A frequency modulation device with frequency event ride-through and seamless on-off grid switching capability

Abstract

This invention discloses a frequency regulation device with frequency event crossover and seamless grid connection / off-grid switching capabilities. This invention relates to the field of power grid technology. It involves real-time monitoring of the main grid frequency, constructing a frequency sequence, calculating the rate of change, plotting a frequency change rate fluctuation curve, and locking the crossover trigger point based on real-time frequency values. A tiered off-grid control strategy is employed, recording the voltage amplitude, frequency, and phase parameters at the grid connection point in real time, and assessing the severity level of the event. When the event reaches the first severity level, the droop coefficient is dynamically increased through an adaptive gain coefficient, and a virtual inertia is introduced to execute a feedback response, achieving dynamic adjustment of the power command. When the event reaches the second severity level, the solid-state switch is immediately disconnected, and the inverter is switched to a locked output state. After completing the off-grid transition, the main grid stability index is continuously assessed. By gradually adjusting the microgrid voltage amplitude, frequency, and phase to synchronize with the main grid, seamless secondary grid connection is ultimately achieved.

Description

Technical Field

[0001] This invention belongs to the field of power grid technology, specifically relating to a frequency regulation device with frequency event ride-through and seamless on-grid / off-grid switching capabilities. Background Technology

[0002] As a key hub connecting distributed energy resources with the main power grid, the smoothness and response speed of microgrid switching between grid connection and off-grid have become important indicators for measuring power supply quality.

[0003] In terms of frequency event detection, traditional methods usually rely on a single frequency threshold or a simple frequency change rate threshold to trigger a response. This is prone to false triggering due to instantaneous disturbances or noise, making it difficult to capture the evolution trend of frequency events in a timely manner. Often, the system switches blindly in the early stages of frequency fluctuations or the response is delayed, resulting in poor coordination between the microgrid and the main grid. Secondly, traditional devices typically only have a single off-grid criterion. Once an anomaly is detected, the connection is immediately disconnected. There is a lack of quantitative assessment and differentiated response to the severity of the event. When a slight frequency disturbance occurs, it may overreact, causing unnecessary islanding operation, increasing the difficulty and energy loss of subsequent grid restoration. It is also unable to maintain grid-connected operation by dynamically adjusting control parameters during mild disturbances, thus reducing the power supply reliability and power quality of the system. Finally, traditional methods often rely on timed reconnection or simple voltage amplitude detection to determine whether the main grid has returned to normal after disconnection. This can easily overlook the synchronization accuracy of frequency and phase, resulting in a large inrush current during secondary grid connection, triggering protection actions. If grid connection is forced before the frequency is fully stable, or if the synchronization adjustment is too coarse, grid connection failure or equipment damage may occur. This makes it difficult to meet the requirements of modern microgrids for high reliability and seamless switching.

[0004] To address the aforementioned issues, this invention proposes a frequency modulation device with frequency event crossover and seamless on-grid / off-grid handover capabilities. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a frequency modulation device with frequency event crossover and seamless grid connection / offline switching capabilities, solving the problems of coarse frequency disturbance response, large impact during grid connection / offline switching, and asynchronous grid connection recovery in existing technologies.

[0006] The objective of this invention can be achieved through the following technical solutions: A frequency modulation device with frequency event traversal and seamless on / off-grid handover capabilities, the frequency modulation device comprising: The frequency event detection terminal monitors the main power grid frequency corresponding to the time in real time, constructs the power grid frequency sequence associated with the main power grid, calculates the frequency change rate between each time based on the numerical performance of the power grid frequency at each time, plots the frequency change rate fluctuation curve associated with the main power grid and the time, and locks the trigger point of frequency event crossing by combining the real-time power grid frequency value. The off-grid control terminal records the voltage amplitude, voltage frequency, and voltage phase of all grid-connected points in real time at the corresponding time of the trigger point, constructs a correlation data set, and assesses the severity level of frequency event crossover. At the first severity level, an adaptive gain coefficient is used to dynamically increase the droop coefficient while simultaneously introducing virtual inertia to execute a feedback response. When the grid connection is restored, the solid-state switch is immediately disconnected and the inverter's control mode is switched to locked output state to achieve off-grid operation when the severity level reaches level two. The grid frequency and voltage after disconnection are monitored in real time, and the stability index of the main grid is continuously evaluated in a monitoring cycle until the main grid enters a stable state. The voltage amplitude, voltage frequency and voltage phase of the microgrid are adjusted to complete the secondary grid connection.

[0007] As a further aspect of the present invention, the specific method for constructing the power grid frequency sequence associated with the main power grid in the frequency event detection terminal is as follows: Identify the main power grid and label it Q; Lock the current time, denoted as t1. Use time t1 as the start time for monitoring the main power grid Q. Align the time sequence to obtain the main power grid frequency and perform a permutation to obtain the power grid frequency sequence H1, H2, ..., Hj corresponding to the time sequence t1, t2, ..., tj, where j represents the total number of time points and the total number of corresponding power grid frequencies. The value of j increases over time.

[0008] As a further aspect of the present invention, in the frequency event detection terminal, based on the numerical performance of the power grid frequency at each moment, the frequency change rate between each moment is calculated, and the specific method for plotting the frequency change rate fluctuation curve of the main power grid associated with the moment is as follows: Obtain an arbitrary power grid frequency Hi, where 1≤i≤j; Take the grid frequency Hk at the next moment after the grid frequency Hi, where k≤j; The rate of change of the power grid frequency between time i and time k is calculated using R(i,k)=|Hk-Hi| / (tk-ti); Similarly, the rate of change of the power grid frequency between each adjacent time point is calculated to obtain the power grid frequency rate of change sequence R(1,2),R(2,3),...,R(j-1,j); Construct a two-dimensional coordinate system XY1 with time as the horizontal axis and the rate of change of power grid frequency as the vertical axis; Extract the grid frequency change rate R(1,2) from the grid frequency change rate sequence R(1,2), R(2,3),...,R(j-1,j). Plot the data points of the grid frequency change rate R(1,2) at the first and second moments on the horizontal axis, and plot the subsequent data points in sequence. Use curve fitting to fit all data points and denote it as the grid frequency change rate fluctuation curve S1 associated with the main grid and time.

[0009] As a further aspect of the present invention, the specific method by which the frequency event detection terminal locks the trigger point of the frequency event crossing in combination with the real-time power grid frequency value is as follows: Obtain the power grid frequency change rate fluctuation curve S1 and the two-dimensional coordinate system XY1; On the power grid frequency change rate fluctuation curve S1, determine the data points corresponding to the first power grid frequency change rate R(1,2) and the second power grid frequency change rate R(2,3), and denot them as d1 and d2 respectively. Construct straight lines perpendicular to the horizontal axis and parallel to the vertical axis through the two data points, denoted as line L1 and line L2; Draw a perpendicular line C1 through the data point d1 on line L1, intersecting line L2 at point d3; Calculate the area of ​​the closed region formed by data point d1, data point d2 and point d3, and denote it as the change characteristic TZ(1-2); Similarly, determine the change characteristics TZ(2-3),...,TZ(j-1,j); Obtain the preset change feature threshold TZ yu If any change characteristic is greater than or equal to TZ yu Then, the time intervals associated with the corresponding change characteristics are formed, and each time interval is marked as the trigger point for the frequency event to cross. Synchronously acquire the power grid frequency sequence H1, H2, ..., Hj. If there exists a power grid frequency ∉ [Hmin, Hmax] at any time, mark that time as the trigger point for the frequency event crossing, where Hmin and Hmax are the preset minimum and maximum safe power grid frequencies, respectively.

[0010] As a further aspect of the present invention, the specific method for evaluating the severity level of frequency event traversal in the graded off-grid control terminal is as follows: Obtain the voltage amplitude, voltage frequency, and voltage phase of the microgrid at all grid-connected points at any trigger point at any given time, and form an associated data set, denoted as {U_v,f_v,θ_v}, where v=1,2,...,V, and V is the total number of grid-connected points; Calculate the voltage amplitude deviation ΔU_v=|U_v-U_N|, voltage frequency deviation Δf_v=|f_v-f_N|, and voltage phase deviation Δθ_v=|θ_v-θ_N| at each grid connection point; U_N, f_N, and θ_N represent the rated voltage amplitude, rated voltage frequency, and rated voltage phase, respectively. The voltage amplitude deviation ΔU_v, voltage frequency deviation Δf_v, and voltage phase deviation Δθ_v are normalized using δU_v=ΔU_v / ΔU_max, δf_v=Δf_v / Δf_max, and δθ_v=Δθ_v / Δθ_max, where ΔU_max is the preset maximum allowable voltage amplitude deviation, Δf_max is the preset maximum allowable frequency deviation, and Δθ_max is the maximum allowable phase deviation. The comprehensive deviation value P_v of each grid connection point is calculated using P_v=α×δU_v+β×δf_v+γ×δθ_v, where α, β, and γ are preset weight coefficients, all of which are greater than 0, and α+β+γ=1. The maximum comprehensive deviation value P_max among all grid connection points is taken as the severity index; Compare P_max with the preset first threshold P1 and second threshold P2, where P1 < P2: If P1≤P_max<P2, it is determined to be the first severity level; If P_max ≥ P2, it is determined to be the second most serious level.

[0011] As a further aspect of the present invention, in the graded off-grid control terminal, the specific method of dynamically increasing the droop coefficient with an adaptive gain coefficient and simultaneously introducing virtual inertia to execute the feedback response at the first severe level is as follows: Obtain the rate of change of the power grid frequency R(i,i+1) at any trigger point corresponding to time i when the first severity level is reached; The dynamic adjustment droop coefficient ef is calculated using ef = ef0 + η × |R(i,i+1)|, where ef0 is the initial droop coefficient, η is the preset adaptive gain coefficient, and ef ≤ ef_max = rated power × 15%. The additional power ΔP is output using ΔP=G×|R(i,i+1)|, where the virtual inertia coefficient G increases dynamically with the increase of |R(i,i+1)|, G=G0+λ×|R(i,i+1)|, G0 is the preset initial virtual inertia coefficient, and λ is the preset inertia gain coefficient; The total power command P_total is calculated using P_total=P0-ef×(f-f_nom)+ΔP, where P0 is the current output power reference, f is the real-time grid frequency, and f_nom is the rated frequency. The total power command P_total is used as the active power reference value for main grid control. Continuously monitor the rate of change of the grid frequency at subsequent times until the subsequent M consecutive times are non-trigger points, adjust ef to ef0, restore G to G0, recalculate the total power command, and execute the main grid control; M is the preset total number of safe redundancy times.

[0012] As a further aspect of the present invention, in the grid-connected recovery terminal, when the severity level reaches the second level, the solid-state switch is immediately disconnected, and the inverter's control mode is switched to a locked output state. The specific method for achieving grid disconnection is as follows: Get the time i corresponding to any trigger point when the second severity level is reached; At time i, the solid-state switch is disconnected, and a mode switching command is sent to the inverter controller simultaneously to switch the inverter's control mode from active / reactive power regulation mode to locked output mode. The inverter's output voltage command is forced to zero to achieve off-grid operation. It also records the microgrid voltage phase, microgrid voltage frequency, and microgrid voltage amplitude corresponding to the off-grid time.

[0013] As a further aspect of the present invention, the specific method for completing the secondary grid connection in the restored grid connection terminal is as follows: The off-grid time is determined, and the grid frequency H(Δt) and grid voltage V(Δt) of the main grid are continuously collected at a fixed monitoring period Δt. The microgrid voltage phase θ(Δt), microgrid voltage frequency f(Δt), and microgrid voltage amplitude U(Δt) of the microgrid are collected simultaneously. Here, H(Δt) represents the grid frequency of the main grid at any time within the monitoring period Δt, and the rest are similar. The stability index SAG(Δt) of the main power grid during the monitoring period Δt is calculated by using SAG(Δt)=ω1×|(H(Δt)-f_nom) / f_nom|+ω2×|(V(Δt)-U_nom) / U_nom|, where U_nom is the rated voltage, ω1 and ω2 are preset weighting coefficients, both of which are greater than 0 and w_1+w_2=1; If the stability index SAG(Δt) of the main power grid is less than SAG_th for N consecutive monitoring periods Δt, the main power grid is determined to have entered a stable state, where N is a preset integer; Conversely, continuous monitoring is required. Once the main grid enters a stable state, the microgrid voltage frequency f(Δt) is adjusted to match the grid frequency H(Δt), with an adjustment step size ≤ 0.01Hz / Δt. Adjust the microgrid voltage amplitude U_mg to match the grid voltage V(Δt), with an adjustment step size ≤ 0.5V / Δt; Similarly, adjust the microgrid voltage phase θ(Δt) to synchronize with the main grid, close the solid-state switch, and complete the secondary grid connection.

[0014] The beneficial effects of this invention are: This invention uses a frequency event detection terminal to map the frequency change rate fluctuation curve in real time, accurately pinpointing the trigger point and thus proactively responding to grid disturbances, improving the initiative and accuracy of frequency event crossover. The tiered off-grid control terminal adopts differentiated strategies based on severity level: at the first severity level, the adaptive gain coefficient dynamically adjusts the droop coefficient and introduces virtual inertia to enhance system damping and inertial support, suppressing frequency fluctuations; at the second severity level, the solid-state switch is disconnected and the inverter output is locked, achieving seamless off-grid connection, preventing fault propagation, and the grid-connected terminal continuously monitors the main grid stability index. Once conditions are met, the microgrid voltage amplitude, frequency, and phase are precisely adjusted to complete a smooth secondary grid connection. It combines the advantages of fast frequency response, tiered adaptive control, seamless switching, and reliable self-recovery, improving the microgrid's operational stability, power supply continuity, and grid-connection coordination capabilities under complex grid conditions. This invention constructs a real-time frequency sequence of the main power grid and calculates the frequency change rate between adjacent time points to form a fluctuation curve. It dynamically captures subtle fluctuations in the power grid frequency, uses the area of ​​a closed region formed by continuous data points as a change feature, and combines it with a preset threshold to accurately locate the trigger point of frequency event crossing. At the same time, it is supplemented by double verification within a safe frequency range to improve the reliability and sensitivity of event detection, avoid the lag and false alarms of single threshold judgment, identify abnormal frequency trends in advance, and provide clear and timely triggering basis for subsequent frequency adjustment actions, thus ensuring the stability and safety of power grid operation. This invention identifies the severity level of frequency events by comprehensively evaluating the multi-dimensional deviations of voltage amplitude, frequency, and phase, avoiding misjudgments that may be caused by a single indicator. This provides a basis for graded control. For the first severity level, an adaptive gain coefficient is introduced to dynamically increase the droop coefficient, and the virtual inertia is adjusted in real time in combination with the frequency change rate, so that the frequency regulation response is both fast and predictable, effectively suppressing the deterioration of frequency fluctuations. Additional power is introduced to enhance the active support capability, and the initial parameters are automatically restored after the event to ensure the steady-state operation of the system. This enhances the frequency stability and robustness of the microgrid in off-grid or disturbance scenarios and reduces the risk of grid disconnection. During the off-grid and reconnection process, when the severity level reaches level two, the solid-state switch is immediately disconnected and the inverter output is forced to zero, quickly isolating the fault and preventing the microgrid from impacting the main grid. At the same time, the off-grid reference parameter set is recorded to provide a reference for subsequent synchronization. During the reconnection phase, by calculating and continuously monitoring a stability index that includes frequency and voltage deviations, synchronization is only initiated after the main grid has truly entered a steady state, avoiding faults caused by grid fluctuations. Step size limits are used during the adjustment process to achieve smooth tracking of frequency, voltage, and phase, reducing electrical shocks and equipment stress during grid connection. Attached Figure Description

[0015] The invention will now be further described with reference to the accompanying drawings.

[0016] Figure 1 This is a schematic diagram of the frequency modulation device described in this invention; Figure 2 This is a flowchart illustrating the method described in Embodiment 2 of the present invention; Figure 3 This is a flowchart illustrating the method described in Embodiment 3 of the present invention. Detailed Implementation

[0017] 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, and 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.

[0018] like Figure 1 As shown, this application provides a frequency modulation device with frequency event crossover and seamless on-grid / off-grid handover capabilities; As an embodiment 1 of this application, it specifically includes: The frequency event detection terminal monitors the main power grid frequency corresponding to the time in real time, constructs the power grid frequency sequence associated with the main power grid, calculates the frequency change rate between each time based on the numerical performance of the power grid frequency at each time, plots the frequency change rate fluctuation curve associated with the main power grid and the time, and locks the trigger point of frequency event crossing by combining the real-time power grid frequency value. The off-grid control terminal records the voltage amplitude, voltage frequency, and voltage phase of all grid-connected points in real time at the corresponding time of the trigger point, constructs a correlation data set, and assesses the severity level of frequency event crossover. At the first severity level, an adaptive gain coefficient is used to dynamically increase the droop coefficient while simultaneously introducing virtual inertia to execute a feedback response. When the grid connection is restored, the solid-state switch is immediately disconnected and the inverter's control mode is switched to locked output state to achieve off-grid operation when the severity level reaches level two. The grid frequency and voltage after disconnection are monitored in real time, and the stability index of the main grid is continuously evaluated in a monitoring cycle until the main grid enters a stable state. The voltage amplitude, voltage frequency and voltage phase of the microgrid are adjusted to complete the secondary grid connection. Example 2

[0019] This embodiment, based on Embodiment 1, further explains and illustrates the steps involved in the frequency event detection terminal, and further discloses a method for detecting power grid frequency event trigger points, such as... Figure 2 As shown, it specifically includes the following: First, the main grid Q, pre-calibrated by the operator, is obtained. It should be noted that the main grid Q also interacts with multiple microgrids through the grid connection point. Lock the current time and mark it as time t1. Use time t1 as the start time for monitoring the main power grid Q. Align the time sequence and sequentially acquire the grid frequencies of the main power grid Q. Sort all acquired grid frequencies in chronological order to obtain the grid frequency sequence H1, H2, ..., Hj corresponding to the time sequence t1, t2, ..., tj, where j represents the total number of time points and also the total number of grid frequencies. The value of j increases over time, meaning that the longer the monitoring time, the larger the amount of data in the grid frequency sequence, which facilitates the real-time addition of new data.

[0020] Obtain any grid frequency Hi from the constructed grid frequency sequence H1, H2, ..., Hj, where i is the counting index and its value range is 1 ≤ i ≤ j; Taking the grid frequency Hi at time i as an example, all other grid frequencies are processed in the same and synchronous manner as the grid frequencies, as follows: Obtain the grid frequency Hk of the grid frequency Hi at the next adjacent time, which is time i+1, i.e., k=i+1, and k≤j; The power grid frequency change rate R(i,k) between time i and time k is calculated using R(i,k)=|Hk-Hi| / (tk-ti). This method is then applied to the remaining power grid frequencies to obtain the power grid frequency change rate between adjacent time points. All power grid frequency change rates are arranged in chronological order and denoted as the power grid frequency change rate sequence R(1,2), R(2,3),...,R(j-1,j). This power grid frequency change rate reflects the severity of frequency fluctuations, normalizes the influence of time intervals, and can handle non-equal interval sampling. By calculating the absolute value, directional interference is avoided, and only the degree of power grid frequency fluctuation per unit time is considered.

[0021] Then, construct a two-dimensional coordinate system with time (timeline) as the horizontal axis and the power grid frequency change rate as the vertical axis, and label it as the two-dimensional coordinate system XY1; Next, the first power grid frequency change rate R(1,2) is extracted from the power grid frequency change rate sequence R(1,2), R(2,3),...,R(j-1,j), and the subsequent power grid frequency change rates are traversed in turn and the following example processing of the power grid frequency change rate R(1,2) is performed. First, determine the time associated with the grid frequency change rate R(1,2), namely the first time and the second time. Then, mark the scale corresponding to the first time and the second time on the horizontal axis. Next, plot the grid frequency change rate R(1,2) at the scale between the first time and the second time on the horizontal axis, where the vertical axis scale represents the value of the grid frequency change rate R(1,2). This yields a data point associated with the grid frequency change rate R(1,2). Then, plot all subsequent data points associated with the grid frequency change rate in sequence. Fit a curve to all data points and denote it as the main grid frequency change rate fluctuation curve S1 associated with the time.

[0022] Then, the constructed power grid frequency change rate fluctuation curve S1 and the two-dimensional coordinate system XY1 are further processed. Obtain the data points corresponding to the first power grid frequency change rate R(1,2) and the second power grid frequency change rate R(2,3) on the power grid frequency change rate fluctuation curve S1, and mark these two data points as data point d1 and data point d2 respectively. Construct two straight lines through data points d1 and d2 respectively, one perpendicular to the horizontal axis and the other parallel to the vertical axis. These two lines are denoted as line L1 and line L2 respectively. Then, draw a perpendicular line C1 through the data point d1 on the straight line L1 to intersect the straight line L2 at point d3. This perpendicular line is parallel to the horizontal axis and is perpendicular to both the straight lines L1 and L2. At this point, data points d1, d2, and d3 will form a triangular closed region. The area of ​​each region is calculated, and the area value is marked as the change feature TZ(1-2) associated with the first power grid frequency change rate R(1,2) and the second power grid frequency change rate R(2,3). The change feature quantifies the rate of change of the power grid frequency change rate. The larger the change feature, the more abrupt the frequency fluctuation changes in a short period of time. It incorporates the time span and can more sensitively capture transient impact events.

[0023] Similarly, the same processing is applied to the subsequent grid frequency change rate, ultimately yielding the change characteristics TZ(2-3),...,TZ(j-1,j) excluding the change characteristic TZ(1-2). Obtain the change characteristic threshold TZ preset by the operator based on actual needs. yu If any change feature exists that is greater than or equal to the change feature threshold TZ yu Then, the change feature and the time associated with the change feature are extracted and combined into a time interval, and each time in the determined time interval is marked as the trigger point for the frequency event to cross. At the same time, the power grid frequency sequence H1, H2, ..., Hj is obtained, and the power grid frequency range [Hmin, Hmax] preset by the operator is obtained, where Hmin and Hmax are the preset minimum and maximum safe power grid frequencies, respectively. If there exists a time when the grid frequency does not belong to the grid frequency range [Hmin, Hmax], then the corresponding time will be marked as the trigger point for the frequency event crossing.

[0024] By using both sudden changes in the rate of frequency change and exceeding the limit of the absolute frequency value for judgment, false alarms of slow drift are avoided, while rapid response to violent fluctuations or extreme frequency values ​​is achieved, thus improving the reliability of the frequency modulation device. Example 3

[0025] This embodiment further discloses a multi-parameter weighted hierarchical off-grid frequency control method based on embodiment 2, such as... Figure 3 As shown, it specifically includes the following: Obtain any time corresponding to any trigger point, and at this time obtain the voltage amplitude, voltage frequency and voltage phase of the microgrids interacting with the main grid at all grid connection points. This forms an associated data group, denoted as {U_v,f_v,θ_v}, where v=1,2,...,V, and V is the total number of grid connection points. Next, the voltage amplitude deviation ΔU_v=|U_v-U_N|, voltage frequency deviation Δf_v=|f_v-f_N|, and voltage phase deviation Δθ_v=|θ_v-θ_N| of the microgrid at each grid connection point are calculated, where U_N, f_N, and θ_N are the rated voltage amplitude, rated voltage frequency, and rated voltage phase, respectively. If the needs of different microgrids are different, the values ​​of rated voltage amplitude, rated voltage frequency, and rated voltage phase can be adaptively adjusted.

[0026] Next, the voltage amplitude deviation ΔU_v, voltage frequency deviation Δf_v, and voltage phase deviation Δθ_v are normalized by using δU_v=ΔU_v / ΔU_max, δf_v=Δf_v / Δf_max, and δθ_v=Δθ_v / Δθ_max, respectively. It should be noted that ΔU_max is the maximum permissible voltage amplitude deviation preset by the operator, Δf_max is the maximum permissible frequency deviation preset by the operator, and Δθ_max is the maximum permissible phase deviation. Finally, the comprehensive deviation value P_v associated with the microgrid at each grid connection point is calculated by using P_v=α×δU_v+β×δf_v+γ×δθ_v. It should be noted that α, β, and γ are all weighting coefficients preset by the operator and are all greater than 0, α+β+γ=1. The maximum comprehensive deviation value P_max among all grid connection points is obtained as the severity index, and the microgrid corresponding to the maximum comprehensive deviation value P_max is processed first (i.e., the highest priority). The comprehensive deviation value P_max is compared with the first threshold P1 and the second threshold P2 preset by the operator, wherein the first threshold P1 is less than the second threshold P2. If the comparison result shows that the comprehensive deviation value P_max is greater than or equal to the first threshold P1 and less than the second threshold P2, then the interaction between the main power grid and the microgrid is determined to be of the first severe level. If the comparison result shows that the comprehensive deviation value P_max is greater than or equal to the second threshold P2, then the interaction between the main power grid and the microgrid is determined to be at the second severe level.

[0027] Next, when the interaction between the main grid and the microgrid reaches the first severe level, an adaptive gain coefficient is used to dynamically increase the droop coefficient while simultaneously introducing virtual inertia to execute a feedback response, as follows: Extract the time corresponding to any trigger point when the interaction between the main grid and the microgrid is determined to be the first severity level, mark it as time i, and obtain the grid frequency change rate R(i,i+1) at time i and time i+1. The dynamic adjustment droop coefficient ef is calculated using the formula: ef = ef0 + η × |R(i,i+1)|, where ef0 is the initial droop coefficient, set by the operator based on actual conditions, and η is the operator's preset adaptive gain coefficient. ef ≤ ef_max = rated power × 15%, with an upper limit of 15% of the rated power. The operator can adjust this percentage according to actual needs. In this way, the droop coefficient increases with the rate of frequency change, achieving adaptive gain. That is, the more severe the fluctuation, the stronger the droop control effect, and the faster the offset is suppressed.

[0028] The additional power ΔP is then calculated and output using ΔP=G×|R(i,i+1)|, where the virtual inertia coefficient G increases dynamically with the increase of |R(i,i+1)|, G=G0+λ×|R(i,i+1)|, G0 is the initial virtual inertia coefficient preset by the operator, and λ is the inertia gain coefficient preset by the operator. The inertia response of the power generation process is simulated through virtual inertia, and the virtual inertia coefficient G also increases dynamically with the rate of change to provide additional power proportional to the rate of change of the grid frequency and damp the frequency oscillation. It should also be noted that when R(i,i+1) in |R(i,i+1)| increases, the power needs to be reduced; when R(i,i+1) decreases, the power needs to be increased. The trend of change can be directly locked in the grid frequency change rate fluctuation curve S1. Finally, the total power command P_total is calculated by using P_total=P0-ef×(f-f_nom)+ΔP. It should be noted that P0 is the current output power reference, which is the actual output power at the moment before the trigger point, f is the real-time grid frequency, and f_nom is the rated frequency. Then, the total power command P_total is used as the active power reference value to regulate the output power of the main grid. The purpose is to regulate the active power to the total power command P_total. After regulation, the grid frequency change rate is continuously monitored at each subsequent moment until there are M consecutive non-trigger points. At this point, a recovery operation is performed, ef is adjusted to ef0, G is restored to G0, and the total power command is recalculated. The main grid regulation is then performed, where M is the preset total number of safe redundancy moments. Example 4

[0029] This embodiment further discloses a method for off-grid and re-grid connection based on a stability index, based on embodiment 3, specifically including the following: When the interaction between the main grid and the microgrid is determined to be at the second severe level, the time i corresponding to any trigger point at the second severe level is obtained in real time (generally, the first time of the second severe level trigger or a buffer time preset by the operator, such as the third time). At time i, the solid-state switch is disconnected. At this time, a mode switching command is sent to the inverter controller simultaneously, and the inverter's control mode is switched from active / reactive power regulation mode to locked output mode. At the same time, the inverter's output voltage command is forcibly set to zero, realizing the off-grid operation of the microgrid and the main grid. The microgrid control mode will be switched to islanded operation mode, and the local power supply will maintain the power supply of important loads. If there is no local power supply, all loads will automatically go into standby mode.

[0030] After the operation is off-grid, the microgrid voltage phase, microgrid voltage frequency, and microgrid voltage amplitude are recorded in real time at the off-grid time and at subsequent times.

[0031] The operator presets a monitoring period Δt, and uses the monitoring period Δt as a cycle to obtain the main grid frequency H(Δt) and grid voltage V(Δt), as well as the microgrid voltage phase θ(Δt), microgrid voltage frequency f(Δt), and microgrid voltage amplitude U(Δt) of the main grid. Here, H(Δt) represents the main grid frequency at any time within the monitoring period Δt, and the rest are obtained in the same way.

[0032] The stability index SAG(Δt) of the main power grid during the monitoring period Δt is calculated using SAG(Δt) = ω1 × |(H(Δt) - f_nom) / f_nom| + ω2 × |(V(Δt) - U_nom) / U_nom|. It should be noted that SAG(Δt) represents the stability index of the main power grid at each moment during the monitoring period Δt, where U_nom is the rated voltage of the main power grid, and ω1 and ω2 are weighting coefficients preset by the operator. Both ω1 and ω2 are greater than 0 and w_1 + w_2 = 1. Next, the stability index of the main power grid is continuously monitored. If the stability index SAG(Δt) of the main power grid is less than SAG_th for N consecutive monitoring periods Δt, the main power grid is determined to have entered a stable state, where N is a preset integer. If the stability index SAG(Δt) of the main power grid is not less than SAG_th for N consecutive monitoring periods Δt, then monitoring continues. It should be noted that after a frequency crossover event occurs in the main power grid, the frequency and voltage may experience fluctuations and recovery processes. Therefore, judging stability based on a single instantaneous value is prone to false triggering. The stability index combines the relative deviations of frequency and voltage, calculates the weighted sum, compares it with the threshold, and requires that the condition be met for N consecutive monitoring periods Δt. This is equivalent to setting a low-pass filter with hysteresis to ensure that the main power grid truly returns to steady state.

[0033] Once the main grid has reached a stable state, the microgrid voltage frequency f(Δt) is adjusted to match the grid frequency H(Δt), with an adjustment step size ≤ 0.01 Hz / Δt. Simultaneously, the microgrid voltage amplitude U_mg is adjusted to match the grid voltage V(Δt), with an adjustment step size ≤ 0.5 V / Δt. In addition, the microgrid voltage phase θ(Δt) needs to be synchronized with the main grid. The specific synchronization accuracy needs to be determined by the operator based on actual needs. After all the above adjustment operations are completed, and the voltage frequency, grid voltage, and voltage phase of the microgrid and the main grid are all in perfect harmony, the solid-state switch is closed to complete the secondary grid connection of the microgrid.

[0034] All data in the formulas described above have been calculated with dimensions removed. Furthermore, any content not described in detail in this specification is existing technology known to those skilled in the art.

[0035] The above description is merely an example and illustration of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described, or use similar methods to replace them, as long as they do not deviate from the invention or exceed the scope defined in the claims, all of which should fall within the protection scope of the present invention.

[0036] It should be stated that all user data collected in this application was collected with the user's consent and authorization. Furthermore, the uses of user data are legal and compliant, and the use and processing of user data comply with the relevant laws, regulations, and standards of the relevant regions.

Claims

1. A frequency modulation device with frequency event crossover and seamless on-grid / off-grid handover capabilities, characterized in that, The frequency modulation device includes: The frequency event detection terminal monitors the main power grid frequency corresponding to the time in real time, constructs the power grid frequency sequence associated with the main power grid, calculates the frequency change rate between each time based on the numerical performance of the power grid frequency at each time, plots the frequency change rate fluctuation curve associated with the main power grid and the time, and locks the trigger point of frequency event crossing by combining the real-time power grid frequency value. The off-grid control terminal records the voltage amplitude, voltage frequency, and voltage phase of all grid-connected points in real time at the corresponding time of the trigger point, constructs a correlation data set, and assesses the severity level of frequency event crossover. At the first severity level, an adaptive gain coefficient is used to dynamically increase the droop coefficient while simultaneously introducing virtual inertia to execute a feedback response. When the grid connection is restored, the solid-state switch is immediately disconnected and the inverter's control mode is switched to locked output state to achieve off-grid operation when the severity level reaches level two. The grid frequency and voltage after disconnection are monitored in real time, and the stability index of the main grid is continuously evaluated in a monitoring cycle until the main grid enters a stable state. The voltage amplitude, voltage frequency and voltage phase of the microgrid are adjusted to complete the secondary grid connection.

2. The frequency modulation device according to claim 1, characterized in that, In the frequency event detection terminal, the specific method for constructing the power grid frequency sequence associated with the main power grid is as follows: Identify the main power grid and label it Q; Lock the current time, denoted as t1. Use time t1 as the start time for monitoring the main power grid Q. Align the time sequence to obtain the main power grid frequency and perform a permutation to obtain the power grid frequency sequence H1, H2, ..., Hj corresponding to the time sequence t1, t2, ..., tj, where j represents the total number of time points and the total number of corresponding power grid frequencies. The value of j increases over time.

3. The frequency modulation device according to claim 2, characterized in that, In the frequency event detection terminal, based on the numerical performance of the power grid frequency at each time moment, the frequency change rate between each time moment is calculated, and the specific method for plotting the frequency change rate fluctuation curve of the main power grid associated with the time moment is as follows: Obtain an arbitrary power grid frequency Hi, where 1≤i≤j; Take the grid frequency Hk at the next moment after the grid frequency Hi, where k≤j; The rate of change of the power grid frequency between time i and time k is calculated using R(i,k)=|Hk-Hi| / (tk-ti); Similarly, the rate of change of the power grid frequency between each adjacent time point is calculated to obtain the power grid frequency rate of change sequence R(1,2),R(2,3),...,R(j-1,j); Construct a two-dimensional coordinate system XY1 with time as the horizontal axis and the rate of change of power grid frequency as the vertical axis; Extract the grid frequency change rate R(1,2) from the grid frequency change rate sequence R(1,2), R(2,3),...,R(j-1,j). Plot the data points of the grid frequency change rate R(1,2) at the first and second moments on the horizontal axis, and plot the subsequent data points in sequence. Use curve fitting to fit all data points and denote it as the grid frequency change rate fluctuation curve S1 associated with the main grid and time.

4. The frequency modulation device according to claim 3, characterized in that, In the frequency event detection terminal, the specific method for locking the trigger point of frequency event traversal by combining real-time power grid frequency values ​​is as follows: Obtain the power grid frequency change rate fluctuation curve S1 and the two-dimensional coordinate system XY1; On the power grid frequency change rate fluctuation curve S1, determine the data points corresponding to the first power grid frequency change rate R(1,2) and the second power grid frequency change rate R(2,3), and denot them as d1 and d2 respectively. Construct straight lines perpendicular to the horizontal axis and parallel to the vertical axis through the two data points, denoted as line L1 and line L2; Draw a perpendicular line C1 through the data point d1 on line L1, intersecting line L2 at point d3; Calculate the area of ​​the closed region formed by data point d1, data point d2 and point d3, and denote it as the change characteristic TZ(1-2); Similarly, determine the change characteristics TZ(2-3),...,TZ(j-1,j); Obtain the preset change feature threshold TZ yu If any change characteristic is greater than or equal to TZ yu Then, the time intervals associated with the corresponding change characteristics are formed, and each time interval is marked as the trigger point for the frequency event to cross. Synchronously acquire the power grid frequency sequence H1, H2, ..., Hj. If there exists a power grid frequency ∉ [Hmin, Hmax] at any time, mark that time as the trigger point for the frequency event crossing, where Hmin and Hmax are the preset minimum and maximum safe power grid frequencies, respectively.

5. The frequency modulation device according to claim 4, characterized in that, In the hierarchical off-grid control terminal, the specific method for assessing the severity level of frequency event traversal is as follows: Obtain the voltage amplitude, voltage frequency, and voltage phase of the microgrid at all grid-connected points at any trigger point at any given time, and form an associated data set, denoted as {U_v,f_v,θ_v}, where v=1,2,...,V, and V is the total number of grid-connected points; Calculate the voltage amplitude deviation ΔU_v=|U_v-U_N|, voltage frequency deviation Δf_v=|f_v-f_N|, and voltage phase deviation Δθ_v=|θ_v-θ_N| at each grid connection point; U_N, f_N, and θ_N represent the rated voltage amplitude, rated voltage frequency, and rated voltage phase, respectively. The voltage amplitude deviation ΔU_v, voltage frequency deviation Δf_v, and voltage phase deviation Δθ_v are normalized using δU_v=ΔU_v / ΔU_max, δf_v=Δf_v / Δf_max, and δθ_v=Δθ_v / Δθ_max, where ΔU_max is the preset maximum allowable voltage amplitude deviation, Δf_max is the preset maximum allowable frequency deviation, and Δθ_max is the maximum allowable phase deviation. The comprehensive deviation value P_v of each grid connection point is calculated using P_v=α×δU_v+β×δf_v+γ×δθ_v, where α, β, and γ are preset weight coefficients, all of which are greater than 0, and α+β+γ=1. The maximum comprehensive deviation value P_max among all grid connection points is taken as the severity index; Compare P_max with the preset first threshold P1 and second threshold P2, where P1 < P2: If P1≤P_max<P2, it is determined to be the first severity level; If P_max ≥ P2, it is determined to be the second most serious level.

6. The frequency modulation device according to claim 5, characterized in that, In the graded off-grid control terminal, the specific method for dynamically increasing the droop coefficient with an adaptive gain coefficient and simultaneously introducing virtual inertia to execute the feedback response at the first severe level is as follows: Obtain the rate of change of the power grid frequency R(i,i+1) at any trigger point corresponding to time i when the first severity level is reached; The dynamic adjustment droop coefficient ef is calculated using ef = ef0 + η × |R(i,i+1)|, where ef0 is the initial droop coefficient, η is the preset adaptive gain coefficient, and ef ≤ ef_max = rated power × 15%. The additional power ΔP is output using ΔP=G×|R(i,i+1)|, where the virtual inertia coefficient G increases dynamically with the increase of |R(i,i+1)|, G=G0+λ×|R(i,i+1)|, G0 is the preset initial virtual inertia coefficient, and λ is the preset inertia gain coefficient; The total power command P_total is calculated using P_total=P0-ef×(f-f_nom)+ΔP, where P0 is the current output power reference, f is the real-time grid frequency, and f_nom is the rated frequency. The total power command P_total is used as the active power reference value for main grid control. Continuously monitor the rate of change of the grid frequency at subsequent times until the subsequent M consecutive times are non-trigger points, adjust ef to ef0, restore G to G0, recalculate the total power command, and execute the main grid control; M is the preset total number of safe redundancy times.

7. The frequency modulation device according to claim 6, characterized in that, In the grid-connected recovery process, when the severity level reaches level two, the solid-state switch is immediately disconnected, and the inverter's control mode is switched to locked output state. The specific method for achieving grid disconnection is as follows: Get the time i corresponding to any trigger point when the second severity level is reached; At time i, the solid-state switch is disconnected, and a mode switching command is sent to the inverter controller simultaneously to switch the inverter's control mode from active / reactive power regulation mode to locked output mode. The inverter's output voltage command is forced to zero to achieve off-grid operation. It also records the microgrid voltage phase, microgrid voltage frequency, and microgrid voltage amplitude corresponding to the off-grid time.

8. The frequency modulation device according to claim 7, characterized in that, The specific method for completing the secondary grid connection in the aforementioned grid-connected terminal is as follows: The off-grid time is determined, and the grid frequency H(Δt) and grid voltage V(Δt) of the main grid are continuously collected at a fixed monitoring period Δt. The microgrid voltage phase θ(Δt), microgrid voltage frequency f(Δt), and microgrid voltage amplitude U(Δt) of the microgrid are collected simultaneously. Here, H(Δt) represents the grid frequency of the main grid at any time within the monitoring period Δt, and the rest are similar. The stability index SAG(Δt) of the main power grid during the monitoring period Δt is calculated by using SAG(Δt)=ω1×|(H(Δt)-f_nom) / f_nom|+ω2×|(V(Δt)-U_nom) / U_nom|, where U_nom is the rated voltage, ω1 and ω2 are preset weighting coefficients, both of which are greater than 0 and w_1+w_2=1; If the stability index SAG(Δt) of the main power grid is less than SAG_th for N consecutive monitoring periods Δt, the main power grid is determined to have entered a stable state, where N is a preset integer; Conversely, continuous monitoring is required. Once the main grid enters a stable state, the microgrid voltage frequency f(Δt) is adjusted to match the grid frequency H(Δt), with an adjustment step size ≤ 0.01Hz / Δt. Adjust the microgrid voltage amplitude U_mg to match the grid voltage V(Δt), with an adjustment step size ≤ 0.5V / Δt; Similarly, adjust the microgrid voltage phase θ(Δt) to synchronize with the main grid, close the solid-state switch, and complete the secondary grid connection.

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