This is a basic summary and explanation of engineering & design processes used during designing power substations - by Matt Cole, 3 Phase Associates
Power Substations
For the most part, electric power substations are viewed as the most integral part of a power utilities’ electric system, with electric systems being comprised of power generation, transmission, and distribution systems. (See A Basic Explanation Summary of How the Electric Power Grid Works)
Substations are power stations that include power transformers, potential or voltage transformers, current transformers, electrical bus, breakers, switches, and so on. A substation is a station that includes a power transformer for power conversions either stepping up or stepping down the supply voltage depending on whether it is a generating substation or transmission/distribution substation. Generating substations step up the voltage from the generator’s lower voltage to a higher voltage which is more suitable, and more economical for transmitting electric power over longer distances with less power losses caused by the impedances of transmission lines. Transmission substations take the incoming higher voltage from transmission lines and step them down to a lower voltage for distribution systems, which is in preparation for end user customers.
A switching station is different from a substation since it does not have a power transformer and does not transform the voltage supply. Within a switching station, the voltage coming in the station equals the voltage going out. A switching station is comprised of various switches and breakers that are used to help protect & control the power flow.
Power Substations are filled with large and very expensive equipment. A power utilities’ greatest expense in substations is the power transformer. Other items and equipment involved in designing substations which are less expensive than power transformers are: power circuit breakers, circuit switchers, line-rupters, manual and motor controlled operated disconnects & bypass switches, potential / current transformers (PTs / CTs), lightning arresters, electric bus, metering equipment, steel structures, foundations, control switch house, main station service (SS), automatic transfer switch (ATS), protection & controls (P&C), relaying/IEDs, SCADA, telecommunications equipment, battery bank, control cables, fencing, grounding, lightning protection, real estate, and so on.
Substations can be designed and constructed for outdoor use, known as air insulated substations (AIS); or for indoor/underground use, known as gas-insulated substations (GIS).
Substations may also be owned and maintained by manufacturing, industrial, or large commercial customers; instead of being owned and maintained by the power utility, if the distribution substation is a direct supply from the transmission entity to the customer. Substations can also operate at many different voltage levels. The most common high and medium voltage (HV / MV) levels are: 765kV, 500kV, 345kV, 230kV, 161kV, 115kV, 69kV, 46kV, 33kV, 25kV, 12kV, 7.5kV, 4kV, and 1.2kV. Some utilities may differ slightly on these voltages with small variations that match their power grid network system.
Although there are many factors to consider when designing a substation, the main items and basics steps used during the design process will be discussed here for distribution substations. Distribution substations operate mainly at voltage levels 69kV and below.
Planning for a New Power Substation
When a utility prepares to add a new power substation to their electric grid, it's usually because the electric load growth in their area has increased by consumers – new growth in businesses, industries, residential, etc. Another reason for adding a new substation is to replace an outdated, aging substation that is obsolete and has reached its end-of-life expectancy. The basic steps a utility may perform in planning and implementing a new substation are:
- Conduct planning meetings for discussing the new power substation.
- Perform load flow power studies.
- Determine the substation size and total footprint required (with equipment), including required transmission right of way (ROW) clearances.
- Determine the substation configuration (Single Bus, Main/Transfer Bus, Ring Bus, etc.).
- Allocate the required funds for constructing the substation - necessary real estate purchase, planning, engineering, construction, implementation, testing, turn-up / energizing, maintenance, etc.
- Determine the best location or area and acquire the real estate including the distribution line (TL) ROW access.
- Create the substation project with scheduled milestones along with the final in-service (energize) date.
- Assign the project team resources with in-house staff, external staff/sub-contractors, or contract the entire project as a turnkey solution - engineer-procure-construct (EPC) option. Engineering & design services usually include electrical, civil & structural - protection & controls (P&C) design, physical engineering design, foundation & structural steel design, telecom engineering design.
- Begin engineering & designing the substation drawing package and deliverables.
- Perform periodic design review meetings along with a final design review meeting and pre-construction meeting.
- Finally construct, test, implement, and energize the new substation.
- Update the utility's power system model with the newly added substation in service.
Designing a New Power Substation
Once the substation planning has been completed with real estate acquired, and the project has kicked off; a scaled site plan (SP) will be created to determine the right of way (ROW) access for roads, transmission lines, distribution lines, and other utility access, such as, water, sewer, gas, and telecommunications. - usually prepared in combination with a civil & electrical (physical) engineer. The SP will also show the entire footprint with substation fencing and major substation equipment location including the control switch house, and/or outdoor cabinets – if equipped. The substation general layout configuration will need to be determined before the site plan can be completed. This will illustrate the substation's configuration of whether a single bus configuration, main & transfer bus, ring bus, breaker-and-a-half scheme, and so on.
Next, an excavation plan may be required in order to show details for leveling out any sloped soil in preparation for foundations - usually prepared in combination with civil & physical engineering. The foundation plan will show the concrete foundations (drawn to scale) that are required for supporting all major equipment, steel structures, cabinets, and control switch house. The foundation plan is usually performed by the physical engineer in combination with a civil/structural engineer.
The grounding plan is required to show the underground, ground mat grid with grounding connections at all major equipment, steel structures, control house, and at various sections of the substation fencing in order to properly bond the substation to ground for protecting humans against electrical faults. Generally, substation grounding plans can be designed based on industry standards that are proven designs without the use of assisted computer aided software as long as fault currents do not exceed ~10,000A. A separate grounding analysis study may be required for higher fault currents that utilizes computer software to ensure proper grounding connections, ground rods, copper conductor sizes, and adequate copper ground grid spacing are utilized.
A separate shielding and lightning protection analysis will be required (with or without software assistance) to ensure all major equipment and lines are properly shielded against possible direct lightning strikes. The analysis will help determine the method for protection by using overhead shield wire (OHSW), lightning rod masts, and/or both, etc.
A general arrangement (GA) plan is required to show the exact arrangement of the substation yard equipment including pull-off connections from the transmission and distribution lines. This is a scaled drawing showing the exact orientation of the substation fencing with access gates, control house, transformers, breakers, circuit switchers, line-rupters, disconnect & bypass switches, electrical bus, foundations, steel structures, cable trenching, etc.
The GA is broken up into multiple scaled equipment and detailed elevation section views as electrical detailed drawings, as required, to illustrate all equipment and connections for major yard equipment, bus & equipment structures, fencing, and the control house. These elevation sections & detailed diagrams are needed for constructing, connecting, operating, and maintaining the substation equipment. The SP, GA, elevation section view details, excavation plan, fencing plan, foundation plan, steel erection plan, grounding plan, conduit & cable list, among others are all part of the physical design and engineering portion of a new substation (with civil/structural engineering assistance).
Equipment mechanism drawings are generally provided by manufacturers or created from vendor specifications. The mechanism drawings show the high voltage (HV), medium voltage (MV) and low voltage (LV) enclosures & compartments along with the internal and external connections. These drawings include various details, mounting requirements, schematics, wiring diagrams, and controls as required for installation and operation of the equipment.
Conduit, power and lighting plans & details are created to show all the necessary underground conduit routes to above ground conduits and to show the required controls power, receptacles and lighting required to properly power and illuminate the yard equipment. Conduit and cable lists are created in conjunction with the conduit, power & lighting plans to show all the outdoor and indoor cables with labels including the conduits, cable trench and/or cable trays, along with cable/conduit types, sizes, and number of conductors in each cable, etc.
The control house drawings are usually provided by the manufacturer or vendor selected based on the utilities' substation size and house requirements - unless the utility decides to construct a custom control house. Control houses usually consist of concrete block / brick, masonry, or metal buildings. Control houses today are becoming more popular with prefabbed masonry or metal buildings that are shipped to the construction site with minimal assembly required on site. Depending on the utilities’ specifications, prefab control houses can arrive equipped with electrical, lighting, HVAC, SS ATS, AC/DC distribution panels / cabinets, battery charger, control or command modules, protection & controls (P&C) module, telecommunications module, battery room, bathroom, cable trays, relaying & controls panels, wiring terminal blocks, telecom racks, and so on.
The P&C equipment is shown on protection & controls drawings - one-line or single line diagrams, protective relaying diagrams, AC & DC schematics, three-line diagrams, DC elementary diagrams, relay & switchboard front / rear view elevations, point-to-point wiring diagrams for panel and equip mechanisms, cable lists, bill of materials (BOM), etc. (See The Importance of Protection & Controls and IEDs for more on P&C Systems).
The SCADA equipment drawings will be created to show remote control functions for all substation and P&C equipment. The SCADA equipment will either be shown on P&C drawings or Telecommunications drawings depending on the utilities’ specifications. These drawings usually include SCADA single lines, schematics, SCADA panel front / rear view elevations, connection / wiring diagrams, cable lists, SCADA points list, etc. (See Is Your Legacy SCADA System Secure? for more on SCADA Systems).
The Telecommunications drawings are created to show communications transport systems (including SCADA systems) and how the communications travel back to the utilities' main or master dispatch control center. These communications transport systems can be owned by the utility or leased by a third-party telecom company. The telecom transport systems multiplex the substation’s individual communication circuits into a single channel and de-multiplex these individual circuits from the transport channel after arriving at the control center or other remote destination. The transport systems can consist of one media or multiple media systems, such as: fiber optics (FO), microwave (MW) radio, 900mHz radio, power line carrier circuits (PLCC), copper / coaxial DS3, DS1, DSN, or twisted copper pair circuits, etc. The telecom drawings typically include single line diagrams, schematics, telecom panel front / rear view elevations, connection / wiring diagrams, cable lists, bill of materials (BOM), etc.
3 Phase Associates has professional electrical engineering specialists with several years of utility and substation design, testing, and implementation experience who are ready to serve you. We would be glad to help you with all your engineering needs.
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