Method and device for calculating equivalent electrical stress of mmc valve and storage medium

Abstract

The present application relates to a kind of MMC valve electric stress equivalent calculation method, device and storage medium, wherein method includes: step S1: based on electromagnetic transient simulation obtains the time-domain waveform of each working condition under flexible converter valve bridge arm current;Step S2: select the time-domain waveform of any phase bridge arm current is obtained by windowed Fourier transform in preset time window, and further extract direct current component and fundamental frequency alternating current component;Step S3: draw electric stress curve;Step S4: based on the electric stress curve under each working condition respectively statistics first eigenvalue of the first characteristic value of the electric stress level of direct current component and the second eigenvalue of the electric stress level of fundamental frequency alternating current component;Step S5: obtain test direct current value and test alternating current value after combining safety factor.Compared with prior art, the present application accurately quantifies the electric stress requirement of flexible converter valve from system operating condition without relying on large-scale physical test.

Description

Technical Field

[0001] This invention relates to the field of power system simulation technology, and in particular to a method, apparatus and storage medium for equivalent calculation of electrical stress of an MMC valve. Background Technology

[0002] With the continuous expansion of offshore wind power, long-distance, high-capacity power transmission between offshore wind farms and onshore power grids often adopts flexible DC transmission systems based on modular multilevel converters (MMCs). As the core equipment of flexible DC transmission systems, the flexible DC converter valve's arms and submodules will withstand complex electrical stresses under various operating conditions, including rated operation, overload operation, and AC / DC short-circuit faults. The level of electrical stress directly affects the safe and reliable operation of the converter valve and the entire system. In existing projects, the performance of flexible DC converter valves is often assessed through a combination of factory type testing and on-site engineering testing. However, due to limitations such as high voltage levels, large capacity, and testing risks and costs, it is difficult to comprehensively cover various operating conditions under real voltage and current levels. In particular, the electrical stress under fault and extreme conditions is often difficult to fully obtain through direct testing.

[0003] To fully validate the novel flexible DC converter valve before project implementation, it is necessary to construct a pilot-scale platform for the valve to conduct long-term equivalent operational tests under conditions close to the engineering parameters. The test voltage, current levels, and functional configuration of the pilot-scale platform must accurately reflect the electrical stress level experienced by the converter valve in the target project. Therefore, it is essential to first quantitatively obtain the electrical stress indicators of the flexible DC converter valve under typical operating and fault conditions at the system level. Relevant standards specify the items and requirements for insulation and operational tests of flexible DC converter valves. However, while current test standards specify typical operating conditions and items, they lack methods for quantitatively obtaining the electrical stress of the converter valve from system operation behavior. Existing equivalent test methods mostly focus on the design of test circuits and topologies, generating the required voltage and current stress through test power supplies, power offsets, or series resonances. They lack a systematic method that starts from the overall simulation of the offshore wind power flexible DC transmission system, performs frequency domain analysis and equivalent processing of the arm current under multiple operating conditions, and determines the test current requirements of the pilot-scale platform accordingly.

[0004] Therefore, it is necessary to propose an equivalent calculation method for electrical stress of MMC valves. Based on the electromagnetic transient simulation model, the current waveform of the converter valve bridge arm is obtained under various representative operating conditions. The electrical stress curves of key components are extracted through frequency domain analysis and digital filtering, thereby forming an equivalent electrical stress index that can be used to guide the test current configuration and functional requirement design of the pilot platform of flexible DC converter valves. Summary of the Invention

[0005] The purpose of this invention is to provide a method, device, and storage medium for equivalent calculation of electrical stress in MMC valves. This method can accurately quantify the electrical stress requirements of flexible DC converter valves based on system operating conditions without relying on large-scale physical tests. It provides a basis for pilot-scale platform capacity configuration and equipment selection, avoids insufficient or excessive test conditions, and improves the rationality and economy of pilot-scale platform design.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] A method for equivalent calculation of electrical stress in an MMC valve includes:

[0008] Step S1: Obtain the time-domain waveform of the flexible DC converter valve arm current under various operating conditions based on electromagnetic transient simulation;

[0009] Step S2: Select the time-domain waveform of any phase arm current and perform windowed Fourier transform within a preset time window to obtain the spectrum components, and further extract the DC component and the fundamental frequency AC component.

[0010] Step S3: Plot the electrical stress curve based on the extracted DC component and fundamental frequency AC component;

[0011] Step S4: Based on the electrical stress curves under each operating condition, statistically analyze the first characteristic value representing the DC component electrical stress level and the second characteristic value representing the fundamental frequency AC component electrical stress level.

[0012] Step S5: Multiply the first characteristic value and the second characteristic value by the safety factor to obtain the test DC current value and the test AC current value.

[0013] Step S1 includes:

[0014] Step S1-1: Build a simulation model of the offshore wind power flexible DC transmission system in the electromagnetic transient simulation platform;

[0015] Step S1-2: Set up multiple representative operating conditions in the simulation model and run the simulation, and record the time-domain waveform of the flexible DC converter valve arm current under each operating condition.

[0016] The offshore wind power flexible DC transmission system includes:

[0017] Offshore wind farms;

[0018] Offshore MMC station connected to offshore wind farm;

[0019] A land-based MMC station connected to the onshore AC power grid;

[0020] DC transmission lines connecting offshore MMC stations and onshore MMC stations;

[0021] The offshore wind power flexible DC transmission system adopts a two-port offshore wind power transmission scheme. The offshore wind farm is collected by the grid and sent to the offshore MMC station through the DC transmission line and then connected to the onshore AC grid.

[0022] Step S2 includes:

[0023] Step S2-1: Select the time-domain waveform of the bridge arm current of any phase's upper or lower bridge arm;

[0024] Step S2-2: For the time-domain waveform of the selected bridge arm current, extract the current waveform within a preset time window and multiply it by a preset window function to obtain the spectral components;

[0025] Step S2-3: Based on the obtained spectral components, further extract the DC component and the fundamental frequency AC component.

[0026] The spectral components are:

[0027]

[0028] in: For the first m Spectral components at each frequency point N For the sample size, This refers to the nth sample value of the discrete sequence of bridge arm currents obtained within the specified time window. This is the preset window function.

[0029] The DC component is:

[0030]

[0031] in: For the DC component of the k-th operating condition, The frequency component at frequency point m=0 corresponds to the DC component of the bridge arm current.

[0032] The communication components are:

[0033]

[0034] in: Let h be the AC component of the h-th harmonic. When h is 1, it represents the fundamental frequency AC component. This is the set of frequency point indices corresponding to the h-th harmonic frequency band.

[0035] The first characteristic value is the maximum value of the DC component current within a preset time window, and the second characteristic value is the effective value of the maximum value of the fundamental frequency AC component current.

[0036] The safety factor is 1.05.

[0037] An equivalent electrical stress calculation device for an MMC valve includes a memory, a processor, and a program stored in the memory, wherein the processor executes the program to implement the method described above.

[0038] A storage medium having a program stored thereon, which, when executed, implements the method described above.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] 1. A model of a flexible DC transmission system for offshore wind power was built on an electromagnetic transient simulation platform. The time-domain waveforms of the flexible DC converter valve arm current were obtained under various representative operating conditions, including rated operating conditions, overload operating conditions, and AC / DC faults. Compared with the method of estimating based solely on nameplate parameters or a single operating condition, this method can more comprehensively and objectively reflect the electrical stress level of the converter valve in actual engineering applications, providing a reliable data basis for setting test conditions on the pilot platform.

[0041] 2. By combining windowed Fourier transform and digital filtering, frequency domain analysis and time domain decomposition of the bridge arm current are performed. The DC component and fundamental frequency AC component are clearly selected as the main equivalent components, and the corresponding electrical stress curves are constructed. Compared with the method of only examining the total current or simple peak value, it can distinguish the different effects of DC and AC components on the device and sub-module, making the configuration of the test DC current value and test AC current value of the pilot platform more targeted.

[0042] 3. Based on the electrical stress curves of each working condition, the characteristic values ​​representing the electrical stress levels of the DC component and the fundamental frequency AC component are statistically analyzed, and an equivalent electrical stress index is formed. Compared with the existing technology of setting the test current according to a single working condition or empirical coefficient, this method can systematically take into account the stress requirements under various typical and extreme working conditions, and avoid insufficient coverage due to small test conditions or redundant equipment selection due to large test conditions.

[0043] 4. The equivalent electrical stress index directly provides the test DC current value and test AC current value of the pilot platform for the flexible DC converter valve. It can be combined with the safety factor for the design of key parameters such as power supply capacity, test current level and valve cooling configuration. Compared with the method of selecting test conditions by relying on experience or analog engineering, it significantly improves the rationality and economy of the pilot platform design. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of the main steps of the method of the present invention;

[0045] Figure 2 This is a schematic diagram of the offshore wind power flexible DC transmission system in an embodiment of the present invention;

[0046] Figure 3 This is a schematic diagram of the flexible DC converter valve bridge arm structure and the selection position of the bridge arm current in an embodiment of the present invention;

[0047] Figure 4 This is a schematic diagram of the time-domain waveform of the flexible DC converter valve arm current under rated operating conditions in an embodiment of the present invention.

[0048] Figure 5 This is a schematic diagram of the current spectrum of the flexible DC converter valve bridge arm under rated operating conditions in an embodiment of the present invention;

[0049] Figure 6 These are the DC component and fundamental frequency component of the flexible DC converter valve arm current under the maximum continuous operating load test condition in this embodiment of the invention.

[0050] Figure 7 These are the DC component and fundamental frequency component of the flexible DC converter valve arm current under the maximum transient overload operation test condition in this embodiment of the invention.

[0051] Figure 8 These are the DC component and fundamental frequency component of the flexible DC converter valve arm current under DC-side fault conditions in this embodiment of the invention.

[0052] Figure 9 These are the DC component and fundamental frequency component of the flexible DC converter valve arm current under AC side fault conditions in this embodiment of the invention. Detailed Implementation

[0053] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0054] Example 1

[0055] A method for equivalent calculation of electrical stress in MMC valves, such as Figure 1 As shown, it includes:

[0056] Step S1: Obtain the time-domain waveforms of the flexible DC converter valve arm currents under various operating conditions based on electromagnetic transient simulation, including:

[0057] Step S1-1: Build a simulation model of the offshore wind power flexible DC transmission system in the electromagnetic transient simulation platform;

[0058] like Figure 2 As shown, the offshore wind power flexible DC transmission system includes:

[0059] Offshore wind farms;

[0060] Offshore MMC station connected to offshore wind farm;

[0061] A land-based MMC station connected to the onshore AC power grid;

[0062] DC transmission lines connecting offshore MMC stations and onshore MMC stations;

[0063] The main parameters of the system are shown in Table 1. The offshore wind power flexible DC transmission system adopts a two-port offshore wind power transmission scheme. The offshore wind farm is collected by the grid and sent to the offshore MMC station through the DC transmission line and connected to the onshore AC grid.

[0064] Table 1

[0065]

[0066] Figure 2 middle, This is the effective value of the AC mains voltage. For the converter transformer inductance, For line inductance, This is the effective value of the output voltage of MMC1. This is the effective value of the MMC2 output voltage. This refers to the DC side current of the MMC. This is the DC side voltage of the MMC. This represents the effective value of the output voltage of the doubly-fed induction wind turbine. This represents the effective value of the output voltage of a direct-drive permanent magnet synchronous wind turbine generator. The active power delivered to the AC system The reactive power supplied to the AC system The active power delivered by the doubly-fed induction generator. The reactive power delivered by the doubly-fed induction wind turbine. The active power delivered by the direct-drive permanent magnet synchronous wind turbine. The reactive power delivered by the direct-drive permanent magnet synchronous wind turbine.

[0067] Step S1-2: Set up multiple representative operating conditions in the simulation model and run the simulation, and record the time-domain waveform of the flexible DC converter valve arm current under each operating condition.

[0068] The simulation step size was set to 50 microseconds. Based on relevant test standards, several representative operating conditions were selected, including at least the rated operating condition, the maximum continuous operating load condition, the maximum transient overload operating condition, the DC-side short-circuit fault condition, and the AC-side short-circuit fault condition. In the maximum continuous operating load condition, the total active power of the wind farm was set to 1.05 times the rated power, and the DC-side voltage was set to 1.05 times the rated DC voltage. In the maximum transient overload operating condition, after the system reached rated steady state, the total active power of the wind farm was increased to 1.10 times the rated power within a set time, and the DC-side voltage was also increased to 1.10 times the rated DC voltage. The DC-side short-circuit fault condition was set as a single-pole ground fault on the DC side, and the AC-side short-circuit fault condition was set as a three-phase ground fault in the line between the offshore MMC station and the high-voltage AC cable.

[0069] Step S2: Select the time-domain waveform of any phase arm current, perform a windowed Fourier transform within a preset time window to obtain the spectral components, and further extract the DC component and the fundamental frequency AC component.

[0070] The spectral components include DC and AC components. In this application, the DC component and the fundamental frequency AC component are selected as the main equivalent components characterizing the electrical stress of the bridge arm current. Due to the use of circulating current suppression, the second harmonic current component is very small and can be ignored. Since the bridge arm currents of each phase are approximately the same under symmetrical operating conditions, any phase current is representative. The bridge arm current can be selected as the upper or lower bridge arm current of any phase in an offshore MMC station or an onshore MMC station, such as... Figure 3 As shown, in Embodiment 1 of the present invention, the upper arm current of phase A of the offshore MMC station is used as an example. For example.

[0071] Figure 3 middle, u a The AC phase voltage of phase A on the MMC valve side. u b This refers to the AC phase voltage of phase B on the MMC valve side. u c This refers to the AC phase voltage of phase C on the MMC valve side. u a1 This refers to the voltage of phase A of the upper arm of the MMC bridge. u a2 This refers to the voltage of phase A of the lower arm of the MMC bridge. i a For the A-phase AC current of the MMC valve, i b For the B-phase AC current of the MMC valve, i c For the C-phase AC current of the MMC valve, i a1This refers to the A-phase current of the upper arm of the MMC bridge. SM2 is the second submodule. n For the nth submodule, R To simulate the equivalent resistance of the MMC bridge arm and converter transformer losses, L For the inductance of the bridge arm reactor, i a2 This refers to the A-phase current of the lower bridge arm of the MMC, where SM1 is the first submodule. i b1 This refers to the B-phase current of the upper arm of the MMC. u b1 This refers to the voltage of phase B of the upper arm of the MMC bridge. u b2 This refers to the voltage of phase B of the lower arm of the MMC bridge. i b2 This refers to the B-phase current of the lower arm of the MMC. i c1 This refers to the C-phase current of the upper arm of the MMC. u c1 The voltage of phase C of the upper arm of the MMC bridge. u c2 This refers to the C-phase voltage of the lower arm of the MMC bridge. i c2 This refers to the C-phase current of the lower arm of the MMC bridge. I d This refers to the DC component of the MMC DC-side current. U d This refers to the DC component of the MMC DC-side output voltage. O This is the neutral point between the positive and negative terminals of the MMC DC voltage.

[0072] Specifically, step S2 includes:

[0073] Step S2-1: Select the time-domain waveform of the bridge arm current of any phase's upper or lower bridge arm;

[0074] Step S2-2: For the time-domain waveform of the selected bridge arm current, the current waveform within a preset time window is extracted and multiplied by a preset window function to obtain the spectral components, so as to reduce spectral leakage and obtain the spectral amplitude at DC, fundamental frequency and integer multiples of frequency.

[0075] Let the time window of condition k be... , t 0 represents the start time of the time window. T ω The time window length is and the sampling frequency is . The sample size is The current of the upper arm of phase A at the offshore MMC station is denoted as a discrete sequence within this window:

[0076]

[0077] in: Let n be the nth sample value of the discrete sequence of bridge arm currents obtained within the specified time window.

[0078] To reduce spectral leakage, discrete sequences need to be windowed. This embodiment uses the Hanning window as an example. Let the window function be... By performing a Fast Fourier Transform on the windowed discrete sequence, we can obtain:

[0079]

[0080] in: For the first m Spectral components at each frequency point N For the sample size, The preset window function, the frequency corresponding to the m-th frequency point. f m as follows:

[0081]

[0082] Step S2-3: Based on the obtained spectral components, further extract the DC component and the fundamental frequency AC component.

[0083] The DC component is:

[0084]

[0085] in: For the DC component of the k-th operating condition, The frequency component at frequency point m=0 corresponds to the DC component of the bridge arm current.

[0086] The communication components are:

[0087]

[0088]

[0089] in: For the first h The AC component of the second harmonic, when h is 1, represents the fundamental frequency AC component. Let h be the set of frequency points corresponding to the h-th harmonic band. The frequency width of the h-th harmonic band is used to define the area surrounding the center frequency of the h-th harmonic. The selection range.

[0090] The waveform of the upper arm current of phase A of the offshore MMC station under rated operating conditions is as follows: Figure 4 As shown in the figure, the horizontal axis represents time, in seconds. Performing a Fast Fourier Transform on the current waveform within this time window yields the spectrum as shown below. Figure 5As shown in the figure, the main components of the arm current under rated operating conditions are the DC component and the fundamental frequency component. Due to the use of circulating current suppression, the contribution of second-order and higher harmonics to electrical stress is relatively minor and can be ignored at the equivalent level. The same conclusion is obtained by performing a Fast Fourier Transform on the arm current under subsequent operating conditions. Therefore, the electrical stress equivalent method proposed in this application only considers the DC component and the fundamental frequency component.

[0091] Step S3: Plot the electrical stress curve based on the extracted DC component and fundamental frequency AC component;

[0092] To improve the accuracy of the acquired DC component and fundamental frequency AC component, digital filtering is required. This digital filtering process includes using a low-pass digital filter to extract the DC component current curve and a band-pass digital filter to extract the fundamental frequency AC component current curve. The cutoff frequency of the low-pass digital filter is 5Hz, and the center frequency of the band-pass digital filter is 50Hz, with a quality factor of 10.

[0093] By simulating and comparing the DC component and harmonic amplitude of the arm current under rated operating conditions, it can be seen that the main components of the arm current are the DC component and the fundamental frequency component. Therefore, the arm current can be... The inner approximation is:

[0094]

[0095] in: To represent the instantaneous current of the bridge arm at time t, The amplitude of the fundamental frequency AC component of the bridge arm current is expressed in amperes, and is used to characterize the magnitude of the fundamental frequency sinusoidal component. For the corresponding angular frequency, This is the initial phase angle of the fundamental frequency AC component.

[0096] Step S4: Based on the electrical stress curves under each operating condition, statistically analyze the first characteristic value representing the DC component electrical stress level and the second characteristic value representing the fundamental frequency AC component electrical stress level.

[0097] The first characteristic value is the maximum value of the DC component current within a preset time window, and the second characteristic value is the effective value of the maximum value of the fundamental frequency AC component current. The first characteristic value and the second characteristic value are then combined to obtain a binary tuple as the equivalent current index.

[0098] Step S5: Multiply the first characteristic value and the second characteristic value by the safety factor to obtain the test DC current value and the test AC current value.

[0099] Since the higher harmonic components with amplitudes lower than 10% of the fundamental frequency AC component amplitude are ignored in the equivalent calculation of electrical stress in step S2, the test DC current value of the pilot platform of the flexible DC converter valve is the product of the equivalent DC current index and the safety factor, and the test AC current value is the product of the equivalent AC current index and the safety factor. In this embodiment, the safety factor is 1.05.

[0100] The DC component and fundamental frequency component of the upper arm current of phase A of the offshore MMC station obtained after filtering under the maximum continuous operating load test condition, maximum transient overload operating test condition, DC side fault, and AC side fault are as follows: Figure 6 , 7 As shown in Figures 8 and 9, the horizontal axis represents time, with the unit being seconds.

[0101] Calculate the maximum value of each component during the change process, and take its effective value. The electrical stress setpoint binary for each working condition can be constructed as follows:

[0102]

[0103]

[0104] in, to It is a binary tuple with 5 different operating conditions.

[0105] The equivalent electrical stresses for each working condition are shown in Table 2.

[0106] Table 2

[0107]

[0108] Example 2

[0109] The electronic device of this invention includes a central processing unit (CPU), which can perform various appropriate actions and processes according to computer program instructions stored in read-only memory (ROM) or loaded from a storage unit into random access memory (RAM). The RAM may also store various programs and data required for device operation. The CPU, ROM, and RAM are interconnected via a bus. Input / output (I / O) interfaces are also connected to the bus.

[0110] Multiple components in the device are connected to the I / O interface, including: input units such as keyboards and mice; output units such as various types of displays and speakers; storage units such as disks and optical discs; and communication units such as network interface cards (NICs), modems, and wireless transceivers. The communication unit allows the device to exchange information / data with other devices through computer networks such as the Internet and / or various telecommunications networks.

[0111] The processing unit executes the various methods and processes described above, such as methods S1 to S5. For example, in some embodiments, methods S1 to S5 may be implemented as computer software programs tangibly contained in a machine-readable medium, such as a storage unit. In some embodiments, part or all of the computer program may be loaded and / or installed on the device via ROM and / or a communication unit. When the computer program is loaded into RAM and executed by the CPU, one or more steps of methods S1 to S5 described above may be performed. Alternatively, in other embodiments, the CPU may be configured to execute methods S1 to S5 by any other suitable means (e.g., by means of firmware).

[0112] The functions described above in this document can be performed, at least in part, by one or more hardware logic components. For example, exemplary types of hardware logic components that can be used, without limitation, include: Field Programmable Gate Arrays (FPGAs), Application-Specific Integrated Circuits (ASICs), Application Standard Products (ASSPs), System-on-Chip (SoCs), Complex Programmable Logic Devices (CPLDs), and so on.

[0113] The program code used to implement the methods of the present invention can be written in any combination of one or more programming languages. This program code can be provided to a processor or controller of a general-purpose computer, special-purpose computer, or other programmable data processing device, such that when executed by the processor or controller, the program code causes the functions / operations specified in the flowcharts and / or block diagrams to be implemented. The program code can be executed entirely on the machine, partially on the machine, as a standalone software package partially on the machine and partially on a remote machine, or entirely on a remote machine or server.

[0114] In the context of this invention, a machine-readable medium can be a tangible medium that may contain or store a program for use by or in conjunction with an instruction execution system, apparatus, or device. A machine-readable medium can be a machine-readable signal medium or a machine-readable storage medium. Machine-readable media can include, but are not limited to, electronic, magnetic, optical, electromagnetic, infrared, or semiconductor systems, apparatus, or devices, or any suitable combination of the foregoing. More specific examples of machine-readable storage media include electrical connections based on one or more wires, portable computer disks, hard disks, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), optical fibers, portable compact disk read-only memory (CD-ROM), optical storage devices, magnetic storage devices, or any suitable combination of the foregoing.

[0115] If the aforementioned functions are implemented as software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of this invention, or the part that contributes to the prior art, or a part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of this invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

Claims

1. A method of electric stress equivalence calculation for MMC valves, characterized in that, The method comprises the following steps: Step S1: obtaining time-domain waveforms of bridge arm currents of the HVDC valve under various operating conditions based on electromagnetic transient simulation; Step S2: selecting a time-domain waveform of a bridge arm current of any phase, performing windowed Fourier transform on the time-domain waveform within a preset time window to obtain a frequency spectrum component, and further extracting a DC component and a fundamental frequency AC component; Step S3: drawing an electrical stress curve based on the extracted DC component and the fundamental frequency AC component; Step S4: respectively counting a first characteristic value representing a DC component electrical stress level and a second characteristic value representing a fundamental frequency AC component electrical stress level based on the electrical stress curves under various operating conditions; Step S5: multiplying the first characteristic value and the second characteristic value by a safety factor to obtain a test DC current value and a test AC current value; The step S2 comprises: Step S2-1: selecting a time-domain waveform of a bridge arm current of an upper bridge arm or a lower bridge arm of any phase; Step S2-2: multiplying a preset window function to a current waveform within a preset time window after intercepting the current waveform to obtain a frequency spectrum component; Step S2-3: further extracting a DC component and a fundamental frequency AC component based on the obtained frequency spectrum component; The frequency spectrum component is: wherein: is a spectral component of the mth frequency point, m is a spectral component of the mth frequency point, N is a sample number, is an nthsample value of a discrete sequence of the bridge arm current sampled within the preset time window, is a preset window function; The first characteristic value is a maximum value of the DC component current within the preset time window, and the second characteristic value is an effective value of a maximum value of the fundamental frequency AC component current.

2. The method of claim 1, wherein, The step S1 comprises: Step S1-1: building a simulation model of the offshore wind power flexible HVDC transmission system in an electromagnetic transient simulation platform; Step S1-2: setting multiple representative operating conditions in the simulation model and running simulation, and recording time-domain waveforms of bridge arm currents of the HVDC valve under various operating conditions.

3. The method of claim 2, wherein, The offshore wind power flexible HVDC transmission system comprises: an offshore wind farm; an offshore MMC station connected to the offshore wind farm; a land MMC station connected to a land AC power grid; a DC transmission line connecting the offshore MMC station and the land MMC station; The offshore wind power flexible HVDC transmission system adopts a two-port offshore wind power transmission scheme, the offshore wind farm is connected to the offshore MMC station through a power collection grid, and the offshore MMC station is connected to the land MMC station through the DC transmission line and is connected to the land AC power grid.

4. The method of claim 1, wherein, The DC component is: wherein: is the direct current component for the kth operating condition, is the spectral component for frequency index m = 0, corresponding to the direct current component of the bridge arm current; The AC component is: wherein: is an alternating current component of the hth harmonic, when h is 1, representing a fundamental frequency alternating current component, is a set of frequency point serial numbers corresponding to the hth harmonic frequency band.

5. The method of claim 1, wherein, The safety factor is 1.

05.

6. An apparatus for electric stress equivalent calculation of an MMC valve, comprising a memory, a processor, and a program stored in the memory, characterized in that, The processor implements the method of any one of claims 1-5 when executing the program.

7. A storage medium having stored thereon a program, characterized by The program is executed to implement the method of any one of claims 1-5.

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