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News

Fibre reinforced H-frame for transmission lines meets environmental challenge

FRP Transmission Innovations : 25 October, 2013  (Application Story)
FRP Transmission Innovations, in collaboration with BC Hydro, is setting a new environmental standard for high voltage infrastructure with the installation of a fibre reinforced polymer H-frame structure built for transmission lines.
Fibre reinforced H-frame for transmission lines meets environmental challenge
Traditionally, Canadian transmission line structures have utilized treated lumber, steel, or concrete as a construction material. Recently it became clear that large quantity of good quality timber for transmission line H-frames was difficult to procure, particularly after major wind and/or ice storms, when suddenly large shipments of timbers are needed for restoration. In addition, high demand creates price inflation for these products.
 
Quality of timber has also declined in recent years as only second growth wood is available, making large size structural pieces harder to come by. Many newly installed wood cross arms fail within a short time and, according to some utilities, the newly installed timber cross arms are less reliable then some old wood cross arms that have been in the field for a long time.
 
Utilities started to use galvanized steel as a wood cross arm alternative. It has worked well for certain applications, but unfortunately, steel is electrically conductive, therefore using it in live line installation is very dangerous. Steel procurement time is currently running at around six months and the cost of steel has increased significantly in recent years due to high demand. Steel cross arms are also very heavy compare to wood. Most of the time, hollow structural shapes (box sections) are specified and their internal corrosion is difficult to monitor.
 
Over the last decade, new materials have been emerging: two-component polyurethanes are now widely known to exhibit superior strength and toughness and may allow manufacturers to cost effectively produce lighter, stronger and more damage tolerant profiles. After initial consultation and evaluation a 20 year collaboration agreement was put in place to do joint BC Hydro and FRP TII research and development, first on new fibre reinforced polymer (FRP) cross arms and later on other products that are needed for the BC Hydro transmission system.
 
Two new designs were developed for H-frames; one was designed for 138kV configurations and another larger size for up to 287kV geometry. Depending upon the load requirements, the larger cross arm could be used for 340kV lines as well.
 
A comprehensive third party testing program was developed and performed by BC Hydro, several cross arms were installed in the field gaining experience with field installation, providing feedback opportunities for line crews and to evaluate field performance and ease of construction.
 
In warm high humidity environments where treated wood does not last very long, steel corrodes very quickly, FRP TII cross arms are a very attractive choice in this situation. An extensive accelerated aging, electrical, mechanical, and structural testing programme was developed and carried out by Powertech, a research subsidiary of BC Hydro. This test programme compared regular wood, steel, Fibre Reinforced Polymer (FRP) composite and other specialty composite material cross arms. These test programs clearly show the benefits of FRP composite cross arms. Test results also proved the viability of FRP cross arms in showing their resistance to lightning strikes, bearing structural loads well, and long term durability.
 
Field installation was carried out as part of the regular cross arm replacement programme and feedback from line crews was collected when they installed several 138, 230 and 287kV H-frame cross arms. Benefits reported include:
  • Light weight. It is easy to lift by two persons, easy to move as lifted on the ground.
  • Light helicopter could be used for lifting providing cost effective installation.
  • Fibre reinforced composite cross arms are electrically non-conductive. When installed on live energized lines they do not zap the crews or cause a fault if intermittent phase to poles or phase to phase contact is made, unlike steel or wood cross arms - there is no induction in the FRP cross arm and there is no electrocution hazard. They are safe from accidental live conductor related flash over or other forms of electrical contact from the cross arm.
  • The surface is smooth but not slippery. Personnel can sit on it and there is a comfortable position for leg support at the lower flange. There are no splinters or other surface items that can potentially harm personnel.
  • Easy to field drill and field cut when needed.
  • Identical hardware used for wood or metal cross arms are used for the FRP cross arms. The only additional item they found useful but not essential in their tool set is a socket extension so they can tighten pole connecting bolts outside the flange plain with 360 degree rotation.
 
Long term performance
 
Using new materials and components always raises questions of the long term performance. Accelerated aging test performed by an independent laboratory confirmed that FRP TII’s two component polyurethane composite cross arms expected life span is over 75 years. This life span is much better than wooden cross arms or galvanized steel with no additional corrosion intervention after the galvanized coating is gone, which is about 35 years in most urban environments.
 
Manufacturing
 
FRP TII has a strategic alliance partnership with Creative Pultrusions in the USA. This facility produces high quality pultruded products, specializing in high pressure injection of polyurethane resin pultrusions. CPI has been in business for forty years and manufactures the cross arms.
 
Cost benefit of using FRP cross arms
 
The largest cost of replacing cross arms is actual replacement cost from crew time going to site (which could be quite expensive if helicopter only site access is the only way due to rough terrain). Wooden cross arms need to be replaced in about 25 to 40 years depending upon environmental conditions and the original quality of timber. FRP cross arms provide the possibility to skip up to two replacement cycles, compared to wooden cross arms. In overall life cycle cost analysis this is a very significant saving.
 
Raptor protection
 
At some transmission operation areas raptor protection is a particular concern, when they land on energized steel transmission cross arms their wings bridge over distances between conductor phases and metal cross arms causing injuries and loss of wild life. Utilities have to pay significant penalties; also it is a public relations challenge when dealing with wildlife death or injury. Using FRP TII cross arms, due to their electrically non-conductive nature such accidents and issues are preventable.
 
 
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