Short-circuit current suppression measure comprehensive optimization configuration method considering safety efficiency cost

Abstract

A short-circuit current suppression measure comprehensive optimization configuration method considering safety performance cost comprises the following steps: step 1, considering two aspects of a grid structure and equipment, performing sensitivity analysis on five measures of circuit breaking, bus split operation, current limiting reactor installation, high-impedance transformer replacement and transformer neutral point small reactor installation; a dimension reduction strategy is provided; 2, establishing a comprehensive optimization configuration model considering the safety performance cost; and step 3, solving the optimal configuration model based on an ordinal optimization algorithm. The invention aims to solve the technical problem that the cost is high when equipment is widely applied due to the fact that the traditional short-circuit current suppression measure only considers the improvement of the system safety but neglects the increase of the installation and maintenance cost of the short-circuit current suppression measure.

Description

Technical Field

[0001] The present invention relates to the field of ultra-high voltage technology, and in particular to a method for comprehensively optimizing the configuration of short-circuit current suppression measures taking into account safety efficiency costs. Background Art

[0002] With the rapid development of UHV technology and supporting projects, the receiving-end power grid is becoming increasingly large and complex in structure, resulting in closer electrical connections between grids and rising short-circuit current levels, which poses a huge challenge to the safe and stable operation of the power grid. At present, in areas with high power grid load density, such as Zhejiang and Jiangsu, the problem of excessive short-circuit current has become one of the main problems restricting the development of the power grid, and it is urgent to take reasonable short-circuit current management measures to effectively limit the short-circuit current. Therefore, in order to improve the reliability of power supply of the power grid and ensure its safe operation level, in-depth research on short-circuit current suppression measures has important theoretical significance and practical value.

[0003] The most commonly used idea at present is to adopt a single short-circuit current suppression measure to suppress the short-circuit current according to the operation experience accumulated by the operators in their actual work. The document "Research on the Application of Small Reactance Grounding at the Neutral Point of 500kV Autotransformer" aims at the problem of single-phase short-circuit current exceeding the standard in Hunan power grid. First, the system single-phase short-circuit current is obtained by calculation, and then a single measure of increasing the reactance value of the small reactance at the neutral point of the main transformer is taken for the exceeding site, and a single-phase short-circuit current suppression plan is formulated. The documents "Research on the Power Grid Partitioning Method Based on the Equivalent Model Short-circuit Current Calculation" and "Research on the Short-circuit Current Problem and Current Limiting Measures of UHV Access to the 500kV Power Grid in Beijing, Tianjin and Hebei" analyze the short-circuit current composition factors of the site through the short-circuit point self-impedance model, and propose a short-circuit current suppression measure selected by the exceeding mechanism. In order to more effectively suppress the short-circuit current, the document "Research on the Optimal Configuration of Power Grid Current Limiting Measures" proposes two methods: increasing the impedance of the step-up transformer in the plant and increasing the impedance of the step-down transformer at the receiving end, and analyzes the suppression effect of the short-circuit current in two single directions. However, a single current limiting measure has a narrow scope of application and is only applicable to specific short-circuit situations. In other complex short-circuit situations, a single current limiting measure may find it difficult to deal with multiple faults at the same time, making it lack flexibility when facing different types of short-circuit currents.

[0004] However, traditional short-circuit current suppression measures only consider improving system safety, but ignore the increase in installation and maintenance costs, which may lead to high costs when the equipment is widely used. In power grid planning and operation decisions, it is necessary to control the short-circuit current so that it does not exceed the rated capacity of the equipment, and it is also necessary to maintain a certain short-circuit capacity to ensure that the power grid has sufficient power supply capacity, which makes the control of short-circuit current and maintaining the stability of system operation a contradiction.

[0005] In view of this, how to comprehensively consider safety and equipment investment costs in actual power grid operation to ensure dynamic association in decision-making, and how to comprehensively consider the benefits and costs of short-circuit current suppression measures while ensuring the safe and stable operation of the system, and establishing a comprehensive optimization configuration method for short-circuit current suppression measures that considers safety efficiency costs are issues that need to be urgently addressed. Summary of the invention

[0006] The purpose of the present invention is to solve the problem that traditional short-circuit current suppression measures only consider improving system safety, but ignore the increase in their installation and maintenance costs, which may lead to high costs when the equipment is widely used. In power grid planning and operation decisions, it is necessary to control the short-circuit current so that it does not exceed the rated capacity of the equipment, and it is also necessary to maintain a certain short-circuit capacity to ensure that the power grid has sufficient power supply capacity. This makes the control of short-circuit current and maintaining the stability of system operation a pair of contradictory technical issues. The unified decision-making of current limiting equipment installation and disconnection measures, as well as the relationship between system safety stability, current limiting scheme effect and investment cost are comprehensively considered, and a comprehensive optimization configuration method for short-circuit current suppression measures considering safety efficiency cost is proposed.

[0007] The comprehensive optimization configuration method of short-circuit current suppression measures considering safety efficiency cost includes the following steps:

[0008] Step 1: Considering the grid structure and equipment, sensitivity analysis is performed on five measures: line disconnection, busbar split operation, installation of current limiting reactors, replacement of high-impedance transformers, and installation of small reactors at the neutral point of transformers, in order to propose a dimension reduction strategy;

[0009] Step 2: Establish a comprehensive optimization configuration model considering safety effectiveness cost;

[0010] Step 3: Solve the optimization configuration model based on the sequential optimization algorithm.

[0011] In step 1, from the perspectives of grid structure and equipment, sensitivity analysis is performed on five measures: line disconnection, busbar split operation, installation of current limiting reactors, replacement of high-impedance transformers, and installation of small reactors at the neutral point of transformers, including:

[0012] 1-1: Perform sensitivity analysis on the disconnected line and define the sensitivity of the disconnected line between nodes i and j to the self-impedance of node k as:

[0013]

[0014] Where: Z kk and Z' kk are the self-impedance of node k before and after the line is disconnected, Z ij is the mutual impedance between node i and node j, Z ii, Z jj are the self-impedances of nodes i and j, respectively, ij is the impedance of line ij, Z ki and Z kj are the mutual impedances of nodes i, j and k respectively, △Z kk is the change in self-impedance of node k after the line is disconnected;

[0015]

[0016] Where: N e is the number of sites with excessive short-circuit current; ω k is the weight factor; I k is the short-circuit current of the exceeding site k. The larger short-circuit current value is taken for the site where both the three-phase short-circuit current and the single-phase short-circuit current exceed the standard at the same time; is the maximum breaking current of the circuit breaker;

[0017] For stations where only the three-phase short-circuit current exceeds the standard, Defined as:

[0018]

[0019] For stations where only single-phase short-circuit current exceeds the limit, Defined as:

[0020]

[0021] Where: are the sensitivities calculated based on the positive sequence network and the zero sequence network respectively;

[0022] For stations where both three-phase short-circuit current and single-phase short-circuit current exceed the standard at the same time, The expression corresponding to the larger short-circuit current value is used, as shown in equations (3) and (4); similarly, if the bus connected to the switch in the substation is regarded as an independent bus, and the switch is represented by a small reactance line, then the busbar split operation is equivalent to breaking the line;

[0023] 1-2: Perform sensitivity analysis on busbar split operation. The calculation method of busbar operation sensitivity and weighted sensitivity is similar to that of busbar disconnection.

[0024] 1-3: Sensitivity analysis of the installation of current limiting reactors:

[0025] The sensitivity of adding series reactors to the self-impedance of excessive sites is defined as:

[0026]

[0027] In order to characterize the current limiting effect of adding series reactors on all over-limit stations in the system, the weighted sensitivity is defined as:

[0028]

[0029] Where: γ * k With λ * k The definition is similar; N e is the number of sites with excessive short-circuit current; ω k is the weight factor; I k is the short-circuit current of the exceeding site k. The site where both the three-phase short-circuit current and the single-phase short-circuit current exceed the standard takes the larger short-circuit current value; I k max is the maximum breaking current of the circuit breaker;

[0030] 1-4: Sensitivity analysis of replacing the high impedance transformer. After replacing the high impedance transformer, equation (1) is rewritten as:

[0031]

[0032] Therefore, the sensitivity of replacing the high impedance transformer to the node exceeding the standard is defined as follows:

[0033]

[0034] In order to consider the current limiting level of the overall excessive-standard site in the system after replacing the high-impedance transformer, the weighted sensitivity is defined as:

[0035]

[0036] Where: * k With λ * k The definition is similar; N e ,ω k ,I k ,I k max The meaning is the same as formula (2).

[0037] 1-5: Perform sensitivity analysis on the installation of a small reactance grounding at the neutral point of the transformer. In the autotransformer, adding a small reactance to the neutral point is equivalent to changing the reactance values ​​of the three zero-sequence network branches at the same time. The sensitivity of the small reactance at the neutral point can be obtained by superimposing it in sequence according to the following formula:

[0038]

[0039] Where: N s is the number of nodes with single-phase short-circuit current exceeding the standard; ω k is the weight factor; ε * kI , ε* kII , ε * kIII are the sensitivities of adding series reactances on the three zero-sequence network branches; I k is the single-phase short-circuit current of the exceeding node k; I k max is the maximum breaking current of the circuit breaker; 1 k , ε 0 k are the sensitivities calculated based on the positive sequence network and the zero sequence network respectively;

[0040] 1-6: Establish a dimensionality reduction strategy that takes into account the sensitivity of current limiting measures, and define the comprehensive sensitivity as follows:

[0041]

[0042] Where: N is the number of current limiting measures to be selected for configuration object i. For lines, busbar switches and transformers, N is 2; α * k The sensitivity when the current limiting measure k is selected;

[0043] The dimension reduction strategy considering the sensitivity of current limiting measures is as follows: first, analyze and calculate the sensitivity of each configuration object (line, bus tie switch, transformer) in the system; then select the configuration objects in descending order according to the sensitivity of the comprehensive current limiting measures, and select the top N l Return line, N b Bus tie switches, N t The transformers form a set of dimension-reduced decision variables;

[0044] In addition, in order to ensure the convergence and efficiency of the calculation, the number of configuration objects N in the dimensionality reduction decision process is l 、N b 、N t The value of should be adjusted according to the overall scale of the computing network.

[0045] In step 2, a comprehensive optimization configuration model of short-circuit current suppression measures considering the three indicators of safety, efficiency and cost is proposed, including:

[0046] 2-1: Obtaining the objective function

[0047] Taking the line, bus tie switch, high impedance transformer, transformer neutral point small reactance and series reactor as decision variables, a current limiting optimization configuration model based on SEC is constructed, and its objective function is as follows:

[0048] F(x,y,z,h)=min[S(x,y,z,h),E(x,y,z,h),C(x,y,z,h)] (12)

[0049] In the formula: x is the disconnection of the line and busbar switch; y is whether to replace the high-impedance transformer; z is the neutral point small reactance of the substation main transformer; h is whether the line is equipped with a series reactor; S, E, and C are safety, efficiency, and cost indicators respectively;

[0050] 2-2: Obtaining the safety index S

[0051] The system transient voltage recovery level is used as the safety index S to measure the transient stability of the system under each scheme. The definition of the safety index S is as follows:

[0052]

[0053] Where: tc is the fault removal time; te is the calculation cut-off time, which can be taken as the 10th second after the expected fault j removal time according to the provisions of the power guidelines [3]; Ri,j(t) is defined as shown in formula (14).

[0054]

[0055] Where: Ui,j(t) is the voltage amplitude of bus i at time t after fault j occurs; To simplify the calculation, this paper selects key nodes in the power grid for index calculation;

[0056]

[0057] Where: M is the number of key nodes in the power grid; N is the number of expected faults; ω i is the weight of busbar i.

[0058] 2-3: Obtaining performance indicator E

[0059] The short-circuit current over-limit index E is used as the performance index to characterize the suppression effect after the implementation of different current limiting optimization measures;

[0060]

[0061] Where: Ii is the three-phase short-circuit current of the ith bus; Iimax is the rated breaking current of the ith bus circuit breaker; ki greater than -1 indicates that the line short-circuit current exceeds the standard, and less than -1 indicates that the short-circuit current is equal to the rated breaking current of the circuit breaker; for the convenience of calculation, ki is normalized;

[0062]

[0063] Where: Nb is the busbar set; after processing (17), E is normalized to the interval [0,1]; the smaller ki is, the closer E is to 0, indicating a better current limiting effect; the larger ki is, the closer E is to 1, indicating a worse current limiting effect.

[0064] 2-4: Obtain cost indicator C

[0065] Taking the total investment cost of current limiting measures as the cost indicator, the calculation formula is:

[0066]

[0067] In the formula: Let N s is the total number of current limiting measures taken; u s =1 means that the current limiting measure s is adopted, and 0 means that it is not adopted; k as , k bs is the investment cost coefficient of the current limiting measure s; s is the equipment parameter. For current limiting measures such as disconnecting lines and busbar segmentation, z s Take 0; for adding series reactor, z s is the series reactance value; for replacing a high impedance transformer, z s is the percentage increment of short-circuit voltage; for the small reactance at the neutral point of the transformer, z s is the reactance value;

[0068] 2-5: Establishing constraints

[0069] The constraints of the comprehensive optimization configuration model considering safety effectiveness cost are as follows:

[0070]

[0071] Where: N b is the total number of nodes; is the upper limit of the short-circuit current of node k; N l is the total number of branches; is the upper power limit of branch l; Respectively represent the upper and lower limits of the voltage at node k; They respectively represent the upper and lower limits of the parameter values ​​of the current limiting device.

[0072] In step 3, the optimization configuration model is solved based on the ordinal optimization algorithm, and step 3 includes the following steps:

[0073] Step 3-1: Generate a representation set;

[0074] Input network data and decision variables, use MATLAB to generate a set of solutions, and use PSASP to calculate the power flow and short-circuit current of each solution; then randomly select N current limiting solutions in the feasible domain according to the constraints to determine the characterization set Θ N ;

[0075] Step 3-2: Rough model solution;

[0076] Formula (17) is used as the rough evaluation model, and this model is used to evaluate the set ΘN Make an evaluation, and obtain the descending order of all solutions based on the evaluation results, obtain the OPC curve, and use it to clarify the type of planning problem;

[0077] Step 3-3: Generate a selected set W;

[0078] According to the type of planning problem determined in step 2, refer to formula (20) to select the first s feasible solutions as the selected set W;

[0079]

[0080] Where: s is a function of k and g; Z0, p, m, η are regression parameters, which are determined according to the typical curve type; g is the number of observed good enough solutions, among which there are k real good enough solutions;

[0081] Step 3-4: accurate model solution;

[0082] The short-circuit current and transient voltage recovery data of the scheme are obtained by PSASP calculation, and an accurate evaluation model is constructed based on formula (12). The model is used to accurately evaluate all feasible solutions in the set W to form a Pareto non-dominated solution set;

[0083] Step 3-5: Manual experience screening;

[0084] Use the working experience of operators to screen out unreasonable solutions in actual projects;

[0085] Step 3-6: Normalize the S, E, and C indicators respectively, and then sum them up to get the SEC indicator. Use the SEC indicator to further sort the Pareto non-dominated solution set that meets the actual engineering requirements, and take the solution with the smallest SEC indicator as the optimal solution; its SEC indicator is shown in formula (21).

[0086] SEC(f n )=S * +E * +C * (twenty one)

[0087] Where: S * 、E * , C * is the normalized evaluation value.

[0088] The normalization formula of S, E, and C indicators is shown in formula (22):

[0089]

[0090] Where: α k is the actual value of indicator α, α min and α maxare the minimum and maximum values ​​of the indicator α respectively.

[0091] The method also includes step 4, which proves the feasibility of the optimal solution by analyzing the short-circuit current level and transient voltage stability of the power grid under the optimal configuration scheme of the optimal current limiting measures;

[0092] Step 4-1: Analysis of the short-circuit current suppression effect of the configuration scheme. Compare the short-circuit current after the current limiting measure scheme is implemented with the rated interrupting current of the circuit breaker.

[0093] Step 4-2: Grid transient voltage stability analysis of the configuration scheme. Use PSASP to perform transient stability calculations on the configuration scheme and calculate the transient voltage recovery curve of the busbar at key sites.

[0094] Compared with the prior art, the present invention has the following technical effects:

[0095] 1) The present invention analyzes the sensitivity relationship between the selected current limiting measures and the short-circuit current of the exceeding station, and proposes a dimensionality reduction strategy that takes into account the sensitivity of the current limiting measures;

[0096] 2) The present invention establishes a comprehensive optimization configuration model of short-circuit current suppression measures taking into account the system safety level, and uses a sequence optimization algorithm to solve the optimization configuration model to find the best current limiting measure configuration scheme;

[0097] 3) Compared with the traditional short-circuit current suppression measures configuration method, the present invention can balance the relationship between safety, efficiency and cost, make optimal configuration decisions with the goal of comprehensive optimization, and effectively avoid the waste of current limiting measures in resource allocation;

[0098] 4) Compared with the traditional configuration method that only considers the optimal short-circuit current suppression effect, the current limiting configuration scheme can effectively ensure the current limiting effect while ensuring the safe and stable operation of the power grid; based on an actual power grid example in a certain area of ​​Hunan, the correctness and effectiveness of the proposed method are verified. BRIEF DESCRIPTION OF THE DRAWINGS

[0099] The present invention will be further described below in conjunction with the accompanying drawings and embodiments:

[0100] Figure 1 It is a schematic diagram of equivalent simulation of breaking circuit in the present invention;

[0101] Figure 2 This is a schematic diagram of busbar split operation in the present invention;

[0102] Figure 3 It is the busbar split operation equivalent diagram in the present invention;

[0103] Figure 4 This is a schematic diagram of the equivalent simulation of adding a current limiting reactor in the present invention;

[0104] Figure 5 This is a schematic diagram of the equivalent simulation of replacing a high impedance transformer in the present invention;

[0105] Figure 6 It is a zero-sequence equivalent circuit diagram of the autotransformer with the neutral point directly grounded in the present invention;

[0106] Figure 7 It is a zero-sequence equivalent circuit of an autotransformer in which the neutral point is grounded via a small reactance in the present invention;

[0107] Figure 8 The flowchart of solving the optimization configuration model by the in-order optimization algorithm of the present invention;

[0108] Fig. 9 It is a typical OPC curve diagram in the present invention;

[0109] Fig.10 It is the OPC curve diagram of the in-order optimization algorithm of the present invention;

[0110] Fig.11 A schematic diagram of a Pareto non-dominated solution set of a selected set in the present invention;

[0111] Fig.12 The transient voltage recovery curve of the busbar at the key site obtained by using PSASP to perform temporary stability calculation on Scheme 5. DETAILED DESCRIPTION

[0112] The present invention proposes a distribution network planning method considering distribution system operators and multi-agent games, which specifically includes the following steps:

[0113] Step 1-1: Perform sensitivity analysis on the disconnected line. Define the sensitivity of the disconnected line between nodes i and j to the self-impedance of node k as:

[0114]

[0115] Where: Z kk and Z' kk are the self-impedance of node k before and after the line is disconnected, Z ij is the mutual impedance between node i and node j, z ij is the impedance of line ij, Z ki and Z kj are the mutual impedances of nodes i, j and k respectively, △Z kk is the change in self-impedance of node k after the line is disconnected.

[0116]

[0117] Where: N e is the number of sites with excessive short-circuit current; ωk is the weight factor; I k is the short-circuit current of the exceeding site k. The larger short-circuit current value is taken for the site where both the three-phase short-circuit current and the single-phase short-circuit current exceed the standard at the same time; It is the maximum breaking current of the circuit breaker.

[0118] For stations where only the three-phase short-circuit current exceeds the standard, Defined as:

[0119]

[0120] For stations where only single-phase short-circuit current exceeds the limit, Defined as:

[0121]

[0122] Where: are the sensitivities calculated based on the positive sequence network and the zero sequence network respectively.

[0123] For stations where both three-phase short-circuit current and single-phase short-circuit current exceed the standard at the same time, The expression corresponding to the larger short-circuit current value is used, as shown in equations (3) and (4). Similarly, if the bus connected to the switch in the substation is regarded as an independent bus and the switch is represented by a small reactance line, the bus split operation is equivalent to breaking the line.

[0124] Step 1-2: Perform sensitivity analysis on bus split operation. The calculation method of bus operation sensitivity and weighted sensitivity is similar to that of bus disconnection.

[0125] Step 1-3: Perform sensitivity analysis on the installation of current limiting reactor.

[0126] The sensitivity of adding series reactors to the self-impedance of excessive sites is defined as:

[0127]

[0128] In order to characterize the current limiting effect of adding series reactors on all over-limit stations in the system, the weighted sensitivity is defined as:

[0129]

[0130] Where: γ * k With λ * k The definition is similar; N e is the number of sites with excessive short-circuit current; ω k is the weight factor; I kis the short-circuit current of the exceeding site k. The site where both the three-phase short-circuit current and the single-phase short-circuit current exceed the standard takes the larger short-circuit current value; I k max It is the maximum breaking current of the circuit breaker.

[0131] Step 1-4: Perform sensitivity analysis on the replacement of high impedance transformer. After the high impedance transformer is replaced, equation (1) is rewritten as:

[0132]

[0133] Therefore, the sensitivity of replacing the high impedance transformer to the node exceeding the standard is defined as follows:

[0134]

[0135] In order to consider the current limiting level of the overall over-standard site of the system when replacing the high-impedance transformer, we define the weighted sensitivity as:

[0136]

[0137] Where: * k With λ * k The definition is similar; N e ,ω k ,I k ,I k max The meaning is the same as formula (2).

[0138] Step 1-5: Perform sensitivity analysis on the transformer neutral point with a small reactance grounding. In an autotransformer, adding a small reactance to the neutral point is equivalent to changing the reactance values ​​of the three zero-sequence network branches at the same time. The sensitivity of adding a small reactance to the neutral point can be obtained by superimposing it in sequence as described in the following formula.

[0139]

[0140] Where: N s is the number of nodes with single-phase short-circuit current exceeding the standard; ω k is the weight factor; ε * kI , ε * kII , ε * kIII are the sensitivities of adding series reactances on the three zero-sequence network branches; I k is the single-phase short-circuit current of the exceeding node k; I k max is the maximum breaking current of the circuit breaker; 1 k , ε 0k are the sensitivities calculated based on the positive sequence network and the zero sequence network respectively.

[0141] Step 1-6: Establish a dimensionality reduction strategy that takes into account the sensitivity of the current limiting measures. Define the comprehensive sensitivity as follows:

[0142]

[0143] Where: N is the number of current limiting measures to be selected for configuration object i. For lines, busbar switches and transformers, N is 2; α * k is the sensitivity when the current limiting measure k is selected.

[0144] The dimension reduction strategy considering the sensitivity of current limiting measures is as follows: first, analyze and calculate the sensitivity of each configuration object (line, bus tie switch, transformer) in the system; then select the configuration objects in descending order according to the sensitivity of the comprehensive current limiting measures, and select the top N l Return line, N b Bus tie switches, N t Transformers form a set of dimension reduction decision variables. In addition, in order to ensure the convergence and efficiency of the calculation, the number of configuration objects N in the dimension reduction decision process l 、N b 、N t The value of should be adjusted according to the overall scale of the computing network.

[0145] Step 2-1: Obtain the target function

[0146] Taking the line, bus tie switch, high impedance transformer, transformer neutral point small reactance and series reactor as decision variables, a current limiting optimization configuration model based on SEC is constructed, and its objective function is as follows:

[0147] F(x,y,z,h)=min[S(x,y,z,h),E(x,y,z,h),C(x,y,z,h)] (12)

[0148] In the formula: x is the opening and closing of the line and the busbar switch; y is whether the high-impedance transformer is replaced; z is the small reactance at the neutral point of the main transformer of the substation; h is whether a series reactor is installed on the line; S, E, and C are safety, efficiency, and cost indicators respectively.

[0149] Step 2-2: Safety Index S

[0150] In this section, the system transient voltage recovery level is used as the safety index S to measure the transient stability of the system under each scheme. The definition of the safety index S is as follows;

[0151]

[0152] Where: tc is the fault removal time; te is the calculation cut-off time, which can be taken as the 10th second after the expected fault j removal time according to the provisions of the power guidelines [3]; Ri,j(t) is defined as shown in formula (14).

[0153]

[0154] Where: Ui,j(t) is the voltage amplitude of bus i at time t after fault j occurs. To simplify the calculation, this paper selects key nodes in the power grid for index calculation.

[0155]

[0156] Where: M is the number of key nodes in the power grid; N is the number of expected faults; ω i is the weight of busbar i.

[0157] Step 2-3: Performance Index E

[0158] This section uses the short-circuit current over-limit index E as the performance index to characterize the suppression effect after the implementation of different current limiting optimization measures.

[0159]

[0160] Where: Ii is the three-phase short-circuit current of the ith busbar; Iimax is the rated breaking current of the ith busbar circuit breaker; ki greater than -1 indicates that the line short-circuit current exceeds the standard, and less than -1 indicates that the short-circuit current is equal to the rated breaking current of the circuit breaker. For ease of calculation, ki is normalized below.

[0161]

[0162] Where: Nb is the busbar set; after processing (17), E is normalized to the interval [0,1]. The smaller ki is, the closer E is to 0, indicating a better current limiting effect; the larger ki is, the closer E is to 1, indicating a worse current limiting effect.

[0163] Step 2-4: Cost Indicator C

[0164] This section uses the total investment cost of current limiting measures as the cost indicator, and its calculation formula is:

[0165]

[0166] In the formula: Let N s is the total number of current limiting measures taken; u s =1 means that the current limiting measure s is adopted, and 0 means that it is not adopted; k as , k bs is the investment cost coefficient of the current limiting measure s; sis the equipment parameter. For current limiting measures such as disconnecting lines and busbar segmentation, z s Take 0; for adding series reactor, z s is the series reactance value; for replacing a high impedance transformer, z s is the percentage increment of short-circuit voltage; for the small reactance at the neutral point of the transformer, z s is the reactance value.

[0167] Step 2-5: Establish constraints

[0168] The constraints of the comprehensive optimization configuration model considering safety effectiveness cost are as follows:

[0169]

[0170] Where: N b is the total number of nodes; is the upper limit of the short-circuit current of node k; N l is the total number of branches; is the upper power limit of branch l; Respectively represent the upper and lower limits of the voltage at node k; They respectively represent the upper and lower limits of the parameter values ​​of the current limiting device.

[0171] Step 3-1: Generate a characterization set. Input network data and decision variables, use MATLAB to generate a set of solutions, and use PSASP to calculate the power flow and short-circuit current for each solution; then randomly select N current limiting solutions in the feasible domain according to the constraints to determine the characterization set Θ N .

[0172] Step 3-2: Rough model solution. This paper uses formula (12) as the rough evaluation model and uses this model to evaluate the set Θ N Make an evaluation and arrange all solutions in descending order based on the evaluation results to obtain the OPC curve, and use it to clarify the type of planning problem in this chapter.

[0173] Step 3-3: Generate a selected set W. According to the type of problem to be solved determined in step 2, refer to formula (20) to select the first s feasible solutions as the selected set W.

[0174]

[0175] Where: s is a function of k and g; Z0, p, m, η are regression parameters, which are determined according to the typical curve type; g is the number of observed good enough solutions, among which there are k real good enough solutions.

[0176] Step 3-4: Accurate model solution. The short-circuit current and transient voltage recovery data of the scheme are obtained by PSASP calculation, and an accurate evaluation model is constructed based on equation (12). This model is used to accurately evaluate all feasible solutions in the set W to form a Pareto non-dominated solution set.

[0177] Step 3-5: Manual experience screening: Use the working experience of operators to screen unreasonable solutions in actual projects.

[0178] Step 3-6: Normalize the S, E, and C indicators respectively, and then sum them up to get the SEC indicator. Use the SEC indicator to further sort the Pareto non-dominated solution set that meets the actual engineering situation, and take the solution with the smallest SEC indicator as the optimal solution. Its SEC indicator is shown in formula (21).

[0179] SEC(f n )=S * +E * +C * (twenty one)

[0180] Where: S * 、E * , C * is the normalized evaluation value.

[0181] The normalization formula of S, E, and C indicators is shown in formula (22):

[0182]

[0183] Where: α k is the actual value of indicator α, α min and α max are the minimum and maximum values ​​of the indicator α respectively.

[0184] Step 4-1: Analysis of the short-circuit current suppression effect of the configuration scheme. Compare the short-circuit current after the current limiting measure scheme is implemented with the rated interrupting current of the circuit breaker.

[0185] Step 4-2: Grid transient voltage stability analysis of the configuration scheme. Use PSASP to perform transient stability calculations on the configuration scheme and calculate the transient voltage recovery curve of the busbar at key sites.

[0186] Example:

[0187] 1) Simulation example

[0188] The calculation network is the load center grid of a province in the central part of China in a certain planning year. The grid consists of 15 500kV hub substations, 99 220kV substations, 5 thermal power plants and 2 hydropower stations. The calculated load of the grid in the large load mode in this planning year is 43,300MW. Based on the national network calculation data and planning data issued by the National Dispatching Center, the Power System Comprehensive Calculation and Analysis Program (PSASP) is used to calculate and analyze the short-circuit current and transient voltage recovery of the grid in this planning year. The short-circuit current calculation shows that the short-circuit current of some station buses in the network exceeds the standard. The specific situation is as follows:

[0189] (1) Three-phase short-circuit current

[0190] In this planning year, the only stations with 500kV busbar three-phase short-circuit current exceeding the standard are HL and SSH. The stations with 220kV busbar three-phase short-circuit current exceeding the standard are CSX, DG1, NX, XC2, HL1 and HL2.

[0191] The exceeding of the three-phase short-circuit current of 500kV bus is shown in Table 1.

[0192] Table 1 500kV busbar three-phase short-circuit current exceeding the standard

[0193]

[0194] The exceeding of the three-phase short-circuit current of 220kV busbar is shown in Table 2.

[0195] Table 2 220kV busbar three-phase short-circuit current exceeding the standard

[0196]

[0197]

[0198] (2) Single-phase short-circuit current

[0199] At the 500kV level, only SSH exceeded the standard for the single-phase short-circuit current of the 500kV busbars of each substation. At the 220kV level, the 220kV single-phase short-circuit current of the CSX, NX, WC and bml busbars all exceeded the standard.

[0200] The 500kV busbar single-phase short-circuit current exceeding the standard is shown in Table 3.

[0201] Table 3 500kV busbar single-phase short-circuit current exceeding the standard

[0202]

[0203] The exceeding of single-phase short-circuit current of 220kV bus is shown in Table 4.

[0204] Table 4 220kV busbar single-phase short-circuit current exceeding the standard

[0205]

[0206] 2) Example simulation results

[0207] (1) Parameter setting

[0208] The maximum interrupting current of 500kV and 220kV circuit breakers in this power grid is 63kA and 50kA respectively. The range of reactance value of series reactor is 7Ω, 14Ω, 21Ω, 28Ω; the range of short-circuit voltage percentage of replacing high-impedance transformer is 18% and 20%; the range of small reactance of neutral point of autotransformer is 10Ω, 15Ω, 20Ω. The cost analysis of short-circuit current suppression measures in reference [4] shows the investment cost coefficient of each measure in Table 5.

[0209] Table 5 Investment cost coefficients of current limiting measures

[0210]

[0211] (2) Determination of the set of decision variables for dimensionality reduction

[0212] In a provincial power grid planning year, considering the number of decision variables, we reduced their dimensionality, including 30-dimensional 220kV lines, 8-dimensional busbar switches, and 16-dimensional transformers. The decision variable set obtained after dimensionality reduction includes 12-dimensional transmission lines, 3-dimensional busbar switches, and 6-dimensional transformers. For example, we take the sensitivity of the comprehensive current limiting measures of the line as an example, and select lines with a sensitivity greater than 0.1 to add to the dimensionality reduction decision variable set, because these lines have strong competitiveness, while lines with a sensitivity less than 0.1 have weak competitiveness. The dimensionality reduction set of line decision variables obtained in this way is s.

[0213] Table 6. Sensitivity ranking of line decision variables

[0214]

[0215] (3) Sequential optimization algorithm to find the optimal solution

[0216] The sensitivity dimension reduction decision variable set of the current limiting measures is used as the original data and parameters to screen the current limiting measures to form a sequence optimization representation set of the current limiting optimization configuration scheme.

[0217] Based on formula (17), a sequence optimization rough evaluation model is established to sort and stratify the solutions in the representation set and generate the OPC curve. Fig.10 .

[0218] Depend on Fig.10It can be seen that the OPC curve using the sequential optimization algorithm is Bell type. According to the parameter comparison table of different OPC curves, the regression parameters Z0, p, m, and η are 8.1998, 1.9164, -2.0250, and 10.00, respectively. Let k = 1 and g = 20, and then according to formula (20), s = 16 is obtained to form the selected set W.

[0219] The specific optimization measures in the selected set W are shown in Table 7 below:

[0220] Table 7 Selected Sets

[0221]

[0222]

[0223] The established SEC model is used to quantitatively evaluate the current limiting schemes in Table 7. First, the corresponding measures of each current limiting scheme are simulated and calculated using PSASP, and then the three SEC indicators are calculated using equations (15), (17) and (18), respectively. The results are shown in Table 8.

[0224] Table 8 SEC indicators of each scheme in the selected set

[0225]

[0226]

[0227] For the selected set W, the exact model is used to solve the Pareto non-dominated solution set. Fig.11 As shown in Table 9, the specific current limiting measures for different non-dominated solution sets are shown in Table 9.

[0228] Table 9 Current limiting optimization measures corresponding to Pareto non-dominated solution set

[0229]

[0230]

[0231] Manually assisted screening of the current limiting measures shown in Table 9 shows that disconnecting the 220kV NX-cw double-circuit line will cause a heavy overload on the ty-cw single-circuit line, which will have a significant impact on the reliability of local power grid power supply. Therefore, Schemes 7, 8, and 16 for disconnecting the NX-cw double-circuit line are not feasible.

[0232] Finally, the SEC values ​​of the Pareto non-dominated solution sets of the five solutions shown in Table 10 are solved to obtain the optimal configuration solution.

[0233] Table 10 SEC values ​​of Pareto non-dominated solution sets

[0234]

[0235] From the analysis of the results in Table 10, it can be seen that the SEC value of Scheme 5 is the smallest, which is the optimal solution for the optimization configuration of current limiting measures.

[0236] 3) Comparative analysis

[0237] (1) Analysis of short-circuit current suppression effect of configuration scheme

[0238] After adopting the current limiting measure scheme 5, the short-circuit current of the over-limited stations in the power grid has been reduced to below the rated interrupting current of the circuit breaker in a certain planning year. The three-phase short-circuit current of the 500kV bus is shown in Table 11, and the three-phase short-circuit current of the 220kV bus is shown in Table 12. The single-phase short-circuit current of the 500kV bus is shown in Table 13, and the single-phase short-circuit current of the 220kV bus is shown in Table 14.

[0239] Table 11 500kV busbar three-phase short-circuit current

[0240]

[0241] Table 12 220kV busbar three-phase short-circuit current

[0242]

[0243] Table 13 500kV busbar single-phase short-circuit current

[0244]

[0245] Table 14 220kV busbar single-phase short-circuit current

[0246]

[0247]

[0248] (2) Transient voltage stability of the power grid in the configuration scheme

[0249] PSASP is used to calculate the transient stability of Scheme 5. The transient voltage recovery curve of the busbar at the key site is as follows: Fig.12 As shown. It can be seen that under the three-period N-1 fault on the HL side of the 500kV XTX-HL line, the bus transient voltage of each key station after configuration scheme 5 can be restored to the normal operating level.

[0250] The comprehensive optimization configuration method of short-circuit current suppression measures considering SEC is applied to a load center power grid in a certain province. Firstly, the SEC rough model is solved by the ordinal optimization algorithm for the power grid, and the candidate set of comprehensive optimization configuration schemes for current limiting measures is obtained. The Pareto non-dominated solution set is further obtained through the precise model; then the optimal current limiting measure optimization configuration scheme is obtained by using the SEC normalized index evaluation; finally, the feasibility of the optimal scheme is proved by analyzing the short-circuit current level and transient voltage stability of the power grid under the optimal current limiting measure optimization configuration scheme, and the effectiveness and rationality of the comprehensive optimization configuration method of current limiting measures proposed in this chapter in actual power grids are also verified.

[0251] This patent comprehensively considers the unified decision-making of current limiting equipment installation and disconnection measures, as well as the relationship between system safety stability, current limiting scheme effect and investment cost, and proposes a method for comprehensive optimization configuration of short-circuit current suppression measures considering safety efficiency cost. First, by analyzing the sensitivity relationship between the selected current limiting measures and the short-circuit current of the exceeding site, a dimension reduction strategy is proposed, which takes into account the sensitivity of the current limiting measures. Then, a mathematical model for the comprehensive optimization configuration of short-circuit current suppression measures considering the system safety level is established. Finally, a sequential optimization algorithm is used to solve the optimization configuration model to find the optimal current limiting measures configuration scheme.

Claims

1. A comprehensive optimization configuration method for short-circuit current suppression measures considering safety efficiency cost, characterized in that: The following steps are involved: Step 1: Considering the grid structure and equipment, sensitivity analysis is performed on five measures: line disconnection, busbar split operation, installation of current limiting reactors, replacement of high-impedance transformers, and installation of small reactors at the neutral point of transformers, in order to propose a dimension reduction strategy; Step 2: Establish a comprehensive optimization configuration model considering safety effectiveness cost; Step 3: Solve the optimization configuration model based on the sequential optimization algorithm; The above steps can realize the comprehensive optimization configuration of short-circuit current suppression measures.

2. The method according to claim 1, characterized in that: In step 1, from the perspectives of grid structure and equipment, sensitivity analysis is performed on five measures: line disconnection, busbar split operation, installation of current limiting reactors, replacement of high-impedance transformers, and installation of small reactors at the neutral point of transformers, including: 1-1: Perform sensitivity analysis on the disconnected line and define the sensitivity of the disconnected line between nodes i and j to the self-impedance of node k as: Where: Z kk and Z' kk are the self-impedance of node k before and after the line is disconnected, Z ij is the mutual impedance between node i and node j, Z ii , Z jj are the self-impedances of nodes i and j, respectively, ij is the impedance of line ij, Z ki and Z kj are the mutual impedances of nodes i, j and k respectively, △Z kk is the change in self-impedance of node k after the line is disconnected; Where: N e is the number of sites with excessive short-circuit current; ω k is the weight factor; I k is the short-circuit current of the exceeding site k. The larger short-circuit current value is taken for the site where both the three-phase short-circuit current and the single-phase short-circuit current exceed the standard at the same time; is the maximum breaking current of the circuit breaker; For stations where only the three-phase short-circuit current exceeds the standard, Defined as: For stations where only single-phase short-circuit current exceeds the limit, Defined as: Where: are the sensitivities calculated based on the positive sequence network and the zero sequence network respectively; For stations where both three-phase short-circuit current and single-phase short-circuit current exceed the standard, The expression corresponding to the larger short-circuit current value is used, as shown in equations (3) and (4); similarly, if the bus connected to the switch in the substation is regarded as an independent bus, and the switch is represented by a small reactance line, then the busbar split operation is equivalent to breaking the line; 1-2: Conduct sensitivity analysis on busbar split operation; 1-3: Sensitivity analysis of the installation of current limiting reactors: The sensitivity of adding series reactors to the self-impedance of excessive sites is defined as: In order to characterize the current limiting effect of adding series reactors on all over-limit stations in the system, the weighted sensitivity is defined as: Where: γ * k With λ * k The definition is similar; N e is the number of sites with excessive short-circuit current; ω k is the weight factor; I k is the short-circuit current of the exceeding site k. The site where both the three-phase short-circuit current and the single-phase short-circuit current exceed the standard takes the larger short-circuit current value; I k max is the maximum breaking current of the circuit breaker; 1-4: Sensitivity analysis of replacing the high impedance transformer. After replacing the high impedance transformer, equation (1) is rewritten as: Therefore, the sensitivity of replacing the high impedance transformer to the node exceeding the standard is defined as follows: In order to consider the current limiting level of the overall excessive-standard site in the system after replacing the high-impedance transformer, the weighted sensitivity is defined as: Where: * k With λ * k The definition is similar; N e ,ω k ,I k ,I k max The meaning is the same as formula (2); 1-5: Perform sensitivity analysis on the installation of a small reactance grounding at the neutral point of the transformer. In the autotransformer, adding a small reactance to the neutral point is equivalent to changing the reactance values ​​of the three zero-sequence network branches at the same time. The sensitivity of the small reactance at the neutral point can be obtained by superimposing it in sequence according to the following formula: Where: N s is the number of nodes with single-phase short-circuit current exceeding the standard; ω k is the weight factor; ε * kI , ε * kII , ε * kIII are the sensitivities of adding series reactances on the three zero-sequence network branches; I k is the single-phase short-circuit current of the exceeding node k; I k max is the maximum breaking current of the circuit breaker; 1 k , ε 0 k are the sensitivities calculated based on the positive sequence network and the zero sequence network respectively; 1-6: Establish a dimensionality reduction strategy that takes into account the sensitivity of current limiting measures, and define the comprehensive sensitivity as follows: Where: N is the number of current limiting measures to be selected for configuration object i. For lines, busbar switches and transformers, N is 2; α * k The sensitivity when the current limiting measure k is selected; The dimension reduction strategy considering the sensitivity of the current limiting measures is as follows: first, analyze the sensitivity of each configuration object in the computing system; then select the configuration objects in descending order according to the sensitivity of the comprehensive current limiting measures, and select the top N l Return line, N b Bus tie switches, N t The transformers form a set of dimension-reduced decision variables; In addition, in order to ensure the convergence and efficiency of the calculation, the number of configuration objects N in the dimensionality reduction decision process is l 、N b 、N t The value of should be adjusted according to the overall scale of the computing network.

3. The method according to claim 1, characterized in that In step 2, a comprehensive optimization configuration model of short-circuit current suppression measures considering the three indicators of safety, efficiency and cost is proposed, including: 2-1: Obtaining the objective function Taking the line, bus tie switch, high impedance transformer, transformer neutral point small reactance and series reactor as decision variables, a current limiting optimization configuration model based on SEC is constructed, and its objective function is as follows: F(x,y,z,h)=min[S(x,y,z,h),E(x,y,z,h),C(x,y,z,h)] (12) In the formula: x is the disconnection of the line and busbar switch; y is whether to replace the high-impedance transformer; z is the neutral point small reactance of the substation main transformer; h is whether the line is equipped with a series reactor; S, E, and C are safety, efficiency, and cost indicators respectively; 2-2: Obtaining the safety index S The system transient voltage recovery level is used as the safety index S to measure the transient stability of the system under each scheme. The definition of the safety index S is as follows: Where: tc is the fault removal time; te is the calculation cut-off time, which can be taken as the 10th second after the expected fault j removal time according to the provisions of the power guidelines [3]; Ri,j(t) is defined as shown in formula (14); Where: Ui,j(t) is the voltage amplitude of bus i at time t after fault j occurs; To simplify the calculation, this paper selects key nodes in the power grid for index calculation; Where: M is the number of key nodes in the power grid; N is the number of expected faults; ω i is the weight of busbar i; 2-3: Obtaining performance indicator E The short-circuit current over-limit index E is used as the performance index to characterize the suppression effect after the implementation of different current limiting optimization measures; Where: Ii is the three-phase short-circuit current of the ith bus; Iimax is the rated breaking current of the ith bus circuit breaker; ki greater than -1 indicates that the line short-circuit current exceeds the standard, and less than -1 indicates that the short-circuit current is equal to the rated breaking current of the circuit breaker; for the convenience of calculation, ki is normalized; Where: Nb is the busbar set; after processing (17), E is normalized to the interval [0,1]; the smaller ki is, the closer E is to 0, indicating a better current limiting effect; the larger ki is, the closer E is to 1, indicating a worse current limiting effect; 2-4: Obtain cost indicator C Taking the total investment cost of current limiting measures as the cost indicator, the calculation formula is: In the formula: Let N s is the total number of current limiting measures taken; u s =1 means that the current limiting measure s is adopted, and 0 means that it is not adopted; k as , k bs is the investment cost coefficient of the current limiting measure s; s is the equipment parameter. For current limiting measures such as disconnecting lines and busbar segmentation, z s Take 0; for adding series reactor, z s is the series reactance value; for replacing a high impedance transformer, z s is the percentage increment of short-circuit voltage; for the small reactance at the neutral point of the transformer, z s is the reactance value; 2-5: Establishing constraints The constraints of the comprehensive optimization configuration model considering safety effectiveness cost are as follows: Where: N b is the total number of nodes; is the upper limit of the short-circuit current of node k; N l is the total number of branches; is the upper power limit of branch l; Respectively represent the upper and lower limits of the voltage at node k; They respectively represent the upper and lower limits of the parameter values ​​of the current limiting device.

4. The method according to claim 1, characterized in that In step 3, the optimization configuration model is solved based on the sequential optimization algorithm, and step 3 includes the following steps: Step 3-1: Generate a representation set; Input network data and decision variables, use MATLAB to generate a set of solutions, and use PSASP to calculate the power flow and short-circuit current of each solution; then randomly select N current limiting solutions in the feasible domain according to the constraints to determine the characterization set Θ N ; Step 3-2: Rough model solution; Formula (17) is used as the rough evaluation model, and this model is used to evaluate the set Θ N Make an evaluation, and obtain the descending order of all solutions based on the evaluation results, obtain the OPC curve, and use it to clarify the type of planning problem; Step 3-3: Generate a selected set W; According to the type of planning problem determined in step 2, refer to formula (20) to select the first s feasible solutions as the selected set W; Where: s is a function of k and g; Z0, p, m, η are regression parameters, which are determined according to the typical curve type; g is the number of observed good enough solutions, among which there are k real good enough solutions; Step 3-4: accurate model solution; The short-circuit current and transient voltage recovery data of the scheme are obtained by PSASP calculation, and an accurate evaluation model is constructed based on formula (12). The model is used to accurately evaluate all feasible solutions in the set W to form a Pareto non-dominated solution set. Step 3-5: Manual experience screening; Use the working experience of operators to screen out unreasonable solutions in actual projects; Step 3-6: Normalize the S, E, and C indicators respectively, and then sum them up to get the SEC indicator. Use the SEC indicator to further sort the Pareto non-dominated solution set that meets the actual engineering situation, and take the solution with the smallest SEC indicator as the optimal solution; its SEC indicator is shown in formula (21); SEC(f n )=S * +E * +C * (21) Where: S * 、E * , C * is the normalized evaluation value; The normalization formula of S, E, and C indicators is shown in formula (22): Where: α k is the actual value of indicator α, α min and α max are the minimum and maximum values ​​of the indicator α respectively.

5. The method according to claim 1, characterized in that: The method also includes step 4, which proves the feasibility of the optimal solution by analyzing the short-circuit current level and transient voltage stability of the power grid under the optimal configuration scheme of the optimal current limiting measures; Step 4-1: Analyze the short-circuit current suppression effect of the configuration scheme; compare the short-circuit current after the current limiting measure scheme is implemented with the rated interrupting current of the circuit breaker; Step 4-2: Analyze the transient voltage stability of the grid for the configuration scheme; use PSASP to perform transient stability calculations on the configuration scheme and calculate the transient voltage recovery curve of the busbar at key sites.

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