Optimal practices for the design of concrete and steel structures in power substations involve accounting for safety clearances, insulation capacity, allowable loads on equipment and structures, as well as environmental criteria. Foundations must be designed and calculated by a civil engineer to support the design loads and adhere to regulatory standards. Additionally, it is crucial to take into account soil conditions, installation expenses, and the selected utility standard design approach.
Designing a substation typically starts with the overall layout, which hinges on the necessary safety clearances and insulation levels, as well as the allowable loads on the substation's equipment and structures. These loads can affect the choice of high-voltage (HV) conductors used, which may, in turn, impact the layout and various other factors.
Once the busbar layout, ratings, and suitable equipment for a substation are determined, several additional aspects related to its construction require meticulous planning. This technical article will discuss the foundations for various high-voltage apparatuses, substation structures, trenches for control and power cables, oil containment measures, and other pertinent subjects.
The designs mentioned are of great importance, and best practices will be explored.
Table of Contents:
1. High-Voltage Equipment Foundations
The expense of civil engineering for a substation encompasses the installation of foundations for AIS equipment components, warranting attention to the most economical approach. Foundations must be designed and computed by a civil engineer to support the design loads and load combinations outlined by the substation designer, adhering to industry or national standards and regulations.
Developing the most cost-effective design may require iterative collaboration. It's essential to apply a uniform design strategy throughout the substation when selecting loads. This could involve designing to match the rated loads of the equipment or choosing loads that align with an optimal combination of design specifications.
A single foundation may be utilized for each element in every phase, or one foundation could encompass all three phases.
There are several options for foundation designs, including shallow raft foundations, block foundations, or deep foundations with pillars for mounting equipment.
Ground conditions, installation costs for different systems, and the selected utility standard design methodology could all influence the decision-making process.
Figure 1 – Grounding of steel structures
Foundation-building methods differ based on soil type and load characteristics.
Concrete that has been poured with or without steel reinforcing
Reinforced concrete prefabricated
Concrete slab (often used for interior substations or GIS)
Drilled (for use in hard soil)
Pile Auger-bored
When designing the layout of a substation, it is crucial for the designer to determine the appropriate level for the top of the foundation, whether it be below, at, or above the finished ground level. The equipment may be supported directly on the foundation, slightly elevated, or positioned at a specific height such as 100 or 200 mm above the foundation.
When equipment support is placed directly on the foundation, it is essential to grout or seal the gap to prevent water accumulation at the interface. Should the support stand taller than the foundation's top, structural requirements might call for grouting the space between the support base and the foundation or constructing a concrete cap encircling the base of the support.
When using a cap, it is crucial to carefully monitor the junction between the cap and the support structure to prevent water accumulation that could lead to corrosion.
Drilled and cast-in-place anchors represent the two fundamental techniques for securing a structure to its foundation.
Anchor bolts are commonly used to connect the support structure to the foundation using two design methods. They can be either cast into the foundation during the pouring process or drilled into it once it has hardened. The two types of drilled anchor bolts are expansion anchors and chemical anchors.
Figure 2 – The structure of the current transformer is secured to the foundation using anchor bolts.
Chemical anchors provide the advantage of locating the support structure and utilizing the baseplate holes as a drilling guide. Nevertheless, it is crucial to exercise additional care to adhere strictly to the supplier's instructions to achieve proper adhesion, which greatly relies on the cleanliness of the drilled hole.
It is advisable to conduct a pullout test on a significant portion of the installed anchors. Since the hole diameter for expansion anchors is usually larger than the bolt holes in the structural baseplate, a distinct drilling template is necessary.
During the installation of foundations, it is crucial to precisely position cast-in bolts because certain structures, like gantries, necessitate exact alignment of bolt positions across multiple foundations. In contrast, drilling anchors into set foundations offers more leeway in their placement.
Certain cast-in bolts are designed to permit a degree of positional adjustment, which can be subsequently fixed in place by grouting once the support structure has been erected.
The steel framework, including the anchor bolts, can be cast into the foundation. In this case, the foundation is poured with a pocket designed to fit the steel framework. The structure must be precisely aligned using additional setting frames before the concrete is poured.
Figure 3 – Typical post-installed anchors
Post-installed anchors are a common choice for securing elements to concrete after it has been set. They include types such as mechanical expansion anchors and adhesive anchors, which are installed into pre-drilled holes in the concrete.
Anchor bolt fastening provides a significant advantage, particularly when making lead connections with tubular conductors. It allows for the adjustment of the structure's height to compensate for any discrepancies at the top of the foundation level.
In addition to the structural fixing method design, the foundation design must also consider the installation of earth conductors and control cables. This can be done either during the foundation's installation or afterwards, using cut-outs, for instance.
Each approach has its benefits and drawbacks. Incorporating design provisions during the foundation installation may slow down the process and increase its cost, yet it can expedite and decrease the cost of later earth conductor or control cable installation. Nonetheless, if the design provisions are not implemented properly, correcting them could be expensive and troublesome.
The alternative approach of post-pouring provision accelerates and lowers the cost of laying foundations, albeit at the expense of subsequent work. However, this subsequent work is guaranteed to be correctly positioned as it can be adjusted to align with the pre-existing equipment support structures.
Generally, the construction of foundations is minimally affected by the presence of high voltages or large currents. However, an exception exists in the case of air-cored reactors, which produce very strong magnetic fields and require specialized foundation design.
The design of reinforcement for these foundations should avoid closed loops of conductive material by using insulating material at the joints of conventional reinforcing bars or by utilizing nonmetallic reinforcing materials.
Figure 4 – Foundations for transformers with rails
Civil works for transformers or oil-filled reactors are designed to fulfil four key functions: to support the transformer and facilitate its movement into and out of service, to contain any leaking transformer oil, to reduce the risk of fire spread, and where necessary, to mitigate acoustic noise propagation.
Civil works for transformers or oil-filled reactors must serve four distinct functions:
To support the transformer and allow it to be moved into and out of service (rails may be required depending on the transformer type).
To contain any transformer oil leaking. Extinguishing oil fires can also be aided by filling the oil containment area with gravel covered by a top layer of broken stones or by connecting it to an underground tank.
To decrease the risk of fire propagation (recommended are fire walls and fire barriers in trenches).
Where necessary, to decrease acoustic noise propagation.
2. Substation Buildings
Substation buildings should be constructed following national and corporate standards. They are primarily intended to house and protect switchgear, protection relays, SCADA systems, auxiliary equipment, batteries, fire protection pumps, among others. The scale of water, sewage treatment, and accommodation facilities needed for the operators is determined by whether the substation is staffed or remote.
The need for a workshop and its size are determined by the accessibility of substations and the utility's maintenance practices. Similarly, the amount of maintenance equipment, including elevating platforms and SF6 handling facilities, that should be permanently stored on-site versus transported as required for particular tasks, must be evaluated using the same criteria.
Ultimately, there may be sound reasons for the installation of a transformer un-tanking hall, for instance.
Economic considerations, such as reducing control cable lengths and cross-sections, lowering auxiliary supply voltage, and minimizing initial investment, may lead to the design of a substation with several dispersed buildings instead of a single central facility.
Substation structures can be constructed from various materials such as reinforced concrete, concrete blocks, bricks, clad steel, or steel plates. The choice of a pitched or flat roof may depend on building costs and planning constraints. Additionally, surface finishes and color treatments can be determined by planning requirements.
Building design should consider lifetime costs, especially regarding protection from moisture infiltration and the prevention of corrosion.
Figure 5 – New piers have been poured and cables installed in preparation for the arrival of the control house.
2.1 Energy Consumption
Substation buildings are substantial energy consumers. New structures can readily comply with the latest energy efficiency regulations. However, retrofitting existing buildings is more complex, necessitating thorough energy assessments.
Site planning, building orientation, wall and roof design, window configuration, solar heat gain, heating, ventilation, air-conditioning systems, electric lighting, and landscaping all represent opportunities for energy conservation.
Figure 6 – An example of a properly ventilated battery room
2.2 Built On-Site or Prefabricated?
Another aspect of building design to consider is whether to construct the structure on-site or to prefabricate it off-site, then deliver it to the location fully equipped and place it onto a prepared foundation. For small substations, the decision may involve determining if the facility should be considered permanent or temporarily relocatable.
When it comes to cabins, containers, or blockhouses used for decentralized control and protection equipment, the decision between on-site and off-site placement becomes particularly crucial. Additionally, it is worth considering whether a separate building is required for each bay, or if a single structure can house multiple bays.
While initial and ongoing costs for a particular country or location are the main factors driving the choice of construction type, security considerations are also crucial. These may require the use of minimal or no windows, or alternatively, protected windows, as well as reinforced non-flammable roofing and doors, among other measures.
In building design, particular attention must be given to flood protection, especially through the implementation of an appropriate floor elevation for prevention.
Figure 7 – Containerized and prefabricated substations
Containerized and prefabricated substations offer a modular, transportable, and quickly deployable solution for electrical distribution needs. These units are factory-assembled, tested, and ready for rapid installation and commissioning upon arrival at the site, making them ideal for a variety of applications, including renewable energy integration and temporary power supply during maintenance or emergencies.
2.3 Control and Power Cables
Designing access for a building's control and power cables is a related concern. It demands extra caution as effectively sealing the cable openings to prevent water ingress poses a significant challenge.
In-room design, accommodating control cable access between equipment cabinets, across rooms, and from the building to the exterior is essential. There are two methods: overhead routing, where cables traverse on trays or ladders hanging from the ceiling, and below-floor routing, where cables run through open sub-floor spaces under the tiles or within built-in floor ducts.
Regardless of the system in use, it is imperative that all points where cables traverse rooms are fire-sealed to inhibit the propagation of fire or smoke.
Figure 8 – Sealing of cable entries
2.4 Environmental Requirements
The level of climate control required in substation buildings depends on the local climate and the specific environmental requirements of the equipment installed. Although temporary measures can be implemented during the construction phase, it is recommended that the environmental conditions inside a control building are conducive to the needs of operational staff.
The installation of climate control technology may be necessary. Besides the needs of the operational crew, many electronic equipment products have strict environmental requirements regarding allowable temperature and humidity ranges.
This is true for batteries as well, with some having a minimum temperature requirement. For instance, valve-regulated lead acid batteries experience a considerable decrease in performance when the ambient temperature rises above 20°C.
Climatic conditions influence the internal layout of buildings, making it advantageous to segregate equipment with distinct climatic needs. This approach helps to minimize the volume within the building that necessitates precise climate control.
2.5 Batteries
Viewed from another perspective, certain equipment can pose risks that necessitate isolation. Given that lead-acid batteries are filled with sulfuric acid, which can partially vaporize, the room housing these batteries must be equipped with acid-resistant surfaces.
For this reason, it is crucial for battery rooms to be equipped with air circulation and extraction capabilities, especially due to the emission of hydrogen during the charging process.
Figure 9 – Battery room on the left and battery chargers on the right.
2.6 Maintenance
In the design phase of a structure's layout, considering the ease of routine maintenance and the frequency of replacing control or protection equipment is crucial. The fundamental layout of the building should allow enough flexibility for future expansions or be designed to facilitate easy additions later on.
2.7 Transformers and Generators
When situating power supplies within a building as opposed to outside, it is essential to allocate sufficient space for housing supply transformers and/or diesel generators. For dry-type transformers, the transformer room needs to ensure sufficient airflow for cooling and, if live terminals are exposed, appropriate clearances and access restrictions must be in place.
The use of oil-filled equipment requires the building to be fireproofed, along with the installation of fire extinguishing and containment systems.
Large ventilation openings are necessary in a diesel generator room to ensure sufficient cooling for the engine, and they should be situated to allow proper airflow throughout the engine area.
The design of the generator base must ensure that vibrations from the generator are not transmitted through the structure.
Figure 10 – Room ventilation for indoor generator operation
2.8 Water supply
Water and sanitation facilities ought to be provided as necessary. In determining the infrastructure to be installed, consideration must be given to the frequency of use by workers and the long-term maintenance costs.
2.9 Fire Alarm
The SCADA system should be integrated with a zoned fire alarm system appropriate for the environment. Conducting a risk assessment is necessary to determine the suitable level of fire suppression required.
Figure 11 – 3D layout of a fire suppression system installed in a control room.
3. Trenches for Control & Power Cables
The configuration of control cable installation, connecting individual high-voltage equipment pieces to the bay marshalling and control points, and further linking these points to the control building(s), represents a critical element in the design of an Air-Insulated Switchyard (AIS).
There are three widely recognized techniques for cable route design. Each technique balances the initial installation costs against the ease and expense of making future adjustments, such as adding new cables.
These are three options:
Direct-buried cables: Cheap installation/expensive changes.
Cables placed in pipe ducts: Modest installation costs can be achieved with adequate access for modifications, provided that pulling ropes are installed and attention is given to pipe junctions to prevent silt blockages and to ensure a surplus of pipes is available.
Surface ducts: Installation may be costly, yet it allows for easy modifications later on. Two construction methods are available: on-site construction with poured concrete or concrete block walls, and prefabricated components made from a type of glass-reinforced plastic that can be rapidly assembled on-site.
Removable covers can be constructed from concrete or timber, each with its own set of pros and cons. For instance, timber may become slippery when wet, while concrete covers often require special lifting equipment. Since these ducts will serve as pedestrian walkways, they need to be built to endure the necessary loads. For example, concrete covers should include a certain degree of reinforcement.
For a secure foothold, the surface finish ought to be slightly roughened. Ensure that any potentially buoyant materials are securely anchored to prevent movement during a flood.
Figure 12 – Cable routes
The design of cable routes should consider the need for vehicle access, including heavy vehicles, around the switchyard. Therefore, the routes must either be engineered to withstand vehicle loads or have specific vehicle crossing points that are provided and prominently marked.
The initial planning of cable routes should include provisions for potential additional cables needed for any future expansion of the substation. Adding new cable routes to a control building at a later stage can be both complicated and costly.
4. Oil Containment
The selection of the location should ensure the protection of the natural drainage network, especially the permanent surface watercourses and groundwater recharge areas, to prevent damage to the subsurface network. All hazardous materials within the substation should be handled and utilized in a manner that prevents them from leaking into the groundwater or extending beyond the substation's boundaries. Additionally, the containers for power and instrument transformers, capacitors, coils, etc., should be leakproof wherever possible.
Most nations require additional safeguards against hazardous compounds.
Oil pits are designed to capture all or part of the oil (or other liquids) and prevent it from igniting. If a central underground tank is used, it needs to be sufficiently large to contain the volume of the largest piece of oil-containing equipment, any accumulated rainwater since the last emptying of the tank, and the amount of water generated by the water spray fire protection system (if used).
Rainwater can be drained when there is no oil leak. If contamination occurs, decontamination is necessary through mechanical separation, filtration, or chemical treatment.
Figure 13 – Transformer oil pit
Although large power transformers are the most conspicuous sources of oil pollution, it is essential to investigate all potential sources, such as diesel generator tanks, substation supply transformers, and bulk-oil circuit breakers, among others.
The probability of an oil spill occurring in a substation is exceedingly low. Nonetheless, some substations are at a greater risk of discharging significant amounts of oil into the environment. This risk is heightened by factors such as proximity to groundwater resources, open water, or designated wetlands, the volume of oil present on-site, the surrounding topography, soil characteristics, among others.
Heightened public awareness of the environmental aspects of substations, coupled with stricter environmental regulations, necessitates that utility companies implement measures to mitigate the environmental impact of new substations and particularly address the effects of older ones.
Source: Basic Design and Analysis of Air-Insulated Substations by Colm Twomey, Hugh Cunningham, Fabio Nepomuceno Fraga, Antonio Varejão de Godoy, and Koji Kawakita
Comments