Rational design of substation civil works

Thread Content

Research phase: During the feasibility study phase, emphasis is placed on factors such as land area required, location of the site, demolition work, foundation, and underground utilities, in order to determine the final location for the station. 　　(1) Floor area Generally, the number of outgoing circuits and the quantity of main transformers are the decisive factors in determining an appropriate layout scale. By properly adjusting the relationship between the outdoor power distribution equipment site and buildings, and by utilizing advanced equipment and layout methods that make use of available space, it is possible to effectively reduce the required floor area. Where possible, buildings should be combined into one integrated structure while still meeting their functional requirements, in order to reduce the land area occupied; this also leads to lower costs for associated retaining walls, fences, and site grading. Through a technical and economic analysis of various layout options for outdoor 220KV substations, based solely on comparisons of construction costs, the use of two main transformers is preferred; there are 7–8 220KV output bays and 13–15 110KV output bays, with the 110KV and 220KV sections arranged symmetrically, resulting in less land usage. If 3 main transformers are used, there will be 8 to 15 220KV outgoing bays, and 18 to 20 110KV outgoing bays; the 110KV and 220KV systems are arranged symmetrically, resulting in less floor space required. Therefore, it is required that those responsible for the initial scientific research and design make a scientific plan; the number of main transformers should be proportional to the number of outgoing busbars. The above are merely personal opinions for reference only. At a substation on the outskirts of Chongqing, the 220KV and 110KV switchgear areas are arranged symmetrically; the main transformer and functional buildings are located in the middle. Firewalls are used on the side facades of the buildings, with the main transformer situated between them. It covers an area of just over 20 mu. 　　(2) Selection of site location The site should be chosen in accordance with the requirements of the planning and land management departments. It should be located near load centers, in an area with flat terrain, short road connections, minimal need for demolition, and easy access to water sources and municipal facilities. Areas with large elevation differences should be avoided as much as possible. In plain areas, the cost of site leveling has a minor impact depending on the location of the station site ； In hilly and gully areas, the cost of site grading has a particularly significant impact on the overall construction cost. Utilizing municipal facilities can save costs related to water supply and drainage, fire protection systems, as well as staff amenities, and it can also reduce expenses associated with roads leading to the site (a cost that cannot be ignored). The author was involved in the design work for a 220KV substation. Due to inadequate consideration during the site selection phase, the substation was located between two small hills, with an alluvial gully in between; the maximum height difference there was nearly 30 meters. After multiple optimizations by the author, a three-step design was adopted, and the total volume of earthwork and rockwork that needed to be removed amounted to 80,000 cubic meters, while the volume of stone retaining walls required was around 6,000 cubic meters. As for the feasibility report for a recently designed 220KV substation, the site was carefully selected during the site-selection phase; it is relatively flat with a maximum elevation difference of about 4 meters. After optimization, the amount of earthwork required for leveling the site is the same as that in plain areas. Only the earthwork related to foundation construction and trench digging is required for site leveling, while the amount of earthwork for stone retaining walls is only 600 cubic meters. From the comparison of the costs for earthwork excavation and retaining walls in these two cases alone, it is evident how important site selection is for project costs. 　　(3) Demolition and compensation for cash crops or trees Once the location is determined, it is necessary to gather information on local land expropriation fees, demolition compensation amounts, and the availability of building materials. When selecting the location, try to minimize the use of economically valuable farmland or timber plantations, and avoid demolishing houses, relocating power lines, or disturbing graves. When necessary, local chamfering, translation, rotation, and other adjustments to the overall layout are employed to reduce compensation costs and lower the total construction expense. 　　(4) Foundation treatment and underground concealed facilities: During the feasibility study phase, it is necessary to understand the conditions underground – whether the area is in a high-fill zone or a landslide-prone area, and whether there are air-raid shelters or major pipelines underground – this can help reduce unnecessary costs associated with foundation treatment and relocation. 　　During the feasibility study phase of a certain 110KV indoor substation, it was found that the thickness of the underground backfill soil was 15 meters; this soil consisted entirely of rubble from excavations, and its density was low, making it unsuitable as a bearing layer for major structures. The cost of dealing with this issue amounted to 1.5 million yuan. In addition, there was a main sewage pipe with a diameter of 1.2 meters buried underground, and the cost of relocating this pipe was 600,000 yuan. As a result, it was decided to choose a new location for the substation. 　　During the feasibility study phase, considerations are mainly drawn from the above four aspects to determine a reasonable site location, thereby achieving cost savings. 　　(II) Preliminary design stage In the preliminary design phase, the focus is on determining the site location. By comparing various design options regarding the overall layout and vertical arrangement, as well as quantities of earthwork to be removed or filled, retaining walls, slopes, buildings, foundations, roads, and water supply and drainage systems, an economically reasonable solution is identified. The final area of land that needs to be acquired for the site is determined; generally, it is advisable to compare three different design options. 　　(1) General layout 　　Optimization is primarily carried out in terms of power inlet and outlet, road connections, drainage connections, safety distances, fire protection, and transportation. In accordance with standards and regulations, the various buildings of the determined scale are arranged reasonably, shared facilities are combined wherever possible to reduce the area occupied by roads, and development is directed upward as much as possible to make the layout more compact and save land use. For example: by combining the capacitor room and the power distribution room in a linear layout, the fire separation distance can be eliminated. The power distribution room, capacitor room, and control room are integrated together, reducing space requirements and eliminating the need to consider fire separation distances between buildings. For example, at a 220KV substation in the suburbs of Chongqing, thorough planar optimization was carried out during its expansion: two main transformers were located on either side of the control building, while a 10KV distribution room and capacitor room were arranged in a row behind it. The layout was compact and rational. The 110KV outdoor distribution equipment was installed in a semi-high configuration, allowing for better use of space and saving approximately 5 mu of land. 　　(2) Vertical layout 　Whether it is a flat or stepped layout, it is necessary to make full use of the existing terrain, minimize land grading and earthwork, and reduce unnecessary excavation. Depending on the elevation differences in the terrain at the site location, a stepped or flat-slope layout is determined to reduce the costs associated with land grading and foundations; the design should take into account the amount of soil and rock that needs to be removed during excavation of the foundation pit, as well as the volume of material required for slope stabilization and retaining walls. For a stepped layout, the height difference between the two levels should preferably not exceed 3.5 meters, to facilitate meeting the slope requirements for road connections within the station. At a 220KV substation in Jiangbei, Chongqing, the original terrain had a height difference of nearly 30 meters; a stepped layout was adopted, with the maximum height difference between two levels being 3.0 meters. The layout consisted of three levels, two of which had a slope of 2%, while one level featured a dual-slope design. This approach helped to reduce the need for retaining walls and the amount of earthwork required, thereby lowering the project cost. 　　For stations located in relatively flat areas, a flat-slope design is adopted, with the slope being as consistent as possible with the original terrain; however, the slope must be at least 0.5% to ensure proper drainage of the site. A 220KV substation in a suburban county features a flat site and is designed with a gentle slope. After multiple calculations using software to optimize the slope, the volume of earthwork required was less than 50 cubic meters, with most of it being reused for backfilling the foundation pit. 　　(3) Retaining walls 　　When the amount of earthwork required changes little, the lower the height of the retaining wall, the better. Retaining walls with a height of less than 6.0 meters are more cost-effective; for heights over 8.0 meters, reinforced concrete counterfort retaining walls are generally used. After horizontal and vertical optimization, the height of the retaining wall is generally also optimized, with different cross-sectional shapes selected depending on the geological conditions and intended use. The height of the excavation area shall not exceed 5 meters; retaining walls should be used as much as possible in such areas to reduce slope excavation and land use. No examples are given here. 　　(4) Slopes 　It is not economical to construct slopes when the excavation depth is within 6 meters. If a slope ratio of 1:1 is used, it requires an additional land area of 6 square meters per meter, as well as an increased amount of earthwork of about 18 cubic meters per meter. In addition, slope protection measures are needed; therefore, it is more economical and reasonable to use rubble stone retaining walls in the excavation area. For heights of over 7 meters, it is appropriate to use slopes; however, retaining walls require a large cross-sectional area, which is not economical. 　　(5) Building structure The building area is determined based on the design scale of the substation; efforts are made to reduce unnecessary auxiliary areas while still meeting the functional requirements. According to current standards, for multi-story buildings with floor heights exceeding 3.6 meters, brick-concrete structures no longer meet seismic requirements. It is recommended that rooms such as capacitor rooms, power distribution rooms, and control rooms be integrated into a single building, which is more reasonable; this helps to reduce the cost per unit area of construction. A frame structure is suitable for this purpose, as it allows for larger spaces that facilitate the placement of electrical equipment. Save floor space. It also saves space for fire separation between buildings, reducing construction costs. 　　(6) Foundation 　When selecting a foundation for buildings and structures, it is necessary to take into account the geological conditions and adapt the design to local circumstances, making full use of natural foundations. Ordinary strip foundations account for 15%~20% of the total cost of a building; therefore, it is essential to optimize the foundation design. Under normal circumstances, natural foundations and strip enlarged foundations are considered first. The foundation is compacted using old clay, located above the water table; a digging depth of around 2.5 meters is considered. 37% lime-soil or mixed soil is used for compaction, either manually or mechanically. Each layer of soil filled in is about 25 centimeters thick, and compaction continues until the thickness reaches 15 centimeters, reaching the bottom surface of the foundation. For foundations with an excessive burial depth, block grouting is used to enlarge the foundation and reduce its burial depth. For natural foundations with a bearing capacity of 150 KPa, it is advisable to use a reinforced foundation made of fill material; generally, graded gravel is used for such reinforced foundations. The compaction coefficient of the fill soil in the backfill area should be no less than 0.94; by measuring its bearing capacity, it is feasible to construct a single-story building directly on top of it. For multi-story buildings, pile foundations are used when the depth of the bearing stratum is greater than 5 meters; while for depths between 3 and 5 meters, grouting with rubble is a more economical option for deep foundations. 　　The preliminary design plan generally takes the above six aspects into consideration, and after comparison, the resulting plan essentially achieves the goal of optimal design. 　　(III) Construction design phase: For the scheme determined through the review of the preliminary design, adjustments are made in accordance with current standards and regulations or **criteria, as well as the dimensions specified for the equipment; these adjustments are carried out first on a partial basis and then on an overall basis, with the principles of adjustment not exceeding those established in the preliminary design. Once all dimensions have been confirmed, detailed calculations and economic comparisons should be carried out for the structural dimensions of various buildings and structures as estimated in the preliminary design, following the principles of adapting to local conditions and using locally available materials. Advanced technical processes and rational cross-sections are employed, along with locally sourced materials, to reduce unnecessary transportation costs. Based on the detailed geological data, the foundation should be designed appropriately; strip foundations are preferred. The required depth of a conventional foundation is around 2.0 meters, and pile foundations are not used when the depth is less than 3 meters. 　　(1) General layout and vertical arrangement The general layout is usually determined based on preliminary principles, requiring no major adjustments; minor adjustments can be made in certain areas. The main focus is on adjusting the vertical arrangement to determine the optimal elevation, thereby minimizing the volume of earthwork required for site leveling as well as the amount of retaining walls and slope protections needed. 　　(2) Retaining walls and slopes The elevation is determined based on the site grading calculations from the construction design, in order to establish appropriate cross-sections for the retaining walls and slopes. The design of these structures should make full use of geological data. The geological conditions on the reverse slope can be handled simply. For slopes with favorable geological conditions, a combination of slope protection and retaining walls can be considered. For retaining walls in filled areas, the materials and cross-sectional design of the walls are determined based on the bearing capacity of the foundation. In areas where the foundation bearing capacity is low, it is advisable to enlarge the foundation base and reduce the depth to which it is buried. Retaining walls of the counterfort type are used when the height is over 8 meters, and natural foundations are preferred for the base. 　　(3) Building structure The architectural design should strive to be in harmony with the surrounding environment, meeting the requirements regarding the facade and functional uses while reducing unnecessary additional building areas. The span and floor height should be reduced according to the operational needs of the equipment. While meeting the regulatory requirements, brick-concrete structures should be used as much as possible to reduce the use of reinforced concrete frame structures. When the calculations for outdoor power distribution installations meet the requirements of the codes, prefabricated reinforced concrete ring poles should be used as much as possible. 　　(4) Foundation The foundation design is a key aspect in optimizing the construction design. It is necessary to study the geological data carefully, make use of natural foundations wherever possible; for areas where the depth is excessive, block grouted foundations should be used. In areas with poor soil conditions, enlarged foundations should be employed. The foundation must meet the requirements for equipment installation and operation, while also being as shallow as possible. Try to use enlarged foundations instead of reinforced concrete pile foundations. In areas where pile foundations are necessary, drill-driven piles with a diameter of 300–400 mm should be used as much as possible, in order to reduce or eliminate the use of manually dug piles. The minimum diameter for manually dug piles is 800 mm, and their cost is significantly higher. 　　(IV) Construction During the construction phase, it is necessary to strengthen the inspection of the foundation pit to ensure that the excavation meets the design requirements regarding the underlying soil layers. In areas with favorable geological conditions, and when the specifications and equipment operation requirements are satisfied, excavation should be minimized. In cases where the designed foundation conditions do not match the actual conditions, temporary exploration pits should be dug on-site or on-site tests should be conducted. If the natural foundation can meet the required bearing capacity, it should be utilized; otherwise, enlarged foundations or temporary local filling methods should be employed. The filling area after construction should be compacted manually or mechanically, with a compaction coefficient ranging from 0.91 to 0.95; the construction party must provide the final test results to ensure that the ground does not settle.