Cables and conductors play a crucial role in the transmission of electrical 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 safety standards.
Testing of cables and conductors serves multiple purposes that are essential for all manufacturers, utilities and end-users. It helps manufacturers ensure that cables and conductors meet specific quality standards and performance criteria, thereby minimising the risk of faulty or substandard components. Testing also ensures that these cables can withstand specified 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 resistance, insulation integrity, capacitance and attenuation. To this end, various tests such as tan delta tests, type tests and surface condition tests are conducted to assist utilities in selecting the most suitable cables for their applications, 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 resistance test measures the resistance between a conductor and its insulation. It identifies any weaknesses or defects that could lead to electrical leakage or breakdowns. Meanwhile, the dielectric strength test 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 assesses the ability of a cable or conductor to store electrical charge, determining its suitability for specific applications and preventing signal distortion or loss. Further, impedance and attenuation measurement tests evaluate the resistance, inductance and capacitance of cables and conductors, 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. Insulation properties change with time, and it is necessary that key parameters are tested accurately to ascertain the deterioration level of the insulation. Cables get damaged due to adverse installation conditions during cable laying. Apart from this, another popular method used to diagnose the health of cables, especially XLPE-insulated power cables, is very low frequency testing, which involves the application of AC voltage at a low frequency of 0.01-1.00 Hz, depending 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 include distributed temperature sensor (DTS) and sheath current monitoring. Temperature detection is very important because 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 fibre 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. Capacitive 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 cables makes it difficult to identify from which 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, shrinkage tests and thermal stability tests. During a type test, samples are taken at regular intervals along the length of the conductor to undergo various examinations. A visual examination involves a physical inspection of the conductor to ensure high quality workmanship and an overall satisfactory surface finish. When conducting a visual examination, it is imperative that the conductor drums be rewound in the presence of the inspecting officer. The inspector meticulously examines the conductor for any signs of scratches, joints or other potential defects. 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 should 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, ensuring they remain in their intended positions 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 dimensions and structural integrity. In some cases, utilities may also conduct stress and strain tests on conductors. However, these tests are typically performed to gather creep data directly from the manufacturer. For this purpose, a conductor sample with a minimum length of 10 metres is used, and it is appropriately compressed using dead-end clamps.
The way forward
In the next few years, technological advancements and the emergence of robotics and autonomous systems are expected to provide increased support for the monitoring, inspection and maintenance of offshore wind farms, transmission lines, underground cables, etc. These developments aim to address the existing health and safety risks faced by human 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 deployed to gather data on the condition of assets and assist technicians in taking necessary actions. Further, machine learning 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 requirements and facilitate thorough evaluations of cables and conductors. This can contribute to enhanced safety, reliable system performance, cost savings, and compliance with regulations.