A fast and smooth handover method for unplanned network disconnection

Abstract

A fast and smooth switching method for unplanned grid disconnection is disclosed, relating to the field of microgrid grid-connected / off-grid switching. Existing microgrid grid disconnection control methods suffer from unstable regulation effects and slow adjustment speeds. This invention establishes a time-domain data cyclic storage area; monitors disturbance data at the microgrid's grid disconnection PCC point and categorizes the state signals triggering grid disconnection; determines the grid disconnection time section; performs error compensation on the microgrid-side power after disconnection based on the grid disconnection state signals; if the frequency deviation exceeds a set frequency adjustment threshold, performs compensatory adjustments on the power after disconnection based on the power compensated for the frequency difference; and sends grid disconnection adjustment power to known micro-sources in the microgrid system based on the grid disconnection section power and the grid disconnection correction power, thereby achieving the switching of microgrid grid-connected and off-grid power and reducing frequency and voltage fluctuations in the microgrid. This invention is mainly used for fast and smooth switching of unplanned grid disconnection.

Description

Technical Field

[0001] This invention relates to the field of microgrid grid-connected / off-grid switching, and more particularly to a fast and smooth switching method for unplanned grid disconnection. Background Technology

[0002] With the rapid development of distributed new energy sources, the advantages of low cost, environmental protection, and convenient on-site deployment have made microgrids an important solution to energy problems. In particular, for remote areas and islands with weak power supply, the application of microgrids has played an important role in power supply stability, which not only increases the reliability of power supply and saves costs, but also improves the ecological environment and reduces the dependence of end users on the power grid.

[0003] As a beneficial supplement to the power supply network, microgrids increase the power supply options in the supply area and improve the power supply reliability of the grid. However, they also increase the complexity of stable operation control for the microgrid itself. In particular, rapid regulation when the microgrid is disconnected from the main grid is crucial for the stable operation of the microgrid.

[0004] Currently, the control method for off-grid microgrids involves real-time acquisition and calculation of frequency and voltage after off-grid operation, followed by power adjustment using droop control algorithms based on changes. The adjustment effect varies depending on the algorithm used, and the stabilization process of a microgrid may take hundreds of milliseconds or even seconds. Excessive instability control time can adversely affect the operation and stability of equipment within the microgrid, and in severe cases, may lead to frequency transient overshoot and drastic voltage fluctuations, potentially causing instability of the entire network and, in extreme cases, grid paralysis.

[0005] Therefore, there is a need for a fast and smooth switching method for unplanned off-grid operation that can accelerate the accurate adjustment speed of off-grid operation, maintain the stable operation of microgrid equipment, and avoid grid paralysis. Summary of the Invention

[0006] To address the shortcomings of existing microgrid off-grid control methods, such as unstable regulation effects and slow regulation speed, this invention provides a fast and smooth unplanned off-grid switching method that can accelerate the accurate regulation speed of off-grid switching, maintain the stable operation of microgrid equipment, and avoid grid paralysis.

[0007] The present invention provides a fast and smooth handover method for unplanned network disconnection, comprising the following steps:

[0008] S1. Establish a time-domain data circular storage area;

[0009] S2. Monitor disturbance data at the off-grid PCC point of the microgrid and classify the status signals that trigger off-grid access.

[0010] S3. Determine the off-grid time segment based on the classification;

[0011] S4. Perform error compensation on the power of the microgrid side after disconnection based on the off-grid status signal to obtain the off-grid corrected power;

[0012] S5. If the frequency deviation exceeds the set frequency adjustment setting value, calculate the compensation power value caused by the frequency difference, and make compensatory adjustments to the power after disconnection based on the power compensated for the frequency difference.

[0013] S6. Based on the off-grid cross-sectional power and off-grid correction power, send off-grid regulation power to the known micro-sources in the microgrid system to realize the switching of on-grid and off-grid power in the microgrid, thereby reducing the frequency and voltage fluctuations of the microgrid.

[0014] Further: In S1, the specific steps for establishing the time-domain data circular storage area include:

[0015] S11. Create a two-dimensional array that includes the number of power points and power data;

[0016] S12. Store the power data corresponding to each power point in real time at preset time intervals; form a time-domain power data cyclic storage area.

[0017] Further: In S2, the specific steps for monitoring the disturbance data at the off-grid PCC point of the microgrid include:

[0018] S21. Collect target node data to form event record information at different time sections, collect disturbance data of PCC points, and collect status signals of planned and unplanned passive disconnection.

[0019] S22. Classify and process different status signals according to different reasons for disconnection.

[0020] Furthermore, the principle of the classification process is:

[0021] For planned off-grid operations, the power at the PCC point is adjusted to zero to achieve uninterrupted or minimally disruptive off-grid operation.

[0022] Unplanned passive disconnection caused by circuit breaker misoperation and manual tampering is classified according to the circuit breaker's tripping time;

[0023] Unplanned passive disconnection caused by protective actions is classified according to the type of protective action and the delay.

[0024] Further: In S3, the specific steps for determining the off-grid time section include:

[0025] S31. Timestamp the corresponding time position in the time domain data circular storage area for different types of off-network reasons;

[0026] S32. Calculate the start time of the power fluctuation that caused the grid disconnection for each type of disconnection based on different timestamps.

[0027] Further: In S4, the error compensation is based on: according to the conditions for triggering disconnection, a threshold for disconnection speed is preset, and the speeds that trigger disconnection are divided into Class I disconnection and Class II disconnection.

[0028] For a certain type of event that triggers disconnection, ignore the micro-volatility error;

[0029] For the second type of triggering off-network events, the rate of change of frequency is calculated in real time and stored in the corresponding storage area of ​​the two-dimensional array. If the rate of change of frequency exceeds the preset rate of change threshold, the impact of frequency change needs to be compensated.

[0030] Further: In S4, the error compensation includes: using an inertial support algorithm to calculate the increase or decrease in active power caused by inertia for off-grid compensation; when the power adjustment is based on the frequency slip at a given time, the corresponding adjustment power is compensated and corrected, thereby realizing error compensation for the off-grid pre-adjustment power.

[0031] The beneficial effects of this invention are:

[0032] This invention proposes a fast and smooth switching method for unplanned grid disconnection, which is based on the judgment criteria for fast grid disconnection power adjustment using a memory method. This method can improve the fast adjustment power balance under both planned and unplanned grid disconnection conditions, improve the accuracy and speed of grid disconnection adjustment, and thus reduce microgrid disturbances.

[0033] This invention is designed for off-grid scenarios of microgrids. It can be used to address abnormal external conditions of microgrids, such as system failures, circuit breaker tripping, and abnormal power outages caused by human intervention. Depending on the situation, it can quickly and accurately adjust power and load to achieve power balance in the off-grid state, thereby improving the smoothness of the microgrid off-grid process, quickly achieving stable operation of the microgrid's frequency and voltage, and enhancing the survivability of the microgrid.

[0034] Based on dynamically stored data, this invention performs targeted and precise time tracing for different off-grid types. It deducts the impact of uncertainties in system data caused by abnormal off-grid actions such as protection actions. By storing historical power data and off-grid scenario classifications in the memory time-domain data cyclic storage area, this invention performs dynamic adjustment calculations for different situations. The amount of calculation and the calculation time are relatively small. The pre-issued power command greatly improves the speed at which the system enters the stable region, increases the probability of rapid stability of the microgrid, and shortens the stability adjustment time of the microgrid. Attached Figure Description

[0035] Figure 1 This is a flowchart illustrating the method.

[0036] Figure 2 This is a schematic diagram illustrating the scenarios in which this method is applicable.

[0037] Figure 3 A diagram illustrating the classification of off-grid types. Detailed Implementation

[0038] The following are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. The embodiments described below are only for explaining the present invention and should not be construed as limiting the present invention. The scope of protection of the present invention should be determined by the scope of the claims. The embodiments of the present invention are described in detail below. In order to facilitate the description of the present invention and simplify the description, the technical terms used in the specification of the present invention should be interpreted broadly, including but not limited to conventional alternatives not mentioned in this application, and including both direct and indirect implementation methods.

[0039] Example 1

[0040] Combination Figures 1-3 This embodiment describes a fast and smooth handover method for unplanned network disconnection, comprising:

[0041] When a microgrid is disconnected from the grid, it needs to be quickly brought to a new power balance point to stabilize its frequency and voltage, enabling it to rapidly enter a stable operating state. According to the conditions for stable power system operation, during steady-state operation, the power absorbed or consumed by the grid is equal; this is governed by the law of power conservation.

[0042] ;

[0043] Among them, P 输入 P represents the input power. 输出 Indicates output power;

[0044] Using the point of common coupling (PCC) of the microgrid as the critical node (i.e., the target node), two systems are formed after the grid connection is removed. Let the power provided by the system at this point be... This value can be positive or negative. The current at this point satisfies Kirchhoff's laws, as does the power at this point. The power at different time sections is recorded with the system's operating time domain as the axis, and the power is calculated with the grid connection point as the node.

[0045] ;

[0046] ;

[0047] in, This indicates the power at the grid connection point; a positive grid connection point power indicates that power is flowing out of the microgrid. Indicates the power of the microgrid system; This indicates the power generation capacity within the microgrid; This indicates the energy storage capacity; a positive energy storage capacity indicates charging, and a negative energy storage capacity indicates discharging. Indicates microgrid load; This represents other power within the microgrid.

[0048] The steps of this method are as follows:

[0049] S1. Establish a time-domain data circular storage area with sufficient margin. The specific steps are as follows:

[0050] S11. Create a two-dimensional array dwMemory[k][n]. The array dimension and length can be changed according to the actual data recording situation. k is the number of power points that need to be specified + 1, + 1 indicates that the time marker buffer needs to be recorded, and n is the width of the circular storage area for recording power and other time domain data that needs to be opened.

[0051] Table 1 shows the circular storage area for time-domain data constructed using this method.

[0052]

[0053] S12. Real-time storage of power data of relevant nodes at a certain time interval ΔT, where ΔT can be the sampling interval or a fixed value can be set according to the time accuracy requirements; real-time monitoring of grid connection points and collected power data, and establishment of a dynamically updated memory time domain data loop storage area; the memory time domain data loop storage area adopts a ring storage structure, the storage depth covers at least the duration of the power system transient process, and the sampling frequency is not less than 1kHz.

[0054] For different types of off-grid events, the process time ΔT from the occurrence of the disturbance to complete off-grid is calculated. The process time ΔT is determined by analyzing the combined characteristics of the protection device's action sequence and the change in electrical quantity.

[0055] The calculation methods for process time ΔT include: for instantaneous disconnection, ΔT is the sum of the inherent operating time of the protection device and the circuit breaker opening time, or it can be approximately equal to the circuit breaker opening time; for continuous disconnection, ΔT is adjusted according to the dynamic response time of the protection operation duration.

[0056] S2. Monitor the disturbance data of the microgrid when it is off-grid, including switch status, voltage, current, power, voltage fluctuation, and off-grid status. Specific operations are as follows:

[0057] S21. The GOOSE data network may be used to collect data from key nodes, forming event logs at different time points. This includes collecting disturbance data at PCC points, the characteristics of internal microgrid protection fault signals, and the system-side interactive fault signal transmission mechanism. During off-grid power regulation, if a fault occurs on an internal line within the microgrid at the time of off-grid, causing a trip, the power change of that line cannot ignore the impact of fault clearing. Therefore, it needs to be considered that internal microgrid faults should be deducted from the regulation after off-grid operation. Internal microgrid fault signals include trip signals from protection devices installed on internal lines or transformers, or signals obtained based on the circuit breaker position signals of key nodes. System-side fault signals can be obtained through directly acquired position signals, communication networks, or specially established communication networks, transmitting fault-related signals (including position, protection trip signals, protection action segments, and time settings) via the GOOSE fast communication mechanism. This also includes collecting planned and unplanned passive off-grid status signals. Key point data refers to power nodes whose power values ​​cannot be ignored; PCC points are among the key points.

[0058] S22. Different status signals are categorized and processed. The main consideration is that planned disconnection requires adjusting the power at the PCC point to zero to achieve disturbance-free or minimal-disturbance disconnection. For cases of circuit breaker tripping or accidental manual switching, only the circuit breaker's opening time is considered. Since the time affecting disconnection is the circuit breaker's opening time, which is short, the power change impact during the opening time can be disregarded. For protection action cases, the protection action type and delay must be considered. The protection action type mainly distinguishes the protection action time. If it exceeds 40ms, the cross-sectional time before the fault impact needs to be calculated based on the action time and start time used by the protection type.

[0059] The system receives off-grid trigger signals and identifies and classifies off-grid event types as instantaneous or continuous off-grid based on the time characteristics of the trigger signals. The trigger signals include voltage drop signals, frequency over-limit signals, external protection action events, or circuit breaker trip signals, etc.

[0060] S3. Determine the time frame for adjusting the off-grid power base value based on the classification. The specific steps are as follows:

[0061] S31. For different types of disconnection reasons, timestamp the corresponding time position in the time-domain data circular storage area, for example: This is the system protection trip time. It is the time when you are offline. It is the disturbance recording time. This includes circuit breaker tripping time, etc.

[0062] S32. Calculate the power fluctuation start time that causes the grid disconnection for different types of disconnection reasons according to different timestamps; store historical power time-domain cross-sectional data and event information in the form of timestamps; the event information refers to information that marks important time sections such as grid disconnection, protection action, and disturbance, which is used to determine the grid disconnection time, disturbance start time, etc.

[0063] For example: Off-network failure caused by a fault in an external microgrid system:

[0064] generally It can be considered as... If the values ​​are the same or similar, the compensation amount may be increased depending on the actual situation.

[0065] ;

[0066] ;

[0067] ;

[0068] in, It is the fault action delay information recorded by the protection trip; It is the protection trip start time; Power adjustment is based on time; The time difference between the two time points is less than a smaller set value;

[0069] Then prove To determine the start time of the off-grid fluctuations, using The time is the power memory point for off-grid operation. Read the corresponding As a basis for the difference in the redistribution of the microgrid after it is off-grid, the power within the scope of the off-grid microgrid is adjusted;

[0070] in, It is the grid-connected power value in the cyclic storage area of ​​the off-grid fluctuation start time memory time domain data;

[0071] Unplanned passive disconnection of circuit breakers due to unauthorized tripping:

[0072] ;

[0073] If a microgrid internal fault trips at the time of disconnection:

[0074] ;

[0075] ;

[0076] ;

[0077] Read Moment power , ;

[0078] but ;

[0079] in, This refers to the fault clearing power within the microgrid. It is the off-grid pre-adjusted power;

[0080] S4. Based on the state at the time of disconnection, error compensation can be performed on the power after disconnection. For rapid disconnection, the basis for compensation can be simplified and ignored for micro-fluctuation errors.

[0081] For events involving relatively long periods of offline activity, the rate of frequency change is calculated in real time. If the rate of frequency change exceeds a set value, compensation is needed to mitigate the impact of the frequency change; that is, if... If the value exceeds the set value, the increase or decrease in active power caused by inertia is calculated using an inertial support algorithm for off-grid compensation.

[0082] ;

[0083] Pick The frequency slip at a given time is used to compensate for and correct the corresponding adjustment power, based on the previously calculated parameters. After error compensation, the corrected adjustment result is:

[0084] .

[0085] in, Inertia affects the increase or decrease in active power. It is the equivalent inertial time constant; It is the rate of change of frequency; It is the rated power of the adjustable microgrid power supply; It is the system's rated frequency; It is the total adjusted power after correcting for the influence of virtual inertia in the off-grid test;

[0086] Based on the conditions for triggering off-grid access, a threshold for off-grid access speed is preset, and the speed of triggering off-grid access is divided into Class I triggering off-grid access and Class II triggering off-grid access. Class I triggering off-grid access is fast off-grid access, and Class II triggering off-grid access is slow triggering off-grid access, such as circuit breaker tripping or trigger signals with a time less than a certain time (which can be adjusted according to the power grid conditions; usually, the action time of instantaneous overcurrent protection is 0ms, and the action signal response time of the circuit breaker is set to 40ms).

[0087] Using the off-grid time T0 as a benchmark, the power data in the memory time domain data loop storage area corresponding to the time period (T0-ΔT) is traced back, and the power value of that time period is extracted as the off-grid power adjustment benchmark value. Based on the power adjustment benchmark value, combined with the real-time load demand of the microgrid and the output of distributed power sources, a power rebalancing allocation strategy is implemented. Power command allocation is completed within the first power frequency cycle after off-grid, realizing a smooth switch between grid-connected mode and off-grid mode.

[0088] The extracted power values ​​include, but are not limited to: the active power value at the grid connection point during the ΔT period before disconnection, and the active power value of each relevant branch.

[0089] S5. If the frequency deviation is large and exceeds the set frequency adjustment value, the power for frequency difference compensation can be calculated according to the formula:

[0090] ,

[0091] in, It is the power for frequency difference compensation. It is the frequency modulation diastolic rate. It is the frequency of the disturbance moment. The definition is the same as above.

[0092] Based on the calculation results, compensatory adjustments are made to the power output after grid disconnection. The adjusted microgrid system has a frequency closer to its rated value.

[0093] Power rebalancing specifically includes: comparing the power value extracted from the memory time-domain data loop storage area with the real-time load demand, the output and capacity of distributed power sources, and using the historical power data from the memory time-domain data loop storage area as the basis for power rebalancing within the capacity range.

[0094] S6. Based on the off-grid cross-sectional power record and off-grid correction power, send off-grid regulation power to the known corresponding micro-sources in the microgrid system. The off-grid regulation power includes the power and frequency deviation of the interaction between the system grid and the microgrid during normal operation. After the off-grid operation, the correction power required within the rated value range of the normal operating frequency of the microgrid is adjusted so that the micro-sources in the microgrid can make up the difference in power, realize the smooth switching of the microgrid's on-grid and off-grid power, and thus reduce the frequency and voltage fluctuations of the microgrid.

[0095] During off-grid switching, the voltage fluctuation of critical loads should not exceed ±10% of the rated value, and the frequency deviation should be controlled within ±0.5Hz.

[0096] This embodiment uses historical power data and off-grid scenario classification in the memory time-domain data cyclic storage area to dynamically adjust and calculate for different situations. When the grid is disconnected, the process time causing the disconnection is calculated based on the data and event information in the memory storage area. The disconnection process time is then subtracted from the actual disconnection time. Since the data is from the memory storage area, a compensation calculation confirmation cycle of 2ms is considered. The off-grid event confirmation time is 10-12ms (half-cycle calculation time for voltage, current, power, etc.; no calculation time is needed if the disconnection is caused by an external signal). The compensation calculation time is 2-6ms (can be confirmed three times), and the cumulative process time is within 18ms. Therefore, the power adjustment calculation time is shortened to less than 20ms, with less calculation workload and time consumption. The power rebalancing command issuance exit time (signal output time, communication or exit time <10ms) is <30ms. Pre-issuing the power command greatly increases the speed at which the system enters the stable region, increases the probability of rapid microgrid stabilization, and shortens the microgrid's stabilization adjustment time.

Claims

1. A fast and smooth handover method for unplanned network disconnection, characterized in that, Includes the following steps: S1. Establish a time-domain data circular storage area; S2. Monitor disturbance data at the off-grid PCC point of the microgrid and classify the status signals that trigger off-grid access. S3. Determine the off-grid time segment based on the classification; In S3, the specific steps for determining the off-grid time section include: S31. Timestamp the corresponding time position in the time domain data circular storage area for different types of off-network reasons; S32. Calculate the power fluctuation start time that causes the grid disconnection for each type of disconnection based on different timestamps; S4. Perform error compensation on the power of the microgrid side after disconnection based on the off-grid status signal to obtain the off-grid corrected power; the error compensation includes inertia power compensation caused by the rate of frequency change. S5. If the frequency deviation exceeds the set frequency adjustment setting value, calculate the compensation power value caused by the frequency deviation, and make compensatory adjustments to the power of the microgrid side after disconnection based on the power compensated for the frequency deviation. S6. Based on the power at the off-grid time section and the off-grid correction power, send off-grid regulation power to the known micro-sources in the microgrid system to realize the switching of on-grid and off-grid power in the microgrid, thereby reducing the frequency and voltage fluctuations of the microgrid.

2. The method for fast and smooth handover of unplanned network disconnection according to claim 1, characterized in that, In S1, the specific steps for establishing the time-domain data circular storage area include: S11. Create a two-dimensional array that includes the number of power points and power data; S12. Store the power data corresponding to each power point in real time at preset time intervals; form a time-domain power data cyclic storage area.

3. The method for fast and smooth handover of unplanned network disconnection according to claim 1, characterized in that, In S2, the specific steps for monitoring the disturbance data of the off-grid PCC point of the microgrid include: S21. Collect target node data to form event record information at different time sections, collect disturbance data of PCC points, and collect status signals of planned and unplanned passive disconnection. S22. Classify and process different status signals according to different reasons for disconnection.

4. The fast and smooth handover method for unplanned network disconnection according to claim 3, characterized in that, The principle of the classification process is: For planned off-grid operations, the power at the PCC point is adjusted to zero to achieve uninterrupted or minimally disruptive off-grid operation. Unplanned passive disconnection caused by circuit breaker misoperation and manual tampering is classified according to the circuit breaker's tripping time; Unplanned passive disconnection caused by protective actions is classified according to the type of protective action and the delay.

5. The method for fast and smooth handover of unplanned network disconnection according to claim 1, characterized in that, In S4, the error compensation is based on: according to the conditions for triggering disconnection, a threshold for disconnection speed is preset, and the speed of triggering disconnection is divided into Class I disconnection and Class II disconnection. For a certain type of event that triggers disconnection, ignore the micro-volatility error; For the second type of triggering off-network events, the rate of change of frequency is calculated in real time and stored in the corresponding storage area of ​​the two-dimensional array. If the rate of change of frequency exceeds the preset rate of change threshold, the impact of frequency change needs to be compensated.

6. The method for fast and smooth handover of unplanned network disconnection according to claim 1, characterized in that, In S4, the error compensation includes: using an inertial support algorithm to calculate the increase or decrease in active power caused by inertia for off-grid compensation; and when the power adjustment is based on the frequency slip at a given time, compensating and correcting the corresponding adjustment power, thereby achieving error compensation for the power on the off-grid side of the microgrid.

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