Transformer Condition Monitoring – Five Need To Knows

Condition monitoring can help prevent unplanned transformer outage with its sudden power loss and cost implications.
Published: Mon 02 Feb 2015

Transformer condition monitoring has become increasingly important over the past two decades, given the financial impact of failure, Dr.-Ing. Rüdiger Kutzner, head of Global Technology Center at Siemens Transformer Lifecycle Management, told participants in a recent Engerati webinar, Transformer Condition Monitoring – A dashboard for your most important Transmission & Distribution assets.

“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.”

Joining the webinar with Kutzner, who discussed the transformer ageing process and potential fail points, were Siemens colleagues Stefan Ettl, head of R&D for Transformer Condition Monitoring, and Anton Wiedenbauer, Product Lifecycle Manager. Ettl gave in-depth insights into the condition monitoring process and Wiedenbauer outlined a condition monitoring integration and implementation strategy.

Notably the webinar participants were divided just about equally 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.

What were some of the key concerns of the webinar participants?

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 can then be ranked and trend analyses done. The strategic value of the transformer in the network also needs to be taken into account.

This is a nice approach, and an earlier step may be to gather groups of data such as loading conditions, bushings measurements, dielectric properties of the insulation fluid, etc.

What is 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 on 675 major transformer failures indicates that about 40% were in the windings, 7% in the lead exit and 3% in the core and DGA has a high potential to detect these, as well as potential failure of the selector switch of the tap changer. However, it doesn’t 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.

Can online monitoring equipment be retrofitted to transformers?

Yes, 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.

Is there a study to indicate that transformer monitoring negates failures?

The number of case studies is limited as still, only a limited proportion 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.

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, etc.

Guidelines and checklists are normally available in the manuals. Cigré has also issued recommendations for transformer maintenance and we as a transformer manufacturer and service partner can provide a programme for the right actions to be taken at the right time.