Adaptive asymmetric impedance reshaping control method for magnetizing inrush of direct current grid-connected system

Abstract

The embodiment of the application provides a self-adaptive asymmetric impedance remodeling control method for excitation inrush current of a direct current grid-connected system, comprising: constructing a positive sequence additional damping controller and a negative sequence additional damping controller, including a high-pass filter link, a phase compensation link and a gain adjustment link connected in series; collecting three-phase voltages of an alternating current side, and calculating a voltage unbalance degree based on the three-phase voltages of the alternating current side; obtaining a background unbalance degree through a sliding window average method according to the voltage unbalance degree; obtaining a self-adaptive control coefficient through self-adaptive smooth activation function calculation with zero point anchoring characteristics based on the voltage unbalance degree, the background unbalance degree and a discrete change rate of the voltage unbalance degree; multiplying the self-adaptive control coefficient with outputs of the positive sequence additional damping controller and the negative sequence additional damping controller respectively to obtain positive sequence additional damping control quantity and negative sequence additional damping control quantity; and superimposing the positive sequence additional damping control quantity and the negative sequence additional damping control quantity to output ends of positive sequence current control loops and negative sequence current control loops respectively to realize accurate remodeling of asymmetric impedance.

Description

Technical Field

[0001] This application relates to the field of inrush current impedance reshaping technology, and in particular to an adaptive asymmetric impedance reshaping control method for inrush current in DC grid-connected systems. Background Technology

[0002] With the large-scale transmission of high-proportion renewable energy through DC transmission systems, the stability issues of converter stations connected to the AC grid are becoming increasingly prominent. In actual operation, inrush currents are easily generated when step-up transformers are closed under no-load conditions or when external faults are cleared, leading to deep saturation of the transformer core. Due to differences in the initial phase angle and residual magnetism distribution of the three phases, the cores of each phase exhibit non-uniform deep saturation characteristics, resulting in asymmetric inrush currents. This severe asymmetric operating condition on the AC side can trigger complex frequency coupling effects: the positive-sequence component of the second harmonic generated by transformer saturation will excite the DC component on the AC side through the hetero-sequence second harmonic coupling mechanism of the converter control loop. This DC component, in turn, acts on the transformer excitation branch, exacerbating its saturation, forming a positive feedback loop of "second harmonic - DC bias - exacerbated saturation," which in turn induces wideband resonance and harmonic amplification, seriously threatening system stability. Existing stability improvement strategies mostly employ fixed positive and negative sequence currents with additional feedback control. However, the degree of asymmetry in inrush currents is highly time-varying and random. If the additional control parameters are fixed and large, they will change the original impedance characteristics of the system under normal operating conditions, increase the risk of oscillation under normal disturbances, and reduce the control margin; if the parameters are small, the impedance cannot be effectively reshaped to block the frequency coupling positive feedback under inrush conditions.

[0003] Therefore, there is an urgent need for an adaptive asymmetric impedance reshaping control method for the inrush current of DC grid-connected systems, which can accurately reshape the asymmetric impedance. Summary of the Invention

[0004] The adaptive asymmetric impedance reshaping control method for inrush current in a DC grid-connected system provided in this application embodiment can accurately reshape the adaptive asymmetric impedance of the inrush current in a DC grid-connected system, including: Construct a positive-sequence additional damping controller and a negative-sequence additional damping controller; both the positive-sequence additional damping controller and the negative-sequence additional damping controller include a high-pass filter, a phase compensation, and a gain adjustment stage connected in series; the input terminal of the high-pass filter receives the current error signal of the corresponding sequence, and the output terminal of the gain adjustment stage outputs the modulation voltage command correction component of the corresponding sequence. Collect the three-phase voltage on the AC side, and calculate the voltage imbalance based on the three-phase voltage on the AC side; based on the voltage imbalance, obtain the background imbalance reflecting the steady-state asymmetry level of the power grid by using the sliding window averaging method; Based on the voltage imbalance, the background imbalance, and the discrete rate of change of the voltage imbalance, the adaptive control coefficients are calculated using an adaptive smoothing activation function with zero-point anchoring characteristics. The adaptive control coefficient is multiplied by the positive sequence modulation voltage command correction component output by the positive sequence additional damping controller to obtain the positive sequence additional damping control quantity; the adaptive control coefficient is multiplied by the negative sequence modulation voltage command correction component output by the negative sequence additional damping controller to obtain the negative sequence additional damping control quantity.

[0005] The positive-sequence additional damping control quantity is superimposed on the output of the positive-sequence current control loop, and the negative-sequence additional damping control quantity is superimposed on the output of the negative-sequence current control loop to complete the adaptive asymmetric impedance reshaping of the inrush current of the DC grid-connected system.

[0006] The technical solution provided in this application provides adjustable damping support for the system by constructing a positive-sequence additional damping controller and a negative-sequence additional damping controller, which include high-pass filtering, phase compensation, and gain adjustment stages. It calculates the voltage imbalance by collecting the three-phase AC voltage and obtains the background imbalance reflecting the steady-state asymmetry level of the power grid using the sliding window averaging method, thus distinguishing between normal power grid fluctuations and transient disturbances and avoiding false triggering of additional control. Based on the voltage imbalance, background imbalance, and the discrete rate of change of the voltage imbalance, adaptive control coefficients are calculated using an adaptive smoothing activation function with zero-point anchoring characteristics, enabling control during normal power grid operation. During symmetrical operation, the control coefficient is strictly zero, allowing the additional control branch to completely shut down. This eliminates the steady-state interference of continuous additional control on the base frequency power control and DC side voltage stability. When asymmetrical disturbances occur, seamless damping injection can be achieved, balancing steady-state control accuracy and transient suppression speed. Finally, the positive-sequence and negative-sequence additional damping control quantities are superimposed on the corresponding current control loop outputs to complete adaptive asymmetrical impedance reshaping. This effectively blocks the asymmetrical positive feedback caused by inrush current, suppresses wideband resonance and harmonic amplification, and improves the stability of the DC grid-connected system under inrush current conditions, while avoiding adverse effects on the system operation under normal conditions. Attached Figure Description

[0007] Figure 1 This is a flowchart of the adaptive asymmetric impedance reshaping control method for inrush current in a DC grid-connected system provided in this application embodiment; Figure 2 This is a schematic diagram of the adaptive asymmetric impedance reshaping control method for inrush current in a DC grid-connected system provided in the embodiments of this application; Figure 3 The system structure diagram provided in the embodiments of this application; Figure 4 The excitation inrush current FFT analysis results and ratio curves provided for embodiments of this application; Figure 5 A diagram of the positive-sequence additional damping control structure provided in the embodiments of this application; Figure 6 A diagram of the negative-sequence additional damping control structure provided in the embodiments of this application; Figure 7 The system simulation waveform diagram provided in the embodiments of this application is shown below. Figure 8 The curve showing the change in the proportion of second harmonics during transformer saturation operation is provided in the embodiments of this application. Figure 9 This is a schematic diagram of an electronic device structure provided in an embodiment of this application. Detailed Implementation

[0008] Complex frequency coupling and broadband resonance problems arise from transformer inrush current and core saturation in new energy DC grid-connected systems. In actual operation, the inrush current generated when the step-up transformer in the converter station is closed under no-load conditions or recovers from a fault leads to deep core saturation, generating a large number of second harmonics. The positive-sequence component of this second harmonic excites the DC component on the AC side through the converter's out-of-sequence second harmonic coupling mechanism. This DC component then reacts on the transformer's excitation branch, exacerbating its saturation, forming a positive feedback loop of "second harmonic - DC component - exacerbated saturation." This, in turn, induces broadband resonance and harmonic amplification in the system, seriously threatening the stable operation of high-proportion new energy DC transmission systems.

[0009] However, existing fixed-parameter asymmetric control strategies cannot simultaneously address the suppression requirements under extreme inrush conditions and the system stability under normal operating conditions. Existing positive and negative sequence additional feedback controls mostly employ fixed-parameter designs. If a large fixed gain is used to cope with extreme inrush, it will alter the system's original single-input, single-output impedance characteristics under normal operating conditions, reducing the system's phase margin and increasing the risk of oscillation under normal disturbances. If a smaller fixed gain is used, it cannot effectively reshape the impedance to block the aforementioned frequency-coupled positive feedback path.

[0010] Furthermore, asymmetric additional control strategies can cause negative disturbances to the system under normal operating conditions. Traditional additional control is often continuously applied under all system operating conditions. Even under normal or slightly unbalanced operating conditions without inrush current, it introduces additional control delays and impedance distortion, leading to a decline in the system's dynamic performance.

[0011] To address the aforementioned issues, this application proposes an adaptive asymmetric impedance reshaping control method for inrush current in DC grid-connected systems. This method can reshape the impedance based on the asymmetric characteristics of inrush current while avoiding adverse interference to the system under normal operating conditions.

[0012] The present application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0013] Figure 1 This is a flowchart of the adaptive asymmetric impedance reshaping control method for inrush current in a DC grid-connected system provided in this application embodiment. Figure 2 This is a schematic diagram of the adaptive asymmetric impedance reshaping control method for inrush current in a DC grid-connected system provided in an embodiment of this application. As can be seen, the adaptive asymmetric impedance reshaping control method for inrush current in a DC grid-connected system of this application includes: S1: Construct a positive-sequence additional damping controller and a negative-sequence additional damping controller; both the positive-sequence additional damping controller and the negative-sequence additional damping controller include a high-pass filter, a phase compensation, and a gain adjustment stage connected in series; the input of the high-pass filter receives the current error signal of the corresponding sequence, and the output of the gain adjustment stage outputs the modulation voltage command correction component of the corresponding sequence.

[0014] The input to the positive-sequence additional damping controller is the positive-sequence current disturbance component, and the input to the negative-sequence additional damping controller is the negative-sequence current disturbance component. The outputs of the positive-sequence additional damping controller and the negative-sequence additional damping controller are fed back to the corresponding current control loop input.

[0015] Specifically, the positive-sequence additional damping controller and the negative-sequence additional damping controller adopt a symmetrical unified structure: a high-pass filter first filters out the fundamental frequency and low-frequency steady-state components in the input signal to avoid interference from the additional control on the fundamental frequency power regulation of the original current loop; then, a phase compensation stage provides advanced phase correction for the target frequency band (such as the band where the second harmonic is located) to offset the inherent phase lag of the control system and improve the phase margin of the damping injection; finally, a gain adjustment stage outputs a damping control quantity of appropriate amplitude. The three stages are connected in series, enabling the controller to accurately act on the harmonic disturbance frequency band caused by inrush current without affecting the system characteristics under normal operating conditions. The positive and negative-sequence additional damping controllers independently generate positive-sequence and negative-sequence additional damping control quantities using the corresponding sequence current error signals, providing basic damping support for subsequent adaptive modulation to reshape impedance for the asymmetric characteristics of the excitation inrush current.

[0016] For example, the transfer function of the additional damping controller is expressed as:

[0017] in, This represents the transfer function of the additional damping controller. This indicates the additional damping control gain. This represents the transfer function of the high-pass filter stage. This represents the transfer function of the phase compensation stage.

[0018] Therefore, the positive-sequence modulated voltage command correction component output by the positive-sequence additional damping controller transfer function can be expressed as: The negative-sequence modulated voltage command correction component output by the negative-sequence additional damping controller transfer function can be expressed as: ,Right now:

[0019] in, This represents the positive-sequence current disturbance component. This is the negative sequence current disturbance component.

[0020] In one embodiment of this application, the output of the high-pass filter is connected to the input of the phase compensation stage, and the output of the phase compensation stage is connected to the input of the gain adjustment stage. The high-pass filter is used to reduce the influence of the fundamental frequency and low-frequency steady-state components on the original control objective.

[0021] The high-pass filter first filters the input signal, removing low-frequency and fundamental frequency components to prevent the steady-state fundamental frequency component from entering the additional control branch. The phase compensation stage adjusts the phase of the signal to compensate for delays in the control loop. The gain adjustment stage proportionally adjusts the signal amplitude to output a suitable damping control quantity. These stages are connected in series, processing the signal step by step to ensure that the positive-sequence and negative-sequence additional damping controllers can accurately generate the required damping control quantities, improving the reliability of the entire control method. Furthermore, the explicit connection relationships facilitate implementation in hardware circuits or software programs, reducing the difficulty of engineering implementation.

[0022] In one embodiment of this application, the expression for the high-pass filtering stage is: In the formula, Let S represent the transfer function of the high-pass filter stage, and let S represent the Laplace complex frequency variable. Indicates the cutoff angular frequency, and , This indicates the cutoff frequency of the high-pass filter.

[0023] This application overcomes the limitations of traditional methods that only consider steady-state imbalance by specifically extracting the discrete rate of change of voltage imbalance to accurately identify the transient and severe characteristics of inrush current. Furthermore, a composite filtering stage targeting the second harmonic and fundamental negative-sequence components of the inrush current—namely, a high-pass filter plus a phase-lead stage—is designed in the additional control branch. This design effectively prevents deep coupling of positive and negative sequences and broadband resonant divergence caused by transformer no-load closing, highly suited to the complex operating environment of high-proportion new energy AC / DC microgrids, and possesses strong robustness against extreme asymmetrical transient conditions such as inrush current. The high-pass filtering stage employs a first-order inertial high-pass filter, which effectively allows high-frequency signals to pass while attenuating low-frequency and DC signals. In DC grid-connected systems, the fundamental frequency current component is the primary component, but the additional damping control focuses on harmonics and unbalanced components induced by inrush current. By setting an appropriate high-pass filter cutoff frequency, steady-state components at the fundamental frequency and lower frequencies can be filtered out, preventing these components from entering the additional control branch and avoiding interference with the fundamental frequency power control of the original current control loop. Furthermore, this high-pass filter has a simple structure, low computational complexity, and is easily implemented in a digital signal processor, meeting the requirements of real-time control. The adjustable high-pass filter cutoff frequency also allows the controller to adapt to different grid frequency environments, enhancing its versatility.

[0024] In one embodiment of this application, the phase compensation stage employs a leading phase compensator, and the expression for the phase compensation stage is as follows: ； In the formula, Let S represent the transfer function of the phase compensation element, and let S represent the Laplace complex frequency variable. Indicates the corner frequency. α is the phase compensation coefficient, and 0 < α < 1, so that the phase compensation stage can provide advanced phase compensation in the target frequency band.

[0025] Phase compensation coefficient With the maximum compensation phase of the leading phase compensator Satisfy the following formula:

[0026] The strength of the compensation needs to be determined by the degree of negative damping in the target frequency band, i.e., the maximum compensation phase of the lead phase compensator. Size.

[0027] Maximum compensation phase of the leading phase compensator corresponding frequency for: The phase compensation stage employs a leading phase compensator, providing leading phase compensation within the target frequency band to offset delays and phase lags present in the control system. In DC grid-connected systems, the current control loop and modulation loop introduce a certain amount of phase lag, especially at higher frequencies, where phase lag can severely impact system stability. By appropriately setting parameters, precise phase compensation can be provided at the target frequency requiring damping injection, increasing the system's stability margin and effectively suppressing wideband resonance. This phase compensation design allows the additional damping control to act more precisely on the frequency band requiring improvement, enhancing inrush current suppression, and the parameter tuning method is clear, facilitating engineering applications.

[0028] S2: Collect the three-phase voltage on the AC side and calculate the voltage imbalance based on the three-phase voltage on the AC side; based on the voltage imbalance, obtain the background imbalance reflecting the steady-state asymmetry level of the power grid by using the sliding window averaging method.

[0029] In one embodiment of this application, the voltage imbalance is calculated based on the three-phase AC voltage, specifically including: Acquire the three-phase voltage on the AC side; for example, acquire the three-phase voltage on the AC side synchronously at the switching frequency.

[0030] Based on the three-phase AC voltage, the positive-sequence voltage base amplitude and negative-sequence voltage base amplitude are obtained through a dual second-order generalized integrator. The voltage imbalance is calculated based on the fundamental amplitude values ​​of the positive-sequence voltage and the fundamental amplitude values ​​of the negative-sequence voltage.

[0031] A dual second-order generalized integrator is employed to extract the fundamental amplitudes of the positive-sequence and negative-sequence voltages. This method offers accurate calculations and a fast dynamic response, enabling precise separation of positive and negative-sequence components even in the presence of harmonics and distortions in the grid voltage, thus yielding a reliable voltage imbalance. This provides accurate input characteristics for subsequent adaptive control, ensuring the reliable operation of the entire adaptive control method and laying the foundation for the correct output of the activation function.

[0032] In one embodiment of this application, the formula for calculating voltage imbalance is:

[0033] in, Indicates the degree of voltage imbalance. This represents the fundamental amplitude of the negative sequence voltage. This represents the amplitude of the positive sequence voltage fundamental wave.

[0034] The voltage imbalance obtained by the above formula can accurately quantify the degree of grid asymmetry, providing a direct basis for subsequent judgment on whether asymmetrical faults such as inrush current have occurred. Furthermore, this ratio is not sensitive to the absolute magnitude of the voltage amplitude, reflecting only the relative degree of asymmetry. Therefore, it has good adaptability, is unaffected by grid voltage fluctuations, and ensures the universality of the control method in systems of different voltage levels.

[0035] To accurately capture the characteristics of extreme transient asymmetrical operating conditions such as inrush current, the discrete rate of change of voltage imbalance is further calculated. .

[0036]

[0037] in, T represents the discrete rate of change of the voltage imbalance at time t. S The sampling period is This represents the degree of voltage imbalance at time t. The voltage imbalance at time tN represents the voltage unbalance, where N is the sliding window step size for calculating the discrete rate of change of the voltage unbalance. N is used as the sampling interval window (transient) for calculating the discrete rate of change of the voltage unbalance, typically ranging from 1 to 5 sampling periods. The discrete rate of change of the voltage unbalance is used to characterize the transient severity of asymmetric disturbances, providing multi-dimensional feature input for subsequent operating condition identification.

[0038] In one embodiment of this application, to characterize the inherent steady-state background asymmetry level of the power grid (such as line parameter asymmetry or slight imbalance caused by long-term single-phase load), the sliding window averaging method is used to calculate the background imbalance degree. The calculation formula is as follows:

[0039] in, This represents the background imbalance, where M is the length of the sliding window used to calculate the background imbalance, and N is... s For sampling interval window, The voltage imbalance is represented by k, which represents time k, and i represents a variable ranging from 0 to M-1 (representing every second).

[0040] In order to completely filter out the pulsating interference within the fundamental period and accurately extract the steady-state DC background value, the value of the sliding window length M for calculating the background imbalance is strictly constrained to be the controller sampling frequency f. c The ratio of the frequency to the fundamental frequency f0 of the power grid is rounded down, i.e.:

[0041] The sliding window averaging method is used to calculate background imbalance, effectively filtering out short-term fluctuations and noise in voltage imbalance and extracting a background value reflecting the long-term steady-state characteristics of the power grid. During normal grid operation, voltage imbalance may fluctuate slightly; the sliding window averaging method provides a stable background value, preventing accidental triggering of additional control measures due to instantaneous fluctuations. The calculation method is simple, requiring only a data buffer and calculating the arithmetic mean, thus having low processor resource requirements and making it suitable for resource-constrained embedded control systems. The window length can be flexibly adjusted to adapt to different response speed requirements. It provides a reliable steady-state reference for subsequent activation threshold adjustments, helping to achieve a balance between steady-state anti-malfunction and rapid transient response.

[0042] S3: Based on the voltage imbalance, background imbalance, and discrete rate of change of voltage imbalance, the adaptive control coefficients are calculated using an adaptive smoothing activation function with zero-point anchoring characteristics. In one embodiment of this application, step S3 specifically includes: S31: The discrete rate of change of voltage imbalance is calculated by the difference between the voltage imbalance at the current moment and the voltage imbalance at the previous sampling moment. S32: Calculate the unbalance activation threshold based on the discrete rate of change of voltage unbalance and background unbalance. S33: Based on the voltage imbalance and the imbalance activation threshold, the adaptive control coefficient is calculated using an adaptive smooth activation function.

[0043] The discrete rate of change of voltage imbalance is used to capture and react to sudden changes in voltage imbalance, thereby quickly identifying transient disturbances such as inrush current. The imbalance activation threshold is dynamically adjustable: in steady state, the discrete rate of change of voltage imbalance is small, and the activation threshold is high to prevent malfunctions; in transient state, the discrete rate of change of voltage imbalance is large, and the activation threshold is lowered to achieve rapid activation. The adaptive smooth activation function ensures a smooth transition of the control coefficient from zero to the target value, avoiding the impact of sudden changes. This ensures that the adaptive control coefficients accurately reflect the grid operating conditions and achieve a smooth switching between zero-interference normal operating conditions and strong transient damping.

[0044] In one embodiment of this application, the formula for calculating the adaptive control coefficient is:

[0045] in, This represents the adaptive control coefficient, where μ is the smoothing transition factor. Indicates the imbalance activation threshold. This indicates the degree of voltage imbalance.

[0046] when hour, Strictly equal to zero, achieving zero interference under symmetrical operating conditions. The adaptive smooth activation function of this application, with zero-point anchoring characteristics, ensures that the adaptive control coefficient is strictly zero during normal symmetrical grid operation, and the additional control branch is completely dormant, eliminating the steady-state interference to baseband power control and DC-side voltage stability caused by continuous operation of traditional additional control. When asymmetrical disturbances occur, the control coefficient can achieve seamless damping injection, balancing steady-state control accuracy and transient suppression speed. This achieves "zero interference" under normal symmetrical operating conditions and "strong damping" under transient operating conditions.

[0047] The adaptive control coefficient formula exhibits zero-point anchoring characteristics, meaning that when the voltage imbalance is zero, the adaptive control coefficient remains strictly zero regardless of the imbalance activation threshold. This characteristic ensures that during normal symmetrical grid operation, the additional control branch remains completely inactive, effectively eliminating interference with baseband power control and DC-side voltage stability. Furthermore, when the voltage imbalance exceeds the threshold, the adaptive control coefficient increases smoothly, achieving a smooth damping injection and avoiding system shocks caused by step changes.

[0048] The smooth transition factor μ is:

[0049] in, This represents the acceptable limit for voltage imbalance. The smooth transition factor determines the steepness of the adaptive control coefficients; by selecting this factor appropriately, both response speed and smoothness can be considered.

[0050] In one embodiment of this application, the formula for calculating the imbalance activation threshold is:

[0051] in, Indicates the imbalance activation threshold. Indicates background imbalance. The discrete rate of change representing the voltage unbalance. This indicates the steady-state anti-maloperation tolerance, and its value is the acceptable limit for voltage unbalance specified in the national standard (GB / T 15543). With the present The difference, The transient attenuation coefficient is represented by the maximum allowable transient response time t of the system. r Decide, Background imbalance reflects the inherent steady-state asymmetry level of the power grid, and the steady-state anti-maloperation tolerance ensures that even slight fluctuations during normal operation will not trigger maloperations. The exponential term causes the threshold to decrease as the rate of change increases: when severe transients such as inrush current occur, the rate of change is large, the exponential term approaches 0, and the imbalance activation threshold approaches the background imbalance, thus achieving rapid activation; when in steady state, the rate of change is small, the exponential term approaches 1, and the imbalance activation threshold increases, preventing maloperation. The transient decay coefficient determines the rate at which the threshold decays with the rate of change. The dynamic threshold balances the needs of steady-state anti-maloperation and rapid transient suppression, significantly improving the adaptability and reliability of the control method.

[0052] S4: Multiply the adaptive control coefficient by the positive-sequence modulation voltage command correction component output by the positive-sequence additional damping controller to obtain the positive-sequence additional damping control quantity; multiply the adaptive control coefficient by the negative-sequence modulation voltage command correction component output by the negative-sequence additional damping controller to obtain the negative-sequence additional damping control quantity. For example, for a positive-sequence current control channel, the positive-sequence additional damping control quantity Represented as: For the negative sequence current control channel, the negative sequence additional damping control quantity It can be represented as: in, The positive-sequence modulated voltage command correction component is added to the output of the positive-sequence damped controller transfer function. The negative-sequence modulated voltage command correction component is added to the output of the negative-sequence damped controller transfer function. This is added to the original output signal of the positive sequence current control loop. To be superimposed on the original output signal of the negative sequence current control loop, i.e. the modulation voltage command output by the current inner loop PI controller, and then sent to the modulation stage to generate the final bridge arm voltage command.

[0053] S5: The positive-sequence additional damping control quantity is superimposed on the output of the positive-sequence current control loop, and the negative-sequence additional damping control quantity is superimposed on the output of the negative-sequence current control loop to complete the adaptive asymmetric impedance reshaping of the inrush current of the DC grid-connected system.

[0054] When only a single dominant channel needs to be controlled, only the positive-sequence modulation voltage command correction component output by the positive-sequence additional damping controller transfer function or the negative-sequence modulation voltage command correction component output by the negative-sequence additional damping controller transfer function can be applied; when both the positive-sequence and negative-sequence impedances have significant negative damping regions, both the positive-sequence modulation voltage command correction component and the negative-sequence modulation voltage command correction component can be applied simultaneously.

[0055] This application incorporates positive-sequence control, negative-sequence control, and their coupling effects into a unified broadband impedance analysis framework. It generates an equivalent additional impedance on the AC side through positive-sequence and negative-sequence additional damping controllers. This method can flexibly select single-sided or dual-sided channel injection based on the negative damping distribution of the target frequency band. Simultaneously, it improves the device's positive-sequence voltage support capability and negative-sequence harmonic suppression capability, avoiding system instability issues caused by blindly superimposing controllers across multiple stages. This achieves precise reshaping and coordinated control of asymmetrical positive and negative-sequence impedances.

[0056] For example, taking the converter in a high-proportion renewable energy DC grid-connected system as the object, the control design is carried out for the typical asymmetrical operating condition of core saturation caused by inrush current generated when the step-up transformer of the converter station is closed under no-load conditions. Figure 3 This is a system architecture diagram provided for an embodiment of this application. Wherein, i mmc u mmc and i cir This represents the sampled grid-connected current, voltage, and circulating current of the Modular Multilevel Converter (MMC). Subscripts "ABC" indicate the corresponding ABC three-phase current, voltage, or circulating current; subscripts "dq" indicate the dq-axis components of the three-phase current, voltage, or circulating current; superscripts "+" and "-" represent the positive and negative sequence components of the dq-axis current, voltage, or circulating current, respectively. mmc This represents the modulation voltage generated by the control system, and base represents the reference value during the per-unit process.

[0057] In this embodiment, when the transformer is switched on under no-load conditions, generating inrush current, the deep saturation of the iron core will excite 1st to 9th harmonic currents with significant amplitudes. For example... Figure 4 As shown, the cumulative proportion of the first nine harmonics of the PCC point current is very close to 100%, indicating that the first to ninth harmonics cover the main harmonic components under transformer saturation conditions, while the contributions of the tenth to nineteenth harmonics to the main spectral characteristics are relatively small. Among them, the positive-sequence current component of the second harmonic (100Hz) reaches more than 20% and has a significant DC component, verifying the theoretical analysis of the "second harmonic-DC bias-aggravated saturation" positive feedback loop caused by transformer saturation. The second harmonic corresponds to 100 Hz, which is the main coupling component of the closed-loop positive feedback formed by the transformer saturation and the MMC out-of-sequence second harmonic coupling. Therefore, it is the target frequency for suppression in this application.

[0058] A wideband impedance model for the converter, considering the aforementioned frequency coupling effects, was established. Sensitivity scanning of the converter control parameters revealed that the proportional coefficients of the positive-sequence and negative-sequence current inner loops have the most significant impact on the impedance phase and amplitude in the 100Hz and 0Hz frequency bands. Therefore, the reference input terminals of the positive and negative-sequence current inner loops were selected as the injection points for additional damping control. Based on the aforementioned impedance characteristic analysis results, this section proposes an additional damping control strategy based on positive and negative sequence current feedback. Its overall control structure is as follows: Figure 5 and Figure 6 As shown. Among them, This represents the transfer function of the additional feedback control loop, which is used to generate corresponding additional control signals based on the positive and negative sequence current disturbances, so as to adjust the AC side impedance characteristics of the converter.

[0059] Figure 7 The following is a system simulation waveform diagram for transformer saturation operation provided in the embodiments of this application. Figure 8The curve showing the change in the proportion of second harmonics during transformer saturation operation is provided in the embodiments of this application. From Figure 7 As can be seen, the 50Hz harmonic source on the DC side is added at t1=5s to trigger transformer saturation, and then this harmonic source is removed at t2=8s. The additional feedback strategy is added at t3=10s. Figure 8 It can be observed that, without any additional feedback control, even after the disturbance is removed at t2=8s, the 100Hz oscillation persists without decay; however, after adding positive-sequence additional damping control and negative-sequence additional damping control respectively, after t... c After a settling time of 0.59s, the oscillation was suppressed, and the second harmonic content was reduced to below 2% of the harmonic limit specified in the national standard, which is consistent with the theoretical analysis conclusion.

[0060] As can be seen from the above implementation process, this application can accurately reshape the target frequency band impedance through adaptive positive and negative sequence additional control without changing the main circuit structure of the system, perfectly balancing zero interference under normal operating conditions and strong damping suppression under extreme operating conditions such as inrush current.

[0061] This application provides adjustable damping support for the system by constructing positive-sequence and negative-sequence additional damping controllers, which include high-pass filtering, phase compensation, and gain adjustment. It calculates the voltage imbalance by collecting three-phase AC voltage data and uses a sliding window averaging method to obtain the background imbalance reflecting the steady-state asymmetry level of the power grid, thus distinguishing between normal grid fluctuations and transient disturbances and avoiding false triggering of additional control. Based on the voltage imbalance, background imbalance, and the rate of change of voltage imbalance, adaptive control coefficients are calculated using an adaptive smoothing activation function with zero-point anchoring characteristics, enabling the system to operate under normal symmetrical power grid conditions. The control coefficient is strictly zero, which puts the additional control branch into complete dormancy, eliminating the steady-state interference of continuous additional control on the base frequency power control and DC side voltage stability. When asymmetric disturbances occur, it can also achieve seamless damping injection, taking into account both steady-state control accuracy and transient suppression speed. Finally, the positive-sequence and negative-sequence additional damping control quantities are superimposed on the corresponding current control loop output terminals to complete adaptive asymmetric impedance reshaping, effectively blocking the asymmetric positive feedback caused by inrush current, suppressing wideband resonance and harmonic amplification, improving the stability of DC grid-connected system under inrush current conditions, and avoiding adverse effects on system operation under normal conditions.

[0062] like Figure 9 As shown in the figure, this application provides an electronic device, including a processor 111, a communication interface 112, a memory 113, and a communication bus 114, wherein the processor 111, the communication interface 112, and the memory 113 communicate with each other through the communication bus 114. Memory 113 is used to store computer programs; In one embodiment of this application, when the processor 111 executes the program stored in the memory 113, it implements the adaptive asymmetric impedance reshaping control method for inrush current in a DC grid-connected system provided in any of the foregoing method embodiments, including: Construct a positive-sequence additional damping controller and a negative-sequence additional damping controller; both the positive-sequence additional damping controller and the negative-sequence additional damping controller include a high-pass filter, a phase compensation, and a gain adjustment stage connected in series; the input terminal of the high-pass filter receives the current error signal of the corresponding sequence, and the output terminal of the gain adjustment stage outputs the additional damping control quantity of the corresponding sequence. Collect the three-phase voltage on the AC side, and calculate the voltage imbalance based on the three-phase voltage on the AC side; based on the voltage imbalance, obtain the background imbalance reflecting the steady-state asymmetry level of the power grid by using the sliding window averaging method; Based on the voltage imbalance, the background imbalance, and the discrete rate of change of the voltage imbalance, the adaptive control coefficients are calculated using an adaptive smoothing activation function with zero-point anchoring characteristics. Multiply the adaptive control coefficient by the output of the positive-sequence additional damping controller to obtain the positive-sequence additional damping control quantity; multiply the adaptive control coefficient by the output of the negative-sequence additional damping controller to obtain the negative-sequence additional damping control quantity; The positive-sequence additional damping control quantity is superimposed on the output of the positive-sequence current control loop, and the negative-sequence additional damping control quantity is superimposed on the output of the negative-sequence current control loop to complete the adaptive asymmetric impedance reshaping of the inrush current of the DC grid-connected system.

[0063] This application also provides a computer-readable storage medium storing a computer program thereon, which, when executed by a processor, implements the steps of the method provided in any of the foregoing method embodiments.

[0064] The device embodiments described above are merely illustrative. The units described as separate components may or may not be physically separate, and the components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the modules can be selected to achieve the purpose of this embodiment according to actual needs.

[0065] Through the above description of the embodiments, those skilled in the art can clearly understand that each embodiment can be implemented by means of software plus a general-purpose hardware platform, and of course, it can also be implemented by hardware. Based on this understanding, the above technical solutions, in essence or the parts that contribute to the related technology, can be embodied in the form of a software product. This computer software product can be stored in a computer-readable storage medium, such as ROM / RAM, magnetic disk, optical disk, etc., including several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute the methods of various embodiments or some parts of embodiments.

[0066] The above embodiments are merely illustrative examples and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations. However, obvious variations or modifications derived therefrom are still within the scope of protection of this application.

Claims

1. An adaptive asymmetric impedance reshaping control method for inrush current in a DC grid-connected system, characterized in that, include: Construct positive-sequence additional damping controllers and negative-sequence additional damping controllers; Both the positive-sequence additional damping controller and the negative-sequence additional damping controller include a high-pass filter, a phase compensation, and a gain adjustment stage connected in series. The input of the high-pass filter receives the current error signal of the corresponding sequence, and the output of the gain adjustment stage outputs the modulation voltage command correction component of the corresponding sequence. Collect the three-phase voltage on the AC side, and calculate the voltage imbalance based on the three-phase voltage on the AC side; based on the voltage imbalance, obtain the background imbalance reflecting the steady-state asymmetry level of the power grid by using the sliding window averaging method; Based on the voltage imbalance, the background imbalance, and the discrete rate of change of the voltage imbalance, the adaptive control coefficients are calculated using an adaptive smoothing activation function with zero-point anchoring characteristics. The adaptive control coefficient is multiplied by the positive sequence modulation voltage command correction component output by the positive sequence additional damping controller to obtain the positive sequence additional damping control quantity; the adaptive control coefficient is multiplied by the negative sequence modulation voltage command correction component output by the negative sequence additional damping controller to obtain the negative sequence additional damping control quantity. The positive-sequence additional damping control quantity is superimposed on the output of the positive-sequence current control loop, and the negative-sequence additional damping control quantity is superimposed on the output of the negative-sequence current control loop to complete the adaptive asymmetric impedance reshaping of the inrush current of the DC grid-connected system.

2. The adaptive asymmetric impedance reshaping control method for inrush current in a DC grid-connected system according to claim 1, characterized in that, The output of the high-pass filter is connected to the input of the phase compensation stage, and the output of the phase compensation stage is connected to the input of the gain adjustment stage.

3. The adaptive asymmetric impedance reshaping control method for inrush current in a DC grid-connected system according to claim 2, characterized in that, The expression for the high-pass filtering stage is: In the formula, Let S represent the transfer function of the high-pass filter stage, and let S represent the Laplace complex frequency variable. Indicates the cutoff angular frequency, and , This indicates the cutoff frequency of the high-pass filter.

4. The adaptive asymmetric impedance reshaping control method for inrush current in a DC grid-connected system according to claim 2, characterized in that, The expression for the phase compensation stage is: ； In the formula, Let S represent the transfer function of the phase compensation element, and let S represent the Laplace complex frequency variable. Indicates the corner frequency. α is the phase compensation coefficient, and 0 < α < 1.

5. The adaptive asymmetric impedance reshaping control method for inrush current in a DC grid-connected system according to claim 1, characterized in that, The voltage imbalance is calculated based on the three-phase AC voltage, specifically including: Collect the three-phase voltage on the AC side; Based on the three-phase AC voltage, the positive-sequence voltage base amplitude and negative-sequence voltage base amplitude are obtained through a dual second-order generalized integrator. The voltage imbalance is calculated based on the positive-sequence voltage fundamental amplitude and the negative-sequence voltage fundamental amplitude.

6. The adaptive asymmetric impedance reshaping control method for inrush current in a DC grid-connected system according to claim 5, characterized in that, The formula for calculating the voltage imbalance is: ； in, Indicates the degree of voltage imbalance. This represents the fundamental amplitude of the negative sequence voltage. This represents the amplitude of the positive sequence voltage fundamental wave.

7. The adaptive asymmetric impedance reshaping control method for inrush current in a DC grid-connected system according to claim 5, characterized in that, The formula for calculating the background imbalance is: ； in, This represents the background imbalance, where M is the length of the sliding window used to calculate the background imbalance, and N is... s For sampling interval window, The voltage imbalance is represented by k, where k represents time k, and i represents the variable.

8. The adaptive asymmetric impedance reshaping control method for inrush current in a DC grid-connected system according to claim 1, characterized in that, Based on the voltage imbalance, the background imbalance, and the discrete rate of change of the voltage imbalance, the adaptive control coefficients are calculated using an adaptive smoothing activation function with zero-point anchoring characteristics, specifically: The discrete rate of change of voltage imbalance is calculated by the difference between the voltage imbalance at the current moment and the voltage imbalance at the previous sampling moment. The imbalance activation threshold is calculated based on the discrete rate of change of the voltage imbalance and the background imbalance. The adaptive control coefficients are calculated using the adaptive smoothing activation function based on the voltage imbalance degree and the imbalance degree activation threshold.

9. The adaptive asymmetric impedance reshaping control method for inrush current in a DC grid-connected system according to claim 8, characterized in that, The formula for calculating the adaptive control coefficient is as follows: ； in, This represents the adaptive control coefficient, where μ is the smoothing transition factor. Indicates the imbalance activation threshold. This indicates the degree of voltage imbalance.

10. The adaptive asymmetric impedance reshaping control method for inrush current in a DC grid-connected system according to claim 8, characterized in that, The formula for calculating the imbalance activation threshold is: ； in, Indicates the imbalance activation threshold. Indicates background imbalance. The discrete rate of change representing the voltage unbalance. This indicates the steady-state tolerance against malfunctions. This represents the transient decay coefficient.

Images