Power asset engineering specialist EA Technology claims large industrial and public sector electricity users could save millions of pounds by adopting the latest best practice for maintaining their oil-filled 11kV switchgear.
The company is urging all switchgear owners to follow the lead set by many asset managers in the UK’s power supply industry, who have moved away from scheduled time-based maintenance, to maintaining switchgear only when its condition indicates that intervention is needed.
It argues that condition-based switchgear maintenance allows operators to extend service intervals by many years, whilst actually improving reliability and safety. By contrast, conventional regimes, based on shutting down equipment regularly to inspect and maintain switches, can reduce reliability, increase health and safety risks, and cause unnecessary downtime and disruption.
Anne McIntosh, EA Technology’s materials and failure investigations lead consultant, explained: “Experience in the UK power industry clearly shows that the need for intrusive servicing of switchgear can be reduced dramatically, by moving from conventional time-based maintenance, to maintenance based on assessing the actual condition of the switchgear and oil.”
“Optimising maintenance regimes on the basis of condition could cut costs by more than 80%, from around £650 per switchgear set to less than £110 – or more than £100 million for the approximately 200,000 units in Britain.”
“Our research proves that, by applying effective condition assessment techniques to switchgear, the vast majority of units will operate safely and reliably for many years, without additional maintenance. This is confirmed by several reliability centred maintenance (RCM) studies.”
“By contrast, shutting down switchgear at regular intervals to inspect and maintain the unit can positively induce faults, and carries risks for maintenance personnel. There are also major implications in terms of isolating the switch, shutting down plant, the expense of the maintenance itself and the cost of bringing in backup power supplies if required.”
EA Technology’s argument is based on its development of a Live Tank Oil Sampling (LTOS) system, which uses the principle that the condition of oil in switchgear gives a highly accurate indication of the state of the unit and when it will need maintenance. The system also enables oil to be sampled and analysed whilst switches are partially energised, removing the need to isolate and totally shut them down.
Tests by the company on oil from more than 500 switchgear units due for maintenance at the end of a 10 year interval revealed that less than 1% actually needed attention within the subsequent six months. Less than 7% indicated the need for re-testing within 30-36 months, whilst the condition of over 90% of units was such that re-testing and maintenance could be postponed by several years.
“The results of oil sampling give an extremely accurate and consistent indication of switchgear condition. We can now give operators a choice of testing individual units using LTOS, or generic sampling. In the latter case, analyses from a representative sample can be applied to a larger group of units, wherever there are sufficient numbers of switches of the same type, operating in similar conditions,” said McIntosh.
“Whether using LTOS testing of individual units, or statistical sampling, operators will have all the data they need to optimise their maintenance regimes on the basis of actual condition, at a far lower cost than time-based servicing.”
“This is an example of condition monitoring at its best, with the prospect that in some cases, invasive maintenance could be postponed well beyond 10 years with a high degree of confidence.”
EA Technology has developed a range of LTOS test kits, tailored to fit most of the 11kV switchgear types currently in service in the UK. They include a product-specific sampling cover for the switch’s oil tank and disposable syringes for taking samples.
Only 50ml of oil is needed for EA Technology to give a detailed assessment of the condition of each unit tested and a recommendation of when maintenance will be needed. Recommendations are based on a combination of laboratory analysis and EA Technology’s database of the known degradation patterns for specific models of switchgear.
EA Technology has developed the LTOS system over the last three years in association with a large UK electricity company. The system has also benefited from EA Technology’s 37 year experience of conducting investigations into the failure of HV equipment, research into the degradation process within oil filled switchgear and its increasing involvement in developing Condition Based Risk Management (CBRM) solutions.
Note: EA Technology’s calculations of projected savings are based on a study of 516 switchgear sets in service in the UK. The estimated cost of routine invasive maintenance was £335,000 (£650 per unit). The estimated cost of LTOS monitoring, analysis and reporting, together with maintenance of only those units requiring attention, was a total of £55,000 (an average of £107 per unit). The overall saving is therefore £280,000 (83.6%, or £547 per unit).