Predictive modelling of overhead lines reliability and lifetime

The Portuguese Transmission System Operator (REN) launched a long term effort to develop a risk based methodology for operation and maintenance to optimise the lifetime and maintenance costs of Overhead Lines (OHL), based on asset information, inspection, maintenance and third party data. The scope of the work considers next-day (for operation) and next-year (for maintenance) risk estimates based on a common framework. To estimate the reliability of each asset, the authors propose combining heterogeneous sources, such as information from airborne inspections, dispatch event records and third party data. Such procedures are common for switching equipment and transformers inside substations and power plants, but it is less so on OHL that spread across wide and heterogeneous territories, and are subject to diverse environmental factors. Therefore, the proposed probabilistic approach considers the interaction of the OHL with its surrounding dynamic environment, i.e. the OHL's space-time context. The methodology is discriminative regarding contingencies related to OHL outages, and this paper presents details on the design of the reliability functions concerning lightnings, storks and vegetation contingencies. In particular, it complies with the vegetation dynamics using a non-stationary approach. For each contingency, the intensity function of the random process is design in a data-driven fashion. Regarding lightning incidents, data shows that the OHL proneness is heterogeneous in both space and time, as well as with respect to the operation voltage. Concerning the outages caused by the storks' usage of the infrastructure, data also shows a non-uniform space-time distribution, although it can be seen that storks (themselves, not incidents) are spreading to new lines. In fact, the number of outages related with storks has been decreasing due to REN's highly effective preventive maintenance policies. With respect to outages related with vegetation, they are very scarce. The major reason is the REN's right-of-way and fuel management policies, which are very efficient regarding the quality of service criteria. This includes, for example, routine airborne inspections and time and condition based maintenance. Such measures have granted high quality of service on the Portuguese transmission grid: in 2013, REN's achieved 0,09 minutes of Average Interruption Time. However, to increase efficiency, there must be a compromise between economic and quality of service criteria. Hence, this paper presents a risk based maintenance viewpoint, with emphasis on inspections, which is deemed to increase asset availability while decreasing costs.