India has an extensive transmission network spanning 425,071 ckt. km at the 220 kV and above voltage levels as of March 2020. Any failure in the transmission network has the potential to disrupt power evacuation and hamper supply to end-consumers. Therefore, timely and precise testing of transmission lines is paramount. To ensure optimal performance, regulations and standards have been set by government authorities from time to time. In addition, companies have their own set of testing and maintenance measures to ensure longevity and efficiency of transmission systems.
In the case of overhead lines, fault identification is simpler. These lines can be inspected using on-the-ground or helicopter-aided visual inspection. Such inspection is often hampered by sun glare, cloud cover and rapidly changing visual circumstances. Overhead lines are more accessible 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 readings sent to graphic displays, which detect any leaks in electricity by monitoring variations in fields. Other monitoring methods include distributed temperature sensors (DTSs) 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 the cross-bonding of transmission cables makes it difficult to identify from which phase the partial discharge signal emanated.
EHV and UHV lines
In order to minimise network losses and right-of-way (RoW) requirements, the use of extra-high voltage (EHV) and ultra-high voltage (UHV) lines is gaining traction. To monitor the health of these lines, aerial line patrolling is increasingly being deployed. Patrolling by helicopters or drones and using state-of-the-art sensors such as light detection and ranging (LIDAR), thermovision, high resolution video and digital camera is being done for detection of defects. The advantage of aerial patrolling is its speed, which enables about 150 km to be covered per day. Aerial patrolling can identify electrical clearance issues and hotspots by using multiple sensors, and reduce manual intervention. Unmanned aerial vehicles (UAVs) have become more affordable and practical for the inspection of high voltage transmission lines as accessing these lines is a time-consuming and risky job due to the high voltage and the height and accessibility of the towers. UAVs are capable of performing fully robotic operation. They have a built-in video link with the operator, a high-precision inertial measurement unit, a multi-global navigation satellite system receiver and an integrated multi-sensor system, along with an on-board computer and storage unit. A consolidated UAV patrol improves the reliability, resilience and safety of transmission networks.
While the majority of transmission lines in India are overhead, utilities are also considering installing underground power cables to avoid issues related to RoW and route aesthetics.
However, fault identification and repairing of underground cables require specialised techniques, and it often takes several days or weeks to find and repair a fault in an underground cable system. In underground cabling, the location of a fault is identified by a pre-location method such as time domain reflectometry. It locates and characterises deflections in the power flow inside a cable that are caused by faults, joints or open connections. The distance to the fault is measured on the basis of the time taken in the process and the velocity of the pulse using time domain reflectometers. Other fault pre-location methods are arc reflection, travelling wave, high voltage bridge and voltage drop.
The way forward
Apart from the methods mentioned above, internet of things (IoT) is now bringing significant advantages in transmission line testing and fault location detection. The advanced sensing and communication technologies of IoT can effectively avoid or reduce the damage caused by natural disasters to power lines, and hence improve the reliability and stability of power transmission systems. UtilisingIoT in overhead lines is beneficial for effective management of the transmission infrastructure. IoT technology used in overhead transmission lines not only carries out line state monitoring but also improves the transmission line’s operation condition by providing an insight into meteorological conditions, ice cover, ground wire breeze vibration, conductor temperature and sag, transmission line windage yaw, tower inclination, etc.
Net, net, to achieve the desired level of performance and prevent failures, utilities need to follow industry best practices for the operation of transmission lines through proper testing.