Cables and conductors are among the most important components of a power transmission network. In the current scenario of increasing power demand, it is imperative to keep these components in good condition through better operations and maintenance (O&M) practices. This requires concerted efforts from power utilities.
Testing and monitoring
The measurement of partial discharge (PD) is fundamental to the evaluation of the health of high voltage cable systems. PD measurements highlight and localise any ongoing degradation that could damage the electrical apparatus insulation and lead to sudden breakdowns. Typically, PD occurs in equipment operating at 3.3 kV phase-to-phase or above voltage.
PD only occurs when there is gas (air, SF6, hydrogen) between two conductors. The conductors must have a high voltage between them, creating an electric stress. If the electric stress in the gas exceeds 3 kV per mm of spacing, then electrons are stripped from the gas atoms. The electric strength of most solid and liquid insulations is 50-100 times higher than most gases. The negative electrons move through the air and bombard the solid or liquid, causing ageing of the insulation. PD is an important cause of failure in equipment using pure organic insulation. If the insulation is an organic/inorganic composite, PD may not cause failure, but it is a symptom of poor manufacturing or certain ageing processes.
There are various PD testing practices that can be adopted. These include offline testing in which the equipment is disconnected from the power system (factory testing for quality and in-situ for condition assessment); online testing in which the equipment is connected to the power system (used for condition assessment); and other test methods.
The offline PD testing technique requires de-energisation of the cable under test and in some cases, a complete removal from the distribution system. It allows close control of the test voltage. If necessary, voltages can be raised above the normal operation voltage. PD measurement necessitates the use of high kVA rating, PD free source transformers in order to drive higher capacitive currents. Alternatives to this include very low frequency, oscillatory wave test set and DAC. While offline testing is a valuable resource, this method is costly owing to the disconnection of cables from the substation.
Online PD measurement detects the discharges at the operating voltage using sensors, without any additional source. The testing is conducted in real operating conditions under typical temperature, voltage stress and vibration levels. It is a non-destructive test and does not use overvoltages that could adversely affect the equipment. Online PD testing is inexpensive compared to offline testing. Online PD testing is performed using current transformers with high frequency, transient earth voltage sensors, coupling capacitors with high voltage and acoustic sensors.
In online PD testing, understanding the cable return time (CRT) plays a vital role. CRT can be defined as the amount of time a single pulse takes to travel along the entire length of the cable that runs through the circuit. After detecting a probable PD in a cable, CRT helps in determining the approximate location where the PD might have been taking place.
Potential discharge signals are usually consistent in phase. In some cases, even noise signals are consistent. However, differentiating between the PD and the noise signals can be a tedious task. Hence it is important to conduct the noise activity measuring test for as long as possible in order to get accurate results.
The health of cables and conductors can also be ensured through other best practices. For instance, patrolling of lines can be undertaken through helicopters, drones, etc. At the country’s biggest transmission utility, Power Grid Corporation of India Limited, regular patrolling is being carried out for ensuring the health of cables and conductors. It undertakes ground patrolling at different cycles for different terrains. For normal terrain, it undertakes patrolling every six months for 400 kV or below transmission lines, and every four months for 765 kV and HVDC lines. For vulnerable terrains, however, it follows quarterly patrolling. Aerial patrolling using helicopters has also been taken up. Besides, patrolling using state-of-the-art sensors like LIDAR, thermovision, high resolution video and digital cameras is being done. App-based patrolling has been introduced for monitoring patrolling progress and updating patrolling data in a standard format.
Net, net, to achieve the desired level of performance and prevent failures, utilities need to follow best practices for the O&M of cables and conductors along with proper testing.