Large Offshore Wind Turbine Transformers

Ilario Scian 1, John Eckerle2
1ABB, Vaasa, Finland, 2ABB, Turgi, Switzerland


Offshore wind turbines size increases and so does the wind turbine transformer. The power and voltage of these large wind turbine transformers increased up to respectively 11MVA and 66kV while dimension and weights are minimized for the good of the turbine mechanical structural constraints and costs.

Outcome of the study is a comparison among the major technical parameters (weights, dimensions, no-load losses, and load losses) of the transformers stepping up the voltage from Medium Voltage (MV, > 1KV) drives or Low Voltage (LV, 1kV) drives to collection grids at 34kV or 66kV. Most suitable choices of cooling type (ONAN, ONAF, OFAF, OFWF), cooling media type (mineral oil vs ester) as well as the most common auxiliary transformer solutions are also explained and discussed over the presentation. Considerations on LV vs MV drivetrain, their impact on losses and on Levelised Cost of Energy (LCoE) are also explained, with a view from the wind turbine transformer perspective.


The transformer design data are generated by using the Common Design System (CDS), the global platform to design transformers within ABB. A team of ABB transformer design experts compared and contrasted the various solutions pointing at the optimum solution for transformers > 8MVA, with HV of 34 or 66kV, and LV above 1kV or below 1kV. Their suggestions are explained with particular focus on technical/operational benefits. Costs and Losses of the various solutions are stated and consolidated into LCoE calculation.
Results are presented in tables and plots.


Comparing 66kV vs 34kV transformer data we can notice an increase of weight and dimensions but not as much as the increase on losses.
MV drivetrain resulted more convenient respect on LV drivetrain (LCoE increased 1.5-3%) even if the initial investment of the MV drivetrain is higher.
66kV transformers are less efficient than 34kV transformers - due to stringent dimensional constraints they are subject to - but they bring a greater benefit to system LCoE by enabling less losses on the collection grid.


The optimum transformer solution for offshore wind turbines is with a HV of 66kV and a LV > 1kV (i.e. the transformer is stepping up the voltage from a MV drivetrain). The MV drivetrain is preferred solution for wind turbine transformers > 8MVA because of the great benefit on the LCoE respect on a LV drivetrain.
The 66kV transformer is less efficient than the 34kV version but it brings a larger benefit to the LCoE thanks to the reduction of losses in the 66kV collection grid respect on the 34kV one.


Delegates will learn about possible transformer solutions for offshore wind, their applicability and their impact on cost and on LCoE. Transformer technical solution are compared and contrasted giving the audience and understanding of the optimal solution based on boundary constraints.
Delegates will have an understanding that a solution with higher initial investment costs and less efficient – such as the 66kV transformer – is only apparently worse than a 34kV transformer because it can bring greater benefits to the overall windfarm system and efficiency.