The electrical power system in existing Balikpapan Refinery is not possible to interconnect with new loads and New Power Plant. This scheme was limited by rating capacity and short circuit. In this paper has describes power system study of optimization in designing Balikpapan Refinery power plant system, through the interconnection between Existing, RDMP phase-I and RDMP phase-II with Single Power Island concept. The main problem to connecting all the power generators become single power island at 33 kV bus is resulting in short circuit rating in excess of 40 kA. In term of process design and constructability, three (3) scheme were purposed and evaluated. Selected scheme with 33 kV synchronization bus installed with the inter-tie transformer to split two groups has been analyzed to meet the minimum requirement of design criteria and power system study. Using ETAP Model with 3 (three) method of studies result that maximum short circuit has been reduced to 32.074 kA from 46.403 kA respectively. Also from this integration concept has been optimized the number of the spare generator from each island power plant become 1 (one) unit common spare generator, which lead to reducing CAPEX of overall Project RDMP phase-I and RDMP phase-II.