Considerations in unit substation design to optimize reliability and electrical workplace safety

Many legacy low and medium-voltage unit substations installed today are based upon older designs that took advantage of reduced first cost “opportunities” allowed by existing installation codes and standards. Fast-forward to how these substation designs fair in safety and reliability today, particularly in industrial process applications found in cement, pulp and paper, petroleum & chemical and others, some of the exercised “opportunities” applied in the past begin to look more like liabilities than assets. Legacy engineering decisions once thought to be prudent take on new meanings today, particularly when these decisions are viewed through the lens of emerging new workplace safety standards. The critical issue of addressing destructive arc-flash hazards associated with persons working on or around energized electrical equipment must now be considered. Because traditional substation designs often appeared to involve some compromise regarding both safety and reliability, a design team of a major process industry user took a fresh look at unit substation design. The design review took place in conjunction with construction of a Greenfield plant built in the spring of 2009 in the USA. This paper will review the design limitations of traditional unit substation configurations, offer an overview of the alternatives considered by the Greenfield site project team, and discuss technical and safety validation of the design that was ultimately selected and installed. Economic comparisons to traditional designs, changes in the owner operating and safety procedures for plant personnel as a result of the engineering design changes, and overall design acceptance by operations will also be reviewed in this paper.