Wind turbines and solar inverters produce electricity at low voltage, typically between 400 V and a few kilovolts. That is far too low to move across a site, let alone into a transmission network. Transformers bridge the gap in two stages: a unit transformer at each generator raises the voltage to a collection level such as 20 kV or 33 kV, and a larger transformer at the grid connection point raises it again.
The two step-up stages in a wind farm
Each turbine has its own step-up transformer, either in the nacelle or at the base of the tower. It takes the generator or converter output and lifts it to the collection voltage, so that the cable running through the field carries a manageable current. Without this stage the internal cabling of a farm would be impossibly thick.
The collection cables converge on a substation where a power transformer steps the voltage up again to the level agreed with the transmission operator. Offshore this arrangement becomes a platform, with corrosion protection, restricted access and repair logistics driving the design as much as the electrical rating.
Solar farms: from inverter output to collection voltage
In a solar plant the panels deliver DC, which inverters convert to AC at low voltage. Step-up transformers, usually one per inverter station, raise this to the collection voltage. The Vattenfall solar park Protrafo supplied illustrates the pattern: eight 20 kV / 3.15 MVA oil-type distribution transformers behind a 45 MW grid connection.
Because inverter stations are spread across the field and often exposed, the units are outdoor-rated, with an enclosure suited to the site and provision for oil containment where a mineral or ester fluid is used.
Designing for a fluctuating, harmonic-rich load
A renewable plant loads its transformers very differently from a factory. Output follows wind and irradiance, so the unit swings between near no-load and full load, sometimes several times a day. Thermal cycling like this puts more stress on gaskets and on the mechanical stability of the winding than a steady load does.
Inverters also inject harmonic currents. These raise eddy-current losses in the windings and can require a derating or a design that tolerates a defined harmonic spectrum. Where the plant sits in an aggressive environment, near the coast, in dusty terrain or in high ambient temperatures, the corrosion category and the temperature rise limits need to be specified explicitly rather than left to a standard offer.
Why losses weigh heavily in renewables
No-load losses run continuously, whether or not the plant is producing. Load losses only occur when power flows. A solar park with a modest number of full-output hours per year therefore feels no-load losses disproportionately, which is one reason the EcoDesign Tier 2 limits matter here more than the purchase price difference suggests.
The comparison worth making is a total cost of ownership calculation over the plant lifetime: capitalise the no-load and load losses at the expected production profile and add them to the purchase price. A lower-loss core often pays for itself well within the operating life.
Distributed generation adds one further point. Rooftop and community systems feed power back into a network originally designed for one-way flow, so the distribution transformer must handle bidirectional flow and help hold voltage within limits at the far end of a feeder. Specify the loss class and the environment first, then let the electrical rating follow from the plant layout.
