India has an extensive transmission network spanning 390,970 ckt. km (at the 220 kV and above voltage levels) as of March 2018. Minor failures at any point in the transmission network can result in disruptions in the evacuation and delivery of power to a large number of consumers.
To ensure that these lines perform optimally, transmission lines are operated in accordance with the regulations and standards set by the Central Electricity Authority (CEA), the Central Electricity Regulatory Commission (CERC) and the respective state electricity regulatory commissions (SERC). In addition, companies have their own set of maintenance measures to ensure longevity and efficiency of transmission systems.
While the majority of transmission lines in India are overhead, utilities are increasingly choosing to install underground power cables rather than overhead lines to avoid issues related to right of way (RoW), electricity theft and route aesthetics. Additionally, individual lines are made to carry more capacity through the use of extra high voltage (EHV), and ultra-high voltage (UHV) lines such as 765 kV direct current transmission lines. High capacity high voltage direct current (HVDC) lines are preferred wherever possible to increase capacity, given the limited space. In very congested areas where access to RoW is prohibitive, EHV cables are put in trenches underground. While power cables offer many benefits, the time and cost involved in addressing disruptions or faults in the underground cable system can be substantial as compared to overhead lines. Therefore, it is imperative that the performance of underground cables is tested and measured on a regular basis to ensure uninterrupted functioning. Overhead lines, underground cables and high capacity lines come with unique challenges.
Underground cables are not as easily accessible as overhead lines, hence locating faults in underground cables requires a number of tests for identifying the exact fault location. Pre-location involves determining the fault distance. The most commonly used method for pre-location is time domain reflectometry. The time domain reflectometer locates and characterises deflections in the power flow inside a cable, which are caused by faults, joints, open connections, etc.
The distance to the fault is measured on the basis of the time taken in the process, and velocity of the pulse and time domain reflectometers are used for calculating the distance. Other fault pre-location methods used are arc reflection, travelling wave, high voltage bridge and voltage drop.
Utilities deploy several testing mechanisms and devices to identify any susceptible breakdown in cable system components as well as to locate the actual fault occurrence. One of the methods 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 dielectric 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 the operator from electrical shock.
Another method used is the tan delta test, which indicates the degree of cable insulation degradation and helps prioritise repair or replacement works. Partial discharge testing is also used to determine cable health. While the tan delta method tests the cable’s overall health, partial discharge testing identifies individual locations of electrical discharge. The test involves the application of elevated AC voltage between the conductor and metallic shield inside the cable through a pulse generator. Subsequently, the transient signals emitted from the discharges are detected through the inbuilt oscilloscope of partial discharge meter. Some of the other methods used are high potential testing and Megohmeter or Megger testing.
Overhead lines are more accessible than underground cables and thus they can be tested without the need to go offline. 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 online or live monitoring methods include distributed temperature sensor (DTS) and sheath current monitoring. Temperature detection is very important because the insulation condition and carrying capacity can be obtained by monitoring the temperature of cables.
A built-in DTS using optical fibre temperature sensor systems can measure in real time, is anti-corrosive, immune to electromagnetic interference and has high sensitivity. Capacitive coupling sensors, magnetic couple/capacitive couple, 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 localise from which of the three phases a partial discharge signal has emanated.
EHV and UHV lines
To monitor the health of EHV transmission systems, aerial line patrolling is increasingly being deployed. Helicopters and, where clearances allow it, drones equipped with light detection and ranging (LIDAR) sensors, thermovision cameras, corona cameras, and highresolution video and digital cameras are deployed for identifying transmission line defects. Power Grid Corporation of India Limited (Powergrid), which owns and operates around 142,989 ckt. km of EHV transmission lines spread over the length and breadth of the country, envisages the aerial patrolling of 17,000 route km of transmission lines with the help of helicopters equipped with gimbal-mounted cameras providing high resolution images.
Powergrid has also received clearances and commenced tower top patrolling of the ± 500 kV HVDC Balia-Bhiwadi line using unmanned aerial vehicles (UAVs) or drones. These UAVs are also equipped with gimbal-mounted, ultra-high definition video cameras, which can take high resolution close photographs and videos of towers and their components for detection of faults and defects, and eliminates the need for tower top patrolling (by climbing the tower) for such inspections.
Net, net, in the next few years, with greater integration of renewable energy, a shift towards higher voltage systems and the addition of generation capacities, India’s transmission network is set to grow even further. Therefore, ensuring its optimal performance is essential for reducing congestion and enabling free flow of power from surplus to deficit regions.