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

Abstract

The invention discloses a self-adaptive load unloading method for a micro-grid. According to the method, power unbalance overload and low-frequency risks are comprehensively considered, when the operation state of the micro-grid is in a grid-connected operation mode, preliminary evaluation calculation of power shortage is carried out in real time, whether overload operation risks exist or not after a main power grid breaks down and the micro-grid is converted into island operation is evaluated, and preliminary unloading control of loads when the main power grid breaks down is carried out accordingly; and then, according to the frequency change rate of the micro-grid, estimating the time margin when the frequency is reduced to a frequency threshold in real time, judging the risk level of the low-frequency margin, and performing further unloading control on the load according to the risk level of the low-frequency margin. The invention further discloses a micro-grid control system. According to the invention, a frequency support function under the condition of isolated island operation of the micro-grid system can be realized, micro-grid protection shutdown caused by power imbalance and frequency drop is avoided, continuous power supply of important loads of the micro-grid system is ensured, the power supply demand of three-level loads is satisfied as much as possible, and the stability and the power supply utilization rate of the micro-grid are improved.

Description

Technical Field

[0001] The invention relates to a microgrid adaptive load unloading method, and belongs to the technical field of microgrid control. Background Art

[0002] A microgrid is composed of distributed power generation equipment, power loads, monitoring / protection and automation devices, and energy storage equipment when necessary. It is a small power network that can achieve a basic balance of internal power. It can be connected to an external power grid or run independently off the grid. Figure 1 A typical structure of a microgrid is shown.

[0003] According to the energy industry standard NBT10149-2019 Microgrid Part 2: Microgrid Operation Guidelines, 4.2.3.2 on frequency response characteristics: When the grid-connected microgrid operates in island mode, it should have load following capability. In island mode, load demand should be met through DER (distributed generation) and load management methods, and the capacity of DER should be large enough to ensure the normal operation of important loads.

[0004] When the microgrid operates in island mode, it should meet the following requirements:

[0005] Power balance between DER output power and load power; frequency measurement and regulation capabilities; load following, load management, and load shedding; the ability to maintain system transient stability when loads change drastically, DER exits, or other internal faults occur.

[0006] Energy industry standard NBT 10645-2021 photovoltaic storage load interactive control operation technical guidelines 6.5 emergency support requirements, when the grid encounters equipment overload and insufficient backup, the interactive object should provide emergency support through automatic control. Interactive objects that can participate in grid emergency support include interruptible flexible loads and electrochemical energy storage systems with discharge margin.

[0007] In the application of microgrid system, it is necessary to ensure the reliability of power supply. In the operation of microgrid, when the main grid fails, the microgrid is converted from the grid-connected mode to the island operation mode. If the output power of the micro power source is not enough to support the current load demand during the conversion, then after the conversion to the microgrid island operation, it may cause the power imbalance overload of the microgrid system and the frequency reduction. If the controllable load is not unloaded in time, it is easy to cause the overcurrent, overload and low-frequency protection of the microgrid, causing the microgrid power supply to trigger the protection trip, thereby causing the important loads in the microgrid system to lose power, affecting production and other losses. On the other hand, if the load capacity cut off is too large, it will also affect the power supply of the tertiary load and reduce the power supply utilization rate of the microgrid system.

[0008] Existing microgrid control technologies usually adopt a load unloading strategy that only considers power imbalance overload or a load unloading strategy that only considers low-frequency reduction, which is difficult to fully and effectively guarantee the various indicators specified in the standard. Summary of the invention

[0009] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a microgrid adaptive load unloading method that comprehensively considers power imbalance overload and low frequency risks, which can realize the frequency support function of the microgrid system when it is converted to island operation, avoid microgrid protection shutdown caused by power imbalance and frequency drop, ensure continuous power supply to important loads of the microgrid system, and try to meet the power supply needs of the third-level loads, thereby improving the stability and power supply utilization of the microgrid.

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

[0011] A microgrid adaptive load unloading method comprises the following steps:

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

[0013] S2. When the main grid fails and the microgrid enters the island operation mode, it first performs preliminary load unloading based on the currently estimated power shortage, unloading the current low-priority loads so that the power shortage after the preliminary load unloading is less than or equal to 0;

[0014] 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 at 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 <=k 1 *t act_min is low frequency and high risk; k 2 *t act_min >t act >=k 1 *t act_min Low frequency medium risk; t act >=k 2 *t act_min or df w >=0 means low frequency and low risk; k 1 , k2 is the preset risk assessment coefficient, and k 1 >=1,k 2 >k 1 ;

[0015] 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.

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

[0017] A microgrid control system includes a microgrid adaptive load unloading subsystem, the subsystem including: a microgrid power shortage estimation unit, used for continuously estimating the power shortage of the microgrid when it enters an island operation mode after a main grid failure in a grid-connected operation state;

[0018] 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 at 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 <=k 1 *t act_min is low frequency and high risk; k 2 *t act_min >t act >=k 1 *t act_min Low frequency medium risk; t act >=k 2 *t act_min or df w >=0 means low frequency and low risk; k 1 , k 2 is the preset risk assessment coefficient, and k 1 >=1,k 2 >k 1 ;

[0019] The load unloading unit is used for, when a fault occurs in the main power grid and the microgrid enters the island operation mode, firstly, performing preliminary load unloading according to the currently estimated power shortage, unloading the current low-priority load, so that the power shortage after the preliminary load unloading is less than or equal to 0; and further unloading the load according to the low-frequency margin risk level assessed by the low-frequency margin risk assessment unit of the microgrid: 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 performed without load unloading.

[0020] Preferably, the power shortage when the microgrid enters the 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 that is put into operation only when the microgrid is in off-grid operation, and m is the total number of micro power sources that are put into operation only when the microgrid is in off-grid operation.

[0021] In one embodiment, 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:

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

[0023] In another embodiment, 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:

[0024] 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 unloading the load; otherwise, estimate the power shortage ΔP required for the second load unloading sys_2 , and then select the corresponding load from the current lowest priority load for the second unloading so that the total power of the load unloaded this time 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 unloading is low-frequency low-risk. If so, stop load unloading; otherwise, estimate the power shortage ΔP required for the third load unloading sys_3 , and then select the corresponding load from the current lowest priority load for the third unloading so that the total power of the load unloaded this time 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.

[0025] In yet another embodiment, 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:

[0026] Select a batch of loads to unload 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, 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.

[0027] Further preferably, the equivalent inertia coefficient H of the microgrid is sys The following method is used for online identification: 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:

[0028] 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 unloading the load; otherwise, estimate the power shortage ΔP required for the second load unloading sys_2 , and then select the corresponding load from the current lowest priority load for the second unloading so that the total power of the load unloaded this time 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 unloading is low-frequency low-risk. If so, stop load unloading; otherwise, estimate the power shortage ΔP required for the third load unloading sys_3 , and then select the corresponding load from the current lowest priority load for the third unloading so that the total power of the load unloaded this time 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 equivalent inertia coefficient is used to calculate the power shortage ΔP required for subsequent multiple load unloading 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, recalculate the equivalent inertia coefficient and perform error checking.

[0029] Preferably, the grid-connected switch, micro power switch and interruptible load switch of the microgrid are networked for communication; and the microgrid adaptive load unloading subsystem is arranged in the intelligent controller of the grid-connected switch.

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

[0031] The microgrid adaptive load unloading method of the present invention comprehensively considers the risks of power imbalance overload and low frequency. When the microgrid is in the grid-connected operation mode, a preliminary evaluation and calculation of the power shortage is performed in real time to evaluate whether there will be an overload operation risk after the microgrid switches to island operation when the main grid fails, and the preliminary load unloading control is performed accordingly when the main grid fails; then, the time margin for the frequency to drop to the frequency threshold is estimated in real time according to the frequency change rate of the microgrid, and the risk level of the low frequency margin is judged, and further load unloading control is performed accordingly; the present invention can provide emergency frequency support for the operation of the microgrid to ensure that the power supply demand of the interruptible load is met as much as possible under the premise of continuous power supply to the important loads of the microgrid system, and reduce the microgrid protection tripping or load overcutting caused by insufficient margin of the micro power supply output power after the microgrid system is in the island operation mode, so as to meet the power safety of the microgrid system and make full use of the power demand of the microgrid; on the other hand, the microgrid system assembly design can also be simplified to reduce costs. Since the hierarchical unloading of the interruptible load is considered, the power supply demand of the three-level load with high priority can be met as much as possible, and the power distribution utilization rate of the microgrid is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a typical structural diagram of a microgrid;

[0033] Figure 2It is a schematic diagram of the networking communication architecture of a microgrid control system in a specific embodiment;

[0034] Figure 3 It is the structural block diagram of the microgrid adaptive load unloading subsystem;

[0035] Figure 4 This is a structural diagram of a microgrid example using the technical solution of the present invention;

[0036] Figure 5 , Figure 6 They are respectively the changes of current and voltage per unit quantity and bus frequency without microgrid load unloading control;

[0037] Figure 7 , Figure 8 They are respectively the changes of current and voltage per unit quantity and bus frequency when the microgrid load unloading threshold control method is adopted;

[0038] Fig. 9 , Fig.10 They are respectively the changes in the per-unit current and voltage and the changes in the bus frequency when the microgrid adaptive load unloading method of the present invention is adopted. DETAILED DESCRIPTION

[0039] In view of the deficiencies in the prior art, the solution of the present invention is to comprehensively consider the risks of power imbalance, overload and low frequency. When the microgrid is in the grid-connected operation mode, a preliminary evaluation and calculation of the power shortage is performed in real time to evaluate whether there will be an overload operation risk when the main grid fails and the microgrid switches to island operation. Based on this, preliminary load unloading control is performed when the main grid fails; then, the time margin for the frequency to drop to the frequency threshold is estimated in real time according to the frequency change rate of the microgrid, and the low-frequency margin risk level is judged, and further load unloading control is performed accordingly.

[0040] The present invention specifically adopts the following technical solutions to solve the above technical problems:

[0041] A microgrid adaptive load unloading method comprises the following steps:

[0042] S1. In the grid-connected operation state, continuously estimating the power shortage when the microgrid enters the island operation mode after the main grid fails;

[0043] S2. When the main grid fails and the microgrid enters the island operation mode, it first performs preliminary load unloading based on the currently estimated power shortage, unloading the current low-priority loads so that the power shortage after the preliminary load unloading is less than or equal to 0;

[0044] S3, according to the real-time detection of the microgrid bus voltage frequency f wCalculate the frequency change rate df of the microgrid w , and at 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 <=k 1 *t act_min is low frequency and high risk; k 2 *t act_min >t act >=k 1 *t act_min Low frequency medium risk; t act >=k 2 *t act_min or df w >=0 means low frequency and low risk; k 1 , k 2 is the preset risk assessment coefficient, and k 1 >=1,k 2 >k 1 ;

[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 shortage estimation unit, used for continuously estimating the power shortage 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 at 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 microgridact_min Evaluate the low frequency margin risk level: t act <=k 1 *t act_min is low frequency and high risk; k 2 *t act_min >t act >=k 1 *t act_min Low frequency medium risk; t act >=k 2 *t act_min or df w >=0 means low frequency and low risk; k 1 , k 2 is the preset risk assessment coefficient, and k 1 >=1,k 2 >k 1 ;

[0048] The load unloading unit is used for, when a fault occurs in the main power grid and the microgrid enters the island operation mode, firstly, performing preliminary load unloading according to the currently estimated power shortage, unloading the current low-priority load, so that the power shortage after the preliminary load unloading is less than or equal to 0; and further unloading the load according to the low-frequency margin risk level assessed by the low-frequency margin risk assessment unit of the microgrid: 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 performed without load unloading.

[0049] In order to facilitate public understanding, the technical solution of the present invention is described in detail below through a specific embodiment and in conjunction with the accompanying drawings:

[0050] In order to make full use of the advantages of fast response and short delay of edge computing at the power distribution user end, the grid-connected switch, micro power switch, and interruptible load switch of the microgrid in this embodiment (all of the above switches are intelligent circuit breakers) are arranged according to Figure 2The network communication is performed in the manner shown; the grid-connected switch and the micro power switch have an embedded intelligent control module and a switch state acquisition subsystem, the grid-connected switch includes a grid-connected interface power acquisition subsystem, the micro power switch includes a bus frequency acquisition subsystem, the intelligent control module is embedded in the circuit breaker and is connected to the circuit breaker body through an internal communication bus, and the state information of the switch body (switch state, power, frequency, etc.) can be obtained through the internal bus. The intelligent control modules form a communication network, and the intelligent control module embedded in the grid-connected switch is provided with the microgrid adaptive unloading subsystem of the present invention, and the microgrid adaptive load unloading subsystem obtains the switch state, power, frequency and other information of other switches through Ethernet, and issues control instructions to the load switch. The intelligent control module in the grid-connected switch includes a 485 interface, which is connected to the 485 interface in the load switch, through which the intelligent control module obtains the state information of the load switch and sends opening and closing command information to the load switch.

[0051] Microgrid adaptive unloading subsystem Figure 3 As shown, it includes a data acquisition unit, a microgrid power shortage estimation unit, a microgrid low-frequency margin risk assessment unit, and a load unloading unit; the data acquisition unit obtains the power, voltage, frequency, switching state of the grid-connected switch of the microgrid grid-connected point, the bus voltage frequency, switch state of each micro power source, and the state information of each load switch, and stores the historical information of the microgrid power; the microgrid power shortage estimation unit is used to continuously estimate the power shortage when the microgrid enters the island operation mode after the main grid fails in the grid-connected operation state; the microgrid low-frequency margin risk assessment unit is used to estimate the power shortage when the microgrid enters the island operation mode when the main grid fails. When the microgrid is in the form of a load unloading unit, the frequency change rate of the microgrid is calculated according to the real-time detected microgrid bus voltage frequency, and when the microgrid voltage frequency continues to decrease, the time margin for the microgrid voltage frequency to drop to the frequency threshold set by the user is estimated; the low-frequency margin risk level is evaluated according to the estimated time margin and the single load unloading action frequency response time of the microgrid; the load unloading unit is used to perform preliminary and further load unloading control on the load according to the power shortage estimated by the microgrid power shortage estimation unit and the low-frequency margin risk level evaluated by the microgrid low-frequency margin risk assessment unit when a main grid failure occurs and the microgrid enters the island operation mode.

[0052] The unloading control process of the microgrid adaptive unloading subsystem is as follows:

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

[0054] (2) In the grid-connected operation state, the grid power shortage estimation unit continuously estimates the power shortage of the microgrid when it enters the island operation mode after the main grid fails; the power shortage estimation unit predicts the grid-side power P according to the power history data of CB0. grid , grid-side power P grid The prediction can be done by using various existing methods such as time series analysis or regression data model prediction. Taking the more commonly used moving arithmetic average method as an example, the n-second sliding window historical power data P of the grid-connected circuit breaker is detected and stored in real time. grid (n), P grid (n-1), ..., P grid (1) P grid (0), the predicted value of grid-side power is: The estimated power P of n micro-power sources in the microgrid that only support grid-connected power generation derG (i) It can be obtained based on the communication data of the micro power source, or it can be obtained based on the predicted power of the microgrid management data center, or it can be obtained based on the power prediction method based on the historical data of the micro power source; the rated power P of the m micro power sources put into operation only when they are off-grid derE (j) is a known parameter; then the initial estimated power shortage when the main power grid fails is As P ce ≤0, it means that in the island operation mode, the generated power exceeds the power required by the load and there is no power shortage.

[0055] (3) When a main grid failure occurs and the microgrid enters the island operation mode, the load unloading unit first performs preliminary load unloading based on the power shortage currently estimated by the grid power shortage estimation unit, and unloads the current low-priority load, so that the power shortage after the preliminary load unloading is less than or equal to 0.

[0056] (4) After the microgrid enters the island operation mode, the low-frequency margin risk assessment unit detects the microgrid bus voltage frequency f in real time. w Calculate the frequency change rate df of the microgrid w , and at 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 value f preset by the user. lim Time margin

[0057] 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 <=k 1 *t act_min is low frequency and high risk; k 2 *tact_min >t act >=k 1 *t act_min It is low frequency and medium risk;

[0058] t act >=k 2 *t act_min or df w >=0 means low frequency and low risk; k 1 , k 2 is the preset risk assessment coefficient, and k 1 >=1,k 2 >k 1 , preferably k 2 >=10k 1 , where the microgrid bus voltage frequency refers to the voltage frequency of the grid connection point; where the frequency change rate df w The calculation can be performed in various existing ways, for example, directly according to the formula: where f k is the frequency at the kth moment, f k-1 is the frequency at the k-1th moment, T s is the frequency detection sampling period; or, in order to improve the anti-interference performance of the frequency, the frequency change rate filtering process or the equivalent incomplete differential replacement form can be used for calculation, and the incomplete differential implementation form is: (N is generally an integer from 1 to 5); filtering can be performed by averaging

[0059] (5) The load unloading unit further unloads loads according to the low-frequency margin risk level assessed by the 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 performed without load unloading.

[0060] The batch unloading strategy at low frequency and medium risk can adopt the following optimal solutions:

[0061] The first one is a batch unloading strategy based on low-frequency margin risk, as follows:

[0062] 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 unloading the load; otherwise, repeat the above process until the low frequency margin risk level is adjusted to low frequency and low risk. The preset number can be one or more.

[0063] The second strategy is based on the detection power shortage estimation. Compared with the first strategy, it has the advantage of fast response speed. The details are as follows:

[0064] 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 unloading the load; otherwise, estimate the power shortage ΔP required for the second load unloading sys_2 , and then select the corresponding load from the current lowest priority load for the second unloading so that the total power of the load unloaded this time 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 unloading is low-frequency low-risk. If so, stop load unloading; otherwise, estimate the power shortage ΔP required for the third load unloading sys_3 , and then select the corresponding load from the current lowest priority load for the third unloading so that the total power of the load unloaded this time 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.

[0065] The third strategy is to use an unloading strategy based on the power shortage estimation based on the equivalent inertia coefficient. Compared with the first strategy, it also has the advantage of fast response speed. The details are as follows:

[0066] Select a batch of loads to unload 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, 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.

[0067] The equivalent inertia coefficient H of the microgrid described in the third unloading strategy is sys It can be obtained in advance through calculation or experiment according to the specific structure of the microgrid, or it can be identified online using the following method in combination with the unloading strategy based on the detective power shortage estimation:

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

[0069] 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 unloading the load; otherwise, estimate the power shortage ΔP required for the second load unloading sys_2 , and then select the corresponding load from the current lowest priority load for the second unloading so that the total power of the load unloaded this time 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 unloading is low-frequency low-risk. If so, stop load unloading; otherwise, estimate the power shortage ΔP required for the third load unloading sys_3 , and then select the corresponding load from the current lowest priority load for the third unloading so that the total power of the load unloaded this time 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;

[0070] 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 equivalent inertia coefficient is used to calculate the power shortage ΔP required for subsequent multiple load unloading 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, recalculate the equivalent inertia coefficient and perform error checking.

[0071] (6) When the main grid is restored, the microgrid is reconnected to the main grid for operation, and load unloading is canceled or put into operation according to the settings allowed by the user and the load priority and interval time.

[0072] In order to verify the technical effect of the technical solution of the present invention, based on Figure 4 The microgrid shown compares the control effects of the following three situations: (1) no microgrid load unloading control (2) microgrid load unloading method using traditional frequency threshold (3) microgrid adaptive load unloading method using the present invention. The frequency threshold set by the user of the microgrid system in the three situations is 48Hz.

[0073] In the absence of microgrid load unloading control, the changes in current and voltage per unit and bus frequency are as follows: Figure 5 , Figure 6 It can be seen that if there is no load unloading frequency support function, the system frequency will drop rapidly when the load is overloaded, and the drop time is very short. If the frequency unloading response is not timely or the command communication delay is long, the frequency drop response time cannot be met, which may easily lead to exceeding the frequency threshold of 48Hz, and then triggering the low-frequency protection shutdown.

[0074] When the microgrid load unloading method using the traditional frequency threshold is used, the changes in the current and voltage per unit and the bus frequency are as follows: Figure 7 , Figure 8 As shown. It can be seen that if the frequency threshold is used for load unloading control, the frequency will drop when the load is overloaded. When the system detects that the frequency reaches the frequency threshold, the load is unloaded. However, due to the delay in the execution of the load unloading action and the delay in the system frequency response after unloading, if the power shortage is large and the frequency drops quickly, the frequency will exceed the frequency threshold of 48Hz by a lot after the load is unloaded. In this example, the lowest frequency point dropped to 45.65Hz, which may also trigger the low-frequency protection shutdown. If the system does not want to touch the frequency threshold protection point, the user needs to adjust the setting range of the frequency threshold by himself to leave enough low-frequency margin. However, the low-frequency change is related to the load excess amount, which is a dynamically changing amount. Therefore, the frequency threshold is set to a fixed value, which is not easy to adapt to the change of load and select the appropriate value.

[0075] When the microgrid adaptive load unloading method of the present invention is adopted, the changes of the current and voltage per unit and the bus frequency are as follows: Fig. 9 , Fig.10 As shown. It can be seen that the present invention estimates the low-frequency margin risk and performs corresponding unloading control according to the frequency threshold set by the user and the frequency change rate, and takes into account the response delay time of low-frequency load unloading. Therefore, after the load is unloaded, the frequency of the microgrid system does not exceed the frequency threshold of 48Hz, and the frequency drops to 49.2Hz at the lowest. The frequency is converted to an upward trend. The frequency does not reach the frequency threshold of 48Hz, and the low-frequency protection shutdown will not be triggered. Since the adaptive frequency unloading scheme of the present invention takes into account the influence of the frequency change rate and the load response delay, and is associated with the dynamic change amount of the load excess, the corresponding load unloading strategy can be dynamically and adaptively adjusted. The user only needs to set the frequency threshold allowed by the microgrid, and the user does not need to repeatedly adjust the set frequency threshold parameters, which simplifies parameter debugging, increases the adaptability of the system, and enhances the operation stability of the microgrid system.

Claims

1. A microgrid adaptive load unloading method, characterized in that: The following steps are involved: S1. In the grid-connected operation state, continuously estimating the power shortage when the microgrid enters the island operation mode after the main grid fails; S2. When the main grid fails and the microgrid enters the island operation mode, it first performs preliminary load unloading based on the currently estimated power shortage, unloading the current low-priority loads so that the power shortage 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 at 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 For low frequency and high risk; k2*t act_min >t act >=k1*t act_min Low frequency 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 unloading is performed 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 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 that is put into operation only when the microgrid is in off-grid operation, and m is the total number of micro power sources 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, unload the current low-priority loads 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 unloading the loads; 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, unload the current low-priority loads 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 and low-risk. If so, stop unloading the loads; Otherwise, estimate the power deficit ΔP required for the second load shedding sys_2 , and then select the corresponding load from the current lowest priority load for the second unloading so that the total power of the load unloaded this time 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 unloading sys_3 , and then select the corresponding load from the current lowest priority load for the third unloading so that the total power of the load unloaded this time 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, unload the current low-priority loads in batches according to the following method: Select a batch of loads to unload 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, 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, unload the current low-priority loads 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 and low-risk. If so, stop unloading the loads; Otherwise, estimate the power deficit ΔP required for the second load shedding sys_2 , and then select the corresponding load from the current lowest priority load for the second unloading so that the total power of the load unloaded this time 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 unloading sys_3 , and then select the corresponding load from the current lowest priority load for the third unloading so that the total power of the load unloaded this time 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 equivalent inertia coefficient is used to calculate the power shortage ΔP required for subsequent multiple load unloading 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, recalculate the equivalent inertia coefficient and perform error checking.

7. A microgrid control system, characterized in that: It includes a microgrid adaptive load unloading subsystem, which includes: A microgrid power shortage estimation unit, used for continuously estimating the power shortage 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 at 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 For low frequency and high risk; k2*t act_min >t act >=k1*t act_min Low frequency 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 for, when a fault occurs in the main power grid and the microgrid enters the island operation mode, firstly, performing preliminary load unloading according to the currently estimated power shortage, unloading the current low-priority load, so that the power shortage after the preliminary load unloading is less than or equal to 0; and further unloading the load according to the low-frequency margin risk level assessed by the low-frequency margin risk assessment unit of the microgrid: 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 performed 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 that is put into operation only when the microgrid is in off-grid operation, and m is the total number of micro power sources 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, unload the current low-priority loads 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 unloading the loads; 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, unload the current low-priority loads 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 and low-risk. If so, stop unloading the loads; Otherwise, estimate the power deficit ΔP required for the second load shedding sys_2 , and then select the corresponding load from the current lowest priority load for the second unloading so that the total power of the load unloaded this time 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 unloading sys_3 , and then select the corresponding load from the current lowest priority load for the third unloading so that the total power of the load unloaded this time 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, characterized in that: When the low-frequency margin risk level is low-frequency medium risk, unload the current low-priority loads in batches according to the following method: Select a batch of loads to unload 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, 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, unload the current low-priority loads 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 and low-risk. If so, stop unloading the loads; Otherwise, estimate the power deficit ΔP required for the second load shedding sys_2 , and then select the corresponding load from the current lowest priority load for the second unloading so that the total power of the load unloaded this time 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 unloading sys_3 , and then select the corresponding load from the current lowest priority load for the third unloading so that the total power of the load unloaded this time 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 equivalent inertia coefficient is used to calculate the power shortage ΔP required for subsequent multiple load unloading 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, recalculate the equivalent inertia coefficient and perform error checking.

13. The microgrid control system according to claim 7, characterized in that: 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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