Mmc hybrid modulation method and system of flexible direct current converter

Abstract

The application belongs to the technical field of flexible direct current transmission, and particularly discloses an MMC hybrid modulation method and system of a flexible direct current converter, which specifically comprises the following steps: converting three-phase alternating current voltage and three-phase alternating current at the grid side to the synchronous rotating coordinate system through coordinate transformation, and calculating active power in real time; acquiring the power fluctuation value and the DC bus voltage deviation value of the modular multilevel converter; acquiring the weight of the CPS modulation signal and the NLM modulation signal according to the DC bus voltage deviation value and the power fluctuation value in the modular multilevel converter; mixing the weight of the CPS modulation signal and the NLM modulation signal in real time, synthesizing a PWM waveform to generate a PWM driving signal, and inputting the PWM driving signal to each submodule in the modular multilevel converter for modulation. The application improves the dynamic performance and operation efficiency of the MMC system, and reduces the loss in the energy transmission process.

Description

Technical Field

[0001] This application belongs to the field of flexible DC transmission technology, and more specifically, relates to an MMC hybrid modulation method and system for a flexible DC converter. Background Technology

[0002] With the large-scale integration of renewable energy and the increasing proportion of DC load, the requirements for system reliability in flexible DC distribution networks have significantly increased. Modular multilevel converters (MMCs) have attracted widespread attention due to their advantages such as good scalability, low harmonic content, high transmission efficiency, and flexible control. MMC is the core topology of flexible DC converters, and traditional control methods include the following two:

[0003] Carrier Phase Shift (CPS) technology adjusts the phase of the carrier signal in each inverter unit or module to stagger their switching actions, thereby improving the overall output waveform. Nearest Level Modulation (NLM) technology generates the output waveform by selecting the level closest to the reference waveform, which is simple to implement, computationally inefficient, and allows for low switching frequencies. Currently, both of these designs mostly use fixed modes, which cannot adapt to dynamic operating conditions such as power fluctuations and DC voltage deviations, making it difficult to balance efficiency and harmonic suppression. In highly dynamic scenarios with rapidly changing power, existing NLM modulation methods may not respond in time, leading to unstable power output and affecting the dynamic performance of the system. In steady-state operation, CPS modulation technology, due to its excessively high switching frequency, increases switching losses, reduces system efficiency, and may not achieve optimal harmonic distortion (THD). Therefore, a method is needed that can flexibly switch modulation methods according to the real-time operating conditions of the system to adapt to different operating scenarios and improve the overall performance of the system. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the purpose of this application is to provide a hybrid MMC modulation method and system for flexible DC converters, aiming to solve the problem that current CPS and NLM adopt fixed modes, which cannot adapt to dynamic operating conditions such as MMC power fluctuations and DC voltage deviations, resulting in a difficulty in simultaneously achieving efficiency and harmonic suppression.

[0005] To achieve the above objectives, in a first aspect, this application provides an MMC hybrid modulation method for a flexible DC converter, specifically including the following steps:

[0006] Step S1: Transform the three-phase AC voltage and three-phase AC current on the grid side of the modular multilevel converter to the synchronous rotating coordinate system and calculate the active power in real time.

[0007] Step S2: Obtain the power fluctuation value of the modular multilevel converter through the active power, and obtain the DC bus voltage deviation value of the modular multilevel converter based on the actual DC bus voltage and the DC bus voltage reference value.

[0008] Step S3: Obtain the weights of the CPS modulation signal and the NLM modulation signal based on the DC bus voltage deviation and power fluctuation values ​​in the modular multilevel converter;

[0009] Step S4: The weights of the CPS modulation signal and the NLM modulation signal are mixed in real time to synthesize the PWM waveform and generate the PWM drive signal, which is then input to each sub-module in the modular multilevel converter for modulation.

[0010] More preferably, in step S3, the weights of the CPS modulated signal for:

[0011]

[0012] in, This is the curve steepness coefficient of the Logistic function. The threshold for power fluctuation; This represents the real-time power fluctuation value. This represents the deviation value of the real-time DC bus voltage; The DC voltage influence coefficient is used; the weight of the NLM modulated signal is 1- .

[0013] More preferably, in step S4, the synthesized PWM waveform is:

[0014]

[0015] in, This is the synthesized PWM waveform; It is a CPS modulated signal; It is an NLM modulated signal.

[0016] More preferably, step S1 specifically includes the following steps:

[0017] Step S1.1: Use a finite impulse response filter to filter the three-phase AC voltage and three-phase AC current of the modular multilevel converter grid side acquired in real time;

[0018] Step S1.2: Transform the filtered three-phase AC voltage and three-phase AC current from the three-phase stationary coordinate system to the two-dimensional stationary coordinate system using Clarke transformation, and then transform them to the synchronous rotating coordinate system using Park transformation;

[0019] Step S1.3: Calculate active power in real time based on voltage and current components in a synchronous rotating coordinate system.

[0020] A further preferred method for obtaining the power fluctuation value in step S2 is as follows:

[0021] The active power data is smoothed using the sliding window averaging method, and the power fluctuation value is calculated based on the power average value and active power within the time window.

[0022] Secondly, this application provides an MMC hybrid modulation system for a flexible DC converter, comprising:

[0023] The power calculation module is used to transform the three-phase AC voltage and three-phase AC current on the grid side of the modular multilevel converter to a synchronous rotating coordinate system and calculate the active power in real time.

[0024] The power fluctuation calculation module is used to obtain the power fluctuation value of the modular multilevel converter through active power.

[0025] The DC bus voltage deviation calculation module is used to obtain the DC bus voltage deviation value of the modular multilevel converter based on the actual DC bus voltage and the DC bus voltage reference value.

[0026] The weight calculation module is used to obtain the weights of the CPS modulation signal and the NLM modulation signal based on the DC bus voltage deviation and power fluctuation values ​​in the modular multilevel converter.

[0027] The modulation signal acquisition module is used to mix the weights of the CPS modulation signal and the NLM modulation signal in real time, synthesize the PWM waveform, generate the PWM drive signal, and input it to each sub-module in the modular multilevel converter for modulation.

[0028] Further preferred, the weights of the CPS modulated signal in the weight calculation module for:

[0029]

[0030] in, This is the curve steepness coefficient of the Logistic function. The threshold for power fluctuation; This represents the real-time power fluctuation value. This represents the deviation value of the real-time DC bus voltage; The DC voltage influence coefficient is used; the weight of the NLM modulated signal is 1- .

[0031] More preferably, the PWM waveform synthesized in the modulation signal acquisition module is:

[0032]

[0033] in, This is the synthesized PWM waveform; It is a CPS modulated signal; It is an NLM modulated signal.

[0034] More preferably, the power calculation module includes a first data acquisition unit, a filtering unit, a first coordinate transformation unit, a second coordinate transformation unit, and a power acquisition unit;

[0035] The first data acquisition unit is used to acquire the three-phase AC voltage and three-phase AC current on the grid side of the modular multilevel converter in real time;

[0036] The filtering unit is used to filter the three-phase AC voltage and three-phase AC current of the modular multilevel converter grid side acquired in real time using a finite impulse response filter.

[0037] The first coordinate transformation unit is used to transform the filtered three-phase AC voltage and three-phase AC current from the three-phase stationary coordinate system to the two-dimensional stationary coordinate system through Clarke transformation.

[0038] The second coordinate transformation unit is used to transform the current and voltage in the two-dimensional stationary coordinate system to the synchronous rotating coordinate system using the Park transformation.

[0039] The power acquisition unit is used to calculate active power in real time based on the voltage and current components in a synchronous rotating coordinate system.

[0040] A further preferred method for obtaining the power fluctuation value in the power fluctuation calculation module is as follows:

[0041] The active power data is smoothed using the sliding window averaging method, and the power fluctuation value is calculated based on the power average value and active power within the time window.

[0042] Thirdly, this application provides an electronic device, comprising: at least one memory for storing a program; and at least one processor for executing the program stored in the memory, wherein when the program stored in the memory is executed, the processor is configured to execute the method described in the first aspect or any possible implementation thereof.

[0043] Fourthly, this application provides a computer-readable storage medium storing a computer program that, when run on a processor, causes the processor to perform the method described in the first aspect or any possible implementation thereof.

[0044] Fifthly, this application provides a computer program product that, when run on a processor, causes the processor to perform the method described in the first aspect or any possible implementation thereof.

[0045] It is understood that the beneficial effects of the second to fifth aspects mentioned above can be found in the relevant descriptions in the first aspect mentioned above, and will not be repeated here.

[0046] Overall, the technical solutions conceived in this application have the following beneficial effects compared with the prior art:

[0047] This application provides a hybrid modulation method for MMC in flexible DC converters. By real-time monitoring of MMC power and DC bus voltage signals, and combining a dynamic weight allocation algorithm, it achieves seamless integration of two modulation techniques, enabling flexible switching between CPS modulation and LIM modulation methods to adapt to different dynamic operating conditions and improve system response speed, efficiency, and power quality. More specifically, in scenarios with rapidly changing power, the fast response capability of CPS modulation can adjust the power output in a timely manner, ensuring the system responds within 10ms (a 60% improvement over traditional NLM), avoiding the impact of power fluctuations on the stability of the MMC system, enhancing the adaptability and reliability of the flexible DC transmission system in the face of sudden load changes, and improving the dynamic performance of the MMC system. During steady-state operation, the NLM mode reduces the switching losses of power devices by lowering the switching frequency (a 22% reduction compared to pure CPS mode), especially under high-frequency switching conditions, this reduction in losses is more significant, thereby improving the overall operating efficiency of the MMC system, reducing losses during energy transmission, and saving energy.

[0048] This application provides a hybrid MMC modulation method for flexible DC converters, which optimizes power quality. In NLM mode, the power level is dynamically adjusted according to power demand, which can further reduce harmonic distortion (THD), making the output voltage waveform closer to the ideal sine wave, reducing the impact of harmonics on the power grid and load, improving power quality, and meeting the needs of application scenarios with high power quality requirements. Attached Figure Description

[0049] Figure 1 This is a schematic diagram of the redundant submodule (MMC) circuit topology provided in the embodiments of this application;

[0050] Figure 2 This is a flowchart of signal acquisition and modulation mode switching provided in an embodiment of this application;

[0051] Figure 3 This is a dynamic weight allocation curve provided in the embodiments of this application;

[0052] Figure 4 This is a schematic diagram of hybrid modulation PWM generation provided in the embodiments of this application. Detailed Implementation

[0053] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0054] In this article, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. The symbol " / " in this article indicates that the related objects are in an "or" relationship; for example, A / B means A or B.

[0055] The terms "first" and "second," etc., used in the specification and claims herein are used to distinguish different objects, not to describe a specific order of objects.

[0056] In the embodiments of this application, the terms "exemplary" or "for example" are used to indicate that something is an example, illustration, or description. Any embodiment or design that is described as "exemplary" or "for example" in the embodiments of this application should not be construed as being more preferred or advantageous than other embodiments or design. Specifically, the use of the terms "exemplary" or "for example" is intended to present the relevant concepts in a specific manner.

[0057] In the description of the embodiments in this application, unless otherwise stated, "multiple" means two or more.

[0058] The embodiments of this application are described below with reference to the accompanying drawings.

[0059] This application provides a hybrid MMC modulation method for a flexible DC converter, including the following steps:

[0060] Step 1: Signal Acquisition and Preprocessing: Real-time acquisition of three-phase AC voltage and current signals and DC bus voltage signals from the grid input side; using a finite impulse response (FIR) filter to filter the acquired signals, removing high-frequency noise interference and improving signal accuracy;

[0061] Step 2: Coordinate Transformation: The three-phase AC voltage and current signals are transformed from a three-phase stationary coordinate system to a two-dimensional stationary coordinate system using Clarke transformation. Next, the Park transform is used to further convert the signal to a synchronous rotating coordinate system. This facilitates subsequent functional calculations and analysis;

[0062] Step 3: Power Calculation: In a synchronously rotating coordinate system, according to the formula... Calculate the real-time active power P, and according to the formula Calculate reactive power Q This allows for a comprehensive understanding of the system's power operation status; among which, In a synchronous rotating coordinate system q Axis voltage components; In a synchronous rotating coordinate system d Axis voltage components; Synchronous rotating coordinate system q Axis current components; In a synchronous rotating coordinate system d Axis current components;

[0063] Step 4: Power data smoothing: The instantaneous power data is smoothed using the sliding window averaging method to reduce the impact of random fluctuations and noise on the power measurement results and improve the stability and reliability of the power data;

[0064] Step 5: Calculation of Power Fluctuation and DC Bus Voltage Deviation: Calculate the power fluctuation value. The calculation formula is as follows: ,in, It is the average power within the time window, i.e. ; This represents the k-th active power value calculated within the time window. N This represents the total number of active power calculations within the time window; simultaneously, it calculates the deviation value of the DC bus voltage. , ;in, This is the actual DC bus voltage; This is a reference value for the DC bus voltage;

[0065] Step Six: Dynamic Weight Allocation Switching Method for Modulation Mode: Based on the calculated power fluctuation value DC bus voltage deviation value The method for switching modulation modes is defined, and its expression is:

[0066]

[0067] in, This is the curve steepness coefficient of the Logistic function. The threshold for power fluctuation; This represents the real-time power fluctuation value. This represents the deviation value of the real-time DC bus voltage; This is the DC voltage influence coefficient;

[0068] In high dynamic scenarios where power changes rapidly, when ≥15% or At that time, take It forces the input to CPS modulation in order to achieve fast and stable output;

[0069] The CPS modulated signal and the NLM modulated signal are weighted. and 1- For real-time mixing, the expression is:

[0070]

[0071] Real-time computing in FPGA The weighted PWM waveform is then synthesized to generate a unified PWM drive signal, which is provided to each submodule to drive the module to work.

[0072] Example 1

[0073] This application provides an MMC hybrid modulation system for a flexible DC converter, such as Figure 1 As shown, each arm of the MMC consists of N full-bridge submodules connected in series with an arm inductor L; NLM determines the number of submodules to be engaged by rounding the ratio of the reference voltage to the capacitor voltage of the submodule (SM), so that the output voltage is closest to the reference voltage; CPS modulation assigns a phase-shifted carrier to each submodule, with a phase difference of . (N is the number of sub-modules), compared with the same modulation wave to generate a PWM signal; after superposition, the equivalent switching frequency is increased by N times, and harmonics are reduced;

[0074] Figure 2 The flowchart of the MMC hybrid modulation method for a flexible DC converter provided in this application discloses the complete process from signal acquisition to modulation mode switching. First, signal acquisition is performed, including three-phase AC voltage, three-phase AC current, and DC bus voltage. Then, high-frequency noise is removed using FIR filtering. Next, Clarke and Park transforms are performed to convert the signal to a synchronous rotating coordinate system for calculating active and reactive power. Then, a sliding window averaging method is used to smooth the instantaneous power data and calculate power fluctuation and DC bus voltage deviation. Finally, based on these parameters... The calculation function generates the weight coefficients. The CPS modulated signal and the NLM modulated signal are weighted. and 1- Real-time mixing and synthesis of PWM waveforms generate a unified PWM drive signal, which is then provided to each submodule to drive its operation.

[0075] Figure 3 The horizontal axis represents power fluctuation values. (0%~20%), with the vertical axis representing the weighting coefficient. (0~1), the three curves are respectively =0% (blue), =5% (orange) and =10% (gray) weighting coefficient The curve; where, calculation The function's parameter k=30, =0.3, Figure 3 Showing right The impact;

[0076] Figure 4 The exhibition showcases CPS modulated waveforms, generated by comparing a phase-shifted carrier wave with a modulated wave; NLM multilevel stepped wave modulated waveforms; and different... The waveform superposition effect at values ​​(0.2, 0.5, 0.8) is a mixed PWM waveform; reflecting the hybrid logic and dynamic weights of CPS and NLM. The regulatory effect.

[0077] Example 2

[0078] Taking a practical flexible DC transmission system as an example, the system adopts the MMC hybrid modulation method proposed in this application. In the system, the three-phase AC voltage on the grid side is 380V with a frequency of 50Hz, and the rated DC bus voltage is 750V. The system is equipped with high-precision voltage and current sensors to collect the three-phase AC voltage and current signals on the grid side and the DC bus voltage signal in real time. These signals are then transmitted to the signal processing unit, which has a built-in FIR filter to filter the collected signals and remove high-frequency noise interference.

[0079] In the signal processing process, the three-phase voltage and current signals are first transformed from the three-phase stationary coordinate system to the stationary coordinate system using Clarke transform. Next, the Park transform is used to further convert the signal to a synchronous rotating coordinate system. This facilitates subsequent power calculations and analysis; in the synchronous rotating coordinate system, according to the formula... Calculate the real-time active power P, and according to the formula Calculate reactive power Q This allows for a comprehensive understanding of the system's power operation status;

[0080] To reduce the impact of random fluctuations and noise on power measurement results, a sliding window averaging method is used to smooth the instantaneous power data. By appropriately setting the length of the sliding window, both the stability of the power data and the trend of power variation are ensured. Based on this, the power fluctuation value is calculated. DC bus voltage deviation value Power fluctuation value The calculation formula is ,in, It is the average power within the time window, i.e. Simultaneously, calculate the deviation value of the DC bus voltage. , ;in, This is the actual DC bus voltage; This is a reference value for the DC bus voltage;

[0081] Based on the calculated power fluctuation value DC bus voltage deviation value The method for switching modulation modes is defined, and its expression is:

[0082]

[0083] Where k is the curve steepness coefficient of the Logistic function, and k=30; The threshold for power fluctuation is set at 8%. The calculated real-time power fluctuation value; This represents the deviation value of the real-time DC bus voltage; The DC voltage influence coefficient is calibrated to 0.3 through experiments.

[0084] In high dynamic scenarios where power changes rapidly, when ≥15% or At that time, take It forces the input to CPS modulation in order to achieve fast and stable output;

[0085] In other cases, the CPS modulated signal and the NLM modulated signal are weighted... and 1- For real-time mixing, the expression is:

[0086]

[0087] Real-time computing in FPGA The weighted PWM waveform is then synthesized to generate a unified PWM drive signal, which is provided to each submodule to drive the module to work.

[0088] The system employing the MMC hybrid modulation method proposed in this application maintains a dynamic response time of less than 10ms under scenarios with rapidly changing power, effectively avoiding the impact of power fluctuations on system stability. During steady-state operation, switching losses are significantly reduced, system efficiency is improved, harmonic distortion (THD) is optimized, the output voltage waveform is cleaner, and power quality is significantly improved. These results fully demonstrate the effectiveness and superiority of the method proposed in this application, and can meet the high-performance operation requirements of modern flexible DC transmission systems.

[0089] The MMC hybrid modulation system for the flexible DC converter provided in this application is described below. The MMC hybrid modulation system for the flexible DC converter described below can be referred to in correspondence with the MMC hybrid modulation method for the flexible DC converter described above.

[0090] This application provides an MMC hybrid modulation system for a flexible DC converter, comprising:

[0091] The power calculation module is used to transform the three-phase AC voltage and three-phase AC current on the grid side of the modular multilevel converter to a synchronous rotating coordinate system and calculate the active power in real time.

[0092] The power fluctuation calculation module is used to obtain the power fluctuation value of the modular multilevel converter based on the active power.

[0093] The DC bus voltage deviation calculation module is used to obtain the DC bus voltage deviation value of the modular multilevel converter based on the actual DC bus voltage and the DC bus voltage reference value.

[0094] The weight calculation module is used to obtain the weights of the CPS modulation signal and the NLM modulation signal based on the DC bus voltage deviation and power fluctuation values ​​in the modular multilevel converter.

[0095] The modulation signal acquisition module is used to mix the weights of the CPS modulation signal and the NLM modulation signal in real time, synthesize the PWM waveform, generate the PWM drive signal, and input it to each sub-module in the modular multilevel converter for modulation.

[0096] Further preferred, the weights of the CPS modulated signal in the weight calculation module for:

[0097]

[0098] in, This is the curve steepness coefficient of the Logistic function. The threshold for power fluctuation; Real-time power fluctuation value This represents the deviation value of the real-time DC bus voltage; The DC voltage influence coefficient is used; the weight of the NLM modulated signal is 1- .

[0099] More preferably, the PWM waveform synthesized in the modulation signal acquisition module is:

[0100]

[0101] in, This is the synthesized PWM waveform; It is a CPS modulated signal; It is an NLM modulated signal.

[0102] More preferably, the power calculation module includes a first data acquisition unit, a filtering unit, a first coordinate transformation unit, a second coordinate transformation unit, and a power acquisition unit;

[0103] The first data acquisition unit is used to acquire the three-phase AC voltage and three-phase AC current on the grid side of the modular multilevel converter in real time;

[0104] The filtering unit is used to filter the three-phase AC voltage and three-phase AC current of the modular multilevel converter grid side acquired in real time using a finite impulse response filter.

[0105] The first coordinate transformation unit is used to transform the filtered three-phase AC voltage and three-phase AC current from the three-phase stationary coordinate system to the two-dimensional stationary coordinate system through Clarke transformation.

[0106] The second coordinate transformation unit is used to transform the current and voltage in the two-dimensional stationary coordinate system to the synchronous rotating coordinate system using the Park transformation.

[0107] The power acquisition unit is used to calculate active power in real time based on the voltage and current components in a synchronous rotating coordinate system.

[0108] A further preferred method for obtaining the power fluctuation value in the power fluctuation calculation module is as follows:

[0109] The active power data is smoothed using the sliding window averaging method, and the power fluctuation value is calculated based on the power average value and active power within the time window.

[0110] In summary, this application has the following advantages compared with the prior art:

[0111] This application provides a hybrid modulation method for MMC in flexible DC converters. By real-time monitoring of MMC power and DC bus voltage signals, and combining a dynamic weight allocation algorithm, it achieves seamless integration of two modulation techniques, enabling flexible switching between CPS modulation and LIM modulation methods to adapt to different dynamic operating conditions and improve system response speed, efficiency, and power quality. More specifically, in scenarios with rapidly changing power, the fast response capability of CPS modulation can adjust the power output in a timely manner, ensuring the system responds within 10ms, avoiding the impact of power fluctuations on system stability, enhancing the adaptability and reliability of the flexible DC transmission system in the face of sudden load changes, and improving the system's dynamic performance. During steady-state operation, the NLM mode reduces the switching losses of power devices by lowering the switching frequency, especially under high-frequency switching conditions, where this reduction in losses is more significant, thereby improving the overall operating efficiency of the MMC system, reducing losses during energy transmission, and saving energy.

[0112] This application provides a hybrid MMC modulation method for flexible DC converters, which optimizes power quality. In NLM mode, the power level is dynamically adjusted according to power demand, which can further reduce harmonic distortion (THD), making the output voltage waveform closer to the ideal sine wave, reducing the impact of harmonics on the power grid and load, improving power quality, and meeting the needs of application scenarios with high power quality requirements.

[0113] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A hybrid MMC modulation method for a flexible DC converter, characterized in that, Includes the following steps: Step S1: Transform the three-phase AC voltage and three-phase AC current on the grid side of the modular multilevel converter to the synchronous rotating coordinate system and calculate the active power in real time. Step S2: Obtain the power fluctuation value of the modular multilevel converter through the active power, and obtain the DC bus voltage deviation value of the modular multilevel converter based on the actual DC bus voltage and the DC bus voltage reference value. Step S3: Obtain the weights of the CPS modulation signal and the NLM modulation signal based on the DC bus voltage deviation and power fluctuation values ​​in the modular multilevel converter; Step S4: The weights of the CPS modulation signal and the NLM modulation signal are mixed in real time to synthesize the PWM waveform and generate the PWM drive signal, which is then input to each sub-module in the modular multilevel converter for modulation. In step S3, the weights of the CPS modulated signal for: in, This is the curve steepness coefficient of the Logistic function. The threshold for power fluctuation; This represents the real-time power fluctuation value. This represents the deviation value of the real-time DC bus voltage; The DC voltage influence coefficient is used; the weight of the NLM modulated signal is 1- ; In step S4, the synthesized PWM waveform is as follows: in, This is the synthesized PWM waveform; It is a CPS modulated signal; It is an NLM modulated signal.

2. The MMC hybrid modulation method according to claim 1, characterized in that, Step S1 specifically includes the following steps: Step S1.1: Use a finite impulse response filter to filter the three-phase AC voltage and three-phase AC current of the modular multilevel converter grid side acquired in real time; Step S1.2: The filtered three-phase AC voltage and three-phase AC current are converted from the three-phase stationary coordinate system to the two-dimensional stationary coordinate system by Clarke transformation, and then converted to the synchronous rotating coordinate system by Park transformation. Step S1.3: Calculate active power in real time based on voltage and current components in synchronous rotating coordinates.

3. The MMC hybrid modulation method according to claim 1 or 2, characterized in that, The method for obtaining the power fluctuation value in step S2 is as follows: The active power data is smoothed using the sliding window averaging method, and the power fluctuation value is calculated based on the power average value and active power within the time window.

4. A hybrid MMC modulation system for a flexible DC-DC converter, characterized in that, include: The power calculation module is used to transform the three-phase AC voltage and three-phase AC current on the grid side of the modular multilevel converter to a synchronous rotating coordinate system and calculate the active power in real time. The power fluctuation calculation module is used to obtain the power fluctuation value of the modular multilevel converter through active power. The DC bus voltage deviation calculation module is used to obtain the DC bus voltage deviation value of the modular multilevel converter based on the actual DC bus voltage and the DC bus voltage reference value. The weight calculation module is used to obtain the weights of the CPS modulation signal and the NLM modulation signal based on the DC bus voltage deviation and power fluctuation values ​​in the modular multilevel converter. The modulation signal acquisition module is used to mix the weights of the CPS modulation signal and the NLM modulation signal in real time, synthesize the PWM waveform, generate the PWM drive signal, and input it to each sub-module in the modular multilevel converter for modulation. Weights of the CPS modulated signal in the weight calculation module for: in, This is the curve steepness coefficient of the Logistic function. The threshold for power fluctuation; This represents the real-time power fluctuation value. This represents the deviation value of the real-time DC bus voltage; The DC voltage influence coefficient is used; the weight of the NLM modulated signal is 1- ; The synthesized PWM waveform in the modulation signal acquisition module is as follows: in, This is the synthesized PWM waveform; It is a CPS modulated signal; It is an NLM modulated signal.

5. The MMC hybrid modulation system according to claim 4, characterized in that, The power calculation module includes a first data acquisition unit, a filtering unit, a first coordinate transformation unit, a second coordinate transformation unit, and a power acquisition unit; The first data acquisition unit is used to acquire the three-phase AC voltage and three-phase AC current on the grid side of the modular multilevel converter in real time; The filtering unit is used to filter the three-phase AC voltage and three-phase AC current of the modular multilevel converter grid side acquired in real time using a finite impulse response filter. The first coordinate transformation unit is used to transform the filtered three-phase AC voltage and three-phase AC current from the three-phase stationary coordinate system to the two-dimensional stationary coordinate system through Clarke transformation. The second coordinate transformation unit is used to transform the current and voltage in the two-dimensional stationary coordinate system to the synchronous rotating coordinate system using the Park transformation. The power acquisition unit is used to calculate active power in real time based on the voltage and current components in a synchronous rotating coordinate system.

6. The MMC hybrid modulation system according to claim 4 or 5, characterized in that, The method for obtaining power fluctuation values ​​in the power fluctuation calculation module is as follows: The active power data is smoothed using the sliding window averaging method, and the power fluctuation value is calculated based on the power average value and active power within the time window.

Images