Medium and low voltage distribution network collaborative optimization operation method considering flexible interconnection transformer area

Abstract

The invention relates to a medium and low voltage distribution network collaborative optimization operation method considering a flexible interconnection zone area, and aims at the operation problem of a medium and low voltage distribution network containing the flexible interconnection zone area, a flexible interconnection zone area flexibility area constraint and a flexible interconnection zone area correlation power upper and lower limit constraint are introduced, and a medium and low voltage collaborative optimization framework is provided. According to the method, the flexible area constraint and the correlation power upper and lower limit constraint of each flexible interconnection area are calculated, the medium-voltage power distribution network adopts a centralized method to solve and issue the area control instruction, the flexible interconnection area adopts a distributed method to solve, and the distributed optimization operation of the flexible interconnection area is realized through boundary information interaction and parameter updating. And the economic operation problem of the medium and low voltage distribution network containing the flexible interconnection transformer area is solved. According to the method, distributed operation of the flexible interconnection transformer area and collaborative optimization of the medium and low voltage power distribution network can be realized under the condition that the transformer area flexible interconnection device is accessed to the transformer area, and economic operation of the power distribution network is ensured.

Description

Technical Field

[0001] The present invention belongs to the technical field of power grid control, and in particular to a method for collaboratively optimizing operation of medium and low voltage distribution networks taking into account flexible interconnected substations. Background Art

[0002] With the high proportion of distributed power sources and power electronic equipment connected, the distribution network urgently needs more complete operation control. The output of distributed power sources is volatile and time-sensitive. The output of wind turbines is affected by seasonal wind speed changes, and the output of photovoltaics is affected by weather and light. The access of power electronic devices realizes the interconnection between different distribution networks, forming a new flexible interconnected distribution network and flexible interconnected substations. However, how to quantify the equivalent of the flexible interconnected substations on the medium-voltage side distribution network and how to describe the relationship between the flexible interconnected substations are urgent problems to be solved.

[0003] With the rapid development of power electronic devices, intelligent soft switches are connected to the medium-voltage distribution network to form a new type of flexible distribution network. On the low-voltage side of the distribution network, the flexible interconnection device of the substation is connected to form a flexible interconnected substation. As a flexible interconnection device between substations, the substation flexible interconnection device can perform fine power control, systematically adjust the flow distribution, provide power mutual assistance, and realize the difference-free adjustment of power transmission within the capacity range of the substation itself. The substation flexible interconnection device has two terminals, three terminals, or even more ports, which can meet the flexible interconnection needs in the power supply scenario of multiple substations.

[0004] The distribution network contains a large number of flexible devices, including energy storage-like devices represented by energy storage, electric vehicles, and controllable loads, generator-like devices represented by distributed power sources and micro generators, and power electronic devices represented by flexible interconnection devices in substations. Flexible devices have corresponding flexibility areas, and substations containing flexible devices also have certain flexibility areas. By calculating the flexibility areas of controllable loads, distributed power sources, and flexible interconnection devices in substations, the flexibility areas of substations can be obtained.

[0005] The methods currently used for distribution network operation optimization are mainly divided into centralized methods and distributed methods. The centralized method is that the medium-voltage distribution network collects information from the low-voltage distribution network and performs calculations to obtain the operation results and send them to the low-voltage side. The results have the characteristics of global optimization. However, large-scale data transmission requires high communication capacity, and the transmission process also faces risks such as bad data. The distributed method divides the distribution network into several sub-areas, thereby decomposing complex large-scale computing problems into parallel small-scale problems. It has the advantages of reducing data communication and computing volume, achieving partition autonomy, and greatly alleviating the computing and data transmission pressure of the medium-voltage distribution network. Distributed optimization obtains an approximate global optimal solution by coordinating the boundary information between adjacent areas. Compared with the centralized method, the distributed method is slightly inferior in computing accuracy and speed, but it eliminates the process of data transmission and can significantly reduce the communication burden between distribution networks.

[0006] Therefore, a coordinated optimization operation method for medium and low voltage distribution networks considering flexible interconnected substations is proposed, which provides a new idea for the operation of medium and low voltage distribution networks containing flexible interconnected substations and helps to achieve the economic operation of distribution networks. Summary of the invention

[0007] The purpose of the present invention is to overcome the shortcomings of the prior art and propose a method for coordinated optimization operation of medium and low voltage distribution networks taking into account flexible interconnected substations. The method can realize the distributed operation of flexible interconnected substations and the coordinated optimization of medium and low voltage distribution networks while taking into account the access of flexible interconnected devices to the substations, thereby ensuring the economic operation of the distribution network.

[0008] The present invention solves the technical problem by adopting the following technical solutions:

[0009] The method for coordinated optimization operation of medium and low voltage distribution networks considering flexible interconnected substations includes the following steps:

[0010] Step 1: For the selected distribution network with flexible interconnected substations, input the parameter information of the distribution network, including network topology connection relationship, line resistance parameters, line reactance parameters, medium voltage node and low voltage substation load parameters, distributed power access location and capacity, substation flexible interconnection device access location and capacity, substation flexible equipment parameters and source-load prediction information;

[0011] Step 2: Calculate the flexibility area constraints of each flexible interconnection area according to the parameter information input in step 1;

[0012] Step 3: For each flexible interconnection area, calculate the upper and lower limit constraints of the power correlation of the flexible interconnection area;

[0013] Step 4: Establish a centralized optimization model for the medium-voltage distribution network. The objective function is set to minimize the sum of the medium-voltage network loss cost and the medium-voltage node voltage over-limit penalty cost. The constraints considered include: medium-voltage distribution network flow constraints, medium-voltage distribution network safe operation constraints, medium-voltage distribution network distributed power constraints, flexible interconnected area flexibility constraints, and flexible interconnected area correlation power upper and lower limit constraints.

[0014] Step 5: The medium-voltage distribution network performs centralized optimization to solve the day-ahead problem, and obtains the day-ahead optimization operation strategy of the medium-voltage distribution network. The day-ahead optimization operation strategy of the medium-voltage distribution network includes control instructions for the flexible interconnected substations, and the instructions include the voltage reference value of the grid connection point of each substation. Active power reference value Reactive power reference value

[0015] Step 6: Based on the voltage reference value of each flexible interconnection area grid connection point Reactive power reference value Generate the day-ahead voltage-reactive adaptive regulation constraints for each substation;

[0016] Step 7: During the current day's operation optimization period, each flexible interconnected substation control device measures the substation grid connection point voltage, and calculates the substation reactive power output target value according to the substation voltage-reactive power adaptive adjustment constraint of the corresponding period generated a few days ago;

[0017] Step 8. Establish a distributed optimization model for each flexible interconnected substation during the current intraday operation optimization period. The objective function is set to minimize the sum of the substation power offset penalty cost, the substation node voltage over-limit penalty cost, and the substation low-voltage network loss cost. The constraints considered include: substation low-voltage network flow constraints, substation safe operation constraints, substation distributed power constraints, and substation flexible interconnection device constraints; set the initial value of the iteration, including the step size τ, the quadratic penalty term coefficient ρ, and the Lagrange multiplier λ. j,i , the number of iterations r = 0;

[0018] Step 9: Each flexible interconnected substation performs the optimization solution of the distributed optimization model of each flexible interconnected substation during the rth operation optimization period of the current day, and exchanges boundary information with adjacent substations;

[0019] Step 10: determine whether the boundary information error of the rth iteration satisfies the preset convergence accuracy ε; if so, output the optimization result; otherwise, update the interval interaction parameter and the number of iterations r=r+1, and return to step 9;

[0020] Step 11: Determine whether the intraday optimization period has reached the maximum value. If so, the calculation ends; otherwise, proceed to the next intraday optimization period and return to step 7.

[0021] Moreover, the calculation method of the flexibility area constraint of each flexible interconnection area in step 2 is:

[0022]

[0023] In the formula, is the uncontrollable active load of the station area i at time T, is the uncontrollable reactive load of the station area i at time T, is the actual active load of the station area i at time T, is the actual reactive load of the station area i at time T, is the capacity of transformer in area i, and are the minimum and maximum active power of the controllable load connected to the station area i, and are the minimum and maximum reactive power of the controllable load connected to the station area i, and are the minimum and maximum active output of distributed photovoltaic power connected to the grid area i, and are the minimum and maximum reactive power output of distributed photovoltaic connected to the grid area i, and are the minimum and maximum active power of the flexible interconnection device connected to the substation i, and are the minimum and maximum active power of the flexible interconnection device connected to the substation i, respectively; and are the minimum and maximum active load of the station area i at time T, and are the minimum and maximum reactive loads of the station area i at time T, is the flexibility area of ​​station i at time T; N CL,i and N PV,i are the number of controllable loads and distributed photovoltaics in the substation i; T is the day-ahead operation optimization period of the medium-voltage distribution network.

[0024] Moreover, the calculation method of the upper and lower limit constraints of the correlation power of the flexible interconnection area in step 3 is:

[0025]

[0026] In the formula, are the active load values ​​of the stations i and j respectively, are the uncontrollable load values ​​of the stations i and j respectively, are the minimum controllable load power of the stations i and j respectively, are the maximum controllable load power of the stations i and j respectively, are the minimum active output of distributed photovoltaic power in the area i and j, are the maximum active output of distributed photovoltaic power in the area i and j, is the total active load value of the area i and j, It is the correlation area of ​​stations i and j.

[0027] Moreover, the centralized optimization model of the medium voltage distribution network in step 4 is:

[0028] minf=f1+f2 (10)

[0029]

[0030]

[0031] Where, f represents the objective function of the medium voltage distribution network, f1 represents the medium voltage network loss cost, and f2 represents the medium voltage node voltage over-limit penalty cost; U T,i represents the voltage value of node i, is the voltage upper and lower limits, the voltage lower limit is U thr,d , the voltage upper limit is U thr,u , for U T,i thr,d or U t,i >U thr,u In the case of over-limit voltage at the medium voltage node, a penalty fee f2 will be generated; C L and C U is the weight coefficient; l T,ij is the square of the current flowing through line ij, is the resistance value of line ij; N T is the total duration of the optimization run on the previous day;

[0032] The medium voltage distribution network power flow constraint in step 4 is expressed as:

[0033]

[0034] In the formula, and are the resistance and reactance of the medium voltage distribution network line ij, P T,ij and Q T,ij Represents the active power and reactive power transmitted on line ij; P T,j and Q T,j represents the active power and reactive power injected by node j; v T,i and v T,j are the squares of the voltages at nodes i and j, respectively; l T,ij Is the square of the current flowing through line ij; Ω b ​It is a collection of medium voltage distribution network lines;

[0035] The safe operation constraints of the medium voltage distribution network are expressed as:

[0036]

[0037] In the formula, and is the maximum and minimum value of the node voltage at node i; is the maximum current allowed to flow through line ij;

[0038] The distributed generation constraints of the medium voltage distribution network are expressed as:

[0039]

[0040] In the formula, It is the minimum value of the active output of the distributed power source; It is the maximum value of the active output of the distributed power source; and is the active output value and reactive output value of the distributed power source, and are the upper and lower limits of the reactive power output of distributed generation. is the capacity of the distributed generation.

[0041] Moreover, the specific implementation method of step 5 is:

[0042] Convex relaxation is used to transform the linearized medium voltage distribution network power flow constraints into second-order cone constraints:

[0043]

[0044] And the cone constraint deviation norm gap is used as a quantitative indicator:

[0045]

[0046] When the deviation norm gap is the smallest, the second-order cone relaxation meets the accuracy requirements, and the control strategy for the flexible interconnected low-voltage substation issued by the medium-voltage distribution network is obtained, including the voltage reference value of the grid connection point of each substation. Active power reference value Reactive power reference value

[0047] Moreover, the day-ahead voltage-reactive adaptive regulation constraint of each substation in step 6 is:

[0048]

[0049] In the formula, It is the reactive power reference value issued by the medium voltage distribution network. is the voltage reference value issued by the medium voltage distribution network; V i,t is the measured voltage value of station i, Q i,t is the reactive power output target value of the substation i; t is the optimal operation period of the substation during the day.

[0050] Moreover, the distributed optimization model of each flexible interconnected area during the current intraday operation optimization period established in step 8 is:

[0051]

[0052]

[0053] Where N LVN is the number of flexible interconnection areas; f LVN represents the objective function of the flexible interconnection area, Represents the power loss cost, which includes network loss cost and area flexible interconnection device loss cost. Indicates the penalty fee for voltage exceeding the limit at the substation node; l t,ij is the square of the current flowing through line ij, is the power loss value of the flexible interconnection device in the substation area, U t,i is the voltage value at node i, are the upper and lower voltage limits; It represents the active power deviation penalty fee of the flexible interconnection area; It is the actual active power value of the substation. It is the active power reference value issued by the medium voltage distribution network; Indicates the reactive power deviation penalty fee of the flexible interconnection area; is the reactive power value of the actual operation of the substation, Q i,t It is the reactive power output target value of the substation area calculated according to the voltage-reactive power adaptive regulation constraint of the substation area; C LV , C UL , C P , C Q is the weight coefficient;

[0054] The low-voltage network power flow constraint in the station area in step 8 is expressed as:

[0055]

[0056] In the formula, and are the resistance and reactance of the line ij in the substation area, P t,ij and Q t,ij represents the active power and reactive power transmitted on line ij, P t,jh and Q t,jh represents the active power and reactive power transmitted on line jh; Pt,j and Q t,j represents the active power and reactive power injected by node j; v t,i and v t,j are the squares of the voltages at nodes i and j, respectively; l t,ij is the square of the current flowing through line ij; It is a collection of area lines;

[0057] The safe operation constraints of the substation are expressed as:

[0058]

[0059] In the formula, and is the maximum and minimum value of the node voltage at node i; is the maximum current allowed to flow through line ij;

[0060] The distributed power generation constraints in the substation are expressed as:

[0061]

[0062] In the formula, It is the minimum value of the active output of the distributed power source; It is the maximum value of the active output of the distributed power source; and is the active output value and reactive output value of the distributed power source, and are the upper and lower limits of the reactive power output of distributed generation. is the capacity of the distributed generation;

[0063] The constraints of the flexible interconnection device in the station area are expressed as:

[0064]

[0065]

[0066] In the formula, is the active power value transmitted at the port β of the flexible interconnection device in the substation area, is the active power loss value at the port β of the flexible interconnection device in the substation area, is the loss coefficient at the port β of the flexible interconnection device in the station area; and are the minimum and maximum active power at port β of the flexible interconnection device in the substation during period t, respectively; and They are the minimum and maximum reactive power at port β of the flexible interconnection device in the substation during period t, respectively.

[0067] Moreover, the specific implementation method of step 9 is:

[0068] Step 9.1: Set the augmented Lagrangian function of the station area objective function as:

[0069]

[0070] In the formula, is the original objective function, is the augmented Lagrangian function of the objective function, λ j,i is the Lagrange multiplier, ρ is the coefficient of the quadratic penalty term, ρ>0; is the active power transmitted by the flexible interconnection device in the substation j in the rth iteration, is the active power of the adjacent area i transmitted by the flexible interconnection device of the area in the rth iteration;

[0071] Step 9.2, taking the above-mentioned augmented Lagrangian function as the objective function, considering the low-voltage network flow constraints of the substation area, the flexible interconnection device constraints of the substation area, the safe operation constraints of the substation area, and the distributed power supply constraints of the substation area, the second-order cone optimization method is used to solve, and the flexible interconnection substation area network loss cost, the flexible interconnection device loss cost of the substation area, the substation area node voltage value, and the boundary information of the interaction between adjacent substations are obtained; the boundary information of the interaction between adjacent substations includes: the active power transmitted by the flexible interconnection device in the substation area j in the rth iteration Active power of adjacent area i transmitted by the flexible interconnection device of the area in the rth iteration

[0072] Moreover, the specific implementation method of step 10 is:

[0073] Calculate the original residual reflecting the feasibility of the original problem after the rth iteration and the dual residual reflecting the feasibility of the dual problem

[0074]

[0075] In the formula, r represents the number of iterations; is the original residual of the rth iteration; is the dual residual of the rth iteration; is the active power transmitted by the flexible interconnection device in the substation i in the rth iteration; is the active power of the adjacent area i transmitted by the flexible interconnection device of the area in the r-1th iteration;

[0076] To determine whether convergence is achieved, the original residual needs to be calculated and the dual residual Whether the value of is small enough, the iterative convergence criterion is as follows:

[0077]

[0078] Where ε is the given convergence accuracy;

[0079] The update interval interaction parameter is expressed as:

[0080]

[0081]

[0082] In the formula, is the active power transmitted by the flexible interconnection device of the substation j in the r+1th iteration, is the active power transmitted by the flexible interconnection device of the substation i in the r+1th iteration; and They are respectively the active power of the flexible interconnection device in the substation j Lagrange multipliers corresponding to the r+1th and rth iterations; τ is the step size.

[0083] The advantages and positive effects of the present invention are:

[0084] The present invention aims at the operation problem of medium and low voltage distribution networks containing flexible interconnected substations, introduces the flexibility regional constraints of flexible interconnected substations and the upper and lower limit constraints of the correlation power of flexible interconnected substations, proposes a medium and low voltage collaborative optimization framework, and establishes a collaborative optimization operation method for medium and low voltage distribution networks considering flexible interconnected substations. The present invention calculates the flexibility regional constraints and the upper and lower limit constraints of the correlation power of each flexible interconnected substation, and the medium voltage distribution network adopts a centralized method to solve and issue substation control instructions. The flexible interconnected substation adopts a distributed method to solve, and through boundary information interaction and parameter update, the distributed optimization operation of the flexible interconnected substation is realized, and the economic operation problem of the medium and low voltage distribution network containing the flexible interconnected substation is solved. In consideration of the access of the flexible interconnection device of the substation to the substation, the distributed operation of the flexible interconnected substation and the collaborative optimization of the medium and low voltage distribution network can be realized to ensure the economic operation of the distribution network. BRIEF DESCRIPTION OF THE DRAWINGS

[0085] Figure 1 It is a flow chart of the method for coordinated optimization operation of medium and low voltage distribution networks considering flexible interconnected substations of the present invention;

[0086] Figure 2 This is a structural diagram of a medium and low voltage distribution network calculation example including a flexible interconnected substation according to an embodiment of the present invention;

[0087] Figure 3 The load fluctuation curve and the distributed power output curve of the embodiment of the present invention;

[0088] Figure 4 is the voltage reference value of the grid connection point of each flexible interconnection area in the embodiment of the present invention;

[0089] Figure 5 It is a diagram of active power reference value and upper and lower limits of flexibility area in a low voltage area according to an embodiment of the present invention;

[0090] Figure 6 It is a diagram of active power reference value and upper and lower limits of flexibility area of ​​low voltage area 2 in an embodiment of the present invention;

[0091] Figure 7 It is a reactive power reference value and upper and lower limits diagram of the flexibility area in a low voltage area according to an embodiment of the present invention;

[0092] Figure 8 It is a diagram of reactive power reference value and upper and lower limits of flexibility area in the low voltage area according to an embodiment of the present invention;

[0093] Fig. 9 The active power value comparison of the flexible interconnection device in the substation area of ​​the second embodiment and the third embodiment of the present invention is shown;

[0094] Fig.10 It is a comparison of reactive power values ​​of the flexible interconnection devices in the substation area in scheme 2 and scheme 3 of the embodiments of the present invention. DETAILED DESCRIPTION

[0095] The present invention is further described in detail below with reference to the accompanying drawings.

[0096] Considering the coordinated optimization operation method of medium and low voltage distribution networks in flexible interconnected areas, such as Figure 1 As shown, the following steps are included:

[0097] 1) For the selected distribution network with flexible interconnected substations, input the parameter information of the distribution network, including network topology connection relationship, line resistance parameters, line reactance parameters, medium voltage node and low voltage substation load parameters, distributed power access location and capacity, substation flexible interconnection device access location and capacity, substation flexible equipment parameters and other basic system parameter information and source-load prediction information;

[0098] 2) According to the input parameters in step 1), the flexibility area constraints of each flexible interconnection area are calculated; specifically, they are expressed as:

[0099]

[0100]

[0101] In the formula, is the uncontrollable active load of the station area i at time T, is the uncontrollable reactive load of the station area i at time T, is the actual active load of the station area i at time T, is the actual reactive load of the station area i at time T, is the capacity of transformer in area i, and are the minimum and maximum active power of the controllable load connected to the station area i, and are the minimum and maximum reactive power of the controllable load connected to the station area i, and are the minimum and maximum active output of distributed photovoltaic power connected to the grid area i, and are the minimum and maximum reactive power output of distributed photovoltaic connected to the grid area i, and are the minimum and maximum active power of the flexible interconnection device connected to the substation i, and are the minimum and maximum active power of the flexible interconnection device connected to the substation i, respectively; and are the minimum and maximum active load of the station area i at time T, and are the minimum and maximum reactive loads of the station area i at time T, is the flexibility area of ​​station i at time T; N CL,i and N PV,i are the number of controllable loads and distributed photovoltaics in the substation i; T is the day-ahead operation optimization period of the medium-voltage distribution network.

[0102] 3) For each flexible interconnection area, calculate the upper and lower limit constraints of the correlation power of the flexible interconnection area; specifically expressed as:

[0103] In the formula, are the active load values ​​of the stations i and j respectively, are the uncontrollable load values ​​of the stations i and j respectively, are the minimum controllable load power of the stations i and j respectively, are the maximum controllable load power of the area i and j respectively, are the minimum active output of distributed photovoltaic power in the area i and j, are the maximum active output of distributed photovoltaic power in the area i and j, is the total active load value of the area i and j, It is the correlation area of ​​stations i and j.

[0104] 4) A centralized optimization model for the medium-voltage distribution network is established. The objective function is set to minimize the sum of the medium-voltage network loss cost and the medium-voltage node voltage over-limit penalty cost. The constraints considered include: medium-voltage distribution network flow constraints, medium-voltage distribution network safe operation constraints, medium-voltage distribution network distributed power constraints, flexible interconnected area flexibility constraints, and flexible interconnected area correlation power upper and lower limit constraints; specifically expressed as:

[0105] The objective function is specifically expressed as:

[0106] minf=f1+f2 (10)

[0107]

[0108] Where, f represents the objective function of the medium voltage distribution network, f1 represents the medium voltage network loss cost, and f2 represents the medium voltage node voltage over-limit penalty cost; U T,i represents the voltage value of node i, is the voltage upper and lower limits, the voltage lower limit is U thr,d , the voltage upper limit is U thr,u , for U T,i thr,d or U t,i >U thr,u In the case of over-limit voltage at the medium voltage node, a penalty fee f2 will be generated; C L and C U is the weight coefficient; l T,ij is the square of the current flowing through line ij, is the resistance value of line ij; N T is the total duration of the optimization run on the previous day;

[0109] The power flow constraint of medium voltage distribution network is expressed as:

[0110]

[0111] In the formula, and are the resistance and reactance of the medium voltage distribution network line ij, P T,ij and Q T,ij Represents the active power and reactive power transmitted on line ij; P T,j and Q T,j represents the active power and reactive power injected by node j; v T,i and v T,j are the squares of the voltages at nodes i and j, respectively; l T,ij Is the square of the current flowing through line ij; Ω b It is a collection of medium voltage distribution network lines;

[0112] The safe operation constraints of the medium voltage distribution network are expressed as: ​

[0113]

[0114] In the formula, and is the maximum and minimum value of the node voltage at node i; is the maximum current allowed to flow through line ij;

[0115] The distributed generation constraints of the medium voltage distribution network are expressed as:

[0116]

[0117] In the formula, It is the minimum value of the active output of the distributed power source; It is the maximum value of the active output of the distributed power source; and is the active output value and reactive output value of the distributed power source, and are the upper and lower limits of the reactive power output of distributed generation. is the capacity of the distributed generation.

[0118] 5) The medium-voltage distribution network performs centralized optimization solution on the day before, and obtains the optimal operation strategy of the medium-voltage distribution network on the day before, which includes the control instructions for the flexible interconnected substations, and the instructions include the voltage reference value of the grid connection point of each substation. Active power reference value Reactive power reference value Specifically expressed as:

[0119] Convex relaxation is used to transform the linearized formula (16) into a second-order cone constraint:

[0120]

[0121] In order to evaluate the accuracy of convex relaxation, the cone constraint deviation norm gap is used as a quantitative indicator:

[0122]

[0123] When the deviation norm gap is small enough, the second-order cone relaxation is considered to meet the accuracy requirements, and the medium-voltage distribution network issues a control strategy for the flexible interconnected low-voltage substation, including the grid connection point voltage reference value of each substation. Active power reference value Reactive power reference value

[0124] 6) Based on the voltage reference value of each flexible interconnection area grid connection point Reactive power reference value Generate the day-ahead voltage-reactive adaptive regulation constraints of each substation, which can be expressed as:

[0125] The day-ahead voltage-reactive adaptive regulation constraint of the substation is expressed as:

[0126]

[0127] In the formula, It is the reactive power reference value issued by the medium voltage distribution network. is the voltage reference value issued by the medium voltage distribution network; V i,t is the measured voltage value of station i, Q i,t is the reactive power output target value of the substation i; t is the optimal operation period of the substation during the day.

[0128] 7) During the current intraday operation optimization period, each flexible interconnected substation control device measures the substation grid connection point voltage, and calculates the substation reactive power target value based on the substation voltage-reactive power adaptive adjustment constraints of the corresponding period generated a few days ago.

[0129] 8) Establish a distributed optimization model for each flexible interconnected substation during the current intraday operation optimization period. The objective function is set to minimize the sum of the substation power deviation penalty cost, the substation node voltage over-limit penalty cost, and the substation low-voltage network loss cost. The constraints considered include: substation low-voltage network flow constraints, substation safe operation constraints, substation distributed power constraints, and substation flexible interconnection device constraints; set the initial value of the iteration, including the step size τ, the quadratic penalty term coefficient ρ, and the Lagrange multiplier λ j,i , the number of iterations r = 0; specifically expressed as:

[0130] The objective function of the station area is expressed as:

[0131]

[0132] Where N LVN is the number of flexible interconnection areas; f LVN represents the objective function of the flexible interconnection area, Represents the power loss cost, which includes network loss cost and area flexible interconnection device loss cost. Indicates the penalty fee for voltage exceeding the limit at the substation node; l t,ij is the square of the current flowing through line ij, is the power loss value of the flexible interconnection device in the substation area, U t,i is the voltage value at node i, are the upper and lower voltage limits; It represents the active power deviation penalty fee of the flexible interconnection area; It is the actual active power value of the substation. It is the active power reference value issued by the medium voltage distribution network; Indicates the reactive power deviation penalty fee of the flexible interconnection area; is the reactive power value of the actual operation of the substation, Q i,t It is the reactive power output target value of the substation area calculated according to the voltage-reactive power adaptive regulation constraint of the substation area; C LV , C UL , C P , C Q is the weight coefficient.

[0133] The power flow constraint of the low-voltage network in the substation area is expressed as:

[0134]

[0135] In the formula, and are the resistance and reactance of the line ij in the substation area, P t,ij and Q t,ij represents the active power and reactive power transmitted on line ij, P t,jh and Q t,jh represents the active power and reactive power transmitted on line jh; P t,j and Q t,j represents the active power and reactive power injected by node j; v t,i and v t,j are the squares of the voltages at nodes i and j, respectively; l t,ij is the square of the current flowing through line ij; It is a collection of area lines;

[0136] The safe operation constraints of the substation are expressed as:

[0137]

[0138] In the formula, and is the maximum and minimum value of the node voltage at node i; is the maximum current allowed to flow through line ij;

[0139] The distributed power generation constraints in the substation are expressed as:

[0140]

[0141] In the formula, It is the minimum value of the active output of the distributed power source; It is the maximum value of the active output of the distributed power source; and is the active output value and reactive output value of the distributed power source, and are the upper and lower limits of the reactive power output of distributed generation. is the capacity of the distributed generation;

[0142] The constraints of the flexible interconnection device in the station area are expressed as:

[0143]

[0144] In the formula, is the active power value transmitted at the port β of the flexible interconnection device in the substation area, is the active power loss value at the port β of the flexible interconnection device in the substation area, is the loss coefficient at the port β of the flexible interconnection device in the station area; and are the minimum and maximum active power at port β of the flexible interconnection device in the substation during period t, respectively; and They are the minimum and maximum reactive power at port β of the flexible interconnection device in the substation during period t, respectively.

[0145] 9) Each flexible interconnected area performs the rth distributed iterative optimization solution and exchanges boundary information with adjacent areas. The specific steps are as follows:

[0146] (a) The augmented Lagrangian function of the station objective function is set as:

[0147]

[0148] In the formula, is the original objective function, is the augmented Lagrangian function of the objective function, λ j,i is the Lagrange multiplier, ρ is the coefficient of the quadratic penalty term, ρ>0; is the active power transmitted by the flexible interconnection device in the substation j in the rth iteration, is the active power of the adjacent area i transmitted by the flexible interconnection device of the area in the rth iteration;

[0149] (b) Taking the above-mentioned augmented Lagrangian function as the objective function, considering the low-voltage network flow constraints, flexible interconnection device constraints, safe operation constraints, and distributed power constraints in the substation area, the second-order cone optimization method is used to solve the problem, and the loss cost of the flexible interconnection substation network, the loss cost of the flexible interconnection device in the substation area, the voltage value of the node in the substation area, and the boundary information of the interaction between adjacent substations are obtained; the boundary information of the interaction between adjacent substations includes: the active power transmitted by the flexible interconnection device in the substation area j in the rth iteration Active power of adjacent area i transmitted by the flexible interconnection device of the area in the rth iteration

[0150] 10) Determine whether the boundary information error of the rth iteration meets the preset convergence accuracy ε; if so, output the optimization result; otherwise, update the interval interaction parameter and the number of iterations r=r+1, and return to step 9); specifically expressed as:

[0151] Calculate the original residual reflecting the feasibility of the original problem after the rth iteration and the dual residual reflecting the feasibility of the dual problem

[0152]

[0153] In the formula, r represents the number of iterations; is the original residual of the rth iteration; is the dual residual of the rth iteration; is the active power transmitted by the flexible interconnection device in the substation i in the rth iteration; is the active power of the adjacent area i transmitted by the flexible interconnection device of the area in the r-1th iteration;

[0154] To determine whether convergence is achieved, the original residual needs to be calculated and the dual residual Whether the value of is small enough, the iterative convergence criterion is as follows:

[0155]

[0156] Where ε is the given convergence accuracy;

[0157] The update interval interaction parameter is expressed as:

[0158]

[0159] In the formula, is the active power transmitted by the flexible interconnection device of the substation j in the r+1th iteration, is the active power transmitted by the flexible interconnection device of the substation i in the r+1th iteration; is the active power of the flexible interconnection device in the substation j Lagrange multiplier corresponding to the r+1th iteration; τ is the step size.

[0160] 11) Determine whether the intraday optimization period has reached the maximum value. If so, the calculation ends; otherwise, proceed to the next intraday optimization period and return to step 7).

[0161] According to the above-mentioned method for coordinated optimization operation of medium and low voltage distribution networks considering flexible interconnected substations, calculations are performed on a certain distribution network to verify the effect of the present invention.

[0162] The present invention uses an improved IEEE 33-node distribution network example and a flexible interconnected area example to verify the method proposed in the present invention: Figure 2As shown in the figure, the medium-voltage distribution network adopts an improved IEEE 33-node example. On the basis of the standard IEEE 33-node example, two low-voltage areas are added. Area 1 and Area 2 are located on different feeders. The secondary sides of the transformers in the two areas are connected through the flexible interconnection device of the area. The two areas are located at nodes 25 and 29 respectively. The distributed photovoltaic parameters connected to the medium-voltage distribution network are shown in Table 1; the reference voltage of the medium-voltage distribution network is 12.66kV. In the flexible interconnection area example, the active load of area 1 is 70.00kW, the reactive load is 71.40kvar, the active load of area 2 is 83.00kW, the reactive load is 81.60kvar, and the reference voltage of the low-voltage area is 0.4kV. Area 1 is set as a residential load and area 2 is set as an industrial load. The photovoltaic capacity of area 1 is 84kWp, the photovoltaic capacity of area 2 is 60kWp, the total photovoltaic capacity of the flexible interconnection area is 144kWp, and the photovoltaic penetration rate is 94.1%. The specific access location and parameters are shown in Table 2. The capacity of the flexible interconnection device in the substation is 100kVA. The load curve of the substation and the fluctuation curve of the distributed power supply are as follows: Figure 3 As shown. Set the convergence accuracy ε = 1×10 -3 , step length The quadratic penalty coefficient ρ = 0.05, the Lagrange multiplier λ j,i =0, the current number of iterations r=0; the optimization time scale T of the medium voltage distribution network is 1 hour, and the optimization time scale t of the substation is 15 minutes.

[0163] Table 1 Distributed photovoltaic access location and parameters on the medium voltage side

[0164]

[0165] Table 2 Distributed photovoltaic access locations and parameters in the substation area

[0166]

[0167] In order to verify the effectiveness of the proposed method for coordinated optimization of medium and low voltage distribution networks considering flexible interconnected substations, the following three schemes are used for comparative analysis:

[0168] Solution 1: The initial operation state of the distribution network is obtained without considering the access of the flexible interconnection device in the substation area;

[0169] Solution 2: Considering the access of flexible interconnection devices in the substation area, a centralized method is used to obtain the operating status of the distribution network;

[0170] Solution three: Considering the access of flexible interconnection devices in the substation area, a coordinated optimization operation method of medium and low voltage distribution networks considering flexible interconnection substations is adopted to obtain the operating status of the distribution network (the technical solution proposed by the present invention).

[0171] The computer hardware environment for performing optimization calculations is Core i7, with a main frequency of 3.20GHz and a memory of 16GB; the software environment is Windows 10 operating system.

[0172] By adopting the coordinated optimization operation method of medium and low voltage distribution networks considering flexible interconnected substations of the present invention, the substation grid connection point voltage reference value, active power reference value and reactive power reference value of two flexible interconnected substations are obtained; the substation grid connection point voltage reference value is as follows: Figure 4 As shown in the figure, the active power reference values ​​and upper and lower limits of the flexibility area of ​​the two substations are as follows: Figure 5 and Figure 6 As shown in the figure, the reactive power reference value and upper and lower limits of the flexibility area of ​​the two substations are as follows: Figure 7 and Figure 8 Table 3 shows the operation results under the three schemes. It can be seen from Table 3 that in Scheme 1, when the low-voltage substations are not interconnected through the substation flexible interconnection device, even if the installation and use costs and loss costs of the substation flexible interconnection device are saved, the loss cost is still the highest. After the substation flexible interconnection device is connected, the distribution network operation cost and the flexible interconnection substation cost are significantly reduced, and the voltage fluctuation is greatly alleviated, which proves the necessity and economy of flexible interconnection of low-voltage substations through the substation flexible interconnection device. Fig. 9 This is a comparison of the active power values ​​transmitted by the flexible interconnection devices in the substation area in Scheme 2 and Scheme 3. Fig.10 It is a comparison of the reactive power values ​​of the flexible interconnection devices in the substations of Scheme 2 and Scheme 3; after adding the flexible interconnection devices in the substations, the loss costs of the distribution network and the flexible interconnected low-voltage substations in Schemes 2 and 3 are significantly reduced compared with Scheme 1; and although the methods used in Scheme 2 and Scheme 3 are different, the results of the proposed method are generally consistent with the centralized method. While reducing the total loss of the distribution network and the loss of the flexible interconnected substation, the voltage fluctuation problem can be alleviated to a certain extent, which proves the effectiveness of the proposed method.

[0173] Table 3 Operation results of different schemes

[0174]

[0175] To sum up, the method for coordinated optimization operation of medium and low voltage distribution networks considering flexible interconnected substations proposed in the present invention can realize the distributed operation of flexible interconnected substations and coordinated optimization of medium and low voltage distribution networks while considering the access of flexible interconnected devices to the substations, thereby ensuring the economic operation of the distribution network.

[0176] It should be emphasized that the embodiments described in the present invention are illustrative rather than restrictive. Therefore, the present invention includes but is not limited to the embodiments described in the specific implementation manner. Any other implementation manners derived by those skilled in the art based on the technical solution of the present invention also fall within the scope of protection of the present invention.

Claims

1. A method for coordinated optimization operation of medium and low voltage distribution networks considering flexible interconnected substations, characterized in that: The following steps are involved: Step 1: For the selected distribution network with flexible interconnected substations, input the parameter information of the distribution network, including network topology connection relationship, line resistance parameters, line reactance parameters, medium voltage node and low voltage substation load parameters, distributed power access location and capacity, substation flexible interconnection device access location and capacity, substation flexible equipment parameters and source-load prediction information; Step 2: Calculate the flexibility area constraints of each flexible interconnection area according to the parameter information input in step 1; Step 3: For each flexible interconnection area, calculate the upper and lower limit constraints of the power correlation of the flexible interconnection area; Step 4: Establish a centralized optimization model for the medium-voltage distribution network. The objective function is set to minimize the sum of the medium-voltage network loss cost and the medium-voltage node voltage over-limit penalty cost. The constraints considered include: medium-voltage distribution network flow constraints, medium-voltage distribution network safe operation constraints, medium-voltage distribution network distributed power constraints, flexible interconnected area flexibility constraints, and flexible interconnected area correlation power upper and lower limit constraints. Step 5: The medium-voltage distribution network performs centralized optimization solution on the day before, and obtains the day-ahead optimization operation strategy of the medium-voltage distribution network. The day-ahead optimization operation strategy of the medium-voltage distribution network includes control instructions for the flexible interconnected substations, and the instructions include the voltage reference value of the grid connection point of each substation. Active power reference value Reactive power reference value Step 6: Based on the voltage reference value of each flexible interconnection area grid connection point Reactive power reference value Generate the day-ahead voltage-reactive adaptive regulation constraints for each substation; Step 7: During the current day's operation optimization period, each flexible interconnected substation control device measures the substation grid connection point voltage, and calculates the substation reactive power output target value according to the substation voltage-reactive power adaptive adjustment constraint of the corresponding period generated a few days ago; Step 8. Establish a distributed optimization model for each flexible interconnected substation during the current intraday operation optimization period. The objective function is set to minimize the sum of the substation power offset penalty cost, the substation node voltage over-limit penalty cost, and the substation low-voltage network loss cost. The constraints considered include: substation low-voltage network flow constraints, substation safe operation constraints, substation distributed power constraints, and substation flexible interconnection device constraints; set the initial value of the iteration, including the step size τ, the quadratic penalty term coefficient ρ, and the Lagrange multiplier λ j,i , the number of iterations r = 0; Step 9: Each flexible interconnected substation performs the optimization solution of the distributed optimization model of each flexible interconnected substation during the rth operation optimization period of the current day, and exchanges boundary information with adjacent substations; Step 10: determine whether the boundary information error of the rth iteration satisfies the preset convergence accuracy ε; if so, output the optimization result; otherwise, update the interval interaction parameter and the number of iterations r=r+1, and return to step 9; Step 11: Determine whether the intraday optimization period has reached the maximum value. If so, the calculation ends; otherwise, proceed to the next intraday optimization period and return to step 7.

2. The method for coordinated optimization operation of medium and low voltage distribution networks considering flexible interconnected substations according to claim 1 is characterized in that: The calculation method of the flexibility area constraint of each flexible interconnection area in step 2 is: In the formula, is the uncontrollable active load of the station area i at time T, is the uncontrollable reactive load of the station area i at time T, is the actual active load of the station area i at time T, is the actual reactive load of the station area i at time T, is the capacity of transformer in area i, and are the minimum and maximum active power of the controllable load connected to the station area i, and are the minimum and maximum reactive power of the controllable load connected to the station area i, and are the minimum and maximum active output of distributed photovoltaic power connected to the grid area i, and are the minimum and maximum reactive power output of distributed photovoltaic connected to the grid area i, and are the minimum and maximum active power of the flexible interconnection device connected to the substation i, and are the minimum and maximum active power of the flexible interconnection device connected to the substation i, respectively; and are the minimum and maximum active load of the station area i at time T, and are the minimum and maximum reactive loads of the station area i at time T, is the flexibility area of ​​station i at time T; N CL,i and N PV,i are the number of controllable loads and distributed photovoltaics in the substation i; T is the day-ahead operation optimization period of the medium-voltage distribution network.

3. The method for coordinated optimization operation of medium and low voltage distribution networks considering flexible interconnected substations according to claim 2 is characterized in that: The calculation method of the upper and lower limits of the power correlation of the flexible interconnection area in step 3 is: In the formula, are the active load values ​​of the stations i and j respectively, are the uncontrollable load values ​​of the stations i and j respectively, are the minimum controllable load power of the stations i and j respectively, are the maximum controllable load power of the stations i and j respectively, are the minimum active output of distributed photovoltaic power in the area i and j, are the maximum active output of distributed photovoltaic power in the area i and j, is the total active load value of the area i and j, It is the correlation area of ​​stations i and j.

4. The method for coordinated optimization operation of medium and low voltage distribution networks considering flexible interconnected substations according to claim 3 is characterized in that: The centralized optimization model of the medium voltage distribution network in step 4 is: min f=f1+f2 (10) Where, f represents the objective function of the medium voltage distribution network, f1 represents the medium voltage network loss cost, and f2 represents the medium voltage node voltage over-limit penalty cost; U T,i represents the voltage value of node i, is the voltage upper and lower limits, the voltage lower limit is U thr,d , the voltage upper limit is U thr,u , for U T,i thr,d or U t,i >U thr,u In the case of over-limit voltage at the medium voltage node, a penalty fee f2 will be generated; C L and C U is the weight coefficient; l T,ij is the square of the current flowing through line ij, is the resistance value of line ij; N T is the total duration of the optimization run on the previous day;​ The medium voltage distribution network power flow constraint in step 4 is expressed as: In the formula, and are the resistance and reactance of the medium voltage distribution network line ij, P T,ij and Q T,ij Represents the active power and reactive power transmitted on line ij; P T,j and Q T,j represents the active power and reactive power injected by node j; v T,i and v T,j are the squares of the voltages at nodes i and j, respectively; l T,ij Is the square of the current flowing through line ij; Ω b It is a collection of medium voltage distribution network lines; The safe operation constraints of the medium voltage distribution network are expressed as: In the formula, and is the maximum and minimum value of the node voltage at node i; is the maximum current allowed to flow through line ij; The distributed generation constraints of the medium voltage distribution network are expressed as: In the formula, It is the minimum value of the active output of the distributed power source; It is the maximum value of the active output of the distributed power source; and is the active output value and reactive output value of the distributed power source, and is the upper and lower limits of the reactive power output of distributed generation. is the capacity of the distributed generation.

5. The method for coordinated optimization operation of medium and low voltage distribution networks considering flexible interconnected substations according to claim 5 is characterized in that: The specific implementation method of step 5 is: Convex relaxation is used to transform the linearized medium voltage distribution network power flow constraints into second-order cone constraints: And the cone constraint deviation norm gap is used as a quantitative indicator: When the deviation norm gap is the smallest, the second-order cone relaxation meets the accuracy requirements, and the control strategy for the flexible interconnected low-voltage substation issued by the medium-voltage distribution network is obtained, including the voltage reference value of the grid connection point of each substation. Active power reference value Reactive power reference value 6. The method for coordinated optimization operation of medium and low voltage distribution networks considering flexible interconnected substations according to claim 5 is characterized in that: The day-ahead voltage-reactive adaptive regulation constraint of each substation in step 6 is: In the formula, It is the reactive power reference value issued by the medium voltage distribution network. is the voltage reference value issued by the medium voltage distribution network; V i,t is the measured voltage value of station i, Q i,t is the reactive power target value of the station area i; t is the daily operation optimization period of the substation.

7. The method for coordinated optimization operation of medium and low voltage distribution networks considering flexible interconnected substations according to claim 6 is characterized in that: In step 8, the distributed optimization model of each flexible interconnected area during the current intraday operation optimization period is established as follows: Where N LVN is the number of flexible interconnection areas; f LVN represents the objective function of the flexible interconnection area, Represents the power loss cost, which includes network loss cost and area flexible interconnection device loss cost. Indicates the penalty fee for over-limit voltage at the substation node; l t,ij is the square of the current flowing through line ij, is the power loss value of the flexible interconnection device in the substation area, U t,i is the voltage value at node i, are the upper and lower voltage limits; It represents the active power deviation penalty fee of the flexible interconnection area; It is the actual active power value of the substation. It is the active power reference value issued by the medium voltage distribution network; Indicates the reactive power deviation penalty fee of the flexible interconnection area; is the reactive power value of the actual operation of the substation, Q i,t It is the reactive power output target value of the substation area calculated according to the voltage-reactive power adaptive regulation constraint of the substation area; C LV , C UL , C P , C Q is the weight coefficient; The low-voltage network power flow constraint in the station area in step 8 is expressed as: In the formula, and are the resistance and reactance of the line ij in the substation area, P t,ij and Q t,ij represents the active power and reactive power transmitted on line ij, P t,jh and Q t,jh represents the active power and reactive power transmitted on line jh; P t,j and Q t,j represents the active power and reactive power injected by node j; v t,i and v t,j are the squares of the voltages at nodes i and j, respectively; l t,ij is the square of the current flowing through line ij; It is a collection of area lines; The safe operation constraints of the substation are expressed as: In the formula, and is the maximum and minimum value of the node voltage at node i; is the maximum current allowed to flow through line ij; The distributed power generation constraints in the substation are expressed as: In the formula, It is the minimum value of the active output of the distributed power source; It is the maximum value of the active output of the distributed power source; and is the active output value and reactive output value of the distributed power source, and is the upper and lower limits of the reactive power output of distributed generation. is the capacity of the distributed generation; The constraints of the flexible interconnection device in the station area are expressed as: In the formula, is the active power value transmitted at the port β of the flexible interconnection device in the substation area, is the active power loss value at the port β of the flexible interconnection device in the substation area, is the loss coefficient at the port β of the flexible interconnection device in the station area; and are the minimum and maximum active power at port β of the flexible interconnection device in the substation during period t, respectively; and They are the minimum and maximum reactive power at port β of the flexible interconnection device in the substation during period t, respectively.

8. The method for coordinated optimization operation of medium and low voltage distribution networks considering flexible interconnected substations according to claim 7 is characterized in that: The specific implementation method of step 9 is: Step 9.1: Set the augmented Lagrangian function of the station area objective function as: In the formula, is the original objective function, is the augmented Lagrangian function of the objective function, λ j,i is the Lagrange multiplier, ρ is the coefficient of the quadratic penalty term, ρ>0; is the active power transmitted by the flexible interconnection device in the substation j in the rth iteration, is the active power of the adjacent area i transmitted by the flexible interconnection device of the area in the rth iteration; Step 9.2, taking the above-mentioned augmented Lagrangian function as the objective function, considering the low-voltage network flow constraints of the substation area, the flexible interconnection device constraints of the substation area, the safe operation constraints of the substation area, and the distributed power supply constraints of the substation area, the second-order cone optimization method is used to solve, and the flexible interconnection substation area network loss cost, the flexible interconnection device loss cost of the substation area, the substation area node voltage value, and the boundary information of the interaction between adjacent substations are obtained; the boundary information of the interaction between adjacent substations includes: the active power transmitted by the flexible interconnection device in the substation area j in the rth iteration Active power of adjacent area i transmitted by the flexible interconnection device of the area in the rth iteration 9. The method for coordinated optimization operation of medium and low voltage distribution networks considering flexible interconnected substations according to claim 8 is characterized in that: The specific implementation method of step 10 is: Calculate the original residual reflecting the feasibility of the original problem after the rth iteration and the dual residual reflecting the feasibility of the dual problem In the formula, r represents the number of iterations; is the original residual of the rth iteration; is the dual residual of the rth iteration; is the active power transmitted by the flexible interconnection device in the substation i in the rth iteration; The active power of the adjacent area i transmitted by the flexible interconnection device of the area in the r-1th iteration; To determine whether convergence is achieved, the original residual needs to be calculated and the dual residual Whether the value of is small enough, the iterative convergence criterion is as follows: Where ε is the given convergence accuracy; The update interval interaction parameter is expressed as: In the formula, is the active power transmitted by the flexible interconnection device of the substation j in the r+1th iteration, is the active power transmitted by the flexible interconnection device of the substation i in the r+1th iteration; and They are respectively the active power of the flexible interconnection device in the substation j Lagrange multipliers corresponding to the r+1th and rth iterations; τ is the step size.

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