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.