The Power System is an interconnected network. It consists various components like synchronous generators, transformers, transmission lines, and switch gears, etc.
All components operates in synchronism. Due to fault or any abnormal condition, if one component derails from synchronism, it will affect the other components.
In normal condition, all components operate between predefined limits. So, this system is a stable system. In abnormal condition, if the fault is not removing immediately, the whole system may lose their synchronism. So, the system may become unstable.
“The stability of the power system defines as the ability of the power system to return back to its steady-state when subjected to the disturbances.” The study of the electrical power system is incomplete without the study of the power system stability.
The stability of the power system categorized into Steady-State, Transient, and Dynamic Stability.
1) Steady-State Stability:
When the system subject to small and slow disturbances, the system must have to operate under a given load condition to retain synchronism. This ability of the system is known as the steady-state stability. The small and slow disturbances are load fluctuation, turbine governors, etc.
2) Transient Stability:
When the system subject to the large disturbances, the system must have to operate under given load condition to retain synchronism. This ability of the system is transient stability. The large disturbances like a short circuit in transmission line or transformer, a sudden change in load, loss of generation, etc. The time period of the transient study is less than one second.
3) Dynamic Stability:
The power system must remain in synchronism after the transient stability period, until the system settles down to the new steady-state condition. If the oscillations do not acquire more than predefined amplitude and settle down quickly, the system a dynamically stable system. In other words, the ability of the system to regains its equilibrium state or operating point at rated frequency, after the large sudden disturbances (Transient stability period). The time period of dynamic system study is 5-10 sec and sometimes it may take up to 30 sec. By the power system stabilizer, dynamic stability can be improve.
“The stability limits define as the maximum power that can be transferred in a network between source and load without loss of synchronism.”
Steady-state stability limit:
When the load increase gradually under steady-state condition, the steady-state limit is the maximum power that transfer without system becoming unstable.
Transient stability limit:
The system suffers from the sudden change or large disturbances. In this condition, the transient stability limit is the maximum power that transfer without the system becoming unstable.
Dynamic stability limit:
The dynamic stability limit increases significantly, if the increase in field current or adjustments in speed setting occurs simultaneously. With the increase of load from the use of automatic voltage regulators (AVR) and speed governors. Hence, the limit under these conditions called as the dynamic stability limit.