Hotline Maintenance

Proven live-line techniques enabling uninterrupted O&M for T&D utilities

By Harshal Malewar, Deputy Manager, Projects, North East Transmission Company Limited

The ever-increasing demand for electric energy necessitates higher reliability in the transmission and distribution (T&D) of electrical energy. The in­creasing complexity of transmission networks, interregional/interstate ties transporting a huge quantum of power and less redundancy in the transmission system have rendered shutdowns of transmission lines/substations almost impossible. Therefore, preventive ma­in­tenance deploying the live-line maintenance technique seems to be the best solution for power T&D utilities.

Live-line working, also known as hotline maintenance, often operates at high voltage while the equipment is energised. Although it is more hazardous for personnel than working on electrical equipment with the power off, live-line maintenance is adopted in the electricity sector to avoid disruption and the hi­gh economic costs of turning off power to customers in order to perform essential periodic maintenance on lines and substations. Cold-line maintenance, wh­i­ch involves working in shutdown mode, is undertaken in the T&D sector as a preventive measure.

Methods of hotline maintenance

Hotline work can be classified into the following categories:

  • Hot stick method: In this method, the lineman is at ground potential working with hot sticks (live-line tools), keeping a safe distance from the line by using a conductive suit and hotline tools.
  • Bare hand method: In this method, the linemen work at the same potential as the conductor, keeping safe cle­arance from the ground. For this, an insulated suit made of 25 per cent mi­croscopic stainless steel and 75 per cent Nomex is used.
  • Combination of hot stick and bare hand methods: This method is used to overcome the constraints in tower top geometry.
  • Insulating glove or rubber glove wor­king: Gloves protect the worker from exposure to the live part being worked upon, also referred to as the first point of contact. Insulating material such as blankets and line hose are employed in rubber glove working to protect the worker from exposure to a part at a different potential sometimes referred to as the second point of contact. Most utilities require work to be performed from an insulating platform to provide isolation from earth/ground potential, hence the term insulate and isolate.
  • Hotline washing: Pressurised demineralised water with resistivity more than 30,000 ohms is used for the washing of insulators on line with the safe distance of 6.1 metres for 400 kV lines and 4.57 metres for 220 kV lines. A polyplastic nylon reinforced non-conductive hose is handled by the crew on tower with proper earthing.
  • Insulated aerial work platform: The insulated bucket truck used for live-line working is an extraordinary tool that is exceptionally safe and highly efficient to perform more number of ta­sks per day. Further, productivity and safety of a live-line crew will be increased manyfold by using an aerial insulated platform.

Hotline operations in transmission lines and substations

Some hotline maintenance operations that can be carried out are as follows:

For lines

  • Changing of insulators
  • Changing of hardware components, spacers and dampers; application of repair sleevs; maintenance and repla­cement of mid-span joints.
  • Replacement/Repair of damaged sections of the conductor
  • Testing of insulators (one line insulator tester, puncture insulator detector [PID])
  • Tension string replacement
  • Changing of crossarm for HV systems
  • Changing of poles
  • Live-line washing
  • Jumper online tightening and nut-bolt changing
  • Strengthening of dead end hardware
  • Replacement of arcing horn and corona ring hardware.

For substations or switchyard

  • Insulator testing strings replacement, live-line washing of switchyard insula­to­rs, change out of jumper and pilot insulator string.
  • Rectification on the switchyard bus, extension of main bus, connection of new bays and feeder, nut-bolt changing, isolator maintenance and replacement, bypass connection in case of urgency, lubrication of the energised rotating mechanism, maintenance of the various hotspots of the switchyard.
  • In switchyard insulators, PID equipment testing has meticulously redefi­ned the insulator diagnostic process and subsequently assisted transmission system engineers in identifying the failure of transmission lines and an­alysing the behaviour of insulators. The computerised graphical output for each insulator string is very handy to determine the maintenance strategy for a specific transmission line, mo­re economically and timely, by utilising the available resources well.
  • An insulated modular scaffold system is being prominently used to work on energised busbars of switchyard and equipment. Nowadays, live-line working is well accepted throughout the country and the world. Live-line compression crimping, busbar repair and maintenance of the isolator’s switching contacts in EHV switchyards, and launching linemen on live wires be­comes extremely easy, safe and effective with the robotic arm of this truck-mounted platform.
  • With an increasing number of UHV tra­nsmission lines being commissi­on­ed in India, this is a decisive phase to ad­van­ce our present live-line working capabilities. The vast 765 kV transmission network established in recent years will definitely demand attention after some years. Most of these transmission networks are based on the TBCB model and the revenue is directly linked to availability with strict regulatory monitoring. In this scenario, the options of uninterrupted maintenance technologies must be explored by all the leading transmission companies.

Advantages of hotline techniques

  • Uninterrupted maintenance of the transmission system and reduction in technical losses.
  • Safe and practical technique to allow continuous flow of power supply.
  • Help save regulatory penalty and prevent costly outages and forced outages.
  • Early attention to the transmission system problems.
  • Reduction in technical losses.
  • Suitable use of manpower as the live-line work is performed every day.
  • Savings of huge expenditure incurred in breakdowns.
  • Increased availability and reliability of transmission lines.

In view of the expansion of our transmission system and the entry of private players in the transmission sector in recent years, we look forward to adopting modern techniques to stay at par with the glo­bal trend. The adoption of a live-line technology will reduce the interruptions and increase the availability of the transmission system, which is vital for generating more revenue and maintaining a stable grid.

Challenges

A deeper study of stringent electrical and mechanical characteristics of transmission lines is required. A deep analysis of transient over voltage and diverse swit­ching surges will help in determining the protective equipment and pot­ential app­roach distances. Further, the dimensions of insulator string yoke pla­tes, ha­rd­ware and insulators will re­quire redesigning of various live-line tools and equipment. After theoretical calculations, 3D graphic models of live-line tools are necessitated to analyse their compatibility with line hardware by a team of experts to look into each electrical and mechanical phenomenon. These newly developed tools must be tested according to standard in­ter­national procedures.  More­over, with every changed design of insulator and hardware, the entire procedure will be repeated. Skilled manpower is also not available for operating hotline tools. Proper training needs to be provided to working technicians and engineers on hotline maintenance.

New trends in hotline

  • Safety and efficiency can be inc­rea­sed by integrating the maintenance aspe­cts into the structure design at the planning stage rather than as an afterthought. Clearances can be pre-established, and working ease can be ass­ured through scaled layouts utilising human factor considerations. This sh­o­uld encompass the structures, in­su­lators, hardware and tools to be used and the approach interfaces of these with the line workers (accessibi­lity, steps, handholds, platforms, attach­me­nts, weight, visibility, etc.).
  • There are several different telerobotic systems for live-line maintenance be­ing developed around the world. Each of these systems is specially de­sig­ned to fulfil a series of requirements for the relevant application. To this end, the de­signers had to work with different sol­utions and approa­ches, some specific to their application and others of general relevance for all the systems. These solutions define the practical and efficient use of the telerobotic system, and have to be adopted with care – being fairly conservative but also looking at the future requirements.
  • The latest techniques also employ helicopters, useful when transmission lines traverse inaccessible terrain or when a bucket truck damages crops or does not have access to roads for the insulated truck.

Conclusion

Live-line maintenance can be crucial to pro­vide interrupted power to customers and avoid costly shutdowns. Further, system availability can be greatly inc­rea­sed keeping the revenue loss due to sh­utdowns and inconvenience to custo­m­e­­rs to a minimum. Live-line maintenance has proved to be a necessity for power utilities. At the same time, it must be kept in mind that electricity is hazardous and, therefore, strict regulations for live working and rigid adherence to protocols must be ensured.

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