Health index assessment of aged oil-filled ring main units

Oil-filled ring main units (RMUs) are considered to pose a medium to high medium risk due to the severity of failure consequences and the ageing population. The obvious solution is to institute a network wide replacement program of oil-filled RMUs with the newer types of RMUs. However, a replacement program covering the total population of tens of thousands will take significant capital expenditure and time. This paper proposes a condition-based Health Index to determine the present condition of oil-filled RMUs and facilitate the decision making for their replacement. Seven condition indicators which cover different major components are used for the modelling of the RMU in the Health Index model. The criteria for determining the Health Index for each condition indicator is derived from manufacturer´s data, IEC standards and other standards like IEEE, NETA and ASTM if the equivalent IEC standards are not available. The overall health index (HO) is then calculated based on the weighted sum of the individual condition indicator Health Index. The proposed Health Index method was then applied to test data collected from actual RMUs.