I read in news paper "heavy jerk in power plant due to breakdown in transmission line" what does it mean why does it happen?
When a transmission line suddenly becomes unavailable (trips due to fault) this can cause sudden frequency change in the system due to imbalance of supply to demand. Some types of plant are very sensitive to frequency and a change of more than a few hz in system frequency can cause significant plant damage. For this reason they are usually equipped with frequency protection and the plant will trip if the system frequency becomes too high or low. The 'jerk' referred to here could be the mechanical effect on the plant caused by the change in frequency.
In the USA, for a generation plant, jerk is the rate of change of ramp rate. Because the plant jerk rate is limited for mechanical generation plant (the ramp rate is fixed, or almost fixed), a transmission system must have automatic load shedding: even if the plant could ramp up the power fast enough to handle the change, it cannot change the ramp rate fast enough to handle fault conditions.
To prevent automatic load shedding, power suppliers are required to meet minimum standards for Plant Jerk Rate Capability.
So a heavy jerk would be a sudden and sharp change in load. However, the word Jerk is not normally used except for power plant specifications.