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Power System Stability

In all power Systems, the larger Machines are of the synchronous type, these include substantially all the Generators and condensers, and a considerable part of the motors.

Power System
Power Transferred System

On such Systems it is necessary to maintain synchronism, otherwise a standard of service to the consumers will not be achieved.

These transient disturbances caused by the changes in loads, switching operations, and particularly, faults and loss of Excitation.

Thus, maintenance of synchronism or equilibrium after a disturbance are of prime importance to the Electrical utilities.

The term "stability" can be interpreted as
"Maintenance of synchronism".
These terms are quite frequently used interchangeably.

The present tends is towards interconnection of the power System; resulting in to increased lengths and increased Reactance of the system, this presented an acute problem of Maintenance of stability of the system.

The terms " power limit" is also sometimes interpreted as "stability" because to have the maximum utility of the system it should be capable of supplying maximum power without causing instability.

Power System Stability, in general terms may be defined as its ability to respond to a disturbance from its normal Operation by returning to a condition where the operation is again normal.

The terms " Stability" and "stability limit" are defined by AIEE as below :-

Stability :- when used with reference to a power System, is that attribute of the system of Part of which enables it to develop restoring forcing between the elements thereof equal to or greater than the disturbing forces so as to restore a state of equilibrium Between the elements.

A stability limit :- A stability limit is the maximum power flow possible through some particular point in the system when the entire system or part of the system to which  the stability limit refers is operating with stability.

Steady state stability :- May be defined as the capability of an electric power System to maintain synchronism between machine within the System and an external tie - line following a small slow disturbance [Normal
Load Fluctuation, the actions of automatic voltage regulators and turbine governors].

The steady stability Limit refers to the maximum power which can be transferred through the system without loss of stability.

Loss of synchronism is , however, possible at loads ( or disturbance ) Below the steady - State limit, if they are suddenly disturbances a concept of transients application of Fault, clearing of faults, sudden load Change, and inadvertent tripping of lines and Generators.

The maximum power which can be transferred through the system without the loss of stability under sudden disturbance is referred as transient stability limit.

Dynamic Stability :- Dynamic stability is the ability of the 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.

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