Optimization method of voltage hierarchy of rural high-voltage distribution network considering load density

Abstract

This invention discloses a method for optimizing voltage levels in rural high-voltage distribution networks considering load density. The method includes: quantitatively analyzing the impact of load density on the selection of voltage levels in rural high-voltage distribution networks, constructing an economic model to obtain a quantitative method for measuring the economic efficiency of 110kV and 35kV voltage levels; selecting voltage levels in rural high-voltage distribution networks using the critical load density value calculated by the economic model, and further deriving the load density threshold conditions for voltage level optimization; if the load density is lower than the critical load density value, a two-stage voltage reduction mode is more economically advantageous for rural high-voltage distribution networks, otherwise a direct voltage reduction mode is adopted; this invention considers the costs of substations and lines uniformly, enabling the integration of existing power grids in the planning area, improving practicality, and saving investment and improving economy by selecting different voltage levels according to different load densities in the planning area.

Description

Technical Field

[0001] This invention relates to the field of high-voltage distribution network optimization technology, specifically a method for optimizing voltage levels in rural high-voltage distribution networks considering load density. Background Technology

[0002] The appropriate selection of voltage level plays a crucial role in the economy of distribution networks. It is necessary to optimize the selection while meeting the requirements of power supply capacity, reliability and power quality to achieve the best economic performance.

[0003] The national high-voltage distribution network generally adopts 110kV and 35kV voltage levels (66kV voltage level is retained in Northeast China). Current standards provide general guidelines for voltage sequence selection in various power supply areas, but do not specify how to optimize voltage level selection. Scholars have conducted some research on voltage level optimization methods. Zhu Lirong et al. assumed that investment cost is a linear function of voltage level, and operating cost is inversely proportional to the square of the voltage level, establishing a relationship between total cost and voltage level and finding the optimal voltage level when the total cost is minimized. Hu Lijuan et al. established a system based on economic factors (considering investment, line loss, and operation and maintenance). With the comprehensive annual cost as the main objective, a multi-objective optimization model that comprehensively considers technical and social factors is used to optimize the voltage sequence configuration from the transmission network to the low-voltage distribution network. Su Weihua et al. proposed substation geomorphic, strip, and area distributions, and compared the economics of different configuration schemes for high-voltage distribution networks with the goal of minimizing the total annual cost per unit load, thus determining the voltage level configuration principles for different regions. Chen Genyong et al. studied the economics of 20kV as a medium-voltage level in rural power grids, but the model they constructed did not consider the impact of 20kV on the layout of high-voltage substations, making it difficult to carry out overall planning and optimization of the planning area.

[0004] Although there has been a lot of research on voltage level selection methods, the following problems still need to be solved for the selection of voltage levels in rural high-voltage distribution networks: (1) When calculating the one-time investment of power grid construction, the existing methods generally consider substations and lines separately, calculate them separately and then sum them up. The line length is calculated based on assumed conditions, and the existing power grid conditions in the planning area cannot be taken into account. The existing research generally uses the following methods to calculate the line length: ① Determine the power supply radius of the substation according to the optimization algorithm, and determine the transmission line length by multiplying the power supply radius by the tortuosity coefficient; ② Pre-assume that the substation distribution follows several specific patterns, and calculate the line length on this basis; ③ Directly assume the transmission line length under different load densities; ④ Pre-set the approximate location of the high-voltage station or the location of the upstream power source; (2) Voltage level selection is closely related to load density. At present, only some methods have studied this problem. The relevant research generally sets several load densities and performs corresponding economic calculations and comparisons to determine the economy. There is a lack of quantitative analysis methods for the relationship between the economic advantages and disadvantages of different voltage levels and load density. Summary of the Invention

[0005] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. In this section, as well as in the abstract and title of this application, simplifications or omissions may be made, and such simplifications or omissions should not be construed as limiting the scope of the invention.

[0006] In view of the above-mentioned problems, the present invention is proposed.

[0007] Therefore, the technical problems solved by this invention are: existing studies do not consider the power grid conditions of the planning area, lack quantitative analysis of the economic advantages and disadvantages of different voltage levels and the relationship between load density, and the cost data is not in line with reality.

[0008] To address the aforementioned technical problems, this invention provides the following technical solution: a method for optimizing voltage levels in rural high-voltage distribution networks considering load density, comprising: constructing an economic model to obtain a quantitative method for measuring the economic efficiency of 110kV and 35kV voltage levels by quantitatively analyzing the impact of load density on the selection of voltage levels in rural high-voltage distribution networks; selecting voltage levels in rural high-voltage distribution networks using the critical load density value calculated by the economic model, and further deriving the load density threshold condition for optimizing voltage level selection; if the load density is lower than the critical load density value, a two-stage step-down mode is more economically advantageous for rural high-voltage distribution networks, and vice versa.

[0009] As a preferred embodiment of the rural high-voltage distribution network voltage level optimization method considering load density described in this invention, the influence of load density on the selection of rural high-voltage distribution network voltage levels includes:

[0010] The calculation of voltage loss on a 10kV line includes,

[0011]

[0012] Where ΔU' represents the final voltage drop, U represents the 10kV rated voltage, and d% represents the 10kV line voltage drop;

[0013] To meet voltage quality requirements, the power supply radius L should satisfy the following conditions:

[0014]

[0015] Where M represents the number of outgoing lines from the substation, and G U The voltage drop coefficient represents the voltage drop coefficient under different load distributions, τ represents the simultaneity rate, σ represents the load density, r represents the resistance per unit length of the line, and x represents the reactance per unit length. This indicates the load power factor angle of a 10kV line;

[0016] Upper limit S of the power supply capacity of a single substation under voltage constraints max_V The calculations include,

[0017]

[0018] As a preferred embodiment of the rural high-voltage distribution network voltage hierarchy optimization method considering load density described in this invention, the construction of the economic model includes:

[0019] An economic model is constructed by collecting statistical data on the cost of power transmission and transformation projects in the planning area and analyzing the power supply capacity of a single substation.

[0020] From the perspective of thermal limits, the calculation of the required 110kV and 35kV transformer capacities for the planned area includes,

[0021]

[0022] Among them, S ∑ This represents the transformer capacity for 110kV and 35kV, where k represents the capacity-to-load ratio, and P... ∑ This indicates the load forecast for the area to be planned. This indicates the power factor angle on the high-voltage side of 110kV and 35kV substations;

[0023] The comprehensive annual cost of the two-level step-down mode and the direct step-down mode is calculated based on the construction cost and operation and maintenance cost of the power distribution network in the planning area.

[0024] As a preferred embodiment of the rural high-voltage distribution network voltage hierarchy optimization method considering load density described in this invention, the calculation of the comprehensive annual cost of the two-stage step-down mode and the direct step-down mode includes,

[0025]

[0026]

[0027] Where F1 represents the comprehensive annual fee for the direct step-down mode, F2 represents the comprehensive annual fee for the two-stage step-down mode, C1 represents the cost of the 110kV / 10kV transmission and transformation project, C2 represents the cost of the 35kV transmission and transformation project, and S N110 This represents the rated capacity of 110kV and 35kV substations, η represents the percentage of the ultimate power supply capacity of 110kV / 10kV substations considering voltage constraints, and k represents the rated capacity of 110kV and 35kV substations. d F represents the annual discount rate factor. MA1 F represents the annual maintenance cost in the direct reduction mode. loss1 F represents the annual depreciation cost in the direct reduction mode. MA2 F represents the annual maintenance cost of the two-stage step-down mode. loss2This represents the annual loss cost of the two-stage step-down mode, where 'a' represents the ratio of the cost of a 110kV two-stage step-down transmission and transformation project to the cost of a 110kV direct step-down transmission and transformation project, and S represents the annual loss cost of the two-stage step-down mode. N35 This indicates the rated capacity of a 35kV substation.

[0028] As a preferred embodiment of the rural high-voltage distribution network voltage hierarchy optimization method considering load density described in this invention, it further includes:

[0029] The difference in loss cost between the direct step-down mode and the two-stage step-down mode is that the two-stage step-down mode has an additional 110kV / 35kV transformer compared to the direct step-down mode. This difference will cause additional substation losses, and the loss cost of the two modes needs to be simplified.

[0030] The loss cost F loss The simplified calculations include,

[0031] F loss =c e ·T loss ·P losss %·S N

[0032] Among them, c e T represents the electricity price. loss P represents the number of hours of maximum load loss. losss % represents additional transformer losses, S N Indicates the rated transformer capacity;

[0033] By analyzing the calculation results of the loss costs in the direct step-down mode and the two-stage step-down mode, it can be seen that F loss1 and F loss2 Compared to the extremely low annual cost of power transmission and transformation projects, and F loss2 Compared to F loss1 The increase is also minimal, therefore F1 is less than F2 when calculating F<sub loss1 and F loss2 It can be ignored.

[0034] As a preferred embodiment of the rural high-voltage distribution network voltage hierarchy optimization method considering load density described in this invention, the calculation of the economic advantages and disadvantages of the two modes, after simplification of loss costs, includes:

[0035]

[0036] Wherein, ΔC(η) represents the economic advantages and disadvantages of the two modes, k1 represents the cost per unit rated capacity of 110kV / 10kV transmission and transformation projects, and k2 represents the cost per unit rated capacity of 35kV transmission and transformation projects.

[0037] As a preferred embodiment of the rural high-voltage distribution network voltage hierarchy optimization method considering load density described in this invention, wherein: the acquisition of the load density threshold condition includes,

[0038] Let f(η) = k2η 2 -(ak1+k2)η+k1, through calculation, we know that f(η)=0 has only one root on η∈(0,1], which can be expressed as,

[0039]

[0040] Where, η * Let f(η) be a root of f(η) on η∈(0,1].

[0041] As a preferred embodiment of the rural high-voltage distribution network voltage hierarchy optimization method considering load density described in this invention, it further includes:

[0042] When η < η * When ΔC(η)>0, the cost of the direct voltage reduction mode is higher, and the cost of the two-stage voltage reduction mode is better.

[0043] When η * When η≤1, ΔC(η)<0, then the cost of the two-stage voltage reduction mode is higher, and the cost of the direct voltage reduction mode is better.

[0044] As a preferred embodiment of the rural high-voltage distribution network voltage hierarchy optimization method considering load density described in this invention, the calculation of the critical load density value includes:

[0045]

[0046] Where, σ * This represents the critical load density value. This represents the ratio of the capacity of a 110kV substation to the number of 10kV outgoing circuits.

[0047] As a preferred embodiment of the rural high-voltage distribution network voltage hierarchy optimization method considering load density described in this invention, the selection of the voltage hierarchy of the rural high-voltage distribution network includes:

[0048] If the load density is lower than the critical load density value, then the rural high-voltage distribution network adopting a two-stage voltage reduction mode is more economically advantageous.

[0049] If the load density is higher than the critical load density value, then the rural high-voltage distribution network adopting the direct reduction mode has a greater economic advantage.

[0050] The beneficial effects of this invention are as follows: This invention provides a method for optimizing the voltage hierarchy of rural high-voltage distribution networks considering load density. In the process of constructing an economic model, it calculates the cost of substations and the costs of 110kV and 35kV lines. The line cost depends on the distance between substations and is closely related to the existing power grid in the planning area. Traditional methods assume a fixed line length, lack a basis, and are difficult to integrate with the current situation in the planning area. This invention considers the costs of substations and lines uniformly based on the cost statistics of 110kV and 35kV transmission and transformation projects in the planning area in recent years, implicitly taking into account the existing power grid. To a certain extent, it can achieve the integration of the existing power grid in the planning area, improving its practicality. Furthermore, this invention uses the economic model to calculate the load density threshold. When the load density is below the threshold, a two-stage voltage reduction is more economical; when it is above the threshold, a direct voltage reduction is preferable. Case studies show that selecting different voltage levels according to different load densities in the planning area can save investment and improve economic efficiency. Attached Figure Description

[0051] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0052] Figure 1 The overall flowchart of the rural high-voltage distribution network voltage hierarchy optimization method considering load density provided by the present invention;

[0053] Figure 2 A diagram illustrating the ultimate power supply capacity of a 110kV substation in the rural high-voltage distribution network voltage hierarchy optimization method considering load density provided by this invention.

[0054] Figure 3 A diagram illustrating the ultimate power supply capacity of a 35kV substation in the rural high-voltage distribution network voltage hierarchy optimization method considering load density provided by this invention.

[0055] Figure 4 The voltage drop diagram of 10kV line when the capacity of 110kV substation is fully utilized in the voltage hierarchy optimization method for rural high-voltage distribution networks considering load density provided by the present invention. Detailed Implementation

[0056] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the protection scope of the present invention.

[0057] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0058] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0059] This invention is described in detail with reference to the schematic diagrams. When detailing the embodiments of this invention, for ease of explanation, the cross-sectional views illustrating the device structure may be partially enlarged, not adhering to the usual scale. Furthermore, the schematic diagrams are merely examples and should not be construed as limiting the scope of protection of this invention. In actual fabrication, the three-dimensional spatial dimensions of length, width, and depth should be included.

[0060] Furthermore, in the description of this invention, it should be noted that the terms "upper," "lower," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are used solely for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. In addition, the terms "first," "second," or "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0061] Unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" in this invention should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; similarly, they can refer to mechanical connections, electrical connections, or direct connections, or indirect connections through an intermediate medium, or internal connections between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0062] Example 1

[0063] Reference Figure 1As one embodiment of the present invention, a method for optimizing the voltage hierarchy of rural high-voltage distribution networks considering load density is provided, comprising:

[0064] S1: By quantitatively analyzing the impact of load density on the selection of voltage levels in rural high-voltage distribution networks, an economic model is constructed to obtain a quantitative method for measuring the economic efficiency of 110kV and 35kV voltage levels. It should be noted that:

[0065] There are two voltage level modes available for rural high-voltage distribution networks: ① Direct step-down mode: The 35kV voltage level of the 110kV substation is eliminated, and 110kV / 10kV is used to directly supply power to the medium-voltage distribution network; ② Two-stage step-down mode: The 110kV substation is equipped with 35kV and 10kV voltage levels. Power is supplied to some loads through the 10kV line, while the 35kV line extends to various load centers in rural areas to supply power to the 35kV substations in the load centers, which in turn supply power to the loads through the 10kV line.

[0066] It should be noted that in areas with high load density, the direct step-down mode can reduce the number of transformer levels, lower construction costs and losses, and has economic advantages; however, in rural areas with low load density, due to the large power supply radius, excessively long 10kV lines are prone to voltage problems, and it is necessary to build more high-voltage substations to meet voltage quality requirements. The cost of 35kV transmission and transformation projects is much lower than that of 110kV, so retaining the 35kV voltage level is still of great significance.

[0067] Furthermore, the substation power supply areas are classified according to load density, and the power supply capacity of different types of power supply areas is analyzed, including...

[0068] According to circuit theory, the calculation of voltage drop in a 10kV line includes:

[0069] ΔU=(PR+QX) / U bus

[0070] Where ΔU represents the voltage drop of the 10kV line, P represents the active power flowing through the line, R represents the equivalent resistance of the line, Q represents the reactive power flowing through the line, X represents the equivalent reactance of the line, and U bus This indicates the bus voltage of the 10kV line in the substation;

[0071] If the substation power supply area is defined as a circular region centered on the substation power supply area, then the calculation of the load carried by a single 10kV outgoing line includes,

[0072]

[0073] Where τ represents the simultaneity rate, σ represents the load density, L represents the power supply radius, and M represents the number of outgoing lines from the substation;

[0074] Substituting the calculation formula for the load carried by a single 10kV outgoing line into the calculation formula for the voltage drop of a 10kV line, we can obtain:

[0075]

[0076] Where r represents the resistance per unit length of the line, in Ω / km, and x represents the reactance per unit length, in Ω / km. This indicates the load power factor angle of a 10kV line;

[0077] Since voltage drop is closely related to load distribution, a voltage loss coefficient for different load distributions is introduced to calculate the final voltage drop. The calculation of the final voltage drop ΔU' includes...

[0078] ΔU′=ΔU×G U

[0079] Among them, G U The voltage loss coefficient represents the voltage drop coefficient under different load distributions.

[0080] It should be noted that the voltage loss coefficient values ​​for different load distributions are shown in Table 1.

[0081] Table 1: Voltage loss coefficients for different load distributions.

[0082] Load distribution <![CDATA[G U ]]> Terminal concentrated load 1.000 Average distributed load 0.500 Gradually increase the load 0.667 Gradually reduce the load 0.333 Intermediate heavy distribution 0.250

[0083] Furthermore, the calculation of voltage loss on a 10kV line includes,

[0084]

[0085] Where d% represents the voltage drop of a 10kV line and U represents the rated voltage of a 10kV line.

[0086] It should be noted that, according to relevant standards, the voltage deviation of three-phase power supply at 20kV and below is ±7% of the nominal voltage, that is, the 10kV voltage cannot be lower than 9.3kV. d% can be obtained by dividing the difference between the 10kV bus voltage and 9.3kV of the substation by 10.

[0087] To meet voltage quality requirements, U is approximated. bus If ≈U, then the power supply radius should meet the following conditions.

[0088]

[0089] Furthermore, the upper limit S of the power supply capacity of a single substation under voltage constraints max_V The calculations include,

[0090]

[0091] It should be noted that the upper limit of the power supply capacity of a single 110kV or 35kV substation is proportional to the load density σ to the power of 1 / 3 and the number of 10kV outgoing circuits M of the substation to the power of 2 / 3. Due to voltage quality constraints, the upper limit of the power supply capacity of a 110kV substation directly stepping down to 10kV in areas with low load density is much smaller than its substation capacity. This means that the power supply capacity of the 110kV direct-step-down substation cannot be fully utilized. To meet the load demand, the number of 110kV substations needs to be increased, which will seriously reduce the economic efficiency of adopting the direct-step-down mode.

[0092] S2: The critical load density value calculated using the economic model is used to select the voltage level of the rural high-voltage distribution network, and the load density threshold condition for optimal voltage level selection is further derived. It should be noted that:

[0093] An economic model was constructed by collecting cost statistics of power transmission and transformation projects in the planning area and analyzing the power supply capacity of a single substation.

[0094] From the perspective of thermal limits, the calculation of the required 110kV and 35kV transformer capacities for the planning area includes,

[0095]

[0096] Among them, S ∑ This represents the transformer capacity for 110kV and 35kV, where k represents the capacity-to-load ratio, and P... ∑ This indicates the load forecast for the area to be planned. This indicates the power factor angle on the high-voltage side of 110kV and 35kV substations;

[0097] The comprehensive annual fee for the two-stage step-down mode and the direct step-down mode is calculated based on the construction and operation and maintenance costs of the distribution network in the planning area. The calculation of the comprehensive annual fee includes:

[0098]

[0099]

[0100] Where F1 represents the comprehensive annual fee for the direct step-down mode, F2 represents the comprehensive annual fee for the two-stage step-down mode, C1 represents the cost of the 110kV / 10kV transmission and transformation project, C2 represents the cost of the 35kV transmission and transformation project, and S N110 This represents the rated capacity of 110kV and 35kV substations, η represents the percentage of the ultimate power supply capacity of 110kV / 10kV substations considering voltage constraints, and k represents the rated capacity of 110kV and 35kV substations. d F represents the annual discount rate factor. MA1 F represents the annual maintenance cost in the direct reduction mode. loss1 F represents the annual depreciation cost in the direct reduction mode. MA2F represents the annual maintenance cost of the two-stage step-down mode. loss2 This represents the annual loss cost of the two-stage step-down mode, where 'a' represents the ratio of the cost of a 110kV two-stage step-down transmission and transformation project to the cost of a 110kV direct step-down transmission and transformation project, and S represents the annual loss cost of the two-stage step-down mode. N35 This indicates the rated capacity of a 35kV substation;

[0101] It should be noted that the annual discount rate factor Where i represents the annual interest rate, which can be 10%, and n represents the operating period, which can be 30.

[0102] It should be noted that since the maximum power supply capacity of the 110kV direct-step transformer distributed through the 10kV line accounts for η of the total transformer capacity, the total number of 110kV direct-step substations in the formula is: If a two-stage voltage reduction mode is adopted, the maximum power supply capacity of the 110kV substation distributed through the 10kV line will still account for η of the total substation capacity. The remaining 1-η substation capacity will be transmitted to the 35kV substation at the load center through the 35kV line. Therefore, the capacity of the 110kV substation can be fully utilized, and the total number calculated according to the capacity limit is... The total number of 35kV substations is

[0103] Furthermore, the difference in loss costs between the direct step-down mode and the two-stage step-down mode lies in the fact that the two-stage step-down mode has an additional 110kV / 35kV transformer, which will cause additional substation losses. Therefore, the loss costs for both modes need to be simplified. The loss cost F... loss The simplified calculations include,

[0104] F loss =c e ·T loss ·P losss %·S N

[0105] Among them, c e T represents the electricity price. loss P represents the number of hours of maximum load loss. losss % represents additional transformer losses, S N Indicates the rated transformer capacity;

[0106] It should be noted that, taking a 110kV transformer with a capacity of 40MVA as an example, calculations using actual data from a western province show that, based on a 50% load rate, the transformer loss cost F... loss1 and F loss2 The annual costs of 110kV transmission and transformation projects are 0.159% and 0.161% respectively, which can be ignored;

[0107] After simplifying the loss costs, the economic advantages and disadvantages of the two models can be approximated by comparing the difference in one-time investment for power grid construction. Therefore, the calculation of the economic advantages and disadvantages of the two models includes...

[0108]

[0109] Wherein, ΔC(η) represents the economic advantages and disadvantages of the two modes, k1 represents the cost per unit rated capacity of 110kV / 10kV transmission and transformation projects, and k2 represents the cost per unit rated capacity of 35kV transmission and transformation projects.

[0110] It should be noted that, according to engineering practice data, k2 must be greater than k1;

[0111] Furthermore, let f(η) = k2η 2 -(ak1+k2)η+k1, since f(0)=k1>0 and f(1)=(1-a)k1<0, f(η)=0 has only one root on η∈(0,1], which is expressed as,

[0112]

[0113] It should be noted that when η < η * When ΔC(η) > 0, the direct-down mode has a higher cost, while the two-stage reduction mode is more economical; when η * When η≤1, ΔC(η)<0, then the cost of the two-stage step-down mode is higher, and the direct step-down mode is more economical;

[0114] Furthermore, the calculation of the critical load density value includes,

[0115]

[0116] Where, σ * This represents the critical load density value. This represents the ratio of the capacity of a 110kV substation to the number of 10kV outgoing circuits.

[0117] S3: If the load density is below the critical load density value, a two-stage step-down mode is more economical for rural high-voltage distribution networks; conversely, a direct step-down mode is more economical. It should be noted that:

[0118] If the load density is below the critical load density value, the two-stage step-down mode is more economical for rural high-voltage distribution networks; if the load density is above the critical load density value, the direct step-down mode is more economical for rural high-voltage distribution networks.

[0119] It should be noted that this invention provides a method for optimizing the voltage hierarchy of rural high-voltage distribution networks considering load density. In constructing the economic model, it calculates the cost of substations and the costs of 110kV and 35kV lines. The line cost depends on the distance between substations and is closely related to the existing power grid in the planning area. Traditional methods assume a fixed line length, lack a basis, and are difficult to integrate with the current situation in the planning area. This invention considers the costs of substations and lines uniformly based on the cost statistics of 110kV and 35kV transmission and transformation projects in the planning area in recent years, implicitly taking into account the existing power grid. To a certain extent, it can achieve the integration of the existing power grid in the planning area, improving its practicality. Furthermore, this invention uses the economic model to calculate the load density threshold. When the load density is below the threshold, a two-stage voltage reduction is more economical; above the threshold, a direct voltage reduction is preferable. Case studies show that selecting different voltage levels according to different load densities in the planning area can save investment and improve economic efficiency.

[0120] Example 2

[0121] Reference Figures 2-4 This is the second embodiment of the present invention. Unlike the first embodiment, this embodiment provides a verification test of the voltage hierarchy optimization method for rural high-voltage distribution networks that takes into account load density, in order to verify and explain the technical effects used in this method.

[0122] Using the method provided by this invention and combining it with the actual situation of a certain province, the effective power supply area of ​​the province accounts for 39.5% of the province's administrative area. The load density varies greatly between urban and rural areas, and the load distribution shows a low average load density and uneven distribution. Among them, rural areas of categories D and E account for 97.3% of the total effective power supply area.

[0123] ① Power supply capacity analysis.

[0124] In areas with low load density, the distribution capacity of 10kV lines is limited due to voltage quality constraints, severely restricting the power supply capacity of 110kV direct-drop 10kV substations. Based on relevant standards and engineering practice, LGJ-150 substations are selected for 10kV lines in Class D and E areas, with the load calculated according to an average distribution pattern along the line, i.e., G... U =0.5, the 110kV substation is configured with two main transformers, each with a capacity of 50MVA, and 20 outgoing 10kV lines; the 35kV substation is configured with two main transformers, each with a capacity of 8MVA, and 8 outgoing 10kV lines. Other parameters are: M=20, τ=0.8, and power factor is 0.9.

[0125] Figure 1 and Figure 2These represent the maximum power supply capacity of 110kV and 35kV substations supplied via 10kV lines under different load densities and voltage quality constraints. The horizontal line represents the power supply capacity limit determined by the substation capacity (power factor is taken as 0.95).

[0126] Depend on Figure 1 and Figure 2 It can be seen that for 110kV substations, in most load density ranges, the power supply capacity of the 10kV lines under voltage constraints is less than the limit of its transformer capacity. This indicates that if 110kV substations only retain the 10kV voltage level, the substations cannot fully play their role, resulting in a power supply bottleneck. For 35kV substations, their power supply capacity under voltage constraints is far higher than their transformer capacity, and voltage does not constitute a limiting factor for the power supply capacity of 35kV substations.

[0127] From another perspective, in order to fully utilize the transformer capacity of the 110kV / 10kV substation, the power supply distance of the 10kV line must be increased to cover a wider area of ​​load, but this will deteriorate the voltage quality and exceed the standard limit. Figure 3 This assumes the 110kV direct-drop substation is operating at 100% capacity, and the voltage drop at the end of the 10kV line (note that line lengths vary depending on load density; the length is calculated based on 100% substation capacity). According to national standards, the 10kV supply voltage deviation cannot exceed ±7%, meaning the end voltage cannot be lower than 9.3kV. Figure 3 It can be seen that the voltage drop exceeds this value in most load density ranges; even if the substation adjusts the 10kV bus voltage to a higher value (such as 10.5kV, where the voltage drop limit is 1.2kV) through on-load tap-changing transformers, a considerable number of load density ranges still exceed this limit.

[0128] ②Voltage level is preferred.

[0129] Since Class D and Class E power supply areas cannot be separated, rural high-voltage distribution network power supply areas may simultaneously include both types of power supply areas. This invention considers Class D and Class E power supply areas uniformly, and sorts out the feasibility study, preliminary design, and construction cost data of 110kV and 35kV transmission and transformation projects approved by a provincial power grid company from October 2018 to July 2022 (130 110kV projects and 155 35kV projects). Cost data for urban areas were excluded. Based on this, the average cost of 110kV and 35kV transmission and transformation projects in Class D and Class E power supply areas of various prefecture-level administrative regions was estimated (110kV two-stage step-down transmission and transformation project: 90.6 million yuan / project, a = 1.065; 35kV transmission and transformation project: 35.23 million yuan / project). Using this cost data, the critical load density value was calculated. When the 10kV line uses LGJ-150, the critical load density value is 0.0411MW / km. 2 .

[0130] The province's power grid covers 15 prefecture-level administrative regions. According to the saturation planning forecast, the annual saturation load density of Class D and E power supply areas in each prefecture-level administrative region is shown in Table 2.

[0131] Table 2: Selection of voltage levels for high-voltage distribution networks in rural power supply areas of a certain province.

[0132]

[0133] As shown in Table 2, the traditional two-stage step-down mode is suitable for the high-voltage distribution network in the rural power supply areas of 8 prefecture-level administrative regions; and the direct step-down mode is more reasonable for 7 regions where the load density is relatively high and the load density adjustment is greater than the critical density.

[0134] To meet the incremental load (saturated annual load minus current annual load), the total investment in the province's power grid construction was calculated using three schemes (capacity-to-load ratio of 1.8): Scheme 1: All prefecture-level power supply areas adopt the traditional two-stage step-down mode; Scheme 2: All areas adopt the direct step-down mode; Scheme 3: Each area selects either the direct step-down mode or the two-stage step-down mode based on local load density and this method. The calculation results are shown in Table 3.

[0135] Table 3: Construction scale and investment required to meet different incremental load scenarios.

[0136]

[0137]

[0138] As shown in Table 3, Scheme 1 has the highest total cost, Scheme 2 is in the middle, and Scheme 3, after voltage level optimization according to the present invention, has the lowest cost, saving 2.3 billion yuan in investment compared to Scheme 1, reducing investment by 8.2%. Although Scheme 2 omits one stage of transformer, the 10kV transmission capacity of the 110kV direct-step-down substation is limited by voltage in areas with low load density, which cannot fully utilize the capacity. Therefore, more 110kV transmission and transformation projects are needed to meet the load, reducing its economic efficiency. Scheme 3 selects the voltage level according to the load density of each region. For load densities above the critical value, the direct-step-down mode is selected, and for areas with low load density, a two-stage step-down mode is selected, which reduces the total investment to the greatest extent.

[0139] Therefore, the method provided by this invention can, to a certain extent, take over the existing power grid in the planning area, thus improving its practicality. Furthermore, the method of selecting different voltage levels according to the different load densities in the planning area can save investment and improve economic efficiency.

[0140] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for optimizing voltage levels in rural high-voltage distribution networks considering load density, characterized in that, include: By quantitatively analyzing the impact of load density on the selection of voltage levels in rural high-voltage distribution networks, an economic model is constructed to obtain a quantitative method for measuring the economic efficiency of 110kV and 35kV voltage levels. The critical load density value calculated by the economic model is used to select the voltage level of the rural high-voltage distribution network, and the load density threshold condition for the optimal selection of the voltage level is further derived. If the load density is lower than the critical load density value, the rural high-voltage distribution network adopts a two-stage step-down mode, which is more economical; otherwise, the direct step-down mode is more economical. The construction of the economic model includes, An economic model was constructed by collecting cost statistics of power transmission and transformation projects in the planning area and analyzing the power supply capacity of a single substation. From the perspective of thermal limits, the calculation of the required 110kV and 35kV transformer capacities for the planning area includes, in, This indicates the transformer capacity for 110kV and 35kV. k Indicates the capacity ratio. This indicates the load forecast for the area to be planned. This indicates the power factor angle on the high-voltage side of 110kV and 35kV substations; The comprehensive annual cost of the two-stage step-down mode and the direct step-down mode is calculated based on the construction cost and operation and maintenance cost of the power distribution network in the planning area. The calculation of the combined annual fee for the two-stage voltage reduction mode and the direct voltage reduction mode includes, in, This indicates the total annual fee under the direct reduction model. This indicates the combined annual fee for the two-stage voltage reduction mode. This indicates the cost of a 110kV / 10kV power transmission and transformation project. This indicates the cost of a 35kV power transmission and transformation project. This indicates the rated capacity of a 110kV substation. This represents the percentage of the ultimate power supply capacity of a 110kV / 10kV substation considering voltage constraints. This represents the annual discount rate factor. This indicates the annual maintenance cost for the direct reduction mode. This indicates the annual loss cost in the direct reduction mode. This indicates the annual maintenance cost for the two-stage step-down mode. This indicates the annual loss cost of the two-stage voltage reduction mode. a This represents the ratio of the cost of a 110kV two-stage step-down transmission and transformation project to the cost of a 110kV direct step-down transmission and transformation project. This indicates the rated capacity of a 35kV substation; The difference in loss cost between the direct step-down mode and the two-stage step-down mode is that the two-stage step-down mode has an additional 110kV / 35kV transformer compared to the direct step-down mode. This difference will cause additional substation losses, and the loss cost of the two modes needs to be simplified. The loss cost The simplified calculations include, in, Indicates electricity price, This indicates the number of hours of maximum load loss. This indicates additional transformer losses. Indicates the rated transformer capacity; By analyzing the calculation results of the loss costs in the direct step-down mode and the two-stage step-down mode, it can be seen that... and Compared to the annual cost of power transmission and transformation projects, the cost is extremely small, and Compared to The increase is also extremely small, therefore the calculation F 1 and F 2 o'clock and Negligible; After simplifying the aforementioned loss costs, the calculation of the economic advantages and disadvantages of the two modes includes: in, This indicates the economic advantages and disadvantages of the two models. This represents the cost per unit rated capacity of a 110kV / 10kV power transmission and transformation project. This indicates the cost per unit rated capacity of a 35kV power transmission and transformation project. The acquisition of the load density threshold condition includes, set up Calculations show that exist There is only one root, represented as, in, express exist One of the roots.

2. The method for optimizing the voltage hierarchy of rural high-voltage distribution networks considering load density as described in claim 1, characterized in that: The impact of load density on the selection of voltage levels for rural high-voltage distribution networks includes: The calculation of voltage loss on a 10kV line includes, in, Indicates the final pressure drop. This indicates a rated voltage of 10kV. This indicates the voltage drop of a 10kV line. To meet voltage quality requirements, the power supply radius L The following conditions must be met: in, M This indicates the number of outgoing lines from the substation. This represents the voltage loss coefficient for different load distributions. Indicates the simultaneous rate, Indicates load density, r This represents the resistance per unit length of the line. x Represents reactance per unit length. This represents the load power factor angle of a 10kV line; Upper limit of power supply capacity of a single substation under voltage constraints The calculations include, 。 3. The method for optimizing the voltage hierarchy of rural high-voltage distribution networks considering load density as described in claim 2, characterized in that: It also includes, when hour, The direct voltage reduction mode would have a higher cost, while the two-stage voltage reduction mode would be more economical. when hour, The two-stage voltage reduction mode is more expensive, while the direct voltage reduction mode is more economical.

4. The method for optimizing the voltage hierarchy of rural high-voltage distribution networks considering load density as described in claim 3, characterized in that: The calculation of the critical load density value includes, in, This represents the critical load density value. This represents the ratio of the capacity of a 110kV substation to the number of 10kV outgoing circuits.

5. The method for optimizing the voltage hierarchy of rural high-voltage distribution networks considering load density as described in claim 4, characterized in that: The selection of voltage levels for rural high-voltage distribution networks includes, If the load density is lower than the critical load density value, then the rural high-voltage distribution network adopting a two-stage voltage reduction mode is more economically advantageous. If the load density is higher than the critical load density value, then the rural high-voltage distribution network adopting the direct reduction mode has a greater economic advantage.

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