In this thesis, a new HVDC-based converter system for wind farms is investigated. It is based on a mutually commutated soft-switching converter system and provides a unique integrated solution for the wind turbine generator drive systems, the wind turbine interconnection, and the power conversion for HVDC transmission.

In a wind farm, the mutually commutated converter system is a distributed system. A medium-frequency collection grid connects the converter station, equipped with a single-phase voltage source converter and a medium-frequency transmission transformer, with the wind turbines, each containing a cycloconverter and a medium-frequency distribution transformer. In this thesis, various system aspects regarding the application of a distributed mutually commutated converter system in a wind farm are investigated. Special attention is paid to the design of a medium-frequency collection grid that has an acceptable level of transient overvoltages, the design of medium-frequency transformers with suitable magnetic, electric and thermal properties, and the development of a strategy to commutate the voltage source converter during low power generation.

In order to adapt the mutually commutated converter system for an application in a wind farm, it had to be further developped. Different carrier-based and space-vector oriented modulation methods have been investigated. It turns out that for any load angle there is a quasi-discontinuous pulse width modulation strategy that can produce the same pulse patterns as space vector modulation. In addition, a modulation strategy has been developed that allows to replace the IGBTs in the cycloconverter with cheap, robust, and reliable fast thyristors, despite their absence of turn-off capability. The feasibility of different modulation strategies for mutually commutated converter systems has been verified on a down-scaled prototype converter system with both IGBT- and thyristor-based cycloconverters.

Finally, a feasible wind farm layout is proposed, which considerably reduces the energy generation costs for large winds farms distant to a strong grid connection point. As a consequence, the proposed solution may facilitate the establishment of remotely located wind farms.

Offshore wind farms have recently emerged as promising renewable energy sources. For increasing distances between offshore generation and onshore distribution grid, HVDC transmission systems based on voltage source converters can be a feasible and competitive solution. This thesis presents a comprehensive evaluation of a novel integrated wind farm topology that includes the generator drive system, the turbine interconnection and the HVDC transmission.

In the proposed concept, every wind turbine is connected to a single-phase medium-frequency collection grid via a distribution transformer and a cycloconverter, which allows the wind turbines to operate at variable speed. The collection grid is connected to an HVDC cable via a transmission transformer and a single-phase voltage source converter. This thesis evaluates in detail the principle of operation, which is also verified with system simulations in PSCAD.

The proposed concept promises several potential benefits. Converter switching losses and stress on the semiconductors for example can be considerably reduced by applying a soft-switched commutation scheme in all points of operation. Single-phase medium-frequency transformers have comparably low losses and their compact size and low weight implies an important benefit in an offshore environment. In addition, the voltage source converter is considerably simplified by the reduction to one phase leg, which implies a substantial cost saving.

Several technical challenges are identified and critically evaluated in order to guarantee the feasibility of the proposed concept. Especially the design of the medium-frequency collection grid is crucial as unwanted system resonances can cause dangerous overvoltages. Most of the technical challenges concern the specific characteristics of the proposed concept. The insulation of the single-phase medium-frequency transformers for example needs to withstand the high voltage derivatives. This thesis contains also considerations regarding the dimensioning and optimization of different system components.

A survey of different transmission systems for the grid connection of wind farms shows the potential of the proposed concept, which addresses several problems associated with electrical systems of wind farms. Both the requirements for variable-speed operation of the wind turbines and an interface for HVDC transmission are fulfilled in a cost-effective way. Compared to conventional voltage source converter based HVDC transmission systems, the initial costs are reduced and the expected annual energy production is increased. In addition, the proposed voltage source converter based HVDC transmission system can fully comply with recent requirements regarding the grid connection of wind farms.