Home / Electrical substations
Electrical substations
A substation is a site, not a machine. The transformer is the biggest thing on it and gets the name, but it is one item among a dozen that all have to work together.
What a substation does
Three jobs. It transforms voltage between one level of the network and the next, it switches circuits in and out, and it protects everything either side of itself when something faults. A site doing only the second of those is a switchyard rather than a substation.
Distribution substations are the ones most people encounter, stepping transmission voltage down to 12.47 kV, 13.2 kV or 34.5 kV for the feeders. From there the voltage reaches a building through a pad mounted transformer, which is the last step in the chain.
Substation equipment, item by item
| Equipment | What it does |
|---|---|
| Power transformer | Steps transmission voltage down to distribution voltage. The largest and most expensive item on the site. |
| Circuit breakers | Interrupt fault current under load. Oil, vacuum or SF6 depending on voltage and age. |
| Disconnect switches | Provide a visible open point for isolation. Not rated to break load current on their own. |
| Current transformers | Scale line current down to something a relay or meter can read, usually to 5 A or 1 A. |
| Potential transformers | Do the same for voltage, typically down to 120 V. |
| Surge arresters | Clamp lightning and switching surges to protect the winding insulation. |
| Buswork | Rigid tube or strain bus tying the equipment together. |
| Protective relays | Decide when to trip. Now almost always microprocessor based rather than electromechanical. |
| Control house | Houses relays, batteries, chargers and communications. Sometimes a prefabricated skid. |
| Station service transformer | Small unit powering lighting, heaters, chargers and the control house itself. |
| Grounding grid | Buried conductor mat limiting step and touch potential across the yard. |
| Capacitor banks and reactors | Correct power factor and manage voltage on long feeders. |
Indoor, outdoor and unit substations
Outdoor is the fenced yard on open steel, cheapest per MVA and the default wherever land allows. Indoor puts the equipment in a building or a gas insulated lineup, which costs far more and is used where land is scarce, the environment is severe, or the site has to be hidden.
A unit substation is a different idea. The transformer is bolted directly to switchgear rather than cabled to it, and the whole lineup is assembled and tested as one piece. Single ended has one transformer. Double ended has two with a tie breaker between them, so either side can carry the full load when the other is out. Plants, hospitals and data centres use them for exactly that reason.
How a substation is designed
The sequence is load study, bus arrangement, equipment ratings, protection scheme, grounding grid, then physical layout and clearances. Bus arrangement is the decision that drives cost and reliability together, running from a simple single bus through main and transfer to a ring bus and finally breaker and a half at transmission voltage.
Clearances come from the voltage class and are not negotiable. So does the grounding grid, which is sized from the available fault current and the soil resistivity so that step and touch potentials stay inside safe limits during a fault.
We supply the last step, not the substation. Our line is three phase pad mounted distribution transformers, 45 to 2500 kVA. For substation class equipment the right route is the manufacturer or an EPC, and we would rather say so than take an enquiry we cannot serve properly.
Related
If the job is a building service rather than a substation, we can price it today. Send the load and the utility voltage or call 786 220 2801.