Using a probabilistic design process to maximize reliability and minimize cost in urban central business districts

The practices currently used by many US utilities in designing urban distribution systems were developed during the early part of the twentieth century. In those days, distribution systems were not as heavily loaded as they are today, assets were not as aged as they now are now, thermal and spatial constraints were less severe, customers were less demanding, and utilities were not faced with the regulatory and competitive pressures that exist in today´s market. In this dynamic market environment, utility managers are starting to search for new approaches to design and operate their systems. Their objectives in this search are to identify approaches and technologies that enable them to deliver an acceptable level of end-customer reliability and satisfaction, at reasonable costs, while managing the political and public relations risks that are an inevitable part of the regulated utility business. This paper will present the case for a new design process in a modern distribution utility based on probabilistic principles. In the past, it was common practice to ensure reliability and simplify engineering by designing to a deterministic standard such as N-1 or N-2 redundancy, regardless of the application or environment. The paper will examine the impact of deterministic standards on the design of the system, and discuss the potential deficiencies of this design process in a constrained and changing environment. The probabilistic design process will be explained and illustrated on an example sub-system and the cost, reliability and risk implications will be discussed.