Increase transformer reliability and availbility: From condition assessment to site repair

Over the last decade there has been a renewed and increased interest in transformer life evaluation and monitoring. The main reason is that a large number of the transformers world population is approaching its expected end-of-life and the need increases for better methods to see whether the transformers are still fit for use or need to be retrofitted or replaced. In case of failure the possibility to reduce the outage time is usually important for the transformer end user. In this paper we will therefore describe Transformer condition assessment methodology - as well as on-site repair solution - as means to increase both the reliability and availability of transformers and therefore the energy efficiency of the network. The output of such a Transformer condition assessment methodology survey is a ranking of the population with priorities for investment and recommended maintenance actions plan for each unit. The advanced diagnosis of the transformer condition is used to identify defects even before it is required to untank the transformer and therefore it helps to improve repair time especially in the case when a transformer is repaired at site. If a transformer has to be repaired in a factory, the transportation time from the end-user site to factory and return plays a major influence on the duration of the project and putting transformer back in service to resume power. Also the cost and risks associated with heavy transport must be considered. To date a total of more than 300 transformers including utility, industrial, HVDC transformers and reactors have been successfully repaired on site. In many cases transformer output power were also upgraded to provide an increased rating using present advanced design tools.