The preceding review leads to a number of important conclusions and observations concerning power system oscillations:

a. Oscillations are due to natural modes of the system and therefore cannot be eliminated. However, their damping and frequency can be modified.

b. As power systems evolve, the frequency and damping of existing modes change and new modes may emerge.

c. The source of ‘‘negative’’ damping is power system controls, primarily excitation system automatic voltage regulators.

d. Inter area oscillations are associated with weak transmission links and heavy power transfers.

e. Inter area oscillations often involve more than one utility and may require the cooperation of all to arrive at the most effective and economical solution.

f. Power system stabilizers are the most commonly used means of enhancing the damping of
inter area modes.

g. Continual study of the system is necessar y to minimize the probabilit y of poorly damped oscillations. Such ‘‘beforehand’’ studies may have avoided many of the problems experienced in power systems.

It must be clear that avoidance of oscillations is only one of many aspects that should be considered in the design of a power system and so must take its place in line along with economy, reliability, security, operational robustness, environmental effects, public acceptance, voltage and power quality, and certainly a few others that may need to be considered.

Fortunately, it appears that many features designed to further some of these other aspects also have a strong mitigating effect in reducing oscillations. However, one overriding constraint is that the power system operating point must be stable with respect to oscillations.

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