Synchronization Control Method for On-Grid Switching of Islanded Microgrids Based on Virtual Generators

Abstract

This invention belongs to the field of power electronic conversion and microgrid control technology, specifically involving a synchronous control method for grid-connected / off-grid switching of an islanded microgrid based on a virtual generator. The method includes transforming the three-phase voltage signals from the microgrid side and the grid side from a three-phase stationary coordinate system to a two-phase stationary coordinate system, obtaining orthogonal components of the microgrid voltage and grid voltage. These orthogonal components are then substituted into a calculation formula to obtain a real-time phase difference signal. This real-time phase difference signal is then fed into a regulator for proportional-integral and filtering processing to generate a phase compensation coefficient. This phase compensation coefficient is then superimposed onto the active frequency control loop of the virtual synchronous generator to correct the phase angle of the virtual synchronous generator's output voltage. This achieves the goal of maintaining the simplicity of the non-phase-locked loop structure while reducing the inrush current during grid-connected / off-grid switching.

Description

Technical Field

[0001] This invention belongs to the field of power electronic conversion and microgrid control technology, specifically relating to a synchronous control method for on-grid switching of islanded microgrids based on virtual generators. Background Technology

[0002] As a key link in the VSG grid connection process, the pre-synchronization control has the core function of achieving precise matching of the phase, frequency and amplitude of the VSG output voltage with the grid voltage, so as to reduce the grid connection inrush current and ensure the safe and stable operation of the system.

[0003] Traditional grid-connected control strategies only consider the grid connection phase difference requirements, neglecting the short-term frequency limit exceedance issue in islanded microgrids during grid-to-off-grid switching. Regarding this short-term frequency limit exceedance phenomenon, existing technologies employ a pre-synchronization optimization control strategy for islanded microgrids that balances voltage fluctuation suppression with frequency limit avoidance. This strategy utilizes the Simplex algorithm to solve for pre-synchronization optimization PI parameters that address both grid connection speed and frequency fluctuation constraints, thus ensuring rapid pre-synchronization while preventing drastic frequency changes during adjustment. However, the Simplex optimization algorithm has a long computation time, making the proportional coefficient and integral time constant of the PI stage unsuitable for online optimization. Although offline optimization combined with online table lookup is used, the offline computation task remains heavy, requiring significant computational resources and time. Summary of the Invention

[0004] Firstly, in view of the shortcomings of the existing technology, the purpose of this application is to provide a synchronous control method for grid-connected and grid-off switching of islanded microgrids based on virtual generators, which retains the simplicity of the non-phase-locked loop structure while reducing the inrush current during grid-connected and grid-off switching.

[0005] The objective of this application can be achieved through the following technical solutions:

[0006] A synchronization control method for on-grid / off-grid switching in an islanded microgrid based on a virtual generator, the method comprising:

[0007] The three-phase voltage signals from the microgrid side and the grid side are collected. The three-phase voltage signals from the microgrid side and the grid side are transformed from a three-phase stationary coordinate system to a two-phase stationary coordinate system through a coordinate transformation module. The orthogonal components of the microgrid voltage and the grid voltage are decoupled to obtain the orthogonal components of the microgrid voltage and the grid voltage.

[0008] The orthogonal components of the microgrid voltage and the orthogonal components of the grid voltage are input into the vector quadrant judgment module. Based on the positive and negative attributes of each component on the coordinate axes of the two-phase stationary coordinate system, the quadrant values ​​of the microgrid voltage and the grid voltage are determined respectively.

[0009] Based on the position combination formed by the microgrid voltage quadrant value and the grid voltage quadrant value, the corresponding calculation formula is matched from the preset phase difference calculation rules, and the orthogonal components of the microgrid voltage and the grid voltage are substituted into the calculation formula to calculate the real-time phase difference signal.

[0010] The real-time phase difference signal is sent to the regulator for proportional-integral and filtering processing to generate a phase compensation coefficient. The phase compensation coefficient is then superimposed on the active frequency control loop of the virtual synchronous generator to correct the phase angle of the output voltage of the virtual synchronous generator.

[0011] Further, the step of inputting the orthogonal components of the microgrid voltage and the orthogonal components of the grid voltage into the vector quadrant determination module, and determining the quadrant values ​​of the microgrid voltage and the grid voltage respectively based on the positive and negative attributes of each component on the coordinate axes of the two-phase stationary coordinate system, includes:

[0012] The orthogonal component of the microgrid voltage or the orthogonal component of the grid voltage is detected in the two-phase stationary coordinate system. The values ​​on the axis;

[0013] In response to the The value on the axis is greater than 0, detecting the orthogonal component of the microgrid voltage or the orthogonal component of the grid voltage in the two-phase stationary coordinate system. The values ​​on the axis, if the stated If the value on the axis is greater than 0, output the first quadrant number; if the value on the axis is greater than 0, output the first quadrant number. If the value on the axis is not greater than 0, output the fourth quadrant number;

[0014] In response to the The value on the axis is not greater than 0, and the orthogonal component of the microgrid voltage or the orthogonal component of the grid voltage is detected in the two-phase stationary coordinate system. The values ​​on the axis, if the stated If the value on the axis is greater than 0, output the second quadrant number; if the value on the axis is greater than 0, output the second quadrant number. If the value on the axis is not greater than 0, output the third quadrant number;

[0015] The quadrant number corresponding to the microgrid voltage in the two-phase stationary coordinate system is denoted as... The quadrant number corresponding to the grid voltage in the two-phase stationary coordinate system is denoted as... .

[0016] Further, based on the position combination formed by the microgrid voltage quadrant value and the grid voltage quadrant value, a corresponding calculation formula is matched from a preset phase difference calculation rule, and the orthogonal components of the microgrid voltage and the grid voltage are substituted into the calculation formula, including:

[0017] Construct by the With the The quadrant combinations formed;

[0018] When the quadrant combination represents that the grid voltage vector and the microgrid voltage vector are in the same quadrant, the first calculation formula is invoked.

[0019] When the quadrant combination represents the grid voltage vector and the microgrid voltage vector crossing adjacent quadrants, the second calculation formula is invoked;

[0020] When the quadrant combination represents the grid voltage vector and the microgrid voltage vector crossing the phase limit, the third calculation formula is invoked.

[0021] Furthermore, the first calculation formula is:

[0022]

[0023] in, The real-time phase difference signal; The phase angle is calculated based on the orthogonal components of the microgrid voltage. The phase angle is calculated based on the orthogonal components of the grid voltage.

[0024] Furthermore, the second calculation formula is:

[0025] 1) When the quadrant combination corresponds to crossing quadrants I and II or crossing quadrants III and IV:

[0026]

[0027] 2) When the quadrant combination corresponds to crossing quadrants two and three or crossing quadrants one and four:

[0028]

[0029] in, The real-time phase difference signal; The phase angle is calculated based on the orthogonal components of the microgrid voltage. The phase angle is calculated based on the orthogonal components of the grid voltage. Pi is a constant.

[0030] Furthermore, the third calculation formula is specifically as follows:

[0031]

[0032] in, The real-time phase difference signal; The phase angle is calculated based on the orthogonal components of the microgrid voltage. The phase angle is calculated based on the orthogonal components of the grid voltage. Pi is a constant.

[0033] Further, the real-time phase difference signal is sent to the regulator for proportional-integral and filtering processing to generate phase compensation coefficients, including:

[0034] The real-time phase difference signal is input into the proportional-integral control unit in the regulator to calculate the intermediate adjustment amount;

[0035] The intermediate adjustment value is used as the input signal, filtered by the low-pass filter unit in the regulator, and the phase compensation coefficient is output. ;

[0036] The transfer function of the low-pass filter unit for:

[0037]

[0038] In the formula, It is a time constant. For the Laplace operator.

[0039] Furthermore, the step of superimposing the phase compensation coefficient onto the active frequency control loop of the virtual synchronous generator specifically includes:

[0040] The frequency synthesis node of the virtual synchronous generator receives the fundamental reference angular frequency signal, the virtual inertia feedback signal, and the phase compensation coefficient. ;

[0041] Perform addition operations to synthesize the target angular frequency signal;

[0042] An integral operation is performed on the target angular frequency signal to generate a voltage phase angle command for driving the space vector pulse width modulation module.

[0043] Furthermore, the method also includes a grid-connected switching step:

[0044] Configure real-time monitoring of the real-time phase difference signal, as well as the voltage amplitude difference signal and frequency difference signal between the microgrid and the power grid;

[0045] When the real-time phase difference signal is detected to be less than a preset phase threshold, the voltage amplitude difference signal is less than a preset amplitude threshold, and the frequency difference signal is less than a preset frequency threshold, a grid connection permission signal is issued.

[0046] The grid-connected circuit breaker is closed according to the grid-connection permission signal, and the input of the phase compensation coefficient is cut off.

[0047] Secondly, addressing the shortcomings of existing technologies, the purpose of this application is to provide a synchronous control system for grid-connected / off-grid switching of an islanded microgrid based on a virtual generator, while retaining the simplicity of the phase-locked loop-free structure and reducing the inrush current during grid-connected / off-grid switching. This objective can be achieved through the following technical solutions:

[0048] A synchronous control system for on-grid switching of an islanded microgrid based on a virtual generator includes:

[0049] Voltage sampling circuit, used to acquire voltage signals between microgrid and mains in real time;

[0050] The processor is connected to the voltage sampling circuit;

[0051] Memory, which stores computer programs;

[0052] When the processor runs the computer program, it performs the method as described in the first aspect.

[0053] The beneficial effects of this application are:

[0054] This application employs voltage vector quadrant judgment and algebraic operation logic based on a two-phase stationary coordinate system to directly calculate the real-time phase difference between the microgrid and the main grid. This approach fundamentally eliminates the inherent nonlinear dynamic adjustment process and delay characteristics of the phase-locked loop (PLL), greatly reducing the computational burden on the digital controller. It not only avoids the risk of synchronization failure caused by PLL instability in weak grid environments but also significantly improves the real-time performance and accuracy of phase detection, ensuring the linearity and monotonicity of the phase difference signal throughout the entire cycle.

[0055] This invention converts the calculated phase difference into a frequency compensation signal, which is then processed by a proportional-integral regulator and a first-order low-pass filter before being introduced into the active power frequency regulation loop of a virtual synchronous generator. Theoretical analysis and verification using a small-signal model show that this closed-loop feedback mechanism incorporating low-pass filtering effectively filters out high-frequency harmonic noise interference in the voltage sampling signal, providing necessary damping support for the system. This results in a smooth and convergent dynamic characteristic during the phase pre-synchronization process, avoiding frequency oscillations caused by over-regulation and ensuring the frequency stability of the microgrid during the off-grid to grid-connected transition.

[0056] This application enables the microgrid output voltage to quickly and flawlessly track the amplitude, frequency, and phase of the grid voltage before the grid-connected circuit breaker operates. This makes the voltage vector difference across the circuit breaker approach zero at the moment of closing, thus achieving true seamless soft grid connection and completely suppressing the closing inrush current that may occur at the moment of grid connection. It not only effectively prevents the physical damage of the inrush current to the inverter power semiconductor devices and filter inductors, extending the service life of key equipment, but also avoids grid connection failure caused by false triggering of overcurrent protection, significantly improving the operational reliability and power quality of the microgrid system. Attached Figure Description

[0057] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0058] Figure 1 This is the pre-synchronization control flow of an embodiment of this application;

[0059] Figure 2 This is a simplified control diagram of a virtual synchronous generator according to an embodiment of this application;

[0060] Figure 3 This is the vector quadrant position determination process according to an embodiment of this application;

[0061] Figure 4 This is the real-time phase difference calculation process according to an embodiment of this application;

[0062] Figure 5 This is a pre-synchronization control small-signal model considering angular frequency compensation in the embodiments of this application;

[0063] Figure 6 The phase pre-synchronization control strategy based on real-time phase angle difference in the embodiments of this application;

[0064] Figure 7 This is the A-phase voltage waveform of the pre-synchronization process in an embodiment of this application;

[0065] Figure 8 This refers to the A-phase inrush current under the presence or absence of a pre-synchronization control strategy in the embodiments of this application.

[0066] Figure 9 Traditional pre-synchronous control is a classic model for off-grid to grid-connected VSG inverters. Detailed Implementation

[0067] 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.

[0068] For virtual synchronous generator control, such as Figure 2 As shown, in the main power circuit, the system input is provided with a stable DC bus voltage by a DC-side energy storage system, which is electrically connected to the DC input side of the three-phase full-bridge inverter. The AC output side of the inverter is filtered by a set of LC filters, which consist of a series resistor and inductor and a parallel capacitor, used to suppress high-frequency switching harmonics and smooth the output voltage waveform. The filtered AC bus is connected to the local load to meet the power supply requirements during islanded operation, and also connected to the main grid through the grid-connected circuit breaker at the point of common coupling (PCC). The system acquires the three-phase voltage signal of the microgrid side at the PCC in real time. and current signal This serves as feedback for subsequent control strategies.

[0069] At the control logic level, the system employs a hierarchical, cascaded closed-loop control strategy. The acquired three-phase AC voltage and current signals are first input to the coordinate transformation module, and then transformed from the stationary coordinate system via Park transformation. Transform into a synchronous rotating coordinate system This achieves decoupling of active and reactive components. The outermost layer of the control system is a power control loop, including an active-frequency loop and a reactive-voltage loop, which are used to simulate the speed governor and excitation controller characteristics of a synchronous generator, respectively, to generate a reference voltage command. This reference command is sent to a dual-loop control unit, where the outer loop is an output voltage feedback loop to maintain the steady-state accuracy of the output voltage; the inner loop is an AC-side inductor current feedback loop to improve the dynamic response speed of the system and provide overcurrent protection. The output signal of the inner current loop is finally sent to the space vector pulse width modulation (SVPWM) module, which generates high-frequency on / off signals to drive the inverter power switches, thereby achieving precise modulation of the output voltage amplitude, phase, and frequency at the physical level. This ensures that the system can quickly adjust its output state to match the grid parameters according to the aforementioned pre-synchronization control strategy when the grid connection command is issued.

[0070] Example 1:

[0071] This application provides a synchronization control method for on-grid and off-grid switching of an islanded microgrid based on a virtual generator, the method comprising:

[0072] The three-phase voltage signals from the microgrid side and the grid side are collected. The three-phase voltage signals from the microgrid side and the grid side are transformed from a three-phase stationary coordinate system to a two-phase stationary coordinate system through a coordinate transformation module. The orthogonal components of the microgrid voltage and the grid voltage are decoupled to obtain the orthogonal components of the microgrid voltage and the grid voltage.

[0073] Specifically, referring to the real-time phase difference calculation flowchart, such as... Figure 4 As shown, the real-time phase difference calculation logic described in this embodiment is first based on the acquisition of voltage signals and coordinate domain transformation. The system acquires the three-phase voltage signals from the microgrid side in parallel through a sampling circuit. and the three-phase voltage signal on the grid side The above two sets of signals are input synchronously to The coordinate transformation module performs Clarke transformation, decoupling the voltage quantities in the three-phase stationary coordinate system into orthogonal components in the two-phase stationary coordinate system, and outputs the microgrid voltage components respectively. , and grid voltage components , .

[0074] In this application, the microgrid voltage quadrature component and the grid voltage quadrature component input vector quadrature judgment module determines the microgrid voltage quadrature value and the grid voltage quadrature value respectively based on the positive and negative attributes of each component on the coordinate axes of the two-phase stationary coordinate system.

[0075] Specifically, after coordinate transformation, the signal flow is divided into a control logic path and a data calculation path. In the control logic path, the grid voltage component... , With microgrid voltage components , The input components are fed into two independent vector quadrant determination modules. These two modules determine the quadrant based on the input components. shaft and The positive and negative attributes of the axis projection values ​​are used to calculate the current quadrant number of the grid voltage vector in parallel. and the current quadrant number of the microgrid voltage vector. .

[0076] In some embodiments, the orthogonal components of the microgrid voltage and the orthogonal components of the grid voltage are input into a vector quadrant determination module. Based on the positive and negative attributes of each component on the coordinate axes of the two-phase stationary coordinate system, the quadrant values ​​of the microgrid voltage and the grid voltage are determined, including:

[0077] like Figure 3 As shown, the orthogonal component of the microgrid voltage or the orthogonal component of the grid voltage is detected in the two-phase stationary coordinate system. The value on the axis; in response to the The value on the axis is greater than 0, detecting the orthogonal component of the microgrid voltage or the orthogonal component of the grid voltage in the two-phase stationary coordinate system. The values ​​on the axis, if the stated If the value on the axis is greater than 0, output the first quadrant number; if the value on the axis is greater than 0, output the first quadrant number. If the value on the axis is not greater than 0, output the fourth quadrant number; in response to the above... The value on the axis is not greater than 0, and the orthogonal component of the microgrid voltage or the orthogonal component of the grid voltage is detected in the two-phase stationary coordinate system. The values ​​on the axis, if the stated If the value on the axis is greater than 0, output the second quadrant number; if the value on the axis is greater than 0, output the second quadrant number. If the value on the axis is not greater than 0, the third quadrant number is output; where the quadrant number corresponding to the microgrid voltage in the two-phase stationary coordinate system is denoted as... The quadrant number corresponding to the grid voltage in the two-phase stationary coordinate system is denoted as... .

[0078] That is, first judge If the value is greater than 0, then the voltage vector is in the first or fourth quadrant. Further judgment is then made. If the value is greater than 0, it belongs to the first quadrant; otherwise, it belongs to the fourth quadrant. If the voltage vector is not greater than 0, it is in the second or third quadrant. Further judgment is then made. If the value is greater than 0, it belongs to the second quadrant; otherwise, it belongs to the third quadrant. This allows us to determine and record the quadrant number of the voltage vector between the power grid and the microgrid. and .

[0079] In this application, based on the position combination formed by the microgrid voltage quadrant value and the grid voltage quadrant value, a corresponding calculation formula is matched from a preset phase difference calculation rule, and the orthogonal components of the microgrid voltage and the grid voltage are substituted into the calculation formula to calculate the real-time phase difference signal.

[0080] Specifically, this invention employs an enumeration method to represent the microgrid port voltage in a two-phase stationary coordinate system. With grid voltage List all possible positional relationships between them, as shown in the table below:

[0081] Table 1 Spatial Vector Positional Relationships

[0082]

[0083] If the conditions in Table 1 are assumed to be consistent with the grid voltage leading the microgrid voltage, then four additional conditions in the same quadrant where the grid voltage lags behind the microgrid voltage are required, as shown in Table 2.

[0084] Table 2 Supplementary Positional Relationships

[0085]

[0086] In summary, the two tables show that before pre-synchronization and grid connection, In the coordinate system, there are a total of 20 combinations of spatial vector positional relationships between grid voltage and microgrid voltage.

[0087] Given that there are 20 possible positional combinations between two voltage vectors, this invention classifies these 20 relationships as follows:

[0088] when When the combination is (1,1), (2,2), (3,3), (4,4), it indicates that the grid voltage leads the microgrid voltage and is in the same quadrant.

[0089] when When the combination is (1,1), (2,2), (3,3), (4,4), it indicates that the microgrid voltage leads the grid voltage and is in the same quadrant;

[0090] when When the combination is (2,1), (1,2), (4,3), (3,4), it indicates that the grid voltage leads the microgrid voltage and spans the first and second quadrants or the third and fourth quadrants.

[0091] when When the combination is (2,3), (3,2), (4,1), (1,4), it indicates that the grid voltage leads the microgrid voltage and spans the second and third quadrants or the first and fourth quadrants.

[0092] when When the combination is (3,1), (1,3), (4,2), (2,4), it indicates that the grid voltage leads the microgrid voltage and spans the first and third quadrants or the second and fourth quadrants.

[0093] When the space vectors of the grid voltage and the microgrid voltage are in the same quadrant, the phase difference is calculated as follows:

[0094]

[0095] In the formula, The angle by which the grid voltage leads the microgrid voltage.

[0096]

[0097]

[0098] In the formula, For the microgrid voltage space vector and The angle between the axes, For the grid voltage space vector and The angle between axes.

[0099] When the spatial vectors of the grid voltage and the microgrid voltage cross the first and second quadrants or the third and fourth quadrants, the phase difference is calculated using the following formula:

[0100]

[0101] When the spatial vectors of the grid voltage and the microgrid voltage cross the second and third quadrants or the first and fourth quadrants, the phase difference is calculated using the following formula:

[0102]

[0103] When the spatial vectors of the grid voltage and the microgrid voltage cross the first and third quadrants or the second and fourth quadrants, the phase difference is calculated using the following formula:

[0104]

[0105] In this application, the real-time phase difference signal is sent to a regulator for proportional-integral and filtering processing to generate a phase compensation coefficient. The phase compensation coefficient is then superimposed on the active frequency control loop of the virtual synchronous generator to correct the phase angle of the output voltage of the virtual synchronous generator.

[0106] In some disclosures, the real-time phase difference signal is fed into a regulator for proportional-integral and filtering processing to generate a phase compensation coefficient. This includes: inputting the real-time phase difference signal into the proportional-integral control unit of the regulator to calculate an intermediate adjustment value; using the intermediate adjustment value as an input signal, filtering it through a low-pass filter unit in the regulator, and outputting the phase compensation coefficient. The transfer function of the low-pass filter unit for: In the formula, It is a time constant. For the Laplace operator.

[0107] Specifically, such as Figure 5 As shown, the control architecture of this embodiment consists of an upper real-time phase difference calculation stage and a lower self-adjusting frequency stage, which are connected by a phase difference compensation coefficient. Achieve coupling.

[0108] In the real-time phase difference calculation stage, the system input receives the microgrid-side voltage. These two sets of signals are processed by the coordinate transformation module, converting them from a three-phase stationary coordinate system to a two-phase stationary coordinate system, and outputting the microgrid voltage components. , The aforementioned components are input into their respective vector quadrant determination modules, where logical operations are performed to output the grid voltage quadrant number. These two numbers are then input into the quadrant combination determination module to identify the current spatial position relationship and drive the phase difference calculation formula module to output the real-time phase difference. (As shown by the red signal line in the figure). This phase difference After compensation stages (such as PI controllers and low-pass filters) ), generate phase difference compensation coefficient .

[0109] The lower self-adjusting frequency stage (i.e., the VSG control loop) first performs active power-frequency droop control and torque calculation. This stage is based on the system reference angular frequency. Actual angular frequency Reference power and actual power Active power droop control (coefficient) ), generating mechanical torque Meanwhile, the actual measured electromagnetic power It also passes through a divider and feedback angular frequency. Divide to obtain electromagnetic torque Furthermore, the damping element is based on the feedback angular frequency. With reference angular frequency The difference, multiplied by the damping coefficient Generates damping torque .

[0110] Subsequently, the system performs a comprehensive torque calculation based on the rotor motion equations. Mechanical torque. As a driving factor, subtract the electromagnetic torque. With damping torque The resulting resultant torque is input into the virtual inertia element. The output of this stage is related to the reference angular frequency. and the phase difference compensation coefficient from the upper stage. The two components are superimposed to generate the final output angular frequency. The output angular frequency First output frequency The second route involves a points-based system. Output phase This allows for complete control over the frequency and phase of the inverter's output voltage.

[0111] The frequency synthesis node of the virtual synchronous generator receives the fundamental reference angular frequency signal, the virtual inertia feedback signal, and the phase compensation coefficient. Perform addition to synthesize the target angular frequency signal; perform integration on the target angular frequency signal to generate a voltage phase angle command for driving the space vector pulse width modulation module.

[0112] This embodiment establishes as follows: Figure 5 The small-signal mathematical model shown considers angular frequency compensation. In actual physical control, the grid voltage phase is calculated in real time through coordinate transformation; however, in this mathematical model, to simplify the analysis process, the grid side is equivalent to a constant frequency source.

[0113] like Figure 5 As shown, in this small-signal mathematical model, the voltage characteristics on the grid side are equivalently treated, that is, by introducing a constant representing the grid's rated angular frequency (marked as 314 rad / s in the figure, corresponding to a 50 Hz power frequency), so that it passes through the transfer function as After integration, a grid voltage reference phase is generated for analysis and comparison. Correspondingly, the model acquires the real-time phase output of the virtual synchronous generator. And a feedback phase is formed after the initial phase of the superimposed microgrid is applied. The difference between the two is calculated using a subtractor, thereby simulating the generation of a phase difference signal at the model level. .

[0114] The phase difference signal It then enters the pre-synchronization control branch, first being regulated by a proportional-integral PI controller to eliminate steady-state error, and then connected in series through a transfer function... A first-order low-pass filter, wherein The inertial time constant is used to simulate the filtering effect of high-frequency noise in actual control, and the final output is the angular frequency compensation amount. In the frequency synthesis stage, this compensation amount... With the system's reference angular frequency of 314 rad / s and via the virtual inertia element ,in For virtual inertia, The damping coefficient is used to superimpose the calculated dynamic power components to synthesize the final output angular frequency of the virtual synchronous generator. This angular frequency After another points-based process Closed-loop generation of output phase This constitutes a complete closed-loop control model.

[0115] The phase compensation coefficient is superimposed on the active frequency control loop of the virtual synchronous generator to correct the phase angle of the output voltage of the virtual synchronous generator, such as... Figure 1As shown, for the pre-synchronization control process, the grid connection control logic begins with the off-grid operation of the virtual synchronous generator (VSG). At this time, the inverter independently supports the local load, and the system continuously monitors whether the host computer or dispatch center issues a grid connection operation command. Once the control system detects a valid grid connection command, it first performs a rapid initial state assessment, that is, determines whether the current grid and microgrid states directly meet the grid connection conditions. If the detection finds that the current voltage parameters are within the allowable closing error range, it directly jumps to the grid connection action; otherwise, it immediately activates the coordinate transformation-based pre-synchronization control strategy described in this invention.

[0116] During the execution of the pre-synchronization strategy, the control system continuously adjusts the output voltage of the microgrid using the aforementioned quadrant judgment and phase difference calculation algorithms, and constructs a real-time monitoring closed-loop feedback loop. This loop frequently compares the current voltage state with preset closing thresholds, which are set as follows: absolute phase difference less than 5 degrees, voltage amplitude difference less than 4V, and frequency difference less than 0.5Hz. If any of these indicators fails to meet the requirements, the system maintains the operation of the pre-synchronization control strategy, continuously correcting the phase and frequency deviations. Once the monitoring data indicates that the above three grid connection conditions are simultaneously met, the system immediately determines that pre-synchronization is complete and triggers the grid connection circuit breaker to close to complete the electrical connection. After confirming the completion of the grid connection action, the system automatically exits the pre-synchronization control algorithm, switches the VSG to grid connection operation mode, and thus ends the entire pre-synchronization process, ensuring the smoothness of the grid connection process and the safety of the system.

[0117] In some public implementations, upon receiving the grid connection command, through... Figure 6 The calculation process can output the real-time phase difference. Meanwhile, in order to improve system stability, The phase difference compensation coefficient needs to be generated through a PI controller and a small inertia circuit. This is then superimposed on the active frequency regulation stage to form a PI-based phase pre-synchronization controller;

[0118] After the superior authority issues the pre-synchronization grid connection signal, the pre-synchronization control is completed according to the following 6 steps:

[0119] : Obtained through coordinate transformation ;

[0120] :according to Judge separately , Record the quadrant position and quadrant number;

[0121] Determine the current quadrant combination by using the quadrant numbers of the two voltage vectors;

[0122] :according to and Quadrant combination, select phase difference The calculation formula;

[0123] : to the voltage vector , Substitute the axis components into the phase difference calculation formula to solve for the current phase difference;

[0124] The calculated phase difference is sent to the pre-synchronization controller to generate phase difference compensation coefficients for pre-synchronization.

[0125] When the synchronization condition is met, the pre-synchronization control algorithm is disconnected at the same time the circuit breaker is closed.

[0126] like Figure 7 As shown, to further intuitively verify the dynamic performance of the control strategy described in this embodiment, the time-domain response waveform of phase A voltage during the pre-synchronization process is illustrated. The solid line in the figure represents the grid-side phase A voltage waveform, which serves as a reference and maintains a constant frequency and amplitude; the dashed line represents the microgrid-side phase A voltage waveform regulated by the control algorithm of this invention.

[0127] As can be seen from the waveform evolution process, in the initial stage of the pre-synchronization control startup, there is a significant phase deviation between the microgrid voltage and the grid voltage. At this time, the two sine curves are offset and do not coincide. As the coordinate transformation-based phase-locked loop-free pre-synchronization control strategy described in this embodiment is put into operation, the system calculates the precise phase difference in real time and generates compensation coefficients which are then superimposed on the active power frequency regulation link to drive the microgrid voltage vector to fine-tune its angular frequency.

[0128] Observing the convergence trend of the curves in the figure, it can be seen that the microgrid voltage waveform exhibits smooth and rapid tracking characteristics during the adjustment process. Its phase gradually approaches the grid voltage phase, and no obvious oscillations, overshoots, or waveform distortions occur during the entire transition period. After a short adjustment period (corresponding to the data in the aforementioned embodiment of the specification, approximately 0.895s), the dashed waveform and the solid waveform completely overlap, indicating that the microgrid voltage and the grid voltage have achieved a high degree of consistency in amplitude, frequency, and phase. At this time, the instantaneous voltage difference between the two approaches zero, satisfying the synchronization condition for circuit breaker closing. This ensures that subsequent grid connection operations can achieve truly seamless switching, effectively avoiding damage to power devices caused by the inrush current due to phase loss at the moment of closing.

[0129] To verify the protection effect of the control algorithm described in this embodiment on the system hardware during grid connection operation, such as... Figure 8As shown in the figure, the time-domain response of the microgrid's A-phase output current is compared under two operating conditions: with and without the pre-synchronization strategy enabled. The red waveform in the figure shows the current change when the system directly performs a closing operation without the pre-synchronization control strategy; the blue waveform shows the current response after adopting the pre-synchronization control strategy proposed in this application.

[0130] Comparative observation reveals that in the traditional mode without effective pre-synchronization measures, a significant inrush current is generated at the moment the circuit breaker closes due to the phase difference or amplitude mismatch between the microgrid voltage and the grid voltage at the moment of closure. As shown by the red curve in the figure, the instantaneous peak value of this inrush current oscillates violently, with an amplitude far exceeding the system's rated operating current, and is accompanied by a long decaying oscillation period. This violent electrical transient can easily cause irreversible physical damage to the inverter's power switching transistors, filter inductors, and circuit breaker contacts, and may even trigger relay protection malfunctions.

[0131] In contrast, when the pre-synchronization control strategy described in this embodiment is activated, the system first enters a precise phase and frequency adjustment phase after the grid connection command is issued. When the closing action occurs, as shown by the blue curve in the figure, the A-phase current waveform maintains a high degree of stability, without any observable spikes or oscillations, and its current amplitude remains within a controlled and safe range. This indicates that the present invention corrects the phase deviation through a precise mathematical model, ensuring that the voltage vectors of the microgrid and the grid almost completely overlap at the time of grid connection. This completely eliminates the mechanism of closing inrush current at the physical level, achieving truly smooth soft grid connection and significantly improving the operational life and system reliability of power electronic equipment during grid-connection and off-grid switching.

[0132] To further verify the validity of this application:

[0133] like Figure 9 As shown, traditional strategies, such as existing conventional pre-synchronization control, are the classic model for off-grid to grid-connected VSG inverters. Their core objective is to achieve frequency, phase, and amplitude matching between the VSG output voltage and the grid voltage through closed-loop regulation, as follows:

[0134] Step 1: Detect the grid voltage using dual PLL modules on both the grid side and the VSG side. and VSG output voltage amplitude ,frequency and phase ;

[0135] Step 2: Calculate the three types of deviations, namely amplitude deviation. Frequency deviation Phase deviation ;

[0136] Step 3: Send the amplitude deviation to the PI controller and output the voltage regulation command. The feedback is then sent to the reactive power-voltage loop; the frequency deviation and phase deviation are fed into the PI regulator and superimposed to output the frequency modulation command. And feed it back to the active-frequency loop;

[0137] Step 4: Adjust the output parameters of VSG through the power loop until the three types of deviations approach zero, and then close the circuit breaker to connect to the grid after pre-synchronization is completed.

[0138] Within the initial phase difference range of 0-90°, experiments showed that the frequency change trends of the traditional strategy and the strategy of this application were consistent. However, during the pre-synchronization period, the maximum frequency difference of the traditional strategy was 0.5643Hz, while that of the strategy of this application was only 0.5363Hz, a reduction of approximately 5%. At the moment of grid connection, the peak frequency difference of the traditional strategy reached 0.1Hz, while that of the strategy of this application was only 0.05Hz, a reduction of approximately 50%. Furthermore, the maximum frequency difference of the virtual current strategy during the pre-synchronization period reached 0.717Hz, and the frequency difference at the moment of grid connection reached 0.113Hz. In comparison, the frequency difference of the strategy of this application was reduced by 25% and 55.7% respectively during the pre-synchronization period and at the moment of grid connection. In summary, the strategy of this application has significant advantages in suppressing frequency deviation during the pre-synchronization grid connection process. Meanwhile, the method proposed in this application achieves the switch from off-grid operation to grid-connected operation in 0.895s, which improves the pre-synchronization speed by 34.36% and 28.4% respectively compared with the traditional method (1.3635s) and the virtual current method (1.25s).

[0139] Example 2:

[0140] This application provides a synchronous control system for grid-connected / off-grid switching of an islanded microgrid based on a virtual generator. While retaining the simplicity of the phase-locked loop-free structure, it reduces the inrush current during grid-connected / off-grid switching; including:

[0141] Voltage sampling circuit, used to acquire voltage signals between microgrid and mains in real time;

[0142] The processor is connected to the voltage sampling circuit;

[0143] Memory, which stores computer programs;

[0144] When the processor runs the computer program, it performs the method as described in Embodiment 1.

[0145] Specifically, the system mainly includes a voltage sampling circuit, a processor, and a memory. The voltage sampling circuit, as the system's front-end sensing unit, is equipped with a high-precision voltage sensor (such as a voltage transformer or Hall effect voltage sensor) and a matching signal conditioning circuit. This circuit is used to acquire the three-phase voltage signals from the inverter output port on the microgrid side and the three-phase voltage signals from the grid-side point of common coupling (PCC). It converts the acquired analog high-voltage signals into low-voltage analog signals adapted to the processor's input range, and after anti-aliasing filtering, transmits them to the processor's analog-to-digital converter (ADC) interface.

[0146] The processor is the core of the entire synchronous control system and can be implemented using a digital signal processor (DSP), microcontroller unit (MCU), or field-programmable gate array (FPGA) or a combination thereof, all with high-speed floating-point operation capabilities. The memory communicates with the processor and is used to non-volatilely store computer-executed instructions, a set of preset phase difference calculation rules, and parameters of the virtual synchronous generator control model.

[0147] During actual system operation, when the processor calls and runs the computer program in memory, it executes the control method described in Embodiment 1 above. The processor first maps the two sets of three-phase voltage data obtained by the sampling circuit to a two-phase stationary coordinate system using its internal algorithm module, and directly locks the spatial position combination of the voltage vectors using vector quadrant judgment logic. Subsequently, based on this position combination, it calls the corresponding algebraic calculation formula to quickly calculate the real-time phase difference between the microgrid and the grid without relying on physical or software phase-locked loops. This real-time phase difference is processed by the digital regulator integrated within the processor (containing proportional-integral and low-pass filtering algorithms), generating a frequency compensation command which is then superimposed onto the active frequency control loop of the virtual synchronous generator.

[0148] Furthermore, the processor is electrically connected to the inverter power module and grid-connected circuit breaker of the microgrid via a control bus. The processor generates a space vector pulse width modulation (SVPWM) drive signal based on the target angle after frequency compensation, controlling the inverter to adjust the phase and frequency of its output voltage to track the grid. Simultaneously, the processor's internal logic comparison module monitors the phase difference, amplitude difference, and frequency difference in real time. Once it detects that all of these parameters meet the preset synchronization and grid-connection conditions, it sends a closing control signal to the grid-connected circuit breaker, thereby achieving a smooth and shockless switching of the microgrid from islanded mode to grid-connected mode.

[0149] In the description of this specification, the references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0150] The foregoing has shown and described the basic principles, main features, and advantages of this application. Those skilled in the art should understand that this application is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of this application. Various changes and modifications can be made to this application without departing from the spirit and scope thereof, and all such changes and modifications fall within the scope of the claims of this application.

Claims

1. A synchronous control method for on-grid / off-grid switching of islanded microgrids based on virtual generators, characterized in that, The method includes: The three-phase voltage signals from the microgrid side and the grid side are collected. The three-phase voltage signals from the microgrid side and the grid side are transformed from a three-phase stationary coordinate system to a two-phase stationary coordinate system through a coordinate transformation module. The orthogonal components of the microgrid voltage and the grid voltage are decoupled to obtain the orthogonal components of the microgrid voltage and the grid voltage. The orthogonal components of the microgrid voltage and the orthogonal components of the grid voltage are input into the vector quadrant judgment module. Based on the positive and negative attributes of each component on the coordinate axes of the two-phase stationary coordinate system, the quadrant values ​​of the microgrid voltage and the grid voltage are determined respectively. Based on the position combination formed by the microgrid voltage quadrant value and the grid voltage quadrant value, the corresponding calculation formula is matched from the preset phase difference calculation rules, and the orthogonal components of the microgrid voltage and the grid voltage are substituted into the calculation formula to calculate the real-time phase difference signal. The real-time phase difference signal is sent to the regulator for proportional-integral and filtering processing to generate a phase compensation coefficient. The phase compensation coefficient is then superimposed on the active frequency control loop of the virtual synchronous generator to correct the phase angle of the output voltage of the virtual synchronous generator.

2. The method according to claim 1, characterized in that, The step of inputting the orthogonal components of the microgrid voltage and the orthogonal components of the grid voltage into the vector quadrant determination module, and determining the quadrant values ​​of the microgrid voltage and the grid voltage respectively based on the positive and negative attributes of each component on the coordinate axes of the two-phase stationary coordinate system, includes: The orthogonal component of the microgrid voltage or the orthogonal component of the grid voltage is detected in the two-phase stationary coordinate system. The values ​​on the axis; In response to the The value on the axis is greater than 0, detecting the orthogonal component of the microgrid voltage or the orthogonal component of the grid voltage in the two-phase stationary coordinate system. The values ​​on the axis, if the stated If the value on the axis is greater than 0, output the first quadrant number; if the value on the axis is greater than 0, output the first quadrant number. If the value on the axis is not greater than 0, output the fourth quadrant number; In response to the The value on the axis is not greater than 0, and the orthogonal component of the microgrid voltage or the orthogonal component of the grid voltage is detected in the two-phase stationary coordinate system. The values ​​on the axis, if the stated If the value on the axis is greater than 0, output the second quadrant number; if the value on the axis is greater than 0, output the second quadrant number. If the value on the axis is not greater than 0, output the third quadrant number; The quadrant number corresponding to the microgrid voltage in the two-phase stationary coordinate system is denoted as... The quadrant number corresponding to the grid voltage in the two-phase stationary coordinate system is denoted as... .

3. The method according to claim 2, characterized in that, Based on the position combination formed by the microgrid voltage quadrant value and the grid voltage quadrant value, a corresponding calculation formula is matched from a preset phase difference calculation rule, and the orthogonal components of the microgrid voltage and the grid voltage are substituted into the calculation formula, including: Construct by the With the The quadrant combinations formed; When the quadrant combination represents that the grid voltage vector and the microgrid voltage vector are in the same quadrant, the first calculation formula is invoked. When the quadrant combination represents the grid voltage vector and the microgrid voltage vector crossing adjacent quadrants, the second calculation formula is invoked; When the quadrant combination represents the grid voltage vector and the microgrid voltage vector crossing the phase limit, the third calculation formula is invoked.

4. The method according to claim 3, characterized in that, The first calculation formula is: in, The real-time phase difference signal; The phase angle is calculated based on the orthogonal components of the microgrid voltage. The phase angle is calculated based on the orthogonal components of the grid voltage.

5. The method according to claim 3, characterized in that, The second calculation formula is: 1) When the quadrant combination corresponds to crossing quadrants I and II or crossing quadrants III and IV: 2) When the quadrant combination corresponds to crossing quadrants two and three or crossing quadrants one and four: in, The real-time phase difference signal; The phase angle is calculated based on the orthogonal components of the microgrid voltage. The phase angle is calculated based on the orthogonal components of the grid voltage. Pi is a constant.

6. The method according to claim 3, characterized in that, The third calculation formula is as follows: in, The real-time phase difference signal; The phase angle is calculated based on the orthogonal components of the microgrid voltage. The phase angle is calculated based on the orthogonal components of the grid voltage. Pi is a constant.

7. The method according to claim 1, characterized in that, The real-time phase difference signal is fed into the regulator for proportional-integral and filtering processing to generate phase compensation coefficients, including: The real-time phase difference signal is input into the proportional-integral control unit in the regulator to calculate the intermediate adjustment amount; The intermediate adjustment value is used as the input signal, filtered by the low-pass filter unit in the regulator, and the phase compensation coefficient is output. ; The transfer function of the low-pass filter unit for: In the formula, It is a time constant. For the Laplace operator.

8. The method according to claim 1, characterized in that, The step of superimposing the phase compensation coefficient onto the active frequency control loop of the virtual synchronous generator specifically includes: The frequency synthesis node of the virtual synchronous generator receives the fundamental reference angular frequency signal, the virtual inertia feedback signal, and the phase compensation coefficient. ; Perform addition operations to synthesize the target angular frequency signal; An integral operation is performed on the target angular frequency signal to generate a voltage phase angle command for driving the space vector pulse width modulation module.

9. The method according to claim 1, characterized in that, The method also includes a grid connection switching step: Configure real-time monitoring of the real-time phase difference signal, as well as the voltage amplitude difference signal and frequency difference signal between the microgrid and the power grid; When the real-time phase difference signal is detected to be less than a preset phase threshold, the voltage amplitude difference signal is less than a preset amplitude threshold, and the frequency difference signal is less than a preset frequency threshold, a grid connection permission signal is issued. The grid-connected circuit breaker is closed according to the grid-connection permission signal, and the input of the phase compensation coefficient is cut off.

10. A synchronous control system for on-grid / off-grid switching of an islanded microgrid based on a virtual generator, characterized in that, include: Voltage sampling circuit, used to acquire voltage signals between microgrid and mains in real time; The processor is connected to the voltage sampling circuit; Memory, which stores computer programs; When the processor runs the computer program, it performs the method as described in any one of claims 1 to 9.

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