System Stability Concepts

DIFFERENT FORMS OF STABILITY

For the purpose of analysis there are mainly three stability conditions which are namely
1. Steady state stability
2. Transient stability
3. Dynamic stability

Steady State Stability

• Steady state stability is defined as the ability of an electric power system to maintain synchronism between machines within the system and an external tie line following a small slow disturbance.
• The small slow disturbance may constitute normal load fluctuations, the actions of automatic voltage regulators and turbine governors.
• The “steady state stability limit” refers to the maximum power which can be transferred through the system without loss of stability.
• Whenever the maximum power transfer exceeds the steady state limit of the system, individual machines or groups of machines cease to operate in synchronism and a wide fluctuations of the voltage occurs.
• System non-linearities are generally ignored in steady-state analysis.
• The study time for steady state analysis is usually few seconds to several hours.

Methods of Improving Steady-State Stability Limit

The maximum power that can be transferred from sending end to the receiving end is given by

Pmax = |E||V| / X

Hence, the steady-state stability limit can be increased by

1. Increasing the excitation of the generator (E) or by operating the system at a higher value of |E| or |V| or both.
2. Reducing the transfer reactance. The transfer reactance can be reduced by

• using parallel lines
• using bundled conductors (so that self GMD is increased resulting in reduction in reactance of the line). Used mainly in EHV lines.
• using series capacitors connected with the lines. These are used mainly in EHV lines and are economical for a transmission distance over 350 km.

Transient Stability

• Transient stability deals with the study of large sudden disturbances occurring in the system.
• Such large sudden disturbances include application of faults, clearing of faults, sudden change in load, and inadvertent tripping of lines and generators.
• “Transient stability limit” refers to the maximum power which can be transferred through the system without the loss of stability under sudden disturbances.
• System non-linearities play a vital role in transient state analysis.
• Transient stability is the ability of the system to remain in synchronism during the period following a disturbance and prior to the time that the governors can act.
• The study time is less than one second for transient stability analysis.

Dynamic Stability

• Dynamic stability is the ability of a power system to remain in synchronism after the “initial swing” (transient stability period) until the system has settled down to the new-steady state equilibrium condition.
• The study time for dynamic stability analysis is usually between 30 minutes to 80 minutes.
• When sufficient time is elapsed after a disturbance, the governors of the prime-movers react to increase or reduce input energy, as may be required, to re-establish a balance between energy input and the existing electrical load.