While hybrid carbon and glass fiber composite materials were developed to offer outstanding performance advantages in products ranging from high pressure storage vessels to armored vehicles and highly stressed aircraft components, the idea of using a hybrid carbon and glass fiber core to support a bare overhead conductor raised more than a few eyebrows.
Many questions were asked. How strong is it? Will it handle cyclic load fatigue? How will it hold up to pollution, ozone, UV radiation, high temperature exposure, etc.? The list of questions seemed endless. In addition to the core itself, how will the conductor - which uses fully annealed trapezoidal shaped aluminum strands - hold up over time? Will the dead-ends and splices, suspension clamps and dampers work? What if they are overheated? What happens if the conductor is exposed to a lightning strike or short circuit event? The questions, for very important reasons, went on and on, but many things were learned.
While a number of industry standard test protocols were previously developed and followed to address many technical questions, several novel test protocols also had to be developed that were appropriate for composite core conductors. Entities such as EPRI, EDF, AEP, Ontario Hydro and many other Utilities, Universities and Labs developed a number of test protocols to study both polymer matrix and metal matrix composite cores. Additionally, with the advent of other "High-Temperature, Low-Sag" conductors, new protocols were developed to explore the impact of extreme temperatures on conductors and hardware.
Part of the learning curve included developing test procedures and controls that allowed successful test completion. After a few failed tests, for instance, "dummy conductors" were added alongside "test conductors" to provide improved thermocouple control. A 500 cycle thermo-mechanical test developed by EDF and EPRI, that included five sustained holds at 70% of the conductor's rated tensile strength (RTS), surprising failed initially at both 180 degrees C and 200 degrees C at exactly 401 cycles.
When the test control problem was identified and resolved, both tests were
repeated successfully. After completing the 200 degree C thermo-mechanical test, the ACCC conductor and dead-ends were pulled to failure. In spite of the relatively arduous test protocol, the conductor and dead-ends achieved over 110% of their rated tensile strength, indicating that the conductor and hardware could be expected to perform well under anticipated service conditions. These were just two of literally hundreds of tests that established the acceptable continuous and emergency operating temperatures of the ACCC conductor (180 and 200 C, respectively). Additional testing at temperatures well above 300 degrees C further confirmed the adequacy of dead-ends and splices.
In some cases, conductor strand anomalies were observed during testing due to the constraints and size of test fixtures. In one recent example, bird-caging was observed during a short circuit test. When the test was repeated on a longer length of conductor to more accurately represent a typical span, no bird-caging was observed. Video Link
Another important aspect of developing new test protocols relates to interpreting the results. While many tests were performed on the ACCC conductor's uni-directional (pultruded) composite core, some test protocols and measurement techniques were initially developed for laminated composite structures using aerospace and/or defense industry criteria. While these test protocols and criteria provided useful information, the differences in applications and configuration needed to be understood so the results could be correctly interpreted.
For instance, glass transition temperature (Tg) testing is commonly used to assess the temperature at which polymer resins begin to soften when substantially heated. As this occurs, the resin's ability to maintain an adequate bond may become compromised. In a laminated structure, where the resin is the only thing holding the laminate together (no Z-axis fiber reinforcement), the rule of thumb is to establish an operating temperature well below the measured Tg. In a uni-directional composite, where all fibers run in the x-axis only, very little bond strength is actually required to effectively transfer loads between the parallel fibers. In this case, as with the ACCC conductor, Tg measurements cannot be used to establish mechanical limits or assess longevity, they can only be used to confirm the adequacy/consistency of the manufacturing process and degree of resin cure.
For more information about the ACCC conductor and the extensive testing it has been subjected to in labs worldwide, please visit CTC Global's website at: www.ctcglobal.com If you have any specific questions about testing or test results please feel free to send an e-mail to firstname.lastname@example.org We will be happy to share test data or other information.