India's transmission system has encountered several problems with respect to nonavailability for live-line work as a result of the inadequate clearances in the geometry of the tower-top configuration. The tower-top geometry was designed based on the weight, volume and cost of towers and foundations, not on the clearances required to facilitate live-line maintenance work. Therefore, the availability of an adequate approach distance dictates the use of live-line techniques.
Utilities around the country erected various configurations of transmission line towers to optimize the cost of transmission lines. Engineers have designed several single- and double-circuit lines with V-string instead of I-string insulator configurations. The reduced approach distances on single-circuit lines with horizontal conductors designed with V-suspension insulators, and for I-suspension insulator strings on double-circuit lines, required special consideration. For these conductor and insulator configurations, Power Grid Corp. of India Ltd. (POWERGRID; New Delhi, India) developed live-line techniques that could satisfy its maintenance needs. Linemen can use these techniques with ease and safety in situations with reduced approach distances. These techniques ensure that the availability of the transmission system is comparable with the world's top transmission utilities.
India's Transmission System
POWERGRID operates and maintains more than 39,000 circuit km (24,250 miles) of 500-kV HVDC, 400-kV AC, 220-kV AC and 132-kV AC transmission lines, and 65 substations.
Its transmission system is divided into seven regions with a management structure established to maximize operational efficiency. Each region has a set of hotline maintenance equipment and a team of trained and skilled engineers and technicians to execute the hotline maintenance work. Though they vary among the regions, the number of workers on a team, which averages four to six members, includes one or two engineers and a lineman.
Hotline maintenance activities are planned for six months each year, with work suspended during foggy weather and the rainy season. The range of work includes insulator replacement and hardware tightening/replacement near the tower position. Techniques for mid-span work such as conductor repair are under development.
Planning Hotline Maintenance
The planning of hotline work is based on the following safety and system operational procedures:
Favorable climatic condition for hotline operation is sunny weather. If the weather forecasts rain or thunderstorms, work will not begin.
Before going to the work site, all equipment and tools will be inspected and checked for correct operation.
All hotsticks and ladders will be cleaned and checked for integrity by the hotstick tester.
All linemen in the hotline team will be equipped with personal protective equipment during the work.
No live-line team members on the tower/conductor will wear any metallic chain, wristwatch or rings to avoid any circulating current.
Auto recloser will be in “OFF” position for the line at both ends.
Work permit will be taken from the terminal substations at each end of the line.
POWERGRID's live-line works require crews to practice the following on-site briefing and safety disciplines before starting work:
The work procedure is discussed with the team members at the tower location and the responsibility of each member is properly defined.
The land in close vicinity to the tower is cleared to provide a site area for the required tools.
All epoxy hotline sticks are cleaned with Methyl-Ethyl-Ketone or Acitone, the metallic sections are cleaned with kerosene and the rotating parts are cleaned with graphite powder before preparing assemblies for the live-line work
All cleaned hotsticks, strain carrier and other assemblies are kept on the hotline tool rack to avoid ground contact.
The team linemen will wear conductive socks, boots, helmets and hand gloves. The “hot-end” lineman will wear complete barehand suit.
V-String Insulator Replacement
For the circuits with V-string insulators, the procedure to replace insulators is similar to I-string replacement with one main exception. In V-string replacement, a V-cradle subassembly is required for removing the V-string from the yoke plate, and the working clearance is not sufficient for shifting the “hot man” to the conductor. Thus, a new method was developed for shifting the hot man on the conductor.
In this method shown on page 48, the ladder is hung on the crossarm near the conductor alignment and the hot-end lineman climbs on the ladder near the V-portion of the tower. The ladder is then moved away from the tower and the hot-end lineman climbs on the ladder to move onto the conductor. After climbing on the conductor, he can crawl toward the working point.
The hot man cannot directly land at the position where the conductor is attached to the insulator string because the reduced approach distance (3060 mm [10 ft]) is only available with live metal parts. Therefore, no margin is available for inadvertent movement of the hot man. Furthermore, while working, the hot man has to use a barrier in the form of an insulated platform so that his legs are not suspended beyond corona ring. With the precise positioning of the hanging ladder, landing of hot man at an accessible place (away from conductor attachment point) and restricting the leg movements of the hot man, V-string insulators can be replaced where the approach distance is 3 to 3.5 m (9.8 to 11.5 ft). This is somewhat less than the normal approach distance of 4 to 4.5 m (13.1 to 14.8 ft).
Replacement of Insulators
In the case of 400-kV transmission line towers, the electrical clearances of the jumpers and insulator strings from the live metal tower body/crossarm are designed based on power frequency and switching overvoltages. Where no additional clearances have been provided at the design stage, it may be difficult to carry out hotline maintenance with the required clearances and degree of safety. It is relatively easy to access the top phase on the tower, because the problem of electrical clearances for the “cold man” who will be sitting on the top crossarm of the conductor does not exist. However, a limitation of electrical clearances when the cold man sits on the middle or bottom crossarm is only 3 to 3.5 m (9.8 to 11.5 ft). This restricts the use of hotline maintenance by conventional methods, so a special method has been developed to overcome this problem prevalent on existing transmission lines.
To make more room for the cold man sitting on the middle or bottom crossarm, the top and middle phase conductors should be pushed off the tower. This operation is easier on tension-type towers because only jumpers not under tension need to be pushed off. However, because of their weight and tension, it is difficult to push the conductors off suspension towers.
Pushing off the conductor/jumper is done with the help of hotsticks that have suitable end connections and a chain pulley block. Where crossarms can take up the load of conductor, the operation can be performed from nearby crossarm. If not, the operation has to be done from the tower body, which involves the use of longer hotline sticks. Linemen must make sure that no jerk load exists on the tower and that the load is equally distributed on all the legs of tower.
Energized Puncture Detection
The presence of punctured insulators in a string cannot be ignored, particularly on transmission lines that have been in service for a long time. As the electrical strength of the insulator string depends on all insulators being healthy before starting hotline maintenance, it is essential that the integrity of the insulator string is healthy and has adequate electrical strength. It is also important to short out the last two insulators to equalize the potential for carrying out hotline maintenance using the bare hand technique. Energized puncture detection of insulators, therefore, is a prerequisite for starting hotline maintenance. To avoid permanent faults, the preventive-maintenance disciplines include insulator scanning to identify punctured insulators.
One of the methods commonly used throughout the world is to measure the electrical clearance across each insulator disc. This requires a scanner for detecting the electrical clearance that should be easy to operate under live-circuit condition. POWERGRID uses a lightweight IBM-compatible damaged-insulator scanner that is compatible with standard hotstick mountings and has a memory capacity for recording tests on 200 strings.
The figure above shows the output of the electrical field across each insulator disc. If one or more insulators are polluted or punctured, a sudden drop of voltage will be shown on the graph. Defective insulators can be replaced under live conditions or during a circuit shutdown, depending on the number of punctured insulators in a string.
Benefits of Live-Line Maintenance
The proper development of hotline maintenance techniques offer the following benefits:
Preventive maintenance of transmission lines can be done under live-condition; shutdown is not required. This increases the availability of the line and decreases the long-term maintenance cost.
Permanent fault outages and component failures can be reduced, thus reducing the cost associated with major circuit outages. Probability of permanent circuit interruptions is also decreased.
By scanning the insulators with the help of a damaged insulator scanner, the required level of safety necessary when performing hotline maintenance work can be achieved. It also identifies the punctured insulators, thus providing data for planning preventive maintenance.
By developing the techniques to carry out hotline maintenance for those situations where sufficient electrical clearances have not been provided in the design of the transmission line, it is possible to take up preventive maintenance under live conditions.
POWERGRID has adopted modern techniques for maintenance, including: hotline maintenance, maintenance under induced voltage conditions on 400-kV double-circuit lines using a special earthing system, emergency restoration system for transmission lines and condition monitoring of EHV equipment for detecting the fault at its initial stage.
POWERGRID has maintained its line availability above 98% since its inception and is ranked among the few top transmission utilities in the world.
Sh. L.N. Agrawal has spent 20 years in the planning, design, engineering, quality/inspection, construction, and operation and maintenance of EHV transmission lines up to 800 kV. Agrawal has presented technical papers at CIGRÉ, ESMO, CEPSI and PowerGen on the important aspects of transmission lines including emergency restoration systems, hotline maintenance and 500-kV HVDC lines. Agrawal is a member of the Expert Committee on Transmission Line Maintenance of the Central Board of Irrigation and Power, India.