Wind turbines are composed of a variety of interdependent equipment functions that are connected with wire conductors. This equipment includes pitch and yaw controls; power electronics, such as inverters; transformers; and a variety of down-tower auxiliary equipment, such as meteorological stations to measure wind speed and direction, rain and humidity; lightning strike surge protection; and aeronautical visual mechanics. There is wiring that also serves as grounding for the entire wind turbine structure, which is connected to a variety of monitors and gauges.
The wiring for all of these functions is managed with a diverse system of cable ties, which bundle wiring to minimize wear from thermal exposure, vibration and installation. Cable ties come from a variety of polymers with mechanical properties that enable their use in the form of load-bearing shapes. However, not all cable ties are equal, and the manufacturing of the thermoplastic polymer resin used to make cable ties involves specific knowledge of polymer chemistries.
Under Chemical Abstracts Service Number 25038-54-5, semi-crystalline polyamide (PA) nylon 6 chemistries offer excellent flexibility, ultraviolet protection, high resistance to abrasion and high resistance to chemical attack under acids and alkaline, making these chemistries ideal material for cable ties.
The various polymers that make up thermoplastic resins affect product performance when used to make cable ties. The ultimate effects of the polymers’ properties on the end-use performance of the product are critical. Among these properties are conductivity, optics, melting point, isomerism, branching, molecular weight and molecular weight distribution. In addition to PA, other thermoplastic polymers have been considered for this application, such as polyethylene (PE) and polyvinyl chemistries.
The polymer chemistries used for cable ties are characterized by extremely large molecules, formed from polymerization of different monomers. A variety of additives and smaller molecular solutions are introduced to provide color, greater flexibility or rigidity, flame and weather resistance, moisture absorption and stabilization. Determining these various formulae involves a complex series of steps in evaluating the chemistries and interrelationships of each combined material and its process, shape and function.
Because of outdoor exposure, cable ties used in wind turbine applications must be made of polymers that resist ultraviolet radiation and extreme temperatures. Wind turbine nacelles also generate extreme heat from not only outdoor exposure, but also the operation of electronics and power-generation equipment in the interior. These conditions are likely to result in a change in the molecular structure of the polymer, causing some cable ties to degrade and become brittle.
The chemistries used for cable ties include PA, PE and carbonyl. For example, PE is produced in four principal grades (high density, low density, linear low density and ultra-high molecular weight), but it is not on the Underwriters Laboratory’s thermal properties list of selected high-temperature plastics.
The carbonyl groups of ketones, esters and carbonates strongly absorb ultraviolet light in the 2,800 Å to 3,200 Å range, leading to polymer instability and poor outdoor aging characteristics. Additionally, branching the molecular thermal bonding at temperatures of 302° F (150° C) and higher creates low-grade polyamide chemistries, which tend to hydrolyze and degrade upon exposure to moisture and sunlight.
In any polymer, molecular weight distribution is useful in characterizing the properties of plastic. The lower the molecular weight is, the more flexible it is. The use of thermal crystallinity properties in polymers (such as PE, PA, polyvinyl and nylons), if not treated correctly, could pose substantial risk of poor performance.
For example, PE has the highest crystallinity of any polymer and should not be used in applications with long-term, accelerated temperature swings or variable frequency vibration. Polyamide chemistries and their many derivatives are used for their cost competitiveness, but some of the additives, impurities and combined chemical structures do not always result in stable cable tie products. As a result, polymer and similar chemistries are known to have short productive cycles under normal circumstances. Over time, many become less flexible and tend to crack and even break.
Nylon 6.6, a polyamide, and several other polymers used for many nonmetallic cable ties can be formulated to be resistant to ultraviolet radiation and extreme temperatures. If installed properly, nylon 6.6 cable ties can be expected to last through the entire 15- to 25-year service life of a wind turbine.
Heterochain polymers can result in high-strength and flexible cable ties that are useful for a variety of applications. These applications include wind turbine down-tower applications in which cable ties are used to hold wiring on raceways and cable trays. Other polymer chemistries have not delivered cable ties with the versatility and performance of those made with heterochain polymers. The more limited performance of other polymers, such as polyvinylchloride or PE, often is the result of the use of certain gases during the molding or liquid formation of the polymer, rendering the polymer weak and non-flexible over time, due to the aging of the crystallization properties.
In addition to heterochain polymers, advanced nylon chemistries and molding techniques contribute to better performance of cable ties (as do other features, such as stainless-steel locking tongues). Nylon 6 series chemistries, in particular, when combined with a variety of other associated chemistries, have proven to be more compatible and stable than other chemistries. These chemistries, known as covalent-bonded materials, also have demonstrated advantages in the production of extruded and molded products.
The quality of the chemistries applied to the manufacture of cable ties will determine the product’s durability, flexibility and service life. Elements and bonding within the unit largely determine the thermal decomposition temperature of a polymer. Thermal decomposition occurs when the primary covalent bonds of the polymers, additives or coatings are ruptured. The decomposition temperature and chemical resistance of the polymer will affect the strength of the bond and eliminate time versus temperature and frequency decomposition.
In wind turbine applications, rapid fluctuations of extreme temperature and multiple vibration frequencies can significantly impair cable tie performance. If the polymer offers dimensional stability at a variety of temperatures and a mix of frequencies, it is possible that the polymer’s processing temperature, service temperature or both could approach the polymer’s upper range for potential decomposition. Using the combination of appropriate chemistries and manufacturing processes, cable ties are available that do not have temperature or frequency faults, with temperature ranges of -40° F to 302° F (-40° C to 150° C) and the ability to handle a wide range of frequency vibration from a variety of prime movers. w
Marketplace: Wire & Cable
Protecting The Ties That Bind In Wind Turbine Applications
By Sammy Germany
Because of outdoor exposure, cable ties used in wind turbines must be made of polymers that resist ultraviolet radiation and extreme temperatures.
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