Unified impedance reconfiguration method for oscillation of multi-inverter grid-connected system

Abstract

The application discloses a unified impedance reconstruction method for oscillation of a multi-inverter grid-connected system, and relates to the field of power electronics. In view of the non-ignorable oscillation instability problem in the grid-connected process of the existing multi-converter system, starting from the applicability and practicability of the system oscillation mechanism and the oscillation suppression strategy, the application establishes a multi-inverter grid-connected system impedance remodeling strategy based on phase remodeling and amplitude remodeling technology, and realizes dynamic adjustment of the parameters of the impedance remodeling module by introducing an adaptive technology, so as to finally form a unified adaptive impedance remodeling strategy capable of realizing dynamic suppression of the oscillation of the multi-inverter grid-connected system under different grid-connected conditions, and improve the system operation stability and output power quality.

Description

Technical Field

[0001] This invention relates to a unified impedance reconstruction method for oscillations in multi-inverter grid-connected systems, belonging to the field of power electronics. Background Technology

[0002] The interaction between multiple inverter systems in a large-scale renewable power generation system during grid-connected operation is stronger than that in a single inverter grid-connected system. This makes the oscillation problem more severe than in a single inverter system, which not only affects the quality of the system's output power but also reduces the system's operational stability.

[0003] According to the oscillation mechanism of multi-inverter grid-connected systems, the basic condition for system oscillation is the existence of an oscillation network and an oscillation source excitation with a matching frequency. Therefore, when it is impossible to eliminate all oscillation sources, reconstructing the equivalent output impedance of the multi-inverter grid-connected system to change its oscillation network characteristics is the main way to solve the system oscillation problem.

[0004] Existing literature largely focuses on optimizing the output characteristics of grid-connected inverter systems, including oscillation suppression, by improving the impedance characteristics of individual inverters. This approach only requires updating the inverter's control structure without installing new equipment. However, the modification process necessitates shutting down the inverters. For applications with multiple inverters, each inverter needs to be modified individually, leading to cumbersome operations, implementation difficulties, and subsequent maintenance challenges, thus reducing equipment utilization hours. To address this, a centralized reshaping strategy for the system's equivalent output impedance has been proposed to suppress oscillations in multi-inverter grid-connected systems. This can be achieved by attaching the impedance reshaping strategy to the existing SVG controller of the grid-connected system, or by operating the multi-inverter grid-connected system as a STATCOM, making it equivalent to a subsynchronous resistor connected in parallel to the system. This achieves cost savings and flexible solutions to specific oscillation problems in grid-connected systems. However, this approach places high demands on the control system and may affect the functionality of the original equipment. Therefore, further in-depth research is needed on how to use centralized reshaping to cope with changes in the state and external environment of multi-inverter systems and accurately reshape the virtual impedance of the system under complex oscillation conditions. Summary of the Invention

[0005] To address the aforementioned problems, embodiments of the present invention provide a unified impedance reconfiguration method for oscillations in multi-inverter grid-connected systems. By configuring a unified impedance reconfigurator at the common connection point of the multi-inverter grid-connected system and combining it with adaptive control technology to improve the dynamic response capability of the impedance reconfigurator to different oscillation conditions, the finally constructed unified adaptive impedance reconfigurator for oscillations in multi-inverter grid-connected systems can effectively cope with the oscillation problems encountered during the multi-inverter grid connection process.

[0006] The technical solution to achieve the purpose of this invention is: a unified impedance reconstruction method for oscillations in multi-inverter grid-connected systems, comprising:

[0007] Step 1: Extract the voltage at the common connection point of the multi-inverter grid-connected system and the harmonic current to eliminate pulsating interference signals;

[0008] Step 2: Based on the harmonic current at the common coupling point of the multi-inverter grid-connected system to eliminate pulsating interference signals, determine whether there is harmonic current oscillation at the common coupling point of the multi-inverter grid-connected system. If there is, proceed to step 4; otherwise, proceed to step 3.

[0009] Step 3: Based on the impedance on both sides of the common coupling point of the multi-inverter grid-connected system, determine the risk of harmonic current oscillation at the common coupling point of the multi-inverter grid-connected system. If there is a risk of harmonic current oscillation at the common coupling point of the multi-inverter grid-connected system, proceed to Step 4; otherwise, return to Step 1.

[0010] Step 4: Perform equivalent impedance reshaping based on the voltage at the common connection point of the multi-inverter grid-connected system obtained in Step 1 to obtain the impedance reshaping signal;

[0011] Step 5: Compare the impedance reshaping signal with the output signal of the PR current controller, and generate a pulse signal by SPWM modulation.

[0012] A unified impedance reconfiguration system for addressing oscillations in multi-inverter grid-connected systems includes:

[0013] The common coupling point information extraction module for multi-inverter grid-connected systems is used to extract the voltage and harmonic current at the common coupling point of multi-inverter grid-connected systems.

[0014] The first harmonic oscillation current judgment module is used to determine whether there is harmonic current oscillation at the common connection point of the multi-inverter grid-connected system based on the harmonic current at the common connection point of the multi-inverter grid-connected system, and to give a corresponding signal.

[0015] The second harmonic oscillation current judgment module is used to judge the risk of second harmonic current oscillation at the common connection point of the multi-inverter grid-connected system based on the impedance on both sides at the common connection point of the multi-inverter grid-connected system when the first harmonic oscillation current judgment module determines that there is no harmonic current oscillation.

[0016] The impedance reshaping module is used to perform equivalent impedance reshaping based on the voltage at the common coupling point of the multi-inverter grid-connected system when there is a risk of harmonic current oscillation, and to obtain an impedance reshaping signal.

[0017] The pulse generation module is used to compare the impedance reshaping signal with the output signal of the PR current controller, and the generated signal is modulated by SPWM to generate a pulse signal.

[0018] Compared with the prior art, the significant advantages of this invention are: the unified adaptive impedance reconfiguration strategy adopted in this invention simplifies the deployment complexity of impedance reconfiguration in multi-inverter grid-connected systems, while enabling the system to take into account both oscillation monitoring performance and dynamic oscillation suppression, greatly improving the system's ability to cope with complex oscillation problems. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the overall structure of a multi-inverter grid-connected system with a unified adaptive impedance reshaping device.

[0020] Figure 2 This is a structural diagram of a unified adaptive impedance reshaping device.

[0021] Figure 3 This is a flowchart of a unified adaptive impedance reshaping scheme.

[0022] Figure 4 This is a block diagram of the inverter control within a unified device with an impedance reshaping module.

[0023] Figure 5 The simulation waveforms are for a multi-inverter system without impedance reshaping devices.

[0024] Figure 6 Simulation waveforms of a multi-inverter system with an impedance reshaping device. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] A unified impedance reconstruction method for oscillation in multi-inverter grid-connected systems includes the following steps:

[0027] Step 1: Extract the voltage at the point of common coupling (PCC) of the multi-inverter grid-connected system and the harmonic current to eliminate pulsating interference signals. The specific method is as follows:

[0028] For the three-phase voltage at PCC ( , , ) is detected, and the three-phase voltage () is converted using the Clarke transform. , , Convert to α and β axis components and The Clarke transform formula is as follows:

[0029]

[0030] Regarding the three-phase current at PCC ( , , ) is detected, and the three-phase current () is converted using the Clarke transform module. , , This is converted to the α and β axis current components at the point of common coupling of a multi-inverter grid-connected system. and The Clarke transform formula is as follows:

[0031]

[0032] Based on the obtained α and β axis current components at the common coupling point of the multi-inverter grid-connected system and A notch filter is used to extract common frequency harmonic current components at the PCC. The transfer function of the notch filter is as follows:

[0033]

[0034] Where ω0 is the fundamental angular frequency; k is the frequency coefficient. This is the complex frequency domain.

[0035] Based on notch filter Extracted common frequency harmonic current α and β axis components of PCC and By using a low-pass filter to remove the pulsating components, the harmonic current α and β axis components, excluding pulsating interference signals, are finally obtained: and The transfer function of the low-pass filter is as follows:

[0036]

[0037] in, The corner frequency is denoted by .

[0038] Step 2: Use the harmonic current α and β axis components at PCC obtained in Step 1 to eliminate pulsating interference signals. and Harmonic oscillation judgment is performed on multi-inverter grid-connected systems. An amplitude threshold (3% of the fundamental amplitude) is set based on multi-inverter grid-connection standards and the suppression effect of the system oscillation suppression module. The acquired [data / signals] are then [processed / adjusted]. and Compare with the set thresholds respectively, and according to and The presence of harmonic current oscillations in a multi-inverter grid-connected system can be determined by whether the amplitude values ​​all exceed the limits.

[0039] If harmonic current oscillation exists at PCC, proceed to step 4; otherwise, proceed to step 3.

[0040] Step 3: Measure the impedance on both sides of the PCC in the multi-inverter grid-connected system to assess the risk of harmonic current oscillation. If Step 2 confirms that no harmonic current oscillation occurs, then use an impedance measuring device to measure the system-side impedance Z at the PCC of the multi-inverter system. s and grid-side impedance Z g Furthermore, the Middlebrook criterion is used to determine whether a multi-inverter grid-connected system poses a risk of harmonic current oscillation. Based on Z... s and Z g The specific method for judging oscillation risk using the Middlebrook criterion is as follows:

[0041] If Z s amplitude Greater than or equal to 0.2 times Z g amplitude If the system oscillates, then there is a risk of oscillation; otherwise, the system operates stably.

[0042] If there is a risk of harmonic current oscillation at PCC, proceed to step 4; otherwise, return to step 1.

[0043] Step 4: Perform equivalent impedance reshaping based on the voltage at the common connection point of the multi-inverter grid-connected system obtained in Step 1 to obtain the impedance reshaping signal, in order to eliminate system oscillation and oscillation risk;

[0044] In a further embodiment, the impedance reshaping signal is specifically:

[0045]

[0046] In the formula, For impedance reshaping transfer function, , These are the α-axis component and β-axis component of the voltage at the point of common coupling, respectively. The transfer function of the bandpass filter. Here is the transfer function of the notch filter. This is the phase reshaping transfer function.

[0047] The transfer function of the bandpass filter is as follows:

[0048]

[0049] In the formula, The fundamental angular frequency; This is the amplitude scaling factor; Let Gmf(s) be the cutoff frequency coefficient. This is the complex frequency domain.

[0050] The transfer function of the notch filter is as follows:

[0051]

[0052] In the formula, for The cutoff frequency coefficient; The oscillation angular frequency, This is the complex frequency domain.

[0053] The phase reshaping transfer function is as follows:

[0054]

[0055] In the formula, a and b are respectively The phase compensation frequency point and the maximum compensation phase angle adjustment coefficient, This is the amplitude correction factor.

[0056] Step 5: Reshape the impedance signals of the α and β axis control loops. and The signal is compared with the output signal of the PR current controller, and the generated signal is sent to the SPWM module to generate the inverter SPWM control pulse.

[0057] A unified impedance reconfiguration system for addressing oscillations in multi-inverter grid-connected systems includes:

[0058] The common coupling point information extraction module for multi-inverter grid-connected systems is used to extract the voltage and harmonic current at the common coupling point of multi-inverter grid-connected systems.

[0059] The first harmonic oscillation current judgment module is used to determine whether there is harmonic current oscillation at the common connection point of the multi-inverter grid-connected system based on the harmonic current at the common connection point of the multi-inverter grid-connected system, and to give a corresponding signal.

[0060] The second harmonic oscillation current judgment module is used to judge the risk of second harmonic current oscillation at the common connection point of the multi-inverter grid-connected system based on the impedance on both sides at the common connection point of the multi-inverter grid-connected system when the first harmonic oscillation current judgment module determines that there is no harmonic current oscillation.

[0061] The impedance reshaping module is used to perform equivalent impedance reshaping based on the voltage at the common coupling point of the multi-inverter grid-connected system when there is a risk of harmonic current oscillation, and to obtain an impedance reshaping signal.

[0062] The pulse generation module is used to compare the impedance reshaping signal with the output signal of the PR current controller, and the generated signal is modulated by SPWM to generate a pulse signal.

[0063] In a further embodiment, the impedance reshaping module uses the α and β axis components of the voltage signal at PCC obtained in step 1. and Perform equivalent impedance reshaping of the system to obtain the α and β axis components of the impedance reshaping signal: and This is to eliminate system oscillations and the risk of oscillations.

[0064] The impedance reshaping signal is specifically:

[0065]

[0066] In the formula, For impedance reshaping transfer function, , These are the α-axis component and β-axis component of the voltage at the point of common coupling, respectively. The transfer function of the bandpass filter. Here is the transfer function of the notch filter. This is the phase reshaping transfer function.

[0067] In a further embodiment, the unified inverter control block diagram of the impedance reshaping module is as follows: Figure 4 The modules and specific reshaping schemes included therein are as follows:

[0068] The phase reshaping module's main function is to adjust the system phase, preventing the phase frequency curve of the equivalent output impedance of the multi-inverter system from intersecting with the boundary line (90° line) of the negative impedance region within the oscillation risk frequency range, while maintaining a certain margin. The phase reshaping module uses the phase transfer function G... pr (s) are as follows:

[0069]

[0070] Where a and b are the phase compensation frequency point and the maximum compensation phase angle adjustment coefficient of the phase reshaping module, respectively, which are mainly determined based on the system's response speed to oscillation suppression and specific oscillation frequency and other conditions and requirements; k is the amplitude correction coefficient.

[0071] The amplitude reshaping module mainly comprises two parts: a bandpass filter module and a notch filter module. The bandpass filter module increases the amplitude of the equivalent output impedance loop on the grid-connected unit side of the power plant at the fundamental frequency, increasing the proportion of fundamental frequency current flowing into the grid and improving the power quality of the power plant. The notch filter module reduces the amplitude of the equivalent output impedance loop on the grid-connected unit side of the power plant at the power plant's oscillation frequency point, providing an oscillation frequency path and reducing or even eliminating the impact of oscillations on the trunking system.

[0072] The transfer functions used in the bandpass filter module and the notch filter module are as follows:

[0073]

[0074]

[0075] In the formula, The fundamental angular frequency; This is the amplitude scaling factor; Let Gmf(s) be the cutoff frequency coefficient; for The cutoff frequency coefficient; ω is the oscillation angular frequency. The larger the amplitude, the stronger the peak gain of the amplitude reshaping module, resulting in an increase in amplitude at the fundamental angular frequency. However, this also affects the phase margin and amplitude-frequency characteristics of the power plant's grid-connected system. With... and As the amplitude increases, the bandwidth of the amplitude reshaping module increases. The amplitude at the fundamental angular frequency remains unchanged, but the amplitude-frequency characteristics and phase-frequency characteristics of other frequency bands will be affected. The transfer function of the bandpass filter. This is the transfer function of the notch filter.

[0076] Based on the above analysis, the impedance reshaping module function shown below is finally constructed. Under the set performance indicators, the relevant parameters in the amplitude and phase reshaping modules are specifically designed according to the system parameters and the grid connection standards and regulations of the inverter system.

[0077]

[0078] Finally, the impedance reshaping transfer function G r (s) The PCC voltage feedback loop of the multi-inverter grid-connected system, located within the unified inverter unit, is based on the α and β axis components of the voltage signal at the PCC obtained in step 1. and Using the impedance reshaping module shown in the above formula Obtain the impedance reshaping signals of the α and β axis control loops. and .

[0079]

[0080] Figure 5 , 6 The simulation waveforms of the multi-inverter system before and after the addition of the impedance reshaping device show that the harmonics of the system output current are greatly improved with the intervention of the unified impedance reshaping device at the PCC, verifying the effectiveness of the unified impedance reshaping device.

[0081] The above implementation schemes are merely technical ideas of the present invention and should not be used to limit the scope of protection of the present invention. Any modifications made to the technical solutions based on the technical ideas proposed in the present invention shall fall within the scope of protection of the present invention.

Claims

1. A unified impedance reconfiguration method for multi-inverter grid- connected system oscillation, characterized in that, include: Step 1: Extract the voltage at the common connection point of the multi-inverter grid-connected system and the harmonic current to eliminate pulsating interference signals; Step 2: Based on the harmonic current at the common coupling point of the multi-inverter grid-connected system to eliminate pulsating interference signals, determine whether there is harmonic current oscillation at the common coupling point of the multi-inverter grid-connected system. If there is, proceed to step 4; otherwise, proceed to step 3. Step 3: Based on the impedance on both sides of the common coupling point of the multi-inverter grid-connected system, determine the risk of second harmonic current oscillation at the common coupling point of the multi-inverter grid-connected system. If there is a risk of harmonic current oscillation at the common coupling point of the multi-inverter grid-connected system, proceed to Step 4; otherwise, return to Step 1. Step 4: Based on the voltage at the point of common coupling of the multi-inverter grid-connected system obtained in Step 1, perform equivalent impedance reshaping to obtain the impedance reshaping signal. The impedance reshaping signal is as follows: ； wherein is an impedance reshaping transfer function, , are the common connection point voltage alpha axis component and the common connection point voltage beta axis component, respectively, is a band pass filter transfer function, is a notch filter transfer function, is a phase reshaping transfer function; The transfer function of the bandpass filter is as follows: ； wherein is the fundamental angular frequency; is the amplitude scaling factor; is G mf the cutoff frequency coefficient of (s), is the complex frequency domain; The transfer function of the notch filter is as follows: ； wherein is a cutoff frequency coefficient; is a cutoff frequency coefficient; is an oscillation angular frequency, is a complex frequency domain; The phase reshaping transfer function is as follows: ； In the formula, a and b are respectively the phase compensation frequency point and the maximum compensation phase angle adjustment coefficient, is the amplitude correction coefficient; Step 5: Compare the impedance reshaping signal with the output signal of the PR current controller, and generate a pulse signal by SPWM modulation. 2.The unified impedance reconfiguration method against multi-inverter grid-connected system oscillation of claim 1, wherein, The specific method for extracting the voltage at the point of common coupling (PCC) of a multi-inverter grid-connected system is as follows: The three-phase voltage at the point of common coupling (PCC) of a multi-inverter grid-connected system is detected, and the three-phase voltage is converted into the α-axis and β-axis components of the PCC voltage using the Clarke transform. The specific formula is as follows: ， wherein (Vabc) is the three-phase voltage at the point of common coupling of the grid-connected system, , , ) is the three-phase voltage at the point of common coupling of the grid-connected system, , are the α-axis component of the point of common coupling voltage and the β-axis component of the point of common coupling voltage, respectively. 3.The unified impedance reconfiguration method against oscillation of a multi-inverter grid-connected system according to claim 1, wherein, The specific method for extracting harmonic currents at the point of common coupling (PCC) of a multi-inverter grid-connected system to eliminate pulsating interference signals is as follows: The three-phase current at the point of common coupling (PCC) of a multi-inverter grid-connected system is detected, and the three-phase current is converted into the α-axis component and β-axis component of the PCC current using the Clarke transform. Based on the α-axis component and β-axis component of the common coupling current, a notch filter is used to extract the harmonic current component of the set frequency at the common coupling point. By using a low-pass filter to filter out the pulsating component in the harmonic current component of a set frequency, the α-axis component and β-axis component of the harmonic current, which are free from pulsating interference signals, are obtained.

4. The unified impedance reconstruction method for oscillations in multi-inverter grid-connected systems according to claim 1, characterized in that, The specific method for determining whether there is harmonic current oscillation at the common coupling point of a multi-inverter grid-connected system is as follows: compare the harmonic current at the common coupling point of the multi-inverter grid-connected system excluding pulsating interference signals with a set threshold. If the harmonic current at the common coupling point of the multi-inverter grid-connected system excluding pulsating interference signals exceeds the set threshold, then it is determined that there is harmonic current oscillation at the common coupling point of the multi-inverter grid-connected system.

5. The unified impedance reconstruction method for oscillations in multi-inverter grid-connected systems according to claim 1, characterized in that, A specific method for assessing the risk of second harmonic current oscillation at the point of common coupling (PCC) of a multi-inverter grid-connected system based on the impedances on both sides of the PCC: If the amplitude of the system-side impedance at the point of common coupling of a multi-inverter grid-connected system is greater than the amplitude at the grid-side impedance at the point of common coupling of the multi-inverter grid-connected system by a set multiple, it is determined that there is a risk of harmonic current oscillation.

6. A unified impedance reconfiguration system for oscillations in a multi-inverter grid-connected system based on the method shown in any one of claims 1 to 5, characterized in that, include: The common coupling point information extraction module for multi-inverter grid-connected systems is used to extract the voltage and harmonic current at the common coupling point of multi-inverter grid-connected systems. The first harmonic oscillation current judgment module is used to determine whether there is harmonic current oscillation at the common connection point of the multi-inverter grid-connected system based on the harmonic current at the common connection point of the multi-inverter grid-connected system, and to give a corresponding signal. The second harmonic oscillation current judgment module is used to judge the risk of second harmonic current oscillation at the common connection point of the multi-inverter grid-connected system based on the impedance on both sides at the common connection point of the multi-inverter grid-connected system when the first harmonic oscillation current judgment module determines that there is no harmonic current oscillation. The impedance reshaping module is used to perform equivalent impedance reshaping based on the voltage at the common coupling point of the multi-inverter grid-connected system when there is a risk of harmonic current oscillation, and to obtain an impedance reshaping signal. The pulse generation module is used to compare the impedance reshaping signal with the output signal of the PR current controller, and the generated signal is modulated by SPWM to generate a pulse signal.

Images