Basics of Designing Power Substations

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 either stepping up or stepping down the supply voltage depending on whether or not 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 economical for transmitting electric power over longer distances with less power losses caused by the impedance 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.

Power Delivery Substations

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 control the power flow.

Substations are filled with large and very expensive equipment. A power utilities’ greatest expense in substations is the power transformers. Other items and equipment involved in designing  substations which are less expensive than power transformers are: circuit breakers, potential/current transformers, electric bus, steel structures, foundations, control house, protection & controls (P&C), relaying/IEDs, SCADA, control cables, fencing, grounding, real estate, etc.

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).

GIS Substations
GIS Substations

Substations may also be owned by manufacturing, industrial, or large commercial customers; instead of being owned 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 voltage levels are: 765kV, 500kV, 345kV, 230kV, 115kV, 69kV, 46kV, 33kV, 25kV, 12kV, and 4kV.

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 33kV 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. The basic steps a utility may perform in planning and implementing a new substation are:

  1. Plan for the new power substation.
  2. Perform load flow power studies.
  3. Determine the substation size and total footprint required (with equipment).
  4. Determine substation configuration (Single Bus, Main/Transfer Bus, Ring Bus, etc.).
  5. Allocate required funds.
  6. Pick the location or area.
  7. Acquire the real estate.
  8. Create the substation project with schedule and in-service date.
  9. Assign the project team resources with in-house staff.
  10. Assign project resources with external contractors or contract entire project as a turnkey option, engineer-procure-construct (EPC).
  11. Begin Engineering & Design.
  12. Construction, Testing, & Implementation.

Designing a New Power Substation

Once the substation planning is completed with real estate acquired, and the project has kicked off; a scaled site plan 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. The site plan will also show the entire footprint with substation fencing and major substation equipment location including the control house/cabinets – if equipped. The substation layout configuration will need to be determined before the site plan can be completed. This will show if the substation will be a single bus configuration, main & transfer bus, ring bus, breaker-and-a-half scheme, etc.

Substation Configuration Scheme
Substation Configuration Schemes

Next, an excavation plan may be required in order to show details for leveling out sloped soil in preparation for foundations. The foundation plan will show the concrete foundations (drawn to scale) that are required for supporting all major equipment, structures, cabinets, and control house. The grounding plan is required to show the underground, ground grid with grounding connections at all major equipment, steel structures, control house, and at various sections of the substation fencing in order to protect against electrical faults. A separate grounding analysis study may be required to ensure proper grounding connections and copper conductor sizes are utilized.

A general arrangement plan is required to show the exact arrangement of the substation yard equipment including connections to the transmission and/or distribution lines. This is a scaled drawing showing the exact orientation of the control house, transformers, breakers, switches, electrical bus, etc. The general arrangement is broken up into multiple scaled section & detailed drawings, as required, to illustrate all major yard equipment, structures, fencing, and control house. The section & detailed diagrams are needed for mounting, installation, connections, operations, and maintenance. A separate shielding analysis will be required to ensure all equipment and lines are properly shielded against possible lightning strikes.

Equipment mechanism drawings are provided by manufacturers or created from vendor specifications. The mechanism drawings show the high voltage and low voltage enclosures along with the internal and external connections. These drawings include various details, mounting requirements, schematics, and wiring diagrams as required for installation and operation of the equipment.

Equipment Drawings
Equipment Drawings

Conduit and lighting plans are created to show all the necessary underground to above ground conduits and to show the above ground lighting required to properly illuminate the yard equipment. Conduit and cable lists are created in conjunction with the conduit plans to show all the outdoor cables with labels including its conduit, along with cable/conduit types, sizes, and number of conductors in each cable.

The control house drawings are usually provided by the manufacturer or vendor selected based on the utilities' substation size and house requirements. Control houses today are usually prefabbed and shipped to the job site with minimal assembly required on site. Depending on the utilities’ specifications, prefab control houses can come equipped with electrical, lighting, HVAC, AC/DC distribution cabinets, command module, protection & controls (P&C) module, telecommunications module, battery room, bath room, cable trays, relaying & controls panels, wiring terminal blocks, telecomm racks, etc.

The P&C equipment will be shown on protection & controls drawings - single line diagrams, AC & DC schematics, front/rear views, wiring diagrams, 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’ specifics. These drawings usually include single lines, schematics, front/rear views, connection/wiring diagrams, cable lists, SCADA points lists, etc. (See Is Your Legacy SCADA System Secure? for more on SCADA Systems).

Electrical and Telecomm Diagrams
Electrical and Telecomm Diagrams

The Telecommunications drawings are created to show communications transport systems (including SCADA systems) and how the communications travel back to the utilities' main dispatch control center. These communications transport systems can be owned by the utility or leased by a third party telecomm company. The telecomm 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 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 (PLC), copper DS3, DS1, DSN, or twisted pair circuits, etc. The telecomm drawings typically include single line diagrams, schematics, front/rear views, 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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