Improved control method and system based on direct connection type flexible direct current converter

Abstract

The invention relates to the technical field of flexible direct-current power transmission, in particular to an improved control method and system based on a direct-connected flexible direct-current converter, and the method comprises the steps: S1, calculating a difference value between the direct-current components of an upper bridge arm and a lower bridge arm, and generating a bridge arm voltage direct-current correction value; s2, extracting a fundamental frequency component and a double frequency component in the bridge arm circulating current, and generating bridge arm voltage circulating current correction; s3, obtaining a DC deviation value based on the sum of the capacitor voltages of the upper and lower bridge arm sub-modules, and generating an AC voltage correction value; s4, superposing each correction value to a bridge arm voltage reference signal generated by active / reactive power control to obtain a final bridge arm modulation signal; and S5, independently executing S1 to S4 on each phase of bridge arm, and inputting the bridge arm modulation signal generated by each phase into the power device driving circuit of the corresponding bridge arm. According to the invention, the problems of bridge arm current imbalance, circulating current fluctuation and capacitor voltage deviation in the operation of the direct connection type flexible direct current converter are effectively solved, split-phase independent control is realized, and the system stability and the control precision are improved.

Description

Technical Field

[0001] This invention relates to the field of flexible DC transmission technology, and in particular to an improved control method based on a direct-connected flexible DC converter. Background Technology

[0002] Direct-coupled flexible DC converters are widely used in high-voltage direct current transmission and new energy grid connection scenarios due to their compact structure, low loss, and direct connection to AC systems. However, because this topology eliminates the isolation of the power frequency transformer, there is significant energy coupling between the upper and lower arms. During operation, it is prone to imbalance of the DC component of the arm current and fundamental and second harmonic circulating currents, leading to device current distortion and increased losses. At the same time, there are harmonic fluctuations in the voltage of the submodule capacitors, resulting in uneven energy distribution in the arms and affecting system stability.

[0003] Traditional control methods often use a single PI regulator or low-order filter to control circulating current and voltage, which makes it difficult to achieve frequency division regulation and independent correction of different frequency components. The control accuracy is limited. When the system is in dynamic operating conditions or running in phases, problems such as bridge arm energy imbalance, insufficient circulating current suppression and enhanced interphase coupling are likely to occur. Summary of the Invention

[0004] This invention provides an improved control method based on a direct-coupled flexible DC converter, which can effectively solve the problems in the background art.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: An improved control method based on a direct-coupled flexible DC converter, the method comprising: S1, calculate the difference between the DC components of the upper and lower bridge arms, and generate the DC correction amount of the bridge arm voltage through the first proportional-integral controller; S2, add the upper and lower bridge arm currents of each phase to obtain the bridge arm circulating current, extract the fundamental frequency component and the second harmonic component in the bridge arm circulating current, and input them into the quasi-resonant controller to generate the bridge arm voltage circulating current correction amount; S3, the DC deviation is obtained based on the sum of the capacitor voltages of the upper and lower bridge arm submodules, and the AC voltage correction is generated by the second proportional-integral controller; S4. The DC correction amount, circulating current correction amount, and AC voltage correction amount of the bridge arm voltage are superimposed on the bridge arm voltage reference signal generated by the active / reactive power control to obtain the final bridge arm modulation signal. S5, S1-S4 are executed independently for each phase arm, and the generated arm modulation signal for each phase is input into the power device drive circuit of the corresponding arm.

[0006] Furthermore, the quasi-resonant controller includes resonant elements respectively set for the fundamental frequency component and the second harmonic component, and the center frequency of each resonant element corresponds to the power grid fundamental frequency and the power grid second harmonic, respectively.

[0007] Furthermore, the quasi-resonant controller performs zero steady-state error tracking of the input AC signal, and the transfer expression for zero steady-state error tracking is: In the formula The transfer function of the quasi-resonant controller; This is the proportionality coefficient. and These are the resonance coefficients of the resonance modulator; and This represents the response bandwidth of the resonant modulator. This is the reference angular frequency.

[0008] Furthermore, the DC deviation is obtained after filtering out the power frequency component and harmonic component using a notch filter.

[0009] Furthermore, the active / reactive power control adopts a dual closed-loop structure, wherein the inner loop is a current control loop and the outer loop is an active / reactive power control loop.

[0010] Furthermore, the active power regulates the DC voltage, and the reactive power regulates the AC voltage.

[0011] Furthermore, the method also includes: when a DC-side short circuit or an AC-side voltage drop is detected, triggering current limiting control and reducing the amplitude of the bridge arm modulation signal.

[0012] Furthermore, the DC components of the upper and lower bridge arms are extracted through low-pass filtering.

[0013] Furthermore, the bridge arm modulation signals generated by each phase bridge arm are superimposed and modulated by a carrier wave to form a driving pulse sequence, which is updated in real time according to the switching state of the power devices.

[0014] Furthermore, the DC correction amount, circulating current correction amount, and AC voltage correction amount of the bridge arm voltage are combined by vector superposition to form the bridge arm voltage reference signal, wherein each correction amount corresponds to a compensation control channel of a different frequency component.

[0015] An improved control system based on a direct-coupled flexible DC converter, applied to the aforementioned improved control method based on a direct-coupled flexible DC converter, the system comprising: The difference between the DC components of the upper and lower bridge arms is calculated, and the DC correction amount of the bridge arm voltage is generated through the first proportional-integral controller. The upper and lower arm currents of each phase are added together to obtain the arm circulating current. The fundamental frequency component and the second harmonic component in the arm circulating current are extracted and input into the quasi-resonant controller to generate the arm voltage circulating current correction amount. The DC deviation is obtained based on the sum of the capacitor voltages of the upper and lower bridge arm submodules, and the AC voltage correction is generated by the second proportional-integral controller. The DC correction amount, circulating current correction amount, and AC voltage correction amount of the bridge arm voltage are superimposed on the bridge arm voltage reference signal generated by the active / reactive power control to obtain the final bridge arm modulation signal. The above steps are performed independently for each phase arm, and the generated arm modulation signal for each phase is input into the power device drive circuit of the corresponding arm.

[0016] The technical solution of this invention can achieve the following technical effects: By establishing independent control channels for the DC component difference between the upper and lower bridge arms, the fundamental and second harmonic components of the bridge arm circulating current, and the capacitor voltage deviation of the submodules, and uniformly superimposing and compensating them in the final bridge arm modulation signal in a vector manner, the converter can simultaneously achieve DC balance regulation, circulating current frequency division suppression, and capacitor voltage stability control during operation. This significantly improves the inherent problems of current imbalance, circulating current fluctuation, and capacitor voltage deviation in direct-coupled flexible DC converters. Due to the introduction of a quasi-resonant compensation mechanism for different frequency components, it can achieve zero steady-state error tracking and effective suppression of the fundamental and second harmonic circulating currents, making the bridge arm current waveform smoother and suppressing internal energy exchange. At the same time, by independently regulating the capacitor voltage deviation with DC flow, the converter can maintain long-term energy balance of each bridge arm submodule, avoiding output voltage asymmetry and drift. Furthermore, this method adopts a phase-independent execution strategy, so that the control process is no longer affected by inter-phase coupling. Each phase can autonomously achieve rapid correction of current and voltage under different operating conditions, improving the robustness and dynamic response capability of the overall system. The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0017] 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 recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1This is a flowchart illustrating an improved control method based on a direct-coupled flexible DC converter. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0020] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0021] Example 1: like Figure 1 As shown, this application provides an improved control method based on a direct-coupled flexible DC converter, the method comprising: S1, calculate the difference between the DC components of the upper and lower bridge arms, and generate the DC correction amount of the bridge arm voltage through the first proportional-integral controller; Specifically, the upper and lower arm currents of each phase arm of the direct-coupled flexible DC converter are sampled and processed in real time. The current signals are acquired by current sensors installed at the output terminals of the arm, and after signal conditioning and analog-to-digital conversion, they are input to the control unit. Since the arm current contains DC components, fundamental frequency components, and high-frequency harmonic components, in order to accurately reflect the energy balance of the arm, the control unit first uses a digital low-pass filter to filter the sampled signal. The cutoff frequency of the low-pass filter is preferably set to 10% to 30% of the grid fundamental frequency to effectively filter out AC and harmonic components and retain only the DC part. After filtering, the control unit calculates the DC components of the upper and lower arm currents respectively and obtains the difference between them. This difference reflects the deviation in energy distribution between the upper and lower arms. When the difference is positive, it indicates that the DC component of the upper arm current is larger and the arm voltage is higher; when the difference is negative, it indicates that the DC component of the lower arm current is larger and the arm voltage is lower. In order to achieve dynamic energy balance of the arm, a first proportional-integral controller is introduced to adjust this difference. The proportional loop is used for rapid response to current deviations, while the integral loop is used to eliminate steady-state errors, making the DC components of the upper and lower bridge arm currents tend to be consistent. The proportional coefficient and integral time constant of the proportional-integral controller can be adjusted online according to operating conditions to adapt to the current variation characteristics under different conditions. For example, when a large rate of change in current deviation is detected, the controller automatically increases the proportional coefficient to improve the response speed. When the system enters steady state, the integral loop gradually strengthens its effect to eliminate residual deviations and achieve long-term current balance. To prevent integral saturation during sudden current changes, the controller is equipped with anti-integral saturation protection, which automatically limits the integral increment when the output exceeds the preset limit value to prevent overshoot. The output signal obtained after adjustment by the proportional-integral controller is the DC correction amount of the bridge arm voltage. This signal is superimposed on the bridge arm voltage reference value generated by the main voltage control loop in subsequent control loops to compensate for voltage deviation caused by DC component imbalance.

[0022] S2, add the currents of the upper and lower bridge arms of each phase to obtain the bridge arm circulating current, extract the fundamental frequency component and the second harmonic component in the bridge arm circulating current, and input them into the quasi-resonant controller to generate the bridge arm voltage circulating current correction amount; Specifically, the arm circulating current reflects the flow of energy between the upper and lower arms inside the converter. It includes DC components, fundamental frequency components, second harmonic components, and high-frequency harmonic components. In order to suppress the main components that affect system stability, the control unit first uses a bandpass filter to perform frequency decomposition on the circulating current signal and extract the fundamental frequency component and the second harmonic component. The center frequencies of the bandpass filters are set to the grid fundamental frequency and the grid second harmonic, respectively. The bandwidth is adjusted according to system parameters and sampling frequency to ensure that the amplitude and phase distortion of the filtered signal are small. The extracted fundamental frequency component and second harmonic component are respectively sent to the two resonant channels of the quasi-resonant controller. The quasi-resonant controller generates high gain at a specified frequency to achieve zero steady-state error tracking and dynamic compensation of the target frequency component. The fundamental frequency channel is used to suppress the fundamental frequency component in the arm circulating current, so that the circulating current tends to zero at the power frequency, thereby reducing AC side current distortion. The second harmonic channel is used to compensate for the second harmonic circulating current caused by the DC link, reduce arm current fluctuations and improve submodule voltage balance. To ensure the stability of the controller under different operating conditions, the resonant bandwidth parameter can be adjusted online. When the grid frequency deviates or the converter operating mode is switched, the control unit can correct the resonant center frequency according to the real-time frequency detection results to avoid compensation mismatch caused by frequency deviation.

[0023] S3, the DC deviation is obtained based on the sum of the capacitor voltages of the upper and lower bridge arm submodules, and the AC voltage correction is generated by the second proportional-integral controller; Specifically, each bridge arm consists of several series-connected sub-modules. The voltage of the capacitors in the sub-modules reflects their energy storage state. During system operation, due to power fluctuations, changes in modulation duty cycle, and circulating current coupling, the capacitor voltages of the upper and lower bridge arm sub-modules are prone to deviation. If not corrected in time, this will cause an imbalance in the bridge arm output voltage, leading to an imbalance in the energy distribution within the converter. The control unit calculates the sum of the capacitor voltages of all sub-modules in the upper and lower bridge arms in each sampling period, and then takes the difference between the two to obtain the DC deviation of the capacitor voltage. To prevent the power frequency and harmonic components contained in this signal from affecting the control accuracy, a notch filter is introduced to filter the difference signal. The center frequency of the notch filter is set to the grid fundamental frequency, and its bandwidth is usually taken as 10% to 20% of the fundamental frequency to effectively filter out the power frequency and harmonic components, retaining only the DC component that characterizes the long-term voltage deviation. The filtered signal is smoother and can accurately reflect the degree of imbalance in the bridge arm voltage energy. The DC deviation obtained after filtering is input to the second proportional-integral controller. The proportional element is used to quickly adjust the voltage deviation, and the integral element is used to eliminate steady-state error, thereby achieving long-term balance of the bridge arm capacitor voltage. To improve the robustness of the control, the parameters of the proportional-integral controller can be adaptively adjusted according to the operating conditions. For example, when the voltage deviation changes significantly, the proportional gain is automatically increased to speed up the response. In the steady-state phase, the controller reduces the proportional coefficient and extends the integral time to enhance the smoothness and stability of the system. When the rate of change of the voltage deviation exceeds a predetermined threshold, the controller will activate the limiting and inverse integral saturation protection to prevent over-adjustment from causing voltage fluctuations or control oscillations. The output signal obtained after proportional-integral adjustment is the AC voltage correction amount. This correction amount reflects the compensation requirement of the bridge arm voltage energy and will be superimposed on the bridge arm voltage reference signal in subsequent control stages for real-time correction of the output voltage deviation.

[0024] S4. The DC correction amount of the bridge arm voltage, the circulating current correction amount of the bridge arm voltage, and the AC voltage correction amount are superimposed on the bridge arm voltage reference signal generated by the active / reactive power control to obtain the final bridge arm modulation signal. Specifically, within each sampling period, the correction values ​​are updated in real time based on the system's operating status. The three compensation signals are then vector-superimposed to generate a comprehensive arm voltage reference signal. This comprehensive signal simultaneously considers the requirements of DC voltage balance, circulating current suppression, and capacitor voltage stability, ensuring dynamic consistency of the arm output voltage across different frequency components. After generation, the comprehensive voltage reference signal is converted into an arm modulation signal by a modulation module. The preferred modulation method is carrier superposition modulation or pulse width modulation to ensure precise matching of the output signal amplitude and phase. Through this control process, multi-stage coordinated compensation can be achieved without increasing hardware complexity, effectively suppressing energy fluctuations within the arm and improving the dynamic response speed and steady-state control accuracy of the direct-coupled flexible DC converter under complex operating conditions.

[0025] S5 executes S1-S4 independently for each phase arm and inputs the generated bridge arm modulation signal for each phase into the power device drive circuit of the corresponding bridge arm.

[0026] Specifically, the three-phase control units only share DC bus voltage and system operating mode information, without directly exchanging current or modulation signal data. This ensures the autonomy and rapid response capability of each phase control. To further reduce inter-phase interference, the control system includes independent filtering and limiting modules in the current detection and voltage calculation stages of each phase to suppress high-frequency noise from other phases. The modulation signal generated by the control calculation for each phase arm is sent to the corresponding power device drive circuit. The drive circuit controls the on / off state of the power semiconductor devices based on the modulation signal, adjusting the number and polarity of sub-modules to achieve precise control of the bridge arm output voltage. To ensure a smooth switching process, the drive circuit uses soft switching or delayed triggering to reduce device stress and switching losses. In a preferred embodiment, the system can dynamically adjust the phase and amplitude of the modulation signal according to the operating state of each phase arm, keeping the three-phase output voltage consistent in amplitude and symmetrical in phase. When a single-phase current anomaly or voltage deviation is detected, the control unit will prioritize adjusting the proportional-integral parameters and circulating current suppression coefficient of that phase to achieve adaptive correction without affecting the operation of other phases.

[0027] This phase-by-phase independent execution and independent drive control strategy can maintain a balanced energy distribution across each phase arm under complex operating conditions, suppress interphase circulating currents and unbalanced voltages, and significantly improve the dynamic stability and voltage output quality of the direct-coupled flexible DC converter. This step enables the entire control system to achieve arm current balance and voltage stability while possessing higher fault tolerance and engineering adaptability.

[0028] This invention effectively solves the problems of unbalanced arm current, circulating current fluctuation, and capacitor voltage deviation in the operation of direct-coupled flexible DC converters, realizes independent phase control, and improves system stability and control accuracy.

[0029] As a preferred embodiment of the above, the quasi-resonant controller includes resonant elements respectively set for the fundamental frequency component and the second harmonic component, and the center frequency of each resonant element corresponds to the power grid fundamental frequency and the power grid second harmonic frequency respectively.

[0030] Specifically, the two resonant elements are designed with center frequencies around the grid's fundamental frequency and second harmonic frequency, respectively, enabling precise compensation and suppression of these two main interference frequencies. During control system operation, the arm circulating current signal typically contains power frequency and harmonic components. These components cause distortion of the arm current waveform and periodic alternating energy flow between the arms. The fundamental frequency component is mainly caused by AC side voltage fluctuations, while the second harmonic component originates from DC bus voltage ripple and energy exchange within the converter. Traditional control methods struggle to effectively suppress both types of components simultaneously. This implementation introduces dual resonant channels in the quasi-resonant controller, enabling compensation for interference signals at different frequencies. The proportional element of the quasi-resonant controller provides overall gain and dynamic response capability, while the two resonant elements each form high-gain characteristics at their center frequencies, allowing the controller to achieve zero steady-state error tracking of error signals at the grid's fundamental and second harmonic frequencies. The fundamental frequency resonant element primarily weakens the influence of the power frequency circulating current, ensuring the sinusoidal nature of the output current; the second harmonic resonant element compensates for the impact of DC side energy fluctuations on the arm circulating current, improving voltage balance and energy distribution.

[0031] As a preferred embodiment of the above, the quasi-resonant controller performs steady-state error-free tracking of the input AC signal, and the transfer expression for steady-state error-free tracking is: In the formula The transfer function of the quasi-resonant controller; This is the proportionality coefficient. and These are the resonance coefficients of the resonance modulator; and This represents the response bandwidth of the resonant modulator. This is the reference angular frequency.

[0032] Specifically, during operation, the arm circulating current signal is filtered and then input to the quasi-resonant controller. When the signal contains fundamental or second harmonic components, the controller's resonant circuit provides strong selective gain at the corresponding frequency, amplifying the error signal and driving the output to generate a constant-amplitude reverse compensation signal. This ensures that the output voltage waveform is completely consistent with the reference signal at these frequencies, effectively eliminating the residual components of the arm circulating current at the power frequency and harmonics, and ensuring that the converter output current maintains ideal sinusoidal characteristics. In the optimal design, the proportional coefficient... Used to adjust the overall response speed and system damping characteristics, its value range is set according to the system sampling frequency and power level; and These parameters determine the controller's amplification capabilities at the fundamental and second harmonic frequencies. When the system detects an increase in circulating current deviation, these two parameters can be appropriately increased to enhance the compensation effect. and Used to determine the resonant bandwidth, the value is typically taken as five to fifteen radians per second to balance frequency selectivity and phase stability; reference angular frequency This is equal to the fundamental angular frequency of the power grid. To further improve the system's stability and adaptability under different operating conditions, the control unit can monitor the power grid frequency offset online and adjust it in real time. Including bandwidth parameters, when the grid frequency deviates, the center frequency of the resonant element automatically tracks the change to prevent the resonant peak from deviating from the target frequency and reducing the compensation effect. In a preferred embodiment, the parameter tuning of the quasi-resonant controller adopts a combination of experimental modeling and online identification. During the initial operation phase, the circulating current response characteristics are obtained and the initial parameters are determined through system identification; during operation, the controller is corrected online according to the actual error amplitude and phase changes. , , and , Parameters are used to maintain the zero steady-state error characteristic.

[0033] As a preferred embodiment of the above, the DC deviation is obtained after filtering out the power frequency component and the harmonic component using a notch filter.

[0034] Specifically, the capacitor voltage difference signal is introduced into a notch filter stage. During operation, the capacitor voltage of the bridge arm submodule is affected by AC current and modulation fluctuations, and the signal typically contains significant power frequency and second harmonic components. If these periodic components are not filtered out, the calculated DC deviation will be too large, thus affecting the voltage balance control accuracy. The center frequency of the notch filter is set to the grid fundamental frequency, and its filtering bandwidth is adjusted according to the system damping and sampling frequency, generally taking 10% to 20% of the fundamental frequency. By setting suppression bands at the power frequency and second harmonic, AC fluctuations and harmonic interference can be effectively weakened, retaining only the long-term DC component of the voltage signal. In a preferred design, the notch filter is implemented digitally, allowing real-time adjustment of the center frequency according to the operating status to adapt to minor grid frequency drifts. The filtered output signal is smooth and stable, accurately reflecting the average deviation of the bridge arm capacitor voltage, providing reliable input data for the second proportional-integral controller, thereby improving the system's control accuracy and response performance in terms of energy balance and voltage stability.

[0035] As a preferred embodiment of the above, the active / reactive power control adopts a dual closed-loop structure, wherein the inner loop is a current control loop and the outer loop is an active / reactive power control loop.

[0036] Specifically, the outer loop uses active and reactive power as control targets. By calculating the instantaneous power of the three-phase voltage and current on the AC side, it obtains the real-time active and reactive power deviations. The outer loop outputs a current command signal, which serves as the reference input for the inner loop, used to maintain stable power output and regulate the DC voltage level. The inner loop uses current control as its core, adjusting the deviation between the actual current and the current given by the outer loop. The current control loop typically employs a component separation method in a synchronous rotating coordinate system, decomposing the current into active and reactive components. Rapid dynamic response is achieved through proportional-integral regulation. To enhance system robustness, feedforward compensation and limiting modules can be added to the inner loop to prevent current overshoot or phase lag. In the preferred design, active power control corresponds to DC voltage regulation. When the DC voltage deviates from the target value, the outer loop adjusts the active power command to restore the DC bus to stability. Reactive power control corresponds to AC side voltage support, achieving dynamic stability of the grid voltage by adjusting the reactive power command. The dual-loop structure enables the system to maintain voltage balance in steady state and provides rapid response and disturbance suppression capabilities under transient conditions.

[0037] As a preferred embodiment of the above, the active power regulates the DC voltage, and the reactive power regulates the AC voltage.

[0038] Specifically, active and reactive power each perform different regulation functions to achieve comprehensive and stable control of the system voltage. Active power regulation is used to maintain a constant DC-side voltage. When a deviation of the DC voltage from the target value is detected, the control unit adjusts the active power component output by the converter to balance the DC bus energy. When the DC voltage rises, the system increases active power output to release excess energy; when the DC voltage falls, the system reduces active power output or increases input power to restore the voltage level. Reactive power regulation is used to stabilize the AC-side voltage. The control unit detects the deviation between the AC-side voltage amplitude and the target value in real time and achieves voltage support by adjusting the reactive power component. When the AC voltage drops, the system injects reactive power to boost the voltage; when the voltage is too high, the system absorbs reactive power to reduce overvoltage, thereby keeping the grid-side voltage within the allowable range.

[0039] As a preferred embodiment of the above, the method further includes: when a DC-side short circuit or an AC-side voltage drop is detected, triggering current limiting control and reducing the amplitude of the bridge arm modulation signal.

[0040] Specifically, when the control unit detects a DC-side short circuit or a rapid drop in AC-side voltage, it determines the fault status by real-time monitoring of the rate of change of DC voltage, current, and AC bus voltage. If the DC current exceeds a preset threshold or the AC voltage falls below a set lower limit, the system immediately triggers current-limiting control logic. Current-limiting control achieves current limitation by rapidly adjusting the amplitude of the bridge arm modulation signal. The control unit first reduces the output amplitude of the bridge arm voltage reference value and introduces a limiting coefficient in the modulation stage, reducing the drive signal of the power devices and thus lowering the converter output voltage. This process is completed within several control cycles, effectively suppressing the rate of rise of the fault current and preventing overcurrent damage to the power devices.

[0041] As a preferred embodiment of the above, the DC components of the upper and lower bridge arms are extracted by low-pass filtering.

[0042] Specifically, using a low-pass filter to extract the DC component of the upper and lower arm currents is primarily to eliminate AC fluctuations and high-frequency harmonic interference in the current signal, thereby accurately reflecting the energy balance of the arms. During operation, the arm current of a direct-connected flexible DC converter not only contains a DC component representing energy flow but also includes fundamental and harmonic components generated by the AC side voltage and modulation signal. Furthermore, it is affected by switching operations and carrier modulation, resulting in high-frequency noise. If the unfiltered current signal is directly processed, the AC and high-frequency components will cause deviations in the DC component calculation, leading to errors in arm voltage correction and even energy distribution imbalance. A low-pass filter effectively isolates these non-DC components, retaining only the stable DC portion of the current signal. Through filtering, the extracted DC component accurately reflects the average level of arm energy transmission, providing a reliable basis for the arm voltage DC correction process.

[0043] As a preferred embodiment of the above, the bridge arm modulation signal generated by each phase bridge arm is superimposed and modulated by a carrier wave to form a driving pulse sequence, and the driving pulse sequence is updated in real time according to the switching state of the power device.

[0044] Specifically, to accurately convert the bridge arm modulation signal into the switching control signal of the power devices, a carrier superposition modulation method is used to generate a drive pulse sequence. After completing voltage correction and control calculations, each phase bridge arm outputs a continuous reference waveform as its modulation signal. The control unit compares the reference waveform with a high-frequency triangular carrier signal and determines the on / off state of the power devices based on their amplitude relationship, thus forming a pulse width modulated drive pulse sequence. This carrier superposition modulation method enables smooth adjustment of the bridge arm voltage, allowing the output voltage to maintain a near-sinusoidal waveform at a limited switching frequency. To improve system real-time performance, the drive pulse sequence is updated in real time according to the switching state of the power devices in each control cycle. When a power device temperature rise, DC bus voltage fluctuation, or circulating current change is detected, the control unit automatically corrects the amplitude and phase of the modulation signal to synchronize the drive pulse adjustment, thereby ensuring the continuity and stability of the converter output voltage.

[0045] As a preferred embodiment of the above, the DC correction amount of the bridge arm voltage, the circulating current correction amount of the bridge arm voltage, and the AC correction amount are combined by vector superposition to form the bridge arm voltage reference signal, wherein each correction amount corresponds to a compensation control channel of a different frequency component.

[0046] Specifically, in the vector superposition process, each correction is treated as a voltage vector component at its corresponding frequency. The DC correction, located in the zero-frequency band, compensates for long-term deviations in the bridge arm current. The circulating current correction eliminates the fundamental and second harmonic circulating currents within the bridge arm. The AC voltage correction maintains capacitor voltage balance and AC output stability. The control unit performs superposition calculations in vector space based on the amplitude and phase relationship of each correction signal, ensuring that the control actions of different frequency channels are independent yet coordinated. This vector superposition strategy enables simultaneous multi-frequency compensation within a unified voltage reference framework, avoiding phase interference and dynamic coupling problems in traditional cascade control structures.

[0047] Example 2: Based on the same inventive concept as the improved control method based on a direct-coupled flexible DC converter in the foregoing embodiments, the present invention also provides an improved control system based on a direct-coupled flexible DC converter, comprising: The difference between the DC components of the upper and lower bridge arms is calculated, and the DC correction amount of the bridge arm voltage is generated through the first proportional-integral controller. The upper and lower arm currents of each phase are added together to obtain the arm circulating current. The fundamental frequency component and the second harmonic component in the arm circulating current are extracted and input into the quasi-resonant controller to generate the arm voltage circulating current correction amount. The DC deviation is obtained based on the sum of the capacitor voltages of the upper and lower bridge arm submodules, and the AC voltage correction is generated by the second proportional-integral controller. The DC correction amount, circulating current correction amount, and AC voltage correction amount of the bridge arm voltage are superimposed on the bridge arm voltage reference signal generated by the active / reactive power control to obtain the final bridge arm modulation signal. The above steps are performed independently for each phase arm, and the generated arm modulation signal for each phase is input into the power device drive circuit of the corresponding arm.

[0048] The control system described above in this invention can effectively realize the improved control method based on the direct-coupled flexible DC converter, and the technical effects it can achieve are as described in the above embodiments, which will not be repeated here.

[0049] Although this application has been described in conjunction with specific features and embodiments, it is obvious that various modifications and combinations can be made thereto without departing from the spirit and scope of this application. Accordingly, this specification and drawings are merely exemplary illustrations of the application as defined herein, and are to be considered as covering any and all modifications, variations, combinations, or equivalents within the scope of this application. Clearly, those skilled in the art can make various alterations and modifications to this application without departing from its scope. Thus, if such modifications and modifications fall within the scope of this application and its equivalents, this application intends to include such modifications and modifications.

Claims

1. An improved control method based on direct connection type flexible HVDC converter, characterized in that, The method comprises: S1, calculating the difference between the DC components of the upper and lower bridge arms, and generating a bridge arm voltage DC correction amount through a first proportional integral controller; S2, adding the upper and lower bridge arm currents of each phase to obtain a bridge arm circulating current, extracting the fundamental frequency component and the double frequency component in the bridge arm circulating current, and inputting the components into a quasi-resonant controller to generate a bridge arm voltage circulating current correction amount; S3, obtaining a DC deviation amount based on the sum of the capacitor voltages of the upper and lower bridge arm sub-modules, and generating an AC voltage correction amount through a second proportional integral controller; S4, superimposing the bridge arm voltage DC correction amount, the bridge arm voltage circulating current correction amount, and the AC voltage correction amount into a bridge arm voltage reference signal generated by active / reactive power control to obtain a final bridge arm modulation signal; S5, independently performing S1-S4 on each phase bridge arm, and inputting the bridge arm modulation signal generated by each phase into the power device driving circuit of the corresponding bridge arm.

2. The improved control method for the direct-connected HVDC converter based on the flexible DC, according to claim 1, characterized in that, The quasi-resonant controller comprises a resonant element respectively arranged for the fundamental frequency component and the double frequency component, and the center frequencies of the resonant elements respectively correspond to the power grid fundamental frequency and the power grid double frequency.

3. The improved control method for the direct-connected HVDC converter based on the flexible DC, according to claim 2, characterized in that, The quasi-resonant controller performs non-static tracking on the input AC signal, and the transfer expression of the non-static tracking is: ； wherein is the transfer function of the quasi-resonant controller; is the proportional coefficient, and are the resonant coefficients of the resonant regulator, respectively; and is the response bandwidth of the resonant regulator; is the reference angular frequency.

4. The improved control method for the direct-connected HVDC converter based on the flexible DC, according to claim 1, characterized in that, The DC deviation amount is obtained after the power frequency component and the multiple frequency component are filtered out by a notch filter.

5. The improved control method for the direct-connected HVDC converter based on the flexible DC, according to claim 1, characterized in that, The active / reactive power control adopts a double closed-loop structure, wherein the inner loop is a current control loop, and the outer loop is an active / reactive power control loop.

6. The improved control method for the direct-connected HVDC converter based on the flexible DC, according to claim 5, characterized in that, The active power adjusts the DC voltage, and the reactive power adjusts the AC side voltage.

7. The improved control method for direct-connected VSC based on the direct- connected VSC according to claim 1, characterized in that, The method further comprises: when a DC side short circuit or an AC side voltage drop is detected, triggering a current limiting control and reducing the amplitude of the bridge arm modulation signal.

8. The improved control method for the direct-connected HVDC converter based on the flexible DC, according to claim 1, characterized in that, The DC components of the upper and lower bridge arms are extracted through low-pass filtering.

9. The improved control method for direct-connected VSC based on the converter as claimed in claim 1, characterized in that, The bridge arm modulation signals generated by each phase bridge arm are carrier superposition modulated to form a driving pulse sequence, and the driving pulse sequence is updated in real time according to the switching state of the power device.

10. The improved control method for the direct-connected HVDC converter based on the flexible DC, according to claim 1, characterized in that, The bridge arm voltage DC correction amount, the bridge arm voltage circulating current correction amount, and the AC voltage correction amount are combined to form the bridge arm voltage reference signal through vector superposition, and each correction amount corresponds to a compensation control channel of a different frequency component.

11. An improved control system for a direct-connected HVDC converter, applied to the improved control method for a direct-connected HVDC converter according to any one of claims 1-10, characterized in that, The system comprises: calculating the difference between the DC components of the upper and lower bridge arms, and generating a bridge arm voltage DC correction amount through a first proportional integral controller; adding the upper and lower bridge arm currents of each phase to obtain a bridge arm circulating current, extracting the fundamental frequency component and the double frequency component in the bridge arm circulating current, and inputting the components into a quasi-resonant controller to generate a bridge arm voltage circulating current correction amount; obtaining a DC deviation amount based on the sum of the capacitor voltages of the upper and lower bridge arm sub-modules, and generating an AC voltage correction amount through a second proportional integral controller; superimposing the bridge arm voltage DC correction amount, the bridge arm voltage circulating current correction amount, and the AC voltage correction amount into a bridge arm voltage reference signal generated by active / reactive power control to obtain a final bridge arm modulation signal; independently performing the above steps on each phase bridge arm, and inputting the bridge arm modulation signal generated by each phase into the power device driving circuit of the corresponding bridge arm.

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