Severe fault on the power system sometimes causes cascaded tripping of the lines (or) generators.
The unbalanced condition of generation of active power and consumption of power will cause fluctuation of the frequency.
If the supply is less than the demand, frequency will fall, whilst if the supply is in excess of the demand, frequency will rise.
If the cascaded tripping of heavily loaded lines along with a power station outage causes the frequency to drop, a special protection scheme will initiate controlled tripping such as load shedding or tripping of a pumped storage generator during pumping to maintain the frequency.
SPS schemes may also try to maintain the correct frequency in an islanded part of the system until it can be reconnected to the rest of the system.
Following actions could be initiated:
• Tripping of generators
• Fast generation reduction (runback) through fast valving (or) water diversion
• HVDC Power transfer control
• Load shedding
• Controlled opening of interconnection to neighboring system to prevent spread of disturbance
• Controlled islanding of local system into spate areas with matching Generation and Load.