On-load tap-changers current experience and future development: Users experience and perspective

On-load tap-changers may represent only 5-100 % of the capital cost of a supergrid power transformer yet their operational performance and reliability is critical to realising the full asset life of the transformer. In addition, tap-changer inspections largely govern the frequency and duration of transformer circuit outages and represent a high proportion of transformer maintenance costs. This paper presents a summary of the National Grid Company´s (UK) experience in such tap-changers in recent years. This experience, the changes in specification and the user-supplier relationships described are believed to be representative of a major electric utility