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Utility encounters situations like when one out of two power transformers in the substation requires maintenance and only one transformer remains in operation. To meet the load requirements of a maximum number of customers, if not all, the only transformer needs to be overloaded. Due to various load types within the service territory, utilities need to understand the customers' maximum dynamic loading amount to overload a transformer in an emergency. We propose the iteration process with real-world substation data, which considers the concept of a coordinated substation for optimal power flow. We found to what extent the only 138 kV//13.8 kV, 30/40/50 MVA transformer can be safely overloaded beyond its nameplate's maximum rating for a minimum time. Within that time, the utility personnel can work on the substation yard and switchgear room safely and comfortably to bring back the other transformer in operation. We found from the perspective of power flow that with increased dynamic loading, transformer secondary current increases. Therefore, under the maximum dynamic loading condition, we should calculate the asymmetric fault current on the 13.2 kV switchgear bus to select the suitable CTs and develop a protection scheme accordingly.

Power systems optimization problems are very difficult to implement because these systems are very large, complex, widely distributed, and are influenced by many unexpected events. The utility industry employs the most reliable optimization methods to take full advantage of simplifying the formulation and implementation of the problem. For the utility, high voltage substation modernization is getting essential to enable the connection of the new loads and diverse generation customers and to ensure that electricity supplied is of acceptable quality, reliable, and affordable. In light of that, the protection scheme must act perfectly to protect the substation as a whole and its equipment from any possible fault. However, relay used in the high voltage substation for protection purposes acts differently when the current transformer is saturated. Here, we present consideration of the maximum loading capacity of a substation along with its CT saturation, of a coordinated substation at the very early phase of design. The substation is comprised of 138 kV transmission and 13.2 kV distribution networks. Power flow and fault current data, along with protection schemes for line, bus, transformer, and switchgear and availability of key equipment in the market are investigated to explore the most suitable substation design. A procedure is developed based on the steps required to plan and design a modern substation. Step-by-step iterative simulation is performed at the planning phase based on generation and transmission sources’ fault current data, X/R ratio of the 138 kV incoming lines to the substation end, and the continuous and emergency loads for present and future. A design decision to construct a substation is made to select a network configuration to maximize redundancy and reliability to ensure optimal power flow through the substation. Additionally, the presented planning process ensures proper coordination between the transmission and distribution systems.