Electricity reaches a socket through four or five voltage levels, and a transformer stands at every transition. Generation happens at 10 kV to 30 kV, transmission at 110 kV to 400 kV, regional distribution at 50 kV or 20 kV, local distribution at 10 kV or 20 kV, and consumption at 230/400 V. Nothing about that chain is arbitrary: each step exists to keep losses and equipment cost in balance.
Why voltage has to go up before power can travel far
Resistive loss in a cable follows the square of the current. Transmit the same power at ten times the voltage and the current falls to a tenth, so the loss falls to a hundredth. That single relationship is the reason high-voltage transmission exists at all.
The counterweight is insulation. Higher voltage demands larger clearances, taller towers, bigger bushings and more expensive switchgear. The voltage levels used in practice are the compromise between these two curves, which is why long distances justify 380 kV while a city district does not.
The step-up at the generation site
A generator, whether driven by a gas turbine, a steam cycle or a wind rotor, produces at a voltage set by its own construction, usually somewhere between 10 kV and 30 kV. A generator step-up transformer raises this to transmission level. These are power transformers: large, oil-filled, with forced cooling and often an on-load tap changer to hold the output voltage as the load varies.
Substations: transmission down to medium voltage
Near the load, transmission voltage is stepped down in stages. A substation transformer might take 150 kV to 20 kV, feeding a regional network of medium-voltage cables and ring main units. This is where most of the grid switching, protection and metering equipment lives, and where an on-load tap changer keeps the medium-voltage busbar within band as demand rises and falls through the day.
The last transformer before the socket
The final step-down is done by a distribution transformer, typically 250 kVA to 1,000 kVA, converting 10 kV or 20 kV to 230/400 V. In the Netherlands these sit in compact stations, in kiosks or in a room inside a building rather than on poles. From there the low-voltage cable runs to the meter cupboard.
Because a distribution transformer is energised permanently but rarely loaded to its rating, its no-load losses dominate its lifetime energy consumption. This is exactly what the EU EcoDesign rules target, and why Tier 2 limits apply to units of this class.
What changes when generation moves closer to the load
The chain above assumes power flows one way. Rooftop solar, battery storage and local wind reverse that flow for part of the day, and the distribution transformer must now handle export as well as supply while keeping voltage inside the permitted band at the end of a long feeder.
As a rule of thumb: the further the power must travel, the higher the voltage that carries it, and the closer it gets to the user, the more the choice is governed by losses, footprint and safety rather than by distance. Anyone specifying a transformer somewhere in that chain is really choosing where in the trade-off between purchase price and thirty years of losses they want to sit.
