Saturation of Current Transformer in a Coordinated Substation towards Optimal Power Flow

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.