Condition monitoring in New Zealand power transformers

Transpower owns and operates New Zealand’s high voltage electricity grid which includes approximately 725 in service power transformers [1]. Presently, condition monitoring of these units is routinely carried out by oil testing (moisture, acidity and dielectric breakdown) and using dissolved gas analysis (DGA), (every year), and winding resistance, insulation resistance, and bushing power factor tests (every four years). However, since the average age of a power transformer in New Zealand is nearly 40 years [1], it is considered that online condition monitoring of important transformers or transformers that have known issues is carried out to identify any incipient faults. The online condition monitoring in existing power transformers is hoped to minimize the risk of sudden failures and thereby prolong the in service life. It is equally important to decide on what to monitor in a power transformer and how to monitor, and these are also governed by the budgetary constraints. Transpower is in the process of acquiring online condition monitoring units for some of the new large power transformers it plans to purchase and will also retrofit such units to some old transformers as required. This paper presents the condition monitoring techniques currently used by Transpower on power transformers, and the online condition monitoring techniques for new and existing power transformers