Investigation into power distribution _grid interfacing for subsea tidal generation - Rectification to a common DC-bus

The research objective was to investigate the generation and collection of power generated in a distributed tidal generation site, and realise the most economic grid connection methodology for a practical installation. Current tidal steam prototype designs utilise variable speed gear boxes, power frequency converters and variable pitch blades in the nacelle, thus requiring complicated, heavy and expensive equipment in the nacelle. Moreover the variable blade pitch mechanism has a large power demand, and has to operate day after day submerged in water at depths between 30m – 80m. The approach taken in this research is to replace major cost components with much cheaper ones, as well as locating as many components as possible onshore where costs are less.
This thesis investigates a novel tidal stream power generation system referred to as “passive rectification to a common DC-bus” where an array of 3-phase synchronous generators operating at diverse speeds are connected to a common DC bus via passive diode rectifiers. The proposed tidal stream topology shows that variable pitched blades, variable speed gearboxes and frequency converters in the nacelle can be replaced by a much simpler system that uses fixed pitch blades, a fixed ratio gearbox, generator excitation control system, and a diode bridge rectifier. These components are generally cheaper, lighter, have a lower power demand and are more reliable than the components they replace. Also some of the voltage and frequency conversion is carried out ashore where costs are less, in addition to the overall reduction in the number of components required as some of the components are shared by multiple devices. As a result system efficiency is maximised, in addition to facilitating a reduction in capital, installation and operational costs.
Power conversion from the tidal generation power system to shore is achieved via passive rectification to a common DC-bus, thus system power regulation is achieved by individual generator field current control, to attain optimum system operation including, maximum power extraction, power limitation and stall control, in turn improving system reliability and controllability. Furthermore passive stall control via generator field current regulation eliminates the need for mechanical brakes which reduces cost and improves reliability.
The feasibility of the proposed power generation system was carried out via computer simulations and later validated via small scale laboratory hardware simulations.