Determining power transformers' sequence of service in power grids

Utilities need to ensure power grids that are highly reliable and cost-efficient as well. Periodic replacement assures reliability but causes high replacement costs. Thus, it is much better to keep power transformers as long as possible in a functional condition and ensure reliability by adequate service (maintenance and repair). Cost-efficient service requires a sequence of service of power transformers. The sequence can be defined according to some criteria that need to be tracked in course of time. The paper introduces service classes and the risk of an outage as criteria. Service classes and risk are defined according to the condition and function of power transformer's components. The paper focuses on the insulation system, which is one of the most important power transformer components. For the inspection of this component the paper introduces dissolved gas analysis and oil characteristics to find insulation system's defects and aging, respectively. For the interpretation of measurements two new interpretation methods are applied, one for dissolved gas analysis and one for the oil characteristics. They overcome certain limits while they map measurements to different defects and aging conditions. The interpretation methods are presented from a user's point of view. Literature is given for a more in depth presentation of the underlying concepts. The service concept will finally be applied to a group of power transformers that belong to a transmission system operator.