Improved phase-locked loop structure of grid-connected inverter under weak power grid and stability enhancing method
By introducing prefilters and linear self-immune controllers into the phase-locked loop structure of the grid-connected inverter, the problem of volatile instability of traditional grid-connected inverters under weak grids is solved, the stability of the system is significantly improved, and the normal operation of the distributed power generation system is ensured.
Patent Information
- Application Number
- CN202510407536.3
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-04-02
- Publication Date
- 2025-06-24
AI Technical Summary
In a complex weak grid environment, traditional grid-connected inverters are prone to become instable due to coupling of grid impedance, current ring and other factors, resulting in frequent oscillation accidents.
By introducing a prefilter into the phase-locked loop structure of the grid-connected inverter and changing the traditional PI controller to a linear self-immune controller, an improved phase-locked loop structure is formed to enhance the stability of the system.
This method effectively reduces the risk of system instability caused by factors such as grid impedance and current ring, improves the stability of grid-connected inverters under weak grids, and ensures the normal operation of distributed power generation systems.
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Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of microgrid stability control for distributed generation systems in complex weak grid environments, and particularly to an improved phase-locked loop structure and stability enhancement method for grid-connected inverters under weak grids. Background Art
[0002] At present, with the increasingly prominent global environmental problems, the pursuit of clean and renewable energy and the maintenance of the sustainability of economic society have become the consensus of countries around the world. This has greatly promoted the extensive use of renewable energy such as wind energy, solar energy, and water energy. In order to more conveniently apply these energies, distributed generation systems play an important role. However, during the process of grid connection of a large amount of renewable energy, the characteristics of traditional power grids have gradually changed, showing the "dual high" characteristics of a high proportion of renewable energy and a high proportion of power electronic devices, which will weaken the grid strength of the power grid. Grid-connected inverters are key devices connecting renewable energy and the power grid. In the process of grid-connected power generation by grid-connected inverters under traditional control modes, the system is prone to oscillation accidents. In the more complex and harsh modern power grids, the phenomenon of oscillation accidents caused by various grid-connected devices in the power grid is not uncommon. The main reason is that when the grid-connected inverter is working, there is mutual coupling among the current loop, the phase-locked loop, and the grid impedance, thus causing system instability.
[0003] Based on this, the present invention is proposed. Summary of the Invention
[0004] The purpose of the present invention is to provide an improved phase-locked loop structure and stability enhancement method for grid-connected inverters under weak grids, and to optimize the stability of grid-connected inverters under weak grids by improving the control structure of the phase-locked loop of grid-connected inverters, so as to avoid oscillation accidents of grid-connected inverters under weak grids.
[0005] To achieve the above purpose, the present invention provides the following technical solutions:
[0006] The present invention provides a stability enhancement method for grid-connected inverters under weak grids in the first aspect, including:
[0007] S1: Obtain the structure of the grid-connected inverter under a weak grid;
[0008] S2: Obtain the phase-locked loop structure of the grid-connected inverter under a weak grid;
[0009] S3: Analyze the reasons for the instability of the grid-connected inverter under a weak grid according to the structure and phase-locked loop structure of the grid-connected inverter under a weak grid;
[0010] S4: Introduce a pre-filter at the front stage of the phase-locked loop of the grid-connected inverter under a weak grid, and change the traditional PI controller in the phase-locked loop to a linear active disturbance rejection controller to obtain an improved phase-locked loop structure and enhance stability.
[0011] Furthermore, in S3, in the structure of the grid-connected inverter under a weak grid, the small disturbance signal of the PCC is at least part of the reason for the instability of the grid-connected inverter under a weak grid. The small disturbance signal of the PCC forms a partial positive feedback phenomenon through the loop coupling of the phase-locked loop. When the grid impedance increases, the system is prone to instability.
[0012] Furthermore, in S4, the pre-filter is used to attenuate the small disturbance signal in advance before it enters the linear active disturbance rejection controller. For the low-frequency disturbance near 50 Hz, the attenuation amplitude of the pre-filter decreases. A linear active disturbance rejection controller is designed, which can further suppress the remaining low-frequency disturbance of the filter.
[0013] As a preferred solution, the pre-filter has the characteristic of not interfering with the amplitude and phase of the fundamental frequency of 50 Hz.
[0014] As a preferred solution of the pre-filter, the pre-filter adopts a complex coefficient filter.
[0015] The present invention provides an improved phase-locked loop structure for a grid-connected inverter under a weak grid, which is obtained from step S4 in the above method. The improved phase-locked loop structure is obtained by introducing a pre-filter at the front stage of the phase-locked loop of the grid-connected inverter under a weak grid.
[0016] In the second aspect, the present invention provides an improved phase-locked loop structure for a grid-connected inverter under a weak grid, which is obtained from step S4 in the above method. The improved phase-locked loop structure is obtained by introducing a pre-filter at the front stage of the phase-locked loop of the grid-connected inverter under a weak grid and changing the PI controller in the phase-locked loop to a linear active disturbance rejection controller, which can further suppress the interference of low-frequency disturbance and further enhance the stability.
[0017] Compared with the prior art, the above technical solution has the following beneficial technical effects:
[0018] By improving the structure of the phase-locked loop, especially by introducing a pre-filter and a linear active disturbance rejection controller into the phase-locked loop, the present invention reduces the risk of system instability caused by factors such as grid impedance and current loop coupling, thereby enhancing the stability of the grid-connected inverter under a weak grid and ensuring the normal operation of the distributed generation system. BRIEF DESCRIPTION OF THE DRAWINGS
[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained according to the provided drawings.
[0020] Figure 1 Flow chart of a method for enhancing the stability of a grid-connected inverter under a weak grid provided by a specific embodiment of the present invention;
[0021] Figure 2 Structural block diagram of a grid-connected inverter under a weak grid;
[0022] Figure 3 Typical phase-locked loop control block diagram;
[0023] Figure 4 Vector difference diagram of physical rotation and control rotation coordinate systems under a weak grid;
[0024] Figure 5 Control block diagram of an improved phase-locked loop of a grid-connected inverter under a weak grid provided by a specific embodiment of the present invention;
[0025] Figure 6 Working process diagram of a pre-filter in an improved phase-locked loop of a grid-connected inverter under a weak grid provided by a specific embodiment of the present invention;
[0026] Figure 7 Bode diagrams of three pre-filters in the structure and stability enhancement method of an improved phase-locked loop of a grid-connected inverter under a weak grid provided by a specific embodiment of the present invention;
[0027] Figure 8 Control block diagram of a linear auto-disturbance rejection controller in the structure and stability enhancement method of an improved phase-locked loop of a grid-connected inverter under a weak grid provided by a specific embodiment of the present invention;
[0028] Figure 9 Output three-phase current waveform diagram of the inverter enhanced by the structure and stability enhancement method of an improved phase-locked loop of a grid-connected inverter under a weak grid provided by a specific embodiment of the present invention. Specific embodiment
[0029] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.
[0030] In a preferred embodiment, a method for enhancing the stability of a grid-connected inverter under a weak grid is provided. Please refer to Figure 1 and is mainly implemented through the following steps:
[0031] S1: Obtain the structure of the grid-connected inverter under a weak grid. Please refer to Figure 2 Figure 2 This is the system block diagram of a grid-connected inverter under a weak grid. As can be seen from the figure, the grid-connected inverter adopting the synchronous rotating coordinate system control scheme can output the correct current signal and transmit energy only when the phase-locked loop is in steady-state operation. Under a weak grid, the grid impedance also passes through the phase-locked loop, affects the grid voltage, and thus affects the stability of the system under a weak grid. Figure 2 In it: U dc is the DC bus voltage; C dc is the DC-side voltage stabilizing capacitor; L1, L2, and L f are the inverter-side filtering inductance of the LCL filter, the grid-side filtering inductance of the LCL filter, and the equivalent inductance of the three-phase grid; C f is the filtering capacitor of the LCL filter; i1 is the three-phase current on the inverter side; i c is the three-phase current of the filtering capacitor; u g is the three-phase voltage of the three-phase grid, u p_abc is the three-phase sampled voltage at the point of common coupling (PPC) of the three-phase grid; i g is the three-phase current of the three-phase grid, i g_abc is the three-phase sampled current at the point of common coupling (PPC) of the three-phase grid; θ is the electrical angle output by the phase-locked loop; u p_dq is the d-axis and q-axis voltages of the three-phase sampled voltage transformed into a two-phase rotating coordinate system; I ref_dq is the given d-axis and q-axis currents; u d_dq is the output signal of the control loop; G di is the transfer function of the resonant controller; G i (s) is the transfer function of the current controller; G del is the transfer function of the delay link;
[0032] S2: Obtain the phase-locked loop structure of the grid-connected inverter under a weak grid. Please refer to Figure 3 and Figure 4 , Figure 3 is the typical phase-locked loop control block diagram, that is, the basic control block diagram of the phase-locked loop, Figure 4 is the influence diagram of the phase-locked loop phase angle error on the control vectors of the two coordinate systems, that is, the vector difference diagram of the physical rotation and the control rotation coordinate systems under a weak grid. Figure 3 In it is the three-phase voltage signal, are the corresponding d- and q-axis components, LP is the PI controller, and VCO is the integrator. Figure 4 In it, U is the synthetic rotating vector of the grid voltage, θ g , θ PLL , Δθ are the actual angle of the grid voltage, the angle obtained by the phase-locked loop, and the error between the two, d c , q c , ds , q s They are respectively the d and q axis components of the voltage with phase-locked error and the d and q axis components of the actual voltage.
[0033] S3: Analyze the reasons for the instability of the grid-connected inverter under a weak grid according to the structure of the grid-connected inverter and the phase-locked loop structure under a weak grid. The reason for the instability of the grid-connected inverter under a weak grid is that the small disturbance signal of the PCC forms a partial positive feedback phenomenon through the loop coupling of the phase-locked loop. When the grid impedance increases, the system is prone to instability.
[0034] S4: Introduce a pre-filter (or called: pre-filter) at the front stage of the phase-locked loop of the grid-connected inverter under a weak grid, and change the PI controller in the phase-locked loop to a linear active disturbance rejection controller to obtain an improved phase-locked loop structure and enhance stability. Considering the characteristics of small disturbance signals, through the improved phase-locked loop structure, this embodiment gives an improved phase-locked loop structure of the grid-connected inverter under a weak grid, which is an improved phase-locked loop based on a pre-filter and a linear active disturbance rejection controller. Please refer to Figure 5 , the q-axis system control block diagram of the improved control loop of the improved phase-locked loop, obtained from step S4 in the method for enhancing the stability of the grid-connected inverter under a weak grid above, that is, the improved phase-locked loop structure is obtained by introducing a pre-filter and a linear active disturbance rejection controller into the phase-locked loop of the grid-connected inverter under a weak grid. As the phase-locked loop of the grid-connected inverter to improve the stability under the grid-connected inverter. In a preferred embodiment, the pre-filter has the characteristic of not interfering with the amplitude and phase of the 50Hz fundamental frequency. Because it is considered that the phase-locked loop only needs to synchronize the signal with a grid voltage frequency of 50Hz, and small disturbances of other frequencies are its harmonic interferences. If a corresponding pre-filter structure is introduced at the front stage of the phase-locked loop, these interference signals can be attenuated from entering the phase-locked loop, thus avoiding its coupling with the subsequent current loop. However, the pre-filter has a poor filtering effect on low-frequency disturbances near 50Hz. Changing the PI controller to a linear active disturbance rejection controller can effectively suppress the interference of low-frequency disturbances, thereby improving the stability of the grid-connected inverter under a weak grid. The linear active disturbance rejection controller can quickly track the changes of the system and has good dynamic response characteristics. When the grid voltage suddenly changes or the frequency jumps, the phase-locked loop (LADRC-PLL) based on linear active disturbance rejection control in this embodiment can quickly adjust the output of the phase-locked loop to minimize the phase tracking error as soon as possible. Figure 5 in is the three-phase voltage signal, are the corresponding d and q axis components, are the corresponding α, β axis components, are the α, β axis components filtered by the pre-filter.
[0035] Please refer to Figure 5 andFigure 6 , the working process of the pre-filter is as follows, that is, the signal flow:
[0036] S10: Obtain the three-phase voltage of the common coupling point between the weak power grid and the grid-connected inverter;
[0037] S11: performing αβ coordinate transformation on the three-phase voltage (i.e., transforming the three-phase coordinate system into a two-phase stationary coordinate system) to obtain the α-axis component and the β-axis component of the three-phase voltage;
[0038] S12: inputting the α-axis component and the β-axis component of the three-phase voltage into a pre-filter for filtering;
[0039] S13: performing a dq transformation on the α-axis component and the β-axis component of the filtered three-phase voltage (i.e., transforming the stationary coordinate system into a two-phase rotating coordinate system) to obtain the d-axis component and the q-axis component of the three-phase voltage;
[0040] S14: The q-axis component of the three-phase voltage is input into the linear anti-disturbance controller for phase locking.
[0041] Based on the above mechanism of improving the stability of the system under weak power grid, the present invention provides three pre-filter structures as pre-filters, as three implementation methods to improve the frequency characteristics of the phase-locked loop, namely low-pass filter (LPF), band-pass filter (BPF), complex coefficient filter (CCF), and their transfer functions are:
[0042]
[0043] Among them, G BPF (s) is the bandpass filter (BPF) transfer function, G CCF (s) is the complex coefficient filter (CCF) transfer function, G LPF (s) is the low-pass filter (LPF) transfer function, ω g is the center angular frequency, ω lpf is the low-pass cutoff frequency of the low-pass filter (LPF), ξ bpf is the damping coefficient of the bandpass filter (BPF), ξ ccf is the damping coefficient of the complex coefficient filter (CCF), j is an imaginary unit, and s is a complex variable. According to the transfer function expression, the corresponding Bode diagram is drawn as follows Figure 7 shown.
[0044] According to the principle of improving the stability of the phase-locked loop system under weak power grid conditions, the pre-filter needs to be able to quickly attenuate small disturbance components and try not to interfere with the amplitude and phase of the 50Hz base frequency. Otherwise, additional devices will be required for correction, increasing the complexity of the system structure. Figure 7It can be seen that the BPF and CCF can have unity gain at the fundamental frequency without introducing a phase lag effect, and do not affect the working characteristics of the PLL itself; while the LPF will cause amplitude attenuation and phase deviation, and an additional correction device is required. From the perspective of harmonic attenuation, the pre-filters using the CCF and LPF structures can attenuate harmonics faster than the BPF, and can better attenuate the coupled small disturbance interference, improving the stability of the system under weak grid conditions. In summary, it can be seen that choosing the CCF as the pre-filter for improving the PLL will not only not affect the PLL's own phase-locking performance, but also greatly improve the stability of the system under weak grid conditions. It is the most preferred one among the three pre-filter implementation schemes, followed by the LPF, and then the BPF.
[0045] According to Figure 7 it can be seen that the gains of the three pre-filters are not zero at frequencies near 50 Hz (such as 100 Hz, 150 Hz), which means that using only the pre-filter cannot effectively filter out low-frequency disturbances. Therefore, in the present invention, the traditional PI controller is changed to a linear active disturbance rejection controller to enhance the real-time anti-interference ability of the PLL. As Figure 8 shown in the control block diagram of the linear active disturbance rejection controller, it is mainly composed of a linear extended state observer (LESO) and a linear state error feedback rate (LSEF). In the figure, ω g is the output frequency of the PLL in the previous cycle, is the output frequency of the corrected PLL, 1 / s is the integral link, is the current output phase angle of the PLL. The LESO can be given in the following state space form:
[0046]
[0047] In the formula, z1 is the estimation of the q-axis component u q of the three-phase voltage, z2 is the estimation of the total disturbance f, b is an adjustable gain coefficient, and β1, β2 are bandwidth parameters. From this, the LSEF can be further constructed as
[0048]
[0049] Where P is the feedback gain coefficient. Thus, the process of the linear active disturbance rejection controller is to estimate the q-axis component of the three-phase voltage at the next moment and the total disturbance in the LESO based on the q-axis component of the three-phase voltage processed by the pre-filter in the input. Then, the two estimated parameters are input into the LESF for operation to obtain the angular frequency and phase of the three-phase voltage. The linear extended state observer (LESO) in the linear active disturbance rejection controller can estimate various disturbances inside and outside the system in real time, such as the fluctuations of the grid voltage, the changes in frequency, harmonic interference, etc. It is difficult for the traditional PI controller to effectively cope with the influence of these disturbances, while the linear active disturbance rejection controller can accurately estimate the harmonic disturbance through the LESO and perform real-time compensation during the control process, so that the phase-locked loop can track the grid phase more stably. From the perspective of the frequency domain, the linear active disturbance rejection controller has good disturbance rejection performance in a relatively wide frequency range, which indicates that it can better suppress the interference signals of different frequencies and ensure the stable operation of the phase-locked loop in a complex grid environment.
[0050] Furthermore, in order to verify the effectiveness of the improved phase-locked loop structure and the method for enhancing the stability of the grid-connected inverter under weak grids of the present invention, simulation analysis is carried out in Matlab / Simulink. A three-phase grid-connected inverter simulation model is built, the grid impedance is set to 10 mH, the grid-side inductor is 1 mH, the filter capacitor is 5 μF, the inverter-side inductor is 2 mH, and the grid impedance is 3 mH. The experimental results are as Figure 9 shown. The figure shows the three-phase current output by the inverter. At t = 0.5, by switching from the traditional phase-locked loop method to the method of selecting CCF as the pre-filter for the improved phase-locked loop and adding a linear active disturbance rejection controller to enhance the stability of the grid-connected inverter under weak grids, it can be seen that the current waveform is significantly improved.
[0051] Through the above implementation manners, the present invention can achieve the following beneficial technical effects:
[0052] 1. Improve system stability: By improving the phase-locked loop structure and introducing a pre-filter, especially CCF, the stability of the grid-connected inverter under weak grids is significantly improved. The risk of system instability caused by the coupling of factors such as grid impedance and current loop is reduced, ensuring the normal operation of the distributed generation system.
[0053] 2. Suppression of low-frequency disturbances; The present invention changes the traditional PI controller to a linear active disturbance rejection controller. For the low-frequency disturbances that cannot be eliminated by the pre-filter, the linear active disturbance rejection controller can effectively suppress them. Among them, the LESO can estimate the grid frequency and disturbances, and combine with the LSEF to quickly adjust the output frequency to ensure the stable operation of the grid-connected inverter under weak grids.
[0054] 3. Optimize the current waveform: The experimental results show that after adopting the method of the present invention, the three-phase current waveforms output by the inverter are significantly improved. This helps to reduce current harmonics, improve the power quality, and reduce the pollution to the power grid.
[0055] 4. Enhance the system adaptability: The method of the present invention is applicable to grid-connected inverters under different grid impedance conditions, improving the system adaptability. In the scenario where the grid strength is weak, the system stability can still be maintained, expanding the application range of the grid-connected inverter.
[0056] Those of ordinary skill in the art can understand that all or part of the processes in the above-described method embodiments can be completed by instructing relevant hardware through a computer program. The program can be stored in a computer-readable storage medium. When the program is executed, it can include the processes of the above-described method embodiments. Among them, the storage medium can be a magnetic disk, an optical disk, a read-only memory (ROM), or a random access memory (RAM), etc.
[0057] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.
[0058] The above-described embodiments only represent several implementation manners of the present invention. The description is relatively specific and detailed, but it cannot be construed as a limitation on the scope of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the appended claims.
Claims
1. A method for enhancing the stability of a grid-connected inverter under a weak power grid, characterized in that: include: S1: Obtain the structure of the grid-connected inverter under weak power grid; S2: Obtain the phase-locked loop structure of the grid-connected inverter under weak power grid; S3: Based on the structure and phase-locked loop structure of the grid-connected inverter under weak power grid, analyze the reasons for the instability of the grid-connected inverter under weak power grid; S4: A pre-filter is introduced into the front stage of the phase-locked loop of the grid-connected inverter under weak power grid, and the PI controller in the phase-locked loop is changed into a linear anti-disturbance controller to obtain an improved phase-locked loop structure and enhance stability.
2. The method for enhancing the stability of a grid-connected inverter under a weak power grid according to claim 1, characterized in that: In S3, the small disturbance signal of the PCC in the structure of the grid-connected inverter under the weak power grid is at least part of the reason why the grid-connected inverter under the weak power grid becomes instable.
3. The method for enhancing the stability of a grid-connected inverter under a weak power grid according to claim 2, characterized in that: In S4, the small disturbance signal is attenuated in advance by the pre-filter and enters the linear active disturbance rejection controller.
4. The method for enhancing the stability of a grid-connected inverter under a weak power grid according to claim 3, characterized in that: The pre-filter has the characteristic of not interfering with the amplitude and phase of the fundamental frequency of 50 Hz.
5. The method for enhancing the stability of a grid-connected inverter under a weak power grid according to any one of claims 1 to 4, characterized in that: The pre-filter is a low-pass filter.
6. The method for enhancing the stability of a grid-connected inverter in a weak power grid according to any one of claims 1 to 3, characterized in that: The pre-filter is a band-pass filter.
7. The method for enhancing the stability of a grid-connected inverter under a weak power grid according to any one of claims 1 to 4, characterized in that: The pre-filter adopts a complex coefficient filter.
8. The method for enhancing the stability of a grid-connected inverter in a weak power grid according to any one of claims 1 to 4, characterized in that: The working process of the pre-filter is: Obtain the three-phase voltage of the common coupling point between the weak power grid and the grid-connected inverter; Perform αβ coordinate transformation on the three-phase voltage to obtain the α-axis component and β-axis component of the three-phase voltage; Inputting the α-axis component and the β-axis component of the three-phase voltage into a pre-filter for filtering; Performing dq transformation on the α-axis component and the β-axis component of the filtered three-phase voltage to obtain the d-axis component and the q-axis component of the three-phase voltage; The q-axis component of the three-phase voltage is input into the linear active disturbance rejection controller for phase locking.
9. An improved phase-locked loop structure of a grid-connected inverter under a weak power grid, characterized in that: The improved phase-locked loop structure is obtained by step S4 in the method described in any one of claims 1 to 8 above, and is obtained by introducing a pre-filter in the front stage of the phase-locked loop of the grid-connected inverter under a weak power grid and changing the PI controller in the phase-locked loop to a linear anti-disturbance controllable controller.