MMC hybrid modulation method and system of flexible DC converter

Abstract

The invention belongs to the technical field of flexible DC power transmission, and particularly discloses an MMC hybrid modulation method and system for a flexible DC converter, and the method specifically comprises the steps: enabling a three-phase AC voltage and a three-phase AC current at a grid side to be converted into a synchronous rotating coordinate system through coordinates, and calculating the active power in real time; obtaining a power fluctuation value and a DC bus voltage deviation value of the modular multilevel converter; obtaining weights of the CPS modulation signal and the NLM modulation signal according to a DC bus voltage deviation value and a power fluctuation value in the modular multilevel converter; and mixing the weights 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 sub-module in the modular multilevel converter for modulation. According to the invention, the dynamic performance and the operation efficiency of the MMC system are improved, and the loss in the energy transmission process is reduced.

Description

Technical Field

[0001] The present application belongs to the field of flexible direct current transmission technology, and more specifically, relates to an MMC hybrid modulation method and system for a flexible direct current converter. Background Art

[0002] With the large-scale integration of renewable energy and the increasing proportion of DC loads, 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. MMCs are the core topology of flexible DC converters. Traditional control methods include the following: Carrier Phase Shift (CPS) technology adjusts the carrier signal phase of each inverter unit or module to stagger their switching operations, thereby improving the overall output waveform. Nearest Level Modulation (NLM) technology generates the output waveform by selecting the level closest to the reference waveform. These technologies are characterized by simple implementation, low computational complexity, and low switching frequency. However, both designs currently employ fixed modes and are unable to adapt to dynamic operating conditions such as power fluctuations and DC voltage deviations, resulting in difficulties in balancing efficiency and harmonic suppression. In highly dynamic scenarios with rapidly changing power, the existing NLM modulation method may not respond promptly, resulting in unstable power output and affecting the system's dynamic performance. In steady-state operation, CPS modulation technology, due to its high switching frequency, increases switching losses, reduces system efficiency, and may also produce suboptimal harmonic distortion (THD). Therefore, a method that can flexibly switch the modulation method based on the system's real-time operating conditions is needed to adapt to different operating scenarios and improve overall system performance. Summary of the Invention

[0003] In response to the defects of the existing technology, the purpose of this application is to provide an MMC hybrid modulation method and system for a flexible DC converter, aiming to solve the problem that the current CPS and NLM adopt a fixed mode and cannot adapt to dynamic working conditions such as MMC power fluctuations and DC voltage deviations, resulting in difficulty in balancing efficiency and harmonic suppression.

[0004] To achieve the above objectives, in a first aspect, the present application provides an MMC hybrid modulation method for a flexible DC converter, which specifically includes the following steps: Step S1: transforming the three-phase AC voltage and three-phase AC current on the grid side of the modular multilevel converter into a synchronous rotating coordinate system, and calculating the active power in real time; Step S2: obtaining a power fluctuation value of the modular multilevel converter through active power, and obtaining a DC bus voltage deviation value of the modular multilevel converter according to an actual DC bus voltage of the modular multilevel converter and a DC bus voltage reference value; Step S3: Obtaining weights 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; Step S4: real-time mixing of the weights of the CPS modulation signal and the NLM modulation signal, synthesizing a PWM waveform to generate a PWM drive signal which is input to each submodule in the modular multilevel converter for modulation.

[0005] Further preferably, in step S3, the weight of the CPS modulation signal for:

[0006] in, is the curve steepness coefficient of the Logistic function, is the threshold value of power fluctuation; is the real-time power fluctuation value; is the deviation value of the real-time DC bus voltage; is the DC voltage influence coefficient; the weight of the NLM modulation signal is 1- .

[0007] Further preferably, in step S4, the synthesized PWM waveform is:

[0008] in, is the synthesized PWM waveform; is the CPS modulation signal; It is the NLM modulation signal.

[0009] Further preferably, step S1 specifically includes the following steps: Step S1.1: Filtering the three-phase AC voltage and three-phase AC current of the modular multilevel converter grid side collected in real time using a finite impulse response filter; Step S1.2: Convert 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 convert them to the synchronously rotating coordinate system using Park transformation; Step S1.3: Calculate the active power in real time based on the voltage component and the current component in the synchronous rotating coordinate system.

[0010] Further preferably, the method for obtaining the power fluctuation value in step S2 is: 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.

[0011] In a second aspect, the present application provides an MMC hybrid modulation system for a flexible DC converter, comprising: 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 into a synchronous rotating coordinate system and calculate the active power in real time; A power fluctuation calculation module, used to obtain a power fluctuation value of the modular multilevel converter through active power; A DC bus voltage deviation calculation module is used to obtain a DC bus voltage deviation value of the modular multilevel converter based on an actual DC bus voltage of the modular multilevel converter and a DC bus voltage reference value; A weight calculation module is used to obtain the weights 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; 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 to generate a PWM drive signal, and input it into each sub-module in the modular multilevel converter for modulation.

[0012] Further preferably, the weight of the CPS modulation signal in the weight calculation module is for:

[0013] in, is the curve steepness coefficient of the Logistic function, is the threshold value of power fluctuation; is the real-time power fluctuation value; is the deviation value of the real-time DC bus voltage; is the DC voltage influence coefficient; the weight of the NLM modulation signal is 1- .

[0014] Further preferably, the PWM waveform synthesized in the modulation signal acquisition module is:

[0015] in, is the synthesized PWM waveform; is the CPS modulation signal; It is the NLM modulation signal.

[0016] Further 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; The first data acquisition unit is used to collect 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 collected 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 a three-phase stationary coordinate system to a 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 into a synchronous rotating coordinate system using Park transformation; The power acquisition unit is used to calculate the active power in real time based on the voltage component and the current component in the synchronous rotating coordinate system.

[0017] Further preferably, the method for obtaining the power fluctuation value in the power fluctuation calculation module is: 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.

[0018] In a third aspect, the present application provides an electronic device comprising: at least one memory for storing programs; and at least one processor for executing the programs stored in the memory. When the program stored in the memory is executed, the processor is used to execute the method described in the first aspect or any possible implementation of the first aspect.

[0019] In a fourth aspect, the present application provides a computer-readable storage medium, which stores a computer program. When the computer program runs on a processor, the processor executes the method described in the first aspect or any possible implementation of the first aspect.

[0020] In a fifth aspect, the present application provides a computer program product, which, when executed on a processor, enables the processor to execute the method described in the first aspect or any possible implementation of the first aspect.

[0021] It can be understood that the beneficial effects of the second to fifth aspects mentioned above can be found in the relevant description of the first aspect mentioned above, and will not be repeated here.

[0022] In general, the above technical solutions conceived by this application have the following beneficial effects compared with the existing technologies: This application provides a hybrid MMC modulation method for a flexible DC converter. By real-time monitoring of MMC power and DC bus voltage signals and combining it with a dynamic weight allocation algorithm, the two modulation techniques are seamlessly integrated. This method enables flexible switching between CPS modulation and LIM modulation to adapt to different dynamic operating conditions, improving the system's response speed, efficiency, and power quality. More specifically, in scenarios with rapidly changing power, the rapid response capability of CPS modulation enables timely adjustment of power output, ensuring a system response within 10ms (a 60% improvement over traditional NLM modulation). This mitigates the impact of power fluctuations on MMC system stability, enhances the adaptability and reliability of the flexible DC transmission system in the face of sudden load changes, and improves the dynamic performance of the MMC system. During steady-state operation, the NLM mode reduces switching losses in power devices by lowering the switching frequency (compared to the pure CPS mode, the switching losses are reduced by 22%). This loss reduction is particularly significant under high-frequency switching conditions, thereby improving the operating efficiency of the entire MMC system, reducing losses during energy transmission, and conserving energy.

[0023] The present application provides an MMC hybrid modulation method for a flexible DC converter, which optimizes the power quality. In the NLM mode, the number of levels is dynamically adjusted according to the power demand, which can further reduce the harmonic distortion (THD) rate, make the output voltage waveform closer to the ideal sine wave, reduce the impact of harmonics on the power grid and load, improve the power quality, and meet the needs of application scenarios with high requirements for power quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a schematic diagram of the redundant submodule (MMC) circuit topology provided in an embodiment of the present application; Figure 2 This is a flow chart of signal acquisition and modulation mode switching provided by an embodiment of the present application; Figure 3 is a dynamic weight distribution curve diagram provided in an embodiment of the present application; Figure 4 This is a schematic diagram of hybrid modulation PWM generation provided in an embodiment of the present application. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical solutions and advantages of this application more clearly understood, the following further describes this application in detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0026] The term "and / or" as used herein describes an association between related objects, indicating that three possible relationships exist. For example, "A and / or B" can represent: A exists alone, A and B exist simultaneously, or B exists alone. The symbol " / " as used herein indicates that the related objects are in an "or" relationship, for example, A / B means either A or B.

[0027] The terms "first" and "second" and the like in the description and claims herein are used to distinguish different objects rather than to describe a specific order of the objects.

[0028] In the embodiments of this application, words such as "exemplary" or "for example" are used to indicate examples, illustrations, or descriptions. Any embodiment or design described as "exemplary" or "for example" in the embodiments of this application should not be interpreted as being preferred or advantageous over other embodiments or designs. Rather, the use of words such as "exemplary" or "for example" is intended to present the relevant concepts in a concrete manner.

[0029] In the description of the embodiments of the present application, unless otherwise specified, “plurality” means two or more.

[0030] The embodiments of the present application are described below in conjunction with the drawings in the embodiments of the present application.

[0031] The present application provides an MMC hybrid modulation method for a flexible DC converter, comprising the following steps: Step 1: Signal acquisition and preprocessing: The three-phase AC voltage and current signals on the grid side, as well as the DC bus voltage signal, are collected in real time. The collected signals are filtered using a finite impulse response (FIR) filter to remove high-frequency noise interference and improve signal accuracy. Step 2: Coordinate transformation: The three-phase AC voltage and current signals are transformed from the three-phase stationary coordinate system to the two-dimensional stationary coordinate system through Clarke transformation ( ); Then, the Park transform is used to further transform the signal into a synchronous rotating coordinate system ( ), in order to facilitate subsequent functional calculation and analysis; Step 3: Power calculation: In the synchronous rotating coordinate system, according to the formula , calculate the real-time active power P, and according to the formula Calculating reactive power Q , so as to fully grasp the power operation status of the system; among them, For synchronous rotation coordinate system q Shaft voltage component; For synchronous rotation coordinate system d Shaft voltage component; Synchronously rotating coordinate system qShaft current component; For synchronous rotation coordinate system d Shaft current component; Step 4: Power data smoothing: Use the sliding window averaging method to smooth the instantaneous power data, reduce the impact of random fluctuations and noise on the power measurement results, and improve the stability and reliability of the power data; Step 5: Calculation of power fluctuation and DC bus voltage deviation: Calculate power fluctuation value , and its calculation formula is: ,in, is the average power within the time window, that is ; is the kth active power value calculated within the time window; N is the total number of active powers calculated within the time window; at the same time, the deviation value of the DC bus voltage is calculated , ;in, is the actual DC bus voltage; is the reference value of the DC bus voltage; Step 6: Modulation mode dynamic weight allocation switching method: based on the calculated power fluctuation value and DC bus voltage deviation , formulate the switching method of the modulation mode, and its expression is:

[0032] in, is the curve steepness coefficient of the Logistic function, is the threshold value of power fluctuation; is the real-time power fluctuation value; is the deviation value of the real-time DC bus voltage; is the DC voltage influence coefficient; In a high-dynamic scenario with rapid power changes, ≥15% or When, take , forced into CPS modulation to achieve the purpose of fast and stable output; The CPS modulated signal and the NLM modulated signal are weighted and 1- Perform real-time mixing, and the expression is:

[0033] Real-time computing in FPGA The weighted synthesized PWM waveform generates a unified PWM drive signal and provides it to each sub-module to drive each module to work.

[0034] Example 1 This application provides an MMC hybrid modulation system for a flexible DC converter, such as Figure 1 As shown in Figure 1, each bridge arm of the MMC consists of N full-bridge sub-modules and a bridge arm inductor L in series. The NLM determines the number of sub-modules to be put into operation by rounding the ratio of the reference voltage to the sub-module (SM) capacitor voltage so that the output voltage is closest to the reference voltage. The CPS modulation assigns a phase-shifted carrier to each sub-module with a phase difference of (N is the number of submodules), and compared with the same modulation wave to generate a PWM signal; after superposition, the equivalent switching frequency is increased by N times, reducing harmonics; Figure 2 The flow chart 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, and then high-frequency noise is removed by FIR filtering; then Clarke transform and Park transform are performed to convert the signal to a synchronous rotating coordinate system to calculate active power and reactive power; then the sliding window averaging method is used to smooth the instantaneous power data, and the power fluctuation value and DC bus voltage deviation value are calculated; finally, according to these parameters, Calculation function generates weight coefficients , the CPS modulated signal and the NLM modulated signal are weighted and 1- Perform real-time mixing and synthesize PWM waveforms to generate unified PWM drive signals and provide them to each sub-module to drive each module to work.

[0035] Figure 3 The horizontal axis is the power fluctuation value (0%~20%), the vertical axis is the weight coefficient (0~1), the three curves are =0% (blue), =5% (orange) and =10% (gray) when weight coefficient The curve of The function parameter k=30, =0.3, Figure 3 Shown right the impact of; Figure 4 The CPS modulation waveform is generated by comparing the phase-shifted carrier with the modulation waveform; the NLM multi-level step wave modulation waveform; and different Waveform superposition effect under values (0.2, 0.5, 0.8) mixed PWM waveform; reflects the hybrid logic and dynamic weight of CPS and NLM regulation effect.

[0036] Example 2 Taking an actual flexible direct current transmission system as an example, this system adopts the MMC hybrid modulation method proposed in this application. In this system, the three-phase AC voltage on the grid side is 380V, the frequency is 50Hz, and the DC bus voltage is rated at 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 for filtering the collected signals to remove high-frequency noise interference. In the signal processing process, the three-phase voltage and current signals are first converted from the three-phase stationary coordinate system to the stationary coordinate system through Clarke transformation ( ); Then, the Park transform is used to further transform the signal into a synchronous rotating coordinate system ( ), in order to facilitate subsequent power calculation and analysis; in the synchronous rotating coordinate system, according to the formula , calculate the real-time active power P, and according to the formula Calculating reactive power Q , so as to fully grasp the power operation status of the system; In order to reduce the impact of random fluctuations and noise on power measurement results, the sliding window averaging method is used to smooth the instantaneous power data; by reasonably setting the length of the sliding window, the stability of the power data is guaranteed while retaining the trend of power changes. On this basis, the power fluctuation value is calculated and DC bus voltage deviation ;Power fluctuation value The calculation formula is ,in, is the average power within the time window, that is ; At the same time, calculate the deviation value of DC bus voltage , ;in, is the actual DC bus voltage; is the reference value of the DC bus voltage; According to the calculated power fluctuation value and DC bus voltage deviation , formulate the switching method of the modulation mode, and its expression is:

[0037] Wherein, k is the steepness coefficient of the logistic function curve, and k=30; is the power fluctuation threshold, which is 8%; is the calculated real-time power fluctuation value; is the deviation value of the real-time DC bus voltage; is the DC voltage influence coefficient, which is calibrated to 0.3 through experiments; In a high-dynamic scenario with rapid power changes, ≥15% or When, take , forced into CPS modulation to achieve the purpose of fast and stable output; In other cases, the CPS modulated signal and the NLM modulated signal are weighted and 1- Perform real-time mixing, and the expression is:

[0038] Real-time computing in FPGA The weighted synthesized PWM waveform generates a unified PWM drive signal and provides it to each sub-module to drive each module to work.

[0039] The system using the MMC hybrid modulation method proposed in this application has a stable dynamic response time of less than 10ms in scenarios with rapid power changes, effectively avoiding the impact of power fluctuations on system stability; and in steady-state operation, switching losses are significantly reduced, system efficiency is improved, harmonic distortion (THD) is optimized, the output voltage waveform is purer, and the power quality is significantly improved; these results fully demonstrate the effectiveness and superiority of the method of this application, and can meet the requirements of modern flexible direct current transmission systems for high-performance operation.

[0040] The MMC hybrid modulation system of the flexible DC converter provided in the present application is described below. The MMC hybrid modulation system of the flexible DC converter described below and the MMC hybrid modulation method of the flexible DC converter described above can be referred to in correspondence with each other.

[0041] The present application provides an MMC hybrid modulation system for a flexible DC converter, comprising: 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 into a synchronous rotating coordinate system and calculate the active power in real time; A power fluctuation calculation module, configured to obtain a power fluctuation value of the modular multilevel converter based on active power; A DC bus voltage deviation calculation module is used to obtain a DC bus voltage deviation value of the modular multilevel converter based on an actual DC bus voltage of the modular multilevel converter and a DC bus voltage reference value; A weight calculation module is used to obtain the weights 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; 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 to generate a PWM drive signal, and input it into each sub-module in the modular multilevel converter for modulation.

[0042] Further preferably, the weight of the CPS modulation signal in the weight calculation module is for:

[0043] in, is the curve steepness coefficient of the Logistic function, is the threshold value of power fluctuation; is the real-time power fluctuation value; is the deviation value of the real-time DC bus voltage; is the DC voltage influence coefficient; the weight of the NLM modulation signal is 1- .

[0044] Further preferably, the PWM waveform synthesized in the modulation signal acquisition module is:

[0045] in, is the synthesized PWM waveform; is the CPS modulation signal; It is the NLM modulation signal.

[0046] Further 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; The first data acquisition unit is used to collect 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 collected 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 a three-phase stationary coordinate system to a 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 into a synchronous rotating coordinate system using Park transformation; The power acquisition unit is used to calculate the active power in real time based on the voltage component and the current component in the synchronous rotating coordinate system.

[0047] Further preferably, the method for obtaining the power fluctuation value in the power fluctuation calculation module is: 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.

[0048] In summary, compared with the prior art, this application has the following advantages: The present application provides an MMC hybrid modulation method for a flexible DC converter, which realizes seamless integration of the two modulation technologies by real-time monitoring of the power and DC bus voltage signals of the MMC and combining a dynamic weight distribution algorithm, thereby realizing flexible switching between the CPS modulation method and the LIM modulation method to adapt to different dynamic working conditions and improve the response speed, efficiency and power quality of the system. More specifically, in the scenario of rapid power changes, the rapid response capability of the CPS modulation of the present application can adjust the power output in a timely manner, ensure that the system responds within 10ms, avoid the impact of power fluctuations on system stability, enhance the adaptability and reliability of the flexible DC transmission system in the face of load mutations, and improve the dynamic performance of the system. During steady-state operation, the NLM mode reduces the switching loss of the power device by reducing the switching frequency. In particular, in the case of high-frequency switching, this loss reduction is more significant, thereby improving the operating efficiency of the entire MMC system, reducing the loss during energy transmission, and saving energy.

[0049] The present application provides an MMC hybrid modulation method for a flexible DC converter, which optimizes the power quality. In the NLM mode, the number of levels is dynamically adjusted according to the power demand, which can further reduce the harmonic distortion (THD) rate, make the output voltage waveform closer to the ideal sine wave, reduce the impact of harmonics on the power grid and load, improve the power quality, and meet the needs of application scenarios with high requirements for power quality.

[0050] The above description is merely a specific embodiment of the present application, but the scope of protection of the present application is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in this application should be included in the scope of protection of this application. Therefore, the scope of protection of this application should be based on the scope of protection of the claims.

Claims

1. An MMC hybrid modulation method for a flexible DC converter, characterized in that: The following steps are involved: Step S1: transforming the three-phase AC voltage and three-phase AC current on the grid side of the modular multilevel converter into a synchronous rotating coordinate system, and calculating the active power in real time; Step S2: obtaining a power fluctuation value of the modular multilevel converter through active power, and obtaining a DC bus voltage deviation value of the modular multilevel converter according to an actual DC bus voltage of the modular multilevel converter and a DC bus voltage reference value; Step S3: Obtaining weights 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; Step S4: real-time mixing of the weights of the CPS modulation signal and the NLM modulation signal, synthesizing a PWM waveform to generate a PWM drive signal which is input to each submodule in the modular multilevel converter for modulation.

2. The MMC hybrid modulation method according to claim 1, characterized in that: In step S3, the weight of the CPS modulated signal for: in, is the curve steepness coefficient of the Logistic function, is the threshold of power fluctuation; is the real-time power fluctuation value; is the deviation value of the real-time DC bus voltage; is the DC voltage influence coefficient; the weight of the NLM modulation signal is 1- .

3. The MMC hybrid modulation method according to claim 2, characterized in that: In step S4, the synthesized PWM waveform is: in, is the synthesized PWM waveform; is the CPS modulation signal; It is the NLM modulation signal.

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

5. The MMC hybrid modulation method according to claim 1 or 4, characterized in that: The method for obtaining the power fluctuation value in step S2 is: 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.

6. An MMC hybrid modulation system for a flexible 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 into a synchronous rotating coordinate system and calculate the active power in real time; A power fluctuation calculation module, used to obtain a power fluctuation value of the modular multilevel converter through active power; A DC bus voltage deviation calculation module is used to obtain a DC bus voltage deviation value of the modular multilevel converter based on an actual DC bus voltage of the modular multilevel converter and a DC bus voltage reference value; A weight calculation module is used to obtain the weights 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; 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 to generate a PWM drive signal, and input it into each sub-module in the modular multilevel converter for modulation.

7. The MMC hybrid modulation system according to claim 6, characterized in that: Weight of CPS modulation signal in weight calculation module for: in, is the curve steepness coefficient of the Logistic function, is the threshold of power fluctuation; is the real-time power fluctuation value; is the deviation value of the real-time DC bus voltage; is the DC voltage influence coefficient; the weight of the NLM modulation signal is 1- .

8. The MMC hybrid modulation system according to claim 7, characterized in that: The PWM waveform synthesized in the modulation signal acquisition module is: in, is the synthesized PWM waveform; is the CPS modulation signal; It is the NLM modulation signal.

9. The MMC hybrid modulation system according to claim 6, 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 collect 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 collected 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 a three-phase stationary coordinate system to a 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 into a synchronous rotating coordinate system using Park transformation; The power acquisition unit is used to calculate the active power in real time based on the voltage component and the current component in the synchronous rotating coordinate system.

10. The MMC hybrid modulation system according to claim 6 or 9, characterized in that: The method for obtaining the power fluctuation value in the power fluctuation calculation module is: 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.

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