Optimized scheduling method for flexible interconnection system of power distribution substation

Abstract

The invention discloses an optimal scheduling method for a flexible interconnection system of a power distribution substation, which belongs to the field of power distribution optimal scheduling and comprises the following steps: S1, constructing the flexible interconnection system of the power distribution substation based on multi-terminal soft switching model control; s2, considering the distribution substation flexible interconnection system based on multi-terminal soft switching model control constructed in the step S1 and a transformer load balance rate, and constructing a flexible interconnection system optimization model; s3, constructing constraint conditions of the flexible interconnection system optimization model; and S4, optimizing the distribution substation flexible interconnection system by using the flexible interconnection system optimization model constrained in the step S3. According to the optimal scheduling method for the flexible interconnection system of the power distribution substation, overload of the sensor is relieved through energy storage and operation of the interconnection system, the purpose of peak clipping and valley filling is achieved, and the problems of the load unbalance rate of a transformer in the power distribution substation system, high loss of a heavy-load substation and the like are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of power distribution optimization scheduling, and in particular to a method for optimizing scheduling of a flexible interconnection system of a power distribution substation. Background Art

[0002] In the current power system, with the continuous access of new low-carbon loads such as end users, large-scale distributed generation (DG), electric vehicles, and energy storage, the operation of the distribution system faces many challenges. Specifically, they are as follows: 1. Increased transformer loss: The access of new loads leads to uneven current distribution in the distribution network, causing transformers in some areas to be in a high-load state for a long time, thereby increasing transformer losses; 2. Intensified load imbalance: The uneven distribution of different types of loads in time and space further aggravates the load imbalance problem of the distribution network, affecting the stability and reliability of the system.

[0003] To solve the above problems, existing technical solutions mainly rely on the dynamic trend control function of power electronic flexible interconnection devices (FID). Existing FID optimizes the operation of distributed networks in the following ways:

[0004] 1. Dynamic power regulation: FID can dynamically regulate active and reactive power by real-time monitoring and adjusting the power flow between nodes, which helps to balance the load in various areas and reduce local overload;

[0005] 2. Voltage support and stability: FID has the ability to respond quickly and can provide voltage support when needed, maintain the voltage stability of the power grid, and prevent additional losses caused by voltage fluctuations;

[0006] 3. Fault isolation and recovery: When a fault occurs, FID can quickly isolate the fault area to prevent the fault from spreading, and quickly restore power supply after the fault is eliminated, thereby improving system reliability and recovery speed;

[0007] 4. Optimize the integration of distributed power sources: FID can better integrate distributed power sources, such as solar and wind power, to achieve efficient use of these renewable energy sources while reducing their impact on the power grid;

[0008] 5. Cross-region load balancing: Through the dynamic control function of FID, relatively balanced load distribution can be achieved in different regions, reducing the overload pressure in a certain area and improving the operating efficiency of the entire system.

[0009] However, the existing technology still has shortcomings in the following aspects:

[0010] 1. Single operation modem: The existing technology generally only considers the single operation modem of FID, and does not involve the research on transformer reconstruction and FID coordination in flexible interconnected partitioned networks, which limits the application effect of FID in more complex network structures;

[0011] 2. Light-load transformer shutdown strategy: The research on the shutdown strategy of light-load transformers in flexible interconnected distribution networks is not in-depth enough, and the research on effective shutdown strategies is still relatively lacking;

[0012] 3. Application of multi-terminal soft switches: Although the existing technology has explored the application of two-terminal soft switches in single-transformer distribution networks, the application research of multi-terminal soft switches in multi-transformer distribution networks is still insufficient.

[0013] In summary, although the dynamic trend control function of FID has optimized the operation of distributed networks to a certain extent, further research and development are still needed in transformer reconstruction, light-load transformer shutdown strategy, and the application of multi-terminal soft switches. Summary of the invention

[0014] The purpose of the present invention is to provide a method for optimizing the scheduling of a flexible interconnection system of a distribution substation to solve the above technical problems.

[0015] To achieve the above object, the present invention provides a method for optimizing the scheduling of a flexible interconnection system of a distribution substation, comprising the following steps:

[0016] S1. Construct a flexible interconnection system of distribution substations based on multi-terminal soft switch model control;

[0017] S2, considering the distribution substation flexible interconnection system based on the multi-terminal soft switch model control constructed in step S1 and the transformer load balance rate, constructing a flexible interconnection system optimization model;

[0018] S3,constraints for constructing the optimization model of flexible interconnection system;

[0019] S4. Optimize the flexible interconnection system of distribution substations using the flexible interconnection system optimization model constrained by step S3.

[0020] Preferably, step S1 specifically includes the following steps:

[0021] S11. Construct a flexible interconnection system for distribution substations. The flexible interconnection system for distribution substations includes multiple distribution areas, and two adjacent distribution areas are interconnected via a DC bus. The DC bus is also connected to the energy storage system ESS and the DC / AC converter. Each distribution area includes a power input terminal, a distribution transformer and a VSC. The power input terminal is connected in series with the distribution transformer and the VSC in turn via an AC side line.

[0022] S12. Control the flexible interconnection system of distribution substations using a multi-terminal soft switch model: The multi-terminal soft switch model adopts a master-slave control mode. At this time, one of the multiple VSCs in step S1 is used as a master port, and the remaining VSCs are used as slave ports. The master port is used to adjust the DC voltage of the flexible interconnection system of distribution substations, and the slave port is used to manage the current power output.

[0023] Sinusoidal pulse width modulation is assumed, and the DC bus voltage and AC side line voltage of the main port VSC have the following cross relationship:

[0024]

[0025] Where U ac,t represents the AC line voltage; μ represents the utilization rate of the DC voltage; M represents the VSC modeling system; U dc,t Indicates the DC bus voltage;

[0026] The DC side voltage of the slave port VSC satisfies the following constraints:

[0027]

[0028] Where U dc,t Represents the DC side voltage of the slave port VSC; They respectively represent the minimum and maximum values ​​of the DC side voltage of the slave port VSC.

[0029] Preferably, step S2 specifically includes the following steps:

[0030] S21. Define an offline strategy: sort the load rates of distribution transformers in multiple distribution areas from large to small, and shut down the distribution transformer with the smallest load rate during the load valley period to optimize the comprehensive loss of the distribution network;

[0031] S22. Constructing a flexible interconnection system optimization model:

[0032] S221. Considering the balance index of the distribution transformer load rate and the operating loss index of the distribution substation flexible interconnection system, the weighted minimum method is used to optimize the target:

[0033] minf=λ1f LB,t +λ2f s,t (3);

[0034] Where minf represents the minimization objective function; λ1 and λ2 represent the weights of the balance index of the distribution transformer load factor and the operating loss index of the distribution substation flexible interconnection system, respectively; f LB,t Indicates the balance index of the distribution transformer load factor; f s,tIt represents the operating loss index of the flexible interconnection system of distribution substations, which is the total loss of the flexible interconnection system of distribution substations;

[0035] in,

[0036]

[0037] In the formula, β i (t) represents the composite factor of the i-th distribution transformer; n represents the total number of distribution transformers; t represents the time period; T represents the number of time periods; I ij represents the current flowing through branch ij; r ij represents the equivalent resistance of the current flowing through branch ij; η1 represents the efficiency of the power transmission port; represents the active power transfer of the ith distribution transformer at time t; P tran,loss represents the transmission power loss; ω1 represents the shared energy storage operation loss coefficient; P ess,t represents the total charging and discharging power of the shared energy storage; ω2 represents the abandoned light loss coefficient; Indicates the rated installed capacity of photovoltaic power; Indicates the actual capacity of grid-connected photovoltaic power generation;

[0038] S222. Calculate the operating loss index f of the distribution transformer s,t :

[0039] f s,t =F s,t / F max (6);

[0040] In the formula, F max Indicates the maximum operating loss of the distribution transformer.

[0041] Preferably, the objectives described in step S221 include reducing network losses and compensation rates.

[0042] Preferably, the constraints described in step S3 include VSC constraints, distribution transformer constraints, photovoltaic power generation and energy storage constraints, AC side constraints and DC side constraints;

[0043] Among them, the VSC constraint is: the power transmission of the VSC port meets the port capacity constraint:

[0044]

[0045]

[0046] In the formula, Ω FID Represents the set of all ports of the flexible interconnection system of distribution substations; ΔS FID,i represents the power change of the ith distribution transformer in the flexible interconnection system of distribution substations; SFID,loss,i represents the power loss of the i-th distribution transformer in the flexible interconnection system of distribution substations; Indicates a fixed proportional coefficient; P FID,i , Q FID,i Respectively represent the active power and reactive power flowing through the DC / AC converter;

[0047] Distribution transformer constraints include operation range constraints, distribution transformer offline constraints under load valley conditions, and load balancing constraints;

[0048] The operation interval constraint expression is as follows:

[0049]

[0050] In the formula, β i represents the load rate of the ith distribution transformer; P t,i represents the active load borne by the ith distribution transformer at time t; S i represents the rated capacity of the i-th distribution transformer; Indicates the power factor of the distribution transformer;

[0051] The offline constraint expression of distribution transformer under load valley condition is as follows:

[0052]

[0053] In the formula, α i Represents the judgment index of the i-th distribution transformer, when α i ≥1, the i-th distribution transformer is online, when α i <1, the i-th distribution transformer is offline; represents the AC side load of the i-th distribution transformer; Indicates the DC bus side load; S1+S2+…S i-1 +S i+1 …S n Indicates the rated capacity of the remaining distribution transformers except the offline distribution transformer;

[0054] The load balancing constraint expression is as follows:

[0055]

[0056] Where n T,t 、n T,t+1 represents the operating status of the distribution transformer at time t and time t+1 under the offline strategy; N represents the number of soft switches in the flexible interconnection system of the distribution substation;

[0057] The photovoltaic power generation and energy storage constraint expressions are as follows:

[0058] Fn +F BL.ch ≤R F,n +F BL,dis (12);

[0059] P n +F BL,ch ≤R n +F BL,dis (13);

[0060]

[0061] H l ≤H l,max (15);

[0062] In the formula, F n Indicates the user's load in charging mode; F BL.ch Represents the load of the energy storage system ESS in charging mode; R F,n Indicates the line capacity of the user in charging mode; F BL,dis represents the load of the energy storage system ESS in the discharge mode; P n Indicates the user's load in discharge mode; R n Indicates the line capacity of the user in the discharge mode; Represents the charge state variable of the energy storage system ESS; Represents the discharge state variable of the energy storage system ESS; F BL,ch,max Indicates the maximum charging power of the energy storage system ESS; F BL,dis,max Indicates the maximum discharge power of the energy storage system ESS; H l Indicates the actual discharge time of the energy storage system ESS; H l,max Indicates the maximum sustainable discharge time of the energy storage system ESS;

[0063] AC side constraints include node voltage and active and reactive power constraints based on power flow calculation, node voltage and branch current constraints;

[0064] The node voltage and active and reactive power constraint expressions based on power flow calculation are as follows:

[0065]

[0066] Where U t,j , U t,o They represent the voltages of node j and node o at time t respectively; R oj represents the resistance of the node branch oj; P t,oj represents the active power flowing out of the node branch oj at time t; X oj represents the reactance of the node branch oj; Q t,ojrepresents the reactive power flowing out from the node branch oj at time t; I t,0j represents the current of the node branch oj at time t; Ω b represents the set of all branch nodes; P t,jh represents the active power flowing out of the node branch jb at time t; P t,j represents the active power injected into node j at time t; Q t,jh represents the reactive power flowing out from the node branch jh at time t; Q t,j represents the reactive power injected into node j at time t; represents the active power injected into node j by the distributed generator connected to node j at time t; represents the active power injected into node j by the soft switch at time t; represents the active power consumed by the load at node j at time t; represents the reactive power injected into node j by the distributed generator connected to node j at time t; represents the reactive power injected into node j by the soft switch at time t; It represents the reactive power consumed by the load at node j at time t.

[0067] Preferably, the node voltage and branch current constraint expressions are as follows:

[0068]

[0069] In the formula, U and They represent the minimum and maximum allowable voltages of node o under online operation; U t,o Indicates the voltage of node o in online operation; Indicates the maximum allowable branch current.

[0070] Preferably, DC side constraint: the DC side node current sequence expression is as follows:

[0071]

[0072] Where, M represents the DC side node set; It represents the power transmitted to the DC side of the distribution transformer through the VSC; Indicates the power generated by the DC side distribution power supply; Indicates the charging and discharging power of the DC side energy storage system ESS; Indicates the power required by the DC side load; represents the voltage of the DC side node m; represents the conductance of the DC side node branch mq at time t; Represents the voltage at the DC side node q.

[0073] Therefore, the present invention adopts the above-mentioned distribution substation flexible interconnection system optimization scheduling method, which has the following beneficial effects:

[0074] 1. Reduce the operating cost of the distribution network: by optimizing energy flow and distribution, reduce unnecessary power loss and equipment maintenance costs;

[0075] 2. Optimize transformer load ratio: Rationally dispatch multi-terminal soft switching power supplies, balance the load distribution in each area, avoid local overload, and thus improve the operating efficiency of the transformer;

[0076] 3. Reduce network losses: Use flexible interconnection technology and energy storage devices to dynamically adjust power flow, reduce line losses, and improve the energy efficiency of the entire distribution system.

[0077] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 A flowchart of a method for optimizing the dispatching of a flexible interconnection system of a power distribution substation according to the present invention;

[0079] Figure 2 This is a schematic diagram of the structure of the flexible interconnection system of the power distribution substation in the simulation experiment of the present invention;

[0080] Figure 3 The photovoltaic output power curve diagram of the simulation experiment of the present invention;

[0081] Figure 4 It is a DSA load rate curve diagram before shutdown of the simulation experiment of the present invention;

[0082] Figure 5 This is a comparison result diagram of the comprehensive losses of the simulation experiment of the present invention. DETAILED DESCRIPTION

[0083] In the description of the present invention, it should be noted that the terms "upper", "lower", "inside", "outside", etc. indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, or the positions or positional relationships in which the invented product is usually placed when in use. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. In the description of the present invention, it should also be noted that, unless otherwise clearly specified and limited, the terms "setting", "installation", and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be a connection between the two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0084] The embodiments of the present invention are described in detail below in conjunction with the accompanying drawings.

[0085] like Figure 1 As shown, a method for optimizing the dispatching of a flexible interconnection system of a distribution substation includes the following steps:

[0086] S1. Construct a flexible interconnection system of distribution substations based on multi-terminal soft switch model control;

[0087] Step S1 specifically includes the following steps:

[0088] S11. Construct a flexible interconnection system for distribution substations. The flexible interconnection system for distribution substations includes multiple distribution areas, and two adjacent distribution areas are interconnected via a DC bus. The DC bus is also connected to the energy storage system ESS and the DC / AC converter. Each distribution area includes a power input terminal, a distribution transformer and a VSC (back-to-back converter voltage source converter). The power input terminal is connected in series with the distribution transformer and the VSC in sequence via an AC side line.

[0089] S12. Control the flexible interconnection system of distribution substations using a multi-terminal soft switch model: The multi-terminal soft switch model adopts a master-slave control mode. At this time, one of the multiple VSCs in step S1 is used as a master port, and the remaining VSCs are used as slave ports. The master port is used to adjust the DC voltage of the flexible interconnection system of distribution substations, and the slave port is used to manage the current power output.

[0090] Sinusoidal pulse width modulation is assumed, and the DC bus voltage and AC side line voltage of the main port VSC have the following cross relationship:

[0091]

[0092] Where U ac,trepresents the AC line voltage; μ represents the utilization rate of the DC voltage, which is 0.866 in this embodiment; M represents the VSC modeling system, with an upper limit of 1 and a lower limit of 0; U dc,t Indicates the DC bus voltage;

[0093] The DC side voltage of the slave port VSC satisfies the following constraints:

[0094]

[0095] Where U dc,t Represents the DC side voltage of the slave port VSC; They respectively represent the minimum and maximum values ​​of the DC side voltage of the slave port VSC.

[0096] S2, considering the distribution substation flexible interconnection system based on the multi-terminal soft switch model control constructed in step S1 and the transformer load balance rate, constructing a flexible interconnection system optimization model;

[0097] Step S2 specifically includes the following steps:

[0098] S21. In actual engineering applications, the nighttime load of the distribution system is generally low. In this case, if all distribution transformers are still put into operation, the loss rate of the distribution transformers will increase due to the light load, which will lead to a decrease in the economic benefits of the entire interconnected system. Therefore, an offline strategy is defined: the load rates of the distribution transformers in multiple distribution areas are sorted from large to small, and the distribution transformer with the smallest load rate is turned off during the load valley period to optimize the comprehensive loss of the distribution network;

[0099] S22. Constructing a flexible interconnection system optimization model:

[0100] S221. Considering the balance index of the distribution transformer load rate and the operating loss index of the distribution substation flexible interconnection system, the weighted minimum method is used to optimize the target:

[0101] minf=λ1f LB,t +λ2f s,t (3);

[0102] Where minf represents the minimization objective function; λ1 and λ2 represent the weights of the balance index of the distribution transformer load factor and the operating loss index of the distribution substation flexible interconnection system, respectively; f LB,t Indicates the balance index of the distribution transformer load factor; f s,t It represents the operating loss index of the flexible interconnection system of distribution substations, which is the total loss of the flexible interconnection system of distribution substations;

[0103] in,

[0104]

[0105] In the formula, β i (t) represents the composite factor of the i-th distribution transformer; n represents the total number of distribution transformers; t represents the time period; T represents the number of time periods; I ij represents the current flowing through branch ij; r ij represents the equivalent resistance of the current flowing through branch ij; η1 represents the efficiency of the power transmission port; represents the active power transfer of the ith distribution transformer at time t; P tran,loss represents the transmission power loss; ω1 represents the shared energy storage operation loss coefficient, in this embodiment, ω1=0.05; P ess,t represents the total charging and discharging power of the shared energy storage; ω2 represents the abandoned light loss coefficient, and in this embodiment, ω2=0.1; Indicates the rated installed capacity of photovoltaic power; Indicates the actual capacity of grid-connected photovoltaic power generation;

[0106] S222. Calculate the operating loss index f of the distribution transformer s,t :

[0107] f s,t =F s,t / F max (6);

[0108] In the formula, F max Indicates the maximum operating loss of the distribution transformer.

[0109] The objectives described in step S221 include reducing network losses and compensation rates.

[0110] S3,constraints for constructing the optimization model of flexible interconnection system;

[0111] The constraints described in step S3 include VSC constraints, distribution transformer constraints, photovoltaic power generation and energy storage constraints, AC side constraints and DC side constraints;

[0112] Among them, the VSC constraint is: the power transmission of the VSC port meets the port capacity constraint:

[0113]

[0114] In the formula, Ω FID Represents the set of all ports of the flexible interconnection system of distribution substations; ΔS FID,i represents the power change of the ith distribution transformer in the flexible interconnection system of distribution substations; S FID,loss,i represents the power loss of the i-th distribution transformer in the flexible interconnection system of distribution substations; Indicates a fixed proportional coefficient; P FID,i , Q FID,iRespectively represent the active power and reactive power flowing through the DC / AC converter;

[0115] Distribution transformer constraints include operation range constraints, distribution transformer offline constraints under load valley conditions, and load balancing constraints;

[0116] The operation interval constraint expression is as follows:

[0117]

[0118] In the formula, β i represents the load rate of the ith distribution transformer; P t,i represents the active load borne by the ith distribution transformer at time t; S i represents the rated capacity of the i-th distribution transformer; Indicates the power factor of the distribution transformer;

[0119] The offline constraint expression of distribution transformer under load valley condition is as follows:

[0120]

[0121] In the formula, α i Represents the judgment index of the i-th distribution transformer, when α i ≥1, the i-th distribution transformer is online, when α i <1, the i-th distribution transformer is offline; represents the AC side load of the i-th distribution transformer; Indicates the DC bus side load; S1+S2+…S i-1 +S i+1 …S n Indicates the rated capacity of the remaining distribution transformers except the offline distribution transformer;

[0122] The load balancing constraint expression is as follows:

[0123]

[0124] Where n T,t 、n T,t+1 represents the operating status of the distribution transformer at time t and time t+1 under the offline strategy; N represents the number of soft switches in the flexible interconnection system of the distribution substation;

[0125] The photovoltaic power generation and energy storage constraint expressions are as follows:

[0126] F n +F BL.ch ≤R F,n +F BL,dis (12);

[0127] Pn +F BL,ch ≤R n +F BL,dis (13);

[0128]

[0129]

[0130] In the formula, F n Indicates the user's load in charging mode; F BL.ch Represents the load of the energy storage system ESS in charging mode; R F,n Indicates the line capacity of the user in charging mode; F BL,dis represents the load of the energy storage system ESS in the discharge mode; P n Indicates the user's load in discharge mode; R n Indicates the line capacity of the user in the discharge mode; Represents the charge state variable of the energy storage system ESS; Represents the discharge state variable of the energy storage system ESS; F BL,ch,max Indicates the maximum charging power of the energy storage system ESS; F BL,dis,max Indicates the maximum discharge power of the energy storage system ESS; H l Indicates the actual discharge time of the energy storage system ESS; H l,max Indicates the maximum sustainable discharge time of the energy storage system ESS;

[0131] In this embodiment, and It indicates the charging status; and It indicates the discharge state.

[0132] AC side constraints include node voltage and active and reactive power constraints based on power flow calculation, node voltage and branch current constraints;

[0133] The node voltage and active and reactive power constraint expressions based on power flow calculation are as follows:

[0134]

[0135] Where U t,j , U t,o They represent the voltages of node j and node o at time t respectively; R oj represents the resistance of the node branch oj; P t,oj represents the active power flowing out of the node branch oj at time t; X oj represents the reactance of the node branch oj; Q t,ojrepresents the reactive power flowing out from the node branch oj at time t; I t,0j represents the current of the node branch oj at time t; Ω b represents the set of all branch nodes; P t,jh represents the active power flowing out of the node branch jh at time t; P t,j represents the active power injected into node j at time t; Q t,jh represents the reactive power flowing out from the node branch jh at time t; Q t,j represents the reactive power injected into node j at time t; represents the active power injected into node j by the distributed generator connected to node j at time t; represents the active power injected into node j by the soft switch at time t; represents the active power consumed by the load at node j at time t; represents the reactive power injected into node j by the distributed generator connected to node j at time t; represents the reactive power injected into node j by the soft switch at time t; It represents the reactive power consumed by the load at node j at time t.

[0136] The node voltage and branch current constraint expressions are as follows:

[0137]

[0138] In the formula, U and They represent the minimum and maximum allowable voltages of node o under online operation; U t,o Indicates the voltage of node o in online operation; Indicates the maximum allowable branch current.

[0139] DC side constraints: The DC side node current sequence expression is as follows:

[0140]

[0141] Where, M represents the DC side node set; It represents the power transmitted to the DC side of the distribution transformer through the VSC; Indicates the power generated by the DC side distribution power supply; Indicates the charging and discharging power of the DC side energy storage system ESS; Indicates the power required by the DC side load; represents the voltage of the DC side node m; represents the conductance of the DC side node branch mq at time t; Represents the voltage at the DC side node q.

[0142] S4. Optimize the flexible interconnection system of distribution substations using the flexible interconnection system optimization model constrained by step S3.

[0143] Simulation experiment

[0144] This simulation experiment uses Figure 2 The flexible interconnection system of distribution substations shown in the figure includes three 110 / 10kV transformers (DSA1, DSA2, and DSA3, where DSA1 is an industrial area with a relatively high and stable load; DSA2 is an administrative area with a low load ratio; and DSA3 is a residential area with a load that changes with the living habits of residents, with obvious peaks in the afternoon and evening.) and 18 load nodes (distributed photovoltaics with capacities of 80kW, 120kW, and 200kW are connected at nodes 3, 6, and 17, respectively, and their 24-hour segmented output ratios are as follows: Figure 3 shown).

[0145] like Figure 4 As shown, during most of the day (from 7:00-22:00), it can be clearly seen that the transformers of DSA1 and DSA3 are operating under overload conditions (load factor greater than 0.7), while the transformer of DSA2 is mostly operating under underload conditions, so DSA2 is chosen to be shut down.

[0146] The result is as follows Figure 5 As shown, the results show that compared with the traditional non-interconnected power grid, the comprehensive loss of the distribution network after the flexible interconnection technology described in the present invention is greatly reduced, thereby verifying the effectiveness of the present invention.

[0147] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for optimizing the dispatching of a flexible interconnection system of a distribution substation, characterized by: The following steps are involved: S1. Construct a flexible interconnection system of distribution substations based on multi-terminal soft switch model control; S2, considering the distribution substation flexible interconnection system based on the multi-terminal soft switch model control constructed in step S1 and the transformer load balance rate, constructing a flexible interconnection system optimization model; S3,constraints for constructing the optimization model of flexible interconnection system; S4. Optimize the flexible interconnection system of distribution substations using the flexible interconnection system optimization model constrained by step S3.

2. The method for optimizing and dispatching a flexible interconnection system of a distribution substation according to claim 1, characterized in that: Step S1 specifically includes the following steps: S11. Construct a flexible interconnection system for distribution substations. The flexible interconnection system for distribution substations includes multiple distribution areas, and two adjacent distribution areas are interconnected via a DC bus. The DC bus is also connected to the energy storage system ESS and the DC / AC converter. Each distribution area includes a power input terminal, a distribution transformer and a VSC. The power input terminal is connected in series with the distribution transformer and the VSC in turn via an AC side line. S12. Control the flexible interconnection system of distribution substations using a multi-terminal soft switch model: The multi-terminal soft switch model adopts a master-slave control mode. At this time, one of the multiple VSCs in step S1 is used as a master port, and the remaining VSCs are used as slave ports. The master port is used to adjust the DC voltage of the flexible interconnection system of distribution substations, and the slave port is used to manage the current power output. Sinusoidal pulse width modulation is assumed, and the DC bus voltage and AC side line voltage of the main port VSC have the following cross relationship: Where U ac,t represents the AC line voltage; μ represents the utilization rate of the DC voltage; M represents the VSC modeling system; U dc,t Indicates the DC bus voltage; The DC side voltage of the slave port VSC satisfies the following constraints: Where U dc,t Represents the DC side voltage of the slave port VSC; They respectively represent the minimum and maximum values ​​of the DC side voltage of the slave port VSC.

3. The method for optimizing and dispatching a flexible interconnection system of a distribution substation according to claim 2, characterized in that: Step S2 specifically includes the following steps: S21. Define an offline strategy: sort the load rates of distribution transformers in multiple distribution areas from large to small, and shut down the distribution transformer with the smallest load rate during the load valley period to optimize the comprehensive loss of the distribution network; S22. Constructing a flexible interconnection system optimization model: S221. Considering the balance index of the distribution transformer load rate and the operating loss index of the distribution substation flexible interconnection system, the weighted minimum method is used to optimize the target: minf=λ1f LB,t +λ2f s,t (3); Where minf represents the minimization objective function; λ1 and λ2 represent the weights of the balance index of the distribution transformer load factor and the operating loss index of the distribution substation flexible interconnection system, respectively; f LB,t Indicates the balance index of the distribution transformer load factor; f s,t It represents the operating loss index of the flexible interconnection system of distribution substations, which is the total loss of the flexible interconnection system of distribution substations; in, In the formula, β i (t) represents the composite factor of the i-th distribution transformer; n represents the total number of distribution transformers; t represents the time period; T represents the number of time periods; I ij represents the current flowing through branch ij; r ij represents the equivalent resistance of the current flowing through branch ij; η1 represents the efficiency of the power transmission port; represents the active power transfer of the ith distribution transformer at time t; P tran,loss represents the transmission power loss; ω1 represents the shared energy storage operation loss coefficient; P ess,t represents the total charging and discharging power of the shared energy storage; ω2 represents the abandoned light loss coefficient; Indicates the rated installed capacity of photovoltaic power; Indicates the actual capacity of grid-connected photovoltaic power generation; S222. Calculate the operating loss index f of the distribution transformer s,t : f s,t =F s,t / F max (6); In the formula, F max Indicates the maximum operating loss of the distribution transformer.

4. The method for optimizing and dispatching a flexible interconnection system of a distribution substation according to claim 3, characterized in that: The objectives described in step S221 include reducing network losses and compensation rates.

5. The method for optimizing and dispatching a flexible interconnection system of a distribution substation according to claim 4, characterized in that: The constraints described in step S3 include VSC constraints, distribution transformer constraints, photovoltaic power generation and energy storage constraints, AC side constraints and DC side constraints; Among them, the VSC constraint is: the power transmission of the VSC port meets the port capacity constraint: In the formula, Ω FID Represents the set of all ports of the flexible interconnection system of distribution substations; ΔS FID,i represents the power change of the ith distribution transformer in the flexible interconnection system of distribution substations; S FID,loss,i represents the power loss of the i-th distribution transformer in the flexible interconnection system of distribution substations; Indicates a fixed proportional coefficient; P FID,i , Q FID,i Respectively represent the active power and reactive power flowing through the DC / AC converter; Distribution transformer constraints include operation range constraints, distribution transformer offline constraints under load valley conditions, and load balancing constraints; The operation interval constraint expression is as follows: In the formula, β i represents the load rate of the ith distribution transformer; P t,i represents the active load borne by the ith distribution transformer at time t; S i represents the rated capacity of the i-th distribution transformer; Indicates the power factor of the distribution transformer; The offline constraint expression of distribution transformer under load valley condition is as follows: In the formula, α i Represents the judgment index of the i-th distribution transformer, when α i ≥1, the i-th distribution transformer is online, when α i <1, the i-th distribution transformer is offline; represents the AC side load of the i-th distribution transformer; Indicates the DC bus side load; S1+S2+…S i-1 +S i+1 …S n Indicates the rated capacity of the remaining distribution transformers except the offline distribution transformer; The load balancing constraint expression is as follows: Where n T,t 、n T,t+1 represents the operating status of the distribution transformer at time t and time t+1 under the offline strategy; N represents the number of soft switches in the flexible interconnection system of the distribution substation; The photovoltaic power generation and energy storage constraint expressions are as follows: F n +F BL.ch ≤R F,n +F BL,dis (12); P n +F BL,ch ≤R n +F BL,dis (13); H l ≤H l,max (15); In the formula, F n Indicates the user's load in charging mode; F BL.ch Represents the load of the energy storage system ESS in charging mode; R F,n Indicates the line capacity of the user in charging mode; F BL,dis represents the load of the energy storage system ESS in the discharge mode; P n Indicates the user's load in discharge mode; R n Indicates the line capacity of the user in the discharge mode; Represents the charge state variable of the energy storage system ESS; Represents the discharge state variable of the energy storage system ESS; F BL,ch,max Indicates the maximum charging power of the energy storage system ESS; F BL,dis,max Indicates the maximum discharge power of the energy storage system ESS; H l Indicates the actual discharge time of the energy storage system ESS; H l,max Indicates the maximum sustainable discharge time of the energy storage system ESS; AC side constraints include node voltage and active and reactive power constraints based on power flow calculation, node voltage and branch current constraints; The node voltage and active and reactive power constraint expressions based on power flow calculation are as follows: Where U t,j , U t,o They represent the voltages of node j and node o at time t respectively; R oj represents the resistance of the node branch oj; P t,oj represents the active power flowing out of the node branch oj at time t; X oj represents the reactance of the node branch oj; Q t,oj represents the reactive power flowing out from the node branch oj at time t; I t,0j represents the current of the node branch oj at time t; Ω b represents the set of all branch nodes; P t,jh represents the active power flowing out of the node branch jh at time t; P t,j represents the active power injected into node j at time t; Q t,jh represents the reactive power flowing out from the node branch jh at time t; Q t,j represents the reactive power injected into node j at time t; represents the active power injected into node j by the distributed generator connected to node j at time t; represents the active power injected into node j by the soft switch at time t; represents the active power consumed by the load at node j at time t; represents the reactive power injected into node j by the distributed generator connected to node j at time t; represents the reactive power injected into node j by the soft switch at time t; It represents the reactive power consumed by the load at node j at time t.

6. A method for optimizing and dispatching a flexible interconnection system of a distribution substation according to claim 5, characterized in that: The node voltage and branch current constraint expressions are as follows: In the formula, U and They represent the minimum and maximum allowable voltages of node o under online operation; U t,o Indicates the voltage of node o in online operation; Indicates the maximum allowable branch current.

7. A method for optimizing and dispatching a flexible interconnection system of a distribution substation according to claim 6, characterized in that: DC side constraints: The DC side node current sequence expression is as follows: Where, M represents the DC side node set; It represents the power transmitted to the DC side of the distribution transformer through the VSC; Indicates the power generated by the DC side distribution power supply; Indicates the charging and discharging power of the DC side energy storage system ESS; Indicates the power required by the DC side load; represents the voltage of the DC side node m; represents the conductance of the DC side node branch mq at time t; Represents the voltage at the DC side node q.

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