Self-adaptive control method of grid-connected converter

Abstract

The adaptive control method of the grid-connected converter solves the problem of subsynchronous oscillation of the grid-connected converter caused by a phase-locked loop synchronous control method, and belongs to the technical field of grid-connected control of new energy converters. The method comprises the following steps: establishing an output power small signal component equation of the grid-connected converter according to the relationship between the power of the grid-connected converter and a synchronization angle, and obtaining an expression of a direct-current voltage synchronization proportionality coefficient according to the equation: detecting and extracting broadband oscillation characteristic quantity of the grid-connected converter on line, and identifying line impedance when subsynchronous oscillation is determined to be generated, and in combination with the expression and the calculated value, determining a phase-locked loop synchronization link proportionality coefficient, and substituting the proportionality coefficient into a synchronization control link to obtain an actual grid-connected angle of the synchronization link to participate in control of the grid-connected converter. According to the invention, main parameters of the controller are optimized, a self-adaptive control framework is constructed, and the stability of the grid-connected converter under a weak power grid short-circuit ratio change working condition is improved.

Description

Technical Field

[0001] This application relates to an adaptive control method for grid-connected converters, belonging to the field of grid-connected control technology for new energy converters. Background Technology

[0002] New energy generating units are generally connected to the grid via power electronic converters. These devices are responsible for converting the DC power or frequency-converted AC power generated by new energy sources into stable AC power synchronized with the grid, serving as the core interface for energy transmission and system stability. However, grid-connected converters can cause subsynchronous oscillations, primarily due to unfavorable negative damping interactions between the phase-locked loop (PLL) synchronization stage and the weak grid impedance in the subsynchronous frequency band. Subsynchronous oscillations not only threaten the stable operation of the converter itself and the new energy generating units, but can also lead to electrical equipment damage or even cascading grid disconnection accidents in severe cases, making them a key factor restricting the stability of high-proportion new energy grid connections to weak grids. Therefore, proposing stable and effective suppression strategies for subsynchronous oscillations caused by grid-connected converters in weak grid environments is crucial for ensuring the safe and reliable operation of new power systems.

[0003] Currently, methods for suppressing subsynchronous oscillations in grid-connected converters can be broadly categorized into two directions: First, the control parameter design approach. This method, based on the converter's own topology or control structure, enhances damping in specific frequency bands by adjusting key parameters of the controller's current loop and phase-locked loop (such as proportional-integral coefficients and filter time constants). Its advantage lies in its relatively simple implementation, but the improvement is limited by the system's inherent stability margin and dynamic performance, and may require readjustment under different operating conditions, thus limiting its adaptability. Second, the control system impedance reshaping approach. This method primarily targets reshaping the equivalent output impedance characteristics of the grid-connected converter in the subsynchronous frequency band, for example, by adding positive resistance or damping to suppress oscillations. Common techniques include introducing additional damping control loops or specific filters. While impedance reshaping can be more precise, its control structure is relatively complex, increasing the difficulty of design and implementation. Summary of the Invention

[0004] To address the problem of subsynchronous oscillations in grid-connected converters caused by phase-locked loop synchronous control methods, this application provides an adaptive control method for grid-connected converters.

[0005] This application discloses an adaptive control method for a grid-connected converter, comprising:

[0006] S1. Based on the relationship between the power of the grid-connected converter and the synchronization angle, establish the small-signal component equation of the output power of the grid-connected converter, and obtain the DC voltage synchronization proportional coefficient from this equation. The expression;

[0007] S2. Online detection and extraction of wideband oscillation characteristics of the grid-connected converter. When subsynchronous oscillation is detected, the system line impedance is identified based on harmonic information. Combined with DC voltage synchronization proportional coefficient The expression is calculated. The value;

[0008] S3. Acquire the DC-side voltage deviation signal of the grid-connected converter, combined with... The value is used to determine the small-signal component of the angle containing DC dynamic characteristics in the synchronization link. ;

[0009] S4, according to Determine the proportional coefficient of the phase-locked loop synchronization element. Collect the output voltage signal of the three-phase grid-connected converter, and combine it with Determine the small signal component of the phase-locked loop angle in the synchronization stage. ;

[0010] S5. Based on the grid connection angle reference value, and combined with the small-signal component of the angle containing DC dynamic characteristics and the small-signal component of the phase-locked loop angle in the synchronization link, the actual grid connection angle of the synchronization link is obtained, and the grid-connected converter is controlled according to the actual grid connection angle.

[0011] As a preferred option, the small-signal component equation for the output power of the grid-connected converter is:

[0012]

[0013]

[0014] In the formula, This indicates the small-signal component of the grid-connected converter's output power. This represents the virtual inertia provided by the control system. Indicates the reference value for grid connection angle. Indicates the system damping coefficient; Represents the system synchronization coefficient. Let s denote the time constant in a first-order inertial element, and s denote the Laplace operator. , Indicates the DC-side capacitor. This represents the steady-state value of the DC-side voltage. Indicates the converter output voltage. This represents the filter inductor.

[0015] As a preferred option, the DC voltage synchronization proportional coefficient for:

[0016]

[0017] in, , Indicates the system damping ratio. It represents the natural angular frequency of oscillation.

[0018] As a preferred method, the broadband oscillation characteristics of the grid-connected converter are detected and extracted online based on the recursive discrete Fourier transform method, and the line impedance is calculated based on the harmonic information. .

[0019] As a preferred option, line impedance for:

[0020]

[0021] in, Indicates the amplitude of current harmonics. Indicates the voltage harmonic amplitude. Indicates the harmonic frequency.

[0022] As a preferred option, the phase-locked loop angle small signal component for:

[0023]

[0024] in, This indicates the output voltage signal of the three-phase grid-connected converter. The actual value of the q-axis component. This represents the proportional parameter of the phase-locked loop PI control circuit. This represents the integral parameter of the phase-locked loop PI control loop.

[0025] As a preferred option, the small-signal component containing DC dynamic characteristics is used. for:

[0026]

[0027] in, This represents the actual value of the DC-side voltage.

[0028] The beneficial effects of this application are as follows: Addressing the subsynchronous oscillation problem caused by grid-connected converters under varying short-circuit ratios in weak power grids, this application collects the DC-side voltage deviation signal of the grid-connected converter, generates a DC voltage synchronization angle component through a first-order inertial element, and synthesizes it with the output angle of the phase-locked loop to obtain the actual grid-connected angle command. This method innovatively integrates online impedance identification technology with a parameter design method based on damping characteristics, constructing a comprehensive closed-loop update mechanism encompassing dynamic impedance detection, adaptive parameter adjustment, and stability-enhancing control, achieving adaptive optimization of control parameters. Compared to traditional fixed-parameter design methods, this application significantly improves the system's adaptability and stability under varying short-circuit ratios in weak power grids. Attached Figure Description

[0029] Figure 1 This is a system control structure diagram of the adaptive control method for the grid-connected converter in this application;

[0030] Figure 2 The control structure diagram for the angle compensation method of the synchronization link;

[0031] Figure 3 This is a schematic diagram of the adaptive control method.

[0032] Figure 4 A diagram showing the power grid impedance identification results for the waveform recorded by the host computer;

[0033] Figure 5 Experimental diagram showing the effect of suppressing subsynchronous oscillations in the control system. Detailed Implementation

[0034] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0035] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other.

[0036] The present application will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the scope of the application.

[0037] The adaptive control method for grid-connected converters in this embodiment, such as Figure 1 and Figure 2 As shown, this embodiment detects and extracts broadband oscillation characteristics of the grid-connected converter online, calculates the actual grid impedance parameters based on harmonic information, and realizes online grid impedance identification technology. Combining the online impedance identification results with a parameter design method based on damping characteristics, the main parameters of the controller are optimized, an adaptive control architecture is constructed, and the stability of the grid-connected converter under conditions of varying short-circuit ratios in weak grids is improved. Specifically, this includes:

[0038] Step 1: Based on the relationship between the power of the grid-connected converter and the synchronization angle, establish the small-signal component equation of the output power of the grid-connected converter, and obtain the DC voltage synchronization proportional coefficient based on this equation. The expression:

[0039] The DC-side voltage deviation signal of the grid-connected converter is collected and processed by a first-order inertial element to obtain a synchronous angle component with DC dynamic characteristics, which is used as the angle compensation amount of the grid-connected converter.

[0040] (1)

[0041] In the above formula, This represents the small signal component of the DC voltage synchronization angle obtained after processing the DC-side voltage deviation signal through a first-order inertial element. and These represent the actual and steady-state values ​​of the DC-side voltage, respectively. This represents the proportional coefficient of the DC voltage synchronization circuit. Let represent the time constant in a first-order inertial element, and s represent the Laplace operator.

[0042] The small signal component of the DC voltage synchronization angle is linearly superimposed with the small signal component of the phase-locked loop output angle using a proportional coefficient to generate the actual angle command of the grid-connected converter control system, thus realizing the angle compensation method of the synchronization link.

[0043] The small-signal expression for the phase-locked loop output angle in a grid-connected converter control system after passing through a proportional element is as follows:

[0044] (2)

[0045] In the above formula, This represents the small signal component of the phase-locked loop output angle. This indicates the output voltage signal of the three-phase grid-connected converter. The actual value of the q-axis component. This represents the proportional parameter of the phase-locked loop PI control circuit. This represents the integral parameter of the phase-locked loop PI control loop. This represents the newly added proportional coefficient in the phase-locked loop synchronization stage.

[0046] Let the DC voltage synchronization proportional coefficient If the sum of the proportional coefficients of the phase-locked loop synchronization elements is 1, then:

[0047] (3)

[0048] DC voltage synchronization angle small signal component Small signal component of phase-locked loop output angle After linear superposition, the actual angle small signal component of the grid-connected converter control system is obtained. .

[0049] (4)

[0050] The above steps enable the implementation of an angle compensation method for the synchronization process.

[0051] Step 2: Detect and extract the broadband oscillation characteristics of the grid-connected converter online. When subsynchronous oscillation is determined, identify the line impedance based on harmonic information. :

[0052] The dynamic equation for the DC bus capacitor of a three-phase current source grid-connected converter can be expressed as:

[0053] (5)

[0054] In the formula, This indicates the DC-side capacitance value. , This indicates the system's input and output power.

[0055] If power loss is ignored, then the converter output power in equation (5) is... for:

[0056] (6)

[0057] In the formula, Indicates the converter output voltage. Indicates the filter inductance. This represents the difference between the converter output angle and the grid angle.

[0058] Combining equations (1)-(3) and equation (5), and after harmonic linearization, the small-signal equation relationship between converter power and synchronization angle can be obtained as follows:

[0059] (7)

[0060] Substituting equation (6) into equation (7), we can obtain the small-signal equation of the second-order equivalent model of the system:

[0061] (8)

[0062] (9)

[0063] In the above formula, This represents the small-signal component of the converter's output power. This represents the virtual inertia provided by the control system. This represents the reference value for the grid connection angle. This represents the system damping coefficient, which has a suppressive effect on oscillations; This represents the system synchronization coefficient, which helps the system regain stability.

[0064] Its characteristic equation can be expressed as:

[0065] (10)

[0066] To analyze the dynamic characteristics of a control system, a natural oscillation frequency can be defined. and the two key parameters of system damping ratio Characteristic roots of the characteristic equation:

[0067] (11)

[0068] (12)

[0069] In the above formula, Indicates the system damping ratio. The values ​​of ω and ω represent the natural oscillation angular frequency, and can be selected based on the system overshoot and the required settling time.

[0070] In equation (12) Online grid impedance identification is required. The actual operating conditions of grid-connected converter control systems under weak grid conditions are quite complex, making online impedance detection technology based on disturbance-free injection a better choice. To achieve grid impedance identification, it is first necessary to detect the harmonic components in the converter. This involves collecting the three-phase output voltage, three-phase output current, and DC-side voltage of the grid-connected current transformer, and then using the Recursive Discrete Fourier Transform (RDFT) to detect the harmonic components in the converter. The specific implementation method is as follows.

[0071] Assume a signal with an arbitrary period of T Let M represent the number of sampling points in one period, then the discrete value of the signal at any given time is... It can be represented as the sum of all frequency components, where n takes values ​​in the range of 0, 1, 2, …, N-1.

[0072] (13)

[0073] (14)

[0074] In the above formula, Indicates the fundamental angular frequency. , express The amplitudes of the sine and cosine components of the i-th harmonic; This represents the sampling time interval of the algorithm, and j represents the imaginary unit.

[0075] Based on equations (13) and (14), the following iterative relationship can be derived:

[0076] (15)

[0077] In the formula, Indicates the latest sampling point. , This indicates the amplitude of the latest harmonic sine and cosine components.

[0078] According to equation (15), the amplitudes of the i-th voltage and current harmonics of the control system can be expressed as follows:

[0079] (16)

[0080] In the formula, Indicates the voltage harmonic amplitude. Indicates the amplitude of current harmonics. This represents the amplitude of the sinusoidal component of the i-th harmonic of the voltage. This represents the amplitude of the i-th harmonic cosine component of the voltage. This represents the amplitude of the sinusoidal component of the i-th harmonic of the current. This represents the amplitude of the i-th harmonic cosine component of the current.

[0081] The formula for calculating actual power grid impedance is as follows:

[0082] (17)

[0083] In the formula, Indicates the inductance of the power grid. Indicates the harmonic frequency.

[0084] The RDFT detection method has high accuracy and speed. It only needs to be initialized when the device is started. The voltage and current data collected by the above iterative equation can be calculated once in each control cycle in the actual controller chip. At the end of each sliding window cycle, the corresponding amplitude signal of the harmonic can be calculated and the actual value of the grid impedance can be output.

[0085] Substituting equation (17) into equation (12), we get:

[0086] (18)

[0087] Under weak grid conditions, the grid impedance exhibits time-varying characteristics. If the relevant parameters in the control strategy are designed as fixed values, it is difficult to effectively adapt to the dynamic changes in grid impedance, thus leading to a decrease in system robustness and an inability to effectively suppress the subsynchronous oscillations generated by the system. Therefore, for the short-circuit ratio variation condition under weak grid conditions, the RDFT detection method can identify the harmonic components of the system output voltage and current, and then calculate the real-time grid impedance data. Substituting this data into equation (18) will yield the main system parameter, the DC voltage synchronization ratio coefficient. The value; This implementation uses a parameter design method based on damping characteristics to optimize the main parameters of the controller, construct an adaptive control architecture, and improve the stability of the grid-connected converter under the condition of changing short-circuit ratio in a weak power grid.

[0088] Step 3: Acquire the DC-side voltage deviation signal of the grid-connected converter. Then, according to equation (1), we get ;

[0089] Step 4: Calculate the DC voltage synchronization proportional coefficient. Next, the output voltage signal of the three-phase grid-connected converter is acquired. Then, combining with formula (3), the proportional coefficient of the phase-locked loop synchronization element is obtained. The value of the small signal component of the phase-locked loop angle in the synchronization link is obtained according to equation (2). ;

[0090] Step 5, Reference Figure 2 Based on the grid connection angle reference value, and combining the small-signal angle components containing DC dynamic characteristics and the small-signal angle components of the phase-locked loop in the synchronization link, the actual grid connection angle of the synchronization link is obtained. ; Figure 2 In this context, uq represents the q-axis component of the output voltage. This represents the small signal component of the phase-locked loop angle in the synchronization circuit. θ0 represents the angular frequency reference value, and θ0 represents the angular reference value. K represents the small-angle signal component containing DC dynamic characteristics in the synchronization link. pll K dc λ represents the proportionality coefficient, λ represents the time constant of the first-order inertial element, and θ represents the actual output angle of the synchronization element.

[0091] refer to Figure 1 The actual grid connection angle through the synchronization link Participate in the closed-loop control of the system. Figure 1 middle, I dc C represents the DC side current. dc Indicates the DC-side capacitance, u dc e represents the DC side voltage. abc U represents the inverter output voltage. abc Indicates the grid-connected voltage, I abc L represents the grid-connected current. f L represents the filter inductance. g U represents the line impedance. g V represents the line impedance. dcref Indicates the DC voltage setpoint, u dc I represents the actual value of the DC voltage. dq E represents the dq-axis component of the output current. dq U represents the dq-axis component of the output voltage, θ represents the actual angle of the synchronization element, and U represents the dq-axis component of the output voltage. dc0The red box indicates that the control system identifies the grid impedance online, and then sends the impedance identification results to the adaptive control method for parameter calculation and outputs the actual grid connection angle through the synchronization link.

[0092] refer to Figure 3 The adaptive control strategy of this implementation first requires collecting the three-phase output voltage, three-phase output current, and DC side voltage of the grid-connected converter. Then, the RDFT detection method is run to calculate the harmonic amplitude of the three-phase output voltage and current according to equation (16). When the control system experiences subsynchronous oscillation, it is judged. If subsynchronous oscillation occurs, the next step is performed; otherwise, the system operates normally. If the next step is performed, the corresponding grid impedance value needs to be calculated in real time according to equation (17). Then, the main parameters of the control system are obtained according to equations (3) and (18). The parameters are substituted into the synchronous control link to output the actual grid connection angle and realize the adaptive control method.

[0093] refer to Figure 4 This figure represents the experimental verification of the online identification technology of power grid impedance in this application. The figure was generated by the upper computer recording in the experimental platform. The recording lasted for 2.5 seconds and showed that the power grid impedance was 18.5mH, which has high accuracy.

[0094] refer to Figure 5 The figure shows the experimental verification of the adaptive control method proposed in this application to improve the stability of grid-connected converters under weak grid conditions. As can be seen from the figure, under weak grid conditions, when the control system generates subsynchronous oscillations, the proposed adaptive control method can detect and respond promptly, suppressing the oscillations in a short time, demonstrating good dynamic performance and improving the stability of the grid-connected converter control system under weak grid conditions.

[0095] While this application has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of this application. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of this application as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. An adaptive control method for grid-connected converters, characterized in that, include: S1. Based on the relationship between the power of the grid-connected converter and the synchronization angle, establish the small-signal component equation of the output power of the grid-connected converter, and obtain the DC voltage synchronization proportional coefficient from this equation. The expression; S2. Online detection and extraction of wideband oscillation characteristics of the grid-connected converter. When subsynchronous oscillation is detected, the system line impedance is identified based on harmonic information. Combined with DC voltage synchronization proportional coefficient The expression is calculated. The value; S3. Acquire the DC-side voltage deviation signal of the grid-connected converter, combined with... The value is used to determine the small-signal component of the angle containing DC dynamic characteristics in the synchronization link. ; S4, according to Determine the proportional coefficient of the phase-locked loop synchronization element. Collect the output voltage signal of the three-phase grid-connected converter, and combine it with Determine the small signal component of the phase-locked loop angle in the synchronization stage. ; S5. Based on the grid connection angle reference value, and combined with the small-signal component of the angle containing DC dynamic characteristics and the small-signal component of the phase-locked loop angle in the synchronization link, the actual grid connection angle of the synchronization link is obtained, and the grid-connected converter is controlled according to the actual grid connection angle.

2. The adaptive control method for a grid-connected converter according to claim 1, characterized in that, The small-signal component equation for the output power of the grid-connected converter is: In the formula, This indicates the small-signal component of the grid-connected converter's output power. This represents the virtual inertia provided by the control system. Indicates the reference value for grid connection angle. Indicates the system damping coefficient; Represents the system synchronization coefficient. Let s denote the time constant in a first-order inertial element, and s denote the Laplace operator. , Indicates the DC-side capacitor. This represents the steady-state value of the DC-side voltage. Indicates the converter output voltage. This represents the filter inductor.

3. The adaptive control method for a grid-connected converter according to claim 2, characterized in that, DC voltage synchronization proportional coefficient for: in, , Indicates the system damping ratio. It represents the natural angular frequency of oscillation.

4. The adaptive control method for a grid-connected converter according to claim 3, characterized in that, This paper utilizes a recursive discrete Fourier transform method to detect and extract broadband oscillation characteristics of grid-connected converters online, and calculates line impedance based on harmonic information. .

5. The adaptive control method for a grid-connected converter according to claim 4, characterized in that, Line impedance for: in, Indicates the amplitude of current harmonics. Indicates the voltage harmonic amplitude. Indicates the harmonic frequency.

6. The adaptive control method for a grid-connected converter according to claim 2, characterized in that, Phase-locked loop angle small signal component for: in, This indicates the output voltage signal of the three-phase grid-connected converter. The actual value of the q-axis component. This represents the proportional parameter of the phase-locked loop PI control circuit. This represents the integral parameter of the phase-locked loop PI control loop.

7. The adaptive control method for a grid-connected converter according to claim 2, characterized in that, Small-signal component containing DC dynamic characteristics for: in, This represents the actual value of the DC-side voltage.

8. A computer-readable storage device storing a computer program, characterized in that, When the computer program is executed by the processor, it implements the steps of the adaptive control method for the grid-connected converter as described in any one of claims 1 to 7.

9. An adaptive control device for a grid-connected converter, comprising a storage device, a processor, and a computer program stored in the storage device and executable on the processor, characterized in that, The processor executes the computer program to implement the steps of the adaptive control method for the grid-connected converter as described in any one of claims 1 to 7.

10. A computer program product, comprising a computer program, characterized in that, When the computer program is executed by the processor, it implements the steps of the adaptive control method for the grid-connected converter as described in any one of claims 1 to 7.

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