Wind power has become an important player in the power industry and an important factor in new employment generation. There is a continuous development of new wind turbines adapted to offshore wind farms. There is a clear tendency towards manufacturing higher and higher power wind turbines, specially in the design of wind turbines for offshore wind farms. Predictions by the EWEA estimate that onshore wind power will stagnate in the next years while offshore wind power will be a booming industry due to higher profitability and future lower initial investment costs. This cost reduction should come from improvement in technology, manufacturing processes, logistics, operation and maintenance techniques.

Despite the promise of higher returns and cost reductions, offshore wind power requires a higher initial investment and important research and development is required to bring costs down. Power transmission is an important factor in the increase of cost of offshore wind farms. HVDC transmission is the only alternative when the distance from the wind farm to shore is beyond 50-80 km, but the converter station costs are very high. Power losses in the distribution and transmission lines have become a concern for wind farm owners and a reduction of a few tenths of a percentage point becomes a major source of revenue and a significant increase in the profitability of the wind farms.

Power transformers are usually one of the bulkiest and most expensive components in power converters and they introduce a power loss of around 1 % in the wind turbines and the HVDC converters. Suitable power transformers are seldom available as standard components and they must be specified and designed during the converter design process. There is a significant increase in the interest of high frequency transformers and DC grids in distribution systems. The opportunities for size, cost and loss reduction with the introduction of high frequency in wind turbines are clear but it is still technologically very challenging to build this type of transformer in the MW range.

Another potential technology for the improvement of power distribution efficiency is the use of DC current instead of AC current within the wind farm inner grids. Lower logistic demand is another effect of DC grids, as lower section cables are needed. High power offshore power generation farms may contribute significantly to frequency and voltage control of the grid if HVDC VSC systems are used. New technologies must be developed in the following fields: Connection between high voltage static submarine cables and floating platforms or vessels; HVDC system cost reduction; cable installation at sea bed depths beyond 1000 m; direct drive of Very High Voltage generators from the DC bus in HVDC VSC systems; low power converters fed from high or medium voltage DC lines.

This thesis aims to introduce high frequency power transformers in the power stage of wind turbines to reduce the size, power loss and cost of the power stage.

This thesis presents a fully DC integrated system for the elimination of powerstage redundancies and an overall reduction of power loss and cost of the windfarm system. The document describes of a SWHFR DC/C converter using high frequency transformer for direct connection to DC lines. The document explains the use of the leakage inductance of the transformer as an inherent element of the converter to shape the current waveforms. Interleaved operation of series connected SWHFR is introduced to achieve low output current and voltage ripple. The SWHFR concept can reduce the power loss in the cabling and in the VSC converter, simplifying the cooling system. The transformer loss can be reduced by an order of magnitude. The transformer size is also reduced by an order of magnitude.

Last, the thesis presents the operation of a controlled three phase rectifier fed from a square wave three phase inverter through a transformer (CSWHFR). The effect of the transformer leakage inductance and the resulting operating modes have been described in detail, as well as the operation in continuous and discontinuous mode. A combination of SWHFR and CSWHFR can be used to obtain very efficient DC/DC conversion for connection to medium and high voltage lines. While most of the power is delivered through very efficient diode rectifiers, input DC bus voltage control can be obtained by means of a low voltage rating CSWHFR converter.