Fiber optics for reliable wind energy

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.

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.

Fiber optic transmitters typically use 650-nm red LEDs and plastic optical fibers for short links. They use 820 or 1310 nm LEDs for longer reaches where multimode cables will be necessary. For off-shore wind farms where wind turbines are far apart, laser diodes serve as transmitter sources because they generate more light.

Transmitters are typically optimized for small-core fiber. The biggest is 980 µm for plastic optical fiber. The smallest is 9 µm for single-mode fibers.

Optical receivers typically include a photodiode (PIN diode), preamp, and quantizer or analog circuit components. Those that have quantizers put out dc-coupled TTL signals directly. Typical applications are for serial and network protocols that include RS232, RS485, Sercos, Interbus-S, Profibus, and Arcnet. Fiber optic components can also be used for installations employing proprietary protocols. Those with analog outputs must be ac-coupled to a comparator or quantizer circuitry to provide digital logic levels. The ac coupling requires encoding of the serial data (i.e., Manchester, 4B/5B, scrambled coding, and so forth), but provides better sensitivity than dc-coupled receivers.

Some kinds of fiber optic transceivers are also designed to work with Fast Ethernet signals and, as such, interface directly with Ethernet protocol ICs. These sorts of devices are particularly appropriate for wind turbines because Ethernet is beginning to replace other protocols in wind applications. One reason is that Fast Ethernet speeds (100 Mb/sec) are useful for control system applications and for communication links from the I/O of controllers in the nacelle to the main controller at the bottom of the tower.

There are two general kinds of fiber optic systems, short haul and long haul. Both find application in wind turbines. Short-haul systems use 650-nm sources, the optimum wavelength for coupling light into the 1-mm plastic fiber these systems generally employ. Typical lengths for short-haul uses are in the tens of meters. The plastic optical fiber attenuates signals at rates in the range of 0.20 dB/m. This plus the limits of the LED output power keeps link lengths to about a 60-m maximum. For longer lengths on the order of kilometers, 850-nm or 1350-nm infrared sources are generally the choice, in combination with silica glass fiber to get minimum loss.

Transmission range today can go from a few centimeters to several kilometers, depending on the data rate, cable material, and the wavelength at which the LED/laser source operates.

Plastic optical fiber is extremely easy to handle. The fiber connector can be terminated correctly in less than a minute. Termination involves no specific tooling and can take place inside the wind turbine if need be.

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.