A Method for Improving the Stability of Grid-Connected Inverters Based on an Improved Phase-Locked Loop

Abstract

The present invention discloses a method for improving the stability of a grid-connected inverter based on an improved phase-locked loop, which includes the following steps: designing a signal processing module based on a complex coefficient filter through matrix transformation and combining with the internal model principle, and this module is a single-input double-output system; preprocessing the module in front of the input end of the signal processing module to obtain an improved orthogonal vertical signal generation module suitable for a single-phase phase-locked loop; placing the improved orthogonal vertical signal generation module in the front stage of the phase-locked loop to obtain a new type of phase-locked loop structure based on the improved orthogonal vertical signal generation module; and applying the new type of phase-locked loop structure to the grid-connected inverter. The orthogonal vertical signal generation module structure based on the complex coefficient filter designed by the present invention can correct the output impedance characteristics of the grid-connected inverter by reshaping the output characteristics of the phase-locked loop, realize the impedance reshaping of the grid-connected inverter, does not increase the complexity of the control structure, improves the adaptability of the inverter to a weak grid, and expands the stable region of the grid-connected system.

Description

Technical Field

[0001] The present invention belongs to the technical field of grid-connected inverter stability control, and particularly relates to a method for improving the stability of a grid-connected inverter based on an improved phase-locked loop. Background Art

[0002] As an energy interface between a new energy power generation system and the power grid, a grid-connected inverter is a key device for injecting electric energy into the power grid with high quality. However, renewable energy is unevenly distributed and is usually deployed in remote areas, requiring a large number of transformers and long transmission lines to access the power grid. Due to reasons such as the leakage inductance of transformers and long-distance transmission lines, the power grid exhibits non-negligible impedance, gradually showing weak grid characteristics. For a weak grid system, the interactive dynamic effects inside the inverter, between inverters, and between the inverter and the power grid exacerbate the risk of grid-connected system oscillation instability.

[0003] In a grid-connected inverter system, a phase-locked loop is often used to obtain the frequency and phase information of the grid voltage to achieve the synchronous operation of the system. However, the negative phase shift introduced by the phase-locked loop in the inverter impedance makes the inverter exhibit negative impedance characteristics, broadening the instability frequency band range of the grid-connected system. This phenomenon is the main reason for inverter instability. To ensure the stable operation of grid-connected inverters under weak grids, domestic and foreign scholars have carried out relevant research, such as the following publicly available documents:

[0004] [1] Xu Jinming, Bian Shenyiyang, Qian Hao, et al. Robust control and optimization method for a single-phase grid-connected inverter delay phase-locked loop under weak power [J]. Proceedings of the Chinese Society for Electrical Engineering, 2020, 40(7): 2062-2070.

[0005] [2] Ling Yang, Yandong Chen, An Luo, et al. Stability Enhancement for Parallel Grid-Connected Inverters by Improved Notch Filter [J]. IEEE Access, 2019, 7: 65667-65678.

[0006] [3] Song Shaojian, Liu Yanyang, Liu Bin, et al. Stability analysis of an electric vehicle grid-connected inverter based on impedance [J]. Electric Machines and Control, 2019, 23(4): 111-119.

[0007] Reference [1] proposed a method to improve the stability of grid-connected inverters based on a pre-stage low-pass filter phase-locked loop. However, this method additionally introduces a phase compensation link in the control system, increasing the complexity of the control structure. Reference [2] enhances the ability of grid-connected inverters to suppress grid-side harmonics by introducing an improved notch filter, but this method also increases the complexity of the control algorithm. Reference [3] weakens the influence of the phase-locked loop on the impedance characteristics of the inverter by compensating the phase in the low-frequency band, improving the robustness of the grid-connected inverter. However, this method requires an additional online impedance measurement link, and its control effect depends on the grid measurement accuracy, and it cannot guarantee a good compensation effect when the grid impedance varies widely. The above methods for improving the stability of grid-connected inverters have deficiencies in terms of control complexity and weak grid adaptability. Summary of the Invention

[0008] Aiming at the problems existing in the prior art, the purpose of the present invention is to provide a method for improving the stability of grid-connected inverters based on an improved phase-locked loop. The designed phase-locked loop can be used in grid-connected inverters to achieve impedance reshaping, without increasing the complexity of the control structure, weakening the adverse effects of the phase-locked loop on the stability of grid-connected inverters, improving the adaptability of the inverter to weak grids, and expanding the stable region of the grid-connected system.

[0009] To achieve the technical objectives of the present invention, the following technical solutions are adopted:

[0010] A method for improving the stability of grid-connected inverters based on an improved phase-locked loop, comprising the following steps:

[0011] Design a signal processing module based on a complex coefficient filter through matrix transformation and in combination with the internal model principle. This module is a single-input double-output system;

[0012] Prepose a preprocessing module at the input end of the signal processing module to obtain an improved orthogonal vertical signal generation module suitable for a single-phase phase-locked loop;

[0013] Place the improved orthogonal vertical signal generation module in the front stage of the phase-locked loop to obtain a new phase-locked loop structure based on the improved orthogonal vertical signal generation module;

[0014] Apply the new phase-locked loop structure to grid-connected inverters.

[0015] Furthermore, the signal processing module based on the complex coefficient filter includes a Park matrix transformation module T dq , an inverse Park transformation matrix module a complex coefficient filter CCF, and a difference calculation link; the input grid voltage signal v passes through the difference calculation link with the output signal v2 and then enters the complex coefficient filter CCF, and outputs the signal v3; the other grid voltage signal v passes through the inverse Park transformation matrix module the complex coefficient filter CCF, and the Park matrix transformation module Tdq , the output signal v2; the back-end feedforward signal is successively connected to the Park matrix transformation module complex coefficient filter CCF and Park matrix transformation module T dq , one path of the output signal and the signal v3 pass through a subtraction link to obtain the output signal v1, and the other path is fed back to the front end to become the back-end feedforward signal; the expressions of the equivalent transfer functions D(s) and Q(s) from the single input to the two output signals of this signal processing module are respectively:

[0016]

[0017]

[0018] In the formula, ω c represents the cut-off angular frequency of the complex coefficient filter, ω represents the fundamental angular frequency, j represents the imaginary unit, and s represents the Laplace operator.

[0019] Furthermore, the preprocessing module is placed at the front end of the signal processing module based on the complex coefficient filter, and the expression of its equivalent transfer function G(s) is:

[0020]

[0021] In the formula, k represents the gain of the preprocessing module.

[0022] Furthermore, the improved orthogonal vertical signal generation module is placed after the input grid voltage, and the output of this module generates the d-axis grid voltage v d (s) and the q-axis grid voltage v q (s) through a coordinate transformation link, and their expressions are:

[0023]

[0024] In the formula, * represents the convolution operation, θ represents the phase angle output by the phase-locked loop, and v(s) represents the frequency-domain expression of the input grid voltage.

[0025] Furthermore, after adopting the novel phase-locked loop structure, the expression of the equivalent transfer function G PLL (s) of the phase-locked loop in the reshaped grid-connected inverter is:

[0026]

[0027] In the formula, k p and k i are respectively the proportional coefficient and the differential coefficient of the proportional-integral controller in the phase-locked loop structure, I m is the amplitude of the current reference value, and U m is the voltage amplitude of the point of common coupling.

[0028] Further, the equivalent output impedance Z qq_PLL0 (s) of the reshaped grid-connected inverter is expressed as:

[0029]

[0030] In the formula, L1 is the inductor on the inverter side, L2 is the inductor on the grid side, C1 is the filter capacitor, G c (s) is the transfer function of the current controller, H i1 represents the active damping coefficient, I m is the amplitude of the grid-connected current reference value, G PLL (s) is the equivalent transfer function of the new phase-locked loop, k PWM is the transfer function gain from the modulation signal to the output voltage of the inverter bridge.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows: A method for improving the stability of a grid-connected inverter based on an improved phase-locked loop does not require introducing an additional control loop in the grid-connected inverter, and the original control loop is not significantly damaged, so it will not further increase the control complexity of the grid-connected inverter system. Through the new phase-locked loop structure based on the improved orthogonal vertical signal generation module, the equivalent transfer function of the phase-locked loop in the inverter can be reshaped, the impedance characteristics of the grid-connected inverter under a weak grid can be corrected, the adverse effects of the phase-locked loop on the stability of the grid-connected inverter can be weakened, the adaptability of the inverter to the weak grid can be improved, and the stable region of the grid-connected system can be expanded. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 is a flowchart of a method for improving the stability of a grid-connected inverter based on an improved phase-locked loop according to the present invention.

[0033] Figure 2 is a schematic structural diagram of a signal processing module based on a complex coefficient filter in the present invention.

[0034] Figure 3 is a schematic structural diagram of an improved orthogonal vertical signal generation module in the present invention.

[0035] Figure 4 is a schematic structural diagram of a traditional synchronous reference frame phase-locked loop.

[0036] Figure 5 is a schematic structural diagram of the new phase-locked loop structure in the present invention.

[0037] Figure 6 is a schematic diagram of the closed-loop control of a grid-connected inverter considering the phase-locked loop in the present invention.

[0038] Figure 7 is the Bode plot of the inverter and grid impedances before and after improvement.

[0039] Figure 8 The simulation waveform of the output current of the grid-connected inverter under a weak grid before improvement.

[0040] Figure 9 The simulation waveform of the output current of the grid-connected inverter under a weak grid according to the present invention. Detailed implementation manners

[0041] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that the embodiments are intended to explain the present invention rather than limit it.

[0042] As Figure 1 shown, a method for improving the stability of a grid-connected inverter based on an improved phase-locked loop according to the present invention specifically includes the following steps:

[0043] S1: Design a signal processing module based on a complex coefficient filter through matrix transformation and in combination with the internal model principle. This module is a single-input double-output system;

[0044] S2: A preprocessing module is placed in front of the input end of the signal processing module to obtain an improved orthogonal vertical signal generation module suitable for a single-phase phase-locked loop;

[0045] S3: Place the improved orthogonal vertical signal generation module in the front stage of the phase-locked loop to obtain a new phase-locked loop structure based on the improved orthogonal vertical signal generation module;

[0046] S4: Apply the new phase-locked loop structure to the grid-connected inverter, which can reshape the equivalent transfer function of the phase-locked loop in the inverter, correct the impedance characteristics of the grid-connected inverter under a weak grid, and weaken the adverse effects of the phase-locked loop on the stability of the grid-connected inverter.

[0047] As Figure 2 shown, the signal processing module based on the complex coefficient filter in step S1 includes a Park matrix transformation module T dq , an inverse Park transformation matrix module a complex coefficient filter CCF, and a difference calculation link; the input grid voltage signal v passes through the difference calculation link with the output signal v2 and then enters the complex coefficient filter CCF, and the signal v3 is output; another path of the grid voltage signal v passes through the inverse Park transformation matrix module the complex coefficient filter CCF, and the Park matrix transformation module T dq , and the signal v2 is output; the subsequent feedforward signal is successively connected to the inverse Park matrix transformation module the complex coefficient filter CCF, and the Park matrix transformation module T dq , one path of the output signal passes through the difference calculation link with the signal v3 to obtain the output signal v1, and the other path is fed back to the front end to become the subsequent feedforward signal; the expressions of the equivalent transfer functions D(s) and Q(s) from the single input of this signal processing module to the two output signals are respectively:

[0048]

[0049]

[0050] In the formula, ω c represents the cut-off angular frequency of the complex coefficient filter, ω represents the fundamental angular frequency, j represents the imaginary unit, and s represents the Laplace operator.

[0051] As Figure 3 shown, the improved quadrature vertical signal generation module is composed of a signal processing module based on a complex coefficient filter and a preprocessing module. The input end of the signal processing module is preceded by the preprocessing module to obtain an improved quadrature vertical signal generation module suitable for a single-phase phase-locked loop. In the figure, G(s) is the equivalent transfer function of the preprocessing module, and its expression is:

[0052]

[0053] In the formula, k represents the gain of the preprocessing module.

[0054] In step S3, after the improved quadrature vertical signal generation module in the new phase-locked loop structure is placed behind the input grid voltage, the output of this module generates the d-axis grid voltage v d (s) and the q-axis grid voltage v q (s) through a coordinate transformation link, and their expressions are:

[0055]

[0056] In the formula, * represents the convolution operation, θ represents the phase angle output by the phase-locked loop, and v(s) represents the frequency-domain expression of the input grid voltage.

[0057] To illustrate the influence of the improved phase-locked loop on grid connection stability, Figure 4 the block diagram of the traditional synchronous reference frame phase-locked loop structure is given. In the figure, QSG represents the quadrature signal generator, Park represents the Park transformation module, PI represents the proportional-integral controller, is the voltage-controlled oscillator module, and θ is the phase angle output by the phase-locked loop. When the grid-connected inverter adopts different types of phase-locked loop structures, its output presents different impedance characteristics, that is, the adaptability of the grid-connected inverter to a weak grid also changes accordingly.

[0058] As Figure 5As shown, the new phase-locked loop (PLL) structure is obtained by placing an improved orthogonal vertical signal generation module in the front stage of the PLL, resulting in a new PLL structure based on an improved orthogonal signal generator. In the new PLL structure, the improved orthogonal vertical signal generation module is located at the front end of the Park transformation module of the PLL and the back end of the input grid voltage signal, and is used in grid-connected inverters to reshape the equivalent transfer function of the PLL in the inverter, correct the impedance characteristics of the grid-connected inverter under weak grids, and weaken the adverse effects of the PLL on the stability of the grid-connected inverter. In the figure, v α and v β are the voltage components of the grid voltage on α and β respectively, v q is the q-axis component of the grid voltage, and θ is the output phase angle of the new PLL. After adopting the new PLL structure designed by the present invention, the expression of the reshaped equivalent transfer function G PLL (s) in the control block diagram of the grid-connected inverter is:

[0059]

[0060] In the formula, k p and k i are the proportional coefficient and differential coefficient of the proportional-integral controller in the PLL structure respectively, I m is the amplitude of the current reference value, and U m is the voltage amplitude of the point of common coupling.

[0061] In the formula, k p and k i are the proportional coefficient and differential coefficient of the proportional-integral controller in the PLL structure respectively, I m is the amplitude of the current reference value, and U m is the voltage amplitude of the point of common coupling.

[0062] In step S4, the expression of the reshaped equivalent output impedance Z qq_PLL0 (s) of the grid-connected inverter is:

[0063]

[0064] In the formula, L1 is the inductor on the inverter side, C1 is the filter capacitor, L2 is the inductor on the grid side, G c (s) is the transfer function of the current controller, H i1 represents the active damping coefficient, I m is the amplitude of the current reference value, G PLL (s) is the equivalent transfer function of the new PLL, and k PWM is the transfer function gain from the modulation signal to the output voltage of the inverter bridge.

[0065] As Figure 6 shown, considering the closed-loop control block diagram of the grid-connected inverter with the PLL, I mis the amplitude of the grid-connected current reference value, I ref (s) is the inverter input reference current, G c (s) is the transfer function of the current controller, H i1 is the active damping coefficient, Z L1 (s) is the transfer function of the inductor on the inverter side, Z c (s) is the transfer function of the filter capacitor, Z L2 is the transfer function of the grid-side inductor, u PCC (s) is the voltage at the point of common coupling, i g (s) is the transfer function of the inverter output grid-connected current.

[0066] As Figure 7 shown, f1 is the intersection frequency of the impedance phase-frequency characteristic curve of the grid-connected inverter before improvement and -90°, f2 is the intersection frequency of the impedance phase-frequency characteristic curve of the grid-connected inverter after improvement and -90°, Z qq_PLL is the inverter impedance before improvement, Z qq_PLL0 is the inverter impedance after improvement, Z g is the equivalent impedance of the power grid, L g is the equivalent inductance of the power grid. It can be seen from the figure that the stable region of the grid-connected inverter is widened from f1 > 124 Hz to f2 > 64 Hz. By comparison, it can be known that the stable region of the grid-connected inverter under a weak grid is greatly widened. Through analysis, it can be known that when the degree of the weak grid deepens after impedance reshaping, the inverter can always maintain stable operation. Even when the equivalent inductance L of the power grid g increases to 10 mH, the grid-connected system can still maintain a large stability margin. In other words, even if the degree of the weak grid further deepens, the grid-connected inverter can still maintain a certain stability margin and adaptability. The above analysis shows that a method for improving the stability of a grid-connected inverter based on an improved phase-locked loop according to the present invention can adapt to wide-range changes in the grid impedance, and the grid-connected inverter can always maintain good adaptability and robustness when the grid impedance changes under a weak grid.

[0067] In addition, a method for improving the stability of a grid-connected inverter based on an improved phase-locked loop according to the present invention reshapes the impedance characteristics of the grid-connected inverter by adopting a new type of phase-locked loop. Since this method does not introduce an additional control loop in the grid-connected inverter and the original control loop is not greatly damaged, it will not further increase the control complexity of the grid-connected inverter system.

[0068] In order to further verify the stable control of the grid-connected inverter by the improved new type of phase-locked loop, a single-phase grid-connected inverter simulation model is built in the Matlab / Simulink simulation software, and the equivalent inductance L of the power grid is set g to be 10 mH. Figure 8For the simulation waveform of the output current of the grid-connected inverter under a weak grid before improvement, it can be observed that when the traditional synchronous reference frame phase-locked loop is used under a weak grid, the grid-connected inverter cannot operate stably, and the harmonic distortion rate of the output current waveform is relatively large. Figure 9 This is the output current waveform of the grid-connected inverter under a weak grid in the present invention. It can be observed that when the novel phase-locked loop of the present invention is used under a weak grid, the sinusoidal degree of the output current of the inverter is high. At this time, the inverter can operate stably and the output power quality is good. Comparing Figure 8 and Figure 9 it can be seen that a method for improving the stability of a grid-connected inverter based on an improved phase-locked loop in the present invention can effectively improve the robustness of the grid-connected inverter to the grid impedance under a weak grid.

[0069] Finally, it should be pointed out that the above content is only an illustration of a specific embodiment rather than a restriction and limitation on the present invention. At the same time, the scope of the rights protected by the present invention is not limited thereto. Any professional in the field can make certain modifications, equivalent replacements and improvements to the specific implementation, but all should be within the scope of the protection of the claims.

Claims

1. A method for improving the stability of a grid-connected inverter based on an improved phase-locked loop, characterized in that, It includes the following steps: By means of matrix transformation and in combination with the internal model principle, design a signal processing module based on a complex coefficient filter, which is a single-input double-output system; Prepose a preprocessing module at the input end of the signal processing module to obtain an improved orthogonal vertical signal generation module applicable to a single-phase phase-locked loop; Place the improved orthogonal vertical signal generation module in the front stage of the phase-locked loop to obtain a new type of phase-locked loop structure based on the improved orthogonal vertical signal generation module; Apply the new type of phase-locked loop structure to a grid-connected inverter; The signal processing module based on the complex coefficient filter includes a Park matrix transformation module T dq , an inverse Park transformation matrix module a complex coefficient filter CCF, and a difference calculation link; one path of the input grid voltage signal v and the output signal v2 enter the complex coefficient filter CCF after passing through the difference calculation link, and the signal v3 is output; the other path of the grid voltage signal v passes through the inverse Park transformation matrix module the complex coefficient filter CCF and the Park matrix transformation module T dq , and the signal v2 is output; the backend feedforward signal is successively connected to the inverse Park matrix transformation module the complex coefficient filter CCF and the Park matrix transformation module T dq , one path of the output signal and the signal v3 pass through the difference calculation link to obtain the output signal v1, and the other path is fed back to the front end to become the backend feedforward signal; the expressions of the equivalent transfer functions D(s) and Q(s) from the single input to the two output signals of this signal processing module are respectively: where ω c represents the cut-off angular frequency of the complex coefficient filter, ω represents the fundamental angular frequency, j represents the imaginary unit, and s represents the Laplace operator; The preprocessing module is placed at the front end of the signal processing module based on a complex coefficient filter, and the expression of its equivalent transfer function G(s) is: In the formula, k represents the gain of the preprocessing module.

2. The method for improving the stability of a grid-connected inverter based on an improved phase-locked loop according to claim 1, wherein After the improved orthogonal vertical signal generation module is placed behind the input grid voltage, the output of this module generates the d-axis grid voltage v d (s) and the q-axis grid voltage v q (s) in the frequency domain through a coordinate transformation link, and their expressions are as follows: In the formula, * represents the convolution operation, θ represents the output phase angle of the phase-locked loop, and v(s) represents the frequency-domain expression of the input grid voltage.

3. A method for improving the stability of a grid-connected inverter based on an improved phase-locked loop according to claim 2, characterized in that After adopting the novel phase-locked loop structure, the equivalent transfer function of the phase-locked loop in the reshaped grid-connected inverter is obtained, and the expression of this equivalent transfer function G PLL (s) is as follows: where k p and k i are the proportional coefficient and the differential coefficient of the proportional-integral controller in the phase-locked loop structure respectively, I m is the amplitude of the reference value of the grid-connected current, and U m is the voltage amplitude of the point of common coupling.

4. A method for improving the stability of a grid-connected inverter based on an improved phase-locked loop according to claim 3, characterized in that The equivalent output impedance Z qq_PLL0 (s) of the reshaped grid-connected inverter is expressed as: Where, L1 is the inductor on the inverter side, L2 is the inductor on the grid side, C1 is the filter capacitor, G c (s) is the transfer function of the current controller, H i1 represents the active damping coefficient, G PLL (s) is the equivalent transfer function of the new phase-locked loop, k PWM is the transfer function gain from the modulation signal to the output voltage of the inverter bridge.

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