Cables and conductors play a crucial role in the transmission of elÂecÂtrical power and data signals, whether in residential, commercial, or industrial facilities. Therefore, the reliability and safety of cables are of utmost importance, making testing an essential step in ensuring their optimal performance, durability and adherence to saÂfeÂty standards.
Testing of cables and conductors serves mulÂtiple purposes that are essential for all manufacturers, utilities and end-usÂers. It helps manufacturers ensure that caÂbÂles and conductors meet specific quÂality standards and performance criteria, thereby minimising the risk of faulty or substandard components. Testing also ensures that these cables can withstand spÂecified voltages, currents and environmental conditions, reducing the risk of system failures, downtime, signal degradation, overheating and fires. In addition to ensuring security, testing assesses the performance characteristics of cables and conductors. These include electrical reÂsistance, insulation integrity, capacitaÂnce and attenuation. To this end, various tests such as tan delta tests, type tesÂts and surface condition tests are conducted to asÂsist utilities in selecting the most suitable cables for their applicatioÂns, ultimately optimising system performance.
Testing parameters
Several testing methods are employed to evaluate the quality and performance of cables and conductors. These testing methods assess various parameters in such as resistance, voltage and impedance. The insulation resistaÂnce test measures the resistance betÂween a conductor and its insulation. It identifies any weaknesses or defects that could lead to electrical leakage or breakdoÂwÂns. Meanwhile, the dielectric strength teÂst determines the ability of cables to withstand high voltage without failing. It ensures that cables can handle the expected voltage levels and do not pose a safety risk. The continuity test verifies the electrical continuity of a cable or conductor by checking for any breaks or open circuits. It is crucial for ensuring proper signal or power transmission. Capacitance testing asseÂsses the ability of a cable or conductor to store electrical charge, determining its suitability for specific applications and preventing signal distortion or loss. Further, impeÂdaÂnce and attenuation measurement tests evaluate the resistance, inductance and capacitance of cables and coÂnductors, providing information about signal integrity and transmission efficiency. They are particularly important for high frequency applications, such as telecommunications and data transmission.
Cable testing
Underground cables: A popular method for examining underground cables is the tan delta test. It is a diagnostic method of testing cables to determine the quality of the cable insulation. This helps in creating a systemised method to analyse the ageing of cables in the long run. It utilises phase shifts caused by displacement current, which is induced due to impurities in the insulation or the stress on cables. The cable insulation is subjected to a positive feedback process, increasing the risk of a temperature breakdown. InsulaÂtion properties change with time, and it is necessary that key parameters are tested accurately to ascertain the deÂteÂrioration level of the insulation. Cables get damaged due to adverse installation conditions during cable laying. ApaÂrt frÂom this, another popular methÂod used to diagnose the health of cables, especially XLPE-insulated power cables, is very low frequency testing, which inÂvolÂves the application of AC voltage at a low frequency of 0.01-1.00 Hz, depenÂding on the cable’s inbuilt voltage. The strength of the insulating material used in cables is tested using the dielectric withstand test, which verifies that the insulation of a product or component is sufficient to protect it from electrical shocks.
Overhead cables: Overhead power lines are periodically inspected using both on-ground and helicopter-aided visual inspection. Factors including sun glare, cloud cover, close proximity to power lines, and rapidly changing visual circumstances make airborne inspection of power lines a particularly hazardous task. Overhead lines are more accessible than underground cables and can be tested with the help of online tests. For example, punctures in insulation on live wires can be monitored using punctured insulator detectors. These are moved along live wires, and send readings to graphic displays, which detect any leaks in electricity by monitoring variations in fields. Other monitoring methods incluÂde distributed temperature sensor (DTS) and sheath current monitoring. TempÂeÂraÂture detection is very important beÂcauÂse the condition of insulation and the carrying capacity of a cable can be obtained by monitoring the temperature. A built-in DTS that uses optical fibÂre temperature sensor systems can measure the cable temperature on a real-time basis. Besides, it is anti-corrosive, is not susceptible to electromagnetic interference and has high sensitivity. CaÂpacitive coupling sensors and other sensors are also used to detect partial discharge signals. However, a problem with online detection of partial discharge is that cross-bonding of transmission cabÂles makes it difficult to identify from whiÂch phases a partial discharge signal has emanated.
Conductor tests
Type tests are most commonly used as routine testing techniques to assess the quality and performance of cables and conductors. These tests encompass a range of methodologies, including partial discharge tests, tan delta measurement, conductor resistance tests, tensile tests, hot deformation tests, hot set tests, shrinÂkage tests and thermal stability tests. During a type test, samples are taÂken at regular intervals along the length of the conductor to undergo various exÂaÂminaÂtions. A visual examination involÂves a phyÂsical inspection of the conductor to ensure high quality workmanship and an overall satisfactory surface finish. When conducting a visual examination, it is imÂperative that the conductor drums be reÂwound in the presence of the inspecting officer. The inspector meticulously examines the conductor for any signs of scratÂches, joints or other potential defeÂcts. Only after a thorough visual examination and confirmation of conformity to specified requirements, such as those outlined in IS 398 (Part II)-1996, can the conductor be considered compliant.
The surface condition test is an additional technique used to test conductors. In this test, a sample of the finished conductor, designed for use in an 11/33 kV system, is selected. The sample shÂould have a minimum length of 5 metres and be equipped with compression-type dead-end clamps that securely hold both ends, allowing the conductor to maintain its natural straight line shape. During the surface condition test, the sample conductor is subjected to a tension equivalent to 50 per cent of its ultimate tensile strength. The purpose of this test is to assess the integrity of the conductor’s surface. It is important that the surface remains cylindrical, without any significant deviations or distortions. The strands of the conductor should not shift or move relative to each other, enÂsuring they remain in their intended poÂsitions and do not disturb the conductor’s longitudinal smoothness.
The measured diameter of the conductor at any given point should not be less than the sum of the minimum specified diameters of the individual aluminium and steel strands. This criterion ensures that the conductor maintains the required ÂdiÂmensions and structural integrity. In soÂme cases, utilities may also conduct strÂeÂss and strain tests on conductors. HoÂwÂever, these tests are typically performed to gather creep data directly from the maÂnufacturer. For this purpose, a conductor sample with a minimum length of 10 meÂtres is used, and it is appropriately comÂpressed using dead-end clamps.
The way forward
In the next few years, technological adÂvancements and the emergence of roÂbotics and autonomous systems are expected to provide increased support for the monitoring, inspection and maiÂntenance of offshore wind farms, transmission liÂnes, underground cables, etc. TheÂse deÂvelopments aim to address the existing health and safety risks faced by huÂman operators while inspecting or monitoring such infrastructure. Both industry and government are driving the development and availability of AI and robotics technologies that can be depÂloyÂed to gaÂther data on the condition of assets and assist technicians in taking necessary actions. Further, machine leÂarÂning techniques provide advanced computational tools to efficiently and accurately process and analyse all the data (corrosion, erosion, fatigue, cracks, etc.) provided by various technologies.
Net, net, these advanced technologies are expected to meet various testing reÂquirements and facilitate thorough evaluations of cables and conductors. This can contribute to enhanced safety, reliable system performance, cost savings, and compliance with regulations.