A weak grid under the group string type photovoltaic inverter stability enhancement control method

Abstract

The application provides a weak power grid under a group string type photovoltaic inverter stability enhancement control method, comprising: collecting electrical operation data of an inverter AC side grid-connected port in real time; based on the electrical operation data, calculating an optimal closed-loop transfer function parameter adapted to a weak power grid environment in real time; according to the optimal closed-loop transfer function parameter, dynamically determining a target value of an equivalent output impedance in front of the grid-connected port; by adjusting a first impedance control parameter, real-time regulating and controlling the equivalent output impedance in front of the grid-connected port, so that it converges to the target value, so as to realize dynamic matching of the inverter output impedance and the weak power grid impedance. The method realizes accurate matching of the inverter output impedance and the weak power grid impedance by real-time dynamic regulation and control of the inverter output impedance, effectively suppresses the resonance problem caused by impedance mismatch under the weak power grid, and improves the stability of the grid-connected system.

Description

Technical Field

[0001] This application relates to the field of photovoltaic grid-connected power generation technology, specifically to a method for enhancing the stability control of string photovoltaic inverters under weak power grid conditions. Background Technology

[0002] Against the backdrop of a global energy transition towards cleaner and lower-carbon energy, the installed capacity of photovoltaic (PV) power generation continues to climb, becoming a core support for the transformation of the global energy structure. String PV inverters, as the core equipment for grid connection of PV systems, are primarily responsible for converting direct current (DC) to alternating current (AC). Their operational stability not only directly determines the energy utilization efficiency of the PV system but also profoundly affects the safe and stable operation of the power grid.

[0003] However, power grids in rural and remote mountainous areas with concentrated photovoltaic projects generally exhibit characteristics of weak grids, such as low short-circuit capacity, high equivalent impedance, and weak anti-interference capability. When the power injection of the photovoltaic system undergoes dynamic changes, such grids are prone to problems such as voltage amplitude fluctuations, current waveform distortion, and unstable power transmission, posing a severe challenge to the grid-connected operation of string photovoltaic inverters. From the perspective of impedance characteristics, string photovoltaic inverters in weak grid environments can be equivalent to power sources with specific output impedances. Once this output impedance does not match the equivalent impedance characteristics of the grid, it is very easy to trigger system resonance, leading to instability in the inverter's grid-connected voltage and current. In severe cases, it may even trigger inverter protection shutdown, directly threatening the operational safety of the regional power grid.

[0004] Resonance phenomena can cause significant distortion and fluctuations in grid-connected voltage and current, which not only reduces the power generation efficiency of photovoltaic systems but may also damage the internal IGBT switching devices of the inverter and exacerbate grid voltage fluctuations, posing a dual threat to the safe and stable operation of both the photovoltaic system and the grid. Current technical solutions for inverter stability control under weak grid conditions only achieve basic impedance matching by adjusting the PI parameters of the inverter's current and voltage loops, failing to fully consider the dynamic impedance changes of weak grids. Weak grids are prone to impedance abrupt changes due to load switching, fluctuations in renewable energy output, etc., which can instantly disrupt the original impedance matching relationship, still triggering resonance and failing to fundamentally guarantee the stable operation of the inverter. Summary of the Invention

[0005] This application aims to solve the technical problem that resonance can easily occur under dynamic changes in the impedance of a weak power grid, causing the inverter and the grid-connected system to fail to operate.

[0006] To address the aforementioned issues, this application provides a method for enhancing the stability control of string photovoltaic inverters under weak power grid conditions, comprising: Real-time acquisition of electrical operation data of the inverter's AC side grid-connected port. The electrical operation data includes at least the response voltage, response current, and reference current signals at the front end of the grid-connected port, as well as the response voltage, response current, and reference current signals at the back end of the grid-connected port. Based on electrical operation data, the optimized closed-loop transfer function parameters adapted to the weak power grid environment are calculated in real time. Based on the optimized closed-loop transfer function parameters, the target value of the equivalent output impedance at the front end of the grid-connected port is dynamically determined; By adjusting the first impedance control parameter, the equivalent output impedance at the front end of the grid-connected port is controlled in real time to converge to the target value, so as to achieve dynamic matching between the inverter output impedance and the weak grid impedance.

[0007] Specifically, the first impedance control parameter is a core adjustment parameter used to achieve basic matching between the inverter impedance at the front end of the grid connection port and the weak grid impedance at the back end of the grid connection port. Specifically, it includes at least one of the following: current loop proportional coefficient, current loop integral coefficient, voltage loop proportional coefficient, and voltage loop integral coefficient.

[0008] Furthermore, the optimized closed-loop transfer function parameters adapted to the weak grid environment are calculated in real time, including: calculating the optimized closed-loop transfer function parameters adapted to the weak grid environment in real time according to the following formula: in, To optimize the closed-loop transfer function, For the response voltage at the front end of the grid connection port, For the response current at the front end of the grid connection port, This is the reference current signal at the front end of the grid connection port. For the response voltage of the grid-connected port backend, For the response current of the grid connection port backend, This is the reference current signal at the back end of the grid connection port.

[0009] Furthermore, after achieving dynamic matching between the inverter output impedance and the weak grid impedance, the process also includes: calculating the critical resonance electrical parameters when the grid-connected system resonates based on the equivalent model of the string photovoltaic inverter and the weak grid-connected system; and adjusting the second impedance control parameter of the inverter to make the real-time electrical parameters of the grid-connected system less than the critical resonance electrical parameters.

[0010] In this application, the equivalent model of the string photovoltaic inverter and the weak grid grid connection system is constructed as follows: the string photovoltaic inverter is equivalent to a composite structure of an ideal current source, a controlled current source and an equivalent admittance connected in parallel, while the weak grid is equivalent to a structure of an equivalent inductor, an equivalent resistance and an equivalent voltage source connected in series, and the two are electrically connected through the grid connection port.

[0011] The critical resonance electrical parameters include at least: total equivalent current source, total equivalent current, and total equivalent susceptance. The total equivalent current source is the superposition of the equivalent current source at the front end of the grid-connected port and the equivalent current source at the back end of the grid-connected port; the total equivalent current is the equivalent short-circuit current at the grid-connected port; and the total equivalent susceptance is the parallel value of the equivalent susceptance at the front end of the grid-connected port and the equivalent susceptance at the back end of the grid-connected port.

[0012] The second impedance control parameter is a supplementary control parameter for the resonance suppression of the grid-connected system, and includes at least one of the following: the damping coefficient at the resonant frequency, the center frequency and attenuation gain of the notch filter, and the adaptive resonance compensation coefficient.

[0013] Furthermore, the critical resonant electrical parameters when the inverter resonates with the weak grid system are calculated, including: calculating the total equivalent current source when the inverter resonates with the weak grid system according to the following formula. Total equivalent short-circuit current Total equivalent susceptance : ; in, This represents the equivalent impedance of a single photovoltaic string. This is the equivalent current on the weak grid side. This is the equivalent voltage on the weak grid side. For the equivalent inductance of the weak grid side, The equivalent admittance of a single photovoltaic string. This refers to the number of photovoltaic strings connected to the inverter.

[0014] Furthermore, after adjusting the second impedance control parameter of the inverter to make the real-time electrical parameters of the grid-connected system less than the critical resonance electrical parameters, the process also includes: calculating the remaining resonance parameters of the grid-connected system, where the remaining resonance parameters are the difference between the equivalent electrical parameters after resonance suppression and the critical resonance electrical parameters at the time of resonance; calculating the three-phase voltage reactive power decoupling vector and the three-phase current reactive power decoupling vector based on the inverter output voltage, inverter output phase, and remaining resonance parameters; calculating the reactive power decoupling vector compensation value based on the three-phase voltage reactive power decoupling vector and the three-phase current reactive power decoupling vector; and superimposing the reactive power decoupling vector compensation value onto the three-phase voltage reactive power decoupling vector and the three-phase current reactive power decoupling vector respectively, thereby enhancing the stability of the grid-connected system.

[0015] Furthermore, the residual resonant parameters include the residual current source resonant parameters. Residual equivalent current resonant parameters and residual admittance resonant parameters The calculation of the residual resonance parameters of the grid-connected system includes: calculating the residual current source resonance parameters according to the following formulas. Residual equivalent current resonant parameters and residual admittance resonant parameters : 、 、 ; The calculation of the three-phase voltage reactive power decoupling vector and the three-phase current reactive power decoupling vector includes the following formulas: in, 、 、 This is the three-phase voltage reactive power decoupling vector. 、 、 This is the three-phase current reactive power decoupling vector. This is the inverter output voltage. For the inverter output phase, The remaining resonant coupling coefficient; Calculating the reactive power decoupling vector compensation value includes: calculating the reactive power decoupling vector compensation value according to the following formula: in, This is the reactive power decoupling vector compensation value. For compensation coefficient, The angle of the grid current; Superimposing the reactive power decoupling vector compensation values ​​onto the three-phase voltage reactive power decoupling vector and the three-phase current reactive power decoupling vector respectively includes: superimposing the reactive power decoupling vector compensation values... l By superimposing these vectors onto the three-phase voltage reactive power decoupling vector and the three-phase current reactive power decoupling vector respectively, the compensated three-phase voltage reactive power decoupling vector and the three-phase current reactive power decoupling vector are obtained, as shown in the following formula: in, This is the compensated three-phase voltage reactive power decoupling vector. This is the three-phase current reactive power decoupling vector after compensation.

[0016] The above-mentioned calculation of critical resonance parameters based on the equivalent model of the grid-connected system provides a clear quantitative threshold for resonance suppression, avoiding the blindness of existing resonance suppression methods. By adjusting the targeted second impedance control parameters and combining various physical control methods, the resonance energy can be actively weakened, effectively addressing the resonance risk that may still be caused by operating condition fluctuations after impedance matching in stage one, and significantly improving the reliability of resonance suppression. Its control logic forms an effective connection with the impedance matching in stage one, relying on the stable foundation laid in stage one, and reducing the processing load for the remaining resonance compensation in stage three, ensuring the coherence and efficiency of the entire control system, while adapting to different string numbers and grid operating conditions.

[0017] The technical advantages of this application are as follows: This application employs a three-stage closed-loop control strategy of "port impedance enhancement - resonance suppression - residual resonance reactive power compensation," combining equivalent modeling, parameter quantization, and physical control techniques to comprehensively enhance inverter stability under weak grid conditions. First, dynamic impedance interference is eliminated by synchronously acquiring data from both ends of the grid-connected port and solving simultaneous equations, generating an optimized closed-loop transfer function adapted to real-time operating conditions. This provides a precise basis for impedance parameter adjustment, achieving accurate impedance matching across the entire frequency band and reducing the risk of resonance. Then, the resonance suppression stage calculates critical resonance parameters based on the equivalent model of the grid-connected system. Combined with targeted impedance control parameter adjustments and various physical control methods, it weakens resonance energy, addressing the risk of fluctuation-type resonance under operating conditions. This is closely integrated with the impedance matching stage to improve suppression efficiency, reduce subsequent compensation load, and enhance the coherence and versatility of the control system. Finally, the residual resonance reactive power compensation stage dynamically generates reactive power decoupling vectors and uses a reverse compensation strategy to improve the long-term operational stability of the inverter under weak grid conditions, ensuring the safety of the photovoltaic system and the grid. Attached Figure Description

[0018] Figure 1 This is a flowchart illustrating a method for enhancing the stability control of string photovoltaic inverters under weak power grid conditions provided in this application. Figure 2a This is the equivalent model circuit diagram of the string photovoltaic inverter and weak grid connection system provided in this application; Figure 2b yes Figure 2a Equivalent transformation circuit diagram using Norton's equivalent transformation in the complex frequency domain; Figure 3 Figure 3(a) shows the voltage and current fluctuation waveforms obtained by using the string photovoltaic inverter stability enhancement control method provided in this application under weak power grid conditions and by using two other similar control methods. Figure 3(b) shows the voltage and current fluctuation waveforms corresponding to the photovoltaic inverter stability enhancement control method based on the improved particle swarm algorithm; Figure 3 (c) shows the voltage and current fluctuation waveforms corresponding to the stability enhancement control method for string photovoltaic inverters in weak power grid environments based on impedance analysis in this application. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] The terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and not to describe a specific order or sequence. It should be understood that such use of data can be interchanged where appropriate so that embodiments of this application can be implemented in orders other than those illustrated or described herein, and the objects distinguished by "first," "second," etc., are generally of the same class and the number of objects is not limited; for example, a first object can be one or more. Furthermore, in the specification and claims, "and / or" indicates at least one of the connected objects, and the character " / " generally indicates that the preceding and following objects are in an "or" relationship.

[0021] The heart rate noise reduction system, method, and apparatus provided in this application will be described in detail below with reference to the accompanying drawings and through specific embodiments and application scenarios.

[0022] Example 1 Figure 1 This is a flowchart illustrating a method for enhancing the stability control of a string photovoltaic inverter under weak power grid conditions, as provided in an embodiment of this application. This method can be executed by the control unit of the string photovoltaic inverter. Figure 1 As shown, the method may include the following steps.

[0023] S1: Obtain real-time electrical operation data of the inverter's AC side grid-connected port.

[0024] Specifically, the grid-connected port front end refers to the inverter side, and the grid-connected port back end refers to the grid side. Real-time electrical operation data from both the grid-connected port front end and back end can be synchronously collected via voltage Hall effect sensors and current Hall effect sensors deployed at the grid-connected port. The real-time electrical operation data includes the response voltage at the grid-connected port front end. Response current Reference current signal and the response voltage at the grid connection port. Response current Reference current signal .

[0025] The collected electrical operation data needs to be converted from analog to digital by an AD sampling module, with a sampling frequency of no less than 1kHz to ensure that the dynamic changes in the power grid impedance can be captured in detail; reference current signal , Preset by the inverter control board for use in the subsequent construction of impedance equations.

[0026] S2: Calculate the optimized closed-loop transfer function parameters adapted to the weak power grid environment in real time based on real-time electrical operation data.

[0027] In this embodiment, the optimized closed-loop transfer function parameters adapted to the weak power grid environment can be calculated in real time according to the following formula: in, To optimize the closed-loop transfer function, For the response voltage at the front end of the grid connection port, For the response current at the front end of the grid connection port, This is the reference current signal at the front end of the grid connection port. For the response voltage of the grid-connected port backend, For the response current of the grid connection port backend, This is the reference current signal at the back end of the grid connection port. s This is the inverter's operating signal.

[0028] The derivation of formula (1) is as follows: First, establish the basic expression for the inverter port impedance characteristics: Based on the correlation between the closed-loop transfer function and the port electrical signal, the inverter port impedance characteristics... The core expression is: in, Let be the initial closed-loop transfer function. This is the response voltage under the excitation of the inverter signal. This is the response current under the excitation of the inverter signal. This is the reference signal for the inverter. This expression reflects the inherent relationship between the inverter port impedance and the response voltage, response current, reference signal, and initial closed-loop transfer function in the initial state.

[0029] Introducing electrical signals at both ends of the port: Considering the impedance interaction between the front and back ends of the grid connection point, the response voltage at the front end of the port is collected separately. Response current Reference current signal and the response voltage at the port back end. Response current Reference current signal At this point, the impedance characteristics at both ends of the port can be expressed as follows: Front-end impedance characteristics: Back-end impedance characteristics: Eliminate port impedance variables Derivation of the optimized closed-loop transfer function: Since the impedance characteristics before and after the port are both determined by the same inverter port impedance... Therefore, by combining the above two equations, we can eliminate the variables. ,get After cross-multiplication and rearrangement, the optimized closed-loop transfer function is obtained. : The optimized closed-loop transfer function By comprehensively considering the electrical operation data at both ends before and after the grid connection port, it can accurately reflect the interaction characteristics between the inverter and the grid in a weak grid environment, and avoid the problem of insufficient matching caused by the initial closed-loop transfer function relying only on a single-end signal.

[0030] S3: Based on the optimized closed-loop transfer function parameters, dynamically determine the target value of the equivalent output impedance at the front end of the grid-connected port.

[0031] In this embodiment, the target value of the equivalent output impedance at the front end of the grid-connected port can be dynamically determined according to the following formula: in, To enhance the port stability impedance of the string photovoltaic inverter.

[0032] S4: By adjusting the first impedance control parameter, the equivalent output impedance at the front end of the grid-connected port is adjusted in real time to converge to the target value, so as to achieve dynamic matching between the inverter output impedance and the weak grid impedance.

[0033] Specifically, the first impedance control parameter includes at least one of the following: current loop proportional coefficient, current loop integral coefficient, voltage loop proportional coefficient, and voltage loop integral coefficient.

[0034] For example, when the first impedance control parameter is the current loop proportional coefficient: if it is necessary to increase the equivalent output impedance at the front end of the grid-connected port to adapt to the high impedance characteristics of a weak grid, the current loop proportional coefficient is increased to improve the instantaneous response sensitivity of the current loop to deviations. By enhancing the current tracking accuracy, impedance interaction fluctuations are suppressed, and the equivalent output impedance converges to the target value. If it is necessary to reduce the equivalent output impedance to avoid overcurrent risks, the current loop proportional coefficient is decreased to reduce the current loop response strength, avoid oscillations caused by over-matching, and achieve precise reduction of the equivalent output impedance.

[0035] When the first impedance control parameter is the current loop integral coefficient: When optimizing the steady-state adaptability of the equivalent output impedance, if the increase in the weak grid impedance leads to steady-state current deviation, the current loop integral coefficient should be increased appropriately to eliminate static error through cumulative compensation and indirectly stabilize the equivalent output impedance; if the grid impedance decreases, the impedance amplitude should be reduced, the current loop integral coefficient should be slightly reduced, the integral compensation intensity should be reduced, and the impedance should be converged to the target value in conjunction with dynamic response adjustment.

[0036] When the first impedance control parameter is the voltage loop proportional coefficient: when the voltage fluctuation caused by the increase of the weak grid impedance requires an increase in the equivalent output impedance, the voltage loop proportional coefficient is increased to enhance the voltage loop's ability to suppress fluctuations and indirectly optimize the impedance matching degree by stabilizing the grid voltage; when the grid impedance decreases and the equivalent output impedance needs to be reduced, the voltage loop proportional coefficient is decreased to avoid impedance imbalance caused by voltage overshoot and ensure smooth impedance convergence.

[0037] When the first impedance control parameter is the voltage loop integral coefficient: if a long-term stable equivalent output impedance is required to adapt to weak grid conditions, and if there is a steady-state voltage deviation that causes impedance mismatch, the voltage loop integral coefficient should be increased to gradually offset the deviation and maintain impedance stability; if it is necessary to reduce the equivalent output impedance, the voltage loop integral coefficient should be reduced to reduce the effect of integral lag and enable the impedance to quickly respond to the target value adjustment.

[0038] This embodiment collects the response voltage, current, and reference signals at both ends of the grid-connected port, eliminates port impedance variables, and derives an optimized closed-loop transfer function. This overcomes the limitations of traditional single-ended signal analysis, accurately captures the dynamic characteristics of weak grid impedance changes, and enhances port stability impedance based on the optimized transfer function. This effectively improves the problem of equivalent impedance rise caused by increased string connection line resistance, improves the port's tolerance to weak grid fluctuations, and achieves real-time dynamic matching between inverter output impedance and grid-side impedance without complex algorithm iterations or additional hardware. This reduces the risk of resonance caused by impedance mismatch.

[0039] Example 2 Based on Example 1, after achieving dynamic matching between the inverter output impedance and the weak grid impedance, the following is also included: S5: Based on the equivalent model of string photovoltaic inverters and weak grid-connected systems, calculate the critical resonance electrical parameters when the grid-connected system resonates.

[0040] To accurately analyze the impedance interaction mechanism and resonance triggering conditions between weak power grids and string photovoltaic inverters, this invention constructs as follows: Figure 2a The diagram shows an equivalent model of a string photovoltaic inverter connected to a weak grid. This model connects the photovoltaic inverter side to the weak grid side through a point of common coupling (PCC), forming a complete grid-connected system.

[0041] The photovoltaic inverter side is equivalent to an ideal current source. Controlled current source With equivalent admittance Parallel structure, in which ideal current source Characterized by the fundamental active current output of the photovoltaic string after power conversion by the inverter, the controlled current source The equivalent admittance characterizes the harmonic current generated by nonlinear factors such as inverter dead time and switching transistor voltage drop. This reflects the basic impedance characteristics of the inverter side. Figure 2b yes Figure 2a The equivalent transformation circuit diagram using Norton's equivalent transformation in the complex frequency domain, specifically, the ideal current source. Controlled current source With equivalent admittance The parallel combination, through the Norton equivalent transformation in the complex frequency domain, is simplified and integrated into an enhanced port stability impedance. This characterizes the overall impedance characteristics of the inverter side.

[0042] The weak grid side is equivalent to the equivalent inductance. Equivalent voltage and grid admittance The combined structure, where the equivalent voltage As a voltage reference for grid-connected synchronization, the equivalent inductance Characterizing the line inductance characteristics and grid admittance of a weak power grid This is the reciprocal of the equivalent impedance of the weak grid, reflecting the weak grid's ability to conduct current. These three factors together determine the impedance characteristics of the weak grid side, and are related to the inverter side's impedance. Together they determine the system's natural resonant frequency.

[0043] The triggering mechanism of system resonance is: when the equivalent impedance on the inverter side... Equivalent impedance (including equivalent inductance) to the weak grid side Grid admittance When the frequency characteristics of the two components are mismatched, impedance interaction occurs at the common coupling point (PCC), causing system resonance, which manifests as a sharp increase in the amplitude of the grid-connected current.

[0044] Critical resonance electrical parameters refer to the total equivalent current source when a grid-connected system is about to reach resonance. Total equivalent current Total equivalent admittance The corresponding critical threshold value is determined by the equivalent model of the grid-connected system and the impedance matching state. When the real-time electrical parameters of the grid-connected system reach or exceed this critical threshold, the inverter's equivalent admittance... With grid admittance Mismatch (current gain) When the voltage and current are both below the critical threshold, the system will exhibit significant resonance, leading to distortion of the grid voltage and current waveforms. Conversely, when the real-time electrical parameters are below this critical threshold, the system can maintain stable operation.

[0045] The calculation steps for the critical resonance electrical parameters are as follows: First, determine the current gain based on the circuit structure of the equivalent model. Stability condition: when = At time 1, the inverter's equivalent admittance With grid admittance Achieving effective matching eliminates the risk of system resonance; when = At 0, the two are mismatched, and the system enters the critical state of resonance.

[0046] Based on the resonant critical state ( =0 The critical resonance electrical parameters are calculated using the following formula: ; in, This represents the equivalent impedance of a single photovoltaic string. This is the equivalent current on the weak grid side. This is the equivalent voltage on the weak grid side. For the equivalent inductance of the weak grid side, The equivalent admittance of a single photovoltaic string. This refers to the number of photovoltaic strings connected to the inverter. s These are variables in the complex frequency domain.

[0047] S6: By adjusting the second impedance control parameter of the inverter, the real-time electrical parameters of the grid-connected system are made to be less than the critical resonance electrical parameters.

[0048] In this embodiment, the second impedance control parameter is a core adjustment parameter specifically used to suppress resonance in the grid-connected system. It is set based on the resonance suppression requirements of the equivalent model of the grid-connected system and specifically includes at least one of the following: damping resistor, filter inductor, filter capacitor, and inverter switching frequency. These parameters directly affect the equivalent impedance characteristics of the grid-connected system. By precisely adjusting them, the amplitude of the system's real-time electrical parameters can be changed, thereby achieving resonance suppression.

[0049] The core objective of adjusting the second impedance control parameter is to reduce the real-time total equivalent current source of the grid-connected system. Real-time total equivalent current Real-time total equivalent admittance Controlled below the critical resonance electrical parameters, i.e. 、 、 The specific adjustment logic is as follows: Initial parameter calibration: Based on the equivalent model of the grid-connected system and the initial impedance characteristics of the weak grid, the initial value of the second impedance control parameter is preset to ensure that the real-time electrical parameters of the system under initial operating conditions are lower than the critical resonance threshold, laying the foundation for subsequent dynamic adjustment.

[0050] When monitored in real time by sensors near At this time, appropriately increasing the damping resistor or adjusting the filter inductor parameters can enhance the circuit's damping characteristics, weaken the accumulation of resonant energy from the current source, and reduce [the impact of the current source's energy accumulation]. Amplitude; when Approaching At the same time, optimizing the filter capacitor parameters or adjusting the inverter switching frequency can change the system's frequency response characteristics, suppress current oscillations, and make... It quickly fell back to a safe range; when near At the same time, by coordinating the adjustment of the filter inductor and capacitor parameters, the equivalent admittance characteristics of the system are optimized, thus avoiding the resonance risk caused by admittance mismatch.

[0051] Suppressing the total equivalent current source after resonance Total equivalent short-circuit current Total equivalent susceptance The calculation formula is as follows: in, 、 、 To suppress resonance 、 、 ; 、 、 for 、 、 The resonant frequency.

[0052] This technical solution achieves quantitative calculation of critical resonance electrical parameters by constructing an accurate equivalent model of the grid-connected system, overcoming the limitations of traditional resonance suppression relying on empirical parameters. Simultaneously, through dynamic adjustment of the second impedance control parameter, the real-time electrical parameters of the system are strictly controlled below the resonance critical value, suppressing resonance phenomena caused by impedance mismatch in weak grid environments from the source. This solution requires no complex algorithm iteration, has a fast response speed, and synergizes with the impedance dynamic matching strategy of Example 1, further improving the operational stability of string photovoltaic inverters in weak grid environments and providing crucial guarantees for grid-connected voltage and current stability.

[0053] Example 3 This embodiment is based on Embodiment 2. After adjusting the second impedance control parameter of the inverter to make the real-time electrical parameters of the grid-connected system less than the critical resonance electrical parameters, it further includes: S7: Calculate the residual resonance parameters of the grid-connected system.

[0054] The residual resonance parameter refers to the residual resonance remaining after the grid-connected system has been adjusted using the second impedance control parameter to achieve real-time electrical parameters lower than the critical resonance electrical parameter. Its value is equal to the difference between the "equivalent electrical parameters after resonance suppression" and the "critical resonance electrical parameter at the time of resonance." Specifically, it includes the residual current source resonance parameter. Residual equivalent current resonant parameters and residual admittance resonant parameters , 、 、 .

[0055] S8: Calculate the three-phase voltage reactive power decoupling vector and the three-phase current reactive power decoupling vector based on the inverter output voltage, inverter output phase and remaining resonance parameters.

[0056] Three-phase voltage reactive power decoupling vector 、 、 ) Reactive decoupling vector of three-phase current ( 、 、 ) It is a key vector characterizing the reactive power distribution of a grid-connected system, used to separate the active and reactive components in power transmission. Among them, the voltage reactive decoupling vector is derived based on the inverter output voltage, output phase, and residual resonant parameters, while the current reactive decoupling vector is obtained by replacing the output voltage in the voltage reactive decoupling vector with the output current. Together, they constitute the control carrier for reactive power compensation.

[0057] Specifically, first obtain the inverter output voltage. Inverter output phase Then the inverter output voltage Inverter output phase Using the remaining resonant parameters as input and considering the symmetrical characteristics of the three-phase circuit, calculate the three-phase voltage reactive power decoupling vector: in, This is the inverter output voltage. For the inverter output phase, This represents the residual resonant coupling coefficient. (The formula is missing from the original text.) Replace with inverter output current The three-phase current reactive power decoupling vector is obtained as follows: The core logic of the above formula is to correct the base voltage / current vector by using the residual resonance parameters, so that the decoupling vector can accurately reflect the impact of the resonance residue on reactive power.

[0058] S9: Calculate the reactive power decoupling vector compensation value based on the three-phase voltage reactive power decoupling vector and the three-phase current reactive power decoupling vector.

[0059] Reactive components are extracted from the reactive decoupling vectors of three-phase voltage and three-phase current, combined with the grid current angle. Calculate the compensation value : In the formula, For compensation coefficient, The grid current angle is the angle of the grid current. Real-time monitoring is used to obtain the data. This formula quantifies the total amplitude of reactive components and combines it with the phase characteristics of the grid current to ensure that the compensation value can accurately offset the reactive power deviation caused by residual resonance.

[0060] S10: The reactive power decoupling vector compensation value is superimposed on the three-phase voltage reactive power decoupling vector and the three-phase current reactive power decoupling vector respectively, thereby enhancing the stability of the grid-connected system.

[0061] Reactive power decoupling vector compensation value l By superimposing these vectors onto the three-phase voltage reactive power decoupling vector and the three-phase current reactive power decoupling vector respectively, the compensated vector is obtained: The compensated vector is fed back to the inverter control loop, replacing the original voltage / current command: where Used to correct the inverter output voltage. This is used to optimize current tracking accuracy. Ultimately, this is achieved by monitoring the compensated equivalent electrical parameters. If satisfied This indicates that the stability of the grid-connected system has been effectively enhanced.

[0062] This embodiment, through precise calculation of the three-phase voltage and current reactive power decoupling vector and compensation value, not only offsets the reactive power deviation caused by resonance residue, but also optimizes the impedance matching accuracy between the inverter output and the weak grid, solving the voltage and current fluctuation problem caused by multiple string connections in the weak grid environment, and significantly improving the long-term stable operation capability of string photovoltaic inverters in the weak grid environment.

[0063] To verify the effectiveness of the control method proposed in this application, an experimental platform was built and an SG100CX photovoltaic inverter was used for testing to simulate the resonance scenario caused by power mutation and impedance fluctuation under weak grid conditions.

[0064] Experimental results for example Figure 3 As shown. Among them, Figure 3 (a) The voltage and current fluctuation waveforms of the string photovoltaic inverter based on the improved LADRC-PI dual closed-loop control are shown in the scenario. Figure 3 (b) shows the wave waveform of a similar control method based on an improved particle swarm optimization algorithm; for comparison, Figure 3 (c) shows the waveform obtained by using the impedance analysis-based stability enhancement control method for string photovoltaic inverters in weak grid environments designed in this application.

[0065] The comparison shows that, under the same test conditions, the method of this application can effectively stabilize the output voltage fluctuation within the range of -80V to +80V and the output current fluctuation within the range of -20A to +20A. Compared with the other two comparative methods, the method of this application demonstrates superior fluctuation suppression capability and stability control effectiveness, verifying its technical effect.

[0066] Finally, it should be noted that: The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A method for enhancing the stability control of string photovoltaic inverters under weak power grid conditions, characterized in that, include: Real-time acquisition of electrical operation data of the inverter's AC side grid-connected port. The electrical operation data includes at least the response voltage, response current, and reference current signals at the front end of the grid-connected port, as well as the response voltage, response current, and reference current signals at the back end of the grid-connected port. Based on electrical operation data, the optimized closed-loop transfer function parameters adapted to the weak power grid environment are calculated in real time. Based on the optimized closed-loop transfer function parameters, the target value of the equivalent output impedance at the front end of the grid-connected port is dynamically determined; By adjusting the first impedance control parameter, the equivalent output impedance at the front end of the grid-connected port is controlled in real time to converge to the target value, so as to achieve dynamic matching between the inverter output impedance and the weak grid impedance.

2. The control method according to claim 1, characterized in that, Real-time calculation of optimized closed-loop transfer function parameters adapted to weak grid environments, including: The optimized closed-loop transfer function parameters adapted to weak grid environments are calculated in real time using the following formula: in, To optimize the closed-loop transfer function, For the response voltage at the front end of the grid connection port, For the response current at the front end of the grid connection port, This is the reference current signal at the front end of the grid connection port. For the response voltage of the grid-connected port backend, For the response current of the grid connection port backend, This is the reference current signal at the back end of the grid connection port.

3. The control method according to claim 1, characterized in that, The first impedance control parameter includes at least one of the following: current loop proportional coefficient, current loop integral coefficient, voltage loop proportional coefficient, and voltage loop integral coefficient.

4. The control method according to claim 1, characterized in that, After achieving dynamic matching between the inverter output impedance and the weak grid impedance, the following steps are also included: Based on the equivalent model of string photovoltaic inverters and weak grid-connected systems, the critical resonance electrical parameters when the grid-connected system resonates are calculated. By adjusting the second impedance control parameter of the inverter, the real-time electrical parameters of the grid-connected system are made to be less than the critical resonance electrical parameters.

5. The control method according to claim 4, characterized in that, The equivalent model of a string photovoltaic inverter and a weak grid grid connection system is constructed as follows: the string photovoltaic inverter is equivalent to a composite structure of an ideal current source, a controlled current source and an equivalent admittance connected in parallel, while the weak grid is equivalent to a structure of an equivalent inductor, an equivalent resistance and an equivalent voltage source connected in series. The two are electrically connected through the grid connection port.

6. The control method according to claim 4, characterized in that, The critical resonance electrical parameters include at least: total equivalent current source, total equivalent current, and total equivalent susceptance, among which, The total equivalent current source is the sum of the equivalent current sources at the front end of the grid-connected port and the equivalent current sources at the back end of the grid-connected port. The total equivalent current is the equivalent short-circuit current at the grid connection port; The total equivalent susceptance is the parallel value of the equivalent susceptance at the front end of the grid-connected port and the equivalent susceptance at the back end of the grid-connected port.

7. The control method according to claim 5, characterized in that, The second impedance control parameter is a supplementary control parameter for the resonance suppression of the grid-connected system, and includes at least one of the following: the damping coefficient at the resonant frequency, the center frequency and attenuation gain of the notch filter, and the adaptive resonance compensation coefficient.

8. The control method according to claim 6, characterized in that, Calculate the critical resonance electrical parameters when the inverter resonates with the weak grid system, including: The total equivalent current source when the inverter resonates with the weak grid system can be calculated using the following formula. Total equivalent short-circuit current Total equivalent susceptance : ； in, This represents the equivalent impedance of a single photovoltaic string. This is the equivalent current on the weak grid side. This is the equivalent voltage on the weak grid side. For the equivalent inductance of the weak grid side, The equivalent admittance of a single photovoltaic string. This refers to the number of photovoltaic strings connected to the inverter.

9. The control method according to claim 7, characterized in that, After adjusting the second impedance control parameter of the inverter to make the real-time electrical parameters of the grid-connected system less than the critical resonant electrical parameters, the following steps are also included: Calculate the residual resonance parameters of the grid-connected system. The residual resonance parameters are the difference between the equivalent electrical parameters after resonance suppression and the critical resonance electrical parameters when resonance occurs. Calculate the three-phase voltage reactive power decoupling vector and the three-phase current reactive power decoupling vector based on the inverter output voltage, inverter output phase and remaining resonance parameters. Calculate the reactive power decoupling vector compensation value based on the three-phase voltage reactive power decoupling vector and the three-phase current reactive power decoupling vector; The reactive power decoupling vector compensation value is superimposed on the three-phase voltage reactive power decoupling vector and the three-phase current reactive power decoupling vector respectively, thereby enhancing the stability of the grid-connected system.

10. The control method according to claim 9, characterized in that, Residual resonance parameters include residual current source resonance parameters. Residual equivalent current resonant parameters and residual admittance resonant parameters The calculation of the residual resonance parameters of the grid-connected system includes: calculating the residual current source resonance parameters according to the following formulas. Residual equivalent current resonant parameters and residual admittance resonant parameters : 、 、 ； The calculation of the three-phase voltage reactive power decoupling vector and the three-phase current reactive power decoupling vector includes the following formulas: in, 、 、 This is the three-phase voltage reactive power decoupling vector. 、 、 This is the three-phase current reactive power decoupling vector. This is the inverter output voltage. For the inverter output phase, The remaining resonant coupling coefficient; Calculating the reactive power decoupling vector compensation value includes: calculating the reactive power decoupling vector compensation value according to the following formula: in, This is the reactive power decoupling vector compensation value. For compensation coefficient, The angle of the grid current; Superimposing the reactive power decoupling vector compensation values ​​onto the three-phase voltage reactive power decoupling vector and the three-phase current reactive power decoupling vector respectively includes: superimposing the reactive power decoupling vector compensation values... l By superimposing these vectors onto the three-phase voltage reactive power decoupling vector and the three-phase current reactive power decoupling vector respectively, the compensated three-phase voltage reactive power decoupling vector and the three-phase current reactive power decoupling vector are obtained, as shown in the following formula: in, This is the compensated three-phase voltage reactive power decoupling vector. This is the three-phase current reactive power decoupling vector after compensation.

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