TRANSFORMER MAINTENANCE Transformer Condition Monitoring Five Need To Knows A dashboard for your most important transmission & distribution assets T ransformer condition monitoring has become increasingly important over the past two decades, given the financial impact of failure, Rüdiger Kutzner, head of the Global Technology Centre at Siemens Transformer Lifecycle Management, told participants during an Engerati webinar earlier this year. “It started with the large units with very high financial impact of failure,” he said, adding: “However, there are still a lot of smaller units for which there isn’t a commitment to spend money on, but we expect this policy to change in the future.” Notably the webinar participants were divided on transformer monitoring in their companies, with 51% indicating no monitoring and 49% indicating some (or all) being monitored. Further, almost a quarter had experienced a transformer failure, of which 3% recorded a complete failure. Five key concerns were raised by webinar participants: 1. How is a transformer ranking done? To develop a ‘health index’ firstly you need to evaluate the condition of the transformers, which would normally involve collecting data obtained on- site, such as visual inspection and oil analysis, and potentially combining it with online monitoring. With the initial conditions collected the data is then ranked and a trend analysis is completed. The strategic value of the transformer in the network also needs to be taken into account. This is a useful approach, and an 60 earlier step may be to gather groups of data including loading conditions, bushings measurements, and dielectric properties of the insulation fluid. 2. What is the most effective sensor diagnostic for sudden faults? Dissolved gas analysis (DGA), which is used to detect single or multiple gases dissolved in transformer oil, is the first choice and will cover about half of the most probable failures. Data on transformer failures compiled by Cigré (the international council on large electric systems) on 675 major transformer failures indicates that about 40% were in the windings, 7% in the lead exit and 3% in the core. DGA has a high potential to detect these, as well as potential failure of the selector switch of the tap changer. However, it does not cover other sub-systems such as the bushings (which account for over a third of transformer fires and explosions), so a comprehensive approach should include DGA and bushing monitoring technology and possibly also tap changer monitoring. 3. Can transformers be retrofitted with online monitoring equipment? Retrofitting can be done to most transformers. For example, online DGA monitoring requires installation of an oil loop. Bushing monitoring will require some design information on the mechanical dimensions of the potential tap of those devices. The placing of temperature sensors and CTs to measure load current is quite easy. Such a package could be easily retrofitted. 4. Is there a study to indicate that transformer monitoring negates failures? The number of case studies is limited as still, only a limited number of transformers are equipped with online monitoring. In cases where transformers are equipped with monitoring often the data is not integrated in policy or even is not evaluated. We know DGA is a powerful diagnostic to support failure prevention and online DGA is even better, so we expect real benefits. 5. What is the ideal transformer maintenance programme? First, you need to collect the information to get a comprehensive view of the health of the transformer. This includes looking at the insulation fluid and for example at auxiliary parts such as the cooling and protection devices to check they are working properly. There are also the gaskets for sealing and avoiding leaks and environmental impacts amongst other items. Guidelines and checklists are normally available in the manuals. Cigré has also issued recommendations for transformer maintenance and transformer manufacturers and service partners can also provide a programme for the right actions to be taken at the right time. ESI ESI Africa thanks Engerati for permission to reprint this article ESI AFRICA ISSUE 2 2015