In a power transmission system, asset management decisions are made on a continuous basis during the lifespan of a transmission line, with emphasis on extending the life of the assets, maintaining or improving reliability and performance, optimising costs and ensuring safe operation of the system. In India, many transmission lines that were built decades ago are beginning to show their age and now approaching the end of their expected lifetime, necessitating asset maintenance and monitoring. Further, the network is undergoing changes in the process of moving towards a smarter grid and integrating large flows of renewable energy. The management of the existing ageing network along with an evolving grid warrants enhanced asset management by utilities.
Asset management is a complex task that requires quality manpower, aims to maintain availability and reliability all the time, and ensures appropriate returns on the capital invested in the grid. With this view, asset management policies have evolved significantly from time-based maintenance to condition-based maintenance and from corrective maintenance to predictive maintenance. Various technologies such as geographic information system (GIS), drones and mobile-based monitoring systems are also in use now to monitor and maintain transmission lines.
Power Line takes a look at the maintenance and monitoring strategies, and the key technologies used in the management of transmission lines…
One of the most simple strategies is corrective maintenance, as per which a component is operated until it fails. Post failure, the condition of the component is assessed, and the decision is taken whether to repair or replace it.
The most traditional approach for maintaining transmission lines is time-based maintenance, which is widely used even today. Under this, inspection is performed at fixed time intervals and certain routine maintenance works are carried out as per a pre-identified schedule. Time is the trigger for this type of maintenance, which is easy to implement. On the other hand, under the condition-based maintenance (CBM) strategy, maintenance activity depends on the component status and is carried out when the condition of a component shows signs of decreasing performance or upcoming failure. At moderate maintenance costs, CBM leads to high availability of transmission assets. A refined version of CBM is reliability-centred maintenance whereby the maintenance schedule is optimised by considering two aspects: the condition of the equipment and the importance of the equipment for the network.
These approaches have relative merits and all are being used under different scenarios. However, maintenance policies of most utilities are time or condition based. A foolproof technique that can accurately predict whether maintenance or replacement should be done is yet to be discovered.
Condition monitoring can detect maintenance issues before they become failures. Some of the condition parameters that must be monitored to improve the performance of transmission lines include temperatures, corona, electrical clearance, live line punctured insulation and tower footing resistance.
Thermovision scanning detects a rise in temperature at an early stage and prevents costly failures. A punctured insulation detection scan measures the electric field or voltage to detect the point of leakage in a defective insulator. Corona is a phenomenon associated with transmission lines. It is created under conditions where a localised electric field exceeds a certain critical value. It is important to reduce the amount of corona because it not only creates noise, but also interferes with radio and TV signals and damages the grid components. Corona cameras help to inspect transmission lines on a regular basis.
Electrical clearance measurement measures the distance between two conductive parts of a transmission line. It helps in ensuring that there is adequate insulation between the two electrical parts in order to make them function smoothly. Another important aspect of condition monitoring is tower footing resistance. It is the resistance offered by the metal parts of the tower combined with the ground resistance to the dissipation of current. The tower footing resistance value should be as low as possible, which means less voltage stress across the line insulation.
Tools and technologies
To ensure effective inspection and maintenance of power lines, it is vital to be continually informed of the running changes in the power line corridor and their surroundings. Various tools and technologies are being used today for the monitoring of transmission lines.
GIS tools are being used for mapping of transmission assets, route alignment during planning of new transmission lines, vulnerability analysis in case of events like floods and fires, post-tripping analysis, studying changes in river courses (for monitoring of safety of lines and towers located near rivers), and disaster management studies.
Tower top patrolling is now being done using unmanned aerial vehicles (UAVs)/ drones, eliminating the need for inspections by climbing the tower. These UAVs are equipped with gimbal-mounted ultra-HD video cameras that can take high resolution close photographs and videos of towers and their components for detection of faults/defects. This has improved the time as well as cost efficiency in the maintenance of transmission lines. Aerial patrolling of lines using helicopters has also begun in India in areas such as deep forests, hilly terrains and snow-bound areas where ground patrolling is difficult and very time-consuming. These helicopters are also equipped with gimbal-mounted LIDAR (light detection and ranging), thermo-vision cameras, high resolution video and digital cameras to identify the defects in the transmission lines. The high resolution imagery obtained through them provides detailed information about towers and lines, including the condition of insulators, earth peak conditions, existence of broken insulators and missing bolts. This is much faster in comparison with the patrolling of lines on foot by human beings, and with regard to taking corrective actions and results in the effective monitoring of critical locations.
Mobile-app-based inspection solutions, such as Patrosoft being used by Power Grid Corporation of India (Powergrid), help locate and monitor the complex electrical grids in real time from the point of generation to consumption as well as in the case of malfunctions. Such an app allows locating and identifying towers based on their GPS location, besides providing biometrics-based authentication for maintenance engineers. It allows reporting issues in real time. Data can also be captured offline and uploaded subsequently. It runs on corporate servers, and generates reports and reminders on the performance of the transmission system.
Being the central transmission utility of India, Powergrid has the mammoth task of maintaining the large and highly complex transmission system of India. Like most successful utilities across the world, Powergrid follows preventive maintenance and has already adopted the technological tools discussed above to maintain its assets. In its approach to asset health management, Powergrid is integrating supervisory control and data acquisition (SCADA) of the online monitoring system with its National Transmission Asset Management Centre, regional transmission asset management centres and local SCADA. It is also using dissolved gas analysis software to perform periodic checks of transformer parameters. It has deployed emergency restoration systems to restore collapsed towers in the shortest possible time. The company has also implemented a module in SAP for condition assessment and integrated the same with intelligent decision support tools for undertaking asset health indexing, preparing asset plans, and planning the work under progress. The company is also using techniques such as live-line working. This service is called hotline maintenance, a highly specialised maintenance activity wherein skilled manpower carries out maintenance activities such as replacement of insulators, vibration dampers, hardware, etc. on live lines, thus avoiding outages of transmission lines. Powergrid is also in the process of acquiring technology for the use of robotics for transmission line inspection and maintenance.
Asset monitoring has a big role to play in avoiding abnormal stress in the transmission system and in obtaining data essential for planning suitable maintenance (or replacement) measures before the occurrence of a failure. With advancement in technology, this is being carried out more efficiently and has also led to an increase in the system availability. Going ahead, efforts would be required to cover all the assets across the country, and to analyse data more effectively for enhanced asset management and operation.