, enabling MMC to be safely and stably connected to the AC power grid.

[0109] This disclosure can be a system, method, and / or computer program product. A computer program product may include a computer-readable storage medium having computer-readable program instructions loaded thereon for causing a processor to implement various aspects of this disclosure.

[0110] Computer-readable storage media can be tangible devices capable of holding and storing instructions for use by an instruction execution device. Computer-readable storage media can be, for example—but not limited to—electrical storage devices, magnetic storage devices, optical storage devices, electromagnetic storage devices, semiconductor storage devices, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of computer-readable storage media include: portable computer disks, hard disks, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), static random access memory (SRAM), portable compact disc read-only memory (CD-ROM), digital multifunction disc (DVD), memory sticks, floppy disks, mechanical encoding devices, such as punch cards or recessed protrusions storing instructions thereon, and any suitable combination of the foregoing. The computer-readable storage media used herein are not to be construed as transient signals themselves, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through waveguides or other transmission media (e.g., light pulses through fiber optic cables), or electrical signals transmitted through wires.

[0111] The computer-readable program instructions described herein can be downloaded from computer-readable storage media to various computing / processing devices, or downloaded via a network, such as the Internet, local area network, wide area network, and / or wireless network, to an external computer or external storage device. The network may include copper transmission cables, fiber optic transmission, wireless transmission, routers, firewalls, switches, gateway computers, and / or edge servers. A network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards them to the computer-readable storage media in the respective computing / processing device.

[0112] Computer program instructions used to perform the operations of this disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-dependent instructions, microcode, firmware instructions, status setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages ​​such as Smalltalk, C++, etc., and conventional procedural programming languages ​​such as the "C" language or similar programming languages. The computer-readable program instructions may execute entirely on the user's computer, partially on the user's computer, as a standalone software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In cases involving a remote computer, the remote computer may be connected to the user's computer via any type of network—including a local area network (LAN) or a wide area network (WAN)—or may be connected to an external computer (e.g., via the Internet using an Internet service provider). In some embodiments, electronic circuitry, such as programmable logic circuitry, field-programmable gate arrays (FPGAs), or programmable logic arrays (PLAs), is personalized by utilizing the status information of the computer-readable program instructions to implement various aspects of this disclosure.

[0113] Various aspects of this disclosure are described herein with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this disclosure. It should be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer-readable program instructions.

[0114] These computer-readable program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, or other programmable data processing apparatus to produce a machine such that, when executed by the processor of the computer or other programmable data processing apparatus, they create means for implementing the functions / actions specified in one or more blocks of the flowchart and / or block diagram. These computer-readable program instructions can also be stored in a computer-readable storage medium that causes a computer, programmable data processing apparatus, and / or other device to operate in a particular manner; thus, the computer-readable medium storing the instructions comprises an article of manufacture that includes instructions for implementing aspects of the functions / actions specified in one or more blocks of the flowchart and / or block diagram.

[0115] Computer-readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable data processing apparatus, or other device to produce a computer-implemented process, thereby causing the instructions executed on the computer, other programmable data processing apparatus, or other device to perform the functions / actions specified in one or more boxes of a flowchart and / or block diagram.

[0116] The flowcharts and block diagrams in the accompanying drawings illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present disclosure. In this regard, each block in a flowchart or block diagram may represent a module, segment, or portion of an instruction containing one or more executable instructions for implementing a specified logical function. In some alternative implementations, the functions marked in the blocks may occur in a different order than those shown in the drawings. For example, two consecutive blocks may actually be executed substantially in parallel, and they may sometimes be executed in reverse order, depending on the functions involved. It should also be noted that each block in the block diagrams and / or flowcharts, and combinations of blocks in the block diagrams and / or flowcharts, may be implemented using a dedicated hardware-based system that performs the specified function or action, or using a combination of dedicated hardware and computer instructions.

[0117] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not 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 modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A method for full-band four-quadrant admittance correction of modular multilevel converters, characterized in that: The method includes the following steps: Step 1: Model the inner loop control process of the modular multilevel converter (MMC) to obtain the equivalent admittance-frequency characteristic relationship of the MMC; Step 2: Establish a full-band admittance correction criterion for stable operation of the power grid system after MMC grid connection; the full-band admittance correction criterion is: the equivalent admittance-frequency characteristic of MMC satisfies: there is no negative damping characteristic in the full-band, that is, the MMC conductance takes a positive value in the full-band. Step 3: Using a virtual damping correction method, based on the equivalent admittance-frequency characteristic relationship, and with the goal of satisfying the full-band admittance correction criterion, perform full-band four-quadrant admittance correction on the MMC.

2. The modular multilevel converter full-band four-quadrant admittance correction method according to claim 1, characterized in that: In step 1, the inner-loop control process of MMC is modeled to obtain i = G r0 ·i r +Y0·e=i inv +Y0·e; Where e and i are the voltage and output current of the MMC equivalent two-port network, respectively; i inv It is an equivalent current source; G r0 The command current i obtained from the outer loop control r The transfer function; Y0 is the MMC equivalent admittance.

3. The modular multilevel converter full-band four-quadrant admittance correction method according to claim 2, characterized in that: In step 1, let s = jω = j2πf and substitute it into Y0 to obtain the real part G and the imaginary part B of the admittance at frequency f. Then, at frequency f, the MMC equivalent admittance can be expressed as Y0 = G + jB. Where s is the Laplace operator, and ω and f are the angular frequency and frequency, respectively.

4. The modular multilevel converter full-band four-quadrant admittance correction method according to claim 3, characterized in that: The frequency f ranges across the entire frequency band, from the fundamental frequency to the Nyquist frequency.

5. The modular multilevel converter full-band four-quadrant admittance correction method according to claim 1, characterized in that: The virtual damping correction method described in step 3 refers to the assumption that a virtual series impedance Z exists between the physical connection point of the MMC and the power grid. v and virtual parallel admittance Y v The output current i of the MMC controller flows into the power grid through a virtual impedance network, and the grid voltage is applied to the MMC controller through the virtual impedance network. The virtual series impedance Z v and virtual parallel admittance Y v A virtual impedance network is formed, and the virtual impedance network, together with the equivalent admittance of the MMC itself, constitutes the MMC admittance after virtual damping correction.

6. The modular multilevel converter full-band four-quadrant admittance correction method according to claim 5, characterized in that: In step 3, 1) let Z in the virtual impedance network v =R v +L v s, Y v =G v +C v If s, then by applying the voltage e and output current i of the MMC equivalent two-port network to the virtual impedance network, we can obtain i v =i+e·Y v e v =i·Z v +e·Z v ·Y v +e; Among them, R v L is the series resistance correction factor. v G is the series reactance correction factor. v C is the parallel conductance correction factor. v denoted as the parallel susceptance correction factor; e and i are the voltage and output current of the MMC equivalent two-port network, respectively. e v i v The two-port voltage and output current of the MMC inner loop control after virtual damping correction; 2) i is obtained based on the inner loop control. v =G r0 ·i r +Y0·e v ; Among them, G r0 The command current i obtained from the outer loop control r The transfer function is Y0, which is the MMC equivalent admittance, Y0 = G + jB; 3) Change i v =i+e·Y v e v =i·Z v +e·Z v ·Y v +e brings in i v =G r0 ·i r +Y0·e v Combining the equivalent admittance-frequency characteristic relationship Y 0v =G vinv +jB vinv The derivation yields Among them, G r0v Y 0v i inv These are the command currents i after virtual damping correction. r The transfer function, MMC equivalent admittance, and equivalent current source; G vinv B vinv The equivalent admittance Y of MMC after virtual damping correction 0v The real and imaginary parts of the admittance at frequency f.

7. The modular multilevel converter full-band four-quadrant admittance correction method according to claim 6, characterized in that: In step 3, based on equation (1), Z is adjusted. v and Y v The equivalent admittance of MMC can be corrected, and the correction results are as follows: 1) When only Z is increased v Y v When = 0, the MMC equivalent admittance is determined by Y. 0v Become 2) When only Y is increased v Z v When = 0, the MMC equivalent admittance is determined by Y. 0v Become 3) When both corrections are used, i.e., Z is increased v and Y v At that time, the MMC equivalent admittance is determined by Y. 0v It becomes: Where Z1 is the corrected MMC equivalent impedance.

8. The modular multilevel converter full-band four-quadrant admittance correction method according to claim 7, characterized in that: In step 3, Z is adjusted according to equation (1). v =R v +L v s, Y v =G v +C v s, then the MMC equivalent admittance correction is As the equivalent admittance correction value changes, the dynamic equivalent admittance-frequency characteristic curve is obtained, i.e., G at frequency f. vinv +jB vinv When the equivalent admittance-frequency characteristic curve satisfies the full-band admittance correction criterion, i.e., G vinv When the entire frequency band is positive, the full-band four-quadrant equivalent admittance correction of the MMC is completed.

9. A modular multilevel converter full-band four-quadrant admittance correction system for implementing the method of any one of claims 1-8, characterized in that: The system includes: The MMC inner loop control modeling module is used to model the MMC inner loop control process of the modular multilevel converter to obtain the MMC equivalent admittance-frequency characteristic relationship. The full-band admittance correction criterion construction module is used to establish a full-band admittance correction criterion for the stable operation of the power grid system after MMC is connected to the grid; The full-band four-quadrant admittance correction module is used to perform full-band four-quadrant admittance correction on MMC by adopting a virtual damping correction method and based on the equivalent admittance-frequency characteristic relationship, with the goal of satisfying the full-band admittance correction criterion.

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