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Power System StabilityStability: The stability of a system refers to the ability of a system to return back to its steady state

when subjected to a disturbance.

Power System Stability: The power system stability is the property of the system which enables

the synchronous machines of the system to respond to a disturbance from a normal operating

condition so as to return to a condition where their operation is again normal.

The electrical power is generated by synchronous generators that operate in synchronism with the

rest of the system. A generator is synchronized with a bus when both of them have same frequency,

voltage and phase sequence. We can thus define the power system stability as the ability of the

power system to return to steady state without losing synchronism.

Classification of Power System Stability

Depending upon the nature and order of magnitude of the disturbance, power system stabilities are

three types: Transient stability, dynamic stability, and steady-state stability.

Transient Stability: Transient stability involves the study of the power system following a majordisturbance.

Transient stability studies constitute the major analytical approach to the study of power system

electro-dynamical dynamic behavior.

Transient stability studies are aimed at determining if the system will remain in synchronism

following major disturbances such as transmission system faults, sudden load changes, loss of

generating units, or line switching.

Following a large disturbance the alternator (synchronous generator) power (load, or torque) angle

changes due to sudden acceleration/deceleration of the rotor shaft. Thus, the amin objective of the

transient stability study is to ascertain whether or not the load angle returns to a steady value

following the clearance of the disturbance.

Transient Stability problems can be subdivided into(i) First-swing stability problem: This is based on a reasonably simple generator model without

representation of control systems. Usually the time period under study is thefirst secondfollowing

a system fault. If the machines of the system are found to remain synchronism within the first

second, the system is said to be stable.

(ii) Multiswing stability problem: This stability problems extend over a longer study period and

therefore must consider effects of generator control systems which affect machine performance

during the extended time period.

Dynamic (or small-signal) Stability: The ability of a power system to maintain stability under

continuous small disturbances is investigated under the name of dynamic stability (also known as

small-signal stability). These small disturbances occur due random fluctuations in loads andgeneration levels.

Steady-State Stability: Steady state stability studies are restricted to small and gradual changes in

the system operating conditions. In this we basically concentrate on restricting the bus voltages

close to their nominal values. We also ensure that phase angles between two buses are not too large

and check for the overloading of the power equipment and transmission lines.

The dynamic and steady-state stability problems are same in nature. They differ only in the degree

of detail used to model the machines.

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In the dynamic stability studies, the excitation system and turbo-governing system are represented

along with synchronous machine models which provide for flux-linkage variation in the machine

air-gap.

In the steady-state stability problems use a vary simple generator model which treats the generator

as a constant voltage source.

In all stability studies, the main objective is to determine whether or not the rotors of the machine

being perturbed return to constant speed operation.