In 2020, Public Service Company of New Mexico initiated a five-year program to upgrade an obsolete microprocessor and solid-state protection system for their line-tapped distribution power transformer. This effort was driven by the failure of multiple relays, the lack of redundancy, and the lack of event reporting data. The successful program has been extended to greenfield projects for line-tapped and bus-tapped distribution power transformers. The configuration of the distribution substation can include up to two power transformers that are connected in a main-tie-main topology at medium voltage, with an indoor metal-clad switchgear assigned to each.
This paper discusses the key protection improvements for more than 50 indoor metal-clad switchgear, including features such as combined overcurrent elements for main-tie-main configurations, integrated breaker failure protection, restricted earth fault protection, arc-flash protection for greenfield substations, and two-zone differential protection for bus-tapped power transformers. The paper also establishes and discusses a standard protection scheme, standard retrofit plates design, standard protection criteria, settings calculation templates, and relay settings templates.