The effect of converter harmonics in offshore high voltage and medium voltage DC networks

The outputs from turbines in current and planned offshore wind farms are generally interconnected using AC networks. However, there are significant potential advantages in using DC interconnection; in particular, the cable losses are greatly reduced. Such a system is likely to require the use of DC-DC converters to step up the rectified generator output voltage to an intermediate (collection) DC voltage, and then to a high DC voltage for transmission to the shore. These DC-DC converters will introduce harmonics into the DC network, and there is a strong possibility of significant circulating currents between the various converters, causing substantial losses. This paper presents the results of simulations studies of this system using Simulink. A model of a system comprising wind-powered permanent magnet generators, active rectifiers, high power DC-DC converters, appropriate filtering and submarine cables is described. The converter harmonics and their associated losses are presented, and possible methods for their reduction is discussed.