Rapid and smooth switching method for non-planed off-network

Abstract

The invention discloses a rapid and smooth switching method for unplanned off-grid, and relates to the field of micro-grid on-grid and off-grid switching. In order to overcome the defects that an existing microgrid off-grid control method is unstable in adjusting effect and low in adjusting speed, a time domain data circulation storage area is established; monitoring disturbance data of an off-grid PCC point of the micro-grid, and classifying off-grid triggering state signals; determining an off-grid time section; performing error compensation on the off-grid micro-grid side power according to the off-grid state signal; if the frequency deviation exceeds a set frequency adjustment set value, performing compensatory adjustment on the off-grid power according to the power compensated by the frequency difference; according to the off-grid section power and the off-grid correction power, off-grid adjustment power is sent to a known micro power source of the micro-grid system, switching of grid-connected power and off-grid power of the micro-grid is achieved, and therefore frequency and voltage fluctuation of the micro-grid are reduced. The method and the device are mainly used for fast and smooth switching of the unplanned off-network.

Description

Technical Field

[0001] The present invention relates to the field of microgrid on-grid and off-grid switching, and in particular to a fast and smooth switching method for unplanned off-grid operation. Background Art

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

[0003] As a beneficial supplement to the power supply network, microgrids have increased the power supply methods in the power supply area and improved the power supply reliability of the power grid. However, they have also increased the complexity of stable operation control for the microgrid itself, especially the rapid regulation when the microgrid is disconnected from the main power grid, which is crucial to the stable operation of the microgrid.

[0004] The current control method for microgrids off-grid is to collect and calculate the frequency and voltage in real time after off-grid, and use droop control algorithms to make corresponding power adjustments according to the changes. The adjustment effects of microgrids vary according to the algorithm. The stabilization process of the microgrid may take hundreds of milliseconds or even seconds. Excessive unstable control time will have an adverse effect on the operation and stability of equipment in the microgrid. In severe cases, frequency transient overshoot and drastic voltage fluctuations will occur, which may cause instability of the entire network. In severe cases, the power grid may be paralyzed.

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

[0006] In order to solve the defects of the existing microgrid off-grid control method such as unstable adjustment effect and slow adjustment speed, the present invention provides a fast and smooth switching method for unplanned off-grid operation that can accelerate the accurate adjustment speed of off-grid operation, maintain stable operation of microgrid equipment, and avoid grid paralysis.

[0007] The present invention provides a method for fast and smooth switching of an unplanned off-grid event, comprising the following steps: S1. Establish a time domain data loop storage area; S2, monitor the disturbance data of the microgrid off-grid PCC point and classify the status signal that triggers off-grid; S3. Determine the off-grid time section based on the classification situation; S4. Perform error compensation on the microgrid side power after the off-grid state signal to obtain the off-grid corrected power; S5. If the frequency deviation exceeds the set frequency adjustment value, the compensation power value caused by the frequency difference is calculated, and the power after the off-grid operation is compensated according to the power compensated by the frequency difference; S6. According to the off-grid cross-sectional power and the off-grid correction power, the off-grid adjustment power is sent to the known micro power source of the microgrid system to realize the switching of the microgrid and off-grid power, thereby reducing the frequency and voltage fluctuations of the microgrid.

[0008] Further: in S1, the specific steps of establishing the time domain data cyclic storage area include: S11, creating a two-dimensional array including the number of power points and power data; S12. The power data corresponding to each power point is stored in real time at a preset time interval to form a time domain power data circular storage area.

[0009] Furthermore: in S2, the specific steps of monitoring the disturbance data of the off-grid PCC point of the microgrid include: S21. Collect data from target nodes to generate event records at different time sections, collect disturbance data from PCC points, and collect status signals of planned and unplanned off-grid disconnections. S22. Classify and process different status signals according to different off-grid reasons.

[0010] Furthermore: the principles of the classification process are: For planned off-grid operation, the power at the PCC point is adjusted to zero to achieve undisturbed or minimally disturbed off-grid operation. Unplanned passive disconnections caused by circuit breaker tripping and manual mis-tripping are classified according to the circuit breaker tripping time. Unplanned passive off-grid events caused by protection actions are classified according to the protection action type and delay.

[0011] Further: in S3, the specific steps of determining the off-grid time section include: S31, respectively, time stamping the corresponding time position in the time domain data loop storage area for different types of off-grid reasons; S32: Calculate the starting time of the power fluctuation causing the grid-off for the corresponding type of grid-off reason according to the different time stamps.

[0012] Further: in S4, the error compensation is based on: according to the off-grid triggering condition, a threshold value of the off-grid speed is preset, and the speed of triggering off-grid is divided into the first type of triggering off-grid and the second type of triggering off-grid; For a type of off-grid triggering event, the micro-fluctuation error is ignored; For the second type of off-grid triggering events, the frequency change rate is calculated in real time and stored in the corresponding storage area of ​​the two-dimensional array. If the frequency change rate exceeds the preset frequency change rate threshold, the impact of the frequency change needs to be compensated.

[0013] Further: in S4, the error compensation includes: using the 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 certain moment in time, compensating and correcting the corresponding adjusted power, thereby achieving error compensation for the off-grid pre-adjusted power.

[0014] The beneficial effects of the present invention are: The present invention proposes a fast and smooth switching method for unplanned off-grid operation, which is a fast off-grid power adjustment judgment basis based on a memory method. It can improve the fast adjustment power balance in planned and unplanned off-grid situations, improve the accuracy and speed of off-grid adjustment, and thus reduce microgrid disturbances.

[0015] The present invention is aimed at off-grid scenarios of microgrids and can be used for abnormal microgrid external abnormal situations such as system failures, circuit breaker tripping, and abnormal human power outages. According to the situation, the power and load can be adjusted quickly and accurately 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 improving the survivability of the microgrid.

[0016] The present invention uses dynamic storage data as a basis to conduct targeted and precise time tracing for different off-grid types, deducting the influence of uncertain system data caused by abnormal off-grid operation such as protection action. The present invention memorizes historical power data and off-grid scenario classification in the time domain data loop storage area, and performs dynamic adjustment calculations for different conditions. The amount of calculation and the time consumption are relatively low. The pre-issuance of power instructions greatly speeds up the system's entry into the stable zone, increases the probability of rapid stabilization of the microgrid, and shortens the stable adjustment time of the microgrid. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Schematic diagram of the process of this method.

[0018] Figure 2 Schematic diagram of the scenario to which this method is applicable.

[0019] Figure 3 Schematic diagram for classification of off-grid types. DETAILED DESCRIPTION

[0020] The following are only preferred specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or replacements that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the scope of protection of the present invention. The embodiments described below are only used to explain the present invention and cannot be interpreted as limiting the present invention. The scope of protection of the present invention should be based on the scope of protection 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 description of the present invention should be interpreted broadly, including but not limited to conventional replacement schemes not mentioned in this application, and also including direct implementation and indirect implementation.

[0021] Example 1 Combine Figure 1-Figure 3 This embodiment describes a method for fast and smooth switching of an unplanned off-grid operation, including: When a microgrid is off-grid, it is necessary to quickly bring the off-grid microgrid to a new power balance point to stabilize the frequency and voltage of the microgrid and enable the microgrid to quickly enter a stable operating state. According to the conditions for stable operation of the power system, in steady-state operation, the power absorbed or consumed by the grid is equal, and the law of conservation of power: ; Among them, P 输入 Indicates input power, P 输出 Indicates output power; The PCC point (Point of Common Coupling) of the microgrid is used as the key node (i.e., the target node) to form two systems after disconnection. The power provided by the system at this point is , this value can be positive or negative. The current at this point satisfies Kirchhoff's law, and so does the power at this point. The power situation at different time sections is recorded with the time domain of system operation as the axis, and the power calculation is performed with the grid connection point as the node.

[0022] ; ; in, Represents the grid connection point power. A positive grid connection point power indicates that power flows out of the microgrid. represents the power of the microgrid system; Indicates the power generation in the microgrid; Indicates energy storage power. A positive value indicates charging, while a negative value indicates discharging. represents the microgrid load; Indicates other power in the microgrid.

[0023] The steps of this method are as follows: S1. Create a time domain data loop storage area with sufficient margin. The specific operations are as follows: 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 a time stamp buffer area is to be recorded. n is the width of the circular storage area for recording power and other time domain data that needs to be opened. Table 1 shows the memory time domain data cycle storage area built by this method

[0024] 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 according to time accuracy requirements; real-time monitoring of the grid connection points and the collected power data, and establishment of a dynamically updated memory time domain data circular storage area; the memory time domain data circular storage area adopts a ring storage structure, the storage depth of which at least covers the duration of the transient process of the power system, and the sampling frequency is not less than 1kHz.

[0025] For different types of off-grid events, the process time ΔT from the occurrence of disturbance to complete off-grid disconnection is calculated respectively. The process time ΔT is determined by analyzing the composite characteristics of the protection device action sequence and the power change; The calculation method of the process time ΔT includes: for instantaneous off-grid, ΔT is taken as the sum of the inherent action time of the protection device and the circuit breaker tripping time, which can also be approximately equal to the circuit breaker tripping time; for continuous off-grid, ΔT is adjusted according to the dynamic response time of the protection action duration.

[0026] S2. Monitor the disturbance data of the microgrid when it is off-grid, including switch status, voltage, current, power, voltage fluctuation, off-grid status, etc. The specific operations are as follows: S21. The GOOSE data network may be used to collect data from key nodes, forming event record information at different time sections, collecting disturbance data at PCC points, characteristics of internal protection fault signals in the microgrid, and the system-side interactive fault signal transmission mechanism. During the off-grid power regulation process, if a fault occurs in a certain internal line within the microgrid at the time of off-grid operation, causing a trip, the change in the power of this line cannot ignore the impact of fault removal. Therefore, it is necessary to consider that the internal fault of the microgrid must be deducted from the regulation after off-grid operation. Internal fault signals of the microgrid include trip signals of protection devices installed on internal lines or transformers, or obtained based on the position signals of circuit breakers at key nodes. Fault signals on the system side can be directly collected through position signals, using a communication network or a specially established communication network, and transmitting fault-point related signals (including location, protection trip signals, protection action stage and time setting, etc.) through the GOOSE fast communication mechanism. Planned off-grid and unplanned passive off-grid status signals are collected. Key point data refers to power nodes whose power values ​​cannot be ignored, and PCC points are key points among them.

[0027] S22. Categorize and process different status signals. Considering planned off-grid conditions, the power at the PCC point must be adjusted to zero to achieve undisturbed or minimally disturbed off-grid conditions. For circuit breaker trips and manual trips, only the breaker trip time is considered. Because the short duration of the off-grid event is the breaker trip time, the impact of power changes during the trip time can be disregarded. For protection actions, the protection action type and delay must be considered. The protection action type primarily distinguishes the duration of the protection action. If it exceeds 40ms, the cross-sectional time immediately before the fault impact must be calculated based on the action time used by the protection type and the action start time.

[0028] Receive an off-grid trigger signal, and identify the type of off-grid event and classify it as instantaneous or continuous off-grid based on the time characteristics of the trigger signal; the trigger signal may include a voltage sag signal, a frequency over-limit signal, an external protection action event, or a circuit breaker trip signal; S3. Determine the time section of the off-grid power adjustment reference value based on the classification. The specific operations are as follows: S31. Timestamp the corresponding time position in the time domain data loop storage area for different types of off-grid reasons, for example: is the system protection tripping time, It's off-grid time. is the disturbance recording time, is the circuit breaker trip time, etc. S32. Calculate the starting time of the power fluctuation that causes the off-grid for different types of off-grid reasons according to different timestamps; store the time domain section data and event information of the historical power in the form of timestamps; the event information refers to the information that marks important time sections such as off-grid, protection action, disturbance, etc., which is used to determine the off-grid time, disturbance start time, etc.

[0029] For example: Off-grid caused by external system failure of microgrid: generally It can be considered that The same or close, the compensation amount can be increased according to the actual situation; ; ; ; in, It is the fault action delay information of the protection trip record; is the protection tripping start time; Is the power adjustment based on time; The time difference between the two times is less than a smaller set value; Then prove is the starting time of off-grid fluctuation, The time is the off-grid power memory point time , read the corresponding , as the difference basis for the redistribution of the microgrid after the off-grid, the power within the microgrid after the off-grid is adjusted; in, It is the grid connection point power value in the time domain data loop storage area of ​​the off-grid fluctuation starting time memory; Unplanned passive off-grid circuit breaker tripping fault: ; If there is a microgrid internal fault trip at the time of disconnection: ; ; ; Read Power at all times , ; but ; in, is the fault clearing power within the microgrid, It is the off-grid pre-adjusted power; S4. According to the state at the time of disconnection, the power after disconnection can be compensated for errors. For fast disconnection, the micro-fluctuation error can be simplified and ignored. For relatively long off-grid events, the frequency change rate is calculated in real time. If the frequency change rate exceeds the set value, the impact of the frequency change needs to be compensated; that is, if If the set value is exceeded, the inertial support algorithm is used to calculate the increase or decrease in active power caused by inertia for off-grid compensation. ; Pick The frequency slip at the moment is used to compensate for the corresponding adjustment power. Based on the previous calculation, After error compensation, the adjusted result is: .

[0030] in, It is the inertia that affects the increase or decrease of active power; is the equivalent inertia time constant; is the rate of change of frequency; is the rated power of the adjustable microgrid power supply; is the rated frequency of the system; It is the total adjusted power after correction for the effect of off-grid virtual inertia; Based on the conditions for triggering off-grid operation, a threshold for off-grid speed is preset, and the speed of triggering off-grid operation is divided into Class I triggering off-grid operation and Class II triggering off-grid operation; Class I triggering off-grid operation is fast off-grid operation, and Class II triggering off-grid operation is slow off-grid operation, such as a circuit breaker tripping or a trigger signal with a duration less than a certain time (which can be adjusted according to the power grid conditions, usually the action time of the fast-trip protection is 0ms, considering that the circuit breaker action signal reaction time is set to 40ms).

[0031] Taking the off-grid moment T0 as the benchmark, the power data in the memory time domain data loop storage area corresponding to the period (T0-ΔT) is traced back, and the power value of this period is extracted as the off-grid power adjustment reference value. Based on this power adjustment reference 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, achieving smooth switching between grid-connected mode and off-grid mode. The extracted power values ​​include but are not limited to: the active power value of the grid connection point during the ΔT period before disconnection, the active power value of each related branch, etc.

[0032] 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: , in, is the power of frequency difference compensation, is the frequency modulation rate, is the frequency of the disturbance moment, Same definition as above.

[0033] According to the calculation results, the power after off-grid is compensated and adjusted. , the frequency of the adjusted microgrid system is closer to the rated value.

[0034] Power rebalancing specifically includes: comparing the power value extracted from the memory time domain data circulation storage area with the real-time load demand, the output of the distributed power source, and the capacity, and using the historical power data in the memory time domain data circulation storage area as the initial adjustment amount to perform power rebalancing within the capacity range.

[0035] S6. Based on the off-grid cross-sectional power record and off-grid correction power, the off-grid adjustment power is sent to the corresponding micro-power source in the microgrid system. The off-grid adjustment power includes the power and frequency deviation of the interaction between the system grid and the microgrid during normal operation, and the correction power required to keep the microgrid within the normal operating frequency rated value after disconnection. The micro-power sources in the microgrid make up the difference in power, realizing smooth switching of the microgrid and off-grid power, thereby reducing the frequency and voltage fluctuations of the microgrid.

[0036] During off-grid switching, the voltage fluctuation of critical loads is kept within ±10% of the rated value, and the frequency deviation is controlled within the range of ±0.5Hz.

[0037] This embodiment uses historical power data and off-grid scenario classification stored in a time-domain data cyclic storage area to dynamically adjust calculations for different conditions. During an off-grid event, the off-grid process time is calculated based on the data and event information in the storage area, and the off-grid process time is deducted from the off-grid time. Since the data is stored in the storage area, the compensation calculation confirmation cycle is 2ms. The off-grid event confirmation time is 10-12ms (half-cycle calculation time for voltage, current, power, etc.; no calculation time is required if the signal is generated by an external off-grid signal), the compensation calculation time is 2-6ms (three confirmations are possible), and the cumulative process time is 18ms. This reduces the power switching calculation time to less than 20ms, reducing the computational complexity and time consumption. The power rebalancing command issuance time (signal output time, communication or export time < 10ms) is less than 30ms. Pre-issuing power commands significantly accelerates the system's entry into the stable zone, increasing the probability of rapid microgrid stability and shortening the microgrid's stable regulation time.

Claims

1. A fast and smooth switching method for unplanned off-grid operation, characterized in that: The steps include: S1. Establish a time domain data loop storage area; S2, monitor the disturbance data of the microgrid off-grid PCC point and classify the status signal that triggers off-grid; S3. Determine the off-grid time section based on the classification situation; S4. Perform error compensation on the microgrid side power after the off-grid state signal to obtain the off-grid corrected power; S5. If the frequency deviation exceeds the set frequency adjustment value, the compensation power value caused by the frequency difference is calculated, and the power after the off-grid operation is compensated according to the power compensated by the frequency difference; S6. According to the off-grid cross-sectional power and the off-grid correction power, the off-grid adjustment power is sent to the known micro power source of the microgrid system to realize the switching of the microgrid and off-grid power, thereby reducing the frequency and voltage fluctuations of the microgrid.

2. The fast and smooth switching method for unplanned off-grid operation according to claim 1, characterized in that: In S1, the specific steps of establishing the time domain data cyclic storage area include: S11, creating a two-dimensional array including the number of power points and power data; S12. The power data corresponding to each power point is stored in real time at a preset time interval to form a time domain power data circular storage area.

3. The fast and smooth switching method for unplanned off-grid operation according to claim 1, characterized in that: In S2, the specific steps of monitoring the disturbance data of the off-grid PCC point of the microgrid include: S21. Collect data from target nodes to generate event records at different time sections, collect disturbance data from PCC points, and collect status signals of planned and unplanned off-grid disconnections. S22. Classify and process different status signals according to different off-grid reasons.

4. The fast and smooth switching method for unplanned off-grid operation according to claim 3, characterized in that: The principles of classification processing are: For planned off-grid operation, the power at the PCC point is adjusted to zero to achieve undisturbed or minimally disturbed off-grid operation. Unplanned passive disconnections caused by circuit breaker tripping and manual mis-tripping are classified according to the circuit breaker tripping time. Unplanned passive off-grid events caused by protection actions are classified according to the protection action type and delay.

5. The method for fast and smooth switching of unplanned off-grid according to claim 1, characterized in that: In S3, the specific steps of determining the off-grid time section include: S31, respectively, time stamping the corresponding time position in the time domain data loop storage area for different types of off-grid reasons; S32: Calculate the starting time of the power fluctuation causing the grid-off for the corresponding type of grid-off reason according to the different time stamps.

6. The method for fast and smooth switching of unplanned off-grid according to claim 1, characterized in that: In S4, the error compensation is based on: according to the off-grid triggering conditions, a threshold value of off-grid speed is preset, and the speed of triggering off-grid is divided into the first type of triggering off-grid and the second type of triggering off-grid; For a type of off-grid triggering event, the micro-fluctuation error is ignored; For the second type of off-grid triggering events, the frequency change rate is calculated in real time and stored in the corresponding storage area of ​​the two-dimensional array. If the frequency change rate exceeds the preset frequency change rate threshold, the impact of the frequency change needs to be compensated.

7. The method for fast and smooth switching of unplanned off-grid according to claim 1, characterized in that: In S4, the error compensation includes: using the 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 certain moment in time, compensating and correcting the corresponding adjusted power, thereby achieving error compensation for the off-grid pre-adjusted power.

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