Power collection and transmission systems

A power collection and transmission system can be used with a plurality of energy devices such as off-shore wind turbines (2), for example. The energy devices are grouped into two or more clusters. The system includes a dc collection network (N1, N2...N14) for each cluster, the dc collection network being provided by a suitable busbar or cable. A primary DC/DC converter (8) is located at each energy device and is connected to the dc collection network. First and second dc transmission lines (20 and 22) are used to transmit any power generated by the energy devices to an onshore converter station. A secondary DC/DC converter (12) is connected between the dc collection networks and the first and second dc transmission lines (20, 22). The secondary DC/DC converter (12) includes a series of interconnected DC/DC converter modules (14) that in one arrangement are co-located at a converter station (10). Each DC/DC converter module (14) has first and second dc terminals (16a, 16b) connected to the dc lines of a respective one of the dc collection networks and third and fourth dc terminal (18a and 18b) that are connected to either one of the first and second dc transmission lines (20 and 22) or to an adjacent DC/DC converter module in the series.