Higher harmonic suppression method and system

Abstract

The invention discloses a higher harmonic suppression method and system, which is applied to the technical field of direct current transmission, and comprises the following steps: obtaining three-phase current data of an LCC converter; performing error analysis processing on the three-phase current data and a preset current instruction value to obtain three-phase harmonic error current data; based on the three-phase harmonic error current data, performing compensation deviation processing on the obtained output current data of the MMC converter to obtain three-phase harmonic compensation current data; after coordinate transformation, a PI controller is adopted to superpose a repetitive controller to obtain output after harmonic suppression; and modulating to generate a control waveform of the MMC converter, thereby realizing harmonic suppression of the SLCC circuit. The core of the method is to introduce a power grid strength adaptive strategy: a harmonic suppression mode is automatically switched according to a power grid strength grade, and adaptive modification of parameters of a repetitive controller is realized. According to the method, unified suppression of full-higher harmonics under power grid working conditions with different intensities is realized.

Description

Technical Field

[0001] This invention relates to the field of DC power transmission technology, and in particular to a method and system for suppressing high-order harmonics. Background Technology

[0002] In high-voltage direct current (HVDC) transmission systems, grid-commutated converters are widely used due to their simple structure, low cost, and high-capacity transmission capabilities. However, during the commutation process, these converters inevitably generate a large number of high-order harmonic currents on the AC side. If these high-order harmonics in HVDC transmission circuits are not effectively suppressed, they will lead to voltage distortion across the entire power grid, equipment overheating, or relay protection malfunctions, seriously threatening the power quality and operational safety of the power system. Therefore, it is essential to suppress these high-order harmonics.

[0003] Existing technologies primarily employ passive high-order harmonic suppression methods that rely on passive filters. The principle behind this approach is to utilize the extremely low impedance path characteristic of LC series resonance at specific harmonic frequencies, causing specific frequency harmonic currents to preferentially flow through the filter, forming local circulating currents. This significantly reduces the harmonic components injected into the AC power grid, achieving the suppression objective. However, this method has a fixed filtering bandwidth and is only effective for preset specific harmonics, failing to achieve unified suppression of all high-order harmonics. Furthermore, under weak grid conditions, the grid's equivalent impedance is large and highly inductive, easily forming a high-Q parallel resonant circuit with the filter's capacitive elements. This amplifies the harmonic current, seriously threatening system voltage stability and operational safety. Summary of the Invention

[0004] This invention provides a method and system for suppressing high-order harmonics, in order to solve the technical problem that passive harmonic suppression methods cannot achieve uniform suppression of all harmonics, thereby achieving the effect of uniform suppression of all high-order harmonics.

[0005] To address the aforementioned technical problems, this invention provides a method for suppressing high-order harmonics, applied to an SLCC DC transmission circuit composed of LCC and MMC, the method comprising: Obtain the three-phase current data of the LCC converter; The three-phase current data is compared with the preset current command value to perform error analysis and processing, and the three-phase harmonic error current data is obtained. Based on the three-phase harmonic error current data, the obtained output current data of the MMC converter is processed to compensate for the deviation, and three-phase harmonic compensation current data is obtained. The three-phase harmonic compensation current data are subjected to coordinate transformation to obtain d-axis orthogonal component data and q-axis orthogonal component data; Based on the RPC periodic repetitive control superposition algorithm, the d-axis orthogonal component data and the q-axis orthogonal component data are processed to obtain a two-phase orthogonal control output signal; The control MMC modulates the two-phase quadrature control output signal and performs high-order harmonic suppression on the SLCC DC transmission circuit based on the obtained harmonic compensation current.

[0006] As one preferred embodiment, the step of performing error analysis processing on the three-phase current data and the preset current command value to obtain three-phase harmonic error current data includes: Based on a low-pass filtering algorithm, the three-phase current data is denoised, and the denoising result is then aligned to obtain the three-phase current data to be analyzed. The three-phase current data to be analyzed is differentially calculated with the preset current command value to obtain the three-phase instantaneous error current data. The three-phase instantaneous error current data is subjected to secondary noise suppression processing to obtain the three-phase harmonic error current data.

[0007] As one preferred embodiment, the step of performing deviation compensation processing on the obtained output current data of the MMC converter based on the three-phase harmonic error current data to obtain three-phase harmonic compensation current data includes: The three-phase harmonic error current data and the obtained output current data of the MMC converter are time aligned to obtain aligned current data. The alignment current data is used to calculate the deviation, and the three-phase compensation deviation current data is obtained. The amplitude limiting processing is performed on the three-phase compensation deviation current data to obtain the three-phase harmonic compensation current data.

[0008] As one preferred embodiment, the coordinate transformation processing of the three-phase harmonic compensation current data to obtain d-axis orthogonal component data and q-axis orthogonal component data includes: The amplitude of the three-phase harmonic compensation current data is normalized to obtain the current data to be transformed. The current data to be transformed is subjected to Clark transformation to obtain the d-axis orthogonal component data and the q-axis orthogonal component data.

[0009] As one preferred embodiment, the RPC-based periodic repetitive control superposition algorithm processes the d-axis orthogonal component data and the q-axis orthogonal component data to obtain a two-phase orthogonal control output signal, including: Based on the RPC periodic repetitive control superposition algorithm, proportional-integral control is performed on the fundamental wave quantity in the d-axis orthogonal component data, and periodic repetitive control is performed on the harmonic quantity in the d-axis orthogonal component data to obtain the d-axis control component. Based on the RPC periodic repetitive control superposition algorithm, the fundamental wave quantity in the q-axis orthogonal component data is subjected to the proportional-integral control, and the harmonic quantity in the q-axis orthogonal component data is subjected to the periodic repetitive control to obtain the q-axis control component. The d-axis control component and the q-axis control component are subjected to Park transform processing to obtain the two-phase quadrature control output signal.

[0010] Another aspect of the present invention provides a high-order harmonic suppression system applied in an SLCC DC transmission circuit composed of an LCC and an MMC, the system comprising: The data acquisition module is used to acquire the three-phase current data of the LCC converter; The error analysis module is used to perform error analysis processing on the three-phase current data and the preset current command value to obtain three-phase harmonic error current data. The compensation deviation module is used to perform compensation deviation processing on the obtained output current data of the MMC converter based on the three-phase harmonic error current data to obtain three-phase harmonic compensation current data. The coordinate transformation module is used to perform coordinate transformation processing on the three-phase harmonic compensation current data to obtain d-axis orthogonal component data and q-axis orthogonal component data. The signal output module is used to process the d-axis orthogonal component data and the q-axis orthogonal component data based on the RPC periodic repetitive control superposition algorithm to obtain a two-phase orthogonal control output signal. The modulation processing module is used to control the MMC to modulate the two-phase quadrature control output signal and to suppress high-order harmonics in the SLCC DC transmission circuit based on the obtained harmonic compensation current.

[0011] As one preferred embodiment, the error analysis module includes: The filtering and denoising unit is used to perform denoising processing on the three-phase current data based on a low-pass filtering algorithm, and to perform data alignment processing on the denoising processing result to obtain the three-phase current data to be analyzed. The differential calculation unit is used to perform differential calculation between the three-phase current data to be analyzed and the preset current command value to obtain the three-phase instantaneous error current data. The noise suppression unit is used to perform secondary noise suppression processing on the three-phase instantaneous error current data to obtain the three-phase harmonic error current data.

[0012] As one preferred embodiment, the deviation compensation module includes: The time alignment unit is used to perform time alignment processing on the three-phase harmonic error current data and the acquired output current data of the MMC converter to obtain aligned current data. The deviation calculation unit is used to perform deviation calculation on the alignment current data to obtain three-phase compensation deviation current data. An amplitude limiting processing unit is used to perform amplitude limiting processing on the three-phase compensation deviation current data to obtain the three-phase harmonic compensation current data.

[0013] As one preferred embodiment, the coordinate transformation module includes: The normalization processing unit is used to perform amplitude normalization processing on the three-phase harmonic compensation current data to obtain the current data to be transformed. The Clark transform processing unit is used to perform Clark transform processing on the current data to be transformed to obtain the d-axis orthogonal component data and the q-axis orthogonal component data.

[0014] As one preferred embodiment, the signal output module includes: The d-axis control component unit is used to perform proportional-integral control on the fundamental wave quantity in the d-axis orthogonal component data based on the RPC periodic repetitive control superposition algorithm, and at the same time perform periodic repetitive control on the harmonic quantity in the d-axis orthogonal component data to obtain the d-axis control component. The q-axis control component unit is used to perform proportional-integral control on the fundamental wave quantity in the q-axis orthogonal component data based on the RPC periodic repetitive control superposition algorithm, and simultaneously perform periodic repetitive control on the harmonic quantity in the q-axis orthogonal component data to obtain the q-axis control component. The Park transform processing unit is used to perform Park transform processing on the d-axis control component and the q-axis control component to obtain the two-phase quadrature control output signal.

[0015] Compared with the prior art, the beneficial effects of the embodiments of the present invention are at least one of the following: This application directly utilizes the three-phase current error and MMC output to construct closed-loop compensation commands, avoiding Fourier decomposition or successive extraction of harmonics, thus significantly reducing computational complexity; This application performs coordinate transformation on the three-phase harmonic compensation current data and deploys a unified control model on the d-axis and q-axis respectively, using only two controllers to uniformly suppress all high-order harmonics generated by the commutator, greatly simplifying the system structure; Based on the active closed-loop control architecture, this application achieves automatic adaptation to different grid strength conditions, ensuring good harmonic suppression effect under all operating conditions, and effectively improving the robustness of harmonic suppression and system stability under weak grid conditions. Attached Figure Description

[0016] Figure 1 This is a flowchart illustrating a high-order harmonic suppression method in one embodiment of the present invention; Figure 2 This is an application diagram of the SLCC circuit in one embodiment of the present invention; Figure 3 This is a control block diagram of a PI controller plus an RPC controller in one embodiment of the present invention; Figure 4 This is a schematic diagram of a high-order harmonic suppression system in one embodiment of the present invention; Figure label: The module includes: 11. Data acquisition module; 12. Error analysis module; 13. Deviation compensation module; 14. Coordinate transformation module; 15. Signal output module; and 16. Modulation processing module. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The purpose of providing these embodiments is to make the disclosure of the present invention more thorough and comprehensive. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0018] In the description of this application, the terms "first," "second," "third," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined with "first," "second," "third," etc., may explicitly or implicitly include one or more of that feature. In the description of this application, unless otherwise stated, "a plurality of" means two or more.

[0019] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal communication between two components. The terms "vertical," "horizontal," "left," "right," "upper," "lower," and similar expressions used herein are for illustrative purposes only and do not indicate or imply that the device or component referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as limiting the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0020] In the description of this application, it should be noted that, unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used in this specification is for the purpose of describing specific embodiments only and is not intended to limit the invention. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0021] In SLCC circuits, line commutated converters (LCCs) have long been widely used in engineering scenarios such as cross-regional grid interconnection and large-scale new energy transmission due to their advantages such as simple topology, low manufacturing cost, and large capacity and long-distance power transmission capabilities. However, LCCs rely on AC system voltage for natural commutation. During the commutation process of their thyristor valve groups turning on and off, a large amount of non-sinusoidal current is inevitably generated on the AC side, which mainly includes characteristic high-order harmonics of the 6k±1st order (k is a positive integer), such as the 5th, 7th, 11th, and 13th harmonics. If these high-order harmonic currents are injected into the AC power grid without effective control, they will cause a series of serious problems: On the one hand, harmonics will cause voltage waveform distortion at the point of common coupling (PCC), reducing power quality and affecting the normal operation of sensitive electrical equipment; on the other hand, harmonics will generate additional copper and iron losses in equipment such as transformers and cables, leading to overheating, accelerated insulation aging, and shortened service life; more seriously, harmonics of certain frequencies may interfere with the sampling and judgment of relay protection devices, causing maloperation or failure to operate, directly threatening the safe and stable operation of the entire power system. Therefore, efficient and reliable suppression of high-order harmonics generated by LCC is a key technical aspect to ensure the operational reliability of SLCC circuits.

[0022] One embodiment of the present invention provides a method for suppressing higher harmonics. For details, please refer to [link to relevant documentation]. Figure 1 , Figure 1 The diagram shown is a flowchart of a high-order harmonic suppression method according to one embodiment of the present invention, the method including steps S1 to S6: S1. Obtain the three-phase current data of the LCC converter; S2. Perform error analysis processing on the three-phase current data and the preset current command value to obtain three-phase harmonic error current data. S3. Based on the three-phase harmonic error current data, the obtained output current data of the MMC converter is processed to compensate for the deviation, and the three-phase harmonic compensation current data is obtained. S4. Perform coordinate transformation on the three-phase harmonic compensation current data to obtain d-axis orthogonal component data and q-axis orthogonal component data; S5. Based on the RPC periodic repetitive control superposition algorithm, the d-axis orthogonal component data and the q-axis orthogonal component data are processed to obtain a two-phase orthogonal control output signal; S6. Control the MMC to modulate the two-phase quadrature control output signal, and suppress high-order harmonics in the SLCC DC transmission circuit based on the obtained harmonic compensation current.

[0023] Furthermore, in step S1, the high-order harmonics generated by the SLCC circuit during actual operation can affect the overall reliability of the circuit. For details, please refer to [link to relevant documentation]. Figure 2 , Figure 2 The diagram illustrates an application of the SLCC circuit in one embodiment of the present invention. Acquiring the three-phase current data of the LCC converter is crucial for real-time monitoring of the actual waveform characteristics of the converter's output current, thereby accurately identifying the harmonic components it contains. Specifically, high-precision current transformers are installed at the AC side outlet of the LCC converter to synchronously sample the instantaneous currents of phases a, b, and c. The analog signals are then converted from analog to digital and sent to the control system for digital processing. The high-precision current transformers include electromagnetic high-precision measurement current transformers, switchable high-precision current transformers, and zero-flux current transformers. This provides the initial basis for subsequent harmonic error calculations, avoiding reliance on model estimation or offline analysis, ensuring the real-time performance and accuracy of compensation commands, and laying the foundation for closed-loop feedback control, effectively improving the dynamic response and steady-state accuracy of the entire SLCC circuit.

[0024] Furthermore, in step S2, error analysis is performed on the three-phase current data and the preset current command value to separate unwanted harmonic components from the actual output current of the LCC converter. Specifically, firstly, the three-phase current data is denoised based on a low-pass filtering algorithm. Then, the denoising result is aligned. Next, a command value representing the ideal sinusoidal fundamental current is set in the power grid control system. This command value is determined jointly by the system power scheduling target and the grid phase information obtained by the phase-locked loop. Then, the pre-processed three-phase current data is subtracted from the command value of the corresponding phase in real time to obtain the three-phase instantaneous error current data containing all harmonic information. Finally, secondary noise suppression processing is performed on the three-phase instantaneous error current data to obtain the three-phase harmonic error current data. This step can directly extract the total amount of harmonics that need to be compensated without frequency domain decomposition or filtering operations. This not only simplifies the calculation process but also provides accurate and complete error input for subsequent closed-loop compensation control, significantly improving the response speed and accuracy of harmonic suppression.

[0025] Furthermore, in step S3, the compensation deviation processing of the output current data of the MMC converter based on the three-phase harmonic error current data is to construct a closed-loop feedback control mechanism to ensure that the compensation current output by the MMC can accurately track and cancel the harmonics generated by the LCC. Specifically, firstly, the three-phase harmonic error current data and the acquired output current data of the MMC converter are time-aligned to obtain aligned current data. Then, the two are compared in real time to calculate the deviation between them, thus obtaining the three-phase compensation deviation current data. Finally, by setting a current limiting threshold in the control system that matches the rated capacity of the MMC and the DC bus voltage, when the amplitude of any phase of the calculated three-phase compensation deviation current exceeds the threshold, its amplitude is proportionally compressed to the allowable range while maintaining the relative phase relationship between each phase, thereby generating safe and feasible three-phase harmonic compensation current data. This step dynamically corrects compensation errors caused by device delays, parameter drift, or grid disturbances, and effectively suppresses steady-state deviations and dynamic overshoot of the SLCC circuit, thereby significantly improving the accuracy of harmonic cancellation and the overall robustness of the SLCC circuit, providing a reliable guarantee for achieving high-quality power output.

[0026] Furthermore, in step S4, the coordinate transformation processing of the three-phase harmonic compensation current data is to convert the AC quantities that are originally coupled in the three-phase stationary coordinate system into DC quantities or periodic components that are decoupled in the synchronous rotating coordinate system, which facilitates high-performance control. Specifically, firstly, the amplitude of the three-phase harmonic compensation current data is normalized by dividing the current value of each phase by the system reference current or the maximum allowable current, scaling the original current data to a dimensionless uniform numerical range to form the current data to be transformed; then, the Clark transformation is performed on the current data to be transformed, using the linear mapping relationship from the three-phase stationary coordinate system to the two-phase stationary coordinate system to convert the a, b, and c phase signals into dq-axis current component data; finally, the Park transformation is performed on the dq-axis current component data, and combined with the grid voltage phase angle obtained in real time by the phase-locked loop, the signal in the two-phase stationary coordinate system is further rotated to a rotating coordinate system synchronized with the grid fundamental wave, finally obtaining the d-axis orthogonal component data and the q-axis orthogonal component data. This step allows the controller to independently adjust the active and reactive harmonic components, eliminating the effects of phase coupling, significantly improving control accuracy and dynamic response speed. It also provides a structural basis for subsequently deploying proportional-integral and repetitive control algorithms on the d-axis and q-axis respectively, thereby efficiently achieving unified suppression of higher-order harmonics.

[0027] Furthermore, in step S5, the orthogonal component data of the d-axis and q-axis are processed based on the RPC periodic repetitive control superposition algorithm to simultaneously achieve accurate tracking of the fundamental current and efficient suppression of all higher harmonics in a synchronously rotating coordinate system. Specifically, a composite control structure consisting of a proportional-integral (PI) controller and a repetitive controller (RPC) connected in parallel is constructed on the d-axis and q-axis respectively. The PI controller is responsible for fast response and eliminating the steady-state error of the fundamental component, while the repetitive controller uses its internal model principle to learn and compensate for the periodic harmonic signal cycle by cycle, providing high gain at the 6k frequency to accurately suppress the 5th, 7th, 11th, and 13th harmonics (6k±1) generated by the SLCC circuit. For details, please refer to [link to relevant documentation]. Figure 3 , Figure 3 The diagram shown is a control block diagram of a PI controller plus an RPC controller in one embodiment of the present invention.

[0028] Preferably, the z-domain transfer function G(z) of the periodic repetitive control superposition algorithm can be expressed as: in, Let be the transfer function of the proportional-integral controller in the z-domain, expressed as: The transfer function of the repetitive controller in the z-domain is expressed as: in, K p This is the proportionality coefficient. K i The integral coefficient is... N To control the ratio of the frequency to the fundamental frequency, Q ( z Typically, a constant less than 1 is chosen to ensure that the poles fall within the unit circle. To guarantee the stability of the control system, a compensation element is also required. C (z), C The expression for (z) is: in, S (z) is a low-pass filter, design... S (z) causes its gain to decay rapidly in the mid and high frequencies, while maintaining its gain in the mid and low frequencies. m It's a lead element used to compensate for phase lag. This is to ensure the repetitive controller only operates at 6... k If it operates at the ±1 (i.e., 5, 7, 11, 13, 17, 19...) second-highest harmonics, then it needs to operate on the dq axis. The RPC controller only targets the 6th harmonic. k The subharmonics have gain, from which the final repetitive controller transfer function can be obtained: Where, k RPC This is the gain of the RPC controller.

[0029] Preferably, to address the strong coupling of weak grids in SLCC circuits, an adaptive strategy based on grid strength detection is introduced, wherein the RPC controller gain scheduling strategy is expressed as: Q The adaptive adjustment strategy for the (z) filter parameters is expressed as: in, k RPC0 As the reference gain, α To adjust the coefficient, β = β 0 (1+ γG si ), γ To adjust the coefficient, the RPC gain is increased under weak power grid conditions to enhance the ability to suppress harmonics. G si The grid strength index is used to adaptively adjust the harmonic suppression of the SLCC circuit by calculating the RPC controller gain under different grid strength indices. The index calculation process is as follows: Among them, △ V , △ f The deviations in voltage and frequency are represented by SCR, which is the short-circuit ratio. x1, x2, and x3 are weighting coefficients. The power grid strength level is classified according to the Gsi value: strong power grid (Gsi < 0.1), medium power grid (0.1 ≤ Gsi < 0.3), and weak power grid (Gsi ≥ 0.3). The sampling frequency is based on the real-time fundamental frequency. The repetition period N can be dynamically adjusted. .

[0030] Furthermore, in step S6, the MMC modulates the two-phase quadrature control output signal to convert the d-axis and q-axis command signals generated by the controller into actual driving power device switching actions, thereby generating the required harmonic compensation current and suppressing high-order harmonics in the SLCC transmission circuit with strong coupling to a weak power grid. Specifically, the MMC converter first modulates the two-phase quadrature control output signal to obtain a modulated wave signal. Then, a carrier phase-shift pulse width modulation method is used to compare the modulated wave with a triangular carrier to generate a trigger pulse sequence for each submodule. This precisely controls the connection and disconnection states of the capacitors in each submodule of the MMC bridge arm, ensuring that the MMC outputs a compensation current with the same amplitude but opposite phase to the LCC harmonic current, thereby suppressing high-order harmonics in the SLCC transmission circuit with strong coupling to a weak power grid. This step enables real-time and proactive cancellation of high-order harmonics injected into the AC system, significantly improving the quality of the grid current waveform, avoiding the resonance risk caused by passive filters under weak grid conditions, and fully leveraging the rapid response and flexible control capabilities of the MMC to achieve a high-order harmonic suppression effect that is simple in structure, highly efficient, and highly adaptable.

[0031] Another embodiment of the present invention provides a high-order harmonic suppression system; for details, please refer to [link to relevant documentation]. Figure 4 , Figure 4 The diagram shown illustrates the structure of a high-order harmonic suppression system according to one embodiment of the present invention. The system includes: Data acquisition module 11 is used to acquire the three-phase current data of the LCC converter; Error analysis module 12 is used to perform error analysis processing on the three-phase current data and the preset current command value to obtain three-phase harmonic error current data. The compensation deviation module 13 is used to perform compensation deviation processing on the obtained output current data of the MMC converter based on the three-phase harmonic error current data to obtain three-phase harmonic compensation current data. Coordinate transformation module 14 is used to perform coordinate transformation processing on the three-phase harmonic compensation current data to obtain d-axis orthogonal component data and q-axis orthogonal component data; Signal output module 15 is used to process the d-axis orthogonal component data and the q-axis orthogonal component data based on the RPC periodic repetitive control superposition algorithm to obtain a two-phase orthogonal control output signal; The modulation processing module 16 is used to control the MMC to modulate the two-phase quadrature control output signal and to suppress high-order harmonics in the SLCC DC transmission circuit based on the obtained harmonic compensation current.

[0032] Furthermore, in the above embodiments, the error analysis module includes: The filtering and denoising unit is used to perform denoising processing on the three-phase current data based on a low-pass filtering algorithm, and to perform data alignment processing on the denoising processing result to obtain the three-phase current data to be analyzed. The differential calculation unit is used to perform differential calculation between the three-phase current data to be analyzed and the preset current command value to obtain the three-phase instantaneous error current data. The noise suppression unit is used to perform secondary noise suppression processing on the three-phase instantaneous error current data to obtain the three-phase harmonic error current data.

[0033] Furthermore, in the above embodiments, the deviation compensation module includes: The time alignment unit is used to perform time alignment processing on the three-phase harmonic error current data and the acquired output current data of the MMC converter to obtain aligned current data. The deviation calculation unit is used to perform deviation calculation on the alignment current data to obtain three-phase compensation deviation current data. An amplitude limiting processing unit is used to perform amplitude limiting processing on the three-phase compensation deviation current data to obtain the three-phase harmonic compensation current data.

[0034] Furthermore, in the above embodiments, the coordinate transformation module includes: The normalization processing unit is used to perform amplitude normalization processing on the three-phase harmonic compensation current data to obtain the current data to be transformed. The Clark transform processing unit is used to perform Clark transform processing on the current data to be transformed to obtain the d-axis orthogonal component data and the q-axis orthogonal component data.

[0035] Furthermore, in the above embodiments, the signal output module includes: The d-axis control component unit is used to perform proportional-integral control on the fundamental wave quantity in the d-axis orthogonal component data based on the RPC periodic repetitive control superposition algorithm, and at the same time perform periodic repetitive control on the harmonic quantity in the d-axis orthogonal component data to obtain the d-axis control component. The q-axis control component unit is used to perform proportional-integral control on the fundamental wave quantity in the q-axis orthogonal component data based on the RPC periodic repetitive control superposition algorithm, and simultaneously perform periodic repetitive control on the harmonic quantity in the q-axis orthogonal component data to obtain the q-axis control component. The Park transform processing unit is used to perform Park transform processing on the d-axis control component and the q-axis control component to obtain the two-phase quadrature control output signal.

[0036] Compared with the prior art, the beneficial effects of the embodiments of the present invention are at least one of the following: This application directly utilizes the three-phase current error and MMC output to construct closed-loop compensation commands, avoiding Fourier decomposition or successive extraction of harmonics, thus significantly reducing computational complexity; This application performs coordinate transformation on the three-phase harmonic compensation current data and deploys a unified control model on the d-axis and q-axis respectively, using only two controllers to uniformly suppress all high-order harmonics generated by the commutator, greatly simplifying the system structure; Based on the active closed-loop control architecture, this application achieves automatic adaptation to different grid strength conditions, ensuring good harmonic suppression effect under all operating conditions, and effectively improving the robustness of harmonic suppression and system stability under weak grid conditions.

[0037] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for suppressing higher harmonics, characterized in that, The method, applied to an SLCC (Supervised Low-Speed ​​Cylinder) DC transmission circuit composed of an LCC (Liquid Cylinder) and an MMC (Medium-Speed ​​Cylinder), includes: Obtain the three-phase current data of the LCC converter; The three-phase current data is compared with the preset current command value to perform error analysis and processing, and the three-phase harmonic error current data is obtained. Based on the three-phase harmonic error current data, the obtained output current data of the MMC converter is processed to compensate for the deviation, and three-phase harmonic compensation current data is obtained. The three-phase harmonic compensation current data are subjected to coordinate transformation to obtain d-axis orthogonal component data and q-axis orthogonal component data; Based on the RPC periodic repetitive control superposition algorithm, the d-axis orthogonal component data and the q-axis orthogonal component data are processed to obtain a two-phase orthogonal control output signal; The control MMC modulates the two-phase quadrature control output signal and performs high-order harmonic suppression on the SLCC DC transmission circuit based on the obtained harmonic compensation current.

2. The method for suppressing higher harmonics as described in claim 1, characterized in that, The step of performing error analysis processing on the three-phase current data and the preset current command value to obtain three-phase harmonic error current data includes: Based on a low-pass filtering algorithm, the three-phase current data is denoised, and the denoising result is then aligned to obtain the three-phase current data to be analyzed. The three-phase current data to be analyzed is differentially calculated with the preset current command value to obtain the three-phase instantaneous error current data. The three-phase instantaneous error current data is subjected to secondary noise suppression processing to obtain the three-phase harmonic error current data.

3. The method for suppressing higher harmonics as described in claim 1, characterized in that, The process of compensating for deviations in the obtained output current data of the MMC converter based on the three-phase harmonic error current data yields three-phase harmonic compensation current data, including: The three-phase harmonic error current data and the obtained output current data of the MMC converter are time aligned to obtain aligned current data. The alignment current data is used to calculate the deviation, and the three-phase compensation deviation current data is obtained. The amplitude limiting processing is performed on the three-phase compensation deviation current data to obtain the three-phase harmonic compensation current data.

4. The method for suppressing higher harmonics as described in claim 1, characterized in that, The coordinate transformation processing of the three-phase harmonic compensation current data to obtain d-axis orthogonal component data and q-axis orthogonal component data includes: The amplitude of the three-phase harmonic compensation current data is normalized to obtain the current data to be transformed. The current data to be transformed is subjected to Clark transformation to obtain the d-axis orthogonal component data and the q-axis orthogonal component data.

5. The method for suppressing higher harmonics as described in claim 1, characterized in that, The RPC-based periodic repetitive control superposition algorithm processes the d-axis orthogonal component data and the q-axis orthogonal component data to obtain a two-phase orthogonal control output signal, including: Based on the RPC periodic repetitive control superposition algorithm, proportional-integral control is performed on the fundamental wave quantity in the d-axis orthogonal component data, and periodic repetitive control is performed on the harmonic quantity in the d-axis orthogonal component data to obtain the d-axis control component. Based on the RPC periodic repetitive control superposition algorithm, the fundamental wave quantity in the q-axis orthogonal component data is subjected to the proportional-integral control, and the harmonic quantity in the q-axis orthogonal component data is subjected to the periodic repetitive control to obtain the q-axis control component. The d-axis control component and the q-axis control component are subjected to Park transform processing to obtain the two-phase quadrature control output signal.

6. A high-order harmonic suppression system, characterized in that, The system, applied in an SLCC (Superconducting Low-Speed ​​Cylinder) DC transmission circuit composed of an LCC (Liquid Crystal Capacitor) and an MMC (Medium-Speed ​​Cylinder), comprises: The data acquisition module is used to acquire the three-phase current data of the LCC converter; The error analysis module is used to perform error analysis processing on the three-phase current data and the preset current command value to obtain three-phase harmonic error current data. The compensation deviation module is used to perform compensation deviation processing on the obtained output current data of the MMC converter based on the three-phase harmonic error current data to obtain three-phase harmonic compensation current data. The coordinate transformation module is used to perform coordinate transformation processing on the three-phase harmonic compensation current data to obtain d-axis orthogonal component data and q-axis orthogonal component data. The signal output module is used to process the d-axis orthogonal component data and the q-axis orthogonal component data based on the RPC periodic repetitive control superposition algorithm to obtain a two-phase orthogonal control output signal; The modulation processing module is used to control the MMC to modulate the two-phase quadrature control output signal and to suppress high-order harmonics in the SLCC DC transmission circuit based on the obtained harmonic compensation current.

7. The high-order harmonic suppression system as described in claim 6, characterized in that, The error analysis module includes: The filtering and denoising unit is used to perform denoising processing on the three-phase current data based on a low-pass filtering algorithm, and to perform data alignment processing on the denoising processing result to obtain the three-phase current data to be analyzed. The differential calculation unit is used to perform differential calculation between the three-phase current data to be analyzed and the preset current command value to obtain the three-phase instantaneous error current data. The noise suppression unit is used to perform secondary noise suppression processing on the three-phase instantaneous error current data to obtain the three-phase harmonic error current data.

8. A high-order harmonic suppression system as described in claim 6, characterized in that, The deviation compensation module includes: The time alignment unit is used to perform time alignment processing on the three-phase harmonic error current data and the acquired output current data of the MMC converter to obtain aligned current data. The deviation calculation unit is used to perform deviation calculation on the alignment current data to obtain three-phase compensation deviation current data. An amplitude limiting processing unit is used to perform amplitude limiting processing on the three-phase compensation deviation current data to obtain the three-phase harmonic compensation current data.

9. A high-order harmonic suppression system as described in claim 6, characterized in that, The coordinate transformation module includes: The normalization processing unit is used to perform amplitude normalization processing on the three-phase harmonic compensation current data to obtain the current data to be transformed. The Clark transform processing unit is used to perform Clark transform processing on the current data to be transformed to obtain the d-axis orthogonal component data and the q-axis orthogonal component data.

10. A high-order harmonic suppression system as described in claim 6, characterized in that, The signal output module includes: The d-axis control component unit is used to perform proportional-integral control on the fundamental wave quantity in the d-axis orthogonal component data based on the RPC periodic repetitive control superposition algorithm, and at the same time perform periodic repetitive control on the harmonic quantity in the d-axis orthogonal component data to obtain the d-axis control component. The q-axis control component unit is used to perform proportional-integral control on the fundamental wave quantity in the q-axis orthogonal component data based on the RPC periodic repetitive control superposition algorithm, and simultaneously perform periodic repetitive control on the harmonic quantity in the q-axis orthogonal component data to obtain the q-axis control component. The Park transformation processing unit is used to perform Park transformation processing on the d-axis control component and the q-axis control component to obtain the two-phase quadrature control output signal.

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