Fail Safe

Cable and conductor testing

Cables and conductors are the most essential components of a power transmission network. Th­ey are crucial for the development of a new network and also for strengthening of the existing grid in areas with inc­reasing power demand, but less or no space available to lay new lines.

Te­s­ting and measurement (T&M) of power eq­uipment is vital for maintaining efficient and optimal operation of equipment and a robust power system. Besi­des, it plays a vital role in ensuring 24×7 power supply in the country. The proper T&M of cables and conductors leads to cost reduction since it is less expensive than repairs. Also, periodic testing will futureproof the infrastructure.

A look at some types of conductor and cable tests that utilities can consider to ensure that the conductors in their networks are robust and reliable…

Testing conductors

Among different types of tests, one of the most commonly used routine testing techniques is type tests. A type test in­vol­ves different testing techniques such as partial discharge test, tan delta measurement, conductor resistance test, tensile test, hot deformation test, hot set test, shrinkage test, and thermal stability test. For a type test, samples are to be taken continuously from the length of the conductor, which has to undergo va­ri­ous te­sts such as visual examination. In a 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 conductor will be considered as conforming to the requirements of the specifica­tions/IS: 398 (Part II)-1996 only after the in­spector checks for scratches, joints, etc.

The measurement of the 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 required specifications. The measurement and checking of the lay ratio of each layer is another test 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. 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 or greater than 100 mm per minute.

Wrapping tests can also be conducted on both aluminium and galvanised steel wires. For aluminium wires, one specimen cut from each aluminium wire sample 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 sample 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 the 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 the ambient temperature. The measured re­sistance is then corrected to the value corresponding to 20 °C. Yet another test for conductors is the galvanising test. This test is conducted on galvanised steel wires only. The uniformity of the zinc coating and the weight of the coating are 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 a 11/33 kV system, with a minimum length of 5 metres and compression-ty­pe dead-end clamps compressed on both ends in 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 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 and st­rain tests. They are considered only for collecting creep data of the conductor from the manufacturer. A conductor sample of minimum 10 metre length, sui­tably compressed with dead-end clamps, is used.

According to the guidelines issued by the Central Electricity Authority (CEA) for rationalised use of high performance con­ductors, the constituent materials used in high performance conductors (HPCs) vary; some cores are common steel strands coated with zinc (galvanis­ed), zinc alloy or aluminium. Other conductors utilise relatively new materials such as fibre-reinforced aluminium co­m­posites or fibre-reinforced polymer co­m­posites. The required tests and test methods will differ depending on materials. At present, the facility for all types of tests is not available in India and many of the type tests are being carried out outside the country, which adds to the cost of the project. Moreover, the CEA recommended that power utilities shou­ld consider the installation of HPCs in existing and new power lines to augment their transmission capacity. Al­though considered to be relatively more expensive than conventional conductors by two to five times, HPCs could be considered economical if other associated costs and loss reductions are accoun­ted for.

Key development

In view of the increasing grievances of manufacturers regarding the validity of type tests mandated by utilities, the CEA has released the “Guidelines for the validity period of type tests conducted on ma­jor electrical equipment in power transmission”, which standardise the duration of the validity of type tests conducted on transmission system equipment. The guidelines state that type tests conducted on the equipment shall remain valid and for the specified period, provided no major change is introduced in the ba­sic design, technology, material and me­chanical construction, etc. For ins­tan­ce, the validity period of type tests of cables is 10 years and for high temperature superconductors, high temperature low sag conductors and accessories for conductor accessories, it years.

According to the Ministry of Power (MoP) committee report on facilitating and handholding 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. 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.

Conclusion

Conductors are the most vital component of a power transmission network. Considering the critical nature of conductors, it is important to ensure their proper testing. A proper testing technique optimises the durability and performance of the conductor, and also prevents future costly failure events. These tests 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.

Though the faults in electrical systems are unavoidable, with proper T&M any fault in the power equipment can be reversed before it becomes an actual problem. This is expected to go a long way in maintaining a reliable and robust power network, and ensuring 24X7 quality power supply in the country.

 

 

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