A control method and system of a high-anti-interference grid-connected converter based on comprehensive strategy

Abstract

The application provides a grid-connected converter control method and system based on a comprehensive strategy, the method comprising: acquiring grid-connected node three-phase voltage, grid-side output three-phase current and bridge arm side filter inductance three-phase current of the grid-connected converter, combining active and reactive power reference values and rated operation parameters to perform synchronous control operation to generate complex reference voltage; inputting the complex reference voltage and the bridge arm side filter inductance three-phase current into a cross control link, combining virtual impedance parameters and current limiting parameters to perform operation to generate complex reference current; inputting the complex reference current into a current control link, based on limited control set, traversing all bridge arm switch combination states, selecting a switch state that minimizes the comprehensive cost of current tracking and power fluctuation to drive the operation of the converter bridge arm switch. The method effectively suppresses harmonic distortion and power fluctuation, improves transient current limiting capability and synchronization stability, and meets the demand of a complex operation scenario of a weak power grid.

Description

Technical Field

[0001] This application belongs to the field of power electronic control technology, and in particular relates to a high disturbance rejection grid-connected converter control method and system based on a comprehensive strategy. Background Technology

[0002] With the continuous increase in the proportion of renewable energy power generation, the application scenarios of grid-connected converters connected to weak power grids are increasing. Grid-connected converters need to cope with the multiple impacts of small disturbances such as load fluctuations and grid harmonics, as well as large disturbances such as short-circuit faults. Existing control strategies for grid-connected converters mostly focus on steady-state power control accuracy. When there is harmonic distortion in the grid, the output current distortion rate is high, and the power fluctuation amplitude is large, making it difficult to meet the requirements of high-quality power supply. Some control schemes that focus on transient current limiting usually directly hard limit the power or voltage reference value, which can easily lead to synchronous instability during and after faults, failing to balance steady-state operation quality and transient disturbance rejection capability. In addition, existing control schemes mostly use linear modulation methods, which make it difficult to simultaneously optimize current tracking accuracy and power fluctuation level under limited switching conditions, thus restricting the operating performance of grid-connected converters in complex power grid environments. Summary of the Invention

[0003] To address the challenge of ensuring low output current harmonic distortion and power fluctuation during steady-state operation of grid-connected converters in weak grid access scenarios, while simultaneously improving current limiting capability and synchronization stability under fault transients, this application provides a high-disturbance-resistance grid-connected converter control method and system based on a comprehensive strategy.

[0004] This application first provides a high-disturbance-immunity grid-connected converter control method based on a comprehensive strategy, including: The grid-connected converter obtains the three-phase voltage of the grid-connected node, the three-phase current of the grid-side output, and the three-phase current of the filter inductor on the bridge arm side. It then performs synchronous control calculations by combining the active power reference value, the reactive power reference value, and the rated operating parameters to generate a complex reference voltage for the synchronous control output. The complex reference voltage and the three-phase current input of the filter inductor on the bridge arm side are cross-controlled, and the virtual impedance parameters and current limiting parameters are combined to calculate and generate the complex reference current of the cross-control output. The complex reference current is input into the current control loop. Based on the finite control set, all bridge arm switch combination states are traversed, and the switch state that minimizes the combined cost of current tracking and power fluctuation is selected to control the operation of the fully controlled power electronic switches of each bridge arm of the grid-connected converter.

[0005] Optionally, the synchronization control operation includes: Positive sequence extraction and coordinate transformation are performed on the three-phase voltage of the grid-connected node and the three-phase output current of the grid side to obtain the positive sequence complex components of voltage and current. By combining the active power reference value, the reactive power reference value and the line impedance angle, the power regulation term and the current suppression term are calculated. The power regulation term, current suppression term, and rated angular frequency integral term are superimposed to generate a complex reference voltage for synchronous control output.

[0006] Optionally, the calculation combining the virtual impedance parameter and the current limiting parameter includes: The complex reference voltage is subjected to amplitude limiting to obtain the voltage polar coordinate components after amplitude limiting; By combining the virtual impedance parameters with the three-phase current of the filter inductor on the bridge arm side, the current feedback correction term is calculated. The voltage polar coordinate components after the limit are combined with the current feedback correction term to generate a complex reference current for the cross-control output.

[0007] Optionally, the step of traversing all bridge arm switch combination states based on a finite control set includes: A discrete prediction model for the filter circuit of the grid-connected converter is established, and the predicted current value under each switching state is calculated based on the complex reference current. By combining optimized weighting coefficients, a comprehensive cost function is constructed that includes current tracking error and power fluctuation. Iterate through all bridge arm switch combinations, calculate the corresponding comprehensive replacement value, and select the switch combination with the lowest replacement value.

[0008] Optionally, the step of performing positive-sequence extraction and coordinate transformation of the three-phase voltage at the grid-connected node and the three-phase output current on the grid side includes: The Clarke transform is performed on the sampled three-phase instantaneous values ​​to obtain the axial components in the two-phase stationary coordinate system; The positive sequence components of the axis components in the two-phase stationary coordinate system are extracted to obtain the positive sequence axis components of voltage and current. Based on the positive sequence voltage axis component and the positive sequence current axis component, complex number representations are constructed respectively to obtain the positive sequence voltage complex component and the positive sequence current complex component.

[0009] Optionally, the limiting process for the complex reference voltage includes: Calculate the magnitude and phase angle of the complex reference voltage; The magnitude value is compared with the preset maximum allowable voltage amplitude value, and the smaller value is taken as the voltage magnitude value after limiting. By combining the voltage magnitude after limiting with the original phase angle, the polar coordinate components of the voltage after limiting are generated.

[0010] Optionally, the construction of the integrated cost function, which includes current tracking error and power fluctuation, includes: The difference between the predicted current value and the complex reference current is calculated to obtain the current tracking error term; Instantaneous power is calculated based on the predicted current and the grid-connected node voltage, and the power fluctuation error term is obtained. By optimizing the weighting coefficients, the current tracking error term and the power fluctuation error term are weighted and summed to generate a comprehensive cost function.

[0011] Optionally, before calculating the current feedback correction term by combining the virtual impedance parameters and the three-phase current of the bridge arm-side filter inductor, the method further includes: Detect the voltage amplitude at the common connection point of the grid-connected converter to determine the current operating status; When the voltage amplitude is lower than the preset threshold, increase the values ​​of virtual resistance and virtual reactance in the virtual impedance; When the voltage amplitude recovers to above the preset threshold, the virtual impedance parameter will fall back to the steady-state preset value.

[0012] Optionally, the control of the operation of each arm of the grid-connected converter's fully controlled power electronic switch includes: The selected switch state is latched at the end of the current sampling period; The switch state is converted into a switch trigger level signal corresponding to each bridge arm; At the start of the next sampling period, the trigger level signal is output to drive the fully controlled power electronic switch to turn on or off.

[0013] This application also provides a high-disruption-immunity grid-connected converter control system based on a comprehensive strategy, including multiple synchronous control units, cross control units, and current control units. The synchronous control unit is used to collect the three-phase voltage of the grid-connected node of the grid-connected converter, the three-phase current of the grid-side output and the three-phase current of the filter inductor on the bridge arm side, and generate a complex reference voltage by combining the power reference value and the rated operating parameters. The cross control unit is used to receive the complex reference voltage and generate a complex reference current by combining the virtual impedance parameter and the current limiting parameter; The current control unit is used to receive the complex reference current, select the optimal switching state based on a finite control set, and output a drive signal to control the operation of the grid-connected converter.

[0014] In the embodiments provided in this application, the control method generates a reference voltage by combining grid-side voltage and current with power reference values ​​through a synchronization control link. It achieves autonomous synchronization without communication by relying on the combined effects of power regulation and current suppression, providing a stable voltage reference for subsequent control links. The cross-control link receives the reference voltage output from the synchronization control and generates a reference current by combining virtual impedance and a limiting mechanism. In fault scenarios, it limits the output current amplitude through voltage limiting and current feedback, while retaining synchronization phase information to prevent instability during and after fault clearance. The current control link uses the reference current output from the cross-control as a reference, traverses switch states through a finite control set, and selects the scheme with the lowest overall cost. It tracks the reference current while smoothing power fluctuations and reducing output current harmonic distortion. The three control links are cascaded and parameter-matched sequentially to achieve low harmonic and low power fluctuation operation in steady state, while improving current limiting capability and synchronization stability under fault transients, adapting to complex operating scenarios in weak power grids. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of this application 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 some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This is a flowchart of the overall process of the high disturbance rejection grid-connected converter control method based on a comprehensive strategy in the embodiments of this application; Figure 2 This is a flowchart of the synchronization control operation sub-process in the embodiments of this application; Figure 3 This is a flowchart of the cross-control operation sub-process in the embodiments of this application; Figure 4 This is a flowchart of the current control sub-process in an embodiment of this application; Figure 5 This is a flowchart of the forward order extraction and coordinate transformation sub-process in the embodiments of this application; Figure 6 This is a flowchart of the limiting processing sub-process in the embodiments of this application; Figure 7 This is a flowchart of the comprehensive cost function construction sub-process in the embodiments of this application; Figure 8 This is a flowchart of the dynamic virtual impedance adjustment sub-process in the embodiments of this application; Figure 9 This is a flowchart of the switch control action in the embodiments of this application. Detailed Implementation

[0017] In the following description, specific details such as particular system architectures and techniques are set forth for illustrative purposes and not limiting, in order to provide a thorough understanding of the embodiments of this application. However, those skilled in the art will understand that this application can also be implemented in other embodiments without such specific details. In other instances, detailed descriptions of well-known systems, apparatuses, circuits, and methods are omitted so as not to obscure the description of this application with unnecessary detail.

[0018] In this embodiment, the grid-connected converter is a three-phase, two-level grid-connected converter, connected to a weak grid environment. The DC side is connected to a new energy power generation unit, and the AC side is connected to the grid's point of common coupling via a filter circuit. The filter circuit adopts an LCL structure, including a bridge arm-side filter inductor, a filter capacitor, and a grid-side filter inductor. A damping resistor is connected in parallel across the filter capacitor. The control cycle of the grid-connected converter is consistent with the sampling cycle. Each sampling cycle sequentially executes sampling, synchronous control calculation, cross-control calculation, current control calculation, and switch signal output. The sampled signal includes the three-phase voltage of the grid-connected node, the three-phase current of the grid-side output, and the three-phase current of the bridge arm-side filter inductor. All calculations are completed within the digital controller.

[0019] In the embodiments of this application, such as Figure 1 As shown, the high disturbance rejection grid-connected converter control method based on integrated strategy of this application includes: S101: Obtain the three-phase voltage of the grid-connected node of the grid-connected converter, the three-phase current of the grid-side output, and the three-phase current of the filter inductor on the bridge arm side. Combine the active power reference value, reactive power reference value, and rated operating parameters to perform synchronous control calculations and generate a complex reference voltage for synchronous control output. S102: The complex reference voltage and the three-phase current input of the bridge arm side filter inductor are combined with the virtual impedance parameters and current limiting parameters to generate the complex reference current of the cross control output. S103: Input the complex reference current into the current control loop, traverse all bridge arm switch combination states based on the finite control set, select the switch state that minimizes the combined cost of current tracking and power fluctuation, and control the operation of the fully controlled power electronic switches of each bridge arm of the grid-connected converter.

[0020] In this embodiment, the execution entity of the control method is the digital controller of the grid-connected converter. The digital controller executes the sampling and calculation process according to a fixed sampling period. Within each sampling period, it sequentially completes signal sampling, synchronous control calculation, cross-control calculation, current control calculation, and switch signal output. The inputs to the synchronous control calculation include the sampled three-phase voltage of the grid-connected node, the grid-side output three-phase current, and preset active power reference values, reactive power reference values, and rated operating parameters. The calculation output is a complex reference voltage of the synchronous control loop. This complex reference voltage includes amplitude and phase information and is used to characterize the voltage reference value expected to be output by the grid-connected converter. The cross-control loop receives the complex reference voltage output by the synchronous control loop and simultaneously inputs the sampled value of the three-phase current of the filter inductor on the bridge arm side. It performs calculations in combination with preset virtual impedance parameters and current limiting parameters and outputs a complex reference current. This complex reference current includes amplitude and phase information and is used to characterize the current reference value expected to be output by the grid-connected converter. The current control loop receives the complex reference current output by the cross control. Based on the finite set of switch states of the grid-connected converter, it iterates through the operating effect corresponding to each bridge arm switch combination state, calculates the comprehensive cost index, selects the switch combination state with the minimum comprehensive cost, and finally outputs the switch drive signal to control the fully controlled power electronic switches of each bridge arm of the grid-connected converter to perform the on or off actions.

[0021] It should be noted that the switching combination state of the bridge arms of the grid-connected converter is determined by the conduction state of the upper and lower transistors of the three-phase bridge arms. For a three-phase two-level grid-connected converter, each phase bridge arm has two conduction states, resulting in a total of eight switching combination states, including two zero-vector states and six non-zero vector states. Each switching combination state corresponds to a set of three-phase voltage values ​​output by the bridge arms. After coordinate transformation, the voltage components in the two-phase stationary coordinate system can be obtained. The range of the finite control set is all feasible switching combination states. The traversal process covers all feasible states, ensuring that the optimal switching state within the current sampling period is selected.

[0022] In the embodiments of this application, such as Figure 2 As shown, the synchronization control operation includes: S201: Perform positive sequence extraction and coordinate transformation on the three-phase voltage of the grid-connected node and the three-phase output current of the grid side to obtain the positive sequence complex components of voltage and current. S202: By combining the active power reference value, the reactive power reference value and the line impedance angle, the power regulation term and the current suppression term are calculated. S203: The power regulation term, current suppression term and rated angular frequency integral term are superimposed to generate a complex reference voltage for synchronous control output.

[0023] Specifically, the execution process of synchronous control operation is divided into three sequential processing stages. The first stage completes the preprocessing of the input signal to obtain the positive sequence complex components of voltage and current. The second stage completes the calculation related to power regulation and current suppression to obtain the corresponding regulation terms. The third stage superimposes each regulation term with the integral term of the rated angular frequency to generate the final complex reference voltage.

[0024] In the first stage, the positive-sequence components of the sampled instantaneous values ​​of the three-phase voltage at the grid-connected node and the instantaneous values ​​of the three-phase current output from the grid side are extracted to remove any possible negative-sequence and zero-sequence components from the sampled signals, resulting in the three-phase positive-sequence voltage and three-phase positive-sequence current. Subsequently, coordinate transformations are performed on the three-phase positive-sequence voltage and current, converting the signals from the three-phase stationary coordinate system to axis components in the two-phase stationary coordinate system. Finally, complex representations are constructed based on the voltage and current axis components in the two-phase stationary coordinate system, yielding the positive-sequence complex components of voltage and current. The purpose of positive-sequence component extraction is to eliminate negative-sequence and zero-sequence interference caused by grid imbalance, ensuring that synchronous control is executed only based on the positive-sequence components, and improving the stability of the synchronization process. The purpose of coordinate transformation is to convert three-phase AC quantities into two-phase AC quantities, reducing the signal dimension, simplifying the subsequent complex number operations, and reducing the computational load.

[0025] In the second stage, the power regulation term is calculated based on the active power reference value, reactive power reference value, and the positive sequence components of voltage and current. The phase and amplitude of the power regulation term are determined by the deviation between the power reference value and the actual operating value, and are used to achieve power tracking and regulation under steady state, ensuring that the output power of the grid-connected converter follows the preset reference value. The current suppression term is calculated based on the positive sequence component of the grid-side output current and the current suppression coefficient. When the output current amplitude increases, the effect of the current suppression term is enhanced, which is used to lower the reference voltage amplitude, limit the current from further increasing, and achieve automatic suppression under overcurrent conditions. The line impedance angle parameter is used to adjust the phase characteristics of the power regulation term, adapt to the power coupling characteristics under different line impedance scenarios, and ensure stable power regulation and synchronous operation under different weak grid strengths.

[0026] In the third stage, the rated angular frequency integral term is used to provide a basic synchronous phase reference. The rated angular frequency is taken as the angular frequency value corresponding to the rated frequency of the power grid. After integration, the basic phase is obtained, ensuring that the frequency of the output voltage is consistent with the rated frequency of the power grid during steady-state operation. The power regulation term, current suppression term, and rated angular frequency integral term are complexly superimposed to obtain the final complex reference voltage. The magnitude of this complex reference voltage corresponds to the amplitude of the reference voltage, and the argument corresponds to the phase of the reference voltage.

[0027] The operational expression for synchronization control is:

[0028] The sampling point number, The sampling period is The complex reference voltage is the output of the synchronous control at the k-th sampling time. This is the gain coefficient for voltage amplitude adjustment. The rated voltage of the grid-connected converter. The magnitude of the positive-sequence complex component of the grid-connected node voltage. , The imaginary unit, This is the power adjustment gain coefficient. These are the active power reference value and the reactive power reference value, respectively. The line impedance angle of the circuit connected to the grid-connected converter. This is the current suppression coefficient. This represents the magnitude of the positive-sequence complex component of the grid-side output current. For the complex exponential phase compensation term, e is the natural constant.

[0029] This expression integrates amplitude and phase adjustment into a single complex number operation: it adjusts the real part (amplitude dimension) of the reference voltage through voltage amplitude deviation, and adjusts the imaginary part (phase dimension) of the reference voltage through power and current deviations, while also incorporating line impedance angle for phase compensation. It achieves autonomous synchronous operation without a separate phase-locked loop and possesses automatic suppression capability when current increases.

[0030] For example, voltage gain coefficient The value of affects the regulation bandwidth of the voltage loop; a larger value results in faster voltage regulation, but excessively large values ​​can easily cause oscillations. A suitable value is usually selected based on the system's damping characteristics. Current gain coefficient The value of this parameter affects the response speed of power regulation; a larger value results in faster power tracking. However, it must also be considered in the design considering system stability. Current suppression coefficient. The value of the current limiting parameter affects the sensitivity of the current limiting. A larger value results in stronger current suppression and a slower current rise during a fault, but also a greater impact on power regulation under steady-state conditions. A trade-off must be struck between the current limiting effect and steady-state performance. The line impedance angle parameter can be set according to the actual impedance characteristics of the lines connected to the grid. For weak grids dominated by inductive forces, the line impedance angle is close to 90 degrees. In this case, active power mainly affects the phase, and reactive power mainly affects the amplitude, which conforms to the power coupling characteristics of traditional droop control. For grids with strong resistivity, the line impedance angle value can be adjusted accordingly to achieve power decoupling and ensure control effectiveness.

[0031] In the embodiments of this application, such as Figure 3 As shown, the calculation combining the virtual impedance parameter and the current limiting parameter includes: S301: The complex reference voltage is subjected to amplitude limiting to obtain the voltage polar coordinate components after amplitude limiting; S302: The current feedback correction term is calculated by combining the virtual impedance parameters and the three-phase current of the filter inductor on the bridge arm side; S303: Combine the voltage polar coordinate components after the limit with the current feedback correction term to generate a complex reference current for the cross-control output.

[0032] For example, the execution process of cross-control operation is also divided into three sequential processing stages. The first stage performs amplitude limiting on the input complex reference voltage to obtain the amplitude-limited voltage polar coordinate components. The second stage calculates the current feedback correction term based on the virtual impedance parameters and the three-phase current of the filter inductor on the bridge arm side. The third stage combines the amplitude-limited voltage polar coordinate components with the current feedback correction term to generate the complex reference current of the cross-control output.

[0033] In the first stage, the magnitude and phase angle of the input complex reference voltage are first calculated, and the reference voltage in complex form is converted into polar coordinate form to separate the amplitude and phase information. Then, the voltage magnitude is limited to a preset maximum allowable amplitude range to prevent the output current from exceeding the allowable range due to an excessively high reference voltage amplitude. The limiting process only applies to the amplitude dimension, while the phase angle remains unchanged to ensure the continuity of phase information and prevent phase jumps caused by the limiting action.

[0034] In the second stage, the positive sequence of the sampled three-phase current of the bridge arm side filter inductor is first extracted and the coordinates are transformed to obtain the positive sequence component of the bridge arm side current in the two-phase stationary coordinate system, and the corresponding complex number representation is constructed. Then, the complex component of the bridge arm side current is multiplied by the virtual impedance parameter to obtain the voltage drop corresponding to the virtual impedance. This voltage drop is the current feedback correction term, which is used to simulate the effect of the virtual impedance in series in the output circuit, increase the equivalent output impedance of the system, limit the rise rate and final amplitude of the fault current, and at the same time improve the system damping and suppress transient oscillations.

[0035] In the third stage, the voltage polar coordinate components after limiting are converted to recursive form, and the voltage drop corresponding to the virtual impedance is subtracted to obtain the equivalent output voltage reference value. Subsequently, based on the output impedance characteristics of the grid-connected converter, the equivalent output voltage reference value is converted into the corresponding current reference value, which is the complex reference current of the cross-control output. The current limiting gain coefficient is used to adjust the voltage-to-current conversion ratio, and combined with the rated voltage parameters, ensures that the amplitude of the complex reference current matches the power reference value under steady state.

[0036] The expression for cross-control is:

[0037] The complex reference current output by the cross-control loop, The magnitude of the voltage reference signal after amplitude limiting. The phase angle of the voltage reference signal after limiting processing. This is the current limiting gain coefficient, used to calibrate the conversion ratio from voltage reference to current reference. The rated voltage of the grid-connected converter. The virtual impedance is composed of virtual resistance and virtual reactance. This represents the positive-sequence complex component of the filter inductor current on the bridge arm side. It is a complex voltage unit vector in polar coordinates, used to preserve phase information after limiting.

[0038] This expression constrains the amplitude of the output current reference value through amplitude limiting and virtual impedance current feedback. The limiting process only acts on the voltage amplitude dimension and retains continuous phase information throughout, avoiding phase jumps and synchronization instability caused by hard limiting, thus achieving a balance between current limiting capability and synchronization stability under fault conditions.

[0039] For example, the maximum amplitude of the limiting voltage can be set based on the rated voltage of the converter and the current limiting target, usually taking a certain percentage of the rated voltage. During a fault, if the reference voltage amplitude exceeds the limiting value, it will be limited to the limiting value, ensuring that the output current does not exceed the maximum allowable value. Virtual impedance consists of two parts: virtual resistance and virtual reactance. Virtual resistance mainly provides damping, suppressing current oscillations. Virtual reactance mainly changes the equivalent output reactance, affecting power distribution characteristics and current limiting capability. The larger the virtual reactance value, the stronger the current limiting effect, but the greater the voltage drop under steady state; it needs to be selected reasonably according to operational requirements. Current limiting gain coefficient. This coefficient is used to calibrate the conversion ratio from voltage reference to current reference. Under steady-state conditions, the current reference value can be adjusted to correspond to the rated current at rated power, thus ensuring the accuracy of power regulation.

[0040] In the embodiments of this application, such as Figure 4 As shown, the process of traversing all bridge arm switch combinations based on a finite control set includes: S401: Establish a discrete prediction model for the grid-connected converter filter circuit, and calculate the predicted current value under each switching state based on the complex reference current. S402: Combine optimized weighting coefficients to construct a comprehensive cost function that includes current tracking error and power fluctuation. S403: Traverse all bridge arm switch combination states, calculate the corresponding comprehensive replacement value, and select the switch state with the lowest replacement value.

[0041] Specifically, the current control loop is executed based on the finite control set model predictive control principle. The entire operation process is divided into three stages. The first stage establishes a discrete prediction model for the filter circuit and calculates the predicted current value for the next moment under each switching state based on the sampled value and reference current at the current moment. The second stage constructs a comprehensive cost function and incorporates the current tracking effect and power fluctuation effect into a unified evaluation index. The third stage traverses all bridge arm switching combination states, calculates the comprehensive cost value corresponding to each state, and selects the switching combination with the minimum cost value as the optimal switching state for the current cycle.

[0042] In the first stage, based on the topology of the LCL filter circuit on the output side of the grid-connected converter, a continuous-domain state-space equation is established according to Kirchhoff's voltage law and current law. The state variables are selected as the filter inductor current on the bridge arm side, the filter capacitor voltage, and the grid-side output current. Then, a discretization method is used to transform the continuous state equation into a discrete state equation, obtaining a prediction expression for the state variables at the next moment based on the current state variables and the bridge arm output voltage. The bridge arm output voltage is determined by the switching combination state; therefore, for each switching combination state, it can be substituted into the discrete state equation to calculate the predicted current value at the next moment. The discretization method can use the forward Euler method, which has low computational complexity, is suitable for real-time execution by digital controllers, and its prediction accuracy meets control requirements.

[0043] In the second stage, the comprehensive cost function is used to quantitatively evaluate the control effect of each switching combination state; the smaller the cost value, the better the control effect. The comprehensive cost function consists of two parts: one is the current tracking error, which characterizes the deviation between the predicted current and the reference current; the smaller the deviation, the better the current tracking performance and the lower the harmonic content of the output current. The other part is the power fluctuation error, which characterizes the deviation between the instantaneous power calculated based on the predicted current and the power reference value; the smaller the deviation, the smaller the fluctuation amplitude of the output power. Optimization weighting coefficients are used to adjust the proportion of the two errors in the comprehensive cost. The weights can be adjusted according to operational requirements; when emphasizing current waveform quality, the weight of the current error is increased; when emphasizing power stability, the weight of the power error is increased.

[0044] In the third stage, the traversal process sequentially substitutes each feasible bridge arm switch combination state into the discrete prediction model to obtain the corresponding current prediction value. Then, it substitutes this value into the comprehensive cost function to calculate the cost value. After recording the cost values ​​for all switch states, the switch combination state with the lowest cost value is selected as the final output. The traversal process covers all feasible switch states, ensuring the selection of the optimal solution within the current cycle. It eliminates the need for complex optimization processes, has a fixed computational flow, and is suitable for implementation with digital controllers.

[0045] The operation of current control includes the following expression:

[0046] In the formula, To optimize the weighting coefficients, the values ​​range from 0 to 1, which are used to allocate the weight ratio of current tracking error and power fluctuation error in the overall cost. This is the state vector of the filter circuit at the current sampling time. The state prediction vector for the next sampling time includes state variables such as the filter inductor current on the bridge arm side, the filter capacitor voltage, and the grid output current. , The coefficient matrix of the discrete state equation of the filter circuit is determined by the filter circuit topology parameters (inductance, capacitance, resistance) and the sampling period. J is the voltage vector output by the converter bridge arm at the current sampling moment, which is determined by the bridge arm switching combination state; J is the comprehensive cost function value, used to quantitatively evaluate the control effect of each switching combination state; For input complex reference current commands in the current control loop; This is the square operation of the vector 2 norm, used to quantify the magnitude of the current tracking deviation; This is a reference value for active power. This is the predicted active power value at the next sampling time. This is a scalar absolute value operation used to quantify power fluctuation deviation; S represents the combined state of a single bridge arm switch. It is the set of all feasible bridge arm switch combinations for grid-connected converters; The optimal bridge arm switch combination state selected for the current control cycle; The operation is performed to find the independent variable that minimizes the objective function.

[0047] This set of expressions is based on the predictive control principle of finite control set model. First, the current value at the next moment corresponding to each switching state is predicted by the discrete state equation of the filter circuit. Then, the control effect of each switching state is quantitatively evaluated by the comprehensive cost function. Finally, all feasible switching combination states are traversed and the switching state with the minimum cost is selected for output, so as to achieve simultaneous optimization of current tracking accuracy and power stability.

[0048] Furthermore, the coefficient matrices A and B of the discrete state equations are determined by the parameters of the filter circuit and the sampling period. Once the filter circuit parameters are determined, the coefficient matrices can be calculated offline and stored in the controller for real-time computation. The optimization weight coefficient λ ranges from 0 to 1. When λ is 1, the cost function only considers the current tracking error, and the control effect is consistent with traditional model predictive current control. When λ is 0, the cost function only considers power fluctuations, and the control effect is consistent with direct power control. By adjusting the value of λ, a balance can be achieved between current waveform quality and power stability to meet the operational requirements of different scenarios. For a three-phase two-level converter, there are eight feasible switching combination states. The computational load of traversing eight states is relatively small, and a common digital controller can complete all calculations within one sampling period, ensuring real-time control.

[0049] In the embodiments of this application, such as Figure 5 As shown, the positive-sequence extraction and coordinate transformation of the three-phase voltage of the grid-connected node and the three-phase output current of the grid side includes: S501: Perform Clarke transformation on the sampled three-phase instantaneous values ​​to obtain the axial components in the two-phase stationary coordinate system; S502: Extract the positive sequence components of the axis components in the two-phase stationary coordinate system to obtain the positive sequence axis components of voltage and current. S503: Construct complex representations of the positive sequence voltage axis component and the positive sequence current axis component respectively to obtain the positive sequence voltage complex component and the positive sequence current complex component.

[0050] For example, the specific execution process of positive sequence extraction and coordinate transformation is divided into three steps. The first step is to perform coordinate transformation on the three-phase instantaneous sampled values ​​to obtain the axis components in the two-phase stationary coordinate system. The second step is to extract the positive sequence components from the axis components in the two-phase stationary coordinate system to obtain the positive sequence axis components. The third step is to construct a complex number representation based on the positive sequence axis components to obtain the positive sequence complex number components.

[0051] The first step, coordinate transformation, converts the signal from a three-phase stationary abc coordinate system to a two-phase stationary αβ coordinate system. For a three-phase three-wire system, the zero-sequence component is zero, so only the α-axis and β-axis components need to be calculated. The physical meaning of coordinate transformation is to convert a three-phase AC signal with a 120-degree phase difference into a two-phase AC signal with a 90-degree phase difference, reducing signal dimensions and simplifying complex number operations. The transformation process follows the principle of equal amplitude transformation, ensuring consistent signal amplitude before and after the transformation, facilitating subsequent amplitude calculation and adjustment. The coordinate transformation calculation method is as follows: the α-axis component equals two-thirds of the A-phase component minus the sum of one-third of the B-phase component and one-third of the C-phase component; the β-axis component equals the difference between the B-phase component and the C-phase component, which is √3 / 3. Under the equal amplitude transformation method, the amplitude of the transformed two-phase component is equal to the amplitude of the original three-phase component, facilitating direct amplitude comparison and limiting processing.

[0052] The second step, positive-sequence component extraction, separates the positive-sequence component from the three-phase signal, removing interference from the negative-sequence and zero-sequence components. Positive-sequence extraction can employ a delay-cancellation method, utilizing the phase difference between the positive and negative-sequence components after a quarter-cycle delay, canceling the negative-sequence component through addition and subtraction operations while retaining the positive-sequence component. Alternatively, a second-order generalized integrator method can be used, extracting the positive-sequence component through a resonant circuit, which also provides filtering to suppress high-frequency noise. The delay-cancellation method has lower computational complexity and is simpler to implement, making it suitable for scenarios requiring high computational speed. The second-order generalized integrator method offers better filtering and stronger anti-interference capabilities, making it suitable for scenarios with high grid harmonic content. The purpose of positive-sequence component extraction is to ensure that synchronous control is executed solely based on the positive-sequence fundamental component of the grid, maintaining stable synchronous phase even in scenarios with grid imbalance and harmonics, and avoiding phase pulsations caused by negative-sequence components.

[0053] The third step, complex number construction, uses the α-axis component as the real part and the β-axis component as the imaginary part, combining them to form a complex number representation. This complex number representation can simultaneously contain amplitude and phase information, facilitating subsequent magnitude calculation, phase calculation, and complex number operations. Compared to handling amplitude and phase separately, the code implementation for complex number operations is simpler and more efficient. The magnitude of a complex number is equal to the square root of the sum of the squares of the real and imaginary parts, corresponding to the signal amplitude; the argument of a complex number is equal to the arctangent of the ratio of the imaginary to the real part, corresponding to the signal phase.

[0054] Specifically, the principle of the delay-cancellation method for extracting the positive-sequence component is as follows: For a signal in a two-phase stationary coordinate system, the positive-sequence component rotates forward at its rated angular frequency, while the negative-sequence component rotates backward at its rated angular frequency. After delaying the original signal by a quarter of a fundamental period, the phase of the positive-sequence component lags by 90 degrees, and the phase of the negative-sequence component leads by 90 degrees. Multiplying the original signal by an imaginary unit and adding it to the delayed signal results in the phases of the negative-sequence components canceling each other out, and the phases of the positive-sequence components superimposing, thus obtaining a pure positive-sequence component. This method only requires storing a quarter-cycle of historical data, occupies little storage space, has a simple computational process, and can quickly obtain the positive-sequence component result, making it suitable for real-time control requirements.

[0055] In the embodiments of this application, such as Figure 6 As shown, the limiting process for the complex reference voltage includes: S601: Calculate the magnitude and phase angle of the complex reference voltage; S602: Compare the magnitude value with the preset maximum allowable voltage amplitude value, and take the smaller value as the voltage magnitude value after limiting; S603: Combine the voltage magnitude after limiting with the original phase angle to generate the voltage polar coordinate components after limiting.

[0056] Specifically, the limiting process for complex reference voltage consists of three steps: First, the magnitude and phase angle of the complex reference voltage are calculated; second, the magnitude is compared with the preset maximum allowable voltage magnitude, and the smaller value is taken as the voltage magnitude after limiting; third, the voltage polar coordinate components after limiting are generated by combining the voltage magnitude after limiting with the original phase angle.

[0057] The first step, magnitude calculation, is based on the real and imaginary parts of a complex number. The magnitude is equal to the square root of the sum of the squares of the real and imaginary parts, and the phase angle is equal to the arctangent of the ratio of the imaginary to the real part. Calculating the magnitude and phase angle is a standard process for converting complex numbers to polar coordinates. The calculated magnitude represents the magnitude of the reference voltage, and the phase angle represents the phase position of the reference voltage. A fast square root algorithm can be used to reduce the computational load for magnitude calculation, while a lookup table method can be used to improve the calculation speed, adapting to the operational characteristics of the digital controller.

[0058] In the second step of the amplitude limiting comparison process, the preset maximum allowable voltage amplitude is the maximum voltage amplitude allowed to be output during normal operation of the converter. This value can be set according to the DC-side voltage and modulation ratio of the converter, or it can be set according to the current limiting target. When the calculated reference voltage magnitude is less than or equal to the maximum allowable amplitude, the amplitude after limiting is equal to the original amplitude, and the amplitude limiting circuit does not operate. When the reference voltage magnitude is greater than the maximum allowable amplitude, the amplitude after limiting is equal to the maximum allowable amplitude, the amplitude limiting circuit takes effect, and the amplitude of the reference voltage is limited from further increasing. The amplitude limiting process only adjusts the amplitude and does not change the phase angle, ensuring the continuity of phase information and avoiding phase abrupt changes caused by the amplitude limiting action, which could lead to synchronization instability.

[0059] The third step, polar coordinate component generation, uses the limited magnitude as the polar radius and the original phase angle as the polar angle to form the limited voltage polar coordinate components. The polar coordinate form facilitates subsequent calculations with the virtual impedance voltage drop and can also be converted to a recursive form for further calculations as needed. The process of converting polar coordinates to complex numbers is as follows: the real part equals the magnitude multiplied by the cosine of the phase angle, and the imaginary part equals the magnitude multiplied by the sine of the phase angle. The converted complex number can directly participate in complex number addition, subtraction, multiplication, and division operations.

[0060] Furthermore, a soft-limiting method can be used in the limiting stage. This involves gradually increasing the limiting strength as the reference voltage magnitude approaches the maximum allowable amplitude, avoiding the abrupt amplitude changes caused by hard limiting, further reducing transient impacts, and improving operational stability. Soft limiting can be implemented using nonlinear functions, such as a hyperbolic tangent function to smoothly transition the limiting process. When the reference voltage magnitude is much smaller than the limiting value, the limiting effect is almost zero, and the output amplitude equals the original amplitude. As the reference voltage magnitude approaches the limiting value, the rate of increase in output amplitude gradually slows down. When the reference voltage magnitude is much larger than the limiting value, the output amplitude approaches the limiting value. Soft limiting avoids the step changes caused by hard limiting, reduces the impact of limiting action on the system, and improves the smoothness of the transient process.

[0061] In the embodiments of this application, such as Figure 7 As shown, the construction of the comprehensive cost function, which includes current tracking error and power fluctuation, includes: S701: Calculate the difference between the predicted current value and the complex reference current to obtain the current tracking error term; S702: Calculate instantaneous power based on predicted current and grid-connected node voltage to obtain power fluctuation error term; S703: By optimizing the weighting coefficients, the current tracking error term and the power fluctuation error term are weighted and summed to generate a comprehensive cost function.

[0062] For example, the construction of the comprehensive cost function consists of three steps: the first step is to calculate the difference between the predicted current value and the complex reference current to obtain the current tracking error term; the second step is to calculate the instantaneous power based on the predicted current value and the grid-connected node voltage to obtain the power fluctuation error term; and the third step is to generate the comprehensive cost function by weighting and summing the two errors by optimizing the weighting coefficients.

[0063] The first step, the current tracking error term, is calculated by squared the magnitude of the current difference. Squaring the magnitude avoids square root calculations, reducing computational complexity, while simultaneously amplifying the impact of the deviation and improving tracking accuracy. A smaller current tracking error term indicates a smaller deviation between the predicted and reference currents, better current tracking performance, and lower harmonic content in the output current. The current difference is calculated using complex number subtraction; the difference between the real and imaginary parts is subtracted separately, and the sum of the squares gives the magnitude square. This simple calculation process is suitable for real-time computation.

[0064] The second step, calculating the power fluctuation error term, is based on instantaneous power theory. Active power equals the product of the voltage α-axis component and the current α-axis component, plus the product of the voltage β-axis component and the current β-axis component. Reactive power equals the product of the voltage β-axis component and the current α-axis component, minus the product of the voltage α-axis component and the current β-axis component. The difference between the calculated predicted active power and the reference active power value is the absolute value of the difference. The smaller this value, the smaller the fluctuation amplitude of the output power and the better the power stability. Instantaneous power calculation does not require low-pass filtering and can obtain the instantaneous power value at each sampling moment, offering a fast response and making it suitable for evaluating power fluctuations in transient processes.

[0065] In the third step of the weighted summation process, the weighting coefficients are optimized to allocate the weight ratio of the two errors. The values ​​of the weighting coefficients can be dynamically adjusted according to the operating scenario. During steady-state operation, the weight of the current tracking error can be appropriately increased to ensure the quality of the output current waveform; during transient faults, the weight of the power fluctuation error can be appropriately increased to ensure the stability of the power output. The comprehensive cost after weighted summation can simultaneously reflect the current tracking performance and power stability, achieving synchronous optimization of the two control objectives. The adjustment of the weighting coefficients can be automatically performed based on the voltage amplitude. When the voltage is normal, steady-state weights are used, and when the voltage drops, it automatically switches to transient weights, without the need for additional mode judgment logic.

[0066] Furthermore, a switching frequency constraint term can be added to the overall cost function to limit the frequency of switching actions and reduce switching losses. This constraint term applies a penalty value to states that initiate switching actions by comparing the current switching state with the previous cycle's switching state, thereby reducing the switching frequency. Adding a switching frequency constraint achieves a balance between control performance and switching losses, adapting to different operational requirements. The weight of the switching frequency constraint term can be adjusted according to loss requirements. For scenarios with high loss requirements, the constraint term weight can be increased to reduce the average switching frequency; for scenarios with high control performance requirements, the constraint term weight can be decreased to improve control accuracy.

[0067] In the embodiments of this application, such as Figure 8 As shown, before calculating the current feedback correction term by combining the virtual impedance parameters and the three-phase current of the bridge arm-side filter inductor, the following steps are also included: S801: Detects the voltage amplitude at the common connection point of the grid-connected converter to determine the current operating status; S802: When the voltage amplitude is lower than the preset threshold, increase the values ​​of virtual resistance and virtual reactance in the virtual impedance; S803: When the voltage amplitude recovers to above the preset threshold, the virtual impedance parameter will be reduced back to the steady-state preset value.

[0068] In addition, the virtual impedance parameter can be dynamically adjusted according to the operating conditions. The adjustment process consists of three steps: First, the voltage amplitude at the common connection point of the grid-connected converter is detected to determine the current operating status; second, when the voltage amplitude is lower than the preset threshold, the virtual resistance and virtual reactance values ​​in the virtual impedance are increased; third, when the voltage amplitude recovers to above the preset threshold, the virtual impedance parameter is reduced back to the steady-state preset value.

[0069] The first step, voltage amplitude detection, is based on the positive-sequence component magnitude of the grid-connected node voltage. The voltage amplitude is detected for each sampling period and compared with a preset threshold. The preset threshold can be set according to the normal fluctuation range of the grid voltage, for example, 90% of the rated voltage. A voltage below this threshold is considered a fault or severe voltage drop condition. Voltage amplitude detection uses a continuous multi-cycle judgment method; parameter adjustment is only triggered when the voltage amplitude is below the threshold for multiple consecutive sampling periods. This avoids erroneous adjustments caused by transient voltage fluctuations and improves the control's anti-interference capability.

[0070] The second step of increasing the virtual impedance can be done gradually, increasing it to the target value over several sampling periods to avoid transient shocks caused by sudden parameter changes. After increasing the virtual impedance, the converter's equivalent output impedance increases, further limiting the amplitude of the fault current. Simultaneously, the damping is enhanced, suppressing transient oscillations and improving operational stability during faults. The virtual resistance and virtual reactance can be increased proportionally, or they can be adjusted separately according to requirements. For example, increasing the virtual reactance when current limiting is emphasized, and increasing the virtual resistance when damping is emphasized. The target value of the virtual impedance can be set according to the current limiting target. A larger target value results in a stronger current limiting effect, but a weaker voltage support capability under fault conditions. A trade-off must be struck between current limiting and voltage support.

[0071] The third step, parameter reduction, also employs a gradual approach. Once the voltage amplitude recovers above the threshold and remains there for a certain period, the virtual impedance parameter is gradually reduced to the steady-state preset value, smoothly transitioning to steady-state operation mode and avoiding power and voltage fluctuations caused by sudden parameter changes. The rate of parameter reduction can be slower than the rate of parameter increase, further reducing transient impacts during the recovery process and ensuring its stability.

[0072] It should be noted that the introduction of dynamic virtual impedance can achieve a balance between steady-state and transient performance. In steady state, the virtual impedance value is small, having little impact on voltage regulation accuracy and power distribution. In transient state, the virtual impedance automatically increases, enhancing current limiting capability and damping characteristics. No additional mode switching logic is required, and the parameter adjustment process is smooth, avoiding transient shocks caused by mode switching. The trigger condition for dynamic adjustment is based solely on the locally sampled voltage amplitude, requiring no communication or external commands. It relies entirely on local control to achieve adaptive adjustment, making it suitable for scenarios with parallel operation without communication.

[0073] In the embodiments of this application, such as Figure 9 As shown, the operation of the fully controlled power electronic switches of each bridge arm of the grid-connected converter includes: S901: Latch the selected switch state at the end of the current sampling period; S902: Convert the switch state into a switch trigger level signal corresponding to each bridge arm; S903: At the beginning of the next sampling period, the trigger level signal is output to drive the fully controlled power electronic switch to turn on or off.

[0074] For example, the control action of the bridge arm switch is executed in three steps: first, the selected switch state is latched at the end of the current sampling period; second, the switch state is converted into the corresponding switch trigger level signal for each bridge arm; and third, the trigger level signal is output at the beginning of the next sampling period to drive the fully controlled power electronic switch to turn on or off.

[0075] The first step, state latching, ensures the switch state remains stable during output, preventing erroneous triggering caused by intermediate states during computation. After completing all calculations within the current sampling period, the optimal switch state is obtained. At the end of the sampling period, this state is latched into the output register. After latching, the switch state remains unchanged, awaiting the output timing. The latching operation is performed by hardware circuitry to ensure the accuracy of the latch timing and avoid output timing deviations caused by software computation delays.

[0076] The second step, level conversion, transforms the coded switching state signal into drive level signals for the upper and lower transistors of each bridge arm. There are six drive signals in total for the three-phase bridge arms, with the drive signals for the upper and lower transistors of each phase arm being complementary to prevent shoot-through and short circuits. A dead time must be added during level conversion to ensure that shoot-through does not occur when switching between upper and lower transistors. The length of the dead time is set according to the turn-off time of the switching device, ensuring that the device is reliably turned off before turning on the other transistor. The insertion of the dead time can be done automatically by the hardware circuit or added by the software when generating the drive signal. A dead time that is too small can easily cause shoot-through faults, while a dead time that is too large will increase output harmonics; therefore, it must be set appropriately according to the characteristics of the devices.

[0077] The third step of signal output is executed at the beginning of the next sampling period. The drive signals of all bridge arms are updated simultaneously to ensure the synchronicity of the three-phase switch operation. The sampling period is consistent with the switch update period; the switch state is updated once per sampling period. The switching frequency is equal to the sampling frequency. For finite set model predictive control, the switching frequency is not fixed; the average switching frequency is related to the sampling frequency and can be adjusted by adjusting the sampling frequency. The output drive signal, after being amplified by the drive circuit, acts on the control electrode of the switching device to achieve switch on / off control.

[0078] Specifically, the function of the drive circuit is to convert the weak-level signal output by the controller into a strong-level signal that can drive the switching devices, while simultaneously achieving electrical isolation between the control side and the power side to prevent high-voltage interference from the power side from affecting the control circuit. The drive circuit typically includes optocoupler isolation devices and power amplification devices, providing sufficient drive current and voltage to ensure reliable switching of the switching devices. The drive circuit may also include overcurrent protection and overtemperature protection functions, quickly blocking the drive signal in the event of a fault to protect the switching devices from damage.

[0079] This application also provides a high-disturbance-resistance grid-connected converter control system based on a comprehensive strategy, including a synchronization control unit, a cross control unit, and a current control unit: The synchronous control unit is used to collect the three-phase voltage of the grid-connected node of the grid-connected converter, the three-phase current of the grid-side output and the three-phase current of the filter inductor on the bridge arm side, and generate a complex reference voltage by combining the power reference value and the rated operating parameters. The cross control unit is used to receive the complex reference voltage and generate a complex reference current by combining the virtual impedance parameter and the current limiting parameter; The current control unit is used to receive the complex reference current, select the optimal switching state based on a finite control set, and output a drive signal to control the operation of the grid-connected converter.

[0080] In this embodiment, the corresponding control system includes three functional units: a synchronization control unit, a cross control unit, and a current control unit. The three units are cascaded in sequence, and data is transmitted sequentially to jointly complete the control function of the grid-connected converter.

[0081] The synchronization control unit comprises a signal sampling module and a synchronization calculation module. The signal sampling module is responsible for acquiring the instantaneous values ​​of the three-phase voltage at the grid-connected node, the three-phase current output from the grid side, and the three-phase current of the filter inductor on the bridge arm side. It converts the analog signals into digital signals and sends them to the synchronization calculation module. The signal sampling module includes voltage sensors, current sensors, and an AD converter. The voltage sensors employ resistive voltage dividers or Hall effect voltage sensors, while the current sensors use Hall effect current sensors or shunts, enabling accurate sampling of AC and DC signals. The AD converter uses a multi-channel synchronous sampling architecture to ensure consistent sampling times for the three-phase voltage and current, avoiding calculation errors caused by sampling phase differences. The synchronization calculation module incorporates a synchronization control algorithm, receives power reference values ​​and rated operating parameters, performs forward sequence extraction, coordinate transformation, and synchronization calculation, and outputs a complex reference voltage to the cross-control unit. The calculation cycle of the synchronization control unit is consistent with the sampling cycle; one sampling and calculation is completed in each sampling cycle, ensuring real-time control.

[0082] The cross-control unit comprises a voltage limiting module, a virtual impedance calculation module, and a current reference generation module. The voltage limiting module receives the complex reference voltage output from the synchronous control unit, performs amplitude limiting processing, and outputs the limited voltage reference value. The virtual impedance calculation module receives the filter inductor current signal from the bridge arm side, calculates the virtual impedance voltage drop based on the virtual impedance parameters, and outputs a current feedback correction term. The current reference generation module combines the limited voltage reference value and the current feedback correction term to calculate the complex reference current, which is then output to the current control unit. The cross-control unit can automatically adjust the virtual impedance parameters according to the voltage amplitude, achieving adaptive switching between steady-state and transient states. The calculations of the cross-control unit are also completed within each sampling period, executed synchronously with the synchronous control unit to ensure timing consistency of the control link.

[0083] The current control unit comprises a prediction module, a cost calculation module, and an optimization output module. The prediction module, based on a discrete model of the filter circuit, calculates the predicted current value for each switching state. The cost calculation module constructs a comprehensive cost function to calculate the comprehensive cost value corresponding to each switching state. The optimization output module iterates through all switching states, selects the switching state with the lowest cost value, and converts it into a drive signal output to the bridge arm switch of the converter. The current control unit's calculations are completed within each sampling period, ensuring real-time updates of the switching states. The current control unit is the execution end of the entire control system, directly determining the final output waveform quality and dynamic response speed; its calculation accuracy and speed directly affect the overall control effect.

[0084] It should be noted that the control system can be implemented using a digital signal processor (DSP), a field-programmable gate array (FPGA), or a combination of both. Sampling and logic output are handled by the FPGA, while computation is performed by the DSP, balancing computational power and response speed. The division of functional units is only a logical functional division; in actual implementation, they can be integrated into the same processor, with each unit's function implemented through software code, or some functions can be implemented using independent hardware circuits. Software implementation offers high flexibility, facilitating parameter adjustment and algorithm upgrades; hardware implementation offers fast computation speed and high timing accuracy, suitable for high-frequency control scenarios. In practical applications, the appropriate implementation method can be selected based on cost, performance, and development cycle requirements.

[0085] In this embodiment, two grid-connected converters with identical rated parameters are selected for parallel operation as the verification scenario. The AC sides of the two converters are connected to a common coupling point, which is connected to a local load. The DC sides are each connected to an independent DC power supply, simulating a new energy power generation unit. Both converters adopt the control method of this application and have no communication connection with each other. They achieve autonomous synchronization and power distribution by relying on their respective local sampling and control algorithms.

[0086] For example, if the initial active power reference values ​​of two converters are set to be the same, during steady-state operation, the active power output of the two converters is basically equal, sharing the load power. The three-phase voltage at the point of common coupling remains stable, with the voltage amplitude maintained near the rated value, and the output current waveform is smooth. When the active power reference value of one converter is adjusted to increase it, the output active power of that converter increases accordingly. The other converter, without external commands, can automatically adjust its output active power based on changes in the voltage and frequency at the point of common coupling. The sum of the output power of the two converters always matches the load power demand, with no significant voltage drop at the point of common coupling and frequency fluctuations within a small range. This scenario verifies that the control method of this application can achieve parallel operation of multiple converters without communication, possess autonomous power distribution capabilities, and support load power demands.

[0087] Furthermore, with the two converters operating in parallel, a sudden increase in the local load power caused a slight transient drop in the point of common coupling voltage, which quickly recovered. The output active power of both converters increased simultaneously, sharing the increased load power, and the output power of the two converters remained essentially evenly distributed with no significant power deviation. During the transient process of increased load, the output current showed no significant overshoot, and the transient oscillation amplitude was small, allowing for a rapid return to a new steady-state operating point. This scenario validates the adaptability of the proposed control method under load fluctuation conditions, demonstrating its ability to quickly respond to load changes and maintain synchronous operation and even power distribution.

[0088] For example, a three-phase short-circuit fault scenario is set up on the grid side. During steady-state operation, the grid-side three-phase short-circuit fault is triggered, and after a period of time, the fault is cleared, restoring normal grid connection. After the fault occurs, the grid-connected node voltage drops rapidly, and the voltage limiting and virtual impedance adjustment of the cross-control loop quickly intervene, limiting the amplitude of the converter output current. The output current amplitude is controlled within the allowable range, preventing significant overcurrent damage to devices. During the fault duration, the synchronization control loop continues to operate, maintaining phase synchronization. The phase of the converter output voltage remains synchronized with the grid phase, without any loss of synchronization. After the fault is cleared, the grid voltage recovers, the converter output current gradually decreases, the virtual impedance parameter gradually returns to its steady-state value, and the converter smoothly transitions to steady-state operation without transient oscillations or loss of synchronization. In contrast, common control methods often result in current amplitudes significantly exceeding allowable values ​​during the fault period, and after the fault is cleared, the converter experiences phase oscillations, ultimately losing synchronization and failing to resume normal operation. This scenario verifies the current limiting and synchronization stability capabilities of the control method in this application under large disturbance fault scenarios, improving the transient immunity performance of the converter.

[0089] Specifically, a grid voltage harmonic distortion scenario is set up by injecting a certain proportion of low-order harmonics into the grid voltage to simulate harmonic pollution in the actual power grid. When using common control methods, the converter output current contains a significant amount of harmonic components, resulting in obvious current waveform distortion and fluctuations in output active power. With the control method proposed in this application, the comprehensive cost function of the current control loop simultaneously optimizes current tracking and power fluctuation, significantly reducing the harmonic content of the output current, making the current waveform closer to a sine wave, and also reducing the fluctuation amplitude of the output active power. This scenario verifies the operating performance of the control method proposed in this application under small disturbances such as harmonic voltage, demonstrating that it can reduce the output current harmonic distortion rate, reduce power fluctuations, and improve steady-state operation quality.

[0090] It should be noted that the control method in this application achieves a balance between steady-state and transient performance through the sequential coordination of three levels of control loops. The synchronization control loop, as the outermost power and synchronization control loop, is responsible for generating the basic voltage reference, ensuring power tracking and synchronous operation under steady-state conditions, and also possesses preliminary current suppression capabilities, providing a reasonable voltage reference input for intermediate loops. Common grid-based control schemes often employ a droop control structure combined with virtual inertia, achieving power distribution and synchronization through active frequency droop and reactive voltage droop. These schemes rely on power calculation results to adjust the phase. In weak grid scenarios, the line impedance is high, and power calculations involve delays and fluctuations, easily leading to phase oscillations and affecting synchronization stability. Some schemes introduce phase-locked loops (PLLs) to assist synchronization, but PLLs are prone to loss of lock-in in weak grids, and the coupling between grid-based control and PLLs can easily cause control conflicts. The synchronous control scheme of this application constructs a synchronous reference directly based on the instantaneous components of the grid-connected node voltage and the output current, and performs steady-state regulation in combination with the power reference value. It integrates phase regulation and amplitude regulation into the same complex number operation, eliminating the need for a separate phase-locked loop structure and a separate droop control loop. The operation structure is simple, and the response speed and steady-state accuracy of phase regulation can be flexibly adjusted through the gain coefficient. At the same time, a current suppression term is added, which can automatically adjust the reference voltage when the current amplitude increases, thus limiting the current growth.

[0091] The cross-control loop, acting as an intermediate link and current limiting mechanism, is responsible for converting the voltage reference to a current reference. It also incorporates amplitude limiting and virtual impedance regulation, intervening in current limiting during transient faults while maintaining phase continuity to prevent synchronization instability. This loop receives the outer voltage reference and outputs the inner current reference, providing an adaptive transition between steady-state and transient modes without requiring additional mode-switching logic, thus avoiding transient shocks caused by mode switching. Common fault current limiting schemes often directly limit the output voltage reference value or hard-limit the current reference value. These methods are prone to abrupt phase changes during amplitude limiting, leading to synchronization phase jumps. After fault clearance, it is difficult to quickly restore synchronization, and may even cause loss of synchronization. Some schemes use virtual impedance current limiting, but this is often directly superimposed on the voltage reference, which affects voltage regulation accuracy in steady state and has limited current limiting response speed in transient states. The cross-control scheme of this application simultaneously performs amplitude limiting and virtual impedance adjustment during the process of converting the voltage reference to the current reference. The voltage limiting only acts on the amplitude dimension, while the phase dimension is continuously adjusted. The virtual impedance is calculated and fed back in real time based on the current on the bridge arm side, which can quickly respond to current changes. During the current limiting process, the phase is always continuously adjusted. After the fault is cleared, it can smoothly transition to the steady-state operation mode without the problem of loss of synchronization caused by phase jump.

[0092] The current control loop, as the innermost execution loop, is responsible for quickly tracking the current reference, optimizing power fluctuations and current harmonics, directly controlling switching actions, and converting control commands into actual power output. The three loops, from the outside in, have progressively increasing control bandwidth. The outer layer's synchronous control bandwidth is lower to ensure steady-state stability; the inner layer's current control bandwidth is higher to ensure dynamic response speed; and the middle cross-control loop adapts to the bandwidth of both layers, achieving smooth transitions. Common grid-connected converter current control schemes often use PI regulators combined with PWM modulation. These schemes offer high steady-state accuracy but limited dynamic response speed, resulting in insufficient tracking speed in transient scenarios. Some schemes employ model predictive control, but these often prioritize minimizing current tracking error, resulting in lower output current harmonic content but larger power fluctuations, failing to simultaneously meet the requirements of current waveform quality and power stability. Other schemes aim for power point tracking, resulting in smaller power fluctuations but higher current harmonic content. The current control scheme of this application incorporates both current tracking error and power fluctuation into the comprehensive cost function. By optimizing the weight coefficients to adjust the weight ratio of the two objectives, and traversing the optimization under limited switching conditions, it can reduce the power fluctuation amplitude while ensuring current tracking performance. It does not require an additional power control loop, has a simple control structure, fast dynamic response speed, and can be adapted to both steady-state and transient operation scenarios.

[0093] The inputs and outputs of the three stages form a complete causal link. The complex reference voltage output of the synchronous control is the input of the cross control. The cross control adjusts the current reference based on this voltage reference and current feedback. The current control selects the optimal switching state based on this current reference to control the converter output voltage and current. The output voltage and current are sampled and fed back to the synchronous and cross control stages, forming a closed-loop control. During steady-state operation, the limiting stage of the cross control does not operate, and the virtual impedance remains at a small value. The entire control link is equivalent to voltage-type network control, which has good steady-state performance. During fault transients, the voltage drop causes the reference voltage amplitude to exceed the limit value, the limiting stage operates, and the virtual impedance automatically increases. The control link automatically switches to current-limiting mode to limit the output current, while the synchronous stage continuously maintains phase to ensure synchronous stability. The entire process does not require mode detection and switching, and relies entirely on the self-adaptive adjustment of the control stage's characteristics, avoiding the risk of transient oscillations and loss of synchronization caused by mode switching.

[0094] Furthermore, the control method of this application is applicable not only to grid-connected operation scenarios but also to off-grid independent operation scenarios. In off-grid scenarios, a voltage reference can be directly set to control the stability of the load voltage, while also providing overload current limiting capability. This method is also applicable to microgrid systems composed of multiple converters connected in parallel, achieving power sharing and synchronous operation without communication, reducing the construction cost of microgrids and improving system reliability. This method can also be extended to various power electronic grid-connected devices such as energy storage converters, photovoltaic inverters, and wind power converters, with a wide range of applications. For grid-connected converters with more complex topologies such as three-level and multi-level converters, only the number of switching states of the finite control set and the discrete prediction model need to be adjusted; the control framework remains unchanged, demonstrating strong scalability.

[0095] In this embodiment, the filter circuit adopts an LCL structure. The function of the bridge arm-side filter inductor is to filter out the current ripple at the switching frequency, smoothing the pulse voltage output by the bridge arm into a continuous current waveform. The function of the filter capacitor is to filter out the voltage ripple at the switching frequency, further smoothing the output voltage waveform. The function of the grid-side filter inductor is to work with the filter capacitor to form a second-order filter structure, improving the attenuation effect of high-frequency ripple. The damping resistor connected in parallel with the filter capacitor is used to suppress the resonance peak of the LCL filter circuit, avoiding gain spikes at the resonant frequency and causing oscillation instability. The value of the damping resistor needs to balance the damping effect and loss. A larger value results in stronger damping but higher loss, while a smaller value results in lower loss but weaker damping. It needs to be reasonably selected according to the system stability requirements.

[0096] The design of the sampling stage must ensure sampling accuracy and synchronization. Voltage sampling typically uses a resistor divider network to convert high-voltage AC signals into low-voltage signals, which are then fed into an AD converter after passing through a follower circuit composed of operational amplifiers and a filter circuit. Current sampling typically uses a Hall current sensor to convert large current signals into small voltage signals, which are then filtered before being fed into the AD converter. The cutoff frequency of the sampling filter circuit must be higher than the control bandwidth and lower than the switching frequency. This ensures that the sampled signal does not attenuate significantly while filtering out high-frequency noise near the switching frequency, thereby improving sampling accuracy. Multi-channel synchronous sampling ensures that the sampling times of the three-phase voltage and three-phase current are completely consistent, avoiding phase errors caused by sampling time differences that could affect the accuracy of power calculation and synchronous control.

[0097] The selection of a digital controller must meet the requirements of computational load and sampling frequency. For three-phase two-level grid-connected converters, the sampling frequency is typically set to several kilohertz to tens of kilohertz. A higher sampling frequency results in higher control bandwidth and faster dynamic response, but also a greater computational load and higher performance requirements for the controller. The controller's computing resources must be sufficient to support all operations within each sampling cycle, including coordinate transformation, positive sequence extraction, synchronous control operations, cross-control operations, prediction operations, cost calculation, and optimization, ensuring that all operations are completed within one sampling cycle and the switching state is output. In practical applications, a suitable controller can be selected based on the converter's power rating and performance requirements. For low-to-medium power scenarios, a lower-cost digital signal processor can be used, while for high-power, high-performance scenarios, a higher-performance processor combined with a field-programmable gate array (FPGA) can be used.

[0098] During parameter debugging, the current control loop should be debugged first. Bypass the cross-control and synchronization control loops, directly apply a current reference value, verify the current tracking performance and harmonic suppression effect, and adjust and optimize the weighting coefficients to achieve the expected current waveform and power fluctuations. Next, debug the cross-control loop, applying a voltage reference value, to verify the current limiting effect and the damping effect of the virtual impedance. Adjust the amplitude limit and virtual impedance parameters to balance the current limiting capability and steady-state voltage drop. Finally, debug the synchronization control loop, connecting it to the grid or operating it in parallel, to verify synchronization stability and power distribution effect, and adjust the voltage gain coefficient, current gain coefficient, and current suppression coefficient to ensure the system's steady-state accuracy and dynamic response meet the requirements. Parameter debugging follows a sequence from the inside out and from simple to complex, which can quickly locate problems and improve debugging efficiency.

[0099] It should be noted that the control method of this application is more adaptable to weak grid scenarios. Weak grids have low short-circuit ratios and high line impedances, making traditional grid control prone to synchronization instability. The synchronization control in this application combines instantaneous voltage and current feedback with power regulation, offering better damping characteristics for phase regulation, maintaining stable synchronous operation even under high line impedance. Simultaneously, the current-limiting and virtual impedance characteristics of cross-control provide sufficient current-limiting capability during faults, preventing fault current from impacting the grid and equipment. The fast-response characteristics of current control can quickly track reference changes, suppressing transient oscillations and further improving operational stability under weak grid conditions. With these three features combined, the converter can operate stably under a wider range of grid strengths, adapting to the operational needs of new power systems with a high proportion of renewable energy integration.

[0100] This application is not limited to these embodiments. The above embodiments are merely exemplary implementations. In practical applications, some aspects can be adjusted according to specific scenarios. For example, when applied in a single-phase grid-connected converter, the coordinate transformation and positive sequence extraction methods can be adjusted to adapt to the signal processing requirements of a single-phase system. When applied in high-voltage, high-power scenarios, the topology of a multi-level converter can be adapted to expand the number of switching states in the finite control set, ensuring control effectiveness. All equivalent adjustments and substitutions based on the technical principles of this application fall within the protection scope of this application. Further details are omitted here.

Claims

1. A control method for a high-disturbance-resistance grid-connected converter based on a comprehensive strategy, characterized in that, include: The grid-connected converter obtains the three-phase voltage at the grid-connected node, the three-phase current at the grid-side output, and the three-phase current at the filter inductor on the bridge arm side. It then performs synchronous control calculations by combining the active power reference value, the reactive power reference value, and the rated operating parameters to generate a complex reference voltage for the synchronous control output. The complex reference voltage and the three-phase current input of the filter inductor on the bridge arm side are combined with the virtual impedance parameters and current limiting parameters to generate the complex reference current of the cross control output. The complex reference current is input into the current control loop. Based on the finite control set, all bridge arm switch combination states are traversed, and the switch state that minimizes the combined cost of current tracking and power fluctuation is selected to control the operation of the fully controlled power electronic switches of each bridge arm of the grid-connected converter.

2. The high-disturbance-resistance grid-connected converter control method based on a comprehensive strategy according to claim 1, characterized in that, The synchronous control operation includes: Positive sequence extraction and coordinate transformation are performed on the three-phase voltage of the grid-connected node and the three-phase output current of the grid side to obtain the positive sequence complex components of voltage and current. By combining the active power reference value, the reactive power reference value and the line impedance angle, the power regulation term and the current suppression term are calculated. The power regulation term, current suppression term, and rated angular frequency integral term are superimposed to generate a complex reference voltage for synchronous control output.

3. The high-disturbance-resistance grid-connected converter control method based on a comprehensive strategy according to claim 1, characterized in that, The calculation combining virtual impedance parameters and current limiting parameters includes: The complex reference voltage is subjected to amplitude limiting to obtain the voltage polar coordinate components after amplitude limiting; By combining the virtual impedance parameters with the three-phase current of the filter inductor on the bridge arm side, the current feedback correction term is calculated. The voltage polar coordinate components after the limit are combined with the current feedback correction term to generate a complex reference current for the cross-control output.

4. The high-disturbance-resistance grid-connected converter control method based on a comprehensive strategy according to claim 1, characterized in that, The process of traversing all bridge arm switch combinations based on a finite control set includes: A discrete prediction model for the filter circuit of the grid-connected converter is established, and the predicted current value under each switching state is calculated based on the complex reference current. By combining optimized weighting coefficients, a comprehensive cost function is constructed that includes current tracking error and power fluctuation. Iterate through all bridge arm switch combinations, calculate the corresponding comprehensive replacement value, and select the switch combination with the lowest replacement value.

5. The high-disturbance-resistance grid-connected converter control method based on a comprehensive strategy according to claim 2, characterized in that, The positive-sequence extraction and coordinate transformation of the three-phase voltage of the grid-connected node and the three-phase output current of the grid side includes: The Clarke transform is performed on the sampled three-phase instantaneous values ​​to obtain the axial components in the two-phase stationary coordinate system; The positive sequence components of the axis components in the two-phase stationary coordinate system are extracted to obtain the positive sequence axis components of voltage and current. Based on the positive sequence voltage axis component and the positive sequence current axis component, complex number representations are constructed respectively to obtain the positive sequence voltage complex component and the positive sequence current complex component.

6. The high-disturbance-resistance grid-connected converter control method based on a comprehensive strategy according to claim 3, characterized in that, The limiting process for the complex reference voltage includes: Calculate the magnitude and phase angle of the complex reference voltage; The magnitude value is compared with the preset maximum allowable voltage amplitude value, and the smaller value is taken as the voltage magnitude value after limiting. By combining the voltage magnitude after limiting with the original phase angle, the polar coordinate components of the voltage after limiting are generated.

7. The high-disturbance-resistance grid-connected converter control method based on a comprehensive strategy according to claim 4, characterized in that, The construction of the comprehensive cost function, which includes current tracking error and power fluctuation, includes: The difference between the predicted current value and the complex reference current is calculated to obtain the current tracking error term; Instantaneous power is calculated based on the predicted current and the grid-connected node voltage, and the power fluctuation error term is obtained. By optimizing the weighting coefficients, the current tracking error term and the power fluctuation error term are weighted and summed to generate a comprehensive cost function.

8. The high-disturbance-resistance grid-connected converter control method based on a comprehensive strategy according to claim 3, characterized in that, Before calculating the current feedback correction term by combining the virtual impedance parameters and the three-phase current of the bridge arm-side filter inductor, the following is also included: Detect the voltage amplitude at the common connection point of the grid-connected converter to determine the current operating status; When the voltage amplitude is lower than the preset threshold, increase the values ​​of virtual resistance and virtual reactance in the virtual impedance; When the voltage amplitude recovers to above the preset threshold, the virtual impedance parameter will fall back to the steady-state preset value.

9. The high-disturbance-resistance grid-connected converter control method based on a comprehensive strategy according to claim 1, characterized in that, The operation of the fully controlled power electronic switches of each arm of the grid-connected converter includes: The selected switch state is latched at the end of the current sampling period; The switch state is converted into a switch trigger level signal corresponding to each bridge arm; At the start of the next sampling period, the trigger level signal is output to drive the fully controlled power electronic switch to turn on or off.

10. A high-disturbance-resistance grid-connected converter control system based on a comprehensive strategy, characterized in that, Includes a synchronization control unit, a cross control unit, and a current control unit: The synchronous control unit is used to collect the three-phase voltage of the grid-connected node of the grid-connected converter, the three-phase current of the grid-side output and the three-phase current of the filter inductor on the bridge arm side, and generate a complex reference voltage by combining the power reference value and the rated operating parameters. The cross control unit is used to receive the complex reference voltage and generate a complex reference current by combining the virtual impedance parameter and the current limiting parameter; The current control unit is used to receive the complex reference current, select the optimal switching state based on a finite control set, and output a drive signal to control the operation of the grid-connected converter.

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