A phase-locked loop based on a pre-filter

Abstract

The application discloses a phase-locked loop based on a pre-filter and belongs to the field of grid voltage synchronous signal extraction. The application solves the problem of poor phase-locked precision caused by high harmonic content of grid voltage, and improves the ability of the phase-locked loop to track grid voltage synchronous information. The application introduces an improved complex filter (ICCF) before the Clarke transformation, thereby eliminating the harmonic components in the input three-phase grid voltage from flowing into the phase-locked loop control loop at the source, and adding a phase compensation module to offset the phase offset introduced by the improved complex filter (ICCF). The application is mainly used for extracting three-phase grid voltage synchronous signals in a grid-connected inverter.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of grid voltage synchronization signal extraction, and particularly relates to a phase-locked loop based on a pre-filter. BACKGROUND

[0002] With the rapid development of industrial technology, environmental problems and the exhaustion of available fossil fuels need to be solved. In order to solve the above problems, people gradually turn their attention to renewable energy represented by solar energy and wind energy. China's energy system will gradually transform towards the goal of safety, cleanliness, low carbon and high efficiency. Renewable energy power generation, as an important part of achieving the goal of building a sustainable development society, has attracted widespread attention from domestic and foreign scholars.

[0003] Due to the fact that new energy systems are often located in remote areas, long-distance transmission lines, transformer leakage inductance and other reasons cause the grid to exhibit weak characteristics. Under the weak grid, the frequency deviation and voltage flicker of the grid during the grid connection process and the high proportion of power electronic load access in the grid will generate a large amount of harmonics. As a key grid connection control link of the inverter, the harmonic suppression ability of the phase-locked loop determines its phase-locked precision in a high-harmonic grid environment. Therefore, some scholars introduce a moving average filter in the preprocessing link of the phase-locked loop to improve the high-frequency harmonic suppression ability of the system by using its good filtering characteristics. Some scholars also add an adaptive multi-notch filter in the control loop of the phase-locked loop to attenuate the signals in a specific frequency band and effectively suppress specific frequency harmonics.

[0004] In summary, as the core control link of grid operation, the phase-locked loop is crucial to ensure that the output power of the new energy power generation system is efficiently connected to the grid and to ensure the safe and stable operation of the grid. Therefore, it is of great engineering significance to study the phase-locked loop with high harmonic suppression ability. SUMMARY

[0005] In view of the problems existing in the prior art, the purpose of the present application is to provide a phase-locked loop control method based on a pre-filter. The phase-locked loop can suppress harmonic components in the input voltage at the source, to a certain extent, eliminate the influence of grid harmonics on the phase-locked precision, and is suitable for use in a new energy grid-connected inverter.

[0006] To achieve the technical objective of the present application, the following technical solutions are adopted:

[0007] The phase-locked loop based on a pre-filter designed according to the present application is composed of a harmonic elimination module based on an improved complex filter (ICCF), a Clarke transformation module, a Park transformation module, a proportional integral link and an integral link. Among them, the three-phase grid voltage v a , v b , v cAn improved complex filter (ICCF) based harmonic elimination module is connected to the access harmonic elimination module, and the harmonic elimination module outputs three-phase grid voltage v' a , v' b , v' c A Clarke transformation module and a Park transformation module are connected.

[0008] The improved complex filter (ICCF) based harmonic elimination module is used as a phase-locked loop pre-filter, and is composed of two control loops in cross-parallel connection, wherein the gains of the two control loops are a pair of conjugate complex numbers, and the transfer function thereof is obtained according to the control mode as follows:

[0009]

[0010] In the formula, ω0 is a fundamental frequency angle frequency, generally 100π, ω c is a cut-off frequency, j is an imaginary unit, U1 and U2 are input and output voltages of the improved complex filter (ICCF) respectively. The module can suppress high-order harmonic current of the grid voltage flowing into the phase-locked loop control loop, and does not introduce the amplitude deviation of the fundamental frequency grid voltage.

[0011] The improved complex filter (ICCF) is a second-order linear system, and the frequency band width and the stability margin in the phase-locked loop system are considered, and the expression of the cut-off frequency ω c is as follows:

[0012]

[0013] In the formula, ξ is a damping ratio, ω n is a natural angular frequency. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 is a phase-locked loop control structure diagram based on a pre-filter;

[0015] Figure 2 is a structure diagram of the improved complex filter (ICCF) harmonic elimination module;

[0016] Figure 3 is a Bode diagram of the transfer function of the improved complex filter (ICCF) harmonic elimination module. DETAILED DESCRIPTION

[0017] The application will be described in detail below in combination with the drawings and specific embodiments, and the following embodiments can help the developers in the related field to further understand the technical solutions of the application. It should be noted that the embodiments are intended to explain the application, but not to limit the application.

[0018] The application discloses a phase-locked loop based on a pre-filter, which is composed of a harmonic elimination module based on an improved complex filter (ICCF), a Clarke transformation module, a Park transformation module, a proportional integral link and an integral link. Figure 1 It is a control structure diagram of the phase-locked loop based on the pre-filter, wherein θ1 represents an output phase angle of the integral link of the phase-locked loop, θ2 represents a compensation phase angle of the phase compensation module, θ represents a final output phase angle of the phase-locked loop, ω represents an estimated angular frequency of a power grid voltage, and ω0 represents an angular frequency of a fundamental frequency. Three-phase power grid voltages v a 、v b 、v c The three-phase power grid voltage v a 、v′ b 、v′ c The three-phase power grid voltage v a and v β , v a and v β The three-phase power grid voltage v

[0019] The harmonic elimination module based on the improved complex filter (ICCF) is used as the pre-filter of the phase-locked loop and is composed of two control links in cross-parallel connection, wherein the gains of the two control links are a pair of conjugate complex numbers. Figure 2 It is a structure diagram of the harmonic elimination module based on the improved complex filter (ICCF), wherein U1 and U2 are input and output voltages of the improved complex filter (ICCF), and the transfer function of the improved complex filter (ICCF) is obtained according to the control mode and is as follows:

[0020]

[0021] In the formula, ω0 is an angular frequency of a fundamental frequency, generally 100π, ω c is a cut-off frequency, and j is an imaginary unit. The module can suppress high-order harmonic currents of the power grid voltage from flowing into a control loop of the phase-locked loop and does not introduce a fundamental frequency power grid voltage amplitude deviation.

[0022] Figure 3 It is a Bode diagram of the transfer function of the harmonic elimination module based on the improved complex filter (ICCF), and it can be seen that the amplitude at the fundamental frequency 50Hz of the input power grid voltage is not changed, that is, the amplitude deviation of the power grid voltage is not introduced, and the slope is -20dB / dec in a high frequency band, thereby verifying that the phase-locked loop based on the pre-filter disclosed by the application can suppress high-order harmonics in the input power grid voltage.

[0023] The improved complex filter (ICCF) is a second-order linear system, and the frequency band width and the stability margin in the phase-locked loop system are considered, and the cut-off frequency ωc The expression is:

[0024]

[0025] In the formula, ξ is a damping ratio, ω n For the response speed and the phase-locked loop filtering performance of the second-order system, ξ is usually taken as At this time, ω c The optimal design is 181.26.

[0026] In summary, the application discloses a phase-locked loop based on a pre-filter, solves the problem of poor synchronization signal extraction precision when the power grid voltage contains harmonics, ensures that the inverter output signal is synchronized with the power grid in the inverter, and improves the grid-connected stability of the new energy power generation unit.

[0027] Finally, it should be noted that the application is not limited to the above embodiments, and those skilled in the art can appropriately modify the schemes described in the embodiments or replace part of the control methods, which should be included in the protection scope of the application.

Claims

1. A pre-filter based phase-locked loop, characterized by, The phase-locked loop is composed of a harmonic elimination module based on an improved complex filter (ICCF), a Clarke transformation module, a Park transformation module, a proportional integral element, and an integral element, wherein three-phase grid voltage v a , b , c The harmonic elimination module based on the improved complex filter is connected, and the harmonic elimination module outputs three-phase grid voltage v′ a , b , c The Clarke transformation module is connected to output α-axis and β-axis grid voltage v a and v β , a and v β The Park transformation module is connected to output d-axis and q-axis grid voltage; the harmonic elimination module based on the improved complex filter serves as a pre-filter of the phase-locked loop and is composed of two control elements in cross-parallel connection, wherein the gains of the two control elements are a pair of conjugate complex numbers, and according to the control mode, the transfer function is: In the formula, ω0 is the fundamental angular frequency, taking 100π, ω c is the cut-off frequency, j is the imaginary unit, U1 and U2 are the input and output voltages of the improved complex filter respectively, the module can suppress the high-order harmonic current of the power grid voltage from flowing into the phase-locked loop control circuit, and the amplitude offset of the fundamental frequency power grid voltage is not introduced.

2. A pre-filter based phase locked loop as claimed in claim 1, wherein, The improved complex filter is a second-order linear system, which considers both the frequency band width and the stability margin in the phase-locked loop system, and the expression of the cut-off frequency ω c is: where ξ is the damping ratio and ωn is the natural angular frequency. n is the natural angular frequency.

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