STABILITY STUDIES

Stability Studies in Electrical Power Systems Engineering
Stability studies is the ability of the Electrical Power System to return to normal operation after a disturbance.
There are three main types of stability studies carried out in the Electrical Power System. The are the Transient Stability studies, the Dynamic Stability Studies and Steady State stability studies.

Transient Stability Studies deals with the ability of the Electrical Power System to retain synchronism following major disturbances such as transmission system faults, sudden load changes, loss of generating units or line switching.

Dynamic and Steady-State Stability studies deal with a few machines undergoing slow or gradual changes in operating conditions.

Steady-State Stability Problems use very simple generator model which treats the generator as a constant voltage source. While the dynamic stability studies uses a model in which the excitation system and turbine-governing system are represented along with synchronous machine models which make provision for flux-linkage variation in the machine air-gap.

The Stability of the System is examined under incremental variations about an equilibrum point.

The Non-Linear differential and algebraic equations for the system can be replaced by a set of linear equations which are then solved by methods of linear analysis to determine whether the machine or machines will remain in synchronism following small changes from the operating point.

Transient Stability Studies involve large disturbances which do not allow the linearization process to be used and the non-linear differential and algebraic equation must be solved by direct method or by iterative step-by-step procedure.

Transient Stability Studies can be subdivided into first-swing and multiswing stability Problems.

The first-swing Stability is based on a reasonably simple generator model without representation of control system. Usually te time period under study is the first second following a system fault. If the machines of the system are found to remain in synchronism within the first seconds; the system is said to be stable. Multiswing Stability Problems extended over a longer study period and therefore must consider effects of generator control systems which affect machine performance during the extended time period. Machine models of greater sophistication must be represented to reflect proper behaviour.

In all Stability Studies, the objective is to determine whether, or not the rotors of the machines being perturbed, return to constant speed operation.