An electrical substation is the set of equipment that transforms voltage, distributes power among lines and loads, and protects and controls the grid. The switchyard holds the power transformers, circuit breakers, disconnect switches, instrument transformers, surge arresters and buses; the control building holds the protective relays, control, communications and the station service that powers all of the above. Before it is energized, every piece of equipment and every function is checked: that is commissioning.
What an electrical substation is
An electrical substation is the node of the grid where power changes voltage, is distributed among circuits and is protected. IEC 60050 defines it as the part of a power system, concentrated in a given place, including mainly the terminations of transmission or distribution lines, switchgear and housing, and possibly transformers. Switching stations, which interconnect lines without transforming, are also called substations.
It does three jobs:
- Transform: raise the voltage at the output of a power plant to transmit power with lower losses, or lower it for distribution and use.
- Connect and distribute: join lines, transformers and feeders to common buses, and allow any of them to be taken out without shutting down the rest.
- Protect and control: detect a fault, open only the necessary breaker within milliseconds, and operate and supervise everything from the control building or from a remote control center.
How power flows
Power enters through a line, goes through the incoming bay, reaches the buses, passes through the power transformer and leaves through the feeders. Each section has its own switching device and its own protection.
- 01Incoming lineSurge arrester, voltage transformer and line disconnect switch with grounding switch
- 02Line bayCircuit breaker, current transformers and disconnect switches
- 03BusesJoin all the circuits; their arrangement defines the substation configuration
- 04Transformer bayBreaker and power transformer that lowers the voltage
- 05FeedersOutgoing switchgear or bays, each with its breaker and relay, toward the loads
The way the bays connect to the buses is the configuration (bus arrangement): single bus, double bus, breaker-and-a-half, ring bus and others. It determines what is lost when a piece of equipment fails and what can be maintained without interrupting service.
Parts of a substation and their function
| Equipment | Function |
|---|---|
| Power transformer | Changes the voltage level |
| Circuit breaker | Makes and breaks load and short-circuit current |
| Disconnect switch | Isolates equipment with a visible gap; operated without load |
| Grounding switch | Grounds the isolated section so it can be worked on |
| Current transformer | Scales down the current for relays and metering |
| Voltage transformer | Scales down the voltage for relays, metering and synchronism |
| Surge arrester | Limits lightning and switching overvoltages |
| Buses | Common conductors to which the bays are connected |
| Grounding grid | Controls step and touch voltages during a fault |
At medium voltage, much of this equipment is housed inside metal-enclosed switchgear. At high and extra-high voltage it can be outdoors or gas-insulated.
The control building
The switchyard equipment does not decide anything by itself: the equipment in the control building does, connected to the switchyard by control cables and, in digital substations, by optical fiber. There you find:
- Protection: protective relays in control and protection panels, one or more per bay.
- Control: the bay controller, the interlocks and the local SCADA and HMI.
- Communications: the station network, the gateway to the control center, teleprotection and time synchronization.
- Metering and recording: revenue meters at the interconnection and fault recorders.
Which protection each element carries (line, transformer, buses, feeders) is summarized in substation protection schemes. How this equipment works together, as a substation automation system (SAS), is in substation automation.
Station service
A substation consumes power to run. The AC station service feeds operating mechanism motors, heating, lighting and chargers, with its own transformer and often a backup diesel generator. The DC system, with batteries and chargers, powers the relays and the trip coils: if it fails, the substation is left without protection. That is why any failure of the essential station service must be attended within the autonomy of the batteries or of the backup generator’s fuel tank.
Types and voltage levels
Substations are classified by their function (step-up, transmission, distribution, industrial or switching), by their construction and by their technology:
- Air-insulated (AIS): the conventional substation, with the equipment outdoors.
- Gas-insulated (GIS): compact, for cities and tight spaces, and the hybrid substation.
- Medium voltage for industry and distribution: the industrial substation, the compact unit substation, the pole-mounted substation and the pad-mounted transformer.
- Digital: the IEC 61850 digital substation, which replaces the copper wiring between the switchyard and the control building with messages over fiber.
Voltage classes vary by country and standard. A common classification in North American usage (ANSI C84.1) is medium voltage above 1 kV up to 100 kV, high voltage above 100 kV up to 230 kV and extra-high voltage above 230 kV, with 345, 500 and 765 kV as typical values; other countries draw the lines elsewhere. What changes at those levels is in high and extra-high voltage substations.
How it is operated
Opening and closing equipment in a substation follows an order, explained in substation switching: the breaker interrupts the current and the disconnect switches only isolate, without load. Substation interlocking prevents the wrong operation, and the control levels define where a command can come from.
Commissioning
Before a new or expanded substation is energized, every piece of equipment and every function is checked to do what the design says. Its stages are described in substation commissioning. It includes:
- Primary equipment tests such as ratio, insulation, contact resistance and timing.
- Secondary tests: injection into the relays, logic, interlocks and signals to SCADA.
- Controlled energization and on-load tests, described in first energization of a new substation.
We can help with your substation
Our crew works in medium, high and extra-high voltage substations and in industrial plants. On site we test and commission the complete substation, in Colombia, or elsewhere on request. Contact us with the scope of your project.
Related guides
- Substation control building
- Substation protection schemes
- Substation automation
- Substation interlocking
- High and extra-high voltage substations
- Substation commissioning: stages and tests
References
- IEC 60050-605, International Electrotechnical Vocabulary – Generation, transmission and distribution of electricity – Substations.
- IEC 61936-1: power installations exceeding 1 kV AC.
- IEC 62271 (series): high-voltage switchgear and controlgear.
- IEC 61850 (series): communication networks and systems for power utility automation.
- ANSI C84.1, Electric Power Systems and Equipment – Voltage Ratings (60 Hz).
- J. D. McDonald (ed.), Electric Power Substations Engineering, CRC Press.
Frequently asked questions
What is an electrical substation for?
To change the voltage level (stepping it up at the output of a power plant or down near consumption), to interconnect and distribute power among several lines, and to automatically isolate the part of the grid that fails. It is also where energy is metered and from where the grid is operated remotely.
What are the main parts of a substation?
The switchyard (power transformers, circuit breakers, disconnect switches, current and voltage transformers, surge arresters and buses), the control building (protection, control, communications and metering), the AC and DC station service, and the grounding grid with the shielding against lightning.
What is the difference between a step-up and a step-down substation?
A step-up substation raises the generation voltage so that power can be transmitted with lower losses; it sits at the output of power plants and of solar and wind farms. A step-down substation lowers the transmission voltage to subtransmission, distribution or industrial levels. There are also switching stations, without transformation, which only interconnect lines.