Inverter extremely weak power grid grid-connected control method based on loop decoupling phase-locked loop

Abstract

The invention discloses an inverter extremely weak power grid grid-connected control method based on a loop decoupling phase-locked loop, and the control method comprises the steps: firstly collecting related voltage and current, and obtaining a voltage component under a two-phase static coordinate system through Clark transformation; inputting the voltage component into a loop decoupling phase-locked loop to obtain a voltage phase angle; a grid-connected reference current is obtained based on the voltage phase angle, and a reference voltage is obtained through a grid-connected current controller; and finally, through VSVPWM modulation, a switching device is driven to realize grid-connected control. According to the loop decoupling phase-locked loop, an auxiliary transfer function is constructed for a transfer function of a traditional phase-locked loop by adopting a multiplication inverse element method in a pre-compensation link, so that the transfer function of the phase-locked loop is irrelevant to the bandwidth of the designed phase-locked loop, and a third-order complex vector filter is introduced on the basis, so that the harmonic suppression capability is improved. Therefore, according to the method, decoupling of the phase-locked loop bandwidth, the current loop and the power grid impedance is achieved, the application range of the power grid impedance is further widened, and the robustness of the inverter system is improved.

Description

Technical Field

[0001] This invention relates to the field of grid-connected inverter control technology, specifically to a control method for achieving stable grid connection under extremely weak grid conditions in grid-connected inverters for new energy sources such as photovoltaic power plants, and particularly to a grid-connected control strategy based on loop decoupling phase-locked loop. Background Technology

[0002] As the global energy structure transitions towards cleaner and lower-carbon energy, renewable energy sources, represented by photovoltaic power generation, have become a core pathway to achieving the "dual carbon" goal. As the energy conversion interface between the renewable energy power generation system and the power grid, the grid-connected inverter's control performance directly affects power quality, system efficiency, and grid stability.

[0003] In low-power photovoltaic power generation scenarios, two-level inverters are widely used due to their low manufacturing cost and simple design. However, due to the limitations of the voltage and overcurrent withstand capabilities of individual semiconductor devices, high-power scenarios require a multi-stage inverter series-parallel structure, but this structure suffers from system complexity and control difficulties. Therefore, high-power multi-level inverters have emerged. Taking three-level inverters as an example, they have significant advantages over two-level structures in terms of higher output power and higher voltage withstand capability, making them more suitable for the grid connection needs of high-power renewable energy.

[0004] When a new energy power generation system is connected to the grid via long-distance transmission lines, the superposition of the transmission line impedance and the impedance of the nonlinear equipment in the local power grid causes the grid to exhibit weak grid characteristics. In this situation, when the phase-locked loop (PLL) detects the voltage phase at the point of common coupling (PCC), the coupling effect between the PLL, current loop, and grid impedance cannot be ignored. The higher the grid impedance, the stronger the coupling, which can lead to grid current distortion at best, and system stability deterioration or even grid connection failure at worst. Grid-connected systems require strict synchronization between the inverter output and the grid, including consistency in frequency, voltage amplitude, phase sequence, and phase. This poses a severe challenge to phase-locked control under weak grid conditions.

[0005] The proportional-resonant (PR) control algorithm, based on the internal model principle, can achieve zero steady-state error tracking of the reference value. However, this algorithm is extremely sensitive to frequency fluctuations; even a small frequency deviation can lead to a significant decrease in control performance. To overcome this deficiency, the quasi-proportional-resonant (QPR) controller has emerged. By introducing a bandwidth parameter, it reduces the sensitivity to frequency fluctuations while maintaining the fundamental frequency gain. In this invention, the current loop uses a QPR controller specifically to adapt to frequency fluctuation scenarios under weak power grid conditions.

[0006] Despite the proliferation of traditional phase-locked loops (PLLs) and their improved strategies for weak power grids, the core issue of insufficient system robustness remains unresolved. Wide-range variations in grid impedance lead to a decrease in the robustness of the PLL itself. Under extremely weak grid conditions, the interaction between the PLL, current loop, and grid impedance intensifies, further compressing the system stability margin and seriously threatening the safe operation of new energy power systems. The bottleneck of existing technologies lies in the inability to decouple the PLL bandwidth from the current loop and grid impedance, resulting in insufficient system adaptability to changes in grid impedance. Summary of the Invention

[0007] To address the problems of poor robustness and insufficient adaptability to grid impedance in grid-connected inverters under extremely weak power grid conditions in existing technologies, this invention provides a grid-connected control method for inverters under extremely weak power grid conditions based on loop decoupling phase-locked loop. By decoupling the coupling relationship between the phase-locked loop and the current loop, the stability and adaptability of the system under extremely weak power grid conditions are improved.

[0008] To achieve the above objectives, the specific solution adopted by the present invention is as follows: A grid-connected control method for inverters in extremely weak power grids based on a loop decoupling phase-locked loop is applied to a grid-connected inverter located between a photovoltaic power station and the public power grid. The grid-connected inverter includes an inverter bridge arm circuit, an LCL filter, a controller, and sensors. The controller includes a loop decoupling phase-locked loop and a quasi-proportional resonant (QPR) controller. The control method includes the following steps: S1. Signal Acquisition and Coordinate Transformation The voltage u at the common coupling point of the grid-connected inverter is collected by sensors. pcc Grid-connected current i a-2 and the capacitor current i of the LCL filter c The α-axis voltage component u in the two-phase stationary coordinate system is obtained by Clark transformation. pccα β-axis voltage component u pccβ Clark transformation converts signals in a three-phase stationary coordinate system into a two-phase stationary coordinate system, laying the foundation for subsequent vector control. S2, Loop decoupling phase-locked loop processing will u pccα u pccβ The input is fed into the loop decoupling phase-locked loop, which includes: The loop decoupling unit decouples the input u. pccα u pccβ Mathematical processing is performed, which is achieved by cascading an auxiliary transfer function and a third-order complex vector filter transfer function. The auxiliary transfer function, being the reciprocal of the phase-locked loop (PLL) transfer function, is used to decouple the PLL from the current loop. The third-order complex vector filter transfer function is used to extract the fundamental positive-sequence component u. α+ u β + Suppress harmonic interference; Phase-locked loop, receiving the fundamental positive-sequence component u α + u β + It also outputs voltage phase angle information θ to provide a reference for phase synchronization of grid-connected current; S3, Grid connection control A reference current i is generated based on the voltage phase angle information θ and the grid-connected current amplitude I2 required by the grid-connected inverter. ref The grid-connected current i a-2 With reference current i ref The difference is used to obtain the error signal, which is then transmitted to the quasi-proportional resonant controller for modulation. After modulation by the quasi-proportional resonant controller, the error signal is compared with i. c Superimposed to generate a reference voltage u ref ; will u ref The signal is fed into the VSVPWM module to generate a PWM signal, which drives the circuit to control the switching devices in the grid-connected inverter.

[0009] Furthermore, the mathematical model G of the phase-locked loop PLL (s) is: In the formula, K p K i These are the proportional coefficient and integral coefficient of the phase-locked loop, U m For grid connection point u pcc The voltage amplitude, ω is the angular frequency of the grid voltage.

[0010] Furthermore, the transfer function G of the loop decoupling unit n2 The expression for (s) is: Among them, G n1 (s) represents the auxiliary transfer function, which is obtained by using the multiplicative inverse method. The auxiliary transfer function is the phase-locked loop transfer function G. PLL The reciprocal of (s), G n0 (s) represents the transfer function of a third-order complex vector filter, where k1 and k2 are zero-pole placement coefficients used to adjust the bandwidth and dynamic response of the third-order complex vector filter, s is the input variable after the complex variable Laplace transform, and j is the complex coefficient factor.

[0011] Furthermore, the transfer function G of the loop decoupling phase-locked loop NPLL The expression for (s) is:

[0012] Beneficial effects: (1) The loop decoupling phase-locked loop in step S2 of this invention includes a loop decoupling unit and a phase-locked loop. The decoupling of the phase-locked loop bandwidth from the current loop and the grid impedance is achieved through the cascading of an auxiliary transfer function and a third-order complex vector filter. In traditional phase-locked loops, changes in the phase-locked loop bandwidth directly affect system robustness. However, this invention, through decoupling design, makes system robustness no longer dependent on the phase-locked loop bandwidth, effectively reducing the interaction between loops under extremely weak grid conditions. For example, when the grid impedance increases from 15mH (SCR = 2.72) to 28mH (SCR = 1.51, extremely weak grid), the system can still maintain stable operation, with total harmonic distortion (THD) of the grid-connected current of 0.35% and 0.47%, respectively, far below the grid connection standard requirements.

[0013] (2) In step S3, a quasi-proportional resonant controller is used, which significantly reduces the sensitivity to frequency fluctuations compared to the traditional PR controller. Even with slight shifts in the grid frequency, it can still maintain a high fundamental frequency gain, achieving zero steady-state error tracking of the grid-connected current. Simulation results show that the system reaches steady state within 0.025s, exhibiting excellent dynamic response speed.

[0014] (3) The grid-connected current feedback active damping technology suppresses the inherent resonance peak of the LCL filter in step S3, avoiding current distortion and system instability caused by filter resonance. By introducing the current feedback signal into the control loop, the phase characteristics of the system output impedance are reshaped, the negative damping characteristics of the inverter are reduced, and the adaptability range to grid impedance is broadened.

[0015] (4) In step S2, the third-order complex vector filter achieves distortion-free extraction of the fundamental positive-sequence component of the PCC voltage, while effectively eliminating the fundamental negative-sequence component and harmonic interference. This process improves the anti-interference capability of the phase-locked loop, enabling the system to maintain unity power factor grid connection even under extremely weak grid conditions, ensuring energy conversion efficiency and grid compatibility.

[0016] (5) After decoupling, the output impedance phase characteristic of the system is reshaped, and the negative damping characteristic is reduced. Figure 13 (Fast convergence when SCR=1.51); The grid impedance adaptation range is widened, and stable grid connection is maintained even when SCR is as low as 1.51 (compared to the traditional scheme where the instability critical SCR≈2.0). Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is the main circuit diagram of the new energy grid-connected inverter in the implementation of this invention.

[0019] Figure 2 This is a control block diagram for an LCL-type grid-connected inverter.

[0020] Figure 3 This is one of the equivalent transformation diagrams of the control block diagram of an LCL-type grid-connected inverter.

[0021] Figure 4 This is the second equivalent transformation diagram of the control block diagram of an LCL-type grid-connected inverter.

[0022] Figure 5 G in this invention n1 (s) Detailed implementation diagram of the transfer function.

[0023] Figure 6 G in this invention n0 (s) Detailed implementation diagram of the transfer function.

[0024] Figure 7 This is a zero-pole distribution diagram of the open-loop transfer function of the PLL of this invention.

[0025] Figure 8 This is the Nyquist distribution diagram of the open-loop transfer function of the PLL of this invention.

[0026] Figure 9 This is a waveform diagram of the grid-connected current when the grid inductance is 15mH.

[0027] Figure 10 The THD diagram of the grid-connected current is shown when the grid inductance is 15mH.

[0028] Figure 11 This is a waveform diagram of the grid-connected current when the grid inductance is 28mH.

[0029] Figure 12 The THD diagram of the grid-connected current is shown when the grid inductance is 28mH.

[0030] Figure 13 This is the frequency response diagram of the present invention when the grid inductance is 28mH (SCR = 1.51). Detailed Implementation

[0031] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and 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 protection scope of the present invention.

[0032] This invention provides a grid-connected control method for inverters operating in extremely weak power grids based on a loop-decoupled phase-locked loop. This control method is applied to grid-connected inverters located between a photovoltaic power plant and the public power grid. Its hardware structure is as follows: Figure 1 As shown below, each part will be explained in detail.

[0033] The grid-connected inverter adopts a diode-clamped three-phase three-level inverter circuit, including an inverter bridge arm circuit, two voltage equalizing capacitors, an LCL filter, a controller, and a sensor module.

[0034] The inverter bridge arm circuit includes six clamping diodes (D1 to D6) and twelve fully controlled switching devices (S1 to S12). The cathodes of clamping diodes D1, D3, and D5 are connected to the emitters of switching devices S1, S5, and S9, respectively, while their anodes are connected to the neutral line. The cathodes of clamping diodes D2, D4, and D6 are connected to the neutral line, while their anodes are connected to the emitters of switching devices S3, S7, and S11, respectively, to achieve a three-level output.

[0035] The voltage equalization capacitors include a first voltage equalization capacitor C1 (positive terminal connected to the positive output of the DC terminal, negative terminal connected to the neutral line) and a second voltage equalization capacitor C2 (positive terminal connected to the neutral line, negative terminal connected to the negative output of the DC terminal), which are used to balance the DC bus voltage.

[0036] The LCL filter consists of an inverter-side filter inductor L1, a grid-side filter inductor L2, and a filter capacitor C. It is used to suppress high-frequency harmonics and improve the quality of grid-connected current. The input terminal of L1 is connected to the AC terminal of the inverter circuit, and the output terminal is connected to C and L2. The output terminal of L2 is connected to the public power grid.

[0037] The controller includes a decoupled phase-locked loop (PLL) and a quasi-proportional resonant controller. The input of the decoupled PLL is connected to the public power grid, and the output voltage phase angle information is sent to the controller. The drive circuit receives the PWM signal output from the controller and drives the switching devices of the inverter circuit.

[0038] The sensor module includes voltage and current sensors, which are used to detect grid-side voltage, grid-side current, filter capacitor voltage, inverter output current, midpoint potential and DC terminal voltage, and transmit the data to the controller.

[0039] The detailed implementation steps of the control method are described below.

[0040] S1. Signal Acquisition and Coordinate Transformation

[0041] The voltage u at the point of common coupling (PCC) is acquired using a voltage sensor (such as a Hall effect sensor). pcc Current sensors (such as closed-loop Hall current sensors) collect grid-connected current i a-2A current sensor (such as a closed-loop Hall current sensor) collects the capacitance current i of the LCL filter. c The acquired three-phase voltage signals are converted into two-phase stationary coordinate system (α-β axis) components using the Clark transformation matrix, thus obtaining the α-axis voltage component u in the two-phase stationary coordinate system. pccα β-axis voltage component u pccβ .

[0042] S2, Loop decoupling phase-locked loop processing

[0043] will u pccα u pccβ The input is fed into the loop decoupling phase-locked loop, which includes: The loop decoupling unit decouples the input u. pccα u pccβ Mathematical processing is performed, which is achieved by cascading an auxiliary transfer function and a third-order complex vector filter transfer function; the auxiliary transfer function is the reciprocal of the phase-locked loop transfer function; the third-order complex vector filter transfer function is used to extract the fundamental positive-sequence component u. α + u β + ; Phase-locked loop, receiving the fundamental positive-sequence component u α + u β + It also outputs the voltage phase angle information θ.

[0044] The derivation process of the transfer function of the loop decoupling phase-locked loop is explained below.

[0045] (1) Establish the transfer function of the traditional phase-locked loop

[0046] The traditional phase-locked loop transfer function expression is: Among them, K p K i These are the proportional coefficient and integral coefficient of the phase-locked loop, U m For grid connection point u pcc The voltage amplitude, ω is the angular frequency of the grid voltage, s is the input variable after the complex variable Laplace transform, and j is the complex coefficient factor.

[0047] This model describes the phase tracking characteristics of a traditional phase-locked loop for the input voltage, but under extremely weak power grids, the lack of decoupling of the loop coupling can easily lead to system instability.

[0048] (2) Deriving the equivalent impedance model of LCL grid-connected inverter

[0049] Figure 2The control block diagram of the LCL grid-connected inverter presents the complete control process from voltage acquisition to PWM signal generation. Figure 2 middle u g (s), u pcc (s) represent the grid voltage and the voltage at the PCC, respectively, L g s is the mains impedance; i 2-a (s) represents the grid-connected current; i ref (s) and I2 are the reference current value and the given grid-connected current amplitude, respectively; cosθ is the PLL output cosine value, K d K is the active damping coefficient of the capacitor current. pwm For pulse width modulation gain; G PLL (s) is the PLL transfer function, G c (s) is the transfer function of the quasi-proportional resonant controller. This is the quantity obtained after the Laplace transform of the inductance on the inverter side of the LCL filter. This is the quantity obtained after the Laplace transform of the inductance on the network side of the LCL filter. This is the value after the Laplace transform of the LCL filter capacitor.

[0050] according to Figure 2 The control block diagram shown above is for... Figure 2 Mathematical simplification yields Figure 3 By deriving the interaction relationship between the system output impedance and the grid impedance through equivalent transformation, this method is used to analyze the stability defects of traditional control strategies, providing a basis for... Figure 5-6 This provides a theoretical basis for decoupling design. Figure 3 The obtained equivalent transfer function G x1 G x2 and output impedance Z out The negative impedance Z introduced by traditional phase-locked loops TPLL The expression: Among them, K d K is the active damping coefficient of the capacitor current. pwm For pulse width modulation gain; G PLL (s) is the PLL transfer function, G c (s) represents the transfer function of the quasi-proportional resonant controller, L1 is the inverter-side inductance of the LCL filter, L2 is the grid-side inductance of the LCL filter, C is the LCL filter capacitor, and G... c (s) is the transfer function of the quasi-proportional resonant controller, s is the input variable after the complex Laplace transform, and Z out The impedance introduced by the current loop, Z TPLL The negative impedance introduced by the traditional phase-locked loop, G x1 G x2 This is the equivalent transfer function.

[0051] Further equivalent to Figure 4 The open-loop transfer function G is derived. -0 The expression for (s) is: Among them, L g s is the mains impedance; Z out The impedance introduced by the current loop, Z TPLL The negative impedance introduced by Z in a traditional phase-locked loop out-TPLL To account for the system output impedance of a traditional phase-locked loop (PLL), this model reveals the mechanism by which system stability is affected by grid impedance under traditional control strategies.

[0052] This function is used in the simulation verification stage of step S4 below to verify the stability criterion that "the phase difference between the inverter output impedance and the grid impedance must be less than 90°". It is found that the traditional strategy causes the phase difference to exceed the limit due to the coupling of the phase-locked loop under weak grid conditions, thereby limiting the grid impedance adaptation range.

[0053] (3) Constructing an auxiliary transfer function

[0054] For G PLL (s) Further optimization yields the expression for the constructed auxiliary transfer function: In the formula, K p K i These are the proportional coefficient and integral coefficient of the phase-locked loop, U m For grid connection point u pcc The voltage amplitude is given by ω, the grid voltage angular frequency is given by s, the input variable is the complex variable after Laplace transform, and j is the complex coefficient factor. This function, as the reciprocal of the phase-locked loop transfer function, is a key component for achieving loop decoupling. Its control block diagram is shown below. Figure 5 As shown.

[0055] The core function of this function is to counteract the coupling effect of the traditional phase-locked loop on the current loop, so that the current loop characteristics remain stable when the phase-locked loop bandwidth changes (e.g., the current loop gain fluctuation is <5% when the phase-locked loop bandwidth is adjusted).

[0056] (4) Cascaded processing of third-order complex vector filters

[0057] Using a third-order complex vector filter G n0 Cascaded auxiliary transfer function G n1 Its control block diagram is as follows Figure 6 As shown, processing the common coupling point voltage can achieve distortion-free extraction of the fundamental positive sequence component and elimination of the fundamental negative sequence component, thereby improving the anti-interference capability of the phase-locked loop control strategy and enabling unity power factor grid connection even in extremely weak power grid systems.

[0058] Third-order complex vector filter G n0 The transfer function expression for (s) is: In the formula, ω is the angular frequency of the grid voltage, s is the input variable after the complex Laplace transform, j is the complex coefficient factor, and k1 and k2 are the zero-pole placement coefficients. This function extracts the fundamental positive-sequence component through bandpass filtering characteristics, while using the characteristics of complex number operations to suppress negative-sequence components and harmonics.

[0059] Loop decoupling unit transfer function G n2 The expression for (s) is:

[0060] This unit uses a "decoupling then filtering" process to first eliminate the coupling between the phase-locked loop and the current loop, and then extract the pure fundamental positive sequence component to ensure that the input signal of the phase-locked loop is not affected by changes in the grid impedance.

[0061] (5) Derivation of the transfer function of the loop decoupling phase-locked loop

[0062] Based on steps (1) to (4), the expression for the transfer function of the loop decoupling phase-locked loop can be obtained as follows:

[0063] By setting k1 and k2, the zeros and poles of the open-loop transfer function are all located in the left half-plane, ensuring system stability. When the grid frequency fluctuates, the phase-locked loop adjusts the phase through the error signal. The error signal is processed by the loop decoupling unit to form negative feedback control, which reduces the phase locking time to 1 / 3 of the traditional scheme.

[0064] The corresponding open-loop transfer function G -0n (s) is: In the formula, L g s is the mains impedance; Z out-nPLL To account for the system output impedance of the decoupled phase-locked loop, this function optimizes the system damping ratio and bandwidth by adjusting the zero-pole placement coefficients (k1, k2). Figure 7 , Figure 8 For G -0n (s) in Z g Pole-zero plot and Nyquist plot for an extremely weak power grid with SCR = 28mH (SCR = 1.51), from... Figure 7 It can be seen from G -0n(s) The system has no poles in the right half-plane, and is in a stable state. According to the Nyquist stability criterion, the necessary and sufficient condition for system stability is that the Nyquist curve does not pass through the point (-1, j0), and the number of counterclockwise loops R around the critical point (-1, j0) is equal to the number of positive real poles P. Since G -0n (s) There are no poles in the right half-plane, the system P equals 0, the Nyquist curve does not enclose the point (-1, j0), the system can reach a stable state, and the system still has a certain degree of robustness.

[0065] S3, Grid connection control

[0066] Based on the voltage phase angle information θ and the grid-connected current amplitude I2 required by the grid-connected inverter, a reference current i is generated. ref The reference current vector is in phase with the grid voltage, ensuring unity power factor grid connection.

[0067] The grid-connected current i a-2 With reference current i ref The difference is used to obtain the error signal (i). ref -i a-2 The signal is then transmitted to a quasi-proportional resonant controller for modulation; the error signal, after being modulated by the quasi-proportional resonant controller, is then compared with i. c Superimposed to generate a reference voltage u ref The voltage signal contains a fundamental component and a compensation component that suppresses harmonics. The reference voltage generates a PWM signal through virtual space vector pulse width modulation (VSVPWM). Its core principle is to decompose the reference voltage vector into the switching states of the three-phase bridge arms. By controlling the IGBT switching timing, the inverter output voltage tracks the reference voltage, thereby achieving high-precision grid-connected control.

[0068] S4, Simulation Verification

[0069] The following simulation experiment demonstrates the effectiveness of the method. The simulation parameters are shown in Table 1. parameter Parameter value grid voltage RMS value 150V Grid voltage frequency 50Hz DC bus voltage 350V Inverter-side inductor 2.4mH Grid-side inductor 0.6mH LCL filter capacitor 10μF sampling frequency 15kHz Grid-connected current reference value 15A

[0070] Figure 9-12 The grid-connected current waveforms and THD diagrams under different grid inductances are shown, demonstrating the effectiveness of the invention under weak and extremely weak grid conditions. Figure 13 The frequency response diagram under extremely weak power grid conditions demonstrates the system's excellent dynamic tracking performance. The simulation results are as follows: Figure 9-13 As shown: When the grid inductance is 15mH (SCR = 2.72, weak grid), the grid-connected current THD is 0.35%, and the waveform is smooth. When the grid inductance increases to 28mH (SCR = 1.51, extremely weak grid), the THD is 0.47%, which still meets the grid connection requirements; The frequency response diagram shows that the system reaches steady state within 0.025s, demonstrating excellent dynamic tracking performance.

[0071] The simulation results of the above operating conditions verify that the inverter grid connection control method based on a novel phase-locked loop with loop decoupling can be applied to extremely weak grids and has certain anti-interference performance, dynamic performance and steady-state tracking performance.

[0072] The control method of this invention is not only applicable to photovoltaic grid-connected inverters, but can also be extended to new energy grid-connected scenarios such as wind power generation and energy storage systems.

[0073] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the invention in any way. All equivalent transformations or modifications made in accordance with the essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for inverter grid connection control in extremely weak power grids based on loop decoupling phase-locked loop, characterized in that, This invention is applied to a grid-connected inverter located between a photovoltaic power station and the public power grid. The grid-connected inverter includes an inverter bridge arm circuit, an LCL filter, a controller, and sensors. The controller includes a loop decoupling phase-locked loop and a quasi-proportional resonant controller. The control method includes the following steps: S1. Collect the common coupling point voltage u of the grid-connected inverter using sensors. pcc Grid-connected current i a-2 and the capacitor current i of the LCL filter c The α-axis voltage component u in the two-phase stationary coordinate system is obtained by Clark transformation. pccα β-axis voltage component u pccβ ; S2, u pccα u pccβ The input is fed into the loop decoupling phase-locked loop, which includes: The loop decoupling unit decouples the input u. pccα u pccβ Mathematical processing is performed, which is achieved by cascading an auxiliary transfer function and a third-order complex vector filter transfer function; the auxiliary transfer function is the reciprocal of the phase-locked loop transfer function; the third-order complex vector filter transfer function is used to extract the fundamental positive-sequence component u. α + u β + ; Phase-locked loop, receiving the fundamental positive-sequence component u α + u β + It also outputs the voltage phase angle information θ; S3. Based on the voltage phase angle information θ and the grid-connected current amplitude I2 required by the grid-connected inverter, generate a reference current i. ref The grid-connected current i a-2 With reference current i ref The difference is used to obtain the error signal, which is then transmitted to the quasi-proportional resonant controller for modulation. After modulation by the quasi-proportional resonant controller, the error signal is compared with i. c Superimposed to generate a reference voltage u ref ; will u ref The signal is fed into the VSVPWM module to generate a PWM signal, which drives the circuit to control the switching devices in the grid-connected inverter.

2. The inverter grid-connected control method for extremely weak power grids based on loop decoupling phase-locked loop according to claim 1, characterized in that, The mathematical model G of the phase-locked loop PLL (s) is: In the formula, K p K i These are the proportional coefficient and integral coefficient of the phase-locked loop, U m For grid connection point u pcc The voltage amplitude, ω is the angular frequency of the grid voltage.

3. The inverter grid-connected control method for extremely weak power grids based on loop decoupling phase-locked loop according to claim 2, characterized in that, The loop decoupling unit transfer function G n2 The expression for (s) is: Among them, G n1 (s) represents the auxiliary transfer function, which is the phase-locked loop transfer function G. PLL The reciprocal of (s), G n0 (s) represents the transfer function of a third-order complex vector filter, where k1 and k2 are zero-pole placement coefficients used to adjust the bandwidth and dynamic response of the third-order complex vector filter, s is the input variable after the complex variable Laplace transform, and j is the complex coefficient factor.

4. The inverter grid-connected control method for extremely weak power grids based on loop decoupling phase-locked loop according to claim 3, characterized in that, Loop decoupling phase-locked loop transfer function G NPLL The expression for (s) is:

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