A power supply guarantee type micro-grid double-inlet intelligent switching method based on dynamic threshold and multi-source cooperation and an intelligent switching system thereof

Abstract

The application relates to a dynamic threshold and multi-source collaborative based power supply type micro-grid double-inlet intelligent switching method and an intelligent switching system thereof, and relates to the field of micro-grid intelligent control. In order to solve the defects of existing power supply type double-inlet micro-grid, such as unstable switching, large energy storage investment, lagging operation state recognition and low synchronization grid connection efficiency, the power quality anomaly of the original inlet branch of the micro-grid is recognized, transient disturbance and continuous fault are distinguished, and the off-grid state is judged; the island stable operation regulation is carried out in the mode of multi-source coordinated control and load hierarchical optimization management; the standby inlet is subjected to wide frequency domain power quality analysis and dynamic impedance detection, and the grid connection compatibility and transient response characteristics are evaluated; under the island stable state, the micro-grid self-adapts to regulate the micro-grid side voltage and phase to meet the quasi-synchronization condition, and the inlet switching is realized. The application is mainly used for intelligent switching of the power supply type micro-grid double-inlet.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of micro-grid intelligent control, and particularly relates to a power supply type micro-grid double-inlet intelligent switching method based on dynamic threshold and multi-source cooperation and an intelligent switching system thereof. BACKGROUND

[0002] At present, the power supply type micro-grid aims to guarantee power supply for important loads, and can be switched from grid-connected to off-grid operation when the main power grid loses power. For particularly important loads, double-inlet design is often used. However, the power supply type micro-grid with double-inlet has the following problems to be solved in inlet switching and operation control:

[0003] Inadequate inlet switching stability: double-inlet switching covers three stages of abnormal off-grid, island stable operation, and standby inlet grid-connected. The existing manual trigger inlet switching mode is difficult to accurately determine the island stable operation condition due to the uncertainty of the trigger time of the standby inlet. Too early switching instruction will lead to unstable island, voltage and frequency out of control, and easy switching failure; too late switching will prolong the island operation time, which not only increases the size of the energy storage system and the construction cost of the micro-grid, but also increases the risk of energy storage depletion and system disconnection.

[0004] Running state recognition lag: the reason for the loss of power of the main power grid is complex, and any abnormality or fault of the branch connected to the grid-connected point of the micro-grid or any level of the upper level will make it enter the island state. Since the power grid fault is usually cut off by the protection device independent of the control system, the control system needs to determine the running state of the micro-grid by judging the state of the multi-level protection device through communication. This method cannot meet the fast transient response demand of the micro-grid in the event of main grid failure, and is prone to delayed island state recognition when the external power is lost, resulting in mismatch between the control strategy and the running state, affecting the stable operation of the micro-grid.

[0005] Low efficiency and reliability of synchronous grid connection: when the micro-grid is switched from the island stable operation stage to the grid-connected stage, synchronous grid connection operation is required, that is, the micro-grid main power source is controlled to realize pre-synchronization with the voltage, frequency and phase of the external main power grid. Although there are pre-synchronization synchronization strategies for micro-grid off-grid to grid-connected, the frequency adjustment in these strategies mostly uses fixed step adjustment method, which is prone to frequency oscillation in actual application, resulting in too long synchronization adjustment time and even causing failure of synchronous grid connection.

[0006] Therefore, a power supply type micro-grid double-inlet intelligent switching method based on dynamic threshold and multi-source cooperation and an intelligent switching system thereof are needed, which can realize intelligent automatic switching of the micro-grid double-inlet, and improve the reliability and stability of the micro-grid under complex working conditions. SUMMARY

[0007] The present application provides a kind of based on dynamic threshold and multi-source collaborative power supply type microgrid double-inlet intelligent switching method and its intelligent switching system, which can realize the intelligent automatic switching of microgrid double-inlet, improve the reliability and stability of microgrid under complex working conditions, and provide strong support for efficient operation of power supply type microgrid.

[0008] The microgrid double-inlet intelligent switching method based on dynamic threshold and multi-source collaboration provided by the present application comprises the following steps:

[0009] S1, abnormal identification of power quality at the inlet side;

[0010] Based on the decision algorithm combining multi-dimensional parameter collaborative dynamic monitoring and fuzzy logic judgment, the abnormal identification of power quality of the original inlet branch of the microgrid is realized, the transient disturbance and the continuous fault are distinguished, and the off-grid state is judged.

[0011] S2, stable control of microgrid island state;

[0012] According to the off-grid state of the microgrid caused by the identification result, the island stable operation adjustment is carried out in the mode of multi-source coordinated control and load hierarchical optimization management, to ensure the real-time balance of island state power and the stability of voltage frequency;

[0013] S3, power quality analysis and dynamic impedance detection of standby inlet;

[0014] The standby inlet is subjected to wide frequency domain power quality analysis and dynamic impedance detection to evaluate its grid compatibility and transient response characteristics.

[0015] S4, adaptive adjustment of microgrid standby inlet;

[0016] Under the island stable state, the microgrid adaptively adjusts the microgrid side voltage and phase to meet the quasi-synchronization condition, and realizes the inlet switching.

[0017] Further, in S1, the abnormal identification of power quality at the inlet side comprises the following steps:

[0018] S11, first construct the synchronous measurement mode of inlet side and microgrid side, and synchronously collect electrical quantities,

[0019] S12, calculate the characteristic vector of off-grid criterion of microgrid affected by the collected electrical quantities;

[0020] S13, reference dual-time scale verification mechanism and dynamic weight distribution, determine the change range of weight according to the influence degree of each electrical quantity on off-grid judgment of microgrid;

[0021] S14, the influence degree of the multi-dimensional parameters of the micro-grid incoming line side and the grid-connected point on the off-grid judgment of the micro-grid is used to realize the off-grid decision of the micro-grid;

[0022] Further, in S2, according to different working conditions, the island stable operation adjustment is carried out in a multi-source coordinated control and load hierarchical optimization management manner; the off-grid state caused by the identification result specifically includes the following steps:

[0023] When the working condition of power shortage occurs at the off-grid moment, that is, the dynamic power difference ΔP>0, the maximum discharge power of the current main power energy storage system is obtained according to the real-time communication of the control system and the battery management system, and the maximum discharge power is compared with the dynamic power difference ΔP to judge whether the adjustment can be relied on the energy storage system;

[0024] When the new energy power surplus occurs at the off-grid moment, that is, the dynamic power difference ΔP<0, the maximum charging power of the current main power energy storage system is obtained according to the real-time communication of the control system and the battery management system, and the maximum charging power is compared with the dynamic power difference ΔP to judge whether the adjustment can be relied on the energy storage system.

[0025] Further, when the dynamic power difference ΔP>0, if the energy storage system has the power generation to make up for the dynamic power difference ΔP, the power shortage is actively made up by the energy storage system converter itself controller; if the energy storage system is insufficient to make up for the dynamic power difference ΔP, the control system controls the load shedding according to the level.

[0026] Further, when the dynamic power difference ΔP<0, if the energy storage system has the power absorption to make up for the dynamic power difference ΔP, the power shortage is actively made up by the energy storage system converter itself controller; if the energy storage system is full of electricity, or the charging power is insufficient to make up for the dynamic power difference ΔP, the control system controls the new energy power generation to reduce the power generation, so as to realize the power balance.

[0027] Further, the wide frequency domain power quality analysis is realized by the time-frequency joint analysis technology according to the voltage signal collected in the full frequency band of the standby incoming line;

[0028] The dynamic impedance detection is based on the voltage / current response data, the equivalent circuit model is fitted, the micro-grid-standby incoming line joint model is constructed based on the detection impedance, and the simulation of the closing transient process is realized.

[0029] If the power quality reaches the preset threshold, and the dynamic impedance matched with the micro-grid load is evaluated, the incoming line meets the requirement of being connected to the micro-grid as a standby incoming line.

[0030] Further, the micro-grid adaptive regulation of micro-grid side voltage and phase to meet the quasi-synchronization condition is to adopt the PCS in the energy storage system to carry out the synchronization and grid connection process, and the voltage / frequency regulation of the micro-grid side is carried out.

[0031] Micro-grid internal voltage control: the micro-grid coordination controller issues a target power to the PCS to control the regulation of the micro-grid bus voltage to be equal to the voltage of the standby incoming line.

[0032] Micro-grid internal frequency control: under the premise that the frequency meets the frequency difference requirement, the phase difference needs to be adjusted.

[0033] The intelligent switching system for implementing the micro-grid dual incoming line intelligent switching method based on dynamic threshold and multi-source cooperation disclosed by the application comprises a synchronous measurement module, a coordination and decision module and an execution module.

[0034] The synchronous measurement module is used for synchronously collecting electrical quantities on the incoming line side and the micro-grid side.

[0035] The coordination and decision module is used for analyzing and judging the collected electrical quantities and outputting control instructions.

[0036] The execution module is used for controlling the execution unit to execute the intelligent switching of the dual incoming line according to the control instructions of the coordination and decision module.

[0037] Further, the coordination and decision module comprises an off-grid judgment module, a dynamic weight distribution model, an island control module, a standby power supply evaluation module and a synchronization closing module.

[0038] The off-grid judgment module is used for adopting the time-varying dynamic weight distribution model to judge the off-grid of the collected electrical quantities.

[0039] The island control module is used for controlling the island power balance.

[0040] The standby power supply evaluation module is used for evaluating the grid connection compatibility of the standby power supply.

[0041] The synchronization closing module is used for the variable step frequency control adjustment mechanism based on the dynamic adjustment of the frequency difference to ensure that the frequency adjustment direction is consistent with the phase difference reduction direction.

[0042] Further, the execution module comprises an energy storage PCS, a load switch and a standby power supply switch.

[0043] The application has the following beneficial effects:

[0044] The application proposes an intelligent switching method based on dynamic threshold and multi-source cooperation, which significantly improves multiple key indicators. Through technical innovation, the application breaks through the limitations of traditional solutions from four dimensions of "efficiency, cost, safety, and adaptability", providing a cost-effective intelligent solution for important load supply. Through intelligent control strategy and multi-technology fusion, the application realizes the following technologies:

[0045] 1. Multi-dimensional coordinated off-grid state judgment mechanism

[0046] The application proposes synchronous collection and fusion analysis based on multi-source electrical quantities (parameters such as voltage, frequency, current, and switching quantity on the line side / microgrid side), constructs a fuzzy logic decision algorithm containing a dynamic weight distribution model and a double-time scale verification mechanism, and solves the problems of traditional single parameter judgment being easily disturbed and communication delay of multi-level protection. The application realizes fast (≤200ms) and accurate (accuracy rate 98%) identification of microgrid off-grid state, and for the first time reduces the misjudgment rate of transient disturbance (such as voltage sag <10ms) to below 2%.

[0047] In terms of line switching stability, through multi-dimensional parameter cooperative monitoring and fuzzy logic judgment algorithm, combined with double-time scale verification mechanism and dynamic weight distribution, the accuracy of microgrid off-grid judgment is improved from about 75% of traditional manual method to above 98%, effectively avoiding misjudgment of transient disturbance.

[0048] 2. Dynamic power balance control strategy for island operation

[0049] The application designs multi-source coordinated control and load hierarchical optimization management method, calculates the dynamic power difference (ΔP) in real time, preferentially calls the energy storage system for fast response (response time ≤50ms), and automatically cuts off the load according to the priority of three-level load (three-level load is cut off within 1s) when insufficient, and cooperates with voltage frequency closed-loop regulation under VF control mode. The application improves the voltage and frequency stability during island operation, and the stability is improved by 40% compared with traditional manual switching, realizes millisecond-level dynamic balance of "energy storage-load-new energy" in island state, solves the problems of line change failure and long island operation time caused by uncertain timing of traditional manual switching, realizes full-automatic intelligent switching between dual power sources, ensures uninterrupted power supply for important loads during power grid failure, and improves power supply continuity and reliability.

[0050] Meanwhile, multi-source coordinated control and load hierarchical optimization management are adopted to change the redundant design of energy storage capacity caused by manual operation, shorten the island operation time through accurate control, reduce the scale requirement of the energy storage system, and balance the economy and reliability. In the island operation stage, the power is balanced in real time, the voltage fluctuation range is controlled within ±10% of the rated voltage, the frequency deviation is stabilized at ±0.5 Hz, the scale requirement of the energy storage system is reduced by about 30%, the construction cost of the micro-grid is greatly reduced, and the configuration and cost of the energy storage system are optimized.

[0051] 3. Wideband compatibility evaluation technology of standby incoming line

[0052] The present application proposes 0.1Hz-20kHz full-band power quality analysis (including steady-state index, transient disturbance, harmonic parameter) and dynamic impedance sweep detection (injecting 0.1-100Hz signal to fit equivalent impedance model), and constructs a standby power supply grid-connected compatibility evaluation logic. The problem of relying on manual experience evaluation for the operation state recognition in the traditional scheme has strong subjectivity, and the present application shortens the compatibility detection time of the standby incoming line to within 200ms through wideband power quality analysis and dynamic impedance detection, that is, the feasibility of the standby power supply access is judged within 200ms, which meets the rapid transient response demand in the case of large grid fault, and provides accurate electrical reference for subsequent synchronous closing.

[0053] 4. Adaptive variable step synchronous closing control algorithm

[0054] The present application designs a variable step frequency control strategy based on frequency difference dynamic adjustment, cooperates with phase difference directional adjustment logic (ensures that the phase difference monotonically decreases), and replaces the frequency oscillation problem caused by traditional fixed step adjustment. The probability of misjudgment of off-grid state during large grid fault is reduced, technical difficulties such as standby incoming line grid connection oscillation are solved, the stable operation ability of the micro-grid in complex scenes such as voltage fluctuation and frequency anomaly is improved, and the adaptability to complex working conditions is enhanced.

[0055] In the synchronous grid connection link, the wideband power quality analysis and dynamic impedance detection are carried out on the standby incoming line, and the adaptive synchronous closing technology is adopted, so that the synchronous grid connection adjustment time is shortened from the average 5-8 seconds of the traditional method to within 1.5 seconds, the synchronous grid connection success rate is increased from about 80% to more than 95%, the “non-oscillation fast pre-synchronization” under the active control of the micro-grid main power supply is realized, and the frequency oscillation problem is avoided. BRIEF DESCRIPTION OF DRAWINGS

[0056] Figure 1 is a schematic diagram of multi-dimensional parameters of the micro-grid as functions of off-grid decision variables;

[0057] Figure 2 is a superimposed schematic diagram of decision variables of multi-dimensional parameters when the grid connection point is disconnected;

[0058] Figure 3 is a multi-dimensional parameter decision variable superposition diagram when the off-grid situation such as power failure of the branch connected by the micro-grid occurs;

[0059] Figure 4 is a stable operation control flow chart in island state;

[0060] Figure 5 is a flow chart of the overall scheme. DETAILED DESCRIPTION

[0061] The following are only preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto, any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the present application, which should be covered within the protection scope of the present application. The following examples are only used to explain the present application, and cannot be explained as the limitation of the present application, and the protection scope of the present application should be subject to the protection scope of the claims. The embodiments of the present application are described in detail below, in order to facilitate the description of the present application and simplify the description, the technical terms used in the specification of the present application should be interpreted broadly, including but not limited to the conventional replacement schemes not mentioned in the present application, and including direct implementation mode and indirect implementation mode.

[0062] Embodiment 1

[0063] In combination with Figures 1-5 In order to illustrate the present embodiment, the present embodiment discloses a power supply type micro-grid double-inlet intelligent switching method based on dynamic threshold and multi-source cooperation, which comprises the following steps:

[0064] S1, abnormal identification of power quality at the inlet side;

[0065] The decision algorithm based on the combination of multi-dimensional parameter cooperative dynamic monitoring and fuzzy logic judgment realizes the abnormal identification of power quality of the original inlet branch of the micro-grid, distinguishes transient disturbance and continuous fault, and realizes the accurate judgment of off-grid state.

[0066] Firstly, the synchronous measurement mode of the inlet side and the micro-grid side is constructed, and the electrical quantities of the inlet side and the micro-grid side are synchronously collected:

[0067] Inlet side electrical quantity: inlet voltage amplitude, frequency, current.

[0068] Micro-grid side electrical quantity: bus voltage amplitude, frequency, grid-connected point current.

[0069] Switching quantity signal: grid-connected point switch opening and closing signal.

[0070] According to the collected electrical quantities, the characteristic vector affecting the off-grid criterion of the micro-grid is calculated: the switch jump opening signal S of the grid-connected point, the inlet side no-voltage flag , frequency abnormal flag Reactive power change rate .

[0071] The membership function of the switch trip signal S at the grid connection point is:

[0072] (1);

[0073] in, Let S be the membership function of the switch trip signal S at the grid connection point. ;

[0074] No pressure mark on the incoming line side The membership function is:

[0075] (2);

[0076] in, No pressure mark on the incoming line side membership function, This refers to the voltage on the incoming power grid side. The rated voltage is given, and λ represents the time constant. The magnitude of λ determines... The degree of change over time is adaptively adjusted based on historical disturbances; t represents the amount of change over time.

[0077] Frequency anomaly flag The membership function is:

[0078] (3);

[0079] in, Frequency anomaly flag membership function, It is the actual frequency of the power grid. This is the frequency offset threshold. k is a calculation coefficient, the magnitude of which determines the frequency shift. The degree of influence, where e is a natural constant. For example Then the exponent of e is -k|Δfset|, and the larger k is, the closer μ is to 0; if The more drastic the offset, the more -k(|fgrid-50|-Δfset) is biased to the left of the coordinate axis. Therefore, when k is large enough, μ will be closer to 1. So k is a sufficiently large calculation coefficient.

[0080] Reactive power change rate The membership function is:

[0081] (4);

[0082] in, The rate of change of reactive power of the membership function of the switch position of the grid-connected point, Q is the current reactive power, is the rated reactive power.

[0083] When the above feature vectors are subjected to instantaneous disturbance, part of the feature vectors will trigger a large response. To avoid the influence of disturbance, while considering the rapidity of judgment, a dual time scale verification mechanism and dynamic weight distribution method are needed to achieve the purpose.

[0084] Time-varying weight dynamic distribution model:

[0085] (5);

[0086] (6);

[0087] (7);

[0088] (8);

[0089] wherein, is the weight function of the membership function of the switch position of the grid-connected point, which is set as a constant function a according to its characteristics; is the weight function of the membership function of the in-line voltage-free flag, which can be changed according to different field requirements, and is set as a linear function b(t) for easy understanding; is the weight function of the membership function of the in-line frequency abnormality flag, which can be changed according to different field requirements, and is set as a linear function c(t) for easy understanding; is the weight function of the membership function of the grid-connected point reactive power change rate flag, which can be changed according to different field requirements, and is set as a linear function (t) for easy understanding.

[0090] The change range of the weight is determined according to the influence degree of each electrical quantity on the off-grid judgment of the microgrid. For example, the switch position of the grid-connected point plays a decisive role in the microgrid on-off grid judgment condition, and the function OFF(t) representing the influence degree of the switch split position on the off-grid judgment of the microgrid is:

[0091] (9);

[0092] is the membership function of each variable, is the weight function.

[0093] The influence degree represented by OFF(t) is always greater than or equal to the influence degree of triggering off-grid, and once it is judged that the grid-connected point is tripped, the microgrid is immediately judged to be off-grid.

[0094] Voltage-free judgment , frequency abnormality , rate of reactive power change The influence degree of the off-grid determination of the micro-grid decreases in turn, and the expression functions are as follows:

[0095] (10);

[0096] (11);

[0097] (12);

[0098] Finally, the influence degree of the multi-dimensional parameters of the micro-grid incoming line side and the grid-connected point on the off-grid determination of the micro-grid is used to realize the off-grid decision of the micro-grid. Island(t) is a decision variable of the multi-dimensional parameters affecting the off-grid of the micro-grid, and the expression function is:

[0099] (13)

[0100] When Island(t) is greater than or equal to the off-grid critical value, the micro-grid executes the off-grid strategy, S is a switch tripping signal of the grid-connected point, is a no-voltage symbol of the incoming line side, is a frequency abnormality symbol, is a rate of reactive power change. Taking the linearity of b(t), c(t) and d(t) as an example, and considering that the influence degree of the no-voltage determination, the frequency abnormality and the rate of reactive power change on the off-grid determination of the micro-grid decreases in turn.

[0101] The decision logic diagram is shown in Figures 1-3 , wherein Figure 1 is a function diagram of the multi-dimensional parameters of the micro-grid on the off-grid decision variable, Figure 2 is a decision variable superposition diagram of the multi-dimensional parameters when the grid-connected point is disconnected, Figure 3 is a decision variable superposition diagram of the multi-dimensional parameters when the upper power loss occurs and other off-grid situations occur.

[0102] This determination scheme not only avoids the influence of disturbance on the grid-connected and off-grid determination of the micro-grid, but also fully considers the rapidity, greatly improves the judgment accuracy of the current state of the control system on the micro-grid and the correctness of the switching of the control command.

[0103] S2, stable control of the island state of the micro-grid;

[0104] According to the result of the abnormal identification of S1, the off-grid state of the micro-grid is determined, the multi-source coordinated control and the load hierarchical optimization management are used for island stable operation regulation, and the real-time power balance and voltage frequency stability of the island state are ensured.

[0105] When S1 detects that the micro-grid enters an unplanned island operation state (including protection device action or control system actively cutting off the grid point switch), since there is power exchange at the grid point in the off-grid moment, the system will generate a dynamic power difference (ΔP). This power difference will cause the following technical challenges:

[0106] (1) Transient power imbalance causes voltage / frequency fluctuation;

[0107] (2) Overcharge / overdischarge risk of the main power storage system in VF (voltage-frequency) control mode;

[0108] (3) Possibility of secondary device protection misoperation (over / under voltage protection action).

[0109] In order to solve the above problems, dynamic stable operation adjustment of the micro-grid needs to be carried out at the coordination control layer.

[0110] When the power shortage occurs in the off-grid moment (ΔP>0), the maximum charging power of the main power storage system is obtained according to the real-time communication between the control system and the battery management system (BMS), and is compared with ΔP to determine whether the storage system can be adjusted in place.

[0111] (14);

[0112] In the formula,

[0113] If the storage system has the power to make up for ΔP, the power shortage is actively made up by the storage system converter itself controller; if the storage system is insufficient to make up for ΔP, the control system controls the load shedding according to the level, and the third-level load is preferentially cut off, followed by the second-level load, and the first-level load is not allowed to be cut off.

[0114] When the new energy power generation power surplus occurs in the off-grid moment (ΔP<0).

[0115] (15)

[0116] If the storage system has the power to make up for ΔP, the power shortage is actively made up by the storage system converter itself controller; if the storage system is full of power or the charging power is insufficient to make up for ΔP, the control system controls the new energy power generation to reduce the power generation power to achieve power balance.

[0117] According to different working conditions, the island stable operation adjustment is carried out in the mode of multi-source coordinated control and load hierarchical optimization management. In the above two working conditions, the steady-state control effect is: the micro-grid internal power balance. The voltage can be kept at 0.9Un~1.1Un. The frequency meets 49.5~f~50.5. The control flow chart is as shown in Figure 4 .

[0118] S3, power quality analysis and dynamic impedance detection of backup feeder;

[0119] Wideband power quality analysis and dynamic impedance detection of backup feeder to evaluate its grid compatibility and transient response characteristics;

[0120] Part I: Multidimensional analysis of wideband power quality.

[0121] A high sampling rate power quality analyzer is used to collect backup feeder voltage signals in the 0.1Hz-20kHz frequency band, and time-frequency joint analysis technology is used to identify power quality.

[0122] Steady-state index detection: voltage signals are collected by the on-site power quality analyzer, and the fundamental component and 2-50 harmonic parameters are extracted based on FFT to monitor the steady-state indicators such as voltage amplitude deviation (≤±5%), frequency fluctuation (≤±0.5Hz), total harmonic distortion (THD≤5%) and etc.

[0123] Transient disturbance identification: the background system in the microgrid station uses wavelet packet transform to decompose the voltage transient (10ms-1s), voltage interruption (>1s) and high-frequency disturbance (1kHz-20kHz) based on the voltage situation in the fixed time window, and accurately captures the non-stationary signal characteristics.

[0124] Comprehensive evaluation model: the microgrid coordination control device constructs an evaluation logic containing voltage amplitude deviation, frequency fluctuation, total harmonic distortion and other parameters to determine whether the backup feeder meets the microgrid access conditions.

[0125] Part II: Dynamic impedance detection and transient response evaluation.

[0126] Impedance identification technology: inject 0.1-100Hz sweep signal under no-load condition of backup feeder, collect voltage / current response data, fit equivalent circuit model, calculate equivalent resistance Rs, reactance Xs and frequency-dependent impedance curve Z(f).

[0127] Transient simulation verification: based on the detected impedance, a microgrid-backup feeder joint model is constructed to simulate the switching transient process, focusing on analyzing: impact current peak (≤1.5 times rated current) or (≤protection current setting); voltage recovery time (≤50ms to reach more than 90% of rated value); frequency oscillation amplitude (≤±0.2Hz). Through comparison of measured and simulated data, the phase synchronization ability and power sudden change adaptability of backup feeder are evaluated.

[0128] If the power quality of the first part reaches the preset threshold, i.e. the power quality is excellent, and the dynamic impedance evaluated by the second part matches the microgrid load, the incoming line meets the requirements of accessing the microgrid as a backup incoming line. This scheme realizes 200ms compatibility detection of backup incoming line through full-band signal acquisition and multi-model fusion analysis, with key parameter identification accuracy of 0.1%, providing accurate electrical reference for adaptive synchronous closing, ensuring transient stability of double incoming line switching from the source, significantly improving switching reliability of microgrid under complex working conditions, and laying a data foundation for lightweight design of energy storage system. Power quality includes steady-state indicators and transient disturbance indicators.

[0129] S4, adaptive synchronous closing technology is used to realize the input of the backup incoming line.

[0130] Under islanding steady state, the microgrid uses adaptive synchronous closing technology to adjust the microgrid side voltage and phase to meet the quasi-synchronous condition, realizing incoming line switching.

[0131] Under the premise that S3 confirms that the microgrid is in a stable state in islanding mode, the control system controls the main power source-energy storage system to perform synchronous grid connection operation. Compared with the traditional power system, the synchronous task depends on the voltage index change caused by the output difference of the power plant connected by the two incoming lines, and the synchronous index judgment and instruction triggering. In the islanding operation mode of the microgrid, the energy storage as the main power source in the VF operation mode can adjust the bus voltage amplitude and frequency of the microgrid according to the instruction, realizing a stable and fast pre-synchronization synchronous grid connection process through autonomous control.

[0132] Like traditional power systems, microgrid synchronous grid connection also requires that the microgrid side voltage and incoming line voltage meet the voltage difference, frequency difference and phase difference to meet the synchronous requirements. On this basis, the microgrid controller issues instructions to the grid-type energy storage power supply controller (PCS) to make it adjust the main power source of the energy storage to meet the requirements of synchronous grid connection.

[0133] Control method:

[0134] During the synchronous grid connection process using the PCS in the energy storage system, the voltage / frequency of the microgrid side is adjusted.

[0135] Microgrid internal voltage control: the microgrid coordination controller issues target power to the PCS to control it to adjust the microgrid bus voltage to be equal to the voltage of the backup incoming line.

[0136] Microgrid internal frequency control: Different from the microgrid internal voltage control, the phase difference needs to be adjusted under the premise that the frequency meets the frequency difference requirement. Therefore, the frequency target value issued by the microgrid coordination controller needs to meet the same period closing frequency difference requirement with the incoming line frequency, and there is an acceptable frequency difference value, so as to control the phase difference between the voltage of the incoming line side and the voltage of the microgrid side to adjust in the direction of becoming smaller. Wherein C is a coefficient ranging from 0 to 1, so the strategy is as follows:

[0137] (16)

[0138] In the formula, is the rated frequency, is the frequency difference value of the same period, is the current frequency difference. Under this strategy, the target frequency and the voltage frequency of the incoming line side have a difference to realize phase adjustment, and the adjustment direction of the target frequency value relative to the voltage frequency of the incoming line side is strictly controlled, so as to avoid the frequency adjustment value frequently crossing the voltage frequency of the incoming line side and causing overshoot and vibration. At the same time, the adjustment coefficient C is used to control the frequency change amplitude of each control period. The algorithm of this variable step length adjustment frequency improves the stability of the same period grid connection and greatly improves the success rate of the microgrid same period grid connection.

[0139] Finally, according to the adjustment effect, the voltage difference, frequency difference and phase difference of the two sides are judged, and the closing is completed under the condition of meeting the same period grid connection requirement, so as to realize the off-grid to grid connection step of the microgrid.

[0140] At this point, the microgrid completes the intelligent switching of the microgrid from the original incoming line to the standby incoming line in the case of power failure of the original incoming line. Figure 5 The flowchart of the overall scheme.

[0141] Embodiment 2

[0142] This embodiment combines embodiment 1 to disclose an intelligent switching system for realizing the microgrid double incoming line intelligent switching method based on dynamic threshold and multi-source cooperation, which comprises a synchronous measurement module, a coordination and decision module and an execution module.

[0143] The synchronous measurement module is used for synchronously collecting electrical quantities of the incoming line side and the microgrid side.

[0144] The coordination and decision module is used for analyzing and judging according to the collected electrical quantities, and outputting control instructions.

[0145] The execution module is used for controlling the execution unit to execute the intelligent switching of the double incoming line according to the control instructions of the coordination and decision module.

[0146] The coordination and decision module comprises an off-grid judgment module, an island control module, a standby power supply evaluation module and a same period closing module,

[0147] The off-grid judgment module is configured to judge the off-grid state of the micro-grid by using a time-varying dynamic weight distribution model.

[0148] The island control module is configured to control the island power balance.

[0149] The backup power supply evaluation module is configured to evaluate the grid-connected compatibility of the backup power supply.

[0150] The synchronous closing module is configured to use a variable step frequency control adjustment mechanism based on dynamic adjustment of frequency difference, so as to ensure that the frequency adjustment direction is consistent with the phase difference reduction direction.

[0151] The execution module includes an energy storage PCS, a load switch, and a backup power supply switch.

[0152] Embodiment 3

[0153] In combination with Embodiment 1, this embodiment discloses a power supply guarantee type micro-grid double-inlet intelligent switching method based on dynamic threshold and multi-source cooperation. The off-grid judgment strategy in S1 is verified by using a MATLAB Simulink simulation model. The off-grid judgment is performed under 3000 different working conditions, including 1000 times of power loss experiments of the upper level of the branch to which the micro-grid is connected, 750 times of voltage disturbance experiments (time < 10 ms, amplitude 0.7-0.9 Un), 750 times of frequency disturbance experiments (Δf is 0.1-1 Hz, and the time is not more than 1 s), and 500 times of grid-connected point tripping experiments. During this process, the reactive power at the grid-connected point changes to different degrees. Under the same simulation environment and test conditions, the off-grid judgment strategy is compared with the traditional independent judgment method based on the grid-connected point position, no-voltage flag and other parameters. The results are as follows:

[0154]

[0155] Note: The disturbance recognition rate is the probability of recognizing the experiment as a disturbance and not performing off-grid judgment. The disturbances that are not recognized by the double-time scale and weight distribution method are a small number of disturbances with large disturbance amplitude and long duration. Since the number is small and does not belong to the off-grid category, it is not included in the total recognition time.

[0156] In summary: The double-time scale and weight distribution method of this embodiment can realize the rapid (≤200 ms) and accurate (accuracy rate 98%) recognition of the off-grid state of the micro-grid. Compared with the traditional off-grid discrimination method, the judgment time of the upper power loss is reduced from seconds to 200 ms, the recognition rate of the transient disturbance is 96%, and the overall misjudgment rate is less than 2%. This is conducive to accurately and quickly recognizing the off-grid state, performing mode switching and power control, and improving the success rate of seamless switching from grid-connected to off-grid.

[0157] Embodiment 4

[0158] This embodiment is described in combination with Embodiment 1. This embodiment discloses a power supply type micro-grid double-inlet intelligent switching method based on dynamic threshold and multi-source cooperation. A MATLAB Simulink simulation model is used to test the control effect of the adaptive variable step size synchronization closing method during synchronization grid connection. The micro-grid internal voltage and standby inlet deviation ΔU is set to be less than 0.05Un, the frequency deviation Δf is less than 1 Hz, and the phase difference is randomly distributed.

[0159] Compared with the existing fixed frequency algorithm calculated according to the fixed value, the adaptive variable step size synchronization closing algorithm has a regulation time of less than 8s. The variable step size method has a regulation time of less than 1.5s. The reason is that the variable step size method has a large step size in the early stage due to the large frequency difference, so the frequency difference converges quickly. In the later stage, the step size is small, and the phase difference cannot be satisfied due to the difference between the two sides. The success rate of the variable step size closing is 95.7%, which is much higher than the 79.5% of the fixed frequency method. The reason is that the variable step size method has a small frequency difference between the two sides in the later stage, the intersection of the command and the closing command is small due to the delay of the command and the execution of the closing command, and the size of the impact current during closing is effectively controlled, thereby improving the success rate of synchronization closing.

[0160] .

Claims

1. A power supply type micro-grid double-inlet intelligent switching method based on dynamic threshold and multi-source cooperation, characterized in that, It comprises the following steps: S1, abnormal identification of power quality at the incoming line side; A decision algorithm based on the combination of multi-dimensional parameter collaborative dynamic monitoring and fuzzy logic judgment is used to realize the abnormal identification of power quality at the original incoming line branch of the microgrid, distinguish transient disturbance from sustained fault, and realize the judgment of off-grid state; S2, stable control of the microgrid island state; According to the results of abnormal identification, the off-grid state of the microgrid is determined, and the island stable operation adjustment is carried out in the mode of multi-source coordinated control and load hierarchical optimization management to ensure real-time balance of island state power and voltage frequency stability; S3, power quality analysis and dynamic impedance detection of the standby incoming line; The standby incoming line is subjected to wide frequency domain power quality analysis and dynamic impedance detection to evaluate its grid-connected compatibility and transient response characteristics; S4, adaptive adjustment of the microgrid standby incoming line; Under the island stable state, the microgrid adaptively adjusts the microgrid side voltage and phase to meet the quasi-synchronization condition, and realizes incoming line switching; In S1, the abnormal identification of power quality at the incoming line side specifically comprises the following steps: S11, first, the synchronous measurement mode of the incoming line side and the microgrid side is constructed, and the electrical quantities are synchronously collected, S12, the characteristic vector affecting the off-grid criterion of the microgrid is calculated according to the collected electrical quantities; S13, a double time scale verification mechanism and dynamic weight distribution are cited, and the weight change range is determined according to the influence degree of each electrical quantity on the off-grid judgment of the microgrid; Time-varying weight dynamic distribution model: （5）； （6）； （7）； （8）； wherein, is a weight function of the grid point switch position membership function; is a weight function of the incoming line no-voltage flag membership function; is a weight function of the incoming line frequency abnormality flag membership function; is a weight function of the grid point reactive power change rate flag membership function; The weight change range is determined according to the influence degree of each electrical quantity on the off-grid judgment of the microgrid, and the function OFF(t) representing the influence degree of the switch split on the off-grid judgment of the microgrid is expressed as: （9）； is a membership function of each variable, is a weight function; No pressure judgment , frequency anomaly , reactive power change rate The influence degree of micro-grid off-grid judgment decreases in turn, and the expression function is in turn: （10）； （11）； （12）； The influence degree of the multi-dimensional parameters of the microgrid incoming line side and the grid-connected point on the off-grid judgment of the microgrid is used to realize the off-grid decision of the microgrid; Island(t) is the decision variable of the multi-dimensional parameters affecting the off-grid of the microgrid, and the expression function is: （13） When Island(t) is greater than or equal to the off-grid critical value, the micro-grid executes the off-grid strategy, S is the switch tripping signal of the grid-connected point, is the in-line side no pressure symbol, is the frequency anomaly symbol, is the reactive power change rate; taking b(t), c(t), d(t) as an example, and considering that the influence degree of no pressure judgment, frequency anomaly, and reactive power change rate on the off-grid judgment of the micro-grid decreases in turn; The influence degree of the multi-dimensional parameters of the microgrid incoming line side and the grid-connected point on the off-grid judgment of the microgrid is used to realize the off-grid decision of the microgrid. 2.The power supply type micro-grid dual-inlet intelligent switching method based on dynamic threshold and multi-source cooperation according to claim 1, characterized in that, In S2, the determination of the off-grid state of the microgrid according to the results of abnormal identification specifically comprises the following steps: When the power shortage occurs at the off-grid moment, i.e. dynamic power difference ΔP>0, the maximum discharge power of the current main power storage system is obtained according to the real-time communication of the control system and the battery management system, and the maximum discharge power is compared with the dynamic power difference ΔP to judge whether the adjustment can be relied on the storage system; When the new energy power surplus occurs at the off-grid moment, i.e. dynamic power difference ΔP<0, the maximum charging power of the current main power storage system is obtained according to the real-time communication of the control system and the battery management system, and the maximum charging power is compared with the dynamic power difference ΔP to judge whether the adjustment can be relied on the storage system. 3.The power supply type micro-grid dual-inlet intelligent switching method based on dynamic threshold and multi-source cooperation of claim 2, characterized in that, When the dynamic power difference ΔP>0, if the storage system has the ability to increase the power to make up for the dynamic power difference ΔP, the power shortage is actively made up by the storage system converter itself controller; if the storage system is insufficient to make up for the dynamic power difference ΔP, the control system controls the load according to the level.

4. The method according to claim 2, wherein, When the dynamic power difference ΔP is less than 0, if the energy storage system can absorb power to make up for the dynamic power difference ΔP, the power shortage is actively made up by the energy storage system converter itself controller; if the energy storage system is full of electricity or the charging power is insufficient to make up for the dynamic power difference ΔP, the control system controls the new energy power generation to reduce the power generation, so as to realize the power balance.

5. The method according to claim 1, wherein, The wide frequency domain power quality analysis is based on the voltage signal collected from the standby incoming line in the full frequency band; the identification of power quality is realized through time-frequency joint analysis technology; The dynamic impedance detection is based on the collection of voltage / current response data, fitting of equivalent circuit model, construction of microgrid-backup incoming line joint model based on detection impedance, and simulation of closing transient process; If the power quality reaches the preset threshold, and the dynamic impedance matches the microgrid load according to the evaluation, the incoming line meets the requirements of the microgrid as a backup incoming line.

6. The method according to claim 1, wherein, The microgrid adaptive adjusts the voltage and phase of the microgrid side to meet the quasi-synchronization condition, which is realized by the voltage / frequency regulation of the microgrid side in the synchronization and grid connection process of the PCS in the energy storage system; Microgrid internal voltage control: the microgrid coordination controller issues a target power to the PCS to control the regulation of the microgrid bus voltage to be equal to the voltage of the backup incoming line; Microgrid internal frequency control: under the premise that the frequency meets the frequency difference requirement, the phase difference needs to be adjusted.

7. An intelligent switching system for implementing a power supply type micro-grid dual-inlet intelligent switching method based on dynamic threshold and multi-source cooperation according to any one of claims 1-6, characterized in that, The system includes a synchronous measurement module, a coordination and decision module, and an execution module. The synchronous measurement module is used for synchronous acquisition of electrical quantities on the incoming line side and the microgrid side; The coordination and decision module is used for analysis and judgment based on the collected electrical quantities, and outputs control instructions; The execution module is used for controlling the execution unit to execute intelligent switching of double incoming lines according to the control instructions of the coordination and decision module. 8.The intelligent switching system of a dual-inlet intelligent switching method of a supply guarantee type micro-grid based on dynamic threshold and multi-source cooperation according to claim 7, wherein, The coordination and decision module includes an off-grid judgment module, an island control module, a backup power supply evaluation module, and a synchronization closing module, The off-grid judgment module is used for off-grid judgment of the collected electrical quantities by using a time-varying dynamic weight distribution model; The island control module is used for island power balance control; The backup power supply evaluation module is used for evaluation of the grid connection compatibility of the backup power supply; The synchronization closing module is used for variable step frequency control adjustment mechanism based on frequency difference dynamic adjustment, to ensure that the frequency adjustment direction is consistent with the phase difference reduction direction. 9.The intelligent switching system of a dual-inlet intelligent switching method of a supply-ensuring micro-grid based on dynamic threshold and multi-source collaboration according to claim 7, wherein, The execution module includes an energy storage PCS, a load switch, and a backup power supply switch.

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