Prevention of Load Encroachment Using Model Predictive Control

Abstract

Tripping of distance relays due to load encroachment is one of the common causes behind major disturbances occur in power systems. Several techniques have been presented in literature to differentiate an occurrence of load encroachment and a fault in a system. However, once an event of load encroachment is observed, loading condition of the power system must be rescheduled to avoid any undesired relay tripping. This paper proposes a receding-horizon Model Predictive Control (MPC) strategy to change load and generation of a power system optimally to take the apparent impedance seen by vulnerable distance relays away from their zone-3 boundaries. To apply MPC technique, a linear state-space model of power system is developed using sensitivity analysis. Rescheduled load and generation are found by minimizing the total cost required for the change, owing to the constraints of power balance, power carrying capacity of lines and relay operational margins. During step changes in inputs, adequate time gap is provided to ensure the stability of the power system. An oscillation detection technique is developed for the above purpose. Several case studies have been presented at the end to demonstrate the performance of the proposed technique under stressed operating conditions.