Sohail Ansari

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Introduction to Power System Stability

According to CIGRE-IEEE, Power system stability is the ability of an electric power system for a given initial operating condition, to regain a state of operating equilibrium under being subjected to  physical disturbance  so that the practically the entire system remain intact. Usually power system stability is categorized in to :

1.Steady State Stability

2.Transient Stability

3. Dynamic Stability.


1.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.

2.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.

3.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.

This is all about introduction of power system stability.

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The Advantages of AC Power over DC Power

Back in the 1880s, that question generated an all-out current war between two geniuses. On the one side, Thomas Edison pushed that direct current was safer and would cause less hazard of electrocution. However, it proved to be cheaper and easier to transmit the power of alternating current over long distances. This was the stance of Edison’s competitor, Nikola Tesla.

1. More Power
Probably the biggest advantage of AC over DC is that you can generate much more power from AC than DC. Alternating current is generated by large turbines. Direct current normally comes from batteries or sometimes from solar panels. Solar panels large enough to power entire cities would take huge amounts of land. Batteries use chemical reactions to produce electricity. Producing a huge battery would be expensive and impractical. Large turbines can be easily built and powered using steam, nuclear or hydraulic power.

2. Long Distance Transmission
The ability to generate higher voltages with AC translates into the ability to transmit that power over longer distances. Alternating current is generated at power stations and transmitted through power lines to substations that can boost AC and keep it going farther distances.

In some operations, direct current may be preferred. However, it is easy to convert AC to DC by using simple transformers. It is much more difficult and costly to convert DC to AC.