Modular multilevel converter parallel capacitor monitoring method and system

Abstract

The application provides a modular multilevel converter parallel capacitor monitoring method and system, and relates to the field of modular multilevel converters.The method comprises the following steps: obtaining an oscillation current signal excited between two parallel capacitors in a sub-module when a power device of the modular multilevel converter sub-module is switched; extracting an inherent frequency from the oscillation current signal, and analyzing a series equivalent capacitance value of the capacitor based on the inherent frequency; solving the series equivalent capacitance value in combination with a parallel total capacitance value of the parallel capacitors to obtain a numerical solution; and identifying a physical corresponding relationship between the numerical solution and the two parallel capacitors as a monitoring result.The application realizes accurate calculation of an independent capacitance value of each parallel capacitor without needing to separately configure a current sensor for each parallel capacitor, has higher robustness and real-time performance, and effectively reduces system complexity and hardware cost.

Description

Technical Field

[0001] This invention relates to the field of modular multilevel converter technology, and more specifically to a method and system for monitoring parallel capacitors in a modular multilevel converter. Background Technology

[0002] Several methods for monitoring the capacitance of modular multilevel converter submodules have been proposed in the prior art. For example, one method estimates the capacitance by injecting a small AC signal into the circulating current and combining it with algorithms such as recursive least squares. However, this type of method requires additional signal injection, increasing control complexity and system losses. Capacitance estimation methods based on Kalman filtering require state modeling and filtering operations on capacitor current and voltage, resulting in a large computational load. Additionally, existing technologies simplify online monitoring of modular multilevel converter submodule capacitance by introducing a reference submodule. The core idea of ​​this method is to force the reference submodule and the monitored submodule to maintain completely identical switching states within one fundamental cycle during monitoring, ensuring they experience the same bridge arm current. Under this condition, the amplitude of their capacitor voltage ripple is inversely proportional to the capacitance value. Therefore, the target submodule capacitance can be calculated by measuring the peak-to-peak ratio of the voltage ripple of the reference submodule and the monitored submodule, combined with the estimated reference submodule capacitance. However, this requires modification of the control method.

[0003] However, the core drawback of all these existing technologies is that they all monitor the parallel capacitors within the submodule as a whole. In practical applications, parallel capacitors undergo non-uniform aging due to manufacturing differences or uneven thermal stress. This means that individual capacitors may have degraded below the safe threshold, but the overall capacitance may still be within the safe range, causing the system to fail to detect hidden faults in a timely manner. To achieve independent capacitance monitoring of parallel capacitors, the closest existing solution is to install a separate current sensor for each parallel branch and combine it with a low-frequency ripple algorithm for calculation. However, this method not only significantly increases hardware costs and wiring complexity, but also makes the monitoring accuracy susceptible to impacts in high-noise environments. Therefore, complex algorithms are needed to avoid substandard accuracy. Summary of the Invention

[0004] This invention addresses the problems existing in the prior art by providing a modular multilevel converter parallel capacitor monitoring method and system, which achieves accurate calculation of the independent capacitance value of each parallel capacitor without the need to configure a separate current sensor for each parallel capacitor.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] When the power devices in the modular multilevel converter submodule perform switching operations, the oscillating current signal generated between two parallel capacitors in the submodule is acquired. The inherent frequency is extracted from the oscillating current signal, and the series equivalent capacitance of the capacitor is analyzed based on the inherent frequency; The numerical solution is obtained by combining the series equivalent capacitance with the total parallel capacitance of the parallel capacitors. The physical correspondence between the numerical solution and the two parallel capacitors is identified as a monitoring result.

[0007] In some embodiments, the process of acquiring the oscillating current signal generated between two parallel capacitors within the submodule when the power devices of the modular multilevel converter submodule perform switching operations includes: An equivalent circuit model is established in the modular multilevel converter submodule, and the two parallel capacitors are the equivalent resonant capacitors. When a switching operation is performed, an oscillating loop is generated in the equivalent circuit model to obtain the oscillating current signal; The oscillation circuit is an RLC circuit that includes an equivalent resonant inductance, an equivalent series resistance, and an equivalent resonant capacitance.

[0008] In some embodiments, the process of extracting the inherent frequency from the oscillating current signal includes: The current sensor is used to collect the current of the parallel capacitor branch and the switching signal of the submodule in real time, and the waveform corresponding to the current of the parallel capacitor branch during the turn-off stage is selected based on the switching signal of the submodule. Remove waveforms with durations below a threshold from the waveforms, and extract the remaining waveforms as valid oscillation waveforms; After windowing and zero-filling the effective oscillation waveform, a fast Fourier transform is performed to extract the actual oscillation frequency corresponding to the peak amplitude. The loop damping ratio is calculated by analyzing the FFT spectrum using the half-power bandwidth method. If the frequency exceeds a preset threshold, the frequency is corrected; otherwise, no correction is made to obtain an approximate natural frequency. The average natural frequency is obtained by averaging the approximate natural frequencies calculated over a period of time.

[0009] In some embodiments, the process of resolving the series equivalent capacitance value of the capacitor based on the natural frequency includes: Pre-measure the equivalent resonant inductance; Construct a simultaneous equation relating the natural frequency, the equivalent resonant inductance, and the series equivalent capacitance, and solve it to obtain the current series equivalent capacitance.

[0010] In some embodiments, the process of solving for a numerical solution by combining the series equivalent capacitance with the total parallel capacitance of the parallel capacitors includes: The total parallel capacitance of the sub-modules is obtained through the MMC capacitor overall monitoring method; By combining the series equivalent capacitance value with the parallel total capacitance value, a system of equations is constructed, and the first and second numerical solutions of the capacitor are obtained by solving the equations.

[0011] In some embodiments, the process of identifying the physical correspondence between the numerical solution and the two parallel capacitors as a monitoring result includes: When the submodule is in working condition, measure the current of the first capacitor branch and obtain the total current of the submodule. The current of the second capacitor branch is calculated by summing the current of the first capacitor branch and the total current of the submodule. Compare the amplitudes of the current in the first capacitor branch and the current in the second capacitor branch in the low-frequency range. The amplitude of the branch current is proportional to the capacitance value. The first and second numerical solutions of the capacitor are mapped back to the physical capacitor as the monitoring results of the independent capacitance value.

[0012] In some embodiments, the formula for solving is as follows: ; in, For series equivalent capacitance, For total parallel capacity, For the first capacitor, This is the second capacitor.

[0013] This invention proposes a system for monitoring parallel capacitors in a modular multilevel converter, comprising: The acquisition module is configured to acquire the oscillating current signal generated between two parallel capacitors within the submodule when the power devices of the modular multilevel converter submodule are performing switching operations. The extraction module is configured to extract the inherent frequency from the oscillating current signal and analyze the series equivalent capacitance value of the capacitor based on the inherent frequency. The calculation module is configured to combine the series equivalent capacitance value with the total parallel capacitance value of the parallel capacitors to obtain a numerical solution; The corresponding module is configured to identify the physical correspondence between the numerical solution and the two parallel capacitors as a monitoring result.

[0014] This invention proposes a computer device, comprising: At least one processor; and a memory storing a computer program executable on the processor, wherein the processor, when executing the program, performs the steps of the method for monitoring parallel capacitors in a modular multilevel converter.

[0015] The present invention provides a computer-readable storage medium storing a computer program that, when executed by a processor, performs the steps of the method for monitoring parallel capacitors in a modular multilevel converter.

[0016] Compared with the prior art, the present invention has the following beneficial effects: This invention proposes a method and system for monitoring parallel capacitors in a modular multilevel converter. The method includes: acquiring an oscillating current signal generated between two parallel capacitors within the submodule when the power devices of the modular multilevel converter submodule are performing switching operations; extracting the natural frequency from the oscillating current signal and resolving the series equivalent capacitance value of the capacitors based on the natural frequency; solving for a numerical solution by combining the series equivalent capacitance value with the total parallel capacitance value of the parallel capacitors; and identifying the physical correspondence between the numerical solution and the two parallel capacitors as the monitoring result.

[0017] This invention utilizes the internal circulating current oscillation signal excited between parallel capacitors by the switching transient of power devices. By accurately extracting the inherent frequency of this signal, the series equivalent capacitance value of the capacitors is analyzed. Combined with the overall capacitance value, the problem is solved simultaneously. Thus, without the need to configure a separate current sensor for each parallel capacitor, the independent capacitance value of each parallel capacitor can be accurately calculated. This has higher robustness and real-time performance, and effectively reduces system complexity and hardware cost.

[0018] This invention aims to solve two core problems in monitoring parallel capacitors in modular multilevel converter submodules: first, existing total capacitance monitoring methods cannot identify hidden individual capacitor faults caused by non-uniform aging of parallel capacitors; second, traditional multi-sensor methods suffer from high hardware costs, high system complexity, and poor robustness. By utilizing the inherent high-frequency oscillation signal excited by the switching transients of power devices, this invention achieves high-precision online estimation of the independent capacitance value of each parallel capacitor without requiring a separate current sensor for each capacitor. This method uses the resonant frequency as a characteristic parameter, ensuring that the monitoring results are independent of system operating conditions, significantly improving the robustness of the method, and effectively reducing the hardware cost and algorithm complexity of the monitoring system. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other embodiments can be obtained based on these drawings without creative effort.

[0020] Figure 1The present invention provides a flowchart of a method for monitoring parallel capacitors in a modular multilevel converter.

[0021] Figure 2 This invention provides a system block diagram for monitoring parallel capacitors in a modular multilevel converter.

[0022] Figure 3 A schematic diagram of the structure of an embodiment of the computer device provided by the present invention.

[0023] Figure 4 This is a schematic diagram of an embodiment of the computer-readable storage medium provided by the present invention.

[0024] Figure 5 This invention provides an equivalent circuit model for a method of monitoring parallel capacitors in a modular multilevel converter.

[0025] Figure 6 A representative curve of the DC capacitor switch oscillation current in a method for monitoring parallel capacitors in a modular multilevel converter provided by the present invention.

[0026] Figure 7 The oscillation current waveform diagram is provided by the present invention for a method of monitoring parallel capacitors in a modular multilevel converter.

[0027] Figure 8 The diagram shows a half-power algorithm for a method of monitoring parallel capacitors in a modular multilevel converter provided by this invention.

[0028] Figure 9 This is a flowchart of an embodiment of a method for monitoring parallel capacitors in a modular multilevel converter provided by the present invention. Detailed Implementation

[0029] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention and should not be construed as limiting the scope of protection of the present invention. It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of this application.

[0030] It should be noted that all uses of "first" and "second" in the embodiments of the present invention are for the purpose of distinguishing two entities or parameters with the same name but different names. It is clear that "first" and "second" are only for the convenience of expression and should not be construed as limiting the embodiments of the present invention. Subsequent embodiments will not explain this in detail.

[0031] This invention proposes a method for monitoring parallel capacitors in a modular multilevel converter. Please refer to [link to relevant documentation]. Figure 1 and Figure 9 ,include: When the power devices in the modular multilevel converter submodule perform switching operations, the oscillating current signal generated between two parallel capacitors in the submodule is acquired. The inherent frequency is extracted from the oscillating current signal, and the series equivalent capacitance of the capacitor is analyzed based on the inherent frequency; The numerical solution is obtained by combining the series equivalent capacitance with the total parallel capacitance of the parallel capacitors. The physical correspondence between the numerical solution and the two parallel capacitors is identified as a monitoring result.

[0032] This invention provides a method for online monitoring of the independent capacitance values ​​of parallel capacitors within a modular multilevel converter submodule based on an internal circulating current oscillation signal excited by switching transients. The core of this scheme lies in utilizing the inherent resonant characteristics of the submodule's power devices at the time of turn-off, accurately extracting the series equivalent capacitance information through frequency domain analysis, and then solving the problem by combining the overall monitoring values.

[0033] During the operation of the modular multilevel converter submodule, the oscillating current signal generated between two parallel capacitors within the submodule during the switching operation of its power devices is effectively acquired. The inherent frequency is extracted from the acquired oscillating current signal, and the series equivalent capacitance value of the capacitors is deduced based on this frequency. The inherent frequency, as an intrinsic parameter of the RLC circuit, is independent of the system operating point, making the deduced series equivalent capacitance value more accurate and reliable. In this way, it is unnecessary to configure a separate current sensor for each parallel capacitor, reducing system complexity and hardware costs while ensuring the accuracy of the capacitor capacitance value deduction.

[0034] By combining the analytically obtained series equivalent capacitance value with the total parallel capacitance of the parallel capacitors, a numerical solution is obtained. Through mathematical operations and equation solving, a more comprehensive understanding of the capacitance relationship between the two parallel capacitors in the overall parallel state can be achieved. Identifying the physical correspondence between the numerical solution and the two parallel capacitors, and using this as a monitoring result, allows capacitor monitoring to move beyond theoretical values ​​and correspond to actual physical equipment. This not only helps maintenance personnel intuitively understand the actual operating status of each capacitor but also enables them to quickly locate problematic capacitors and take timely maintenance measures when anomalies occur.

[0035] In some embodiments, please refer to Figure 5 and Figure 6 The process of acquiring the oscillating current signal generated between two parallel capacitors within the submodule when the power devices of the modular multilevel converter submodule perform switching operations includes: An equivalent circuit model is established in the modular multilevel converter submodule, and the two parallel capacitors are the equivalent resonant capacitors. When a switching operation is performed, an oscillating loop is generated in the equivalent circuit model to obtain the oscillating current signal; The oscillation circuit is an RLC circuit that includes an equivalent resonant inductance, an equivalent series resistance, and an equivalent resonant capacitance.

[0036] In practical implementation, the oscillation mechanism is first analyzed and an equivalent circuit model is established. For example, 5 and... Figure 6 In a modular multilevel converter, when the power device performs switching operations, the extremely high rate of current change will excite the DC side via parallel capacitors. and Equivalent series inductance, busbar stray inductance The RLC resonant circuit, composed of parasitic parameters such as the equivalent series resistance, generates a high-frequency damped oscillating current. This oscillating circuit can be simplified to a series... , and The RLC circuit, in which the equivalent resonant inductance Composed of series-connected DC capacitors ESL1 and ESL2, and DC bus stray inductance, it can be considered a constant value under the premise of fixed submodule structure, equivalent to resonant capacitance. That is, the series equivalent capacitance of parallel capacitors , For the first capacitor, For the second capacitor, it provides the equivalent resonant resistance for attenuation. The equivalent series resistance of a DC capacitor and DC bus resistance Composition. Generally, when the resistance is small, the actual oscillation frequency is approximately equal to the natural frequency under undamped conditions. The actual period of this oscillation process has an analytical relationship with the capacitance value: ; In the formula, It is the damped oscillation frequency, and also the actual oscillation frequency. It is the damped oscillation angular frequency. It is the natural frequency under undamped conditions.

[0037] The equivalent circuit model and its expression establish a definite relationship between the oscillation frequency and the circuit parameters. Because It can be obtained through prior measurement, therefore once the natural frequency is accurately measured... This allows us to deduce... Then solve and The capacitance value.

[0038] In most conventional power electronics and busbar designs, stray inductance can be considered largely unaffected by temperature. Stray inductance primarily depends on the geometry of the circuit, while temperature effects are indirect and minimal.

[0039] When a switching operation is performed, an RLC oscillating circuit containing equivalent resonant inductance, equivalent series resistance, and equivalent resonant capacitance is generated in the equivalent circuit model, and the oscillating current signal is obtained. The existence of the RLC oscillating circuit causes the circuit to produce a specific oscillation phenomenon at the moment of switching. Based on the characteristics of the RLC circuit, the oscillating current signal can be used to deduce relevant parameters such as the equivalent resonant capacitance, thereby realizing the monitoring and evaluation of the capacitor state.

[0040] In some embodiments, please refer to Figure 7 and Figure 8 The process of extracting the inherent frequency from the oscillating current signal includes: The current sensor is used to collect the current of the parallel capacitor branch and the switching signal of the submodule in real time, and the waveform corresponding to the current of the parallel capacitor branch during the turn-off stage is selected based on the switching signal of the submodule. Remove waveforms with durations below a threshold from the waveforms, and extract the remaining waveforms as valid oscillation waveforms; After windowing and zero-filling the effective oscillation waveform, a fast Fourier transform is performed to extract the actual oscillation frequency corresponding to the peak amplitude. The loop damping ratio is calculated by analyzing the FFT spectrum using the half-power bandwidth method. If the frequency exceeds a preset threshold, the frequency is corrected; otherwise, no correction is made to obtain an approximate natural frequency. The average natural frequency is obtained by averaging the approximate natural frequencies calculated over a period of time.

[0041] The first step is the acquisition and filtering of oscillating current signals, which are collected in real time by a current sensor from the parallel capacitor. Current on The switching signal S of the submodule is used to filter the shutdown phase based on the switching signal Sap of the submodule. Remove waveforms with excessively short durations. Figure 7 (a) shows the oscillating current waveforms at the turn-on and turn-off times. Figure 7 (b) Comparison of current magnitudes in the low-frequency band. Figure 7 (c) is the oscillating current waveform at the turn-off moment. At this time, only the high-frequency oscillating circulating current exists in the circuit, and the signal-to-noise ratio is the highest. Based on this, the effective oscillating waveform is extracted and waveforms with too short a duration are removed.

[0042] The second step is the extraction and correction of the oscillation frequency, based on the selected frequency. After windowing and zero-filling, the transient oscillation waveform is subjected to a Fast Fourier Transform (FFT) to extract the actual oscillation frequency corresponding to the peak amplitude of the oscillating current. Subsequently, the half-power bandwidth method is used to analyze the FFT spectrum and calculate the damping ratio of the loop. : ; like Figure 8 , It is the lower cutoff frequency, located to the left of the peak value, at the frequency corresponding to when the amplitude drops to 1 / √2 (approximately 0.707 times) of the peak value.

[0043] It is the upper cutoff frequency, located to the right of the peak value, and corresponds to the frequency at which the amplitude drops to 1 / √2 (approximately 0.707 times) of the peak value.

[0044] like If it exceeds the preset threshold, then proceed according to... To obtain a frequency closer to the undamped natural frequency, corrections are made. If the value exceeds the threshold, there is no need to adjust the extraction frequency. Finally, the values ​​calculated over a period of time... Take the average value to obtain the final natural frequency. .

[0045] The system collects the parallel capacitor branch current and submodule switching signals in real time using current sensors. Based on the switching signals, it accurately filters the waveforms corresponding to the parallel capacitor branch current during the turn-off phase, ensuring the data source is phase-specific. The turn-off phase contains key current characteristics of the capacitor under specific operating conditions; filtering the waveforms in this phase eliminates the influence of interference information from other phases. From the filtered waveforms, waveforms with durations below a threshold are removed, and the remaining waveforms are extracted as valid oscillation waveforms. Waveforms with excessively short durations may contain more noise or atypical characteristics; these are removed to obtain more representative valid oscillation waveforms.

[0046] After windowing and zero-filling the effective oscillation waveform, a Fast Fourier Transform (FFT) is performed to convert the time-domain information to the frequency-domain information, thereby extracting the actual oscillation frequency corresponding to the amplitude peak. The complex oscillation waveform is decomposed into different frequency components, accurately identifying the frequency corresponding to the amplitude peak, i.e., the actual oscillation frequency. The loop damping ratio is calculated using the half-power bandwidth method to analyze the FFT spectrum, and frequency correction is determined based on whether the damping ratio exceeds a preset threshold, ultimately obtaining an approximate natural frequency that is closer to the ideal state. The average value of the approximate natural frequencies calculated over a period of time is taken as the natural frequency, effectively reducing random errors and improving the accuracy and reliability of the natural frequency. This period can be a unit of time or the time span for more than 10 approximate natural frequency calculations.

[0047] In some embodiments, please refer to Figure 9The process of resolving the series equivalent capacitance value of the capacitor based on the natural frequency includes: Pre-measure the equivalent resonant inductance; Construct a simultaneous equation relating the natural frequency, the equivalent resonant inductance, and the series equivalent capacitance, and solve it to obtain the current series equivalent capacitance.

[0048] The equivalent resonant inductance is a crucial parameter in the equivalent circuit model of a modular multilevel converter submodule. In actual circuit systems, the equivalent resonant inductance can be affected by various factors, such as circuit layout and component characteristics. Obtaining its true value through precise measurement ensures that the parameters used in the subsequent simultaneous equations are reliable and consistent with reality, thus avoiding calculation errors caused by inaccurate inductance values.

[0049] The natural frequency, as an important parameter reflecting the oscillation characteristics of a circuit, has a close physical relationship with the equivalent resonant inductance and the series equivalent capacitance. By constructing a simultaneous equation to organically combine these three key factors, the current series equivalent capacitance can be accurately solved using mathematical calculations. This allows for rapid and accurate results in practical applications.

[0050] In some embodiments, the process of solving for a numerical solution by combining the series equivalent capacitance with the total parallel capacitance of the parallel capacitors includes: The total parallel capacitance of the sub-modules is obtained through the MMC capacitor overall monitoring method; By combining the series equivalent capacitance value with the parallel total capacitance value, a system of equations is constructed, and the first and second numerical solutions of the capacitor are obtained by solving the equations.

[0051] Next, the independent capacitance values ​​are calculated by solving the simultaneous equations, and the average natural frequency is determined. and the pre-measured Substitution Calculate the current series equivalent capacitance. Simultaneously, the total parallel capacitance value of this submodule can be obtained through any existing mature MMC capacitor overall monitoring solution. = + ; This establishes a system containing and The system of two linear and two quadratic equations in two variables is solved to obtain the numerical solution for the two capacitors. and ,in, This is the first numerical solution. This is the second numerical solution.

[0052] Based on typical component specifications and established literature for high-power MMC applications, the capacitance of a single submodule capacitor ranges from 1mF to 10mF, and its ESL is typically between 10nH and 100nH. Considering the above parameter range and the stray inductance between parallel capacitors, the switching oscillation frequency in the MMC system is calculated to be between 5kHz and 50kHz.

[0053] The total parallel capacitance of the submodules is obtained by using the MMC capacitor overall monitoring method, which reflects the overall capacitance characteristics of the parallel capacitors.

[0054] By combining the series equivalent capacitance value with the parallel total capacitance value, a system of equations is constructed and solved to obtain the first and second numerical solutions for the capacitor. The series equivalent capacitance value and the parallel total capacitance value reflect the characteristics of the capacitor from different perspectives. By constructing a system of equations, specific numerical solutions are obtained. The obtained first and second numerical solutions correspond to the two parallel capacitors, accurately determining the capacitance value of each capacitor, reflecting the relationship between them, and ensuring the stable and reliable operation of the modular multilevel converter submodule.

[0055] In some embodiments, please refer to Figure 7 The process of identifying the physical correspondence between the numerical solution and the two parallel capacitors as monitoring results includes: When the submodule is in working condition, measure the current of the first capacitor branch and obtain the total current of the submodule. The current of the second capacitor branch is calculated by summing the current of the first capacitor branch and the total current of the submodule. Compare the amplitudes of the current in the first capacitor branch and the current in the second capacitor branch in the low-frequency range. The amplitude of the branch current is proportional to the capacitance value. The first and second numerical solutions of the capacitor are mapped back to the physical capacitor as the monitoring results of the independent capacitance value.

[0056] The final step is to identify the physical correspondences in order to determine the numerical solution. and With actual physical capacitors , The correspondence, such as Figure 7 (b) When the submodule is put into operation, the total current IARM of the submodule and the measured current should be used. Branch current Calculate Branch current Subsequently, and For low-frequency current amplitude analysis, since capacitive reactance dominates in the low-frequency range, the branch current amplitude is proportional to the capacitance value. The numerical solution can be obtained by comparing the amplitudes of the two. and Accurately mapped back to physical capacitors , Above, complete independent capacitance monitoring.

[0057] When the submodule is in working condition, the current of the first capacitor branch is measured and the total current of the submodule is obtained. The module being in working condition means that the capacitors have begun to perform their corresponding electrical functions, and the current data measured at this time can accurately reflect the working state of the capacitors. The current of the first capacitor branch directly reflects the current characteristics of one capacitor, while the total current of the submodule contains comprehensive current information of the entire submodule, avoiding analytical errors caused by data deviating from reality.

[0058] The current in the second capacitor branch is calculated using the current in the first capacitor branch and the total current of the submodule. The amplitudes of these two currents in the low-frequency range are compared to map the numerical solution back to the physical capacitor as a monitoring result. In the low-frequency range, the branch current amplitude is proportional to the capacitance value. By accurately calculating and comparing the current amplitudes, the differences in current characteristics between the two capacitors can be clearly distinguished, thus accurately mapping the calculated first and second numerical solutions to the actual physical capacitors. This mapping method establishes a direct link between the originally abstract numerical solution and the specific physical device, allowing for an intuitive understanding of the independent capacitance value of each capacitor and timely detection of performance changes and potential faults.

[0059] In some embodiments, the formula for solving is as follows: ; in, For series equivalent capacitance, For total parallel capacity, For the first capacitor, This is the second capacitor.

[0060] In the complex circuits of modular multilevel converter submodules, the relevant capacitance effects can be equivalent to series equivalent capacitance values, which can greatly simplify the circuit model. The complex capacitance distribution and interaction can be characterized by a single parameter, making circuit analysis more intuitive and convenient.

[0061] It reflects the total capacitance of the parallel capacitors in the submodule as a whole. By monitoring the total parallel capacitance, the overall performance status of the parallel capacitor bank can be quickly understood, determining whether it meets the system's capacitance requirements and whether there is any performance degradation or failure of capacitors in the parallel capacitor bank.

[0062] By solving these two numerical solutions, the capacitance value of each independent capacitor can be monitored, and performance changes or faults of individual capacitors can be detected in a timely manner. It is possible to detect whether the capacitance value of a certain capacitor deviates from the normal range, so as to carry out targeted repairs or replacements and avoid the failure of a single capacitor affecting the operation of the entire system.

[0063] This invention proposes a system for monitoring parallel capacitors in a modular multilevel converter. Please refer to [link to relevant documentation]. Figure 2 ,include: The acquisition module is configured to acquire the oscillating current signal generated between two parallel capacitors within the submodule when the power devices of the modular multilevel converter submodule are performing switching operations. The extraction module is configured to extract the inherent frequency from the oscillating current signal and analyze the series equivalent capacitance value of the capacitor based on the inherent frequency. The calculation module is configured to combine the series equivalent capacitance value with the total parallel capacitance value of the parallel capacitors to obtain a numerical solution; The corresponding module is configured to identify the physical correspondence between the numerical solution and the two parallel capacitors as a monitoring result.

[0064] The independent capacitance monitoring method for parallel capacitors based on switching transient oscillation signals provided by this invention can be widely applied in large-scale modular multilevel converter applications such as high-voltage direct current transmission projects and offshore wind power grid connection. After applying this technical solution, the control system can periodically utilize the switching actions of submodules during normal operation to automatically trigger the monitoring process. The high-speed digital signal processor first collects the transient oscillation current when the selected submodule is turned off, and then quickly completes frequency domain analysis and frequency correction in the FPGA to calculate the internal parallel capacitor value in real time. and The system records the independent capacitance value of each capacitor and uploads this data to the arm controller. If the system detects that the capacitance value of one of the capacitors has dropped beyond the threshold, it can immediately issue a precise warning, pinpointing the fault to a specific parallel capacitor in a specific submodule, and proceeding with the next step of maintenance.

[0065] First, this method extracts and analyzes the inherent high-frequency oscillating current signal generated by the switching action itself. This feature directly avoids the additional power loss caused by injecting test signals, and eliminates the need for complex filters to extract weak low-frequency ripples, ensuring monitoring functionality without affecting system operating efficiency.

[0066] Second, the method innovatively utilizes the inherent oscillation frequency to analytically derive the series equivalent capacitance value, which characterizes the series relationship of capacitors. And combined with the total parallel capacity obtained through conventional methods A solution for each parallel capacitor was successfully constructed. and A system of two equations for independent capacitance values. This technical approach overcomes the challenge of individualized monitoring without requiring a separate current sensor for each parallel capacitor, reducing hardware costs and system intrusion.

[0067] Third, this method uses the resonant frequency of the oscillating current as the core monitoring characteristic parameter, which brings unique robustness and high accuracy. The resonant frequency is an intrinsic property of the RLC resonant circuit, and its value is determined only by a fixed equivalent inductance. and the series equivalent capacitance to be tested This technical feature means that the monitoring results are essentially unaffected by changes in the system's operating point, thus solving the problem of accuracy degradation in traditional low-frequency ripple-based methods under light loads or drastic changes in operating conditions. Simultaneously, by introducing the half-power bandwidth method for online calculation and frequency correction of the damping ratio, the influence of loop parasitic resistance on frequency extraction is further eliminated, ensuring measurement accuracy under various actual parameters.

[0068] To achieve the invention's objective of monitoring the independent capacitance of parallel capacitors within an MMC submodule, the technical solutions typically require simultaneously obtaining information about... and The solution is obtained by solving for two independent pieces of information. Without changing the submodule hardware topology, the closest approximate alternative is a combination of dual sensors and low-frequency ripple analysis. This solution is implemented using a parallel capacitor. and Each capacitor is equipped with an independent current sensor to measure the current in each capacitor. and Combining capacitor voltage and low-frequency impedance models, parameter identification algorithms such as recursive least squares are used to calculate the following: and The capacitance value. Although this scheme can also achieve the purpose of independent monitoring, it requires adding sensors and acquisition channels to the parallel branches of each submodule, resulting in a significant increase in hardware cost and wiring complexity. Furthermore, the low-frequency ripple signal is susceptible to noise interference, the algorithm complexity is high, and the accuracy is easily reduced with changes in system operating conditions (such as light load). In summary, although the alternative scheme is feasible, it achieves independent monitoring by sacrificing hardware cost and increasing system complexity. The advantage of the present invention is that it innovatively utilizes the inherent high-frequency resonance eigenvalue of the system as an independent information source, thereby achieving the same independent monitoring purpose as the dual-sensor scheme without increasing hardware cost (requiring only one sensor). Therefore, the technical solution of the present invention is a superior, more economical, and robust solution.

[0069] The key technical point of this invention lies in minimizing hardware configuration requirements and utilizing the inherent high-frequency oscillation signal generated in the parallel capacitor circuit during the transient turn-off of the submodule power devices as the core monitoring signal source. Based on this signal, the key technology of this invention is to accurately extract the inherent frequency of this circuit through Fast Fourier Transform (FFT). The damping ratio is calculated using the half-power bandwidth method for frequency correction, thereby obtaining the series equivalent capacitance of the capacitor stably and accurately. Based on this, the present invention creatively combines... By combining the total parallel capacitance value obtained by conventional external methods with a system of two equations, and with only one current sensor required in each submodule, the independent numerical solutions of the parallel capacitors are obtained analytically.

[0070] Therefore, the core technical solution protected by this invention is a method for monitoring the health status of parallel capacitors within a modular multilevel converter submodule. This method extracts the resonant frequency by detecting the oscillating current signal generated in the parallel capacitor branch during switching operations of the submodule. Based on the analytical relationship between the resonant frequency and the series equivalent capacitance, combined with the overall parallel capacitance value of the submodule, the independent capacitance value of each parallel capacitor is calculated. This method uses the inherent high-frequency oscillation period as a characteristic parameter, is independent of the load, has a certain degree of robustness, and reduces the hardware burden added by traditional solutions for achieving independent monitoring.

[0071] Based on the same inventive concept, according to another aspect of the present invention, such as Figure 3 As shown, an embodiment of the present invention also provides a computer device 30, which includes a processor 310 and a memory 320. The memory 320 stores a computer program 321 that can be run on the processor. When the processor 310 executes the program, it performs the steps of the method described above.

[0072] Based on the same inventive concept, according to another aspect of the present invention, such as Figure 4 As shown, embodiments of the present invention also provide a computer-readable storage medium 40, which stores a computer program 410 that, when executed by a processor, performs the methods described above.

[0073] Embodiments of the present invention may also include a corresponding computer device. The computer device includes a memory, at least one processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes any of the methods described above when executing the program.

[0074] The memory, as a non-volatile computer-readable storage medium, can be used to store non-volatile software programs, non-volatile computer-executable programs, and modules, such as program instructions / modules in the embodiments of this application. The processor executes various functional applications and data processing of the device by running the non-volatile software programs, instructions, and modules stored in the memory, thereby implementing the above-described method.

[0075] The memory may include a program storage area and a data storage area. The program storage area may store the operating system and applications required for at least one function; the data storage area may store data created based on the use of the device. Furthermore, the memory may include high-speed random access memory and non-volatile memory, such as at least one disk storage device, flash memory device, or other non-volatile solid-state storage device. In embodiments, the memory may optionally include memory remotely located relative to the processor, which can be connected to the local module via a network. Examples of such networks include, but are not limited to, the Internet, corporate intranets, local area networks, mobile communication networks, and combinations thereof.

[0076] Finally, it should be noted that those skilled in the art will understand that all or part of the processes in the above embodiments can be implemented by a computer program instructing related hardware. The program can be stored in a computer-readable storage medium, and when executed, it can include the processes of the embodiments of the methods described above. The storage medium for the program can be a magnetic disk, optical disk, read-only memory (ROM), or random access memory (RAM), etc. The above computer program embodiments can achieve the same or similar effects as any of the corresponding foregoing method embodiments.

[0077] Those skilled in the art will also understand that the various exemplary logic blocks, modules, circuits, and algorithm steps described in conjunction with the disclosure herein can be implemented as electronic hardware, computer software, or a combination of both. To clearly illustrate this interchangeability between hardware and software, the functionality of various illustrative components, blocks, modules, circuits, and steps has been generally described. Whether this functionality is implemented as software or as hardware depends on the specific application and the design constraints imposed on the system as a whole. Those skilled in the art can implement the functionality in various ways for each specific application, but such implementation decisions should not be construed as departing from the scope of the embodiments disclosed herein.

[0078] The above are exemplary embodiments disclosed in this invention. However, it should be noted that various changes and modifications can be made without departing from the scope of the embodiments of this invention as defined by the claims. The functions, steps, and / or actions of the methods according to the disclosed embodiments described herein do not need to be performed in any particular order. The sequence numbers of the disclosed embodiments of this invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments. Furthermore, although the elements disclosed in the embodiments of this invention may be described or claimed individually, they may be understood as multiple unless explicitly limited to a singular number.

[0079] It should be understood that, as used herein, the singular form “a” is intended to include the plural form as well, unless the context clearly supports an exception. It should also be understood that, as used herein, “and / or” refers to any and all possible combinations of one or more of the associated listed items.

[0080] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention (including the claims) is limited to these examples. Within the framework of the invention, technical features of the above embodiments or different embodiments can be combined, and many other variations of different aspects of the invention exist, which are not provided in the details for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the invention should be included within the protection scope of the invention.

Claims

1. A method for monitoring parallel capacitors in a modular multilevel converter, characterized in that, include: When the power devices in the modular multilevel converter submodule perform switching operations, the oscillating current signal generated between two parallel capacitors in the submodule is acquired. The inherent frequency is extracted from the oscillating current signal, and the series equivalent capacitance of the capacitor is analyzed based on the inherent frequency; The numerical solution is obtained by combining the series equivalent capacitance with the total parallel capacitance of the parallel capacitors. The physical correspondence between the numerical solution and the two parallel capacitors is identified as a monitoring result.

2. The method for monitoring parallel capacitors in a modular multilevel converter according to claim 1, characterized in that, The process of acquiring the oscillating current signal generated between two parallel capacitors within the submodule when the power devices of the modular multilevel converter submodule perform switching operations includes: An equivalent circuit model is established in the modular multilevel converter submodule, and the two parallel capacitors are the equivalent resonant capacitors. When a switching operation is performed, an oscillating loop is generated in the equivalent circuit model to obtain the oscillating current signal; The oscillation circuit is an RLC circuit that includes an equivalent resonant inductance, an equivalent series resistance, and an equivalent resonant capacitance.

3. The method for monitoring parallel capacitors in a modular multilevel converter according to claim 1, characterized in that, The process of extracting the inherent frequency from the oscillating current signal includes: The current sensor is used to collect the current of the parallel capacitor branch and the switching signal of the submodule in real time, and the waveform corresponding to the current of the parallel capacitor branch during the turn-off stage is selected based on the switching signal of the submodule. Remove waveforms with durations below a threshold from the waveforms, and extract the remaining waveforms as valid oscillation waveforms; After windowing and zero-filling the effective oscillation waveform, a fast Fourier transform is performed to extract the actual oscillation frequency corresponding to the peak amplitude. The loop damping ratio is calculated by analyzing the FFT spectrum using the half-power bandwidth method. If the frequency exceeds a preset threshold, the frequency is corrected; otherwise, no correction is made to obtain an approximate natural frequency. The average natural frequency is obtained by averaging the approximate natural frequencies calculated over a period of time.

4. The method for monitoring parallel capacitors in a modular multilevel converter according to claim 3, characterized in that, The process of resolving the series equivalent capacitance value of the capacitor based on the natural frequency includes: Pre-measure the equivalent resonant inductance; Construct a simultaneous equation relating the natural frequency, the equivalent resonant inductance, and the series equivalent capacitance, and solve it to obtain the current series equivalent capacitance.

5. The method for monitoring parallel capacitors in a modular multilevel converter according to claim 1, characterized in that, The process of combining the series equivalent capacitance with the total parallel capacitance of the parallel capacitors to obtain a numerical solution includes: The total parallel capacitance of the sub-modules is obtained through the MMC capacitor overall monitoring method; By combining the series equivalent capacitance value with the parallel total capacitance value, a system of equations is constructed, and the first and second numerical solutions of the capacitor are obtained by solving the equations.

6. The method for monitoring parallel capacitors in a modular multilevel converter according to claim 5, characterized in that, The process of identifying the physical correspondence between the numerical solution and the two parallel capacitors as monitoring results includes: When the submodule is in working condition, measure the current of the first capacitor branch and obtain the total current of the submodule. The current of the second capacitor branch is calculated by summing the current of the first capacitor branch and the total current of the submodule. Compare the amplitudes of the current in the first capacitor branch and the current in the second capacitor branch in the low-frequency range. The amplitude of the branch current is proportional to the capacitance value. The first and second numerical solutions of the capacitor are mapped back to the physical capacitor as the monitoring results of the independent capacitance value.

7. The method for monitoring parallel capacitors in a modular multilevel converter according to claim 1, characterized in that, The formula for solving this problem is as follows: ； in, For series equivalent capacitance, For total parallel capacity, For the first capacitor, This is the second capacitor.

8. A system for monitoring parallel capacitors in a modular multilevel converter, characterized in that, include: The acquisition module is configured to acquire the oscillating current signal generated between two parallel capacitors within the submodule when the power devices of the modular multilevel converter submodule are performing switching operations. The extraction module is configured to extract the inherent frequency from the oscillating current signal and analyze the series equivalent capacitance value of the capacitor based on the inherent frequency. The calculation module is configured to combine the series equivalent capacitance value with the total parallel capacitance value of the parallel capacitors to obtain a numerical solution; The corresponding module is configured to identify the physical correspondence between the numerical solution and the two parallel capacitors as a monitoring result.

9. A computer device, comprising: At least one processor; And a memory storing a computer program executable on the processor, characterized in that, when the processor executes the program, it performs the steps of a method for monitoring parallel capacitors of a modular multilevel converter as described in any one of claims 1 to 7.

10. A computer-readable storage medium storing a computer program, characterized in that, When the computer program is executed by the processor, it performs the steps of a method for monitoring parallel capacitors in a modular multilevel converter as described in any one of claims 1 to 7.

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