Modelling a large 11 kV network to optimize protection and improve safety and reliability

The north tees site on Teesside is a large control of major accident hazard [COMAH] facility that includes a one million tonnes per year aromatics complex, Ethylene liquefaction units, Ammonia storage, the Petroplus Crude Oil Refinery and a new BOC hydrogen plant all supplied by twin 132 kV/11 kV Grid Transformers each rated at 60 MVA. COMAH sites require very high levels of infrastructure systems support with accurate and comprehensive documentation, and because parts of the site were over 40 years old and electrical distribution records had not always been fully maintained as the site developed, it was necessary to review and update records for the entire system. This provided an opportunity to optimize protection before producing revised protection schedules and up to date time current characteristic curves [TCC's] for some 400 protection relays, and 150 sets of significant HV and LV three phase fuses on the North Tees network. Following detailed assessments of various tools and approaches available to address this requirement, it was decided to model the entire network using SKM Power Tools Software, in conjunction with a local specialist electrical consultancy TAS Ltd. The exercise was carried out in a carefully managed sequence over a two year period and is described in detail below, together with updated guidance for unrestricted earth fault protection settings to limit arc-flash energy levels on LV switchboards and motor control centres.