Inverter output control system based on reconstruction controller and control method

Abstract

This invention discloses an inverter output control system and method based on a reconfigurable controller, relating to the field of power electronics technology. It measures the grid-connected current and subtracts it from a reference current using a first subtractor to generate a current deviation signal. This current deviation signal is then input to a current loop proportional resonant controller, which selects and switches between a differential controller and a lead compensator controller based on the system short-circuit ratio to generate a first control signal. The grid-connected current is input to an active damping module to generate a damping signal, which is then superimposed on the first control signal and input to a second subtractor. The common coupling point voltage is acquired and a feedforward signal is generated using a feedforward module. This feedforward signal is then superimposed on the output signal of the second subtractor to generate a modulation signal. The modulation signal is converted into a drive signal via a pulse width modulation module to control the inverter output current. This method has the advantage of improving system stability and harmonic suppression capabilities under weak grid conditions by dynamically adjusting the controller combination mode.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to an inverter output control system and control method based on a reconfigurable controller. Background Technology

[0002] As the proportion of renewable energy in the power grid continues to increase, the characteristics of weak power grids are becoming increasingly prominent, mainly manifested in the non-negligible grid impedance and increased background harmonic content, leading to a decrease in the stability margin of grid-connected inverters and a deterioration in grid current quality. Among existing methods, while feedforward point of common coupling (PCC) voltage can suppress harmonics, direct feedforward introduces positive feedback from the grid impedance, exacerbating system instability. Strategies relying on grid impedance measurement require injecting disturbances or sacrificing power generation efficiency, limiting their practicality. On the other hand, while nonlinear control methods can reshape the output impedance, their parameter design is complex and lacks versatility, making widespread application difficult. Therefore, there is an urgent need for a strategy based on conventional control elements that is easy to implement in engineering, in order to balance the needs of improving the stability margin of systems under weak power grid conditions with harmonic suppression. Summary of the Invention

[0003] The purpose of this application is to provide an inverter output control system and method based on a reconfigurable controller, which has the advantages of improving system stability and harmonic suppression capability under weak power grid by dynamically adjusting the controller combination mode.

[0004] This application provides an inverter output control system based on a reconfigurable controller, the technical solution of which is as follows: An inverter output control system based on a reconfigurable controller includes: a first subtractor, a current loop proportional resonant controller, a differential controller and its switching switch, a lead compensation controller and its switching switch, an active damping module, a second subtractor, a grid voltage feedforward module, a first adder, a second adder, and a pulse width modulation module; the input terminal of the first subtractor is connected to the reference current and the feedback grid current, and its output terminal is respectively connected to the input terminals of the current loop proportional resonant controller, the differential controller switching switch, and the lead compensation controller switching switch; the differential controller switching switch and the lead compensation controller switching switch... The output of the positive controller switching switch is connected to the input of the differential controller and the lead compensator, respectively; the outputs of the current loop proportional resonant controller, the differential controller, and the lead compensator are connected to the input of the first adder to generate the first control signal; the input of the active damping module is connected to the grid current, and its output, together with the output of the first adder, is connected to the input of the second subtractor; the input of the grid voltage feedforward module is connected to the common coupling point voltage, and its output, together with the output of the second subtractor, is connected to the input of the second adder to generate a modulation signal; the modulation signal is converted into a drive signal by the pulse width modulation module to control the inverter power devices to output the grid current.

[0005] In some embodiments, the combination mode of the current loop proportional resonant controller, the differential controller, and the lead compensation controller is switched according to the system short-circuit ratio, which is the ratio of the square of the common coupling point voltage to the product of the inverter output power and the grid impedance.

[0006] In some embodiments, when the system short-circuit ratio is greater than 2, the current loop proportional resonant controller and the derivative controller are activated; when the system short-circuit ratio is less than or equal to 2, the current loop proportional resonant controller, the derivative controller, and the lead compensation controller are activated.

[0007] In some embodiments, the active damping module employs a negative high-pass filter or a negative band-pass filter, which is equivalent to a second-order differential element at the resonant frequency.

[0008] In some embodiments, the grid voltage feedforward module supports proportional feedforward or full feedforward modes, which are dynamically switched according to the background harmonic content of the grid.

[0009] In some embodiments, the grid voltage feedforward module is used to adopt a proportional feedforward mode when the grid background harmonic content is lower than a preset value, and to switch to a full feedforward mode when the grid background harmonic content is higher than a preset value.

[0010] In some embodiments, an inverter output control method based on a reconfigurable controller includes the following steps: measuring the grid-connected current and subtracting it from a reference current using a first subtractor to generate a current deviation signal; inputting the current deviation signal to a current loop proportional resonant controller, and selecting a switching differential controller and a lead compensator controller based on the system short-circuit ratio to generate a first control signal; inputting the grid-connected current to an active damping module to generate a damping signal, which is then superimposed on the first control signal and input to a second subtractor; acquiring the common coupling point voltage and generating a feedforward signal using a feedforward module, which is then superimposed on the output signal of the second subtractor to generate a modulation signal; and converting the modulation signal into a drive signal using a pulse width modulation module to control the inverter output current.

[0011] In some embodiments, the switching conditions for the differential controller and the lead compensator are as follows: when the system short-circuit ratio is greater than 2, only the differential controller is activated; when the system short-circuit ratio is less than or equal to 2, both the differential controller and the lead compensator are activated simultaneously.

[0012] In some embodiments, the method further includes: measuring the inverter output power and grid impedance; dividing the square of the common coupling point voltage by the product of the inverter output power and the grid impedance to obtain the system short-circuit ratio.

[0013] In some embodiments, the proportional feedforward or full feedforward mode of the feedforward module is dynamically adjusted according to the background harmonic content of the power grid.

[0014] As can be seen from the above, the inverter output control system and method based on the reconfigurable controller provided in this application achieves the matching optimization of controller parameters and grid impedance under weak grid conditions by combining a current loop proportional resonant controller with a dynamically switched differential controller and a lead compensation controller, as well as an adaptively adjusted grid voltage feedforward module. It has the advantage of improving system stability and harmonic suppression capability under weak grid conditions by dynamically adjusting the controller combination mode.

[0015] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the description, claims, and drawings. Attached Figure Description

[0016] The accompanying drawings are provided to further understand the technical solutions of the present invention and constitute a part of the specification. They are used together with the embodiments of the present invention to explain the technical solutions of the present invention, and do not constitute a limitation on the technical solutions of the present invention.

[0017] The present invention will be further described below with reference to the accompanying drawings and embodiments;

[0018] Figure 1 An overall block diagram of an inverter output control device based on a reconfigurable controller is provided for an embodiment of the present invention;

[0019] Figure 2 A control block diagram of an inverter output control device based on a reconfigurable controller provided in an embodiment of the present invention;

[0020] Figure 3 A simplified control block diagram of an inverter output control device based on a reconfigurable controller is provided for an embodiment of the present invention.

[0021] Figure 4 A flowchart of an inverter output control method based on a reconfigurable controller provided in an embodiment of the present invention;

[0022] Figure 5 This is a Bode plot of the system using a conventional approach.

[0023] Figure 6 Bode plot of the system when using PR+DC controller in an embodiment of the present invention.

[0024] Figure 7 The Bode diagram of the PR+DC+LC controller system is used in the embodiment of the present invention.

[0025] Figure 8 This serves to verify the effectiveness of the strategy proposed in this invention under weak power grid conditions.

[0026] Figure 9 This serves as a verification of the effectiveness of the strategy proposed in this invention under extremely weak power grid conditions. Detailed Implementation

[0027] This section will describe in detail specific embodiments of the present invention. Preferred embodiments of the present invention are shown in the accompanying drawings. The purpose of the drawings is to supplement the textual description with graphics, so that people can intuitively and vividly understand each technical feature and overall technical solution of the present invention, but they should not be construed as limiting the scope of protection of the present invention.

[0028] In the description of this invention, "several" means one or more, "multiple" means two or more, "greater than," "less than," "exceeding," etc. are understood to exclude the number itself, while "above," "below," "within," etc. are understood to include the number itself. "Any one" refers to one or more, and "at least one of the following" and similar expressions refer to any combination of these items, including any combination of single or multiple items. If "first" or "second" is used in the description, it is only for the purpose of distinguishing technical features and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the order of the indicated technical features.

[0029] It should be noted that the terms "setting," "installing," and "connecting" in the embodiments of this invention should be interpreted broadly. Those skilled in the art can reasonably determine the specific meaning of the above terms in the embodiments of this invention in conjunction with the specific content of the technical solution. For example, the term "connection" can be a mechanical connection, an electrical connection, or a connection that allows for mutual communication; it can be a direct connection or an indirect connection through an intermediate medium.

[0030] In traditional grid-connected inverter control systems, the non-negligible grid impedance and increased background harmonic content under weak grid conditions reduce system stability margin. Directly introducing the feedforward point of common coupling voltage creates a positive feedback path of grid impedance, further exacerbating system oscillation risks. Strategies relying on grid impedance measurement require actively injecting disturbance signals or adjusting inverter operating states, resulting in power generation efficiency losses or additional hardware costs, making practical application impractical.

[0031] In this regard, refer to Figure 1 , Figure 1This application presents an overall block diagram of an inverter output control device based on a reconfigurable controller, according to an embodiment of the present invention. The device includes: a first subtractor 1, a current loop proportional resonant controller 2, a differential controller 5 and its switching switch, a lead compensation controller 6 and its switching switch, an active damping module 7, a second subtractor 9, a grid voltage feedforward module 10, a first adder 8, a second adder 11, and a pulse width modulation module 12. The input terminal of the first subtractor 1 is connected to a reference current and a feedback grid current, and its output terminal is connected to the input terminals of the current loop proportional resonant controller 2, the differential controller switching switch 3, and the lead compensation controller switching switch 4, respectively. The output terminals of switch 3 and the lead compensation controller 4 are respectively connected to the input terminals of differential controller 5 and lead compensation controller 6; the output terminals of current loop proportional resonant controller 2, differential controller 5 and lead compensation controller 6 are connected to the input terminal of first adder 8 to generate first control signal; the input terminal of active damping module 7 is connected to grid current, and its output terminal and the output terminal of first adder 8 are connected to the input terminal of second subtractor 9; the input terminal of grid voltage feedforward module 10 is connected to common coupling point voltage, and its output terminal and the output terminal of second subtractor 9 are connected to the input terminal of second adder 11 to generate modulation signal; the modulation signal is converted into drive signal by pulse width modulation module 12 to control the inverter power device to output grid current.

[0032] The first subtractor 1 is an operational amplifier circuit used to calculate the deviation between the reference current and the feedback grid current. Its function is to transmit the current error signal to the subsequent controller for adjustment. The current loop proportional resonant controller 2 is a composite controller combining proportional gain and resonant regulator. It can track and suppress specific harmonics by setting the resonant frequency point, thereby improving the system's tracking accuracy and harmonic suppression capability for the fundamental current. The differential controller 5 and its switching device are components containing a differential operation unit and an electronic switch. They can be implemented using a field-effect transistor-based switching circuit, used to selectively introduce phase lead compensation based on grid strength conditions. The phase lead compensation controller 6 and its switching device are devices containing a phase lead network and relay contacts. They can be constructed using an RC network to build the phase lead network. The compensation circuit is used to enhance the system phase margin under weak grid conditions. The active damping module 7 is a circuit that generates an equivalent negative impedance through active control, specifically implemented using an operational amplifier to construct a negative high-pass filter, used to cancel the resonance peak caused by grid impedance. The second subtractor 9 is an operational unit used to synthesize the control signal and the damping signal, specifically implemented using a differential amplifier circuit, its function is to superimpose the damping compensation amount onto the main control loop. The grid voltage feedforward module 10 is a circuit that collects the voltage at the common coupling point for feedforward compensation, specifically implemented using a proportional amplifier or a bandpass filter, used to cancel the disturbance of grid voltage fluctuations to the system. The pulse width modulation module 12 is a device that converts the modulation signal into a switching drive signal, specifically generating a PWM waveform through a carrier comparator circuit, used to control the conduction timing of the inverter power devices.

[0033] Understandably, by combining the switchable differential controller 5 and the lead compensation controller 6, along with the active damping module 7 and dynamic coordination with grid voltage feedforward, a composite control architecture adaptable to changes in grid strength is constructed. This architecture, while retaining conventional proportional resonant control, achieves dynamic reconfiguration of the control strategy through switching devices. This maintains control efficiency under strong grid conditions while enhancing system stability under weak grid conditions through additional correction stages. Simultaneously, it utilizes active damping to suppress resonance and feedforward compensation to eliminate grid disturbances, forming a multi-dimensional stability enhancement mechanism. Furthermore, the inverter output control system based on the reconfigurable controller includes multiple functional modules that work collaboratively to achieve precise control of the inverter output current.

[0034] refer to Figure 2 and Figure 3 , Figure 2 A control block diagram of an inverter output control device based on a reconfigurable controller provided in an embodiment of the present invention; Figure 3 A simplified control block diagram of an inverter output control device based on a reconfigurable controller is provided for embodiments of the present invention; it is understood that, Figure 1 This application provides a design example for improving the grid-connected dynamic characteristics of a current-source inverter. Figure 2 This is a system control block diagram, in which... Figure 2 After simplification, such as Figure 3 As shown, in Figure 3 In the middle, G x1 (s) and G x2 The expressions for (s) are as follows:

[0035]

[0036] In the formula, K p K is the proportionality coefficient. r Here, ω is the resonant coefficient, ω0 is the fundamental angular frequency, and ω r To consider the resonant term bandwidth required by -3dB, i.e. at ω0±ω r The gain of the resonant term is 0.707K. r The allowable frequency deviation of the power grid is no more than 0.5Hz. To ensure that the PR controller has sufficient gain when the power grid frequency fluctuates, this paper selects ω. r =2πΔf = πrad / s. According to... Figure 3 The open-loop transfer function of the system can be obtained:

[0037] G op (s)=G x1 (s)G x2 (s) (3)

[0038] Because the PR controller has a negative phase shift characteristic, a corner frequency f is required for the PR controller. L Less than the system crossover frequency f c f c It needs to be less than the resonant frequency f r Therefore, in the analysis of f, which is higher than or equal to f c When dealing with the amplitude-frequency response, G PR (s) can be approximated by K p Replace. Because in f c The open-loop transfer function magnitude gain of the system is 1, i.e., |G| = 1. op (j2πf c )|=1, and consider |G i (j2πf c )|≈K p Substituting into equation (3), we can obtain K. p The expression:

[0039]

[0040] according to Figure 3 i can be calculated g expression

[0041]

[0042] Considering s 2 L1C<<1 holds true, therefore the following equation holds true:

[0043] G i (s)K pwm -G x1 (s)=G i (s)K pwm (s 2 L1C / (1+s 2 L1C))≈0 (6)

[0044] Combining equations (5) and (6), we can obtain the expression for the error current:

[0045]

[0046] Since the error mainly considers the frequency response at the fundamental frequency of the power grid, the capacitance component characterizing the high-frequency properties of the system can be ignored. Let the system error at the fundamental frequency be E. g The error of (j2πf0) is η. According to equation (7), we can obtain G. op (s) Minimum amplitude frequency response at the fundamental frequency:

[0047]

[0048] Further derivation can be made:

[0049]

[0050] K can be calculated from (9). r Lower limit of value:

[0051]

[0052] The phase-frequency characteristic of the quasi-PR controller can be expressed as:

[0053]

[0054] K r Increasing the phase gain of the PR controller improves system control accuracy and dynamic response speed, but it causes the phase gain to approach -90° in the mid-to-high frequency range, resulting in a decrease in the corrected system phase margin. Therefore, a trade-off needs to be made between system control accuracy, response speed, and stability. Assuming the system phase margin is PM, the PR controller at f c At a phase angle gain of -θ, the following relationship must be satisfied to ensure system stability:

[0055] π-PM-θ>0 (12)

[0056] According to equations (11) and (12), we can obtain:

[0057]

[0058] According to equation (13), K can be calculated. r Maximum value:

[0059]

[0060] Based on the above derivation, the PR controller parameters can be calculated by substituting the system parameters.

[0061] PCC voltage u under weak and extremely weak power grids PCC Full feedforward results in full feedforward of the grid impedance voltage, severely reducing the system stability margin. The inverter output impedance using proportional feedforward and full feedforward can be expressed as follows:

[0062]

[0063] In the formula G d (s) represents the error that exists after the control delay following repeated compensation.

[0064] Based on the above formulas (1) to (16), the implementation logic of this application can be understood as follows: the first subtractor receives the reference current and the feedback grid-connected current, calculates the current deviation, and inputs the current deviation signal to the current loop proportional resonant controller, the differential controller switching switch, and the lead compensation controller switching switch, respectively; the differential controller switching switch and the lead compensation controller switching switch determine whether to activate the corresponding controller according to the system operating state; the outputs of the current loop proportional resonant controller, the differential controller, and the lead compensation controller are superimposed by the first adder to generate the first control signal; the active damping module receives the grid-connected current and outputs a damping signal. The damping signal is subtracted from the first control signal in the second subtractor, and the grid voltage feedforward module receives the common coupling point voltage to generate a feedforward signal. The feedforward signal is superimposed with the output of the second subtractor in the second adder to form the final modulation signal; the modulation signal is converted into a drive signal by the pulse width modulation module to control the inverter power device to output the grid-connected current. This multi-level control structure can flexibly cope with different grid conditions and improve system stability and current quality.

[0065] As a preferred embodiment, the solution of this application is specifically implemented as follows: The inverter output control system based on the reconfigurable controller consists of multiple functional modules; the first subtractor is implemented using an operational amplifier differential circuit, with its positive input terminal connected to a reference current signal source and its negative input terminal connected to the grid current signal collected by a current sensor; the current loop proportional resonant controller is composed of a parallel proportional element and a resonant element; the differential controller and the lead compensation controller are implemented using an operational amplifier differential circuit and an RC network, respectively; both controllers are equipped with MOSFET switches as switching switches; the active damping module is implemented using a second-order high-pass filter circuit; the second subtractor and the two adders are all implemented using operational amplifier summing circuits; the grid voltage feedforward module is composed of a voltage sensor and a programmable gain amplifier; the pulse width modulation module uses a triangular wave comparator to generate PWM signals; all analog signal processing circuits are built on the same PCB using discrete components such as operational amplifiers, resistors, and capacitors; the digital control signal is generated by a DSP and drives each switch after optocoupler isolation.

[0066] Through the above-described scheme, this application effectively improves the stability margin of the inverter under weak grid conditions while suppressing current harmonics. The reconfigured controller structure decouples impedance reshaping from harmonic suppression, avoiding positive feedback issues from the grid impedance. The multi-stage control signal superposition path enhances the system's adaptability to changes in grid parameters. The feedforward mode dynamic switching mechanism maintains good harmonic suppression performance under various grid conditions. The overall scheme is easy to implement in engineering, requiring no additional grid parameter measurements, thus reducing system complexity and cost.

[0067] In some of the solutions described above in this application, the switching criteria for the combined mode need to be clear and quantifiable in order to avoid system stability fluctuations caused by ambiguity in the controller switching conditions.

[0068] In some embodiments, the combination mode of the current loop proportional resonant controller, the differential controller, and the lead compensation controller is switched according to the system short-circuit ratio, which is the ratio of the square of the common coupling point voltage to the product of the inverter output power and the grid impedance.

[0069] The system short-circuit ratio is calculated by real-time measurement of the common coupling point voltage, inverter output power, and grid impedance, serving as a quantitative indicator of grid strength. The critical value for switching between combined modes is set to 2. When the grid impedance increases or the output power rises, causing a decrease in the system short-circuit ratio, the controller automatically reconfigures the combined mode. Grid impedance can be obtained through online identification or offline calibration, and the inverter output power is calculated from the DC-side voltage and grid-connected current.

[0070] Specifically, the system monitors the square of the voltage at the point of common coupling in real time and simultaneously obtains the product of the inverter output power and the grid impedance. After obtaining the system short-circuit ratio through division, it is logically compared with a preset threshold. If the short-circuit ratio is greater than 2, only the proportional resonant controller and the differential controller combination is retained, and the system operates in a strong grid state. If the short-circuit ratio drops to 2 or below, the lead compensation controller is automatically added, forming a three-controller collaborative working mode. This switching mechanism is implemented through a digital signal processing unit, and the grid impedance parameters are updated online through an impedance scanning device or recursive least squares method. The proportional resonant controller is responsible for fundamental frequency tracking, the differential controller suppresses high-frequency disturbances, and the lead compensation controller compensates for phase lag under weak grid conditions. The dynamic adjustment of the combination of the three maintains the stability margin of the system under different grid strengths.

[0071] In some of the schemes described above in this application, the combined mode is switched based on the system short-circuit ratio. However, when the short-circuit ratio is lower than a certain threshold, relying solely on the current loop proportional resonant controller and the differential controller cannot effectively address the phase lag problem caused by grid impedance under weak grid conditions, resulting in insufficient system stability margin.

[0072] In some embodiments, when the system short-circuit ratio is greater than 2, the current loop proportional resonant controller and the derivative controller are activated; when the system short-circuit ratio is less than or equal to 2, the current loop proportional resonant controller, the derivative controller, and the lead compensation controller are activated.

[0073] The system short-circuit ratio is calculated as the ratio of the square of the common coupling point voltage to the product of the inverter output power and the grid impedance. This ratio reflects the relative relationship between grid strength and inverter output capability. A current loop proportional resonant controller eliminates fundamental current tracking error, a differential controller suppresses resonance peaks by introducing high-frequency gain, and a lead compensation controller provides phase compensation to offset phase lag caused by grid impedance. The short-circuit ratio threshold is set to 2, corresponding to the critical state of grid strength transitioning from a strong to a weak grid.

[0074] Specifically, when the real-time calculated short-circuit ratio of the system is greater than 2, it indicates that the grid impedance is low and the system is in a strong grid state. At this time, only the current loop proportional resonant controller and the differential controller are activated. The proportional resonant stage achieves zero steady-state error current tracking, while the differential stage suppresses high-frequency resonance. When the system short-circuit ratio drops to 2 or lower, the grid impedance increases, and the system enters a weak grid state. At this time, an additional lead compensation controller is activated. Its phase lead characteristic compensates for the phase lag caused by the grid impedance, preventing a decrease in system stability margin. The output of the current loop proportional resonant controller is superimposed with the outputs of the differential controller and the lead compensation controller in the first adder to form a comprehensive control signal. This signal is then processed by the active damping module and the grid voltage feedforward module to generate a modulation signal. This switching logic uses a threshold comparator to determine the short-circuit ratio status in real time and controls the switching switch to achieve dynamic adjustment of the controller combination.

[0075] As a preferred embodiment, the solution of this application is implemented as follows: When the system short-circuit ratio is greater than 2, the current loop proportional resonant controller and the differential controller are activated. When the system short-circuit ratio is less than or equal to 2, the current loop proportional resonant controller, the differential controller, and the lead compensation controller are activated. Specifically, when the system short-circuit ratio is greater than 2, the grid impedance is small, the system stability is good, and only the differential controller needs to be activated to provide sufficient damping. When the system short-circuit ratio is less than or equal to 2, the grid impedance is large, the system stability is poor, and it is necessary to activate the differential controller and the lead compensation controller simultaneously to provide a stronger damping effect and ensure system stability; wherein, the system short-circuit ratio can be calculated by measuring the common coupling point voltage, the inverter output power, and the grid impedance.

[0076] Through the above technical solution, this application realizes adaptive controller reconfiguration based on the system short-circuit ratio. Therefore, under strong grid conditions, unnecessary control loops can be avoided, reducing system complexity; under weak grid conditions, system stability can be enhanced by activating additional control loops. Furthermore, this adaptive reconfiguration method can adjust the control strategy in real time according to changes in grid conditions, improving the system's robustness and adaptability.

[0077] In some of the solutions described above in this application, the active damping module has the problem that the damping effect is limited by the frequency characteristics when suppressing the resonance of the grid current. Conventional damping methods may not be able to accurately match the phase characteristics of the resonant frequency band, resulting in high-frequency noise amplification or insufficient low-frequency suppression.

[0078] In some embodiments, the active damping module employs a negative high-pass filter or a negative band-pass filter, equivalent to a second-order differential element at the resonant frequency. The negative high-pass filter filters out low-frequency components by setting a cutoff frequency and forms a second-order differential characteristic at the resonant frequency, thereby suppressing current oscillations in the high-frequency range. The negative band-pass filter enhances the second-order differential effect within the target resonant frequency band by limiting the passband range, while avoiding signal interference from other frequency bands. Both filters cancel out the positive feedback effect near the resonant point in the frequency domain through the second-order differential term in the transfer function. In specific implementations, the cutoff frequency of the negative high-pass filter is set to 0.5-1.2 times the resonant frequency, the center frequency of the negative band-pass filter is consistent with the system resonant frequency, and the bandwidth is set to ±5% of the resonant frequency.

[0079] Through the above technical solution, this application achieves effective suppression of harmonics near the resonant frequency in the inverter output current. This improves the stability and output current quality of the inverter under weak grid conditions. Furthermore, by employing a negative high-pass filter or a negative band-pass filter as an active damping module, the power loss problem in traditional damping methods is avoided, thus improving the system's energy efficiency.

[0080] In some of the solutions described above in this application, the feedforward mode of the grid voltage feedforward module remains fixed, making it difficult to adapt to the dynamic changes in the background harmonic content of the grid. When the background harmonic content is low, using the full feedforward mode may introduce unnecessary phase delays and increase system complexity; while in high harmonic environments, if only proportional feedforward is used, it cannot effectively compensate for the interference of grid voltage harmonics on the modulation signal, resulting in a decrease in grid-connected current quality.

[0081] In some embodiments, the grid voltage feedforward module supports either proportional feedforward or full feedforward modes, which are dynamically switched based on the grid background harmonic content. The proportional feedforward mode directly amplifies the common coupling point voltage linearly by setting a proportional gain coefficient, while the full feedforward mode achieves complete tracking of the grid voltage amplitude and phase through an all-pass filter. Switching between the two modes is triggered by real-time monitoring of the total harmonic distortion rate of the grid background, with the switching threshold preset based on the inverter's rated capacity and grid standards. The proportional feedforward mode reduces computational complexity in low-harmonic scenarios, while the full feedforward mode eliminates modulation signal deviations through phase compensation in high-harmonic scenarios.

[0082] Specifically, the grid voltage feedforward module incorporates a harmonic analysis unit to continuously acquire the voltage signal at the common coupling point and calculate its total harmonic distortion (THD). When this distortion rate is below a preset threshold, the feedforward channel only activates the proportional gain stage, superimposing the grid voltage onto the modulation signal generation node with a fixed coefficient. When the detected harmonic distortion rate exceeds the threshold, the feedforward channel automatically switches to a full feedforward path including an all-pass filter, reconstructing the grid voltage waveform through a complex domain transfer function. This switching process uses a hysteresis comparator to avoid frequent switching, ensuring the continuity of the feedforward signal. The proportional feedforward mode maintains system stability by simplifying the control structure, while the full feedforward mode improves the grid-connected current quality through precise harmonic cancellation. Together, they achieve adaptive optimization of the system under different grid environments.

[0083] As a preferred embodiment, the solution of this application is implemented as follows: The grid voltage feedforward module supports proportional feedforward or full feedforward modes, and its mode is dynamically switched according to the background harmonic content of the grid. Specifically, the grid voltage feedforward module includes a harmonic analysis unit, a mode switching unit, and a feedforward signal generation unit. The harmonic analysis unit performs real-time harmonic analysis on the voltage at the point of common coupling and calculates the background harmonic content of the grid. The mode switching unit compares the harmonic analysis results with a preset threshold to determine whether to adopt proportional feedforward or full feedforward mode. When the harmonic content is lower than the threshold, the mode switching unit selects the proportional feedforward mode; when the harmonic content is higher than the threshold, it switches to the full feedforward mode. The feedforward signal generation unit processes the voltage at the point of common coupling according to the selected mode to generate a feedforward signal. In the proportional feedforward mode, only the fundamental component is fed forward; in the full feedforward mode, the entire frequency band signal is fed forward. Thus, the grid voltage feedforward module can adaptively adjust the feedforward strategy according to grid conditions and effectively suppress harmonic interference in different harmonic environments.

[0084] Through the above technical solution, this application achieves adaptive adjustment of the grid voltage feedforward module, effectively coping with different harmonic environments. When the grid harmonic content is low, a proportional feedforward mode is adopted to avoid introducing additional harmonic interference; when the harmonic content is high, it switches to a full feedforward mode to enhance harmonic suppression capability. This dynamic switching mechanism improves the system's adaptability to grid harmonic variations, ensuring grid-connected current quality while reducing system instability risks.

[0085] In some of the solutions described above in this application, if the grid voltage feedforward module adopts a fixed feedforward mode, it is difficult to adapt to the dynamic changes in the background harmonic content of the grid. When the harmonic content is low, the full feedforward mode may introduce unnecessary positive feedback of the grid impedance; while when the harmonic content is high, simple proportional feedforward cannot effectively suppress the impact of harmonic current on the inverter.

[0086] In some embodiments, the grid voltage feedforward module supports proportional feedforward or full feedforward modes, which are dynamically switched according to the background harmonic content of the grid.

[0087] The proportional feedforward mode linearly amplifies the fundamental component of the common coupling point voltage, while the full feedforward mode directly tracks the common coupling point voltage without error. The mode switching threshold is set to the range of 5%–7% total harmonic distortion (THD), and mode switching is determined by real-time monitoring of the grid voltage's harmonic spectrum. The two feedforward signal generation channels are connected in parallel via a high-speed electronic switch, maintaining signal continuity during the switching process.

[0088] Specifically, when the grid harmonic distortion rate is below 5%, the feedforward module only activates the proportional feedforward channel. In this mode, the feedforward signal is 1.2 times the fundamental voltage amplitude, preventing amplification of high-frequency harmonic components. When the 3rd or 5th harmonic component is detected to exceed a preset threshold, the module immediately switches to full feedforward mode. At this time, the feedforward coefficient is adjusted to 0.8-1.0, and phase compensation is performed on all frequency components. The feedforward signal and the current control loop output are superimposed in an adder to ultimately form the inverter modulation wave. This dynamic switching mechanism suppresses low-frequency harmonic currents and avoids deterioration of high-frequency impedance characteristics, ensuring the system maintains a stability margin of no less than 6dB even with grid impedance fluctuations.

[0089] As a preferred embodiment, the solution of this application is implemented as follows: The grid voltage feedforward module is used to adopt a proportional feedforward mode when the grid background harmonic content is lower than a preset value, and to switch to a full feedforward mode when the grid background harmonic content is higher than the preset value. Specifically, the grid voltage feedforward module includes a harmonic analysis unit, a comparison unit, and a switching unit. The harmonic analysis unit monitors the grid voltage waveform in real time and calculates the total harmonic distortion (THD). The comparison unit compares the calculated THD with a preset threshold. The switching unit controls the switching of the feedforward mode according to the comparison result. For example, when the THD is lower than 3%, the switching unit selects the proportional feedforward mode; when the THD is higher than or equal to 3%, the switching unit selects the full feedforward mode. Further, in the proportional feedforward mode, only the fundamental component is feedforward compensated; in the full feedforward mode, both the fundamental and harmonic components are feedforward compensated.

[0090] Through the above technical solution, this application achieves adaptive switching of the grid voltage feedforward module under different grid background harmonic content conditions. Therefore, when the grid harmonic content is low, using the proportional feedforward mode can reduce computation and improve system response speed; when the grid harmonic content is high, switching to the full feedforward mode can effectively suppress harmonic interference and improve grid-connected current quality. This dynamic switching strategy ensures stable system operation under different grid conditions and optimizes harmonic suppression.

[0091] In some of the solutions mentioned above in this application, the existing control methods are difficult to balance the requirements of improving stability margin and suppressing harmonics under weak power grids. Especially in scenarios where the grid impedance cannot be ignored and the background harmonic content changes, there are problems such as the introduction of positive feedback of grid impedance by feedforward signal, unclear controller switching conditions, and insufficient dynamic response.

[0092] refer to Figure 4 , Figure 4 This is a flowchart of an inverter output control method based on a reconfigurable controller, provided by an embodiment of the present invention. In some embodiments, an inverter output control method based on a reconfigurable controller includes the following steps: measuring the grid-connected current and subtracting it from a reference current using a first subtractor to generate a current deviation signal; inputting the current deviation signal to a current loop proportional resonant controller, and selecting to switch a differential controller and a lead compensation controller according to the system short-circuit ratio to generate a first control signal; inputting the grid-connected current to an active damping module to generate a damping signal, which is then superimposed with the first control signal and input to a second subtractor; acquiring the common coupling point voltage and generating a feedforward signal through a feedforward module, which is then superimposed with the output signal of the second subtractor to generate a modulation signal; converting the modulation signal into a drive signal via a pulse width modulation module to control the inverter output current.

[0093] The system short-circuit ratio is calculated as the ratio of the square of the voltage at the common coupling point to the product of the inverter output power and the grid impedance. When the system short-circuit ratio is greater than 2, only the differential controller is activated; when it is less than or equal to 2, both the differential controller and the lead compensator are activated simultaneously. The active damping module is equivalent to a second-order differential element at the resonant frequency through a negative high-pass filter or a negative band-pass filter. The feedforward module dynamically switches between proportional feedforward and full feedforward modes based on the background harmonic content of the grid. The combination mode of the current loop proportional resonant controller, the differential controller, and the lead compensator is implemented through a switching switch to achieve logic control, and after being superimposed with the damping signal output from the active damping module, a closed-loop control circuit is formed.

[0094] Specifically, by measuring the deviation signal between the grid-connected current and the reference current in real time, the current loop proportional resonant controller completes fundamental frequency tracking, the differential controller dynamically compensates for the phase margin based on the short-circuit ratio threshold, and the lead compensation controller further optimizes the dynamic response under low short-circuit ratio conditions. The superposition of the damping signal and the first control signal eliminates the resonant component caused by grid impedance through the second subtractor. The feedforward signal selects proportional or full feedforward mode according to the harmonic content to suppress the influence of common coupling point voltage disturbances on the modulation signal. The modulation signal is converted into a drive signal by the pulse width modulation module to control the switching state of the inverter power devices, ultimately achieving a stable output of the grid-connected current. Through controller combination switching triggered by the short-circuit ratio threshold, the system reduces control complexity under high grid strength and enhances dynamic adjustment capability under weak grid conditions. At the same time, combined with the adaptive adjustment of the feedforward mode, it suppresses the influence of grid background harmonics on the output current.

[0095] In some embodiments, only the differential controller is activated when the system short-circuit ratio is greater than 2, and both the differential controller and the lead compensation controller are activated simultaneously when the system short-circuit ratio is less than or equal to 2. Specifically, when the grid impedance decreases, causing the short-circuit ratio to drop to 2, the positive feedback effect of the grid impedance is enhanced. At this time, the lead compensation controller is superimposed on the phase compensation of the differential controller to form a composite phase compensation network. For example, when the short-circuit ratio decreases from 3 to 1.5, the total phase margin of the control loop is reduced from 45° to 25°. The lead compensation controller supplements the 20° phase compensation, allowing the system to restore a 45° stability margin. The synergistic effect of the two controllers is achieved through a hardware switching switch to achieve disturbance-free switching. The output signal of the differential controller is always connected to the control loop, while the lead compensation controller is only connected in parallel when the short-circuit ratio threshold is triggered. This dynamically enhances the system's anti-disturbance capability while maintaining the basic phase compensation.

[0096] In some of the solutions described above in this application, the accurate calculation of the system short-circuit ratio is the key basis for realizing controller mode switching. However, in practical applications, due to the dynamic changes in grid impedance and inverter output power, it is difficult to obtain the short-circuit ratio in real time, which leads to inaccurate judgment of controller switching conditions and affects system stability.

[0097] In some embodiments, the inverter output power and grid impedance are measured, and the system short-circuit ratio is obtained by dividing the square of the common coupling point voltage by the product of the inverter output power and the grid impedance. Specifically, the inverter output power is measured by acquiring the inverter's DC input parameters in real time using DC-side voltage and current sensors, and calculating the instantaneous power value in conjunction with the modulation signal. The grid impedance is measured using a non-intrusive estimation method based on current disturbance injection or harmonic analysis, establishing an equivalent impedance model by detecting the harmonic response characteristics of the common coupling point voltage and the inverter output current. When calculating the system short-circuit ratio, the square of the effective value of the common coupling point voltage is calculated as the ratio of the product of the inverter output power and the grid impedance.

[0098] In some of the solutions described above in this application, the fixed feedforward mode of the feedforward module is difficult to adapt to the dynamic changes in the background harmonic content of the power grid. If the full feedforward mode is directly adopted in the low harmonic scenario, it may introduce additional high-frequency noise. However, the proportional feedforward mode under the high harmonic condition cannot effectively suppress harmonic interference, resulting in an imbalance between system stability and current quality.

[0099] In some embodiments, the proportional feedforward or full feedforward mode of the feedforward module is dynamically adjusted according to the background harmonic content of the power grid.

[0100] The feedforward mode switching is achieved by real-time monitoring of the total harmonic distortion (THD) of the grid voltage. When the harmonic content is below a preset threshold, a proportional feedforward mode is used, compensating only for the fundamental component. When the harmonic content exceeds the threshold, a full feedforward mode is switched to fully feedforward the fundamental and high-frequency harmonic components in the grid voltage. The preset threshold is set to 5% THD based on grid operation standards. A hysteresis comparator is used during mode switching to avoid frequent switching. In the proportional feedforward mode, the feedforward coefficient is set to an adjustable parameter between 0.8 and 1.2. In the full feedforward mode, a second-order generalized integrator is used to extract harmonic components.

[0101] Specifically, during the grid voltage harmonic content detection phase, the total harmonic distortion rate (THD) is calculated in real time using Fast Fourier Transform (FFT). When the detected value remains below 5%, the feedforward module only multiplies the fundamental component of the common coupling point voltage by a proportional coefficient before adding it to the control loop, avoiding amplification of high-frequency noise. When the detected value exceeds 5%, the feedforward module switches to full feedforward mode, using a bandpass filter bank to separate specific harmonic components, which are then combined with the fundamental component to form the feedforward signal. This dynamic adjustment process is implemented through the comparator module of the digital signal processor, with the switching delay controlled within one grid cycle. The proportional feedforward mode reduces the risk of impedance positive feedback by lowering the high-frequency gain, while the full feedforward mode improves harmonic suppression capability by accurately reconstructing the grid voltage harmonic components. The coordinated application of these two modes enables the system to maintain stability margin and improve current waveform quality under weak grid conditions.

[0102] refer to Figures 5 to 9 , Figure 5 To use a conventional method for system Bode plotting; Figure 6 Bode plot of the system when using a PR+DC controller in an embodiment of the present invention; Figure 7 Bode diagram of PR+DC+LC controller system is used in the embodiments of the present invention; Figure 8 This serves to verify the effectiveness of the strategy proposed in this invention under weak power grid conditions. Figure 9 This is to verify the effectiveness of the strategy proposed in this invention under extremely weak power grid conditions; wherein, G is used as the reference. d Taking (s) as an example where the control delay is 1%, Bode plots of equations (15) and (16) are drawn as follows: Figure 5 As shown. When using full feedforward, even if z g Even at very small values, the system remains unstable; reducing the full feedforward to a proportional feedforward at z... g The system can remain stable when the z value is small, but as z increases... g Even with increased capacity, the system will still become unstable.

[0103] Understandably, to ensure system stability, two approaches can be taken: firstly, to allow z... g With z out z at the intersection of amplitude and frequency curves outThe phase gain is between -90° and 90°, while increasing z... out Amplitude gain, let z g With z out There is no intersection of the amplitude-frequency curves. It can be seen from equations (15) and (16) that at k... AD2 and G N (s) Once the parameters are designed, they are relatively difficult to change, so it is possible to start from G. i (s) is the starting point. Considering G i (s) in z out On the molecule, and the differential element increases the amplitude gain with increasing frequency, therefore an attempt was made to... i Add a differential controller to (s).

[0104] Furthermore, Figure 6 (a) When using a PR+DC controller, what is the effect on u? g Full feedforward back z g With z out The relationship curve shows that adding a differential controller can effectively improve z. out The amplitude gain is high in the low-frequency range, but then it is related to the presence of z in the high-frequency range. g The unstable intersection point proves that adding a differential controller when using full feedforward cannot stabilize the system. Figure 6 (b) When using a PR+DC controller, the value of u is given. g Proportional feedforward and backward z g With z out The relationship curve shows that adding a differential controller increases the system's stable region and increases the inverter's output impedance in the low-frequency range, which is beneficial for suppressing grid background harmonics; however, as z... g Increasing the SCR, especially when it decreases to 3, will still cause the system to become unstable; although increasing the differential controller coefficient can further increase the system stability margin, it will reduce the phase margin of the system in the mid-frequency range, and at high frequencies [f a f b Within this range, the addition of a differential controller reduces the output impedance amplitude of the inverter, and there is even a risk that it may become negative, which significantly suppresses the system's ability to suppress harmonics in this frequency band.

[0105] Furthermore, from Figure 6 (b) It can be seen that u g With proportional feedforward and the addition of a differential controller, the system at z g Instability occurs when the value is large, mainly below f. i within the frequency band z out This is caused by an excessively low angle. Increasing the z-axis within this frequency band would help. outPhase angle helps improve system stability margin. Considering that LC can effectively increase system phase margin, a differential controller is connected in series with the LC, and the bandwidth center frequency of the LC is set to f. i Meanwhile, if the LC leads the bandwidth center by 45°, the designed LC can be expressed as:

[0106]

[0107] Furthermore, Figure 7 Give the coefficients K of the series LC branch of the differential controller. d Different times z g With z out The relationship curve shows that K... d Too small a value will cause the system to become unstable in the low-frequency range; as K... d Increasing the z value can stabilize the system, but on the one hand... out In the low-frequency range (from the fundamental frequency to around 400Hz), the amplitude gain decreases, thus reducing the ability to suppress low-order harmonics. On the other hand, in the high-frequency range... s Near / 2, the system phase margin is less than 30°; with K d Continue to increase, z out It has high amplitude gain across the entire frequency band, but in the z-band... g When the value is large, the system at f s The phase gain at point / 2 is less than 30°. Continue increasing K... d , z g With z out The amplitude-frequency curves have no intersection points, and the system can remain stable.

[0108] comprehensive Figures 5-7 It can be seen that for a system using GCF-AD, when u g When only background harmonics are present, PR+u can be used. g Full feedforward strategy; when u g When there are background harmonics and the power grid is not an extremely weak grid, PR+DC+u can be used. g Proportional feedforward strategy; when u g When there are background harmonics and the power grid is an extremely weak grid, PR+DC+LC+u can be used. g Proportional feedforward strategy.

[0109] Figure 8 (a) The experimental waveforms are given when only the PR controller is used under a weak power grid. It can be seen that the system has obvious oscillations. Figure 8 (b) The experimental waveforms when using the PR+DC controller are shown, and it can be seen that the oscillations are effectively suppressed.

[0110] Furthermore, in Figure 8 Based on this, a sudden increase in grid impedance, such as Figure 9 As shown in (a), it can be seen that using PR+DC is no longer sufficient to guarantee system stability, and system oscillation and instability protection is needed. Therefore, at the instant of a sudden increase in grid impedance, a PR+DC+LC controller is used, such as... Figure 9 As shown in (b), it can be seen that the current fluctuates only at the moment of sudden change in grid impedance, but the system does not become unstable. This proves that the present invention can effectively improve the stability margin of grid-connected inverters under weak and extremely weak grid conditions.

[0111] The above are merely embodiments of this application and are not intended to limit the scope of protection of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. An inverter output control system based on a reconfigurable controller, characterized in that, include: The system comprises a first subtractor, a current loop proportional resonant controller, a differential controller and its switching switch, a lead compensator controller and its switching switch, an active damping module, a second subtractor, a grid voltage feedforward module, a first adder, a second adder, and a pulse width modulation module. The input terminal of the first subtractor is connected to the reference current and the feedback grid current, and its output terminal is connected to the input terminals of the current loop proportional resonant controller, the differential controller switching switch and the lead compensation controller switching switch, respectively. The output terminals of the differential controller switching switch and the lead compensator switching switch are respectively connected to the input terminals of the differential controller and the lead compensator. The output terminals of the current loop proportional resonant controller, the differential controller, and the lead compensator controller are connected to the input terminal of the first adder to generate a first control signal; The input terminal of the active damping module is connected to the grid current, and its output terminal is connected to the input terminal of the second subtractor together with the output terminal of the first adder. The input terminal of the grid voltage feedforward module is connected to the common coupling point voltage, and its output terminal and the output terminal of the second subtractor are connected together to the input terminal of the second adder to generate a modulation signal; The modulation signal is converted into a drive signal by the pulse width modulation module to control the inverter power device to output grid-connected current. The combination mode of the current loop proportional resonant controller, the differential controller, and the lead compensation controller is switched according to the system short-circuit ratio, which is the ratio of the square of the common coupling point voltage to the product of the inverter output power and the grid impedance. When the system short-circuit ratio is greater than 2, the current loop proportional resonant controller and the differential controller are activated; when the system short-circuit ratio is less than or equal to 2, the current loop proportional resonant controller, the differential controller, and the lead compensation controller are activated.

2. The inverter output control system based on a reconfigurable controller according to claim 1, characterized in that, The active damping module employs a negative high-pass filter or a negative band-pass filter, which is equivalent to a second-order differential element at the resonant frequency.

3. The inverter output control system based on a reconfigurable controller according to claim 1, characterized in that, The grid voltage feedforward module supports proportional feedforward or full feedforward modes, and the mode is dynamically switched according to the background harmonic content of the grid.

4. The inverter output control system based on a reconfigurable controller according to claim 3, characterized in that, The grid voltage feedforward module is used to adopt a proportional feedforward mode when the grid background harmonic content is lower than a preset value, and to switch to a full feedforward mode when the grid background harmonic content is higher than a preset value.

5. An inverter output control method based on a reconfigurable controller, characterized in that, Includes the following steps: The grid-connected current is measured and subtracted from the reference current using the first subtractor to generate a current deviation signal; The current deviation signal is input to the current loop proportional resonant controller, and the switching differential controller and lead compensation controller are selected according to the system short-circuit ratio to generate the first control signal; the grid current is input to the active damping module to generate the damping signal, which is then superimposed with the first control signal and input to the second subtractor; The voltage at the common coupling point is acquired and a feedforward signal is generated through the feedforward module. This signal is then superimposed with the output signal of the second subtractor to generate a modulated signal. The modulation signal is converted into a drive signal by the pulse width modulation module to control the inverter output current; The switching conditions for the differential controller and the lead compensator are as follows: When the short-circuit ratio of the system is greater than 2, only the differential controller is activated; When the short-circuit ratio of the system is less than or equal to 2, the differential controller and the lead compensation controller are activated simultaneously. The method further includes: Measure the inverter output power and grid impedance; The system short-circuit ratio is obtained by dividing the square of the common coupling point voltage by the product of the inverter output power and the grid impedance.

6. The inverter output control method based on a reconfigurable controller according to claim 5, characterized in that, The proportional or full feedforward mode of the feedforward module is dynamically adjusted according to the background harmonic content of the power grid.

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