Virtual synchronous generator-based pre-synchronization control method for network-forming converter

Abstract

The invention discloses a pre-synchronization control method for a network-building converter based on a virtual synchronous generator, and the method comprises the steps: collecting the three-phase voltage of the output end of the virtual synchronous generator VSG and the three-phase voltage of a power grid side, converting the three-phase voltage into a voltage component in a two-phase static coordinate system through Clark conversion, and extracting a positive-sequence component; calculating a phase difference compensation coefficient based on the positive sequence component; based on the phase difference compensation coefficient, adaptively setting a PI parameter; and calculating and adjusting the amplitude of the internal potential to finish pre-synchronization. In the working process, through self-adaptive PI parameter setting and amplitude accurate judgment, the pre-synchronization time is shortened, the grid-connected impact current is reduced, and the grid-connected success rate of the micro-grid in a weak grid scene is improved.

Description

Technical Field

[0001] This invention relates to the field of grid-connected control technology, and in particular to a pre-synchronization control method for grid-connected converters based on virtual synchronous generators. Background Technology

[0002] Isolated microgrids have attracted much attention due to their ability to locally absorb renewable energy and ensure reliable power supply in remote areas. In isolated microgrids, Virtual Synchronous Generator (VSG) technology significantly improves the grid support capability of power electronic interface inverters by simulating the inertia and damping characteristics of traditional synchronous generators, enabling them to perform grid connection, frequency regulation, and voltage regulation functions, thereby enhancing the stability and reliability of the microgrid.

[0003] Before being connected to a microgrid, VSGs require pre-synchronization control to ensure that their output voltage is consistent with the grid voltage in amplitude, frequency, and phase. This prevents inrush currents caused by voltage differences during grid connection, which could damage power electronic equipment. Traditional pre-synchronization control strategies typically employ phase difference control, which relies on a phase-locked loop (PLL) to detect the grid voltage phase in real time and then uses a PI controller to adjust the VSG's output phase to synchronize it with the grid.

[0004] However, this traditional method suffers from poor stability due to the introduction of a phase-locked loop (PLL). Specifically, as a nonlinear control system, the dynamic response characteristics of a PLL are closely related to the grid strength. In weak grid environments, the PLL is prone to coupling with grid impedance, leading to low-frequency oscillations or even instability. Furthermore, the introduction of a PLL increases the complexity of the control system and reduces its robustness. Summary of the Invention

[0005] To address the above problems, this invention provides a pre-synchronization control method for grid-connected converters based on virtual synchronous generators, which can improve the dynamic response performance and grid-connected stability of the system without the need for a PLL.

[0006] The technical solution of this invention is: a pre-synchronization control method for a grid-connected converter based on a virtual synchronous generator, comprising the following steps: S1. Collect the three-phase voltage at the output terminal of the virtual synchronous generator (VSG) and the three-phase voltage on the grid side, and convert them into voltage components in a two-phase stationary coordinate system through Clark transformation, and then extract the positive sequence components. S2. Calculate the phase difference compensation coefficient based on the positive sequence component; S3. Adaptively tune the PI parameters based on the phase difference compensation coefficient; S4. Calculate and adjust the internal potential amplitude to complete the pre-synchronization. In step S1, the three-phase voltage at the output terminal of the VSG is collected. and the three-phase voltage on the grid side The Clark transformation is used to convert it into voltage components in a two-phase stationary coordinate system. and Specifically: ; In the formula, For the microgrid side voltage space vector in Components on the axis, For the microgrid side voltage space vector in Components on the axis, This refers to the phase A voltage on the microgrid side. This refers to the B-phase voltage on the microgrid side. This refers to the C-phase voltage on the microgrid side.

[0007] The positive-sequence voltage component is extracted using the quarter-cycle power grid delay method, and the positive-sequence component is extracted according to the following rules: ; ; In the formula, For the grid voltage space vector in Components on the axis, For the grid voltage space vector in Components on the axis, , The output voltage of VSG is respectively at and Positive-sequence components on the axis, , The grid voltage is respectively at and Positive sequence component on the axis; T is the grid voltage period. This represents a historical sampling time that is one-quarter of a power grid cycle earlier than the current sampling time t.

[0008] In step S2, the phase difference compensation coefficient is calculated. for: .

[0009] In step S3, based on the phase difference compensation coefficient, the two intervals are divided and the proportional coefficient is adjusted, specifically as follows: When the phase difference compensation coefficient The absolute value lies in the interval At that time, the scaling factor is coarsely adjusted, including: ; In the formula, This is the proportional coefficient after coarse adjustment. This represents the maximum scaling factor for the coarse adjustment range; When the phase difference compensation coefficient The absolute value lies in the interval At that time, the scaling factor is fine-tuned, including: ; In the formula, This is the adjusted scaling factor. This is the benchmark ratio coefficient for fine-tuning the interval.

[0010] Step S3 also includes compensation based on the initial phase difference coefficient. Select the integral coefficients Specifically: like ,but , The value range is 1 to 5; like ,but , The value range is 0.01 to 0.1.

[0011] Step S4 includes: Set the initial value of the internal potential amplitude ; The internal potential amplitude is calculated as follows: ; In the formula, The magnitude of the internal potential. The initial value of the internal potential amplitude, For the disturbance amplitude, For the perturbation frequency, t It is a continuous variable that changes with time for the disturbance signal.

[0012] Step S4 also includes: Calculate the amplitude of the fluctuation component Continuously compare the amplitude of the fluctuation components With respect to preset tolerance It continuously corrects the internal potential until it approaches the voltage amplitude of the power grid.

[0013] Calculate the amplitude of the fluctuation component ,include: The reactive power Q at the grid connection point is monitored and calculated synchronously, and a bandpass filter is used to extract the reactive power caused by the injected disturbance. Directly excited reactive power fluctuation component at the same frequency And calculate the amplitude of the component. Specifically: ; In the formula, , for Components in a two-phase stationary coordinate system.

[0014] like If the current VSG internal potential setting is not equal to the grid voltage amplitude, the VSG internal potential is adjusted. The specific adjustment formula is as follows: ; In the formula, For the first The internal potential amplitude after the second adjustment; For the first The amplitude of the secondary internal potential; This is the step size coefficient.

[0015] like If the voltage amplitude of the VSG is found to be consistent with the voltage amplitude of the grid, the internal voltage adjustment is stopped.

[0016] In operation, this invention utilizes adaptive... Precise parameter tuning and amplitude determination shorten the pre-synchronization time, reduce grid connection inrush current, and improve the grid connection success rate of microgrids in weak grid scenarios. Attached Figure Description

[0017] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0018] The present invention is as follows Figure 1 As shown, a pre-synchronization control method for a grid-connected converter based on a virtual synchronous generator includes the following steps: S1. Collect the three-phase voltage at the output terminal of the virtual synchronous generator (VSG) and the three-phase voltage on the grid side, and convert them into voltage components in a two-phase stationary coordinate system through Clark transformation, and then extract the positive sequence components. S2. Calculate the phase difference compensation coefficient based on the positive sequence component; S3. Adaptively tune the PI parameters based on the phase difference compensation coefficient; S4. Calculate and adjust the internal potential amplitude to complete the pre-synchronization. This method uses adaptive... Precise parameter tuning and amplitude determination shorten the pre-synchronization time, reduce grid connection inrush current, and improve the grid connection success rate of microgrids in weak grid scenarios.

[0019] In step S1, the three-phase voltage at the output terminal of the VSG is collected. and the three-phase voltage on the grid side The Clark transformation is used to convert it into voltage components in a two-phase stationary coordinate system. and Specifically: ; In the formula, For the microgrid side voltage space vector in Components on the axis, For the microgrid side voltage space vector in Components on the axis, This refers to the phase A voltage on the microgrid side. This refers to the B-phase voltage on the microgrid side. This refers to the C-phase voltage on the microgrid side.

[0020] The positive sequence voltage component is extracted using the quarter-cycle delay method: This method is used to extract the VSG output voltage in the two-phase stationary coordinate system at the current sampling time t. , With grid-side voltage , Extract the orthogonal components according to the following rules: ; In the formula, For the grid voltage space vector in Components on the axis, For the grid voltage space vector in Components on the axis, , The output voltage of VSG is respectively at and Positive-sequence components on the axis, , The grid voltage is respectively at and Positive sequence component on the axis; T is the grid voltage period, and the grid frequency is... , , This represents a historical sampling time that is one-quarter of a power grid cycle earlier than the current sampling time t.

[0021] In step S2, the phase difference compensation coefficient is calculated. for: .

[0022] The conventional method for calculating the phase difference compensation coefficient is as follows: Phase-locked loop (PLL) devices are configured on both the microgrid and grid sides; the amplitude, phase, and frequency information of the voltages on both sides are acquired and tracked in real time using the PLLs; and the phase of the microgrid-side voltage is extracted from the PLL output. Phase with grid-side voltage By calculating the difference between the two The phase-locked loop (PLL) is used to obtain the real-time phase difference. This phase difference is then input into a closed-loop control system, such as a proportional-integral (PI) controller. After adjustment and calculation by the controller, the phase difference compensation coefficient is finally obtained. However, the use of PLLs can easily lead to frequency instability and grid-connected power spikes. Moreover, it is susceptible to voltage fluctuations when operating in a weak grid environment, which can cause loss of lockout or exacerbate system oscillations. In addition, phase jumps can easily occur during microgrid grid-connected / off-grid switching, causing frequency fluctuations and inrush currents, reducing system robustness.

[0023] This invention performs pre-synchronization grid connection without using a phase-locked loop (PLL). Synchronization grid connection is achieved by generating a phase difference compensation coefficient through simple mathematical calculations, which effectively avoids the disadvantages caused by using a PLL.

[0024] This invention proposes a method for calculating the phase difference compensation coefficient, based on the microgrid voltage and grid voltage obtained in step S1. Calculate the phase difference compensation coefficient for components on the axis. .

[0025] For the operating condition where both the microgrid and grid voltage vectors are in the first quadrant, firstly... Two-phase stationary coordinate transformation decoupling yields the grid voltage vector in The positive sequence component of the axis projection is and The microgrid voltage vector is The positive sequence component of the axis projection is and .

[0026] At this point, regardless of whether the voltage vectors of the microgrid and the grid exhibit a leading or lagging phase relationship, the two voltage vectors are... The projection onto the axis will inevitably produce a difference, as shown in the following formula: ; but and The sign of the difference changes with the phase relationship between the two voltages. Therefore, to eliminate the interference of the difference in the sign of the difference on the calculation of the compensation coefficient, this invention completes voltage information acquisition and coordinate transformation in step S1, and utilizes the voltage at both ends in... The corresponding real-time difference is calculated from the axis projection components. , Then, both are squared separately to eliminate the influence of the sign.

[0027] Finally, by summing the two squared terms, the phase difference compensation coefficient, which characterizes the degree of phase deviation, is obtained. The details are as follows: ; The phase difference compensation coefficient is fed into a first-order filter, and the filtered output signal is used as the input of the PI controller to finally generate a phase compensation signal for the active frequency modulation stage.

[0028] In step S3, based on the phase difference compensation coefficient, the two intervals are divided and the proportional coefficient is adjusted, specifically as follows: When the phase difference compensation coefficient The absolute value lies in the interval At that time, the scaling factor is coarsely adjusted, including: ; In the formula, This is the proportional coefficient after coarse adjustment. This represents the maximum scaling factor for the coarse adjustment range; its value ranges from 10 to 30. It is the hyperbolic tangent function.

[0029] When the phase difference compensation coefficient The absolute value lies in the interval At that time, the scaling factor is fine-tuned, including: ; In the formula, This is the adjusted scaling factor. This is the benchmark ratio coefficient for fine-tuning the interval, with a value range of 0.5 to 5.

[0030] In existing technologies, traditional pre-synchronization controllers typically employ fixed PI parameters. However, during the pre-synchronization process, the system's dynamic characteristics change significantly: large phase differences need to be quickly eliminated in the initial stage of synchronization, while fine adjustments are required at the end of the synchronization process to avoid overshoot and oscillation. Fixed-parameter PI controllers struggle to balance response speed and stability throughout the entire process, resulting in either slow synchronization or the risk of frequency overshooting or oscillation near synchronization.

[0031] Based on the calculated phase difference compensation coefficient This step provides an adaptive adjustment strategy for the proportional (P) and integral (I) parameters based on the phase difference interval division.

[0032] The difference between this invention and existing technologies lies in the proposal of a dual adjustment mechanism based on high-precision phase difference interval division and initial state adaptation. Its core innovations are as follows: 1. Refined dual-interval division and proportional coefficient ( Adaptive Strategy: This invention is not a simple multi-segment switching, but rather relies on the phase difference compensation coefficient. The values ​​are precisely divided into "coarse adjustment zone" and "fine adjustment zone".

[0033] Coarse adjustment region: when the phase difference compensation coefficient The absolute value lies in the interval At this point, the system determines that it is in the initial stage of synchronization with a large deviation. The controller then uses a significantly increased proportional gain. Its purpose is to provide a powerful regulating effect, thereby significantly reducing the time required for the initial stage of the synchronization process.

[0034] Fine-tuning region: when the phase difference compensation coefficient The absolute value enters At this point, the system determines that synchronization is about to be completed. At this time, the proportional coefficient... It is no longer a fixed value, but rather depends on the current phase difference. A function value that is calculated in real time and changes at a predetermined slope, i.e. . Value follows The proportional gain decreases continuously or in segments. This design makes the proportional effect extremely weak when the phase difference is very small, thus effectively avoiding frequency overshoot or power oscillation caused by excessive proportional gain near the synchronization point, ensuring smoothness and stability during grid connection.

[0035] This partitioned adaptive strategy enables the PI controller to dynamically match the dynamic response requirements of different stages of the pre-synchronization process, prioritizing speed in the initial stage and ensuring stability and accuracy in the later stage. It not only significantly improves the pre-synchronization speed but also effectively avoids the risks of frequency exceeding limits and closing impact, enhancing the robustness and reliability of grid-connected converters during grid connection, and possesses broad engineering applicability and versatility.

[0036] 2. Integral coefficients based on the initial state ( Adaptive Strategy: This invention innovatively introduces an initial phase difference compensation coefficient. Considering this, it is used as the integral coefficient. The basis for selection.

[0037] It is the initial phase difference compensation coefficient. It is the phase difference compensation coefficient, "initial" indicates It is the initial phase difference compensation coefficient obtained by the compensation algorithm when the pre-synchronization process starts, which is the phase difference between the VSG output voltage and the positive sequence component of the grid voltage.

[0038] If the initial phase difference compensation coefficient A relatively large coefficient indicates that the synchronization process begins with a large deviation. To accelerate the elimination of steady-state errors and ensure the speed of the synchronization process, the system automatically selects a large integral coefficient. This is to enhance the strength of the integral action and eliminate residual bias as quickly as possible.

[0039] If the initial phase difference compensation coefficient Smaller ( This indicates that the system's initial state is close to synchronization. At this point, the system automatically switches to a very small integral coefficient. The purpose is to prevent frequency overshoot caused by excessive integration at the end of synchronization and to strictly ensure that the frequency is within the threshold range permitted by the grid connection.

[0040] Specifically, the adaptive formula for the integral coefficient is as follows: like ,but , The value range is 1 to 5; like ,but , The value range is 0.01 to 0.1; in, The initial phase difference compensation coefficient is calculated from the voltage phase difference at the pre-synchronization start-up time and is the initial deviation amount automatically detected by the system.

[0041] Step S4 includes: Set the initial value of the internal potential amplitude ; The internal potential amplitude is calculated as follows: ; In the formula, The magnitude of the internal potential. The initial value of the internal potential amplitude, The value represents the disturbance amplitude, ranging from 1.5V to 9.3V. For the perturbation frequency, t It is a continuous variable that changes with time, used to characterize the dynamic process of sinusoidal disturbances.

[0042] Step S4 also includes: Calculate the amplitude of the fluctuation component Continuously compare the amplitude of the fluctuation components With respect to preset tolerance It continuously corrects the internal potential until it approaches the voltage amplitude of the power grid.

[0043] Calculate the amplitude of the fluctuation component ,include: Synchronous monitoring of reactive power Q at the grid connection point, using the center frequency and disturbance frequency. A consistent bandpass filter with a bandwidth of 0.1~0.5Hz is used to extract the injected disturbance. Directly excited reactive power fluctuation component at the same frequency Calculated using Fast Fourier Transform amplitude The formula is: ; in, , for Components in a two-phase stationary coordinate system.

[0044] like If the current VSG internal potential setting is not equal to the grid voltage amplitude, the VSG internal potential is adjusted. The specific adjustment formula is as follows: ; In the formula, For rated reactive power , The rated reactive power of VSG, For the first The internal potential amplitude after the second adjustment; For the first The amplitude of the secondary internal potential; This is the step size coefficient, with a range of values. ; The sign function determines the direction of adjustment.

[0045] like If the voltage amplitude of the VSG is found to be consistent with the voltage amplitude of the grid, the internal voltage adjustment is stopped.

[0046] The method for calculating the internal potential amplitude in this invention mainly includes four steps, as follows: The first step involves starting the pre-synchronization mode and initializing the system. The converter operates stably in off-grid mode, and the internal potential amplitude setpoint is assigned an initial value. .

[0047] The second step is to inject a periodic voltage disturbance with extremely small amplitude and extremely low frequency into the internal potential reference signal. Its mathematical expression is as follows: ; in, The magnitude of the internal potential. The initial value of the internal potential amplitude, The disturbance amplitude ranges from 0.5% to 1.5% of the rated voltage. The disturbance frequency ranges from 0.5Hz to 2Hz, which is much lower than the fundamental frequency and the bandwidth of the control system.

[0048] The third step involves synchronously monitoring and calculating the reactive power Q at the grid connection point, and using bandpass filtering technology to accurately extract the reactive power Q caused by the injected disturbance. Directly excited reactive power fluctuation components of the same frequency And calculate the amplitude of the component. , which serves as the core observation component.

[0049] The fourth step is the core control stage, which uses a gradient descent algorithm to optimize the setting of the internal potential by minimizing the effective value of the reactive power fluctuation component.

[0050] This step involves continuously comparing the amplitude of the fluctuation components. With respect to preset tolerance It continuously adjusts the internal potential until it infinitely approaches the amplitude of the grid voltage. If If the current VSG internal potential setting is not equal to the grid voltage amplitude, the gradient descent algorithm is adopted, based on... The magnitude and trend of change of the VSG adjust the internal potential. The direction of adjustment is determined by... Compared to The changing gradient information determines the adjustment of the step size and... The magnitude is proportional to the value of the convergence, so as to achieve a balance between fast convergence and stability.

[0051] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for pre-synchronization control of a grid-forming converter based on a virtual synchronous generator, characterized in that, The method comprises the following steps: S1, collecting three-phase voltage of a virtual synchronous generator (VSG) output end and three-phase voltage of a power grid side, and converting the voltage into voltage components in a two-phase stationary coordinate system through Clark transformation, and then extracting positive sequence components; S2, calculating a phase difference compensation coefficient based on the positive sequence components; S3, adaptively setting PI parameters based on the phase difference compensation coefficient; S4, calculating an internal electromotive force amplitude, and completing pre-synchronization.

2. The pre-synchronization control method of the grid-forming converter based on the virtual synchronous generator according to claim 1, characterized in that, In step S1, the three-phase voltage at the output terminal of the VSG is collected. and the three-phase voltage on the grid side The Clark transformation is used to convert it into voltage components in a two-phase stationary coordinate system. and Specifically: ； wherein is the component of the microgrid-side voltage space vector on the axis, is the component of the microgrid-side voltage space vector on the axis, is the microgrid-side A-phase voltage, is the microgrid-side B-phase voltage, is the microgrid-side C-phase voltage.

3. The pre-synchronization control method of the grid-forming converter based on the virtual synchronous generator according to claim 2, characterized in that, The voltage positive sequence components are extracted by using a quarter grid cycle delay method, and the positive sequence components are extracted according to the following rules: ； wherein is the component of the grid voltage space vector on the axis, is the component of the grid voltage space vector on the axis, , are the positive sequence components of the VSG output voltage on the and axis, respectively, , are the positive sequence components of the grid voltage on the and axis, respectively. T is the grid voltage period, denotes the historical sampling instant that is a quarter of the grid period ahead of the current sampling instant t.

4. The pre-synchronization control method of the grid-forming converter based on the virtual synchronous generator according to claim 3, characterized in that, In step S2, the phase difference compensation coefficient is calculated is: 。 5. The pre-synchronization control method of the grid-forming converter based on the virtual synchronous generator according to claim 1, characterized in that, In step S3, based on the phase difference compensation coefficient, the double-interval and proportional coefficient adjustment are divided, and specifically: When the absolute value of the phase difference compensation coefficient is located in the interval , the proportional coefficient is coarsely adjusted, including: ； In the formula, is a proportional coefficient after coarse adjustment, is the maximum proportional coefficient of the coarse adjustment interval; When the absolute value of the phase difference compensation coefficient is located in the interval , the proportional coefficient is finely adjusted, including: ； In the formula, is the fine-tuned proportional coefficient, is the fine-tuned interval reference proportional coefficient.

6. The pre-synchronization control method of the grid-forming converter based on the virtual synchronous generator according to claim 1, characterized in that, In step S3, the initial phase difference compensation coefficient is also included , the selection integral coefficient is performed , specifically: If then , the value range of is 1~5; If then , has a value in the range of 0.01 to 0.

1.

7. The pre-synchronization control method of the grid-forming converter based on the virtual synchronous generator according to claim 1, characterized in that, In step S4, the following steps are included: Setting internal potential amplitude initial value ; The internal electromotive force amplitude is calculated, and specifically: ； wherein is the internal potential amplitude, is the internal potential amplitude initial value, is the perturbation amplitude, is the perturbation frequency, t is the continuous variable of the perturbation signal over time.

8. The pre-synchronization control method of the grid-forming converter based on the virtual synchronous generator according to claim 1, characterized in that, In step S4, the following steps are also included: Computing the amplitude of the fluctuation component Continuously comparing the amplitude of the fluctuation component With a preset tolerance Continuously correcting the internal potential until the grid voltage amplitude is approximated.

9. The pre-synchronization control method of the grid-forming converter based on the virtual synchronous generator according to claim 8, characterized in that, Computing the amplitude of a fluctuation component comprising: Synchronously monitor and calculate the reactive power Q at the grid-connected point, and extract the reactive power fluctuation component of the same frequency caused by the injected disturbance using a band-pass filter directly excited reactive power fluctuation component of the same frequency , and calculate the amplitude of the component , specifically: ； wherein , is in the two-phase stationary coordinate system.

10. The pre-synchronization control method of the grid-forming converter based on the virtual synchronous generator according to claim 8, characterized in that, If , it is determined that the internal potential setting value of the current VSG is not equal to the grid voltage amplitude, and the internal potential of the VSG is adjusted. The specific adjustment formula is as follows: ； wherein is the first adjusted inner potential amplitude; is the first adjusted inner potential amplitude; is the first adjusted inner potential amplitude; is the first adjusted inner potential amplitude; is the step coefficient; If then it is determined that the amplitude of the internal potential in the VSG is consistent with the amplitude of the grid voltage, and the adjustment of the internal potential is stopped.

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