For over 100 years most power lines over 11 kilovolts have been built using steel reinforced aluminum “ACSR” conductors. These legacy conductors offered reasonable strength and conductivity at very low cost. Over time, however, as demand for electricity increased, the limitations of these conductors became more apparent.
Sag is a Drag
The power flow capacity of conductors is generally limited by electrical resistance and thermal constraints (not including system limitations). As conductors carry increasing levels of current, the electrical resistance of the conductive aluminum strands causes them and the inner steel wires to heat up. The relatively high coefficient of thermal expansion of the aluminum and steel wires causes thermal elongation which results in conductor sag. When conductors sag, they can come into contact with underbuilt wires, trees, or other structures that can trigger short circuit events and blackouts. The major East Coast Blackout of 2003 is one noteworthy example. During a heatwave when demand was very high, poorly calibrated monitoring devices and inadequate communications between grid operators led to overloading and a sag-trip outage of a 345 kilovolt transmission line. This in turn caused other 345 kV lines in the system to also become overloaded which led to a cascading series of sag-trip outages throughout the grid. The blackout lasted nearly three days and the economic loss was estimated at ~$9 billion dollars.
Introducing Advanced Conductors
Advanced Conductors, such as ACCC® Conductor, also use conductive aluminum wires but have replaced the central steel core wires with a carbon and glass fiber composite core. The coefficient of thermal expansion of the composite core is about one-tenth of the steel core so when the electrical current is increased the core doesn’t expand which would otherwise lead to conductor sag. Because the composite core is 70% lighter than steel, the ACCC Conductor can incorporate nearly 30% more conductive aluminum without a weight or diameter penalty (using compact trapezoidal shaped wires).
Greatly reduced thermal sag and added aluminum content allow this conductor to carry twice the current of legacy steel core ACSR conductors - not only to accommodate growing demand but also to improve grid reliability. The grid can be described as a spider web of interconnected lines. When one of these lines is taken out of service – either planned or unplanned, the ACCC Conductor’s higher capacity can prevent overloading and sag-trip outages, by simply rerouting power around the impacted area. The same capability allows grid operators to reroute power around high-risk fire areas during dry, windy conditions to help reduce fire risk.
Resilience Benefits
While Advanced Conductors are less susceptible to degradation from Aeolian Vibration, cyclic load fatigue and corrosion - which help improve longevity and reliability, their higher strength, flexibility and toughness also help improve grid resilience. Just weeks ago, when Hurricane Ian slammed into Florida causing massive damage and blackouts, not a single ACCC Conductor failure was reported in any of the twenty ACCC transmission lines operating in the State. However, a tree fell on one of them which caused the aluminum wires to overheat and partially melt. Because the heat capacity of the composite core is twice that of steel, the composite core was undamaged which allowed linemen to simply install a conductive aluminum repair sleeve and quickly move on.
In 2013, an EF-5 Tornado struck Moore, Oklahoma causing extensive damage. The path of the tornado went directly over a newly installed 138 kV ACCC transmission line evacuating power from the McClain power plant. The new line utilized 125’ tall steel monopoles. One of the poles was struck by a flying oil tank which caused the pole to buckle. The impact and shock wave snapped the conductive aluminum strands on the ACCC Conductor but the high-strength and elastic composite core was undamaged. After replacing the pole, linemen in two bucket trucks were able to cut out and replace about twenty feet of conductor using full tension splices - quickly putting the line back into service. If the core had snapped and the conductor fell to the ground. Linemen would have needed to bring in full reels of conductor and pulling equipment and the repair would have taken much longer. These are only two examples of resilience, but they represent experience and lessons learned at more than 1,100 ACCC Conductor installations in over 60 countries.
As we work to decarbonize the grid, meet sustainability goals, link more renewables and mitigate grid congestion, the use of Advanced Conductors will certainly play an important role and hopefully help make the job easier.