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Modern refinery process units require electrical power systems that do not have power interruptions. The backbone of a highly satisfactory 120-MVA system combines local generation with a power company captive dual substation (110 kV/12 kV) that serves dual feeders to area buses. It must be designed with the aid of computers to properly anticipate short-circuit duties, select and set protective relays, and provide adequate voltage regulation, especially during faults and switching. An adequate return on the investment in the system requires that the maintenance be organized, thorough, and technically competent. The system and its operating procedures must provide fault isolation in first zones of protection, and complete restoration of power to process units must be rapid and complete.