POWER SYSTEM STABILITY

The study of the electrical power system is incomplete without the study of the power system stability. The definition of power system stability is the ability of the power system to return back to a steady state without losing synchronism.

The power system network is a very large network and millions of equipment are operating together in parallel. This all equipment is working in synchronism. If one of the equipment goes out of synchronism by an abnormal condition. It will affect the other equipment and thereby in the worst condition it is affecting the entire network.

Let us consider a small system having five generators. If one generator is going out of synchronism due to any fault, it will affect the other generators. Due to this, the entire system will face connivances. So, in this condition, the faulty generator immediately switched out from the system. If the faulty generator is not switched out, then it will affect the voltage profile of the system. There will be large fluctuation in the voltage.

The power system stability is categorized into a steady state, transient and dynamic stability. The stability limit is defined as the maximum power that can be transferred in a network between source and load without loss of synchronism.

1) Steady state stability: It is the ability of the power system when operating under given load condition to retain synchronism when subjected to small and slow disturbances. The load fluctuation and turbine governors are examples of slow disturbances. The steady state limit is defined as the maximum power that can be transferred without making the system unstable. When the load is increased gradually under steady state condition.

2) Transient stability: It is the ability of the power system when operating under a given load condition to retain synchronism when subjected to large disturbances. The loss of generation, sudden changes in loads, a short circuit in the transmission line and short circuit in the transformer are the examples of the large disturbances in the power system. The time period of the transient study is very less. The transient stability limit is the maximum power that can be transferred without making the system unstable in the large disturbance condition. The transient stability limit is almost lower than the steady-state limit.

3) Dynamic stability: It is the ability of the power system to remain in synchronism after the initial swing until the system has settled down to the new steady state equilibrium condition. If the oscillation does not acquire more amplitude and die out quickly, then the system is said to be a dynamically stable system. The dynamic stability can be improved by the power system stabilizer. The time period of dynamic system study is 5-10 sec and some time it may be up to 30 sec. With an increase of load from the use of automatic voltage regulators and speed governors, if the increase in field current or adjustments in speed setting occur, the stability limit would be increased significantly. The limit under this condition is called the dynamic stability limit.