Clean sweep

The growth in wind energy has encouraged UK engineers to investigate a new generator system that could eliminate maintenance challenges and improve a turbine's fault ride-through (FRT) capabilities.

To connect to the National Grid, large turbines often use a doubly-fed induction generator (DFIG) system comprising brushes and slip rings. These require regular maintenance which can be difficult to carry out, particularly offshore or in poor weather conditions.

So engineers at Newcastle and Northumbria universities are looking at a brushless system in collaboration with Scottish and Southern Energy.

'A brushless doubly-fed reluctance machine (BDFRM) is a comparatively little-studied generator system and we want to examine its performance in a wind power application. Fundamentally, it offers a benefit over existing technology in that the generator itself does not need brushes and slip rings,' said Newcastle's Dr Dave Atkinson.

Another problem with the DFIG system is how it responds to grid faults, which usually manifest themselves as voltage dips that can cause large overcurrents to flow in the turbine generator system; for example, overhead transmission line cables might swing and clash together in windy situations and cause short-circuit faults.

'The existing technology tends to deal with severe grid faults by switching off to protect itself — and that is not acceptable. It is OK when you do not have many wind turbines around, but when these things start to become a significant fraction of your overall energy system, losing a wind farm could be a catastrophic event and could cause all sorts of instability problems on the Grid,' said Atkinson.

These faults last approximately a few hundred milliseconds, so the aim is to develop a system that allows the turbine to 'ride through' those milliseconds and then restore full power, which it cannot do if it switches itself off.

'It is not the brushless feature that provides the FRT capabilities. We do not know if it has superior capabilities — we are just saying it is worth investigating,' said Atkinson.

DFIG has been used in wind energy applications mainly because it is a smaller and cheaper power converter compared with more expensive power electronic devices that could be used in variable speed generator systems (a necessary feature because wind speeds are not constant).

In order to extract the maximum amount of energy from the wind, the turbine rotational speed needs to match the wind speed. When the turbine is connected to the Grid, which has a fixed frequency of 50Hz, a frequency changer or power converter is needed to match the speed of the generator to the grid's frequency.

A straightforward system would be to use a power electronic device, which Atkinson said is expensive compared with a DFIG because it would need to have the same rating as the generator. the DFIG set-up would allow a power converter to be made to be a fraction of the rating of the generator. The cost benefit is also present in the BDFRM system.

'Typically, 25-33 per cent would be the rating of the converter relative to the generator, so there is a cost reduction in the DFIG scheme,' said Atkinson.

The researchers will also explore the use of direct power to control a BDFRM, which would include the removal of an expensive and delicate shaft position sensor in the generator.

'Direct power control was first developed on variable speed drives for motor control and we are looking to applying it to a BDFRM. Unlike a motor, we are not trying to control the torque as the main variable but attempting to control the output power into the grid without the use of a sensor on the machine.

'The basic way it is controlled is that incoming gusts of wind are used to accelerate the turbine over a speed range and effectively store that energy in the inertia of the hub, by increasing the speed. The power converter is then used to draw the energy into the grid in a smoother form,' said Atkinson. He also explained how researchers would eradicate the shaft position sensors by using other measurements to estimate the shaft position.

'In power converters you already have a requirement to measure the current flowing between the converter and the generator to control it and to prevent damage to the equipment. a popular way to estimate the position of the shaft is to run a mathematical model of the machine in real-time inside the control computer,' he said.