Maintenance policies for energy systems subject to complex failure processes and power purchasing agreement

Power purchase agreement (PPA) has emerged as a new support contracting approach and received much attention in recent years. In this paper, a novel maintenance model under PPA is developed for a complex energy generation system, whose objective is to maximize the expected net revenue of energy suppliers. The energy system undergoes two competing and dependent failure processes, namely, soft failure process and hard failure process. The former reduces the production rate of the system, whereas the latter immediately causes the stoppage of the system. The dependence is characterized by the random increment of soft/hard hazard rate due to the occurrence of hard/soft failures. The system is minimally repaired upon a hard failure or the detection of a soft failure. Additionally, a preventive replacement is immediate when the system operational age attains the pre-determined age. The target of this article is to maximize the expected net revenue of energy suppliers via the optimization of the inspection interval and number. A case study on wind turbine system (WTS) is provided to validate the effectiveness of the adopted profit-centric approach.