Frequency and voltage stability assessment of a power system during emergency service states

Emergency service states of an electric power system such as island operation, black start or fall in to a blackout are accompanied by rapidly increased requirements of system control and ability to withstand major power, frequency and voltage changes. This paper presents an example of an electric power system (basic connection of city of Pilsen on the 110 kV voltage level) falling in to an island operation and examines influence of rapid load/generation balance changes on the system frequency and voltage. It shows advantages and threats of Rate of Change of Frequency (RoCoF) application instead of solid frequency limits for both — protection of generators and Under Frequency Load Shedding (UFLS). The paper also deals with the question of decreased frequency and its influence on the connected load.