Optimal Power Flow Control and Emergency Voltage Control with A Unified Power Flow Controller (UPFC) to Improve The Performance of The Power System

The increasing global energy demand has intensified pressure on existing transmission infrastructure, giving rise to thermal overloads, voltage instability, and elevated power losses. Flexible Alternating Current Transmission System (FACTS) devices have emerged as a technically viable solution to enhance the controllability and power transfer capability of existing networks without significant infrastructural expansion. Among FACTS controllers, the Unified Power Flow Controller (UPFC) is distinguished as the most versatile device, capable of regulating active power flow, reactive power flow, and bus voltage magnitude simultaneously. This paper presents a comprehensive simulation study of UPFC performance using the IEEE 9-bus benchmark transmission network, which was modeled in DIgSILENT PowerFactory. The voltage stability indices and line stability factors identify Bus 8 as the optimal point for UPFC installation. Four simulation scenarios are investigated: the base case without UPFC, UPFC integrated at Bus 8 under normal full-load conditions, N-1 line contingency with three compensation strategies compared, and a generator outage response. The findings indicate that the integration of UPFC results in a 19.64% decrease in active power losses and a 32.74% decrease in reactive power losses. In the context of the N-1 line contingency, the uncompensated system permits a voltage collapse of Bus 8 to 0.909 pu. However, UPFC effectively restores the voltage to above 1.020 pu within approximately 20 seconds, thereby demonstrating a substantial performance enhancement in comparison to the fixed capacitor bank compensation. During generator outages, the UPFC has been shown to limit the Bus 7 voltage dip to 0.986 pu with rapid stabilization in comparison to a sustained depression to 0.967 pu without compensation. The findings confirm the technical superiority of UPFC as a multi-functional FACTS controller for modernizing voltage-constrained transmission networks.