A method for evaluating the resilience of farm distribution networks supported by solar-powered islands

Abstract

The present invention discloses a method for evaluating the elasticity of an active distribution network based on photovoltaic distributed energy island support. First, the dynamic recovery characteristics of the island system containing photovoltaic energy are considered, and a dynamic photovoltaic energy island support model is established according to the operation information of the distribution network. Then, based on the dynamic characteristics of the active distribution network after a fault, a method for quantifying the elasticity index of the active distribution network is established. Finally, a comprehensive scenario island support capability analysis is performed on the active distribution network, the comprehensive elasticity expectation of the system is calculated, and the elasticity evaluation of the active distribution network is realized. The present invention can convert abstract elasticity indicators into specific comprehensive elasticity expectation values, can be embedded in various elasticity indicator systems, system evaluation systems and system optimization systems, has good universality, reliability and objectivity, and can be widely used in various engineering practices.

Description

Technical Field

[0001] The present invention belongs to the technical field of power system evaluation, and in particular relates to an active distribution network elasticity evaluation method based on photovoltaic distributed energy island support. Background Art

[0002] With the development of renewable energy technology and the improvement of environmental protection requirements, modern small-scale distribution networks, especially farm distribution networks, contain a large number of distributed energy sources, mainly photovoltaic and biogas. Since the installed capacity of renewable energy in such distribution networks is relatively large and the load demand is relatively small, distributed energy can maintain the operation of the system for a long time, making the system elastic. Establishing an elasticity analysis method that considers the operating characteristics of such distribution networks is of great significance for evaluating the system status, exploring the system potential, and optimizing the system operation.

[0003] At present, the distribution network resilience assessment method mainly conducts a comprehensive resilience assessment through the resources contained in the distribution network and the dynamic changes of system-related indicators during failures. Although this method can provide a subjective description of system resilience, due to the lack of system fault recovery methods, it only makes judgments based on static indicators and fault data, lacking the objective authenticity of system resilience assessment. In recent years, with the increase in the use of distributed energy and the development of information fusion technology, the research on the resilience assessment of active distribution networks has been further developed.

[0004] At present, the main problems of the active distribution network resilience assessment method are: the system resilience is difficult to quantify when the distribution network fails; the construction of the resilience index system is highly subjective, and there is a lack of a universal resilience assessment system; the resilience assessment and improvement measures are poorly compatible with the existing distribution network assessment and optimization methods. Therefore, studying the active distribution network fault recovery method and extracting relevant indicators to quantify the system resilience is not only of theoretical research value, but also of important practical significance for engineering practice. Summary of the invention

[0005] The purpose of the present invention is to solve the problems raised in the above-mentioned background technology, and to propose a farm distribution network elasticity assessment method for isolated island support of photovoltaic and biogas energy, so as to quantify the effect of distributed photovoltaic power sources and biogas generators on the elasticity of the distribution network in terms of isolated island support, and to analyze the influence of changes in different elasticity assessment indicators on the distribution network containing photovoltaic and biogas energy.

[0006] In order to achieve the purpose of the present invention, the present invention discloses an active distribution network elasticity assessment method based on solar-powered distributed energy island support, comprising the following steps:

[0007] Step 1: Obtain the operation information of the distribution network;

[0008] Step 2: According to the operation information of the distribution network obtained in step 1, a distribution network island fault recovery model containing solar energy is constructed. The island fault recovery model constructs a system stability criterion by inputting the operation state quantity and equipment state quantity of the island, and determines the dispatching strategy adopted in the island fault recovery stage based on the criterion, including the source load dispatching quantity, the switching status of the power quality management device, and the protection action status of the distribution line and the transformer;

[0009] Step 3: According to the island fault recovery model established in step 2, an island support model of the distribution network containing solar energy is constructed. The island support model constructs an island networking capability detection criterion according to the operation procedures and source-load operation characteristics of the distribution network, and selects islands with networking capability according to the operation state quantity, equipment state quantity and fault location before the distribution network fault; then the relevant information of these islands is input into the fault recovery model to determine the dynamic characteristics of the islands in the fault; finally, the stability process and stable state of the islands are calculated through the expected fault recovery time;

[0010] Step 4: According to the distribution network island support model containing solar energy established in step 3, a method for calculating elasticity coefficient is constructed. The elasticity coefficient calculation method calculates the stability process and the stability state of the distribution network respectively for the cases where solar energy is supported by the solar energy and is not supported by the solar energy according to the distribution network island support model under the given fault scenario of the distribution network, so as to calculate the elasticity coefficient;

[0011] Step 5. According to the elasticity coefficient calculation method established in step 4, a system comprehensive elasticity expectation calculation method is established. The system comprehensive elasticity expectation calculation method generates a system fault scenario set through the line fault conditions in the system, and calculates the elasticity coefficient of each fault scenario according to the elasticity coefficient calculation method, and then calculates the system comprehensive elasticity expectation to realize the elasticity evaluation of the distribution network.

[0012] Furthermore, in step 1, the operation information of the power grid includes the allowable fluctuation range of system frequency and voltage, the predicted load of each node, the importance of the load, the maximum load dispatching amount and the minimum load switching amount, the rated capacity of each distribution line and transformer, the failure probability and expected fault repair time of each line, the type, quantity and location of root-grid and grid-type distributed energy, the transient stability margin of grid-type distributed energy, and the maximum output power of distributed energy and energy storage.

[0013] Furthermore, in step 2, the island failure recovery model is as follows:

[0014] The Don Raphson method is used to analyze the elasticity of the farm distribution network supported by the solar energy island to determine whether the power flow converges. If the power flow equation does not converge, it is considered that the island is facing voltage collapse, and the network is subjected to load shedding. By relaxing the system frequency, node voltage and equipment capacity constraints and optimizing the power flow calculation, the initial state of the power flow can be obtained, and the dispatching plan of the distribution network is returned as the load shedding plan, the initial state is updated, and the power flow analysis of the island is performed again; if the optimized power flow still has no solution, it is considered that the island cannot be powered, all the equipment on the island is shut down, and the current island is marked as stable;

[0015] According to the obtained initial state of the solvable power flow, the power flow analysis is performed on the island, and the obtained system state quantities are compared with the system frequency, voltage, and load stability criteria respectively, the system state quantities that do not meet the constraints are determined, and a recovery plan for the corresponding indicators is constructed. The current island is marked as a fault-continuing state and the corresponding scheduling plan is returned. Fault recovery is performed again until all constraints are met. The current island is marked as a fault-ending state and the corresponding scheduling plan is returned as the final fault recovery plan for the island.

[0016] Furthermore, in step 2, the system stability criterion is as follows:

[0017] The operating states that need to be paid attention to in the stability of the distribution network include frequency, voltage and power. Based on the above three system state quantities, the distribution network state judgment criteria are established.

[0018]

[0019] Where P L,i and P G,i is the load and distributed generation active power of node i, ΔP L,i and ΔP G,i is the load and distributed generation active power adjustment of node i, V i is the voltage at node i, V i,max and V i,min is the voltage offset range of node i, S B,j and S Bmax,j is the load and rated capacity of line j, S T,m and S Tmax,m is the load and rated capacity of the transformer m.

[0020] Furthermore, in step 3, the specific calculation steps of the distribution network island support model containing solar energy are as follows:

[0021] Step 3-1, when a system failure occurs and causes a line, determine the part connected to the upper grid and the island part;

[0022] Step 3-2: Apply the distributed energy network building capability detection criteria to perform a network building capability test on each island determined in step 3-1, select the ones that pass the test and number them;

[0023] Step 3-3, the parts connected to the upper power grid and the islands detected by step 3-2 are respectively substituted into the island fault recovery model to determine whether each area meets the system stable operation requirements and the scheduling plan required for system stability recovery when the requirements are not met;

[0024] Step 3-4: Collect the return information of the parts connected to the upper power grid and the islands in step 3-3. If there are islands that do not meet the stable operation requirements, update the system fault scenario according to the dispatching schemes corresponding to these islands in step 3-3, and jump to step 3-1 to restart fault recovery; if all independent parts meet the stable operation requirements, it is considered that the distribution network fault recovery is successful, and jump to step 3-5;

[0025] Step 3-5: Calculate the power supply shortfall of the part connected to the upper grid and the isolated island.

[0026] Furthermore, in step 3-2, the distributed energy network construction capability detection criteria are as follows:

[0027] The criterion for network building capability detection is: if the island has network-building distributed energy and the adjustable power of the source and load in the system meets the network building requirements, then the island is considered to have independent network building operation capabilities; the network building requirements include power balance requirements and regulation margin requirements, and the power balance requirements are:

[0028]

[0029] Where S L,i and S G,i is the load and distributed generation power of node i, ΔS L,i and ΔS G,i is the load and distributed generation power adjustment of node i. This formula indicates that after the source load of the isolated island is adjusted, the distributed generation should be able to supply power to all loads. The adjustment margin constraint is

[0030]

[0031] In the formula, S Gmax,i is the maximum output of distributed energy at node i, and γ is the regulation margin required for distributed power to stabilize voltage and frequency.

[0032] Furthermore, in step 4, the elastic coefficient calculation method is as follows:

[0033] Step 4-1: Divide the distribution network load into K levels according to the actual importance assessment system on site, and assign weight λ to the load with importance k∈Kk ; According to the distribution network island support model containing solar energy, the total power consumption W of the load forecast of each importance level of the distribution network from the beginning to the end of the fault is calculated f.k And the actual total power consumption of the load W r.k ;

[0034] Step 4-2: Calculate the elastic power supply index Γ according to the weight, load forecast value and actual value obtained in step 4-1. The calculation method is as follows:

[0035]

[0036] Step 4-3: Keep the fault scenario of step 4-1 unchanged, set all distributed energy resources in the distribution network to be unable to participate in the network construction, and calculate the total power consumption W′ of the load forecast of each importance level from the beginning to the end of the fault in the distribution network again. f.k and the actual total power consumption of the load W′ r.k , and calculate the corresponding elastic power supply index Γ′;

[0037] Step 4-4: Subtract the elastic power supply index obtained in step 4-2 from that obtained in step 4-3 to obtain the ΔΓ elastic coefficient of the fault scenario. The calculation method is as follows:

[0038] ΔΓ=Γ-Γ′.

[0039] Furthermore, in step 5, the system comprehensive elasticity expectation calculation method is as follows:

[0040] Step 5-1: Obtain the failure probability σ of all lines in the system based on the historical statistical data and prediction information of the system j and the elastic coefficient ΔΓ corresponding to the line fault j ;

[0041] Step 5-2: Calculate the comprehensive elasticity expectation index ζ of the active power distribution system according to the failure probability and elasticity coefficient obtained in step 5-1. The calculation method is as follows:

[0042] ζ=∑σ j ΔΓ j

[0043] The elasticity evaluation of active distribution network can be realized by using the comprehensive elasticity expectation index ζ.

[0044] Compared with the prior art, the significant progress of the present invention lies in that: while realizing the fault recovery of the isolated support of photovoltaic distributed energy in the active distribution network, the present invention can convert the abstract elasticity index into a specific comprehensive elasticity expectation value, can be embedded in various elasticity index systems, system evaluation systems and system optimization systems, has good universality, reliability and objectivity, and can be widely used in various engineering practices.

[0045] In order to more clearly illustrate the functional characteristics and structural parameters of the present invention, further description is given below in conjunction with the accompanying drawings and specific implementation methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of this application. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0047] Figure 1 It is a flow chart of the present invention.

[0048] Figure 2 The topology diagram of the improved IEEE 33-node standard test system.

[0049] Figure 3 This is a flow chart for calculating the supporting capacity of the solar energy island in the distribution network.

[0050] Figure 4 Calculate the flow chart for the island failure recovery strategy. DETAILED DESCRIPTION

[0051] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0052] A typical trunk distribution network topology is as follows: Figure 2 As shown, the present invention proposes an active distribution network elasticity assessment method based on solar-powered distributed energy island support, such as Figure 1 As shown, the following steps are included:

[0053] (1) Obtain the operating information of the active distribution network, including the allowable fluctuation range of system frequency and voltage, the predicted load of each node, the importance of the load, the maximum load dispatching amount and the minimum load switching amount, the rated capacity of each distribution line and transformer, the failure probability and expected fault repair time of each line, the type, quantity and location of root-grid and grid-type distributed energy, the transient stability margin of grid-type distributed energy, and the maximum output power of distributed energy and energy storage.

[0054] (2) Based on the failure probability of each line in the distribution network obtained in step (1), a distribution network failure scenario set is established, which includes the system structure information and corresponding probability of each line being out of operation due to a failure.

[0055] (3) According to the distribution network fault scenario set established in step (2), perform island support capability analysis for each fault scenario. First, determine the part connected to the upper power grid and the island part, and then apply the distributed energy networking capability detection criterion to perform networking capability detection on each island, screen out the parts that pass the detection and number them. The networking capability detection criterion is: if the island has a networking type distributed energy, and the source and load adjustable power in the system meets the networking requirements, then the island is considered to have independent networking operation capabilities. The networking requirements include power balance requirements and adjustment margin requirements. The power balance requirement is:

[0056]

[0057] In the formula, S L,i and S G,i is the load and distributed generation power of node i, ΔS L,i and ΔS G,i is the load and distributed generation power adjustment of node i. This formula indicates that after the source load of the isolated island is adjusted, the distributed generation should be able to supply power to all loads. The adjustment margin requirement is:

[0058]

[0059] In the formula, S Gmax,i is the maximum output of distributed energy at node i, and γ is the regulation margin required for distributed power to stabilize voltage and frequency.

[0060] (4) According to the relevant content of step (3), the island information under each fault scenario can be obtained, including the structure of the island and the set of islands that can be supported by photovoltaic energy. Considering whether photovoltaic energy participates in island support, fault recovery analysis is performed on each island separately. First, according to the operation information of the distribution network obtained in step (1), the Newton-Raphson method power flow analysis is performed on the island to determine whether the power flow converges. If the power flow equation does not converge, it is considered that the island is facing voltage collapse, and the network is subjected to load shedding. By relaxing part of the system constraints and optimizing the power flow calculation, the initial state of the power flow can be obtained, and the dispatching plan of the distribution network is returned as the load shedding plan, the initial state is updated, and the power flow analysis of the island is performed again. If the optimized power flow still has no solution, it is considered that the island cannot supply power, all the equipment on the island is shut down, and the current island is marked as stable.

[0061] (5) According to the initial state of the tidal current obtained in step (4), considering whether the solar energy is involved in the island support, the tidal current analysis is performed on the island and the part connected to the main grid respectively, and the obtained system state quantity is compared with the system stability criterion respectively to determine the system state quantity that does not meet the constraint conditions. The system stability criterion is:

[0062]

[0063] Where P L,i and P G,i is the load and distributed generation active power of node i, ΔP L,i and ΔP G,i is the load and distributed generation active power adjustment of node i, V i is the voltage at node i, V i,max and V i,min is the voltage offset range of node i, S B,j and S Bmax,j is the load and rated capacity of line j, S T,m and S Tmax,m is the load and rated capacity of the transformer m.

[0064] (6) According to step (5), the system state quantity that does not meet the constraint conditions in each area is obtained. For the area that does not meet the requirements, the optimization power flow analysis is continued to be carried out to determine the dispatching strategy that should be adopted in the fault recovery stage of the area. For the dispatching strategy, frequency recovery is achieved by adjusting the active power output of the source and load so that the active power of the source and load is balanced after recovery and the generator output does not exceed the rated limit; voltage recovery is achieved by adjusting the reactive power output of the source and load and the reactive compensation equipment in the system so that the voltage value of each node and the reactive power adjustment of the equipment do not exceed the adjustment range; power recovery is achieved by tripping the overloaded lines and transformers so that the transmission power of each line and transformer after recovery does not exceed its rated capacity.

[0065] (7) According to the dispatch strategy of each area calculated in step (6), the fault status of each area is updated, and the process jumps to step (4) to determine whether a new island is generated and whether the system state quantity of each area after fault recovery meets the constraint conditions. If the current state of the distribution network still does not meet the above constraint conditions, it is necessary to continue the fault recovery from step (4) to step (6) until all areas meet the constraint conditions in step (5).

[0066] (8) According to step (7), the fault state that satisfies the system state quantity constraint conditions in all regions can be calculated considering whether the solar energy participates in the island support or not. The fault state update record of each time in the loop of step (7) is taken as the stable process of the fault, and the fault state that satisfies the system constraint conditions is taken as the stable state of the fault. Repeat steps (4) to (7) until the stable process and stable state of the fault are calculated for all fault scenarios when the solar energy participates in the island support or not.

[0067] (9) Based on the stable process and stable state obtained in step (8) and the predicted load of each node, the importance of the load and the expected fault repair time obtained in step (1), the distribution network load is divided into K levels according to the actual importance evaluation system on site, and the load with importance k∈K is assigned a weight λ k . According to whether the solar energy participates in the island support under each fault scenario, the stable process and stable state of the fault. Calculate the total power consumption W of the load with the corresponding importance in the fault process f.k And the actual total power consumption of the load W r.k The calculation formula is as follows:

[0068]

[0069] In the formula, and represents the predicted and actual loads of importance level k at time t, T0 and T e It is the fault start time and fault repair time.

[0070] (10) According to the load power consumption obtained in step (9), the elastic power supply index Γ and Γ′ corresponding to whether the solar energy source participates in the island support in each fault scenario are calculated respectively. The calculation method is as follows:

[0071]

[0072] (11) According to the elastic power supply index obtained in step (10), the ΔΓ elastic coefficient of all fault scenarios is calculated as follows:

[0073] ΔΓ=Γ-Γ′

[0074] This method gives a distribution network fault scenario and, based on the distribution network island support model, calculates the power consumption during the fault process when the distribution network considers whether photovoltaic energy participates in island support, thereby calculating the elasticity coefficient.

[0075] (12) Based on the elasticity coefficient of each fault scenario calculated in step (11), the occurrence probability of each fault scenario is obtained according to the historical statistical data and prediction information of the system, and the comprehensive elasticity expectation index ζ of the active distribution system is calculated. The calculation method is as follows:

[0076] ζ=∑σ j ΔΓ j

[0077] In the formula, ΔΓ j and σ j is the elasticity coefficient and occurrence probability of the fault scenario corresponding to the fault of line j.

[0078] The various methods used in the present invention are as follows:

[0079] 1. Operation characteristics of grid-connected photovoltaic virtual synchronous machine

[0080] This application mainly considers a grid-connected virtual synchronous machine (VSG) composed of photovoltaics and energy storage. The core components are composed of distributed energy, energy storage equipment and synchronous inverters. Distributed energy and energy storage equipment are connected to the AC power grid and the main grid through synchronous inverters after filtering to exchange electric energy. In the process of exchanging electric energy with the main grid, by controlling the control mode of the synchronous inverter, the VSG has external characteristics similar to those of a synchronous generator on the AC side, thereby achieving frequency and voltage support for the AC power grid. The output characteristic of VSG is that when the voltage or frequency of the distribution network fluctuates, the VSG adjusts the active and reactive outputs to achieve dynamic support and control of voltage and frequency. Since the output of photovoltaic distributed energy is relatively uncontrollable, the virtual synchronous machine characteristics of VSG are maintained mainly by controlling the charging and discharging power of energy storage, and its output variation range is as follows:

[0081] P PV,i +I Smin,i V PCC,i ≤ΔS VSG,i ≤P PV,i +I Smax,i V PCC,i

[0082] In the formula, ΔS VSG,i is the output range of the i-node VSG, P PV,i is the output power of the photovoltaic power station at node i, I Smax,i and I Smin,i is the maximum discharge and charge current of the energy storage at node i, V PCC,i The voltage of the common coupling point in the VSG at node i. It can be seen from this formula that the instantaneous output power of the energy storage device limits the output power adjustment range of the VSG, and the energy storage charge limits the power adjustment time of the VSG. The output power constraint and adjustment time constraint of the VSG are:

[0083]

[0084] In the formula, C EV (T) is the charge throughput of energy storage at node i, T max is the maximum adjustment time of VSG, SOC is the initial charge of energy storage, SOC max and SOC min are the maximum and minimum charges of the energy storage.

[0085] 2. Initial state determination method based on optimized power flow

[0086] The initial state determination method based on optimized power flow adopted in this application is as follows:

[0087] Since the state quantity that changes is the active and reactive power of each node in the system, and considering that the system should supply power to as many loads as possible, this application takes the minimum power change ΔP before and after the system structure change as the objective function to establish an optimization model. The objective function is:

[0088] minΔP=∑ΔP i

[0089] Constraints

[0090] This method is a method for determining the initial conditions of the power flow equation. It does not care whether the network state meets the operation constraints of the distribution network. The constraints only need to consider the energy balance constraints and power flow convergence constraints. The power flow solution method uses the Newton-Raphson method. The Newton-Raphson method power flow equation can be written as

[0091] N=f(V)

[0092] Where f() is the power flow equation; N is the power vector of the node, including the active power and reactive power of the node; V is the node voltage vector, including the voltage amplitude and voltage phase angle.

[0093] The current equation at this time is a nonlinear group. In order to linearize the current equation, equation (2-10) is expanded using the Taylor series:

[0094] N0+ΔN=f(V0)+f′(V0) -1 ΔV+…

[0095] In the formula, N0 and ΔN are the initial values ​​of node power and node voltage; V0 and ΔV are the changes of node power and node voltage. Ignoring the higher-order terms above the first order, the formula can be written as:

[0096] ΔN=J0ΔV

[0097] Where J0 is the Jacobian matrix at the end of the flow calculation iteration. The Newton-Raphson flow convergence criterion is used to determine whether the flow converges. The convergence criterion can be written as:

[0098] ΔN≤ε

[0099] Where ε is the maximum tolerable error.

[0100] The optimized solution is the load reduction of each node in the system and the voltage of the node containing the distributed power source, which is substituted into the power flow equation to calculate the solvable initial state. In addition, in a few cases, the optimized power flow has no solution. At this time, it is considered that the system has no network construction capability, and the system is shut down, and this state is used as the solvable initial state.

[0101] 3. Calculation method of the support capacity of the solar energy island in the distribution network

[0102] The island support capability of the distribution network is reflected in the ability to ensure that the load continues to be powered when a fault occurs. Therefore, it is necessary to calculate the stable process and stable state of the fault when the photovoltaic energy is involved in the island support in all fault scenarios. The calculation method of the stable process and stable state of the fault is established as the calculation method of the island support capability. The calculation process is as follows: Figure 3 The specific steps of this method are as follows:

[0103] S1. Input a distribution network fault scenario set containing N scenarios, set a fault scenario count i, and let i=1.

[0104] S2. Perform island support capability analysis on the i-th fault scenario. First, determine the part connected to the upper power grid and the island part, then apply the distributed energy network capability detection criterion, consider whether the solar energy participates in the island support, perform network capability detection on each island, and select M islands that meet the conditions. The islands that do not meet the conditions are considered to be unable to operate independently and are directly shut down. Set the island count j, and let j=1.

[0105] S3. Perform fault recovery analysis on the jth island and record whether the island meets the system stable operation requirements and fault recovery strategy.

[0106] S4. Determine whether the stable process and stable state of M isolated islands have been obtained. If j<M, there are still isolated islands that have not been analyzed for fault recovery. Set j=j+1 and jump back to step S3 until j≥M.

[0107] S5. Collect the island status recorded in step S3. If there is an unstable island, update the island status according to the corresponding fault recovery strategy, substitute the updated island status into step S2, and perform fault recovery again; if all islands are stable, record the stable process and stable state of each island when the light marsh energy participates in the island support.

[0108] S6. Determine whether the stable process and stable state of N fault scenarios have been obtained. If i<N, there are still fault scenarios that have not been analyzed. Set i=i+1 and jump back to step S2 until i≥N.

[0109] S7. Output all the obtained stable processes and stable states, and end the island support capability analysis.

[0110] 4. Calculation method of island failure recovery strategy

[0111] The goal of the fault recovery phase is to ensure that each island after fault isolation meets the basic operation requirements of the distribution network. However, in most cases, the basic operation requirements of the distribution network cannot be met immediately after the island is separated from the main network. In order to ensure the stable power supply of the system, it is necessary to temporarily dispatch the system source and load to restore the island system from the state after the fault to a stable state. This process is defined as the fault recovery process, and the corresponding dispatching strategy is the fault recovery strategy.

[0112] The specific implementation method of the fault recovery phase is as follows: Figure 4 As shown. First, the power flow analysis is performed on the island, and the obtained system state quantity is compared with the constraints related to frequency, voltage, and load respectively. The system state quantity that does not meet the constraints is determined, and the recovery plan of the corresponding index is constructed. The current island is marked as a fault-continuing state and the corresponding scheduling plan is returned. Fault recovery is performed again until all constraints are met. The current island is marked as a fault-ending state and the corresponding scheduling plan is returned as the final fault recovery plan for the island. In addition, when performing power flow analysis, there may be a situation where the power flow equation does not converge. The non-convergence situation is generally due to the serious over-limit of the island operation. Its power flow equation is difficult to solve in the given initial state, and the island faces voltage collapse. At this time, it is necessary to perform load shedding on the network. In order to find a solvable initial state, a part of the system constraints are relaxed and the optimized power flow calculation is performed to obtain the initial state where the power flow can be solved. The dispatching plan of the distribution network is returned as the load shedding plan, the initial state is updated, and the current island is marked as a fault-continuing state. The power flow analysis of the island is performed again. If the optimized power flow still has no solution, it means that the island cannot supply power. The island is closed and the current island is marked as a fault-ending state.

[0113] When the distribution network can perform power flow analysis, it is necessary to analyze whether the various operating states of the system meet the requirements. The operating states that the distribution network needs to pay attention to are frequency, voltage and power. According to the above three system state quantities, the distribution network state judgment criteria are established. Frequency over-limit is mainly caused by the imbalance of mechanical power and electromagnetic power. Therefore, frequency recovery mainly focuses on the balance of source and load active power after recovery and the generator output does not exceed the rated limit, which is written as the frequency constraint as follows:

[0114] P G -∑P L =ΔP G -ΔP L

[0115] ΔP Gmin ≤ΔP G ≤ΔP Gmax

[0116] ΔP Lmin ≤ΔP L ≤ΔP Lmax

[0117] Where ΔP Gmax and ΔP Gmin is the maximum and minimum output dispatch of the power source, ΔP Lmax and ΔP Lmin is the maximum and minimum dispatching amount of the load. Voltage over-limit is mainly caused by excessive or low reactive power in the system. Therefore, voltage recovery mainly focuses on the voltage value of each node after recovery and the reactive adjustment of source and load not exceeding the adjustment range, which is written as the voltage constraint as follows:

[0118] V i,min ≤V i ≤V i,max

[0119] ΔQ=ΔQ DG +ΔQ L

[0120] ΔQ DGmin ≤ΔQ DG ≤ΔQ DGmax

[0121] ΔQ Lmin ≤ΔQ L ≤ΔQ Lmax

[0122] Where ΔQ DGmax and ΔQ DGmin is the maximum and minimum change in power reactive power, ΔQ Lmax and ΔQ Lmin is the maximum and minimum dispatching amount of reactive power compensation equipment. Power over-limit is mainly caused by the transmission power in the system exceeding the rated capacity of the line or transformer. Therefore, power recovery mainly focuses on whether the transmission power of each line and transformer exceeds its rated capacity after recovery, which is written as the capacity constraint as follows:

[0123]

[0124] If the current state of the distribution network does not meet the above constraints, the system is considered unstable and fault recovery is still required. During the fault recovery process, the distribution network needs to continuously adjust the source and load output of each node, and dispatch various power quality management equipment to change the active and reactive power values ​​of each node until all stable operation constraints are met.

[0125] Since the system has different tolerance and tolerable time for frequency, voltage, and power exceeding the limit, this application divides the process into three steps. First, the frequency of the island is checked. If the frequency constraint is not met, the frequency recovery is started. The active power of the load and distributed power source is used as an adjustable quantity to optimize the power flow calculation of the system until the frequency constraint is met. When the frequency constraint is met, the voltage is checked. If the voltage constraint is not met, the voltage recovery is started. The reactive power of each node is used as an adjustable quantity to optimize the power flow calculation of the system until the voltage constraint is met. When the voltage constraint is met, the power is checked. If the power constraint is not met, all overloaded transformers and lines are opened and tripped until the power constraint is met. The objective function of optimizing the power flow during fault recovery is to minimize the total dispatch amount in the system. The constraints for optimizing the power flow include equipment operating status constraints and power quality constraints. The equipment operating status constraints in the active distribution network are power flow constraints, load dispatching constraints, and operation constraints of energy storage and distributed energy. The power quality constraints are voltage offset constraints, frequency offset constraints, line current carrying capacity constraints, and transformer current carrying capacity constraints.

[0126] ① Equipment operation status constraints

[0127] The power flow constraints are as follows:

[0128]

[0129] Where P i is the active power of node i, Q i is the reactive power of node i, V i and θ are the voltage amplitude and phase angle of node i, G ij and B ij are the conductance and susceptance of lines i and j.

[0130] The load dispatch constraints are as follows:

[0131] 0≤ΔS L,i ≤S L,i

[0132] In the formula, S L,i is the load of node i, ΔS L,i is the maximum schedulable amount of the i-node load.

[0133] Energy storage operation constraints include energy storage charging and discharging constraints and charge constraints. The energy storage charging and discharging constraints are as follows:

[0134]

[0135] In the formula, is the charge and discharge capacity of distributed energy storage, and The distribution is the maximum discharge and charge of the energy storage.

[0136] The energy storage charging and discharging constraints are as follows:

[0137]

[0138] In the formula, and is the energy storage charge of the node in period k and period k-1, η i is the efficiency of energy storage i. and are the maximum and minimum charges of the energy storage.

[0139] The new energy operation constraints are shown below:

[0140]

[0141] Where S NG,i is the power generation of renewable energy in period k, S NGmax,i It is the maximum power generation capacity of new energy power generation equipment.

[0142] Example

[0143] In order to verify the effectiveness and reliability of the present invention, an improved IEEE 33-node standard test system model was built on Matlab, as shown in Figure 2 The scenario of each line failure is simulated in the system, and the photovoltaic installed capacity and biogas installed capacity are changed. The power supply expectation and the system comprehensive elasticity expectation of the system are calculated with and without considering fault recovery. The experimental results are shown in Table 1. Defined by:

[0144]

[0145] In the above formula, σ j is the failure probability of the jth line, W r.k is the actual total power consumption of the system when the jth line fails. In Table 1, by comparing the expected power supply of the system under and without considering fault recovery, it can be concluded that after a fault occurs, the power supply of the system increases significantly after considering fault recovery, and the distribution system has the ability to repair the fault autonomously during a fault, and the system resilience is improved. The system's comprehensive resilience is expected to increase with the increase in the duration of the fault, and with the increase in the installed capacity of photovoltaic power generation and biogas power generation. It can be concluded that the system resilience of the active distribution network will be more significant when a high proportion of distributed energy is connected and the fault time is longer.

[0146]

[0147]

[0148] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0149] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for evaluating the elasticity of an active distribution network based on solar-powered distributed energy island support, characterized in that: The following steps are involved: Step 1: Obtain the operation information of the distribution network; Step 2: According to the operation information of the distribution network obtained in step 1, a distribution network island fault recovery model containing solar energy is constructed. The island fault recovery model constructs a system stability criterion by inputting the operation state quantity and equipment state quantity of the island, and determines the dispatching strategy adopted in the island fault recovery stage based on the criterion. The dispatching strategy includes source-load dispatching quantity, switching status of power quality management device, and protection action status of distribution lines and transformers; Step 3: According to the island fault recovery model established in step 2, an island support model of the distribution network containing solar energy is constructed. The island support model constructs an island networking capability detection criterion according to the operation procedures and source-load operation characteristics of the distribution network, and selects islands with networking capability according to the operation state quantity, equipment state quantity and fault location before the distribution network fault; Then, the relevant information of these islands is input into the fault recovery model to determine the dynamic characteristics of the islands during the fault. Finally, the stable process and stable state of the islands are calculated through the expected fault recovery time. Step 4: According to the distribution network island support model containing solar energy established in step 3, a method for calculating elasticity coefficient is constructed. The elasticity coefficient calculation method calculates the stability process and the stability state of the distribution network respectively for the cases where solar energy is supported by the solar energy and is not supported by the solar energy according to the distribution network island support model under the given fault scenario of the distribution network, so as to calculate the elasticity coefficient; Step 5. According to the elasticity coefficient calculation method established in step 4, a system comprehensive elasticity expectation calculation method is established. The system comprehensive elasticity expectation calculation method generates a system fault scenario set through the line fault conditions in the system, and calculates the elasticity coefficient of each fault scenario according to the elasticity coefficient calculation method, and then calculates the system comprehensive elasticity expectation to realize the elasticity evaluation of the distribution network.

2. According to claim 1, a method for evaluating the elasticity of an active distribution network based on solar-marsh distributed energy island support is characterized in that: In step 1, the operation information of the power grid includes the allowable fluctuation range of system frequency and voltage, the predicted load of each node, the importance of the load, the maximum load dispatching amount and the minimum load switching amount, the rated capacity of each distribution line and transformer, the failure probability and expected fault repair time of each line, the type, quantity and location of root-grid and grid-type distributed energy, the transient stability margin of grid-type distributed energy, and the maximum output power of distributed energy and energy storage.

3. According to claim 1, a method for evaluating the elasticity of an active distribution network based on solar-marsh distributed energy island support is characterized in that: In step 2, the island failure recovery model is as follows: The Newton-Raphson method is used to analyze the elasticity of the farm distribution network supported by the solar energy island to determine whether the power flow converges. If the power flow equation does not converge, it is considered that the island is facing voltage collapse, and the network is subjected to load shedding. By relaxing the system frequency, node voltage and equipment capacity constraints and optimizing the power flow calculation, the initial state of the solvable power flow is obtained, and the dispatching plan of the distribution network is returned as the load shedding plan, the initial state is updated, and the power flow analysis of the island is performed again; if the optimized power flow still has no solution, it is considered that the island cannot be powered, all the equipment on the island is shut down, and the current island is marked as stable; According to the obtained initial state of the solvable power flow, the power flow analysis is performed on the island, and the obtained system state quantities are compared with the system frequency, voltage, and load stability criteria respectively, the system state quantities that do not meet the constraints are determined, and a recovery plan for the corresponding indicators is constructed. The current island is marked as a fault-continuing state and the corresponding scheduling plan is returned. Fault recovery is performed again until all constraints are met. The current island is marked as a fault-ending state and the corresponding scheduling plan is returned as the final fault recovery plan for the island.

4. According to claim 1, a method for evaluating the elasticity of an active distribution network based on solar-marsh distributed energy island support is characterized in that: In step 2, the system stability criteria are as follows: The operating states that need to be paid attention to in the stability of the distribution network include frequency, voltage and power. Based on the above three system state quantities, the distribution network state judgment criteria are established. Where P L,i and P G,i is the load and distributed generation active power of node i, ΔP L,i and ΔP G,i is the load and distributed generation active power adjustment of node i, V i is the voltage at node i, V i,max and V i,min is the voltage offset range of node i, S B,j and S Bmax,j is the load and rated capacity of line j, S T,m and S Tmax,m is the load and rated capacity of the transformer m.

5. According to claim 1, a method for evaluating the elasticity of an active distribution network based on solar-marsh distributed energy island support is characterized in that: In step 3, the specific calculation steps of the distribution network island support model containing solar energy are as follows: Step 3-1: When a system failure occurs and the line is disconnected, determine the part connected to the upper grid and the island part; Step 3-2: Apply the distributed energy network building capability detection criteria to perform a network building capability test on each island determined in step 3-1, select the ones that pass the test and number them; Step 3-3, the parts connected to the upper power grid and the islands detected by step 3-2 are respectively substituted into the island fault recovery model to determine whether each area meets the system stable operation requirements and the scheduling plan required for system stability recovery when the requirements are not met; Step 3-4: Collect the return information of the parts connected to the upper power grid and the islands in step 3-3. If there are islands that do not meet the stable operation requirements, update the system fault scenario according to the dispatching schemes corresponding to these islands in step 3-3, and jump to step 3-1 to restart fault recovery; if all parts substituted into the island fault recovery model meet the stable operation requirements, it is considered that the distribution network fault recovery is successful, and jump to step 3-5; Step 3-5: Calculate the power supply shortfall of the part connected to the upper grid and the isolated island.

6. According to claim 5, a method for evaluating the elasticity of an active distribution network based on solar-marsh distributed energy island support is characterized in that: Step 3-2, the application of distributed energy network construction capability detection criteria are as follows: The criterion for network building capability detection is: if the island has network-building distributed energy and the adjustable power of the source and load in the system meets the network building requirements, then the island is considered to have independent network building operation capabilities; the network building requirements include power balance requirements and regulation margin requirements, and the power balance requirements are: Where S L,i and S G,i is the load and distributed generation power of node i, ΔS L,i and ΔS G,i is the load and distributed generation power adjustment of node i. This formula indicates that after the source load of the isolated island is adjusted, the distributed generation should be able to supply power to all loads. The adjustment margin constraint is In the formula, S Gmax,i is the maximum output of distributed energy at node i, and γ is the regulation margin required for distributed power to stabilize voltage and frequency.

7. According to claim 1, a method for evaluating the elasticity of an active distribution network based on solar-marsh distributed energy island support is characterized in that: In step 4, the elastic coefficient calculation method is as follows: Step 4-1: Divide the distribution network load into K levels according to the actual importance assessment system on site, and assign weight λ to the load with importance k∈K k ; According to the distribution network island support model containing solar energy, the total power consumption W of the load forecast of each importance level of the distribution network from the beginning to the end of the fault is calculated f.k And the actual total power consumption of the load W r.k ; Step 4-2: Based on the weights obtained in step 4-1, the total load power consumption W is predicted f.k And the actual total power consumption of the load W r.k Calculate the elastic power supply index Γ, the calculation method is as follows: Step 4-3: Keep the fault scenario of step 4-1 unchanged, set all distributed energy resources in the distribution network to be unable to participate in the network construction, and calculate the total power consumption W′ of the load forecast of each importance level of the distribution network from the beginning to the end of the fault again. f.k and the actual total power consumption of the load W′ r.k , and calculate the corresponding elastic power supply index Γ′; Step 4-4: Subtract the elastic power supply index obtained in step 4-2 from that obtained in step 4-3 to obtain the ΔΓ elastic coefficient of the fault scenario. The calculation method is as follows: ΔΓ=Γ-Γ′.

8. According to claim 1, a method for evaluating the elasticity of an active distribution network based on solar-marsh distributed energy island support is characterized in that: In step 5, the calculation method of the system comprehensive elasticity expectation is as follows: Step 5-1: Obtain the failure probability σ of all lines in the system based on the historical statistical data and prediction information of the system j and the elastic coefficient ΔΓ corresponding to the line fault j ; Step 5-2: Calculate the comprehensive elasticity expectation index ζ of the active power distribution system according to the failure probability and elasticity coefficient obtained in step 5-1. The calculation method is as follows: ζ=∑σ j DG j The elasticity evaluation of active distribution network can be realized by using the comprehensive elasticity expectation index ζ.

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