One of the differences between a medical doctor and an electrical engineer (besides income) is that engineers have the luxury to completely turn off any system or equipment needing repair. This of course is not an option for doctors – unless they happen to be pathologists. But this difference is now changing. Power interruptions – no matter how brief – represent lost revenues for utilities and, depending on the affected customers, can cost millions. Working with the system energized (also referred to as live-line or hot-line working) has therefore become a modern-day necessity.
Large equipment such as generators, power transformers and circuit breakers have sophisticated on-line and off-line monitoring systems, which help in their timely maintenance. They are removed from service before any work is done on them and for that reason spares are almost always kept on hand. Moreover, at most substations, the bus configuration allows for a part of the station to be taken out of service for maintenance without interrupting power to customers. On the line side of the business, by contrast, there are virtually no condition assessment techniques at the network component level. Moreover, redundancy is typically minimal. Looped lines and networks make it possible to take a section out for maintenance most of the time, without affecting the end user. But there are exceptions. In times of peak demand it is difficult to schedule any outage since lines are carrying close to their capacity. Taking a line out under such circumstances means that the current in other lines might exceed their thermal limit, causing problems such as increased sag. In addition, many lines are radial feeds and single circuit configurations, meaning no redundancy at all.
There are many reasons why constant maintenance of overhead lines is necessary. Common ones include: tightening loose hardware, repairing damaged splices, spacers and conductor sections (due to vibration, vandals, arcing, etc.), re-routing a line section and, of course, replacing insulators.
There are basically two methods when it comes to live-line working on transmission lines – hotstick and barehand. In the hotstick method, the lineman stands in an insulated bucket or on the tower and relies on tools attached to an insulated stick in order to perform repairs. In the latter method, the hotstick is dispensed with and repairs are done with bare hands. However, the lineman’s entire body, including hands, is covered with a conductive suit and therefore at the same potential as the energized line. It’s easy to visualize that work with bare hands is far simpler than relying on long sticks. However, there are unresolved issues such as the health effects to workers exposed to very high electrical fields. At low and intermediate voltages (i.e. up to 46 kV), a combination of insulated gloves, barriers, and hotsticks are used for live line working. All these methods follow well-documented procedures.
Now, in regard to insulators, there are three important aspects to live-line working. The first is to ascertain before a power interruption, that an insulator has deteriorated enough to warrant replacement; second, that there is adequate dielectric strength remaining in the insulator to ensure no flashover while the lineman is working; and finally, that the insulator being used for replacement is itself sound. Details of these aspects happen to be quite different for the three insulator technologies available today (namely porcelain, toughened glass and composite). With porcelain insulators, damage is observed by visual inspection either from the ground or by aerial patrols. Such damage can take the form of cracked, chipped or broken bells, corroded hardware or burn marks to the glaze due to flashover. To ensure that live-line work can be performed safely, tools such as the buzzer or the electric field probe are used to identify punctured units and establish that the number of healthy bells exceed some critical minimum.
For example, a large utility in Arizona performs barehand work on 230 kV and 500 kV lines. Their criterion is that up to one-third of all bells in a string can be defective (i.e. damaged and/or punctured). This translates to at least 8 and 17 healthy bells for 230 kV and 500 kV strings respectively. If new bells are used for replacement, there is no concern about their integrity since every unit has already been tested electrically and mechanically at the factory of the manufacturer. Toughened glass insulators make it relatively simple to diagnose any defect warranting replacement. If the shell of any unit is intact, the insulator is good. If the shell is missing – and this can be seen from the ground – even then the overall electrical and mechanical integrity is almost as good as a fully intact string. When a number of shells are missing, replacement is necessary as creepage distance is reduced and this adversely affects contamination performance.
Composite insulators are the most complicated from these points of view. Defects compromising the integrity of such insulators are usually internal and can neither be seen visually nor detected with complete reliability – even with devices such as corona and infrared cameras or using electric field probes. As such, it is difficult to confirm whether or not there remains adequate dielectric strength in the intact portion of the insulator. Moreover, not every new composite insulator is tested electrically in the factory. Only a small number of samples undergo electrical testing as a type or design test. There is the chance (admittedly quite small) that the insulator being used for replacing a bad composite insulator is itself defective and will fail electrically upon installation, with the lineman in close proximity. This last concern is in fact the easiest to resolve since it is not a question of technology but of cost.
Users can chose to test composite insulators at their own facilities or have them tested by the manufacturer for some additional cost. I believe that every composite insulator used for live-line working should be tested at their rated voltage. These insulators should then be properly tagged and stored separately in the warehouse. It is important to remember in this regard that we are dealing with organic epoxy resin in the fiberglass core of these insulators. Upon degradation, conductive carbon will be produced. And such degradation could be initiated during manufacturing – even before the insulator is put into service.
Prof. Ravi Gorur