Conductors are the most vital component of a power transmission network. Faults in electrical systems are unavoidable, but it is essential that they undergo crucial tests in order to ensure the reliability of power distribution systems. A proper testing technique will not only optimise the durability and performance of the conductor, but also prevent future costly failure events, as these tests help the utility to better understand failure causes. A look at the types of conductor tests that utilities can consider to ensure robust and reliable conductors in their networks…
Testing conductors
One of the most commonly used routine testing techniques is type tests. For this test, samples are to be taken continuously from the length of the conductor, which has to undergo various tests such as visual examination. In visual examination, the conductor is first examined physically for good workmanship and general surface finish of the conductor. The conductor drums are required to be rewound in the presence of the inspecting officer and the inspector will initially check for scratches, joints, etc., in the conductor and only then it will conform to the requirements of the specifications/IS: 398 (Part II)-1996.
The measurement of diameters of individual aluminium and steel wires is another type test. The diameters of individual aluminium and steel wires are checked to ensure that they conform to the requirement specifications. The measurement of the lay ratio of each layer is another test in which lay ratios of each layer of the conductor are measured and checked to ensure that they conform to the requirements of this specification and IS:398 (Part-II)-1996. Another test is the breaking load test on individual aluminium and galvanised steel wires. This test can be conducted on both aluminium and galvanised steel wires. The breaking load of one specimen cut from each of the samples taken is determined by means of a suitable tensile testing machine. The load is applied gradually and the rate of separation of the jaws of the testing machine should not be less than 25 mm per minute and not greater than 100 millimetre per minute.
Wrapping tests can also be chosen. This test can be conducted on both aluminium and galvanised steel wires. For aluminium wires, one specimen cut from each of the samples of aluminium wires is wrapped around a wire of its own diameter to form a close helix of eight turns. Six turns are then unwrapped and closely wrapped in the same direction as before. The wire should not break or show any crack. In the case of galvanised steel wires, one specimen cut from each of the samples of galvanised steel wire is taken and then wrapped around a mandrel of diameter equal to four times the wire diameter to form a close helix of eight turns. Six turns should be then unwrapped and again closely wrapped in the same direction as before. The wire should not break. Another conductor test is resistance test. This test can be conducted on aluminium wires only, conforming to the procedure as per IEC:889. The electrical resistance of one specimen of aluminium wire cut from each of the samples taken is measured at ambient temperature. The measured resistance is then corrected to the value corresponding to 20 °C. Another test for conductors is galvanising test. This test is conducted on galvanised steel wires only. The uniformity of zinc coating and the weight of coating is required to be in accordance with IS: 4826-1979.
The surface condition test is another testing technique. For this test, a sample of the finished conductor for use in 11/33 kV system, having a minimum length of 5 metres with compression-type dead-end clamps compressed on both ends in such a manner that permits the conductor to take its normal straight line shape, is subjected to a tension of 50 per cent of the ultimate tensile strength of the conductor. The surface should not depart from its cylindrical shape nor should the strands move relative to each other so as to get out of place or disturb the longitudinal smoothness of the conductor. The measured diameter at any place should not be less than the sum of the minimum specified diameters of the individual aluminium and steel strands. Utilities can also perform stress strain tests. The test is contemplated only to collect the creep data of the conductor from the manufacturer. A sample of a conductor of minimum 10 metre length is suitably compressed with dead-end clamps.
Conclusion
By understanding the extent of actual damage and failure mode, utilities can make more informed and confident decisions related to their assets. According to a Ministry of Power (MoP) committee report to facilitate and hand-hold states for mobilising major material/equipment under the Deendayal Upadhyaya Gram Jyoti Yojana and the Integrated Power Development Scheme, the type test reports should not be older than five years and should be valid up to the expiry of offer validity offer. In case of a failure in any type test, the manufacturer is required to either manufacture a fresh sample lot and repeat all tests successfully once or repeat that particular type of test three times successfully on the sample selected from the already manufactured lot at the manufacturer’s own expense.
Net, net, considering the critical nature of conductors, it is important to ensure proper testing. These tests would provide the actual information regarding a fault in the conductor and quantify its severity, thus helping reduce fault recurrence and ensuring a higher uptime of utility networks. N