Modern power systems are characterized by extensive system interconnections and increasing dependence on control for optimum utilization of existing resources. The supply of reliable and economic electric energy is a major determinant of industrial progress and consequent rise in the standard of living. The increasing demand for electric power coupled with resource and environmental constraints pose several challenges to system planners. The generation may have to be sited at locations far away from load centres (to exploit the advantages of remote hydro power and pit head generation using fossil fuels).
However, constraints on right of way lead to overloading of existing transmission lines and an impetus to seek technological solutions for exploiting the high thermal loading limits of EHV lines . With deregulation of power supply utilities, there is a tendency to view the power networks as highways for transmitting electric power from wherever it is available to places where required, depending on the pricing that varies with time of the day. Power system dynamics has an important bearing on the satisfactory system operation.
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It is influenced by the dynamics of the system components such as generators, transmission lines, loads and other control equipment (HVDe and SVC controllers). The dynamic behaviour of power systems can be quite complex and a good understanding is essential for proper system planning and secure operation.
Power System Stability
Stability of power systems has been and continues to be of major concern in system operation [2-7]. This arises from the fact that in steady state (under normal conditions) the average electrical speed of all the generators must remain the same anywhere in the system. This is termed as the synchronous operation of a system. Any disturbance small or large can affect the synchronous operation. For example, there can be a sudden increase in the load or loss of generation. Another type of disturbance is the switching out of a transmission line, which may occur due to overloading or a fault.
The stability of a system determines whether the system can settle down to a new or original steady state after the transients disappear. The disturbance can be divided into two categories (a) small and (b) large. A small disturbance is one for which the system dynamics can be analysed from linearized equations (small signal analysis). The small (random) changes in the load or generation can be termed as small disturbances. The tripping of a line may be considered as a small disturbance if the initial (pre-disturbance) power flow on that line is not significant.
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However, faults which result in a sudden dip in the bus voltages are large disturbances and require remedial action in the form of clearing of the fault. The duration of the fault has a critical influence on system stability. Although stability of a system is an integral property of the system, for purposes of the system analysis, it is divided into two broad classes . It is important to note that, while steady-state stability is a function only of the operating condition, transient stability is a function of both the operating condition and the disturbance(s).
This complicates the analysis of transient stability considerably. Not only system linearization cannot be used, repeated analysis is required for different disturbances that are to be considered. Another important point to be noted is that while the system can be operated even if it is transiently unstable, small signal stability is necessary at all times. In general, the stability depends •upon the system loading. An increaSe in the load can bring about onset of instability. This shows the importance of maintaining system stability even under high loading conditions.