Phase Locking Method and System for Improving the Stability of Grid-Connected Systems under Weak Grid Conditions

Abstract

The present invention discloses a phase-locking method and system for improving the stability of a grid-connected system under weak grid conditions, including: acquiring the three-phase AC voltage at the grid connection port; performing coordinate transformation on the three-phase AC voltage to convert the three-phase AC voltage in the stationary coordinate system abc into a d-axis DC voltage vector and a q-axis DC voltage vector in the rotating coordinate system; sending the q-axis DC voltage vector into a phase-locking link to obtain real-time grid phase information, and recording the interruption cycle number when the phase information reaches 2π; when it is detected that the grid undergoes a deep voltage dip, switching to a self-built phase mode, and obtaining a self-built grid phase based on the frequency value before the dip, the interruption cycle number when the phase information reaches 2π, and the current interruption cycle number. The present invention can suppress the large frequency deviation caused by the Uq amplitude fluctuation due to the positive and negative sequence separation of the dq axes during the voltage sag process.

Description

Technical Field

[0001] The present invention relates to the technical field of grid connection of AC systems, and particularly to a phase-locking method and system for improving the stability of a grid-connected system under weak grid conditions. Background Art

[0002] The statements in this part only provide background technical information related to the present invention and do not necessarily constitute prior art.

[0003] With the large-scale application of new energy power electronic devices in the power system and the gradual increase in the penetration rate, the new power system is developing towards the trend of high proportion of new energy and high proportion of power electronic devices. This change trend has changed many characteristics of the traditional power system and will inevitably lead to new stability problems. Under weak grid conditions, the magnitude of the grid impedance, the bandwidths of the phase-locked loop and the voltage loop will all affect the system stability, and the frequency coupling phenomenon affected by these factors will also have an impact on the stability. During the grid voltage fault ride-through period, the transient response requires rapid reactive current support, and the active current also needs to have a relatively fast response speed to maintain the power angle balance. The large and rapid phase shift at the grid connection port caused thereby impacts the phase-locked loop, easily causes large-scale frequency fluctuations, and even instability.

[0004] The prior art analyzed the cause of frequency coupling from the perspective of the asymmetry characteristics of the DC voltage loop and the phase-locked loop, and proposed a set of compensation algorithms, but the algorithms are complex and not easy to be digitally implemented.

[0005] The prior art proposed a method of injecting additional voltage disturbance to offset the voltage disturbance introduced by the phase-locked loop to suppress frequency fluctuations. Its essence is equivalent to introducing a low-pass filter in the phase-locked loop, reducing the sensitivity of the phase-locked loop, while reducing the dynamic response characteristics of the phase-locked loop, and the calculation is complex. Summary of the Invention

[0006] To solve the above problems, the present invention proposes a phase-locking method and system for improving the stability of a grid-connected system under weak grid conditions, which can well suppress frequency offset during the voltage fault ride-through period under weak grid conditions, has high phase-locking accuracy, good dynamic performance, and the algorithm is simple and easy to implement.

[0007] In some embodiments, the following technical solutions are adopted:

[0008] A phase-locking method for improving the stability of a grid-connected system under weak grid conditions, comprising:

[0009] Obtaining the three-phase AC voltage at the grid connection port;

[0010] Perform a coordinate transformation on the three-phase AC voltage to convert the three-phase AC voltage in the stationary coordinate system abc into a DC voltage vector Ud+ on the d-axis and a DC voltage vector Uq+ on the q-axis in the rotating coordinate system;

[0011] Send the DC voltage vector Uq+ on the q-axis into the phase-locked loop to obtain real-time grid phase information, and record the interruption cycle number when the phase information reaches 2π;

[0012] When it is detected that the grid undergoes a deep voltage dip, switch to the self-built phase mode, and based on the frequency value before the dip, the interruption cycle number when the phase information reaches 2π, and the current interruption cycle number, obtain the self-built grid phase.

[0013] When it is detected that the grid undergoes a deep voltage dip, input the real-time DC voltage vector Uq+ on the q-axis into the phase compensation regulator, output a compensation amount Δθ, and add the self-built grid phase to the compensation amount Δθ to obtain the final grid phase information.

[0014] Among them, obtaining the self-built grid phase based on the frequency value before the dip, the interruption cycle number when the phase information reaches 2π, and the current interruption cycle number is specifically as follows:

[0015]

[0016] Among them, waveindex is the current interruption cycle number, waveindex_old is the interruption cycle number when the phase information reaches 2π, ω old is the frequency value before the dip, and T s represents the interruption cycle.

[0017] In some other embodiments, the following technical solution is adopted:

[0018] A phase-locked system for improving the stability of a grid-connected system under weak grid conditions, including:

[0019] A data acquisition module for acquiring the three-phase AC voltage at the grid connection port;

[0020] A coordinate transformation module for performing a coordinate transformation on the three-phase AC voltage to convert the three-phase AC voltage in the stationary coordinate system abc into a DC voltage vector Ud+ on the d-axis and a DC voltage vector Uq+ on the q-axis in the rotating coordinate system;

[0021] A phase-locked control module for sending the DC voltage vector Uq+ on the q-axis into the phase-locked loop to obtain real-time grid phase information, and recording the interruption cycle number when the phase information reaches 2π;

[0022] The self-built power grid phase module is used to switch to the self-built phase mode when it detects a deep voltage dip in the power grid, and obtain the self-built power grid phase based on the frequency value before the dip, the interruption cycle number when the phase information reaches 2π, and the current interruption cycle number.

[0023] The phase compensation module is used to, when detecting a deep voltage dip in the power grid, input the real-time q-axis DC voltage vector Uq+ into the phase compensation regulator, output the compensation amount Δθ, and add the self-built power grid phase to the compensation amount Δθ to obtain the final power grid phase information.

[0024] In some other embodiments, the following technical solution is adopted:

[0025] A terminal device includes a processor and a memory. The processor is used to implement instructions; the memory is used to store multiple instructions, and the instructions are suitable for being loaded and executed by the processor to perform the above-mentioned phase-locking method for improving the stability of the grid-connected system under weak grid conditions.

[0026] In some other embodiments, the following technical solution is adopted:

[0027] A computer-readable storage medium stores multiple instructions, and the instructions are suitable for being loaded and executed by the processor of the terminal device to perform the above-mentioned phase-locking method for improving the stability of the grid-connected system under weak grid conditions.

[0028] Compared with the prior art, the beneficial effects of the present invention are:

[0029] (1) When a deep voltage dip occurs in the power grid, the present invention obtains the self-built power grid phase based on the frequency value before the dip, the interruption cycle number when the phase information reaches 2π, and the current interruption cycle number; it can suppress the large frequency deviation caused by the Uq amplitude fluctuation caused by the positive and negative sequence separation of the dq axes during the voltage sag process.

[0030] (2) Since the change of the AC active current Id in the rotating coordinate system during the fault ride-through process will cause a phase change at the grid connection point, resulting in a small phase deviation, which affects the control accuracy of the active and reactive currents. The present invention performs dynamic phase compensation on the self-built power grid phase, can keep the power grid frequency unchanged, and uses phase closed-loop tracking to compensate for the phase deviation at the grid connection point caused by the change of the active current, improving the phase tracking accuracy.

[0031] (3) The algorithm of the present invention is simple and easy to implement, consumes less computing power resources, and is easy to be deployed and implemented on a digital platform.

[0032] Other features and advantages of the present invention will be partially given in the following description, partially will become obvious from the following description, or will be understood through the practice of this aspect. Description of the Drawings

[0033] Figure 1 Schematic diagram of the process of the phase-locking method for improving the stability of the grid-connected system under weak grid conditions in the embodiment of the present invention;

[0034] Figure 2 Schematic diagram of the traditional phase-locked loop structure;

[0035] Figure 3 Schematic diagram of the frequency offset when the voltage of the traditional phase-locked loop drops deeply;

[0036] Figure 4 Schematic diagram of the frequency offset of the method in the embodiment of the present invention when the voltage drops deeply. Specific implementation manner

[0037] It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present application belongs.

[0038] It should be noted that the terms used herein are only for describing specific implementation manners and are not intended to limit the exemplary implementation manners according to the present application. As used herein, unless the context clearly indicates otherwise, the singular forms are also intended to include the plural forms. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0039] Embodiment 1

[0040] The traditional phase-locked loop structure is as Figure 2 shown. The grid-connected port voltage Upcc is separated into positive and negative sequences, and after park transformation, Uq+ is obtained. The difference is taken with the reference voltage 0 to obtain the phase-locking error; then it is adjusted through a PI regulator, and the fundamental frequency w0 is superimposed to obtain the phase-locking frequency value w. Integrating w can obtain the phase information of the power grid; at the same time, the phase information is sent to the park transformation link for transformation from the rotating coordinate to the stationary coordinate. In this way, a phase-locked loop is formed.

[0041] Figure 3 The schematic diagram of the frequency offset when the voltage of the traditional phase-locked loop drops deeply is given. It can be seen that during the voltage drop process of the traditional phase-locked loop, the frequency has a large range of offset and cannot return to the stable state. The frequency fluctuates and oscillates throughout the process, causing large fluctuations in the positive-sequence voltage Ud and the reactive current Iq, and the crossing is not smooth, which may cause the problem of power angle instability.

[0042] Based on this, in one or more embodiments, a phase-locking method for improving the stability of the grid-connected system under weak grid conditions is disclosed, in combination with Figure 1, which specifically includes the following processes:

[0043] S101: Obtain the three-phase AC voltage at the grid connection port;

[0044] S102: Perform Clark transformation on the three-phase AC voltage to convert the three-phase AC voltages Ua, Ub, and Uc in the stationary coordinate system into Uα and Uβ in the stationary coordinate system. Then, through a second-order generalized integrator (SOGI), perform positive and negative sequence separation to obtain the positive sequence components Uα+ and Uβ+. After Park transformation, convert them into the d-axis DC voltage vector Ud+ and the q-axis DC voltage vector Uq+ in the rotating coordinate system.

[0045] In this embodiment, both the d-axis DC voltage vector Ud+ and the q-axis DC voltage vector Uq+ are positive sequence components.

[0046] S103: Feed the positive sequence component Uq+ of the q-axis voltage vector Uq into the phase-locked loop to obtain the real-time grid phase information.

[0047] In this embodiment, in the phase-locked loop, subtract the q-axis DC voltage vector Uq+ from 0 and send it into a PI regulator, and then superimpose the fundamental angular velocity ω 0 , which is set to 50 Hz here, to obtain the output of the phase-locked loop. Integrate the output frequency to obtain the real-time grid phase information θ pll .

[0048] The specific formula of the PI regulator is as follows:

[0049]

[0050] where G PI is the transfer function of this PI regulator, s represents the Laplace operator, K p is the proportional coefficient, and the value in this embodiment is 300; K i is the integral coefficient, and the value in this embodiment is 300.

[0051] At the same time, record the frequency value w old and the interruption cycle number waveindex_old when the phase information reaches 2π.

[0052] In this embodiment, both w old and waveindex_old are the values 40 ms before the moment when the grid voltage drop is determined.

[0053] S104: When it is detected that the grid undergoes a deep drop, switch to the self-built phase mode, and based on the frequency value before the drop, the interruption cycle number when the phase information reaches 2π, and the current interruption cycle number, obtain the self-built grid phase.

[0054] In this embodiment, when it is detected that the drop depth of the d-axis DC voltage vector Ud+ is lower than 0.2 p.u., it is determined that a deep voltage dip has occurred in the power grid, and the self-built phase mode is switched in; once the self-built phase mode is switched in, it will not be switched out until the d-axis DC voltage vector Ud+ returns to normal.

[0055] In the self-built phase mode, frequency modulation and phase locking are no longer performed through Uq+, but the self-built phase θset is obtained according to the recorded frequency value before the voltage dip, the interruption cycle number waveindex_old when reaching 2π, and the current interruption cycle number waveindex, as follows:

[0056]

[0057]

[0058] where waveindex is the current interruption cycle number, waveindex_old is the interruption cycle number when the phase information reaches 2π, ω old is the frequency value before the voltage dip, and T s represents the interruption cycle.

[0059] At the same time, the phase compensation regulator is started, and the real-time Uq value is sent to the phase compensation regulator. After PI regulation, the compensation amount Δθ is output; the self-built phase θset and the compensation phase Δθ are added together to avoid phase deviation, and the accurate power grid phase θ pll .

[0060] Figure 4 The frequency offset curve during voltage deep dip obtained by using the method of this embodiment is given. It can be seen that the frequency stability is good during the entire crossing process without oscillation fluctuations; therefore, the positive-sequence voltage Ud and the reactive current Iq are both very stable without oscillation fluctuations, and the crossing performance index is good.

[0061] The method of this embodiment can achieve frequency stability well under weak grid conditions through self-built phase, without large frequency deviation caused by the transient process of fault crossing; and precise compensation is carried out through real-time phase compensation to ensure the phase tracking accuracy during the fault crossing process.

[0062] Embodiment 2

[0063] In one or more embodiments, a phase-locked system for improving the stability of a grid-connected system under weak grid conditions is disclosed, specifically including:

[0064] A data acquisition module for acquiring the three-phase AC voltage at the grid connection port;

[0065] A coordinate transformation module, configured to perform coordinate transformation on the three-phase AC voltage, and convert the three-phase AC voltage in the stationary coordinate system abc into a d-axis DC voltage vector Ud+ and a q-axis DC voltage vector Uq+ in the rotating coordinate system;

[0066] A phase-locked control module, configured to send the q-axis DC voltage vector Uq+ into a phase-locked loop to obtain real-time grid phase information, and record the interruption cycle number when the phase information reaches 2π;

[0067] A self-built grid phase module, configured to, when detecting that the grid undergoes a deep voltage dip, switch to the self-built phase mode, and obtain a self-built grid phase based on the frequency value before the dip, the interruption cycle number when the phase information reaches 2π, and the current interruption cycle number.

[0068] A phase compensation module, configured to, when detecting that the grid undergoes a deep voltage dip, input the real-time q-axis DC voltage vector Uq+ into a phase compensation regulator, output a compensation amount Δθ, and add the self-built grid phase and the compensation amount Δθ to obtain the final grid phase information.

[0069] It should be noted that the specific implementation manners of the above modules have been described in Embodiment 1, and will not be elaborated here.

[0070] Embodiment 3

[0071] In one or more embodiments, a terminal device is disclosed, including a server. The server includes a memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the program, it implements the phase-locking method for improving the stability of the grid-connected system under weak grid conditions in Embodiment 1. For the sake of brevity, it will not be elaborated here.

[0072] It should be understood that in this embodiment, the processor may be a central processing unit CPU, and the processor may also be other general-purpose processors, digital signal processors DSP, application-specific integrated circuits ASIC, field-programmable gate arrays FPGA, or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor may be a microprocessor or the processor may also be any conventional processor, etc.

[0073] The memory may include a read-only memory and a random access memory, and provide instructions and data to the processor. A part of the memory may also include a non-volatile random access memory. For example, the memory may also store information about the device type.

[0074] In the implementation process, each step of the above method may be completed by the integrated logic circuit in the hardware of the processor or the instructions in the form of software.

[0075] Embodiment 4

[0076] In one or more embodiments, a computer-readable storage medium is disclosed, in which multiple instructions are stored, and the instructions are adapted to be loaded and executed by a processor of a terminal device to perform the phase-locking method for improving the stability of a grid-connected system under weak grid conditions described in the first embodiment.

[0077] Although the specific embodiments of the present invention are described above in conjunction with the accompanying drawings, it is not a limitation on the protection scope of the present invention. Those skilled in the art should understand that based on the technical solutions of the present invention, various modifications or deformations that can be made by those skilled in the art without creative efforts are still within the protection scope of the present invention.

Claims

1. A phase-locking method for improving the stability of a grid-connected system under weak grid conditions, characterized in that, it includes: Obtain the three-phase AC voltage at the grid connection port; Perform coordinate transformation on the three-phase AC voltage to convert the three-phase AC voltage in the stationary coordinate system abc into the d-axis DC voltage vector Ud+ and the q-axis DC voltage vector Uq+ in the rotating coordinate system; Send the q-axis DC voltage vector Uq+ into the phase-locking link to obtain real-time grid phase information, and record the interruption cycle number when the phase information reaches 2π; When it is detected that the grid undergoes a deep voltage dip, switch to the self-built phase mode, and based on the frequency value before the dip, the interruption cycle number when the phase information reaches 2π, and the current interruption cycle number, obtain the self-built grid phase; The obtaining of the self-built grid phase based on the frequency value before the dip, the interruption cycle number when the phase information reaches 2π, and the current interruption cycle number is specifically: Among them, waveindex is the current interruption cycle number, waveindex_old is the interruption cycle number when the phase information reaches 2π, ω old is the frequency value before the drop, and T s represents the interruption cycle; When it is detected that the grid undergoes a deep voltage dip, input the real-time q-axis DC voltage vector Uq+ into the phase compensation regulator, output the compensation amount Δθ, and add the self-built grid phase to the compensation amount Δθ to obtain the final grid phase information.

2. The phase-locking method for improving the stability of a grid-connected system under weak grid conditions according to claim 1, characterized in that, The process of sending the q-axis DC voltage vector Uq+ into the phase-locking link to obtain real-time grid phase information is as follows: The difference between the q-axis DC voltage vector Uq+ and 0 is sent into a PI regulator, and the output of the PI regulator is superimposed with the fundamental angular velocity w 0 , to obtain the output of the phase-locked loop, and the output is integrated to obtain the real-time grid phase information.

3. The phase-locking method for improving the stability of a grid-connected system under weak grid conditions according to claim 1, characterized in that, Perform clark transformation on the three-phase AC voltage to convert the three-phase AC voltages Ua, Ub, Uc in the stationary coordinate system into Uα, Uβ in the stationary coordinate system, then separate the positive and negative sequences to obtain the positive sequence components Uα+, Uβ+, and through park transformation, convert them into the d-axis DC voltage vector Ud+ and the q-axis DC voltage vector Uq+ in the rotating coordinate system.

4. The phase-locking method for improving the stability of a grid-connected system under weak grid conditions according to claim 1, characterized in that, When it is detected that the drop depth of the d-axis DC voltage vector Ud+ is lower than 0.2 p.u., it is determined that the grid undergoes a deep voltage dip, and switch to the self-built phase mode; once switched to the self-built phase mode, it will not be switched out until the d-axis DC voltage vector Ud+ returns to normal.

5. A phase-locking system for improving the stability of a grid-connected system under weak grid conditions, characterized in that, it includes: A data acquisition module for obtaining the three-phase AC voltage at the grid connection port; A coordinate transformation module for performing coordinate transformation on the three-phase AC voltage to convert the three-phase AC voltage in the stationary coordinate system abc into the d-axis DC voltage vector Ud+ and the q-axis DC voltage vector Uq+ in the rotating coordinate system; A phase-locking control module for sending the q-axis DC voltage vector Uq+ into the phase-locking link to obtain real-time grid phase information, and recording the interruption cycle number when the phase information reaches 2π; A self-built power grid phase module, which is used to switch to the self-built phase mode when it detects a deep voltage dip in the power grid, and obtain the self-built power grid phase based on the frequency value before the dip, the interruption cycle number when the phase information reaches 2π, and the current interruption cycle number; the obtaining of the self-built power grid phase based on the frequency value before the dip, the interruption cycle number when the phase information reaches 2π, and the current interruption cycle number is specifically as follows: Among them, waveindex is the current interruption cycle number, waveindex_old is the interruption cycle number when the phase information reaches 2π, ω old is the frequency value before the drop, and T s represents the interruption cycle; A phase compensation module, which is used to input the real-time q-axis DC voltage vector Uq+ into the phase compensation regulator when it detects a deep voltage dip in the power grid, output a compensation amount Δθ, and add the self-built power grid phase to the compensation amount Δθ to obtain the final power grid phase information.

6. A terminal device, which includes a processor and a memory. The processor is used to implement instructions; the memory is used to store multiple instructions, characterized in that, the instructions are suitable for being loaded and executed by the processor to perform the phase-locking method for improving the stability of the grid-connected system under weak grid conditions according to any one of claims 1-4.

7. A computer-readable storage medium, in which multiple instructions are stored, characterized in that, the instructions are suitable for being loaded and executed by the processor of the terminal device to perform the phase-locking method for improving the stability of the grid-connected system under weak grid conditions according to any one of claims 1-4.

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