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More stable grid connection for wind turbines

Through designing methods for detecting positive sequences, wind turbines operating at high potential could obtain more stable grid connection, according to a recent PhD thesis.

By Renewable Energy Focus staff

For the study Automation Control and Electronics Engineer Eider Robles opted for a direct operation mechanism with a converter for all potentials. The thesis put forward the permanent magnet synchronous generator (PMSG) with external rotor as the most appropriate.

Because PMSG does not require chafing rings to transmit the electricity, the circular movement of the wind turbine blades do not cause friction, the loss of copper is reduced and maintenance needs are reduced.

Furthermore, the external positioning of the wind turbine blade allows direct coupling of the blades, minimising the weight of the structure.

Higher power

The thesis also proposes an alternative for working at greater power. As semiconductor devices are currently limited in voltage and current, the incorporation of converters for all voltages would be impossible in wind turbines of various megawatts.

To overcome this problem, Robles suggests using multi-level converters, given that these enable the use of semiconductors in series, it could be possible to overcome the limitations and obtain higher voltages.

Furthermore, Robles proposes to add a system counteracting external disruptions such as variations in wind. The system consists of feed-forward controls that can react to changes in their surroundings.

Stability and efficiency

Grid stability and greater efficiency in the grid connection would be achieved through methods for detecting positive sequence (grid tension is made up of positive sequence, but also imbalances and distortions). The detectors used by Robles, which are made up of algorithmic sequences, isolate the positive sequence.

Disadvantages

Robles says the main disadvantage found in the study, is that in general, the detectors are specialised in one type of perturbation (disturbance, variation) or another, but not simultaneously in all of them.

Robles solved this problem by employing four positive sequence detectors, based on filters known as Moving Average Filters (MAF). These detectors have a rapid and constant settling time in the presence of any type of fluctuation – even if several perturbations occur simultaneously.

Robles’ thesis, Grid Connection and Control and Control of Multiple Synchronous Wind Turbines, has been defended at the University of the Basque Country.

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