Fiber optics for reliable wind energy

Jan 1, 2010 12:00 PM,

Mickaël Marie Industrial Fiber Products Div. Product Marketing Manager EMEA & Asia-Pac, Avago T

Advanced wind turbines sport a large number of sensors whose signals are prone to contamination from electrical noise. Fiber optics to the rescue.

Inside the control system

It is helpful to examine a typical wind turbine control system as a means of understanding the possible role of fiber optics. Besides handling wind turbine functions, the turbine controller continuously monitors the condition of the wind turbine and collects statistics on its operation. As the name implies, the controller also manages a large number of switches, hydraulic pumps, valves, and motors within the wind turbine.

There is usually a controller both at the bottom of the tower and in the nacelle. On recent wind turbine designs (especially for 2-MW machines and larger), the communication between these two controllers usually takes place using fiber optics instead of copper links. On some recent models, a third controller sits in the hub of the rotor and manages the pitch of the blades. That unit usually communicates with the nacelle unit using serial communications through a cable connected with slip rings and brushes on the main shaft.

Newer, large machines typically employ redundant computers and sensors in all areas related to safety. The controller continuously compares the readings from measurements throughout the wind turbine to ensure that both the sensors and the computers themselves are OK. All in all, it is possible to monitor or set somewhere between 100 and 500 parameter values in a modern wind turbine.

Among these hundreds of factors, a controller might check the rotational speed of the rotor, and the voltage and current from the generator, as well as noting lightning strikes and their intensity. Wind turbines also contain a variety of temperature sensors for factors that include the air outside and in the electronic cabinets, oil in the gearbox, the generator windings, and the gearbox bearings. Also monitored are the hydraulic pressure and the pitch angle of each rotor blade for pitch-controlled or active-stall-controlled machines. Ditto for the thickness of linings in the brakes used to stop the rotor for maintenance.

The controller typically monitors the yaw angle by counting the teeth on the yaw wheel, and the wind direction and speed, as well as the size and frequency of vibrations in the nacelle and the rotor blades.

As wind turbines have risen in size to megawatt-scale machines, it has become crucial that they exhibit a high availability and function reliably all the time. In that regard, one of the turbines will usually be equipped with a PC from which it collects data from the rest of the wind turbines in the park. This data increasingly is routed over fiber optic lines back to the central PC. The PC then communicates with the owner or operator of the wind turbine via a telephone or radio link, sending alarms or requests for service.

A point to note is that many wind turbine manufacturers consider several kinds of information collected from turbine farms this way to be proprietary. That is because turbine makers are tight-lipped about the specific reliability of their wind turbines. In this environment, fiber optics has an advantage over RF links because it is almost impossible to eavesdrop on optical signals sent over a glass or plastic cable. The data will also be coded by the Scada system.

For the sake of reliability, fiber optic components are also candidates for retrofitting onto existing wind turbines. For example, maintenance companies have begun adding fiber optic components on older wind turbines. Reliability is increasingly an issue on these units. Though original circuitry was based on copper wiring, a redesign using fiber optic components drives IGBTs in the power converter.

In any case, the wind turbine manufacturer defines the level of reliability the turbines should reach. However, offshore wind turbines, for example, should exhibit availability not much less than 99%. Applications within the turbine that have been using fiber optic links include power inverters (IGBT/IGCT), circuit breakers, control system, pitch and yaw control systems, power supplies, switches, routers, and several others. Many such fiber optic links insure that the machine has a high reliability.

In the end, the goal is clearly to get a return on investment as soon as possible. All in all, wind farm operators can control the reliability of their wind turbines most directly and fiber optics can aid in this endeavor. The less the need for maintenance, the less the cost and the more the availability of the turbines to generate electricity and give an ROI.