CN116565932A - MMC control method, electronic equipment, storage medium and direct-current transmission system - Google Patents
MMC control method, electronic equipment, storage medium and direct-current transmission system Download PDFInfo
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Classifications
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/12—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load
- H02J3/16—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load by adjustment of reactive power
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
- H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]
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- Supply And Distribution Of Alternating Current (AREA)
Abstract
The invention belongs to the technical field of flexible direct-current transmission, and particularly relates to an MMC control method, electronic equipment, a storage medium and a direct-current transmission system. The MMC uses a virtual synchronous machine control strategy, and comprises a virtual rotor mechanical equation part, a virtual excitation control part, a voltage outer ring control part and a current inner ring control part. The current command of the current inner loop control part is not only a voltage outer loop output value, but also a feedforward value is superimposed on the voltage outer loop output value, a value processed for an active power set value is superimposed on a d-axis current command value, a value processed for a reactive power set value is superimposed on a q-axis current command value, so that the power regulation speed is increased, the purpose of quickly tracking power is achieved, the power frequency modulation and inertia characteristics of a power grid are met, quick response during power regulation is ensured, power decoupling control is realized, a friendly power grid access scheme for new energy power generation is realized, and the current type control is converted into voltage type control.
Description
Technical Field
The invention belongs to the technical field of flexible direct-current transmission, and particularly relates to an MMC control method, electronic equipment, a storage medium and a direct-current transmission system.
Background
In recent years, with the increasing rise of energy crisis and environmental problems, solar energy has been rapidly developed as clean renewable energy. Micro-grid power generation systems are often used to address the power supply requirements of remote areas, while the power grids at the end of the power grids in remote areas, islands, etc. are usually designed for relatively small loads, and the power supply is mostly of medium-low voltage level. In order to meet the load demands of scattered users, most of power transmission lines are long, line reactance is large, and therefore the power grid can show high impedance characteristics. The synchronous generator in the large power grid has excellent inertia and damping characteristics, can participate in the regulation of the voltage and the frequency of the power grid, and has the advantage of being natural and friendly to the power grid. By taking reference to the operation experience of the traditional power system and the characteristics of the synchronous generator, friendly access of a power grid can be realized, and a plurality of problems and challenges of new energy grid connection can be solved to a great extent.
The virtual synchronous generator technology is a technology which simulates the electromechanical transient characteristics of a synchronous generator set, so that a power supply adopting a converter has the external characteristics of grid-connected operation such as inertia, damping, primary frequency modulation, reactive voltage regulation and the like of the synchronous generator set. Moreover, the virtual synchronous machine does not need to be transformed on a large scale, so that the virtual synchronous machine is high in practicability and economical and practical. The virtual synchronous technology mainly simulates the characteristics of inertia, active frequency modulation, reactive voltage regulation and the like of the synchronous generator according to a mathematical model of the synchronous generator, so that a following-net type control strategy is converted into a net-structured operation mechanism, and the operation characteristic comparable with that of the traditional synchronous generator is realized.
If the modular multilevel converter MMC adopts a virtual inertia control strategy, the virtual synchronization technology has the same inertia as the traditional synchronous generator after being disturbed, and then the virtual synchronization technology can participate in primary frequency modulation; the MMC is controlled by adopting a power electronic device, so that the response speed of the virtual synchronization technology is faster. The moment of inertia of a conventional synchronous generator is a physical quantity related to its size, and generally increases with increasing power. However, the virtual inertia of the virtual synchronization technique is not fixed, but is closely related to the configuration of the controller structure, so that the selection of the virtual inertia of the virtual synchronization technique is more flexible. The frequency stability of the distributed power supply access system can be effectively improved, and the distributed power supply access system has wide application prospect. Of course, the direct application of the virtual synchronous machine control strategy to the MMC is not perfect, and has some drawbacks, such as slow power adjustment and tracking speed, and long time for the system to reach steady state.
Disclosure of Invention
The invention aims to provide an MMC control method, electronic equipment, a storage medium and a direct current transmission system, which are used for solving the problem of low power tracking speed of an MMC applying a traditional virtual synchronous machine control strategy.
In order to solve the technical problems, the invention provides an MMC control method based on a virtual synchronous machine, which is used for obtaining MMC control signals by using the virtual synchronous machine control, wherein the virtual synchronous machine control comprises a virtual rotor mechanical equation part, a virtual excitation control part, a voltage outer ring control part and a current inner ring control part;
the virtual rotor mechanical equation part is used for obtaining a phase angle of a power grid;
the virtual excitation control part is used for obtaining a d-axis voltage command value in the voltage outer loop control part;
the voltage outer ring control part is used for carrying out d-axis and q-axis power grid voltage decoupling control according to the d-axis voltage command value and the power grid phase angle to respectively obtain a d-axis voltage output value and a q-axis voltage output value; the active power given value in the virtual rotor mechanical equation part and the reactive power given value in the virtual excitation control part are multiplied by a set coefficient to obtain a d-axis current feedforward value and a q-axis current feedforward value respectively, wherein the set coefficient is related to the power grid current and the power grid voltage; further, adding the d-axis voltage output value and the d-axis current feedforward value to obtain a d-axis current command value, and adding the q-axis voltage output value and the q-axis current feedforward value to obtain a q-axis current command value;
the current inner loop control part is used for carrying out d-axis and q-axis power grid current decoupling control according to the d-axis current instruction value, the q-axis current instruction value and the power grid phase angle to obtain modulated waves of the MMC, and modulating the modulated waves to control the MMC.
The beneficial effects are as follows: the invention improves the traditional virtual synchronous machine control strategy, the current command of the current inner loop control part is not only a voltage outer loop output value, but also a feedforward value is superposed on the voltage outer loop output value, a value after the active power set value is processed is superposed on the d-axis current command value, a value after the reactive power set value is processed is superposed on the q-axis current command value, so that the power regulation speed is increased, the purpose of quickly tracking the power is achieved, the frequency modulation and inertia characteristics of the power grid are met, the quick response and the power decoupling control are realized during the power regulation are ensured, the friendly power grid access scheme for new energy power generation is realized, and the current type control is converted into the voltage type control.
Further, the set coefficient is the power grid current and the power grid currentThe ratio of the grid voltages.
Further, the virtual synchronous machine control further includes a phase compensation control section for performing adjustment control on a difference between the q-axis voltage command value and the q-axis voltage feedback value in the voltage outer loop control section to obtain an angular velocity compensation value, and superimposing the angular velocity compensation value to the angular velocity output value in the virtual rotor mechanical equation section.
The beneficial effects are as follows: the power control calculation delay causes deviation of the virtual mechanical equation power loop phase during power adjustment, so that the power grid voltage phase angle can be tracked in real time for realizing the current active phase angle, the phase compensation control part is added, the response delay of the control loop is made up, and the power is ensured to be tracked rapidly.
Further, the regulation control is performed by using a PID regulator.
The method has the following effects: by adopting the PID regulator, the integral control link in the regulator can realize accurate adjustment of the deviation change rate.
Further, the q-axis voltage command value is 0.
Further, the modulation is NLM modulation.
In order to solve the technical problems, the invention also provides electronic equipment which comprises a memory and a processor, wherein the processor is used for executing computer executable instructions stored in the memory to realize the MMC control method based on the virtual synchronous machine and achieve the same beneficial effects as the method.
In order to solve the technical problems, the invention also provides a computer readable storage medium, wherein the storage medium stores computer executable instructions, and the computer executable instructions are used for realizing the MMC control method based on the virtual synchronous machine and achieving the same beneficial effects as the method when being executed.
In order to solve the technical problems, the invention also provides a flexible direct current transmission system which comprises an MMC, wherein the MMC is used for controlling by adopting the MMC control method based on the virtual synchronous machine, and achieves the same beneficial effects as the MMC control method.
Drawings
FIG. 1 is a control block diagram of the MMC control method based on the virtual synchronous machine of the present invention;
fig. 2 is a control block diagram of the power tracking control of the present invention.
Detailed Description
The main conception of the invention is that the current feedforward instruction is obtained by utilizing the power calculation of the three-phase symmetrical power grid, the phase angle compensation is dynamically increased during the power tracking, and the control loop response delay is compensated to realize the rapid power tracking. The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent.
System embodiment:
an embodiment of a flexible direct current transmission system according to the present invention includes a modular multilevel converter (hereinafter abbreviated as MMC), and the control method adopted by the flexible direct current transmission system is a virtual synchronous machine-based MMC control method according to the present invention, and the method is based on a virtual synchronous machine (VSG), and the control block diagram of the method is shown in fig. 1, and includes a virtual rotor mechanical equation portion, a virtual excitation control portion, an unbalanced phase-locked loop control portion, a voltage outer loop control portion, a current inner loop control portion, and a phase compensation control portion (the phase compensation control portion is not shown in fig. 1, and is described later in conjunction with fig. 2), which are specifically described below.
1. Virtual excitation control section.
This portion coincides with the virtual excitation control portion of the conventional VSG. Collecting three-phase voltage and current of a power grid, calculating reactive power Q, and setting the reactive power Q according to the current ref Performing closed-loop control on the deviation between the two points, and ensuring that the steady-state error is zero in order to ensure the integration link in the PI regulator with reactive control precision; the voltage sag control is added with a proportion regulator to ensure that the capacitive reactive power is output to raise the voltage of the power grid when the voltage is reduced, and the inductive reactive power is output to lower the voltage of the power grid when the voltage is increased; and adding the two regulator output values to obtain the VSG output voltage amplitude. At the same time, the feedforward value E of the voltage amplitude is increased for improving the response speed of the control loop 0 The component is obtained by detecting the instantaneous amplitude of the power grid voltage, so that the slow speed regulation through control loop integration is avoided, and the stability of the system is ensured. Superposition E 0 Finally obtained E p Is the d-axis voltage command value U in the voltage outer loop control part sd 。
2. Virtual rotor mechanical equations section.
This part is partially consistent with the virtual rotor mechanical equations of conventional VSGs. The active power control is based on the conversion equations of the synchronous machine power, torque and frequency. According to the mathematical model of the traditional synchronous generator, a VSG mathematical model of the grid-connected inverter can be obtained, and a mechanical equation of the VSG can be expressed as follows:
wherein J represents virtual inertia of VSG, kg.m 2 The method comprises the steps of carrying out a first treatment on the surface of the ω represents VSG angular velocity; omega 0 Representing the reference angular speed of the power grid, and rad/s; t (T) m 、T e 、T d Mechanical, electromagnetic and damping torques of VSG, N.m; d is a damping coefficient, N.m.s/rad. Wherein the electromagnetic torque T e Can be determined by the internal potential e of VSG abc And output current i abc And (3) calculating to obtain:
wherein P is e Is VSG electromagnetic power. Meanwhile, a primary frequency modulation algorithm is added to carry out frequency fine adjustment, and finally, the phase angle theta required by control is obtained according to a virtual mechanical equation.
3. A voltage outer loop control section and a current inner loop control section.
The motor is a three-phase symmetrical system, the physical quantity is alternating current quantity, and the variables are mutually coupled, so that the design of a control system is not facilitated. Therefore, coordinate conversion can be performed, wherein the above formula is a three-phase stationary coordinate system abc, and the three-phase stationary coordinate system abc is converted into a synchronous rotation coordinate system dq. After such coordinate transformation, the fundamental wave sine quantity such as three-phase voltage, three-phase current and the like in the three-phase coordinate system can be converted into the direct current quantity in the synchronous rotation coordinate system. The control alternating current quantity is converted into the control corresponding direct current quantity, the number of variables is reduced, the structure of the system is simplified, and the reliability and the flexibility of the control system are enhanced. In addition, similar to the current control inner ring of the traditional grid-connected inverter, after the electric equation is transformed by the rotation coordinate, the fundamental wave sine quantity in the three-phase static symmetrical coordinate system is converted into the direct current quantity in the synchronous rotation coordinate system, and the dq axis variables are mutually coupled, so that the i cannot be controlled dref And i qref The individual control is performed, which makes the design of the regulator difficult. To this end, i is introduced dref And i qref Feedforward decoupling control of u d And u q The feedforward compensation is carried out, and the virtual resistance R is far smaller than the inductance in the system, so the resistance is negligible, and the electromagnetic equation under the three-phase coordinate is converted into the equation under the synchronous rotation coordinate:
E d ＝U d +ωLi q_ref
E q ＝U q -ωLi d_ref
wherein L is the synchronous inductance of VSG; e (E) d 、E q Virtual internal potential e for VSG abc The dq-axis component of (2); u (U) d 、U q Terminal voltage u for VSG abc The dq-axis component of (2); i.e dref 、i qref The dq-axis component of the current is counted for VSG. The system control block diagram of the current inner loop command value output by the PI regulator is shown in figure 1.
The power transmission model of VSG can be equivalent to VSG output voltage connected to grid-connected point via line impedance series, E 0 Outputting a voltage effective value for VSG; taking into account U g For the grid-tie voltage, delta is E assuming that its phase is the reference phase 0 And U g Phase difference between them. Similar to a conventional synchronous motor, the difference angle between the internal potential and the motor end voltage is defined as the power angle, and the phase difference angle delta between the VSG output voltage and the grid voltage is defined as the power angle. The power can be expressed as:
wherein X is eq Is the equivalent inductance value of the decoupled circuit. It is observed that in case of large enough power angle, the active power and reactive power are controlled by the VSG output voltage and the power angle at the same time, and there is power coupling. The power coupling condition is gradually increased along with the increase of the power angle, and the power angle value is related to the active power transmitted by the line and is not fixed, and further is the powerThe decoupling control brings inconvenience. Therefore, the invention realizes the decoupling of the active power and the reactive power input by the rectifier by considering the coordinate transformation, and the d-axis current i d Is the current corresponding to active power, q-axis current i q The current corresponding to reactive power changes alternating current in a three-phase coordinate system into power calculation formulas in a synchronous rotation coordinate system are as follows:
u when the phase angle is consistent with the positive a-phase order sq =0, the above reduces to:
and d-axis components of the current output are adjusted to be in phase with the grid voltage, and output active power and reactive decoupling are obtained according to a three-phase instantaneous power formula.
The voltage outer loop control part utilizes the function of no static difference of an integral link in the PI regulator to control the input and output of the voltage loop to be consistent after the loop is stable, when the q-axis component of the power grid voltage is zero, the phase angle output by the current virtual mechanical equation and the phase of the positive sequence of the power grid can be ensured to be consistent, and the active reactive current is independently controlled to realize the power decoupling effect.
In order to accelerate the power adjusting speed and the adjusting speed, the output of the voltage outer ring is used as the command of the current inner ring, and the power is indirectly controlled by controlling the current command. The invention adopts power command feedforward to realize the function, and is specific in that: performing d-axis and q-axis power grid voltage decoupling control to respectively obtain a d-axis voltage output value and a q-axis voltage output value; setting the active power to a given value P in the mechanical equation part of the virtual rotor ref And reactive power given value Q in virtual excitation control section ref Multiplying by I/(1.732. Times. U) (I and U are power grid current and voltage respectively) respectively to obtain a d-axis current feedforward value and a q-axis current feedforward value; further, the d-axis voltage output value and the d-axis current feedforward value are added to obtain a d-axis current command value, and the q-axis voltage output value and the q-axis current feedforward value are obtainedThe q-axis current command value is obtained by adding the values, so that the aim of rapid power tracking is fulfilled.
After voltage and current double closed-loop control, three-phase modulation waves are calculated, NLM is used for adjusting and outputting to generate MMC submodule triggering signals, the on-off of a power switch device is controlled, and power control, active frequency modulation and reactive voltage regulation are achieved.
4. And a phase compensation control section.
When the power is adjusted, the power control calculation delay causes the deviation of the phase of the virtual mechanical equation power loop, so that the phase angle of the power grid voltage can be tracked in real time for realizing the current active phase angle, and the equipment can enable the phase compensation regulator after receiving the power adjustment command. The q-axis component of the grid voltage is added to the angular velocity of the virtual mechanical equation via deviation, when Uq is greater than 0, the phase lag needs to increase the angular velocity, and conversely, the angular velocity is reduced. In order to prevent the power control accuracy from being affected in the steady state, the phase compensation regulator is a PID regulator, and the differential part is used for regulating the deviation change rate. As shown in fig. 2, in particular: for the difference DeltaU between the q-axis voltage command value and the q-axis voltage feedforward value in the voltage outer loop control section q And (3) performing adjustment control through a PI regulator to obtain an angular velocity compensation value, and superposing the angular velocity compensation value on an angular velocity output value in the mechanical equation part of the virtual rotor.
5. An unbalanced phase locked loop control section.
And an unbalanced phase-locked loop is adopted to monitor the phase angle inspection of the power grid, the frequency inspection of the power grid and the detection of the positive sequence component of the power grid, and a virtual mechanical equation is initialized to output the phase angle when the phase angle of the power grid is suddenly changed, so that the current fluctuation caused by the phase deviation is restrained.
It should be noted that the method is a control strategy adopted under the condition of normal operation of the flexible direct current transmission system. In the event of a system fault (e.g., voltage drop), a change-over switch is provided in fig. 1, where the change-over switch is located in the "voltage drop" position in fig. 1, and the changed-over control strategy is not essential to the present invention and will not be described again. In addition, a change switch is also arranged at the position of power setting change in fig. 2, and other control strategies are adopted under other working conditions of the flexible direct current transmission system, which is not the key point of the invention and is not developed. In addition, the determination of the grid fault by using the grid positive sequence ud+ in fig. 1 is not an endpoint of the present invention, and will not be described further.
In summary, the invention realizes that NLM adjustment output is used for generating MMC submodule trigger signals, controlling the on-off of a power switch device, realizing power control and active frequency modulation and reactive voltage modulation, adopting voltage outer loop PI non-difference tracking and power feedforward decoupling, considering dynamic increase phase angle compensation during power tracking, compensating response delay of a control loop, and ensuring rapid power tracking.
Method embodiment:
the corresponding control block diagrams of the MMC control method embodiment based on the virtual synchronous machine are shown in fig. 1 and fig. 2, and specific contents of the control method are described in detail in the system embodiment, which is not repeated.
Electronic device embodiment:
the embodiment of the invention relates to an electronic device, which comprises a memory, a processor and an internal bus, wherein the processor and the memory are communicated with each other and data interaction is completed through the internal bus. The memory stores computer executable instructions, and the processor is configured to execute the computer executable instructions stored in the memory to implement an MMC control method based on a virtual synchronous machine in the system embodiment of the invention. The processor may be a microprocessor MCU, a programmable logic device FPGA, or other processing device. The memory may be various memories for storing information by using electric energy, such as RAM, ROM, etc.; of course, other types of memory are also possible.
Storage medium embodiment:
in one embodiment of the present invention, a computer readable storage medium stores computer executable instructions, and the computer executable instructions are used to implement an MMC control method based on a virtual synchronous machine in a system embodiment when executed.
Finally, it should be noted that the above embodiments are only for illustrating the technical solution of the present invention and not for limiting the same, and those skilled in the art, after reading the present application, may make various modifications or alterations to the present invention with reference to the above embodiments, all falling within the scope of the appended claims.
Claims (9)
1. The MMC control method based on the virtual synchronous machine is characterized in that a virtual synchronous machine control is adopted to obtain a control signal of the MMC, and the virtual synchronous machine control comprises a virtual rotor mechanical equation part, a virtual excitation control part, a voltage outer ring control part and a current inner ring control part;
the virtual rotor mechanical equation part is used for obtaining a phase angle of a power grid;
the virtual excitation control part is used for obtaining a d-axis voltage command value in the voltage outer loop control part;
the voltage outer ring control part is used for carrying out d-axis and q-axis power grid voltage decoupling control according to the d-axis voltage command value and the power grid phase angle to respectively obtain a d-axis voltage output value and a q-axis voltage output value; the active power given value in the virtual rotor mechanical equation part and the reactive power given value in the virtual excitation control part are multiplied by a set coefficient to obtain a d-axis current feedforward value and a q-axis current feedforward value respectively, wherein the set coefficient is related to the power grid current and the power grid voltage; further, adding the d-axis voltage output value and the d-axis current feedforward value to obtain a d-axis current command value, and adding the q-axis voltage output value and the q-axis current feedforward value to obtain a q-axis current command value;
the current inner loop control part is used for carrying out d-axis and q-axis power grid current decoupling control according to the d-axis current instruction value, the q-axis current instruction value and the power grid phase angle to obtain modulated waves of the MMC, and modulating the modulated waves to control the MMC.
2. The MMC control method of claim 1, wherein the set coefficients are grid current and powerThe ratio of the grid voltages.
3. The MMC control method based on a virtual synchronous machine according to claim 1, characterized in that the virtual synchronous machine control further comprises a phase compensation control section for performing adjustment control of a difference value between the q-axis voltage command value and the q-axis voltage feedback value in the voltage outer loop control section to obtain an angular velocity compensation value, and superimposing the angular velocity compensation value to the angular velocity output value in the virtual rotor mechanical equation section.
4. A MMC control method based on a virtual synchronous machine according to claim 3, characterized in that the regulation control is performed using a PID regulator.
5. The MMC control method of virtual synchronous machine-based system of claim 3, wherein the q-axis voltage command value is 0.
6. The MMC control method of virtual synchronous machine-based according to any one of claims 1-5, characterized in that the modulation is NLM modulation.
7. An electronic device comprising a memory and a processor for executing computer-executable instructions stored in the memory to implement the virtual synchronous machine-based MMC control method of any of claims 1-6.
8. A computer readable storage medium having stored therein computer executable instructions for implementing the virtual synchronous machine based MMC control method of any of claims 1-6 when executed.
9. A flexible direct current transmission system comprising an MMC, characterized in that the MMC is configured to control by using the MMC control method based on a virtual synchronous machine according to any one of claims 1-6.
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