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Plastic Optical Fiber (POF) for for wind turbine and wind farm applications

Author: Date:2012-10-11 12:00:28

Electricity generation by wind turbine generators, or WTGs, is a proven green energy technology in both land and offshore environments. However, wind farms located either onshore or offshore are often in remote and not easily accessible locations. Additionally, their height above ground can pose unique maintenance, repair and lightning strike challenges that must be addressed to make wind power renewable energy reliable and economical.

Fiber optics (FO) technology is probably best known for use in high-speed, high-bandwidth telecommunication applications. But today fiber optics data and control links have replaced copper links in wind turbines and farms making them a critical part of a wind farm operator’s solutions for minimizing costly downtime and service interruption.

Plastic optical fiber(POF) is the most suitable—and in some cases the only acceptable—technology in high electrical noise environments for electrical generator/turbine control, power conversion and wind farm wide-area communications. The characteristics and reliability benefits of POF components—POF patch cords,receivers, transmitters, transceivers and POF cable—are applicable in wind farms and wind turbines, as well as overall wind farm and wind park operation. 

Wind Turbine Environment

Unlike conventional electricity generating facilities that use coal or natural gas as their energy source, wind turbines operate outdoors and in regions with temperature extremes, corrosive spray (e.g., salt), dust, lightning strikes, snow and rain. In offshore wind park installations, weather may prevent maintenance and repair for extended time periods. 

Offshore wind turbines are usually larger than onshore installations and also generate more power because wind speeds are generally stronger and steadier, which means the WTGs must be more powerful. Higher wind speeds, larger mechanical loads, and corrosive elements in sea-based wind parks mean even higher reliability is needed.

Because wind turbines operate in rugged environments, both from a physical and electrical perspective, high levels of noise and electromagnetic interference (EMI) are generated inside the wind turbine nacelle from motors, solenoids, power lines, inverters and generators. Lightning strikes are prevalent in wind farm installations, and fiber’s inherent galvanic isolation adds to system reliability. 

The wind turbine itself is a complex assembly of mechanical components, including a tower, gearboxes, brake systems and blades. It also consists of electronic and other components such as motors, frequency inverters, rectifiers, isolated gate bipolar transistors (IGBTs), power converters, programmable logic controllers (PLCs), sensors, pitch and yaw systems—and all must be interconnected to generate reliable power. Inside the turbine’s nacelle are the power generation electronics, generators, blade pitch control, system controllers, motor, Ethernet/Profinet protocol devices, frequency inverters, circuit breakers and more. 

Fiber optic links are well suited for this short reach environment and are also the best choice for sending data and control signals from an individual wind turbine to the wind farm central monitoring station. They are also the optimal choice within the wind farm thanks to fiber’s advantages of high-bandwidth, galvanic isolation, long transmission distances, high noise immunity and near-perfect EMI immunity. In comparison, traditional copper data links cannot match the overall capability of a fiber optic based system when judged on reliability and operation in rugged environments. Likewise, doubled shielded CAT5/CAT6 copper cables make the solution more expensive than using fiber optics solutions.

 

Figure 1 shows a basic fiber optical link where photons reflected in the core of the fiber cable replace electrons moving in copper cable in the transmission path. By eliminating the conductive copper cable, very high galvanic isolation levels can be achieved.  

 

Figure 1
Figure 1: Fiber optic data communication link with inherent voltage isolation.

 

The power-generation electronics, such as the IGBT/IGCT inverter power switches, are controlled over high-noise-immune, EMI-resistant fiber optic control paths (Figure 2). Fiber data links connect the nacelle’s remote controllers to the turbine’s main controller at the base of the platform and then to the wind park over a redundant fiber data and control communication link (Figure 3). 

 

Figure 2
Figure 2: Real time, noise and EMI-resistant fiber optic communication technology is used for wind turbine power generation, control and communications subsystems.

A wind farm must rely on constant, reliable data flow for peak performance, reliability and safety even in installations covering large areas that are subject to local weather variations. Sensors monitor blade operation, system variables such as vibration, and outside environmental factors such as ice—all of which can impact power generation and system safety. The data from system sensors can also be fed into the SCADA systems for preventive maintenance action. 

Communication links must often run alongside power carrying conduits, and fiber cables are immune to crosstalk from power cables. As shown in Figure 3, fiber-based communication links inside the nacelle, between wind turbines and back to the wind farm control station, all benefit from using optical fiber.

Plastic Optical Fiber

Providing isolation and reliable communications make wind farm management and operation safer and more efficient. Many different fiber cable types (POF, HCS, Multimode, Singlemode) can be used in WTGs, with plastic being the least expensive. Cost-effective plastic optical fiber (POF) and glass optical fiber solutions are used worldwide in existing wind farm installations. Higher reliability and easier maintenance all play a role in the economics of wind power; getting a turbine online quickly and having it running reliably without interruption are critical concerns.

Fiber communication links consist of short-link POF, e.g. 60 meters, in individual turbines or multimode cables coupled to discrete transmitters/receivers or transceivers. Fiber cables are lightweight and both robust and resistant to harsh environments. One advantage of the POF is also its flexibility. For the handling of POF cables, 25 mm is the minimum bend radius (measured to the inside curvature), at which the cable can be bent safely without any damages during installation or even shortening the cable’s life. Under minimum tension, the minimum long-term bend radius is 35 mm. These are necessary characteristics for vertical cabling in towers that can be over 200 meters tall and where several hurdles in different parts of the WTG are to be bypassed. Fiber optics termination also offers safe and robust connections, either with POF, HCS or MM. It is very important to make sure that no cable will be pulled out of any equipment by mistake when technicians operate on any components of the turbine.

For industrial and renewable applications, optical fiber solutions with solid connectors are preferred. In effect, a good retention is required in numerous applications as vibrations may inhibit an optimized coupling of the light to either the source-cable or cable-receiver interfaces.  A commonly used solution is a versatile link transmitter/receiver family. With this solution, a connection is made by inserting the connector into the transmitter and receiver ports. Both the transmitter and receiver have a slot on the top of the housing that enables the use of a latching system. This system allows a retention force of typically 80 newtons (80N), or approximately ten times more than a connector with no latching system. 

For industrial applications with very high operating temperatures, latching connectors are the preferred solution since the connector retention decreases with the temperature. The storage and operating temperature ranges for the plastic connectors are from -40° to +85°C. Moreover, the retention force remains unchanged even after 2000 insertions of the connector into the transmitter or receiver. The force required to insert the connector varies from 30 to 51N maximum based on the type of plastic connector used. Destruction occurs typically from 178N. 

The connector-cable tensile force is approximately 50N for a solution without a ring and from 22 to 35N using a crimping ring. A crimpless connector is less expensive not only because it avoids the purchase of tools and reduces the time spent on the termination, but it also reduces the yield loss due to installation errors.

The connectors, like the POF cable, can be simplex or duplex. The duplex connectors are keyed to prevent any bad connection to a system of duplex modules. A duplex configuration is easily achieved by snapping together two non-latching simplex connectors: the upper part of one of the simplex is inserted into the ferrule of the second. In this way, the POF are definitively connected to the connector and cannot move. From a practical point of view, the connectors used can be color coded in order to facilitate the task of technicians insofar as they can more easily identify the cable to be connected to a transmitter or a receiver.

 

Fiber optic components can be selected to match the link length and data rates needed for each specific environment. Low-rate, short-span applications can be served by 650 nm LED-based components where up to 1 MBaud data rates, a span out to 45 meters and cost-effective plastic fiber cable are suitable. The LED-based components have a reach of 2,700 meters over multimode fiber (MM) and a 20 MBaud data rate, but can also operate up to 160 MBaud over 500 meters. Other rates and reaches are served by components such as those shown in Table 1.

 

Table 1
Table 1: Fiber optic component selection for data rate/link distance.

Many types of equipment in WTGs use Ethernet communications, including the control system, Ethernet-based circuit breakers and the switches used for the networking of the farm. Each control system of each individual wind turbine will be connected to a Local Area Network (LAN) operating with Ethernet that is also connected to a remote control center for monitoring purposes. Using fiber optics solutions for communication will make the wind farm even more reliable, especially in the harsh, offshore farm environment. A digital monitoring interface (DMI) and an industrial 10/100 Ethernet POF and HCS transceiver with DMI can be used. The DMI allows a full real-time monitoring of the FO link through a two-wire serial interface. In addition to the monitoring of the LED drive current and photodiode current, the interface also monitors the transmitter supply voltage and temperature.  

Copyright owner: Avago Technologies
San Jose, CA. 
(408) 435-7400. 
[www.avagotech.com].

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