A Proposed Strategy of Implementation for Load Shedding and Load Recovery with Dynamic Simulations

Electric load shedding is basically the act of rapidly removing loads from a power system under certain predetermined conditions. It is implemented to maintain the generation power margin at nominated level when load demand is higher than electric supply. This prevents the widespread of system collapse when a fault occurs. The objective of the project is to design and implement a load shedding and load recovery scheme for power system. The scope of study consists of system analysis, design and simulation. Dynamic simulations are performed using the ERACS Power Analysis Software. Underfrequency load shedding is more popular compared to undervoltage due to its efficiency and robustness. The design of underfrequency load shedding scheme is strongly related to the system frequency and the amount of generation or load as system frequency will decline when overload occurs. When the frequency falls below preset level at a certain rate, a predetermined amount of load will be removed to restore the system frequency. Important design considerations for this scheme are the maximum anticipated overload, number of load shedding steps, size of load shed at each step and frequency relay settings. Load restoration after load shedding can only be executed after the system has recovered completely and its normal frequency is restored. Loads should be restored in small blocks sequenced by time delay between successive restorations to allow frequency stabilization. A case study is carried out on PETRONAS Penapisan Melaka, whereby a load shedding scheme is designed for its upcoming cogeneration plant. The maximum generation loss designed for by the scheme is 50% or 48 MW at system peak. There are four stages, removing 12.5% of load at each stage. The frequency relay settings are 49.5, 48.77, 48.22 and 47.86 Hz. Dynamic simulations performed indicate that the load shedding design is feasible and able to halt frequency collapse due to generation loss. The design strategy, calculations and justification are presented in this report