22-Module 7_ Stability Studies-25-Mar-2019Reference Material I_Module-VII.ppt - EEE3003 POWER SYSTEM ENGINEERING By M.JANAKI Asso.Prof School of

# 22-Module 7_ Stability Studies-25-Mar-2019Reference Material I_Module-VII.ppt

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EEE3003 EEE3003 POWER SYSTEM ENGINEERING POWER SYSTEM ENGINEERING By M.JANAKI Asso.Prof School of Electrical Engineering.

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Module-VII Power System Stability
Stability study (analysis) is used to evaluate the impact of disturbances

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Power system is a dynamic system . What is meant by dynamic system? The system whose describing equations are functions of time. Whenever a disturbance occurs, the dynamic system variables temporarily vary and reach steady state as time increases. The power system (dynamic system) may be a stable or unstable system.
The stability of a system refers to the ability of a system to return back to its steady state when subjected to a disturbance. 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. Usually power system stability is categorized into Steady State , Transient and Dynamic Stability.

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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. These checks are usually done using power flow studies. Transient Stability involves the study of the power system following a major disturbance. Following a large disturbance the synchronous alternator the machine power (load) angle changes due to sudden acceleration of the rotor shaft. The objective of the transient stability study is to ascertain whether the load angle returns to a steady value following the clearance of the disturbance . 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 and generation levels. In an interconnected power system, these random variations can lead catastrophic failure as this may force the rotor angle to increase steadily .

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In general, the describing equations of dynamic power systems are nonlinear differential equations and algebraic equations.
The steady-state stability studies examine the stability of the system under small incremental variations

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