Fiber optics for reliable wind energy

A block diagram of typical wind turbine components shows how fiber optic components are generally deployed in megawatt-scale turbines.
Select figure to enlarge.

Consider some of the logistics for a typical wind farm. The megawatt-scale turbines in these installations typically have towers anywhere from 40-to 100-m high. A blade tip at its highest point can be 440 ft off the ground or higher. The turbines could well be situated at a high point of local geography to better catch wind gusts. All these factors make such structures inviting targets for lightning strikes. Turbines of this size typically contain lightning rods in their blades and protective circuitry to minimize the harm to the electrical system from a strike. But there is still a possibility of damage to sensitive circuitry within the turbine nacelle even with lightning arrestors. This circuitry is typically found in various systems within the nacelle associated with ice sensors, temperature sensors, speed & wind direction laser sensors, and Fiber Bragg Grating sensors for monitoring bending loads on the blades, connected via I/O devices to controllers.

It is increasingly common to use fiber optics to galvanically isolate such interfaces. This not only limits the damage of any lightning strikes but also can help reduce the effects of power line noise on sensitive sensor readings. Fiber optics are used for both galvanic isolation purposes and data communications. For example, power generated by the turbine typically gets routed down the tower at levels of 690 V and then is converted into utility grid voltages in the range of 3 to 6 kV. Any data signals on copper wires routed next to these power lines are subject to induced noise. But data signals sent through fiber optic lines are not subject to such concerns.

There is another issue with wind turbines in off-shore wind farms. Such turbines may be five miles from the nearest point on land. For example, the distance is 5.2 miles in the case of the Cape Wind installation proposed for Nantucket Sound. Turbines in these installations may be spaced apart by about six football fields or so.

The rectifier in a wind turbine system converts noisy ac to dc power, while the inverter converts dc to clean and reliable ac power. The switching of these devices is usually controlled by a DSP embedded controller via a plastic optical fiber (POF) link to provide high galvanic isolation. Depicted here are two typical configurations as commonly used with an IGBT and emitter turn-off thyristor (ETO).
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All these factors make it challenging to run routine maintenance or to monitor the operating conditions on such facilities. So it is becoming common practice to route readings from sensors on the wind turbines to a PC situated in one of them that functions as a data collector and centralized controller, then beam these to a remote monitoring site over a radio link. It is also increasingly the case that sensor readings get routed to the data collector through fiber optic lines as a way to guard against problems from induced noise. And the high cost of maintenance for off-shore wind farms — as well as uncertain weather conditions and difficulty in getting to the turbines — is forcing operators to collect more status information. The point is to better characterize part replacement schedules and similar matters. Fiber optics are attractive as a means of ensuring these operational statistics are reliable. Further, fiber optics communication networks often link Scada computers handling off-shore installations and each individual wind turbine within those wind farms.