Seamless switching method for enhancing stable operation under microgrid main network fault

Abstract

The invention discloses a seamless switching method for enhancing stable operation under a micro-grid main network fault, and relates to the technical field of micro-grid management, and the method comprises the steps: obtaining the voltage, current, frequency and phase of each node of a micro-grid, judging the operation state of a main power grid through combined collection, and obtaining the electrical parameter information of a plurality of set position monitoring devices at the same time; 2, aligning the electrical parameters obtained in the step 1 according to a unified time reference, judging the response time difference of each monitoring device to a fault event, and selecting a node where the monitoring device is located as a main control node according to a preset rule; and the main control node sends a control instruction to the auxiliary control node, and the auxiliary control node is switched to an auxiliary control state according to a preset priority after receiving the control instruction. According to the invention, rapid and stable switching of a micro-grid control structure can be realized under the condition that a main power grid has a fault, and the output consistency of each control node is ensured through coordinated adjustment of voltage, frequency and active power, so that the continuity and stability of system operation are maintained.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of micro-grid management, in particular to a seamless switching method for enhancing stable operation under micro-grid main grid fault. BACKGROUND

[0002] With the development of distributed energy and micro-grid technology, micro-grid gradually has the ability of autonomous operation and flexible access to the main grid; when the main grid is running normally, the micro-grid is usually connected to the main grid to obtain stable power supply and realize bidirectional flow of electric energy; when the main grid fails, the micro-grid needs to realize seamless switching to island operation state to ensure continuous power supply for critical loads; The existing seamless switching scheme usually relies on single-point fault detection and pre-configuration of control nodes, and some methods trigger island mode by voltage drop detection, and then use energy storage systems to maintain system stability; however, due to the simple response mechanism of control node switching, insufficient prediction of grid connection process, and communication link not participating in switching strategy evaluation, problems such as voltage fluctuation, frequency drift or grid connection current impact may occur during the switching process, thereby affecting system stability and equipment safety; Therefore, how to comprehensively consider the control response time, energy storage capacity and communication quality, dynamically select the optimal control node, and improve the operation stability and grid connection reliability of the micro-grid under the main grid fault scenario by adjusting the parameter consistency control and grid connection trend prediction judgment has become an important technical direction in the field of micro-grid operation control; and then, the present application provides a seamless switching method for enhancing stable operation under micro-grid main grid fault. SUMMARY

[0003] The present application aims to provide a seamless switching method for enhancing stable operation under micro-grid main grid fault to solve the problems mentioned in the background.

[0004] The present application can be implemented by the following technical scheme: a seamless switching method for enhancing stable operation under micro-grid main grid fault, comprising the following steps: Step one, acquire the voltage, current, frequency and phase of each node of the micro-grid, judge the operation state of the main grid through joint acquisition, and acquire the electrical parameter information of a plurality of set position monitoring devices; Step two, align the electrical parameters obtained in step one according to a unified time reference benchmark, judge the response time difference of each monitoring device to the fault event, and select the node where the monitoring device is located as the main control node according to a preset rule; Step three, the main control node sends a control instruction to the auxiliary control node, the auxiliary control node receives the control instruction and switches to the auxiliary control state according to a preset priority, and tracks the voltage output and frequency output of the main control node; Step four, on the basis of the completion of the control state switching, the voltage amplitude, frequency reference value and its adjustment rate of the main control node and the auxiliary control node are adjusted, so that the output parameters of the control nodes tend to be consistent within a set time window; Step five, after the adjustment in step four is completed, the load side current change is monitored, and before the load current rising amplitude reaches the preset load current rising threshold, the active power reference value of each electric energy conversion device is corrected, and the discharge power of the energy storage device is adjusted to meet the energy storage adjustment amplitude threshold; Step six, after the microgrid is stably operated, the state of the switch connected with the main grid, the voltage amplitude difference and the phase difference are detected, the switch is controlled to be put into operation under the condition that the grid-connected voltage amplitude difference threshold and the phase difference threshold are met, and the amplitude limiting control is performed on the grid-connected instantaneous current, and the amplitude limiting is performed according to the grid-connected current rising rate threshold.

[0005] Further technical improvements of the application are that the preset rules for selecting the main control node in step two include: S1, receiving the local time stamp of each control node detecting the main grid voltage drop event, and taking a set unified time reference as a reference, calculating the time difference between the voltage drop event occurrence time and the local detection time to obtain the corresponding response time; S2, obtaining the current voltage value, current current value, nominal capacity value and residual capacity value of the energy storage device connected with each control node, and calculating the sustainable power supply time through the following formula: Sustainable power supply time = residual capacity value ÷ current current value; And the sustainable power supply time is taken as the energy storage output capacity score value; S3, in the communication link initialization stage, a handshake request data frame containing numbering information is broadcasted to all control nodes in turn, and each control node returns a handshake response data frame containing the same numbering information immediately after receiving the handshake request data frame; The number of successfully returned handshake response data frames of each control node within a set time window and the average round-trip communication time are counted, and the communication score value is calculated according to the following formula: Communication score value = number of handshake response data frames - round-trip communication time × penalty coefficient; S4, according to the response time, energy storage output capacity score value and communication score value of each control node, the scores are comprehensively scored according to the preset weighted calculation rule, and the control node with the highest score is selected as the main control node.

[0006] Further technical improvements of the application are that the setting of the time window in step four includes the following steps: A1, setting a reference time window length; A2, if any of the following conditions is met, a lengthening amount is added to the reference time window length: a21, the communication quality does not meet the requirement: the average retransmission number of the handshake request data frame is greater than the set handshake retransmission number threshold, or the noise level value of the communication channel is greater than the set communication noise level threshold, or the round trip communication time standard deviation of the handshake response data frame of the control node is greater than the set communication stability determination threshold; a22, there is a short-time voltage disturbance: the duration of the microgrid public connection point voltage disturbance is less than the set disturbance duration determination threshold, and the voltage recovery slope is greater than the set voltage recovery slope threshold; A3, if any of a21 and a22 in A2 is not met, and the equivalent virtual moment of inertia of all access power sources in the microgrid is less than the set equivalent moment of inertia threshold, and the total active power change rate is greater than the set active power change rate threshold, then the reference time window length is reduced by a shortening amount; A4, the final length of the time window is determined by the numerical addition and subtraction of the length result based on the reference time window length set in A1 according to the lengthening amount corresponding to the lengthening condition met in A2 and the shortening amount corresponding to the shortening condition met in A3.

[0007] Further technical improvements of the application are that in step six, the amplitude limiting control mode of the grid-connected instantaneous current includes: At a preset advance time before the control switch is closed, the grid-connected instantaneous current value at the grid-connected switch is collected, and continuous sampling is performed according to a preset time interval, and the current rise rate is calculated through the current difference value between adjacent sampling points; At the moment when the switch is closed, the output voltage change rate in the control loop is adjusted to form an inhibitory effect on the grid-connected instantaneous current rise rate; Wherein, the output voltage change rate is set according to the current grid-connected instantaneous current rise rate; When the current rise rate is between the first current change rate threshold and the second current change rate threshold, the output voltage change rate decreases with the increase of the current rise rate; When the current rise rate exceeds the second current change rate threshold, the output voltage change rate is not lower than the preset lower limit of the change rate; After detecting that the current rise rate continuously decreases below the current stability determination threshold and the state lasts for more than a preset stability determination time window, the original voltage regulation rate of the control loop is restored.

[0008] The further technical improvement of the application is that after the auxiliary control node switches to a new control instruction receiving node, the auxiliary control node proportionally decreases the control reference value before the switching by a fixed time step in a reference value switching transition time window through linear interpolation, and proportionally increases the control reference value provided by the new receiving node until the control output completely adopts the control reference value of the new receiving node.

[0009] The further technical improvement of the application is that after the communication is recovered, the auxiliary control node determines whether to switch back to the original main control node according to the locally stored control node priority table. The control node priority table determines the priority order based on the node identification number, the historical stability score and the weighted value of the communication quality score.

[0010] The further technical improvement of the application is that the auxiliary control node dynamically updates its communication path priority table according to the broadcast information of other control nodes in each broadcast period. The broadcast information includes the node identification number, the current communication delay value, the data packet loss rate and the communication channel noise level value.

[0011] The further technical improvement of the application is that in step six, the operation of judging whether the grid-connected voltage amplitude difference threshold and the phase difference threshold are met includes: After detecting that the current voltage amplitude difference and the phase difference meet the set grid-connected condition, the voltage amplitude and the phase value of the target control node are periodically collected in a sampling time window, and the change rate between each sampling time is calculated. According to the change rate, the voltage amplitude and the phase value at the predicted time point are calculated in combination with the preset prediction time interval, and the voltage amplitude difference and the phase difference at the corresponding time of the predicted time point are obtained. When the calculation result shows that the voltage amplitude difference threshold and the phase difference threshold are still met at the predicted time point, the switch closing instruction is sent at the advance time point before the predicted time point, so that the switch is closed at the predicted time point.

[0012] Compared with the prior art, the application has the following beneficial effects: The application can realize the rapid and stable switching of the micro-grid control structure under the condition of the main grid fault, and ensure the consistency of the output of each control node through the coordinated adjustment of the voltage, the frequency and the active power, so as to maintain the continuity and stability of the system operation. And, the application introduces a scoring mechanism of three dimensions of response time, energy support capacity and communication quality in the control node selection process, and sets a normalized weighting calculation rule, so that the selection of the master control node is more globally adaptive and stable, avoiding misjudgment and control failure caused by a single indicator; the stability of the master control node under sudden disturbance is enhanced, and the reliability of the overall control is effectively improved. On the other hand, based on the grid-connected control mechanism of trend prediction, the application can judge the voltage and phase change trend in the future time period on the basis of meeting the current grid-connected condition, and issue control instructions in advance to ensure that the switch is closed at the appropriate time point when it is ensured that the grid-connected condition is still met at the predicted time; at the same time, combined with the amplitude limiting regulation mechanism of the grid-connected current rising rate, the transient current impact of grid connection is effectively avoided, and the power quality and equipment protection level in the microgrid reclosing process are improved. BRIEF DESCRIPTION OF DRAWINGS

[0013] In order to facilitate the understanding of those skilled in the art, the application will be further described below with reference to the drawings.

[0014] Figure 1 The method logic diagram of the application. DETAILED DESCRIPTION

[0015] In order to further illustrate the technical means and effects adopted by the application to achieve the predetermined application purpose, the specific embodiments, structures, features and effects according to the application are described in detail below with reference to the drawings and preferred embodiments.

[0016] Example 1 Please refer to Figure 1 As shown in the drawings, the application provides a seamless switching method for enhanced stable operation under microgrid main grid fault, including the following steps: Step one, obtaining the voltage, current, frequency and phase of each node of the microgrid, judging the main grid operation state through joint collection, and obtaining the electrical parameter information of a plurality of set position monitoring devices; Step two, aligning the electrical parameters obtained in step one according to a unified time reference, judging the response time difference of each monitoring device to the fault event, and selecting the node where the monitoring device is located as the master control node according to a preset rule; The preset rule for selecting the master control node includes: S1, receiving the local time stamp of each control node detecting the voltage drop event of the main grid, and taking a set unified time reference as the reference, calculating the time difference between the voltage drop event time and the local detection time to obtain the corresponding response time; Specifically, after the voltage drop event of the main grid occurs, a unified reference time of the whole network is set, for example, the coordinated universal time standard provided by the network time module is taken as the time reference, which is set to ; Each control node records the local timestamp of the first detection of voltage sag ; Then, for each control node, the response time is calculated ; For example, if the unified time reference is 1000.000 milliseconds, and the detection time of a certain control node is 1002.037 milliseconds, then the response time of this node is 2.037 milliseconds.

[0017] S2, obtain the current voltage value, current current value, nominal capacity value and remaining capacity value of the energy storage device connected to each control node, and calculate the sustainable power supply time by the following formula: Sustainable power supply time = remaining capacity value ÷ current current value; And take this sustainable power supply time as the energy storage output capacity score value; Specifically, after obtaining the response time of each control node, the current state parameters of the energy storage device connected to each control node are further obtained; The current voltage value and the current current value are directly read by the local measurement device, the nominal capacity value is pre-stored as an inherent parameter of the device, and the remaining capacity value can be obtained by multiplying the ratio of the current remaining capacity to the nominal capacity value.

[0018] For example, the energy storage device parameters of a certain node are: Current voltage value: 48V; Current current value: 20A; Nominal capacity value: 100Ah; Remaining capacity value: 60Ah; Then the sustainable power supply time is: 60Ah ÷ 20A = 3 hours.

[0019] Take this 3 hours as the energy storage output capacity score value of this node, which represents its ability to maintain power supply alone under the current load condition.

[0020] S3, in the communication link initialization stage, a handshake request data frame containing numbering information is broadcast to all control nodes in turn, and each control node returns a handshake response data frame containing the same numbering information immediately after receiving the handshake request data frame; The number of successfully returned handshake response data frames and the average round-trip communication time of each control node within a set time window are counted, and the communication score value is calculated according to the following formula: Communication score value = number of handshake response data frames - (round-trip communication time × penalty coefficient); Specifically, a handshake request data frame is sent to all control nodes according to the number, and the frame format includes a fixed identifier (such as 0xAA55) and a unique number (such as frame number 001).

[0021] Each control node returns a handshake response data frame immediately after receiving the corresponding data frame, and the response frame contains the same number information as the request frame.

[0022] For example, 10 handshake request data frames are broadcast to control node A, 9 response frames are successfully returned by A, the average round-trip communication time is 45 milliseconds, and the penalty coefficient is set to 0.2.

[0023] The communication score value is 9-(45x0.2)=0. If another node B successfully responds to 10 frames, the average round-trip communication time is 20 milliseconds, and the communication score value is 10-(20x0.2)=6. The communication score value quantifies the timeliness and stability of the communication link, which is used as a key basis for subsequent weight score selection together with steps one and two.

[0024] S4, according to the response time, energy storage output capacity score value and communication score value of each control node, the highest score control node is selected as the master control node according to the preset weighted calculation rule. Specifically, the weight coefficients in this embodiment are set as follows: Response time weight: W1=0.4; Energy storage capacity weight: W2=0.3; Communication score weight: W3=0.3; And in this embodiment, the three core parameters for calculating the comprehensive score—response score, energy storage output capacity score value and communication score value—have different original dimensions (for example, response time is measured in milliseconds or seconds, and energy storage capacity is measured in hours); Therefore, before these parameters are substituted into the weighted summation formula, all parameters must be normalized.

[0025] Normalization converts all parameters into dimensionless values, ensuring that the final weighted calculation is based on a unified dimension; At the same time, in order to unify the scoring direction, the response time is normalized and inverted (the shorter the response time, the higher the score), and the final comprehensive score formula is: Comprehensive score=W1x(1 / response time)+W2x energy storage output capacity score value+W3x communication score value; For example, the response time of a control node is rounded to 2 milliseconds, the energy storage score value is 3 hours, and the communication score value is 6. Then the comprehensive score is: 0.4 x (1 / 2) + 0.3 x 3 + 0.3 x 6 = 0.2 + 0.9 + 1.8 = 2.9.

[0026] If multiple nodes are calculated at the same time, the node with the highest score is determined as the main control node for subsequent voltage frequency control in the microgrid operation.

[0027] Step three, the main control node sends control instructions to the auxiliary control node, and the auxiliary control node receives and switches to the auxiliary control state according to the preset priority, and tracks the voltage output and frequency output of the main control node; After the auxiliary control node switches to the new control instruction receiving node, the auxiliary control node decreases the control reference value before the switching by a fixed time step in the reference value switching transition time window through linear interpolation, and increases the control reference value provided by the new receiving node by a fixed time step, until the control output completely uses the control reference value of the new receiving node.

[0028] Specifically, when the auxiliary control node switches to the new control instruction receiving node after the communication interruption, in order to avoid the control output impact caused by the too fast change of the control reference value, the following linear interpolation mechanism is adopted: First, the control reference value output by the control instruction receiving node before the switching of the auxiliary control node is obtained, which is denoted as the initial reference value; then the control reference value output by the new control instruction receiving node is obtained, which is denoted as the target reference value.

[0029] Then, a reference value switching transition time window is set, for example, divided into several equally spaced sampling points in seconds.

[0030] In this time window, the control reference value is gradually changed from the initial reference value to the target reference value in the linear progression manner with equal time steps, and the current reference value at each sampling point is calculated as the initial reference value plus the time step proportion multiplied by the target difference value.

[0031] During the whole transition process, the auxiliary control node controls the output according to the linearly interpolated reference value, thereby reducing the impact of instantaneous reference value change and ensuring smooth transition of the control process.

[0032] After the communication is restored, the auxiliary control node determines whether to switch back to the original main control node according to the locally stored control node priority table. The control node priority table determines the priority order based on the node identification number, the historical stability score and the weighted value of the communication quality score.

[0033] Specifically, after the auxiliary control node switches to the new control instruction receiving node due to the communication interruption, the system continuously monitors the communication state of the main control node.

[0034] When the main control node communication recovers, the auxiliary control node judges whether to perform the back-switching operation according to the locally stored control node priority table.

[0035] In the control node priority table, the priority order of each control node is determined based on the following indexes: the identification number of the control node, the historical control stability score and the communication quality score.

[0036] The historical control stability score includes indexes such as the number of control switching, the fluctuation degree of voltage frequency adjustment, etc.; the communication quality score includes communication delay, data packet loss rate, communication channel signal-to-noise ratio, etc.

[0037] In this embodiment, it is to be noted that the communication quality score item contained in the control node priority table is a medium-long term, smoothed score value obtained after statistical averaging, standard deviation analysis or filtering processing of the communication path priority table or the instantaneous broadcast data relied thereon over a long time window. This processing mode avoids the control back-switching decision relying on instantaneous network fluctuation data, thereby ensuring the stability and reliability of the decision.

[0038] All score items are respectively given a preset weight, and the comprehensive score value is obtained by weighted summation. The auxiliary control node compares the comprehensive score values of the current control instruction receiving node and the original main control node, and if the score of the original main control node is higher and the score difference exceeds a preset priority difference threshold, the back-switching operation is performed to make the control flow follow the original main control node again.

[0039] The auxiliary control node dynamically updates its communication path priority table according to the information broadcast by other control nodes in each broadcast period; The broadcast information includes node identification number, current communication delay value, data packet loss rate and communication channel noise level value.

[0040] Specifically, to ensure that the auxiliary control node can quickly and reliably select the best control instruction receiving node when the main control node communication is interrupted, the following communication path priority table dynamic updating mechanism is adopted: In each broadcast period, the auxiliary control node listens to the data packets broadcast by each control node in the network. The broadcast content includes the identification number of the control node, the current communication delay value, the data packet loss rate and the communication channel noise level value.

[0041] The auxiliary control node scores the above communication parameters, and the scoring method includes: the communication delay value is scored by linear segmentation, the shorter the delay, the higher the score; the data packet loss rate is calculated according to the inverse function, the lower the packet loss rate, the higher the score; the noise level value is processed by hierarchical deduction according to the preset level.

[0042] Each score is weighted and summarized according to the preset weight to obtain the current communication quality score of the control node.

[0043] The score result is used to update the priority ranking of all control nodes in the communication path priority table.

[0044] The node with higher priority is preferentially selected as the control instruction receiving node in subsequent communication interruption.

[0045] The above dynamic updating mechanism ensures that the communication path priority table can reflect the current state of the network in real time, thereby improving the decision reliability of the auxiliary control node in abnormal situations.

[0046] Step four, on the basis of completing the control state switching, adjusting the voltage amplitude, frequency reference value and its adjustment rate of the main control node and the auxiliary control node, so that the output parameters of the control nodes tend to be consistent within a set time window; The setting of the time window includes the following steps: A1, set a reference time window length; Specifically, a standard time window value is first preset as the maximum response delay reference allowed for the synchronization switching between control nodes after the main grid fault of the microgrid. The reference time window length can be set according to system design parameters, for example, the initial value is 100 ms, and can be initialized and corrected according to the microgrid scale, communication topology, load response characteristics, etc.

[0047] A2, if any of the following conditions is met, add an extension to the reference time window length, in this embodiment, the extension is set to 30 ms: a21, the communication quality does not meet the requirements: the average retransmission number of the handshake request data frame is greater than the set handshake retransmission number threshold, or the noise level value of the communication channel is greater than the set communication noise level threshold, or the standard deviation of the round trip communication time of the handshake response data frame of the control node is greater than the set communication stability determination threshold; a22, there is a short-time voltage disturbance: the duration of the microgrid point of common coupling voltage disturbance is less than the set disturbance duration determination threshold, and its voltage recovery slope is greater than the set voltage recovery slope threshold; A3, if none of the extension conditions in a21 and a22 is met, and the equivalent virtual moment of inertia of all access power sources in the microgrid is less than the set equivalent moment of inertia threshold, and the total active power change rate is greater than the set active power change rate threshold, then reduce a shortening from the reference time window length; A4, the final length of the time window is determined by adding and subtracting the length of the reference time window set in A1 based on the extension corresponding to the extension condition met in A2 and the shortening corresponding to the shortening condition met in A3.

[0048] In this embodiment, the reference time window length is 100 ms, if the conditions of a21 of A2 and A3 are triggered, the final time window length is: 100 ms+30 ms-30 ms=100 ms, which remains unchanged; If A2 is not triggered and A3 is triggered, it is 70 ms; If A2 triggers two items and A3 does not trigger, it is 160 ms, and so on.

[0049] Step five, after the adjustment in step four is completed, the load side current change is monitored, before the load current rise amplitude reaches the preset load current rise threshold, the active power reference value of each electric energy conversion device is corrected, and the discharge power of the energy storage device is adjusted to meet the energy storage adjustment amplitude threshold.

[0050] Step six, after the microgrid is stably operated, the state of the connection switch to the main grid, the voltage amplitude difference and the phase difference are detected, the switch is controlled to be put into operation under the condition that the grid-connected voltage amplitude difference threshold and the phase difference threshold are met, and the grid-connected instantaneous current is limited in amplitude, and the limiting is performed according to the grid-connected current rise rate threshold; The operation of judging whether the grid-connected voltage amplitude difference threshold and the phase difference threshold are met includes: After it is detected that the current voltage amplitude difference and the phase difference meet the set grid-connected condition, the voltage amplitude and the phase value of the target control node are periodically collected in a sampling time window, and the change rate between each sampling time is calculated; According to the change rate, in combination with a preset prediction time interval, the voltage amplitude and the phase value at a prediction time point are calculated, and the voltage amplitude difference and the phase difference at the corresponding time of the prediction time point are obtained according to the calculation results; When the calculation result shows that the grid-connected voltage amplitude difference threshold and the phase difference threshold are still met at the prediction time point, a switch closing instruction is sent at an advance time point before the prediction time point, so that the switch is closed at the prediction time point.

[0051] Specifically, a sampling time window is set, and the voltage amplitude and the voltage phase value at the target control node are periodically collected in the time window according to a fixed time step, to form continuous voltage sampling sequences and phase sampling sequences.

[0052] For each pair of adjacent sampling time points in the voltage sampling sequence and the phase sampling sequence, the difference values of the voltage value and the phase value are calculated respectively, and the difference values are divided by the sampling interval time to obtain the voltage change rate and the phase change rate in the time interval. The voltage change rate is defined as: the voltage value at the next time minus the voltage value at the previous time, and then divided by the time interval between the two times; the phase change rate is calculated in the same way.

[0053] Based on the numerical values of the current voltage change rate and the phase change rate, in combination with a preset prediction time interval, the voltage amplitude and phase values at the prediction time point are recursively predicted using an equal-step numerical derivation method (e.g., each step is equal to the current change rate multiplied by the step time) based on the current sampling point.

[0054] The predicted voltage and phase values are compared with the current grid reference voltage and reference phase to calculate the voltage amplitude difference and phase difference at the prediction time.

[0055] The predicted voltage amplitude difference and phase difference are compared with the grid-connected voltage amplitude difference threshold and phase difference threshold, respectively: If both are within the set threshold range, it is considered that the voltage condition and phase condition at the prediction time remain stable and have the trend of continuously meeting the grid-connected condition.

[0056] If any difference exceeds the set threshold, it is considered that the current voltage and phase temporarily meet the condition, but the trend is unstable, and the grid-connected operation is delayed.

[0057] When the difference at the prediction time meets the condition, the control operation of the switch is triggered at the preset advance time point before the prediction time point is reached.

[0058] The setting of the advance time point takes into account the response delay time required for the grid-connected switch to receive control instructions and physically close, to ensure that the switch is accurately closed before the prediction time, achieving disturbance-free grid connection.

[0059] Through the above steps, it is ensured that the grid-connected control not only considers the current state, but also introduces prediction and judgment of the future short-time voltage and phase change trend, thereby improving the accuracy of grid-connected time selection and the stability of the grid-connected process, effectively reducing the risk of grid-connected impact due to sudden changes.

[0060] The amplitude limiting control method for the grid-connected instantaneous current includes: At a preset advance time before the switch is closed, the grid-connected instantaneous current value at the grid-connected switch is collected, and continuous sampling is performed according to a preset time interval, and the current rise rate is calculated by the current difference between adjacent sampling points; At the moment of switch closing, the output voltage change rate in the control loop is adjusted to form an inhibitory effect on the grid-connected instantaneous current rise rate; Wherein, the output voltage change rate is set according to the current grid-connected instantaneous current rise rate; When the current rise rate is between the first current change rate threshold and the second current change rate threshold, the output voltage change rate decreases with the increase of the current rise rate; When the current rise rate exceeds the second current change rate threshold, the output voltage change rate is not lower than a preset lower limit of change rate; After detecting that the current rise rate continuously is lower than the current stability judgment threshold, and the state lasts more than a preset stability judgment time window, the original voltage regulation rate of the control loop is restored.

[0061] Specifically, in this embodiment, before the control switch is ready to close, at a preset advance time, for example, twenty milliseconds in advance, the grid-connected instantaneous current is continuously sampled at the grid-connected switch; The sampling is performed at a time interval of once per millisecond, and five output current values are continuously collected; For any two adjacent sampling time points, the difference between the grid-connected instantaneous current values thereof is calculated, and the difference is divided by the corresponding time interval to obtain a rise rate sequence of the grid-connected current.

[0062] Then, the maximum value is selected from the sequence as the representative grid-connected current rise rate, which is used as a reference for subsequent voltage change rate adjustment.

[0063] Subsequently, at the moment when the grid-connected switch is closed, the controller adjusts the output voltage reference value change rate of the target control node according to the size of the representative grid-connected current rise rate, to limit the change amplitude of the grid-connected current.

[0064] The setting rule of the output voltage change rate is as follows: When the current rise rate is less than the first current change rate threshold, the output voltage changes at a default rate; When the current rise rate is between the first current change rate threshold and the second current change rate threshold, the output voltage change rate gradually decreases with the increase of the current rise rate, and the decrease mode is linearly related to the current rise rate; When the current rise rate exceeds the second current change rate threshold, the output voltage change rate is limited to be not lower than a preset minimum change rate, to enhance the suppression ability of the severe current impact.

[0065] The first current change rate threshold is used to identify moderate-intensity grid-connected impact, and the second current change rate threshold is used to identify severe impact, both of which can be set based on grid-connected operation historical data, power supply access characteristics, controller bandwidth and other factors, and can be dynamically updated.

[0066] After the switch is closed, the controller continuously monitors the change of the grid-connected current rise rate; When the current rise rate is detected to be lower than the current stability judgment threshold in a plurality of consecutive sampling periods, and the state lasts more than a preset stability judgment time window, it is determined that the grid-connected current has tended to be stable, at which time the normal change rate of the output voltage is restored, and the limiting control process is terminated.

[0067] Embodiment 2 Compared with embodiment 1, the seamless switching method for enhanced stable operation under micro-grid main grid fault of embodiment 2 comprises the following steps: Step one, obtain the voltage, current, frequency and phase of each node of the micro-grid, judge the operation state of the main grid through joint collection, and obtain the electrical parameter information of the monitoring device at multiple set positions; Introduce the default master node mechanism in the system initialization or off-grid start-up stage, including: When the system is constructed, all control nodes store a same, pre-set priority list, which is created based on the hard indicators of node resources (such as the maximum battery energy storage capacity, unique hardware ID or calculation ability evaluation).

[0068] The node ranked first in the list is defined as the default master node.

[0069] The main role of the default master node is to provide an explicit, decentralized start-up anchor point in the case of system disconnection or uncoordinated communication, and to ensure that the system can perform basic control tasks.

[0070] Step two, align the electrical parameters obtained in step one according to the unified time reference benchmark, judge the response time difference of each monitoring device to the fault event, and select the node where the monitoring device is located as the master control node according to the pre-set rule; And after the main grid fault is detected, if the node does not receive any instruction or announcement from the master control node within the pre-set emergency timeout time, the node will start the local promotion arbitration procedure, including: Local evaluation: the node evaluates based on the local, non-communication dependent data (such as response time, current energy storage state) owned by itself.

[0071] Arbitration decision: the node ranks according to the ranking in the pre-set priority list, if it is the node with the highest ranking and normal function in the current list, the node automatically promotes to the master control node and starts broadcasting control instructions to the whole network.

[0072] Dynamic adjustment: after establishing the basic master control, the system immediately implements the pre-set rules of S1-S4 in embodiment 1 to select the node with the best performance as the master control node.

[0073] Step three, the master control node sends control instructions to the auxiliary control node, and the auxiliary control node switches to the auxiliary control state according to the pre-set priority after receiving, and tracks the voltage output and frequency output of the master control node; In the absence of external communication connection, all nodes will follow the control instruction of the current promoted voltage amplitude default master node voltage amplitude according to the arbitration result of step two, to ensure the control consistency of the system at the moment of failure. After the dynamic election is completed, the auxiliary node switches to track the new optimal master control node.

[0074] Step four, on the basis of the completion of the control state switching, adjust the voltage amplitude, frequency reference value and its adjustment rate of the master control node and the auxiliary control node, so that the output parameters of the control node tend to be consistent within a set time window; Step five, after the adjustment in step four is completed, monitor the load side current change, and before the load current rise amplitude reaches the preset load current rise threshold, correct the active power reference value of each power conversion device and adjust the discharge power of the energy storage device to meet the energy storage adjustment amplitude threshold; Step six, after the microgrid is stably running, detect the state of the connection switch to the main grid, the voltage amplitude difference and the phase difference, control the switch to be put into operation under the condition of meeting the grid-connected voltage amplitude difference threshold and phase difference threshold, and perform amplitude limiting control on the grid-connected instantaneous current. The amplitude limiting is performed according to the grid-connected current rise rate threshold.

[0075] The above formulas are dimensionless numerical calculations, and the formulas are obtained by software simulation of a large amount of data to obtain a formula of the most recent real situation. The preset parameters and threshold values in the formula are set by a person skilled in the art according to the actual situation.

[0076] The above is only a preferred embodiment of the present application, and does not limit the present application in any form. Although the present application has been disclosed as above, it is not intended to limit the present application. Any person skilled in the art can make some changes or modifications to the above disclosed technical content without departing from the scope of the technical solution of the present application, and any simple modification, equivalent change and modification of the above embodiments according to the technical essence of the present application are still within the scope of the technical solution of the present application.

Claims

1. A seamless handover method for enhanced stable operation under microgrid master grid failure, characterized in that, The method comprises the following steps: Step one, obtaining the voltage, current, frequency and phase of each node of the microgrid, judging the operation state of the main power grid through joint collection, and obtaining the electrical parameter information of the monitoring device at multiple set positions; Step two, aligning the electrical parameters obtained in step one according to a unified time reference, judging the response time difference of each monitoring device to the fault event, and selecting the node where the monitoring device is located as the main control node according to the preset rule; Step three, the main control node sends a control instruction to the auxiliary control node, the auxiliary control node receives the control instruction and switches to the auxiliary control state according to the preset priority, and tracks the voltage output and frequency output of the main control node; Step four, on the basis of completing the control state switching, adjusting the voltage amplitude, frequency reference value and adjustment rate of the main control node and the auxiliary control node, so that the output parameters of the control nodes tend to be consistent within a set time window; Step five, after the adjustment in step four is completed, monitoring the load current change, and before the load current rises to a preset load current rise threshold, correcting the active power reference value of each power conversion device and adjusting the discharge power of the energy storage device to meet the energy storage adjustment amplitude threshold; Step six, after the microgrid is stably operated, detecting the state, voltage amplitude difference and phase difference of the switch connected with the main grid, controlling the switch to be put into operation under the condition that the voltage amplitude difference threshold and the phase difference threshold of the grid-connected voltage are met, and limiting the amplitude of the grid-connected instantaneous current, and the limiting is performed according to the grid-connected current rise rate threshold.

2. The seamless handover method for enhanced stable operation under failure of a master network of a microgrid according to claim 1, characterized in that, The preset rule for selecting the main control node in step two includes: S1, receiving the local time stamp of each control node detecting the main grid voltage drop event, and taking the set unified time reference as the reference, calculating the time difference between the voltage drop event occurrence time and the local detection time to obtain the corresponding response time; S2, obtaining the current voltage value, current current value, nominal capacity value and residual capacity value of the energy storage device connected with each control node, and calculating the sustainable power supply time by the following formula: Sustainable power supply time = residual capacity value ÷ current current value; And taking the sustainable power supply time as the energy storage output capacity score value; S3, in the communication link initialization stage, a handshake request data frame containing number information is broadcasted to all control nodes in turn, and each control node returns a handshake response data frame containing the same number information immediately after receiving the handshake request data frame; The number of successfully returned handshake response data frames and the average round-trip communication time of each control node within a set time window are counted, and the communication score value is calculated according to the following formula: Communication score value = number of handshake response data frames - round-trip communication time × penalty coefficient; S4, according to the response time, energy storage output capacity score value and communication score value of each control node, the scores are integrated according to the preset weighted calculation rule, and the control node with the highest score is selected as the main control node.

3. The seamless handover method for enhanced stable operation under failure of a master in a microgrid according to claim 1, characterized in that, The setting of the time window in step four includes the following steps: A1, set a reference time window length; A2, if any of the following conditions is met, add an extension to the reference time window length; a21, the communication quality does not meet the requirement: the average retransmission number of the handshake request data frame is greater than the set handshake retransmission number threshold, or the noise level value of the communication channel is greater than the set communication noise level threshold, or the standard deviation of the round trip communication time of the handshake response data frame of the control node is greater than the set communication stability determination threshold; a22, there is a short-time voltage disturbance: the duration of the microgrid public connection point voltage disturbance is less than the set disturbance duration determination threshold, and the voltage recovery slope is greater than the set voltage recovery slope threshold; A3, if any of the extension conditions in a21 and a22 in A2 is not met, and the equivalent virtual moment of inertia of all access power supplies in the microgrid is less than the set equivalent moment of inertia threshold, and the total active power change rate is greater than the set active power change rate threshold, then the reference time window length is reduced by a shortening amount; A4, the final length of the time window is determined by numerically adding and subtracting the length result based on the reference time window length set in A1, according to the extension amount corresponding to the extension condition met in A2 and the shortening amount corresponding to the shortening condition met in A3.

4. The seamless handover method for enhanced stable operation under failure of a master in a microgrid according to claim 1, characterized in that, In step six, the amplitude limiting control mode of the grid-connected instantaneous current includes: At a preset advance time before the control switch is closed, the grid-connected instantaneous current value at the grid-connected switch is collected, and continuous sampling is performed according to a preset time interval, and the current rise rate is calculated by the current difference value between adjacent sampling points; At the moment when the switch is closed, the output voltage change rate in the control loop is adjusted to form an inhibitory effect on the grid-connected instantaneous current rise rate; Wherein, the output voltage change rate is set according to the current grid-connected instantaneous current rise rate; When the current rise rate is between the first current change rate threshold and the second current change rate threshold, the output voltage change rate decreases with the increase of the current rise rate; When the current rise rate exceeds the second current change rate threshold, the output voltage change rate is not lower than the preset lower limit of the change rate; After detecting that the current rise rate continuously falls below the current stability judgment threshold and the state lasts for more than a preset stability judgment time window, the original voltage adjustment rate of the control loop is restored.

5. The seamless handover method for enhanced stable operation under failure of a master in a microgrid according to claim 1, characterized in that, After the auxiliary control node switches to a new control command receiving node, the auxiliary control node decreases the control reference value before the switch by a fixed time step in a reference value switching transition time window through linear interpolation, and increases the control reference value provided by the new receiving node by a fixed time step, until the control output completely uses the control reference value of the new receiving node.

6. The seamless handover method for enhanced stable operation under failure of a master in a microgrid according to claim 5, characterized in that, After the communication is restored, the auxiliary control node determines whether to switch back to the original main control node according to the control node priority table stored locally; Wherein, the control node priority table determines the priority order based on the node identification number, the weighted value of the historical stability score and the communication quality score.

7. The seamless handover method for enhanced stable operation under master network failure of a microgrid according to claim 6, characterized in that, The auxiliary control node dynamically updates its communication path priority table according to the information broadcast by other control nodes in each broadcast period; The broadcast information includes node identification number, current communication delay value, data packet loss rate and communication channel noise level value.

8. The seamless handover method for enhanced stable operation under failure of a master in a microgrid according to claim 4, characterized in that, In step six, the operation of judging whether the grid-connected voltage amplitude difference threshold and the phase difference threshold are met comprises: After detecting that the current voltage amplitude difference and the phase difference meet the set grid-connected condition, periodically collecting the voltage amplitude and phase values of the target control node within a sampling time window, and calculating the change rate between each sampling time; According to the change rate, in combination with a preset prediction time interval, the voltage amplitude and phase values at a prediction time point are calculated, and the voltage amplitude difference and the phase difference at the time corresponding to the prediction time point are obtained; When the calculation result shows that the grid-connected voltage amplitude difference threshold and the phase difference threshold are still met at the prediction time point, a switch closing instruction is sent at an advance time point before the prediction time point, so that the switch is closed at the prediction time point.

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