Welcome to this module on MV Substation- Civil Design and Construction. The first question that features in mind is what is the role of civil engineering in an electrical substation? The answer can be found out from the substation features itself. The substation consists of various types of buildings and various outdoor electrical equipment installed in the substation yard. There are certain electrical equipment and panels that need to be located indoor, hence buildings are necessary. The outdoor featured equipment are mounted on structures. All these buildings and structures are subjected to various kinds of equipment loads and exposed to live load, wind load and seismic actions. Against all these exposures, the substation building must be safely located and designed for serviceability. The support structure for various equipment along with its foundation is required for safe operation and execution. All these activities fall under the scope of civil engineering. This topic covers site development plan and layout finalization, familiarization of buildings in a substation, cable trenches, duct banks and cable tray support. The first and foremost activity in any substation is to identify the substations land. Once the land is identified, a contour survey shall be initiated to obtain the details of the allocated land like blood boundary dimensions and coordinates of the plot where the substation is located. Profile of land that is whether the provided land is flattering or undulating terrain or hill or valley. Location of approach road to the substation from an existing road nearby, any other obstructions in the plot like transmission or distribution lines, nallah or water stream, etcetera. The contour inputs play a major role in finalizing the substation layout arrangement and finished ground level or FGL of the substation. After completing the contour survey, a geotechnical investigation shall be done to obtain the engineering properties of soil and hydrological conditions, especially data on soil properties and groundwater table through borehole test. Soil strength like safe bearing capacity, wild capacity through borehole test to decide the foundation type for various structures and civil works. The site selected for locating substation maybe undulated are with local pits and visitations. All vegetation is have to be cleared and the surface to be leveled. The early civil works include finalization of finished ground level, cutting off earth filling with cut earth or imported earth, compaction first stage road approach road and fence. These early works play a crucial role in the construction of the switchyard and finalization of the substation layout. Substation shall generally be placed in a flat ground with uniform level that has finished ground level or FGL. If the substation location is highly undulated option of adopting multiple finished ground levels may be tried, provided it meets the electrical equipment. FGL shall be higher than the nearby highway road and the maximum flood level of that location. The control building should be located preferably at a higher level of terrain based on contour to avoid flooding at the building location. The finished floor level of the building shall be a minimum of 500 from FGL. And the same may be increased based on the high flood level to avoid flooding inside the building, the building is generally not cited in filled up soil. There are few buildings in the substation and switch yard. The construction of these buildings plays a critical part in the project execution. It is imperative that an engineer should acquire good knowledge about the functionality and intricacy of the buildings, construction sequence and construction material. MV substation consists of two types of buildings. Main electrical building, also called as control come switchgear building houses on indoor electrical equipment. Security building or guard room is included as the substation is a restricted area and it is usually built near the entry of the substation plot. Various rooms accommodated in main electrical building or MV switchgear room, LV switchgear room, control and relay panel room, battery room, cable center, store and pantry room, office room, toilet. Building dimensions and arrangements are considered as per the final electrical equipment layout. All buildings are designed as reinforced cement concrete framed structure with reinforced concrete roof or steel roof. The electrical equipment rooms found in a substation are switchgear room, LV switchgear room, operation and control room has the main computer control system, control and relay panels are usually mounted in this room. They can be mounted in a separate room as well. Battery room houses all batteries required for DC UPS and PC UPS. Cable celler is planned below switchgear and electrical rooms wherever necessary for smaller substations cable trenches beneath the panels are provided instead of cable cellar. The other utility rooms in a substation electrical building are the office room, pantry, toilet and staircase loading platforms will also be provided to facilitate the unloading of panels from transport trucks. Monorail for movement of panels for maintenance can be provided if felt necessary. The other works in the building are water supply system, sanitation system and architectural works. The water supply system includes a bore well or sump to building overhead tank and water supply, distributing pipes to toilets and earth pits. Construction, portable water shall be sourced either from a bore well or another source. Sanitation lines include soil, raised and vent pipes from the toilet to septic tanks. Other building drains are also connected to building sewer or sanitation lines except for rainwater drain with their devices, appurtenances and connections inside the building and outside the building within the property line. Rainwater drain will be connected either to stormwater drain or to rain water harvesting structure. Architectural design at the front side of the building to have aesthetics, landscaping works may also be included within the substation. Once the substation building and the switchyard equipment layouts are finalized, cable trench layout will be finalized based on the voltage grade cable trench type, the number of cables, size of cables. Dimension of trench and its roots are firmed up by electrical engineer on cable layouts, which is also called power layout. The cable layout will also take into consideration of cable bending radius. Cables are routed normally in the body trench, concrete trench and an unplasticized PVC duct banks in accordance with local practice. Different types of cable trenches are adopted, in body trench, cables are directly buried in the excavated earth. Buried trench is generally used for distribution cables or outdoor substation cables. In concrete trench cables are installed either directly on soil filling or it may also have supported on angles and trace. Concrete trench with support angles is generally being used in the substation at indoor and outdoor cable laying. Pipe ducts are being used at road crossing locations and majorly used at substation roads. Horizontal directional drilling or HDD method will be used when conventional excavation is not possible for a cable crossing, especially for the roads with heavy traffic or at nallah or river of smaller vent 150 meters maximum. Cable trenches are provided with concrete covers for outdoor installations and checkered plates for indoor installations. Cable trenches may have cable trays fixed onto the side walls to support cables or cables may be laid on a sand cushion at the bottom of the concrete trenches. Similarly, uPVC ducts may be buried directly in the ground or embedded in concrete. Instead of you PVC ducts, RCC ducts can also be placed directly buried. Local practices will normally be governed by a selection of such choices. However, for road crossings, concrete embedded duct banks are mostly preferred as they have to withstand loads of traffic and transport. Road crossing concrete trenches are also provided within the plant-areas, as per some local practices. Along the barrette cable trench route excavation of soil shall be carried to the specified dimensions, length into breadth into depth. At the bottom of the excavated trench, cables are placed over saved sand bedding. After installing cables, sieved sand will be filled for 75 millimeters. Cable tiles, concrete, precast or plastic are placed over the sand bed. Cable tile will protect cables from physical damage. After that, it will be backfilled with sieved soil, leaving 300 mm from the ground level. The yellow warning tape cautions the presence of cables. Again excavated soil shall be backfilled, compacted and leveled to the required ground level. Concrete cable markers shall generally be placed at every 50 meters on linear routes and at bends and crossings. Important aspects in cable trenches. The concrete cable trench shall be laid in a minimum slope of 1 is to 250 in the direction perpendicular to the run of the trench. And a maximum slope of 1 is to 500 shall be given along the run of the cable trench for effective drainage of rainwater ingress. The bed level of the indoor trench shall be higher than the bed level of the outdoor trench to prevent the entry of water inside the building through trenches, pipe sleeves provided. For the interconnection of outdoor strange and building shall be properly sealed with a proper sealant after laying cables to avoid water seepage inside the building from outdoor trench. The downstream end of the concrete cable trench shall be connected through pipe drains to the nearby sump pit or water drainage system to avoid water stagnation in the trench. The cable trays maybe of ladder type or perforated sheet type. Cable trays may be made of galvanized steel or aluminum or stainless steel, as you may have guessed that the material of the tray is stated in the order of increasing cost. Accordingly, only a few preferred to have stainless steel trays and the majority prefer galvanized steel trays, trays are usually bought out items. Trays supports, cantilever supported from the side wall or ISA angle sections welded to the concrete inserts. Such a construction type involves extensive site welding. It is also a usual practice to go for pre-engineered cable trays and accessories, which require only bolting accessories without any need for site building. Wherever the cables are to be supported without the trays the spacing of cable support angle shall be 600 millimeters minimum and up to 1000 millimeters. While at all other places with trays the interval of support maybe up to 2000 millimeters. Generally the cable trays are 3 meters in length and they are supported at every 1000 millimeters or 2000 millimeters based on the loading and strength capacity of structural steel used. Apart from concrete trenches, trays are also used to support cables rooted above ground. See the photo where cable trays are installed in the cable cellar or basement, pipe or duct for use as duct bank. The duct bank consists of sufficient diameter. A number of pipes encased in RCC are reinforced cement concrete, are PCC or plain cement concrete. The pipes shall be PVC or RCC pipes. The provision of additional pipes in the duct bank for future cable also shall be considered. From the trench to the final equipment location, pipes are Usually installed at a depth of 300 millimeters below FGL. If cables are required to cross heavy traffic or river permission to cut and install the duct bank will not be given in case of international projects. In such cases, a method HDD, horizontal direct drilling will be adopted. Wherein from one side of the road or river bank, horizontal drilling at a depth of say 2 meters or 2.5 meters shall be adopted using horizontal boring machine, galvanized steel pipes or HDPE pipes sleeves can be used as ducts. The drilling is carried out below the nallah. In case off road, the drilling will be almost horizontal. Here is a quick recap of what you have learned thus far. Substation shall generally be placed in a flat ground with uniform level that has finished ground level or FGL. If the substation location is highly undulated option of adopting multiple finished ground levels may be tried, provided it meets the electrical requirement. Main electrical building, also called as control com switchgear building houses all indoor electrical equipment. Various rooms accommodated in this building are MV switchgear room, LV switchgear room, control and relay panel room, battery room, cable cellar, storeroom, pantry room, office room and toilet. Security building or guard room is usually built near the entry of the substation plot as the substation is a restricted area. After the finalization of substation building or switchyard equipment layouts, cable layouts are firmed up by electrical engineer based on the number of cables, voltage grade, size of cables, cable trench type, dimensions and trench routes. Cables are routed normally in body trench concrete trench and in uPVC or unplasticized PVC, duct banks in accordance with local practice. The duct bank consists of sufficient diameter a number of pipes encased in RCC are reinforced cement concrete, or PCC or plane cement concrete. If cables are required to cross busy traffic HDD or horizontal direct drilling will be adopted from one side of the road with galvanized steel pipes. Our HDP pipe sleeves, the bed level of indoor trench shall be higher than the bed level of outdoor trench to prevent entry of water inside building through trenches, pipe sleeves provided at the inter condition of outdoor trench and building shall be properly sealed with a proper sealant after laying off cables to avoid water seepage inside the building from the outdoor trench.
