Micro-grid adaptive load shedding method and micro-grid control system

Abstract

The application discloses a kind of micro-grid adaptive load unloading methods.The method comprehensively considers power imbalance overload and low-frequency risk, when micro-grid operating state is grid-connected operation mode, real-time preliminary assessment calculation of power shortage is carried out, whether there is overload operation risk when main power grid fails and micro-grid is converted to island operation is evaluated, and preliminary load unloading control when main power grid fails is carried out accordingly;Then, according to the frequency change rate of micro-grid, the time margin of frequency drop to frequency threshold is estimated in real time, and the low-frequency margin risk level is judged, and further load unloading control is carried out accordingly.The application also discloses a kind of micro-grid control system.The application can realize the frequency support function of micro-grid system under island operation condition, avoid micro-grid protection shutdown caused by power imbalance and frequency drop, ensure the continuous power supply of important load of micro-grid system, and try to meet the power supply demand of three-level load, improve the stability and power supply utilization rate of micro-grid.

Description

TECHNICAL FIELD

[0001] The present application relates to a micro-grid adaptive load unloading method, belonging to the technical field of micro-grid control. BACKGROUND

[0002] The micro-grid is composed of distributed power generation equipment, power load, monitoring / protection and automation devices, and energy storage equipment if necessary, and is a small power network that can achieve internal power balance. It can operate in parallel with the external power grid or independently in island mode. Figure 1 A typical structure of a micro-grid is shown.

[0003] According to the energy industry standard NBT10149-2019 Micro-grid Part 2: Micro-grid operation guide 4.2.3.2 on frequency response characteristics requirements: grid-connected micro-grid operating in island mode should have load following capability. In island mode, DER (distributed power) and load management means should be used to meet the load demand, and the capacity of DER should be large enough to ensure the normal operation of important loads.

[0004] The micro-grid operating in island mode should meet the following requirements:

[0005] Power balance between DER output power and load power; frequency measurement and regulation capability; load following, load management, load shedding; ability to maintain transient stability of the system when load changes dramatically, DER exits or other internal faults.

[0006] Energy industry standard NBT 10645-2021 Photovoltaic and Energy Storage Interactive Control Operation Technology Guide 6.5 Emergency Support Requirements: When the power grid has equipment overload and insufficient backup, etc., the interactive object should provide emergency support through automatic control. The interactive objects that can participate in grid emergency support include interruptible flexible load and electrochemical energy storage system with discharge margin.

[0007] In the application of micro-grid system, the power supply reliability needs to be ensured. When the main power grid fails, the micro-grid changes from grid-connected mode to island operation mode. If the output power of the micro-source is insufficient to support the current load demand during the conversion, the power imbalance and overload of the micro-grid system may occur after the micro-grid is converted to island operation, and the frequency may decrease. If the controllable load is not unloaded in time, it is easy to cause overcurrent, overload and low-frequency protection of the micro-grid, triggering the protection trip of the micro-grid power supply, thereby causing the important load in the micro-grid system to be powered off, affecting production and other losses. On the other hand, if the capacity of the cut-off load is too large, it will also affect the power supply of the three-level load and reduce the power supply utilization rate of the micro-grid system.

[0008] The load unloading strategy of the prior art micro-grid control technology usually separately considers the power imbalance overload or the load unloading strategy of low frequency reduction, and it is difficult to fully and effectively ensure the indicators specified in the standard. SUMMARY

[0009] The technical problem to be solved by the present application is to overcome the deficiencies of the prior art, provide a micro-grid adaptive load unloading method considering power imbalance overload and low frequency risk, realize the frequency support function of the micro-grid system in the case of converting to island operation, avoid the micro-grid protection shutdown caused by power imbalance and frequency reduction, ensure the continuous power supply of important loads of the micro-grid system, and try to meet the power supply demand of the three-level load, improve the stability and power utilization rate of the micro-grid.

[0010] The present application specifically adopts the following technical solutions to solve the above technical problems:

[0011] A micro-grid adaptive load unloading method, comprising the following steps:

[0012] S1. In the grid-connected operation state, continuously estimate the power shortage when the micro-grid enters the island operation mode after the main grid fails;

[0013] S2. When the main grid fails and the micro-grid enters the island operation mode, first, according to the current estimated power shortage, perform preliminary load unloading, unload the current low-priority load, so that the power shortage after preliminary load unloading is less than or equal to 0;

[0014] S3. According to the real-time detected micro-grid bus voltage frequency f w , calculate the frequency change rate df w of the micro-grid, and when the micro-grid voltage frequency f w continuously decreases, estimate the time margin t lim when the micro-grid voltage frequency decreases to the frequency threshold f act ; According to the estimated time margin t act_min and the single load unloading action frequency response time t act of the micro-grid, evaluate the low frequency margin risk level: t act_min <= k1*t act_min is a high risk of low frequency; k2*t act > t act_min is a medium risk of low frequency; t act >= k2*t act_min or df w >= 0 is a low risk of low frequency; k1, k2 are preset risk evaluation coefficients, and k1 >= 1, k2 > k1;

[0015] S4, further load shedding according to the assessed low frequency margin risk level: if the low frequency margin risk level is low frequency high risk, then shed all interruptible load that can be shed; if the low frequency margin risk level is low frequency medium risk, then shed the current low priority load in batches until the low frequency margin risk level is adjusted to low frequency low risk; if the low frequency margin risk level is low frequency low risk, then continuously observe and do not shed load.

[0016] Based on the same inventive concept, the following technical solutions can also be obtained:

[0017] A micro-grid control system, comprising a micro-grid adaptive load shedding subsystem, which comprises: a micro-grid power deficiency estimation unit, configured to continuously estimate the power deficiency when the micro-grid enters an island operation mode after the main grid fails in a grid-connected operation state;

[0018] a micro-grid low frequency margin risk assessment unit, configured to, when the main grid fails and the micro-grid enters an island operation mode, assess the low frequency margin risk according to the real-time detected micro-grid bus voltage frequency f w , the frequency change rate df w of the micro-grid, and when the micro-grid voltage frequency f w continuously decreases, estimate the time margin t lim for the micro-grid voltage frequency to decrease to a frequency threshold f ; and according to the estimated time margin t act and the single load shedding action frequency response time t act_min of the micro-grid, assess the low frequency margin risk level: t act <= k1*t act_min is low frequency high risk; k2*t act_min >t act >= k1*t act_min is low frequency medium risk; t act >= k2*t act_min or df w >= 0 is low frequency low risk; k1 and k2 are preset risk assessment coefficients, and k1 >= 1 and k2 > k1.

[0019] The load unloading unit is configured to, when the micro-grid enters the island operation mode due to the fault of the main grid, firstly perform preliminary load unloading according to the current estimated power shortage, unload the current low-priority load, and make the power shortage after the preliminary load unloading less than or equal to 0; and further unload the load according to the low-frequency margin risk level evaluated by the low-frequency margin risk evaluation unit: if the low-frequency margin risk level is high-frequency risk, unload all the interruptible load; if the low-frequency margin risk level is medium-frequency risk, unload the current low-priority load in batches until the low-frequency margin risk level is adjusted to low-frequency risk; and if the low-frequency margin risk level is low-frequency risk, continuously observe and do not unload the load.

[0020] Preferably, the power shortage of the micro-grid when the micro-grid enters the island operation mode after the fault of the main grid wherein, P grid is the grid-side power of the micro-grid grid-connected point estimated according to the historical information of the micro-grid grid-connected operation power, P derG (i) is the power of the i-th micro-source in the micro-grid which only supports grid-connected power generation, n is the total number of the micro-sources in the micro-grid which only support grid-connected power generation, P derE (j) is the rated power of the j-th micro-source in the micro-grid which is only put into operation when off-grid, and m is the total number of the micro-sources in the micro-grid which is only put into operation when off-grid.

[0021] In one embodiment, when the low-frequency margin risk level is medium-frequency risk, the current low-priority load is unloaded in batches according to the following method:

[0022] A preset number of loads are selected from the current lowest-priority load for unloading, and it is judged whether the low-frequency margin risk level after the unloading is low-frequency risk; if yes, the load unloading is stopped; otherwise, the above process is repeated until the low-frequency margin risk level is adjusted to low-frequency risk.

[0023] In another embodiment, when the low-frequency margin risk level is medium-frequency risk, the current low-priority load is unloaded in batches according to the following method:

[0024] A preset number of loads are selected from the current lowest-priority load for first unloading, and it is judged whether the low-frequency margin risk level after the first unloading is low-frequency risk; if yes, the load unloading is stopped; otherwise, the power shortage ΔP sys_2 required for second load unloading is estimated, and then corresponding loads are selected from the current lowest-priority load for second unloading so that the total power of the loads unloaded this time is greater than or equal to the estimated power shortage ΔP sys_2; whether the low-frequency margin risk level after the second load shedding is low-frequency low risk, if yes, stop load shedding; otherwise, estimate the power shortage ΔP required for the third load shedding sys_3 Then select the corresponding load from the current lowest priority load for the third load shedding so that the total power of the load in this load shedding is greater than or equal to the estimated power shortage ΔP sys_3 ; and so on until the low-frequency margin risk level is adjusted to low-frequency low risk; the power shortage ΔP required for the kth load shedding df k , df k-1 respectively represent the frequency change rate of the microgrid before the kth, k-1th load shedding, P sys_k-1 represents the total power of the load in the k-1th load shedding, k = 2, 3, K.

[0025] In yet another embodiment, when the low-frequency margin risk level is low-frequency medium risk, the current low-priority load is batched and unloaded according to the following method:

[0026] Select a batch of loads for unloading in order of priority from low to high, so that the total power of the batch of loads is greater than or equal to the power shortage ΔP sys_all = 2H sys df, wherein H sys is the equivalent inertia coefficient of the power source of the microgrid, and df is the frequency change rate of the microgrid after the last load shedding.

[0027] Further preferably, the equivalent inertia coefficient H sys is obtained online using the following method: when the low-frequency margin risk level is low-frequency medium risk, the current low-priority load is batched and unloaded according to the following method:

[0028] Select a preset number of loads from the current lowest priority load for the first load shedding, and determine whether the low-frequency margin risk level after the first load shedding is low-frequency low risk, if yes, stop load shedding; otherwise, estimate the power shortage ΔP required for the second load shedding sys_2 Then select the corresponding load from the current lowest priority load for the second load shedding so that the total power of the load in this load shedding is greater than or equal to the estimated power shortage ΔP sys_2 ; determine whether the low-frequency margin risk level after the second load shedding is low-frequency low risk, if yes, stop load shedding; otherwise, estimate the power shortage ΔP required for the third load shedding sys_3 Then select the corresponding load from the current lowest priority load for the third load shedding so that the total power of the load in this load shedding is greater than or equal to the estimated power shortage ΔP sys_3; and so on until the low frequency margin risk level is adjusted to low frequency low risk; power shortage required for kth load unloading df k , df k-1 respectively represent the frequency change rate of the micro-grid before kth and (k-1)th load unloading, P sys_k-1 represents the total power of the k-1th load unloading, k=2, 3, K; at the same time, the equivalent inertia coefficient of the micro-grid at the second load unloading is calculated synchronously and the power shortage ΔP sys_all required for subsequent multiple load unloading calculated by using the equivalent inertia coefficient is calculated sys_k Error check is performed on the corresponding ΔP sys_k If the error check results of preset times are all within the preset error range, the current equivalent inertia coefficient is taken as the final equivalent inertia coefficient; otherwise, the calculation and error check of the equivalent inertia coefficient are performed again.

[0029] Preferably, the grid-connected switch, micro-power supply switch and interruptible load switch of the micro-grid perform networking communication; and the micro-grid adaptive load unloading subsystem is arranged in the intelligent controller of the grid-connected switch.

[0030] Compared with the prior art, the technical scheme of the present application has the following beneficial effects:

[0031] The micro-grid adaptive load unloading method of the present application comprehensively considers power imbalance overload and low frequency risk, performs preliminary evaluation and calculation of power shortage in real time when the micro-grid is in grid-connected operation mode, estimates whether there is overload operation risk when the main grid fails and the micro-grid is in island operation mode, and performs preliminary load unloading control according to the estimation; then, the time margin of frequency drop to the frequency threshold is estimated in real time according to the frequency change rate of the micro-grid, and the low frequency margin risk level is judged, and further load unloading control is performed according to the judgment; the present application can provide emergency frequency support for the operation of the micro-grid, so as to ensure the continuous power supply of important loads of the micro-grid system and meet the power supply demand of interruptible loads as much as possible, reduce the micro-grid protection trip or load overcut caused by insufficient margin of micro-power supply output power when the micro-grid system is in island operation mode, and meet the power safety of the micro-grid system and fully utilize the power demand of the micro-grid; on the other hand, the design of the micro-grid system can be simplified and the cost can be reduced; since the hierarchical unloading of interruptible loads is considered, the power supply demand of high-priority three-level loads can be met as much as possible, and the power distribution utilization rate of the micro-grid can be improved. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 is a schematic diagram of a typical structure of a micro-grid;

[0033] Figure 2A networking communication architecture schematic diagram of a micro-grid control system in specific embodiments;

[0034] Figure 3 A structural block diagram of a micro-grid adaptive load shedding subsystem;

[0035] Figure 4 A structural diagram of a micro-grid example adopting the technical scheme of the present application;

[0036] Figure 5 , Figure 6 The current and voltage unit quantity change situation and bus frequency change situation under the condition of no micro-grid load shedding control are shown in Figures 1 and 2 respectively;

[0037] Figure 7 , Figure 8 The current and voltage unit quantity change situation and bus frequency change situation under the condition of micro-grid load shedding threshold control are shown in Figures 3 and 4 respectively;

[0038] Figure 9 , Figure 10 The current and voltage unit quantity change situation and bus frequency change situation under the condition of the micro-grid adaptive load shedding method of the present application are shown in Figures 5 and 6 respectively. DETAILED DESCRIPTION

[0039] In view of the deficiencies of the prior art, the solution idea of the present application is to comprehensively consider power imbalance overload and low-frequency risk, to perform real-time preliminary assessment calculation of power deficiency when the micro-grid is in grid-connected operation mode, to assess whether there is an overload operation risk when the main grid fails and the micro-grid is in island operation mode, and to perform preliminary load shedding control of the main grid failure; then to estimate the time margin of the frequency threshold when the micro-grid frequency changes in real time, to judge the low-frequency margin risk level, and to perform further load shedding control of the load accordingly.

[0040] The present application specifically adopts the following technical scheme to solve the above technical problems:

[0041] A micro-grid adaptive load shedding method, comprising the following steps:

[0042] S1. In the grid-connected operation state, continuously estimate the power deficiency when the micro-grid enters the island operation mode after the main grid fails;

[0043] S2. When the main grid fails and the micro-grid enters the island operation mode, first perform preliminary load shedding according to the current estimated power deficiency, and shed the current low-priority load, so that the power deficiency after preliminary load shedding is less than or equal to 0;

[0044] S3. According to the real-time detected micro-grid bus voltage frequency f wCalculate the frequency change rate df of the microgrid w , and in the microgrid voltage frequency f w When the voltage and frequency of the microgrid continue to decrease, it is estimated that the voltage and frequency of the microgrid decrease to the frequency threshold f lim Time margin According to the estimated time margin t act And the single load unloading action frequency response time t of the microgrid act_min Evaluate the low frequency margin risk level: t act <=k1*t act_min Low frequency and high risk; k2*t act_min >t act >=k1*t act_min Low frequency and medium risk; t act >=k2*t act_min or df w >=0 means low frequency and low risk; k1 and k2 are preset risk assessment coefficients, and k1>=1, k2>k1;

[0045] S4. Further load unloading is carried out according to the assessed low-frequency margin risk level: if the low-frequency margin risk level is low-frequency high risk, all unloadable and interruptible loads are unloaded; if the low-frequency margin risk level is low-frequency medium risk, the current low-priority loads are unloaded in batches until the low-frequency margin risk level is adjusted to low-frequency low risk; if the low-frequency margin risk level is low-frequency low risk, continuous observation is carried out without load unloading.

[0046] A microgrid control system includes a microgrid adaptive load unloading subsystem, the subsystem including: a microgrid power deficit estimation unit for continuously estimating the power deficit of the microgrid when it enters an island operation mode after a main grid failure in a grid-connected operation state;

[0047] The microgrid low frequency margin risk assessment unit is used to detect the microgrid bus voltage frequency f in real time when the main grid fails and the microgrid enters the island operation mode. w Calculate the frequency change rate df of the microgrid w , and in the microgrid voltage frequency f w When the voltage and frequency of the microgrid continue to decrease, it is estimated that the voltage and frequency of the microgrid decrease to the frequency threshold f lim Time margin According to the estimated time margin t act And the single load unloading action frequency response time t of the microgrid act_min Evaluate the low frequency margin risk level: t act <=k1*t act_min Low frequency and high risk; k2*t act_min >t act >=k1*tact_min Low frequency, medium risk; t act >= k2*t act_min or df w >= 0 Low frequency, low risk; k1, k2 are preset risk assessment coefficients, and k1 >= 1, k2 > k1;

[0048] The load unloading unit is configured to, when the micro-grid enters the island operation mode due to the fault of the main grid, firstly perform preliminary load unloading according to the current estimated power shortage, unload the current low-priority load, so that the power shortage after the preliminary load unloading is less than or equal to 0, and further unload the load according to the low-frequency margin risk level evaluated by the low-frequency margin risk evaluation unit: if the low-frequency margin risk level is low frequency high risk, all interruptible loads are unloaded; if the low-frequency margin risk level is low frequency medium risk, the current low-priority load is unloaded in batches until the low-frequency margin risk level is adjusted to low frequency low risk; and if the low-frequency margin risk level is low frequency low risk, the load is not unloaded.

[0049] In order to facilitate the public to understand, the technical solutions of the present application are described in detail below through a specific embodiment and in combination with the drawings:

[0050] In order to fully utilize the advantages of fast response and short time delay of the edge computing of the power distribution user end, the grid-connected switch, the micro-power supply switch and the interruptible load switch (all the switches are intelligent circuit breakers) of the micro-grid in the embodiment are networked and communicated in the manner shown in Figure 2 The grid-connected switch and the micro-power supply switch have embedded intelligent control modules and contain switch state acquisition subsystems. The grid-connected switch contains a grid-connected interface end power acquisition subsystem, and the micro-power supply switch contains a bus frequency acquisition subsystem. The intelligent control modules are embedded in the circuit breakers and are in communication connection with the circuit breaker bodies through an internal communication bus. The state information (switch state, power, frequency, etc.) of the circuit breaker bodies can be obtained through the internal bus. The intelligent control modules form a communication network. The intelligent control module embedded in the grid-connected switch is provided with the micro-grid adaptive unloading subsystem of the present application. The micro-grid adaptive load unloading subsystem obtains the switch state, power, frequency, etc. of other switches through Ethernet and sends control instructions to the load switch. The intelligent control module in the grid-connected switch includes a 485 interface which is connected with the 485 interface in the load switch. Through the interface, the intelligent control module obtains the state information of the load switch and sends the on-off command information to the load switch.

[0051] The micro-grid adaptive unloading subsystem is shown in Figure 3As shown, including data acquisition unit, micro-grid power shortage estimation unit, micro-grid low frequency margin risk assessment unit, load shedding unit; data acquisition unit obtains the power, voltage, frequency of micro-grid grid-connected point, switch state of grid-connected switch, bus voltage frequency, switch state of each micro-source, each load switch state information, and stores the historical information of micro-grid power; micro-grid power shortage estimation unit is used for continuously estimating the power shortage of the micro-grid entering island operation mode when the main grid fails in grid-connected operation state; micro-grid low frequency margin risk assessment unit is used for calculating the frequency change rate of the micro-grid according to the real-time detected micro-grid bus voltage frequency when the main grid fails and the micro-grid enters island operation mode, and estimating the time margin of the micro-grid voltage frequency falling to the frequency threshold set by the user when the micro-grid voltage frequency continuously falls; the low frequency margin risk level is evaluated according to the estimated time margin and the single load shedding action frequency response time of the micro-grid; the load shedding unit is used for preliminary and further load shedding control according to the power shortage estimated by the micro-grid power shortage estimation unit and the low frequency margin risk level evaluated by the micro-grid low frequency margin risk assessment unit when the main grid fails and the micro-grid enters island operation mode.

[0052] The load shedding control process of the micro-grid adaptive load shedding subsystem is as follows:

[0053] (1) The intelligent control module of the grid-connected circuit breaker switch continuously acquires the grid-side power data sequence of the micro-grid grid-connected circuit breaker CB0 through the data acquisition unit, and stores the historical data and acquires the switch state information.

[0054] (2) In the grid-connected operation state, the grid power shortage estimation unit continuously estimates the power shortage of the micro-grid entering island operation mode after the main grid fails; the power shortage estimation unit predicts the grid-side power P grid of CB0 according to the power historical data. grid The prediction of the grid-side power P grid may use various existing methods such as time series analysis or regression data model prediction. Taking the commonly used moving arithmetic average method as an example, the n-second sliding window historical power data P grid (n), P grid (n-1), …, P grid (1), P derG (0) are detected and stored in real time, and the predicted value of the grid-side power is obtained as follows: The estimated power P derG(i) can be obtained according to micro power communication data, or can be obtained according to predicted power of micro grid management data center, or can be obtained according to power prediction method of micro power history data; the power generation rated power P of the m micro powers which are only put into operation when off-grid operation derE (j) is a known parameter; then the power shortage preliminarily estimated when the main power grid fails is If P ce ≤0, it is indicated that in the island operation mode, the power generation power exceeds the required power of the load, and there is no power shortage.

[0055] (3) When the main power grid fails and the micro grid enters the island operation mode, the load unloading unit first performs preliminary load unloading according to the power shortage preliminarily estimated by the grid power shortage estimation unit, and unloads the current low-priority load, so that the power shortage after preliminary load unloading is less than or equal to 0.

[0056] (4) After the micro grid enters the island operation mode, the low-frequency margin risk assessment unit calculates the frequency change rate df w of the micro grid according to the real-time detected micro grid bus voltage frequency f w , and when the micro grid voltage frequency f w continuously decreases, estimates the time margin t lim when the micro grid voltage frequency decreases to the frequency threshold f act preset by the user.

[0057] According to the estimated time margin t act and the single load unloading action frequency response time t act_min of the micro grid, the low-frequency margin risk level is assessed: t act <=k1*t act_min is high-frequency high-risk; k2*t act_min >t act >=k1*t act_min is low-frequency medium-risk;

[0058] t act >=k2*t act_min or df w >=0 is low-frequency low-risk; k1 and k2 are preset risk assessment coefficients, and k1 >=1, k2>k1, and preferably k2 >=10k1, wherein the micro grid bus voltage frequency refers to the voltage frequency of the grid connection point; wherein the frequency change rate df w can be calculated in various ways, for example, can be directly calculated according to the formula: Where f k is the frequency at the kth moment, f k-1 is the frequency at the (k-1)th moment, and T sThe sampling period is the frequency detection sampling period; or, in order to improve the anti-interference of the frequency, a frequency change rate filtering process or an equivalent incomplete differential replacement form can be used for calculation, and the incomplete differential implementation form is as follows: (N is generally an integer from 1 to 5); the filtering process can be an average value process

[0059] (5) The load unloading unit further unloads the load according to the low-frequency margin risk level evaluated by the low-frequency margin risk evaluation unit: if the low-frequency margin risk level is a high low-frequency risk, all interruptible loads that can be unloaded are unloaded; if the low-frequency margin risk level is a medium low-frequency risk, the current low-priority load is unloaded in batches until the low-frequency margin risk level is adjusted to a low low-frequency risk; if the low-frequency margin risk level is a low low-frequency risk, the load is continuously observed and no load unloading is performed.

[0060] The batch unloading strategy in the medium low-frequency risk state can adopt the following preferred schemes:

[0061] The first scheme is a batch unloading strategy based on the low-frequency margin risk, and the specific process is as follows:

[0062] A preset number of loads are selected from the current lowest priority load for unloading, and it is judged whether the low-frequency margin risk level after unloading is a low low-frequency risk; if yes, the load unloading is stopped; otherwise, the above process is repeated until the low-frequency margin risk level is adjusted to a low low-frequency risk. The preset number can be one or more.

[0063] The second scheme is an unloading strategy based on the exploratory power shortage estimation, which has the advantage of fast response speed compared with the first scheme, and the specific process is as follows:

[0064] A preset number of loads are selected from the current lowest priority load for the first unloading, and it is judged whether the low-frequency margin risk level after the first unloading is a low low-frequency risk; if yes, the load unloading is stopped; otherwise, the power shortage ΔP sys_2 required for the second load unloading is estimated, and then the corresponding load is selected from the current lowest priority load for the second unloading so that the total power of the load unloading is greater than or equal to the estimated power shortage ΔP sys_2 ; it is judged whether the low-frequency margin risk level after the second unloading is a low low-frequency risk; if yes, the load unloading is stopped; otherwise, the power shortage ΔP sys_3 required for the third load unloading is estimated, and then the corresponding load is selected from the current lowest priority load for the third unloading so that the total power of the load unloading is greater than or equal to the estimated power shortage ΔP sys_3 ; and so on, until the low-frequency margin risk level is adjusted to a low low-frequency risk; the power shortage ΔP dfk , df k-1 respectively represent the frequency variation rate of the micro-grid before the k, k-1th load unloading, P sys_k-1 represents the total power of the k-1th load unloading, k=2, 3, K.

[0065] The third: the unloading strategy based on the equivalent inertia coefficient estimation power shortage, compared with the first strategy also has the advantage of fast response speed, as follows:

[0066] According to the order of priority from low to high, a batch of loads are selected for unloading, so that the total power of the batch of loads is greater than or equal to the power shortage ΔP sys_all =2H sys df, wherein H sys is the equivalent inertia coefficient of the power supply of the micro-grid, and df is the frequency variation rate of the micro-grid after the last load unloading.

[0067] The equivalent inertia coefficient H sys of the micro-grid in the third unloading strategy can be obtained by calculation or experiment in advance according to the specific structure of the micro-grid, or can be obtained online by identification in combination with the unloading strategy based on the detection power shortage estimation using the following method:

[0068] When the low-frequency margin risk level is low-frequency medium risk, the current low-priority load is unloaded in batches according to the following method:

[0069] A preset number of loads are selected from the current lowest priority load for the first unloading, and it is judged whether the low-frequency margin risk level after the first unloading is low-frequency low risk. If yes, the load unloading is stopped; otherwise, the power shortage ΔP sys_2 required for the second load unloading is estimated, and then the corresponding load is selected from the current lowest priority load for the second unloading so that the total power of the load unloading is greater than or equal to the estimated power shortage ΔP sys_2 ; it is judged whether the low-frequency margin risk level after the second unloading is low-frequency low risk. If yes, the load unloading is stopped; otherwise, the power shortage ΔP sys_3 required for the third load unloading is estimated, and then the corresponding load is selected from the current lowest priority load for the third unloading so that the total power of the load unloading is greater than or equal to the estimated power shortage ΔP sys_3 ; and so on, until the low-frequency margin risk level is adjusted to low-frequency low risk; the power shortage ΔP df k , df k-1 respectively represent the frequency variation rate of the micro-grid before the k, k-1th load unloading, P sys_k-1 represents the total power of the k-1th load unloading, k=2, 3, K.

[0070] At the same time, the equivalent inertia coefficient of the micro-grid at the second load unloading is calculated synchronously And the power shortage ΔP required for subsequent multiple load unloadings calculated using the equivalent inertia coefficient sys_all corresponding ΔP sys_k Error checking is performed, and if error checking results of a preset number of times are all within a preset error range, the current equivalent inertia coefficient is taken as the final equivalent inertia coefficient; otherwise, the calculation of the equivalent inertia coefficient and the error checking are performed again.

[0071] (6) When the main grid is restored, the micro-grid is reconnected to the main grid for operation, and according to the setting allowed by the user, load unloading is cancelled or put into operation according to the priority of the load and the interval time.

[0072] In order to verify the technical effects of the technical scheme of the present application, based on the micro-grid shown in Figure 4 , the control effects of the following three cases are compared: (1) no micro-grid load unloading control (2) micro-grid load unloading method using a traditional frequency threshold (3) micro-grid adaptive load unloading method of the present application. The frequency threshold set by the user of the micro-grid system in the three cases is 48 Hz.

[0073] In the case of no micro-grid load unloading control, the changes of current and voltage per unit and bus frequency are respectively shown in Figure 5 , Figure 6 It can be seen that if there is no load unloading frequency support function, the system frequency will quickly decrease when the load is overloaded, and the time of decrease is very short. If the response of frequency unloading is not timely or the communication delay of the instruction is long, the response time of frequency decrease cannot be met, which easily leads to exceeding the frequency threshold 48 Hz, and thus triggering low-frequency protection shutdown.

[0074] In the case of using the micro-grid load unloading method of the traditional frequency threshold, the changes of current and voltage per unit and bus frequency are respectively shown in Figure 7 , Figure 8It can be seen that the frequency is lowered when the load is overloaded, and the load shedding is performed when the system detects that the frequency reaches the frequency threshold. However, due to the load shedding execution delay and the system frequency response delay, if the power shortage is large and the frequency drops rapidly, the frequency after load shedding exceeds the frequency threshold 48Hz by a large margin, and in this example, the lowest frequency drops to 45.65Hz, which may trigger low-frequency protection shutdown. If the system does not trigger the frequency threshold protection point, the user needs to adjust the frequency threshold setting range to leave sufficient low-frequency margin. However, the low-frequency change is related to the load excess, which is a dynamic change, so the frequency threshold is set as a fixed value, which is not easy to adapt to the change of the load and select the appropriate value.

[0075] In the case of using the adaptive load shedding method of the micro-grid of the present application, the changes of the current and voltage standard units and the bus frequency are shown in FIGS. 4 and 5, respectively. Figure 9 、 Figure 10 It can be seen that the present application estimates the low-frequency margin risk according to the frequency threshold set by the user and the frequency change rate, and performs corresponding load shedding control. The response delay time of low-frequency load shedding is considered, so the frequency of the micro-grid system after load shedding does not exceed the frequency threshold 48Hz, and the lowest frequency drops to 49.2Hz. The frequency is in an upward trend, and the frequency does not reach the frequency threshold 48Hz, so the low-frequency protection shutdown is not triggered. Since the adaptive frequency shedding scheme of the present application considers the frequency change rate and the influence of the load response delay, it is related to the dynamic change of the load excess, and can dynamically adjust the corresponding load shedding strategy. The user only needs to set the frequency threshold allowed by the micro-grid, and does not need to repeatedly adjust the set frequency threshold parameter, which simplifies the parameter debugging, increases the adaptability of the system, and enhances the operation stability of the micro-grid system.

Claims

1. A microgrid adaptive load unloading method, characterized in that: The following steps are involved: S1. In a grid-connected operation state, continuously estimating the power deficit of the microgrid when it enters an island operation mode after a main grid failure; S2. When a main grid failure occurs and the microgrid enters the island operation mode, it first performs preliminary load unloading based on the current estimated power deficit, unloading the current low-priority loads so that the power deficit after the preliminary load unloading is less than or equal to 0; S3, according to the real-time detection of the microgrid bus voltage frequency f w Calculate the frequency change rate df of the microgrid w , and in the microgrid voltage frequency f w When the voltage and frequency of the microgrid continue to decrease, it is estimated that the voltage and frequency of the microgrid decrease to the frequency threshold f lim Time margin According to the estimated time margin t act And the single load unloading action frequency response time t of the microgrid act_min Assess the low frequency margin risk level: t act <=k1*t act_min Low frequency and high risk; k2*t act_min >t act >=k1*t act_min Low frequency and medium risk; t act >=k2*t act_min or df w >=0 means low frequency and low risk; k1 and k2 are preset risk assessment coefficients, and k1>=1, k2>k1; S4. Further load shedding is performed based on the assessed low-frequency margin risk level: if the low-frequency margin risk level is low-frequency high risk, all interruptible loads that can be shelved are shelved; If the low-frequency margin risk level is low-frequency medium risk, the current low-priority loads will be unloaded in batches until the low-frequency margin risk level is adjusted to low-frequency low risk; If the low-frequency margin risk level is low frequency and low risk, continuous observation will be performed without load unloading.

2. The microgrid adaptive load unloading method according to claim 1, characterized in that: Power shortage when the microgrid enters island operation mode after the main grid fails Among them, P grid is the grid-side power of the microgrid grid-connected point predicted based on the historical power information of the microgrid grid-connected operation, P derG (i) is the power of the i-th micro power source in the microgrid that only supports grid-connected power generation, n is the total number of micro power sources in the microgrid that only support grid-connected power generation, P derE (j) is the rated power of the jth micro power source in the microgrid that is put into operation only when the microgrid is in off-grid operation, and m is the total number of micro power sources in the microgrid that are put into operation only when the microgrid is in off-grid operation.

3. The microgrid adaptive load unloading method according to claim 1, characterized in that: When the low-frequency margin risk level is low-frequency medium risk, the current low-priority loads are unloaded in batches according to the following method: Select a preset number of loads from the current lowest priority loads for unloading, and determine whether the low-frequency margin risk level after unloading is low frequency and low risk. If so, stop load unloading; Otherwise, repeat the above process until the low frequency margin risk level is adjusted to low frequency low risk.

4. The microgrid adaptive load unloading method according to claim 1, characterized in that: When the low-frequency margin risk level is low-frequency medium risk, the current low-priority loads are unloaded in batches according to the following method: Select a preset number of loads from the current lowest priority loads for the first unloading, and determine whether the low-frequency margin risk level after the first unloading is low-frequency low-risk. If so, stop load unloading; Otherwise, estimate the power deficit ΔP required for the second load shedding sys_2 Then, the corresponding load is selected from the current lowest priority loads for the second unloading so that the total power of the unloaded loads is greater than or equal to the estimated power shortage ΔP sys_2 ; Determine whether the low-frequency margin risk level after the second unloading is low-frequency low-risk. If so, stop load unloading; Otherwise, estimate the power deficit ΔP required for the third load shedding sys_3 Then, the corresponding load is selected from the current lowest priority loads for the third unloading so that the total power of the unloaded loads is greater than or equal to the estimated power shortage ΔP sys_3 ; And so on, until the low frequency margin risk level is adjusted to low frequency low risk; The power shortage required for the kth load unloading df k 、df k-1 They represent the frequency change rate of the microgrid before the kth and k-1th load unloading, P sys_k-1 Represents the total load power of the k-1th load unloading, k = 2, 3, K.

5. The microgrid adaptive load unloading method according to claim 1, characterized in that: When the low-frequency margin risk level is low-frequency medium risk, the current low-priority loads are unloaded in batches according to the following method: Select a batch of loads to be unloaded in order of priority from low to high, so that the total power of the loads is greater than or equal to the power deficit ΔP sys_all =2H sys df, where H sys is the power equivalent inertia coefficient of the microgrid, and df is the frequency change rate of the microgrid after the last load unloading.

6. The microgrid adaptive load unloading method according to claim 5, characterized in that: The equivalent inertia coefficient H of the microgrid sys The following method is used to identify it online: When the low-frequency margin risk level is low-frequency medium risk, the current low-priority loads are unloaded in batches according to the following method: Select a preset number of loads from the current lowest priority loads for the first unloading, and determine whether the low-frequency margin risk level after the first unloading is low-frequency low-risk. If so, stop load unloading; Otherwise, estimate the power deficit ΔP required for the second load shedding sys_2 Then, the corresponding load is selected from the current lowest priority loads for the second unloading so that the total power of the unloaded loads is greater than or equal to the estimated power shortage ΔP sys_2 ; Determine whether the low-frequency margin risk level after the second unloading is low-frequency low-risk. If so, stop load unloading; Otherwise, estimate the power deficit ΔP required for the third load shedding sys_3 Then, the corresponding load is selected from the current lowest priority loads for the third unloading so that the total power of the unloaded loads is greater than or equal to the estimated power shortage ΔP sys_3 ; And so on, until the low frequency margin risk level is adjusted to low frequency low risk; The power shortage required for the kth load unloading df k 、df k-1 They represent the frequency change rate of the microgrid before the kth and k-1th load unloading, P sys_k-1 represents the total load power of the k-1th load unloading, k = 2, 3, K; At the same time, during the second load unloading, the equivalent inertia coefficient of the microgrid during the second load unloading is calculated synchronously. The power shortage ΔP required for subsequent multiple load unloading is calculated using this equivalent inertia coefficient. sys_all With the corresponding ΔP sys_k Perform error checking. If the error checking results of the preset number of times are all within the preset error range, the current equivalent inertia coefficient is used as the final equivalent inertia coefficient; otherwise, the equivalent inertia coefficient is recalculated and error checking is performed again.

7. A microgrid control system, characterized in that: It includes a microgrid adaptive load unloading subsystem, which includes: A microgrid power deficit estimation unit, configured to continuously estimate the power deficit of the microgrid when it enters an island operation mode after a main grid failure in a grid-connected operation state; The microgrid low frequency margin risk assessment unit is used to detect the microgrid bus voltage frequency f in real time when the main grid fails and the microgrid enters the island operation mode. w Calculate the frequency change rate df of the microgrid w , and in the microgrid voltage frequency f w When the voltage and frequency of the microgrid continue to decrease, it is estimated that the voltage and frequency of the microgrid decrease to the frequency threshold f lim Time margin According to the estimated time margin t act And the single load unloading action frequency response time t of the microgrid act_min Evaluate the low frequency margin risk level: t act <=k1*t act_min Low frequency and high risk; k2*t act_min >t act >=k1*t act_min Low frequency and medium risk; t act >=k2*t act_min or df w >=0 means low frequency and low risk; k1 and k2 are preset risk assessment coefficients, and k1>=1, k2>k1; The load unloading unit is used to, when a fault occurs in the main power grid and the microgrid enters the island operation mode, first perform preliminary load unloading based on the current estimated power shortage, unload the current low-priority load, so that the power shortage after the preliminary load unloading is less than or equal to 0; and further perform load unloading based on the low-frequency margin risk level assessed by the microgrid low-frequency margin risk assessment unit: if the low-frequency margin risk level is low-frequency high risk, all unloadable and interruptible loads are unloaded; if the low-frequency margin risk level is low-frequency medium risk, the current low-priority loads are unloaded in batches until the low-frequency margin risk level is adjusted to low-frequency low risk; if the low-frequency margin risk level is low-frequency low risk, continuous observation is carried out without load unloading.

8. The microgrid control system according to claim 7, characterized in that: Power shortage when the microgrid enters island operation mode after the main grid fails Among them, P grid is the grid-side power of the microgrid grid-connected point estimated based on the historical power information of the microgrid grid-connected operation, P derG (i) is the power of the i-th micro power source in the microgrid that only supports grid-connected power generation, n is the total number of micro power sources in the microgrid that only support grid-connected power generation, P derE (j) is the rated power of the jth micro power source in the microgrid that is put into operation only when the microgrid is in off-grid operation, and m is the total number of micro power sources in the microgrid that are put into operation only when the microgrid is in off-grid operation.

9. The microgrid control system according to claim 7, characterized in that: When the low-frequency margin risk level is low-frequency medium risk, the current low-priority loads are unloaded in batches according to the following method: Select a preset number of loads from the current lowest priority loads for unloading, and determine whether the low-frequency margin risk level after unloading is low frequency and low risk. If so, stop load unloading; Otherwise, repeat the above process until the low frequency margin risk level is adjusted to low frequency low risk.

10. The microgrid control system according to claim 7, characterized in that: When the low-frequency margin risk level is low-frequency medium risk, the current low-priority loads are unloaded in batches according to the following method: Select a preset number of loads from the current lowest priority loads for the first unloading, and determine whether the low-frequency margin risk level after the first unloading is low-frequency low-risk. If so, stop load unloading; Otherwise, estimate the power deficit ΔP required for the second load shedding sys_2 Then, the corresponding load is selected from the current lowest priority loads for the second unloading so that the total power of the unloaded loads is greater than or equal to the estimated power shortage ΔP sys_2 ; Determine whether the low-frequency margin risk level after the second unloading is low-frequency low-risk. If so, stop load unloading; Otherwise, estimate the power deficit ΔP required for the third load shedding sys_3 Then, the corresponding load is selected from the current lowest priority loads for the third unloading so that the total power of the unloaded loads is greater than or equal to the estimated power shortage ΔP sys_3 ; And so on, until the low frequency margin risk level is adjusted to low frequency low risk; The power shortage required for the kth load unloading df k 、df k-1 They represent the frequency change rate of the microgrid before the kth and k-1th load unloading, P sys_k-1 Represents the total load power of the k-1th load unloading, k = 2, 3, K.

11. The microgrid control system according to claim 7, wherein: When the low-frequency margin risk level is low-frequency medium risk, the current low-priority loads are unloaded in batches according to the following method: Select a batch of loads to be unloaded in order of priority from low to high, so that the total power of the loads is greater than or equal to the power deficit ΔP sys_all =2H sys df, where H sys is the power equivalent inertia coefficient of the microgrid, and df is the frequency change rate of the microgrid after the last load unloading.

12. The microgrid control system according to claim 11, characterized in that: The equivalent inertia coefficient H of the microgrid sys The following method is used to identify it online: When the low-frequency margin risk level is low-frequency medium risk, the current low-priority loads are unloaded in batches according to the following method: Select a preset number of loads from the current lowest priority loads for the first unloading, and determine whether the low-frequency margin risk level after the first unloading is low-frequency low-risk. If so, stop load unloading; Otherwise, estimate the power deficit ΔP required for the second load shedding sys_2 Then, the corresponding load is selected from the current lowest priority loads for the second unloading so that the total power of the unloaded loads is greater than or equal to the estimated power shortage ΔP sys_2 ; Determine whether the low-frequency margin risk level after the second unloading is low-frequency low-risk. If so, stop load unloading; Otherwise, estimate the power deficit ΔP required for the third load shedding sys_3 Then, the corresponding load is selected from the current lowest priority loads for the third unloading so that the total power of the unloaded loads is greater than or equal to the estimated power shortage ΔP sys_3 ; And so on, until the low frequency margin risk level is adjusted to low frequency low risk; The power shortage required for the kth load unloading df k 、df k-1 They represent the frequency change rate of the microgrid before the kth and k-1th load unloading, P sys_k-1 Represents the total load power of the k-1th load unloading, k = 2, 3, K; at the same time, during the second load unloading, the equivalent inertia coefficient of the microgrid during the second load unloading is calculated synchronously The power shortage ΔP required for subsequent multiple load unloading is calculated using this equivalent inertia coefficient. sys_all With the corresponding ΔP sys_k Perform error checking. If the error checking results of the preset number of times are all within the preset error range, the current equivalent inertia coefficient is used as the final equivalent inertia coefficient; otherwise, the equivalent inertia coefficient is recalculated and error checking is performed again.

13. The microgrid control system according to claim 7, wherein: The grid-connected switch, micro power switch and interruptible load switch of the microgrid are networked and communicated; the microgrid adaptive load unloading subsystem is arranged in the intelligent controller of the grid-connected switch.

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