**4. Conclusion**

Power system is a complex dynamic system exposed to constant disturbances of varying intensity. Most of these disturbances are common operator's activities, for example, swich turning on or off system elements, and such disturbances do not have a major influence on the system. However, some disturbances can cause major problems in the system, and the subsequent development of events and cascading tripping of system elements can lead to a system's collapse. One of the most severe disturbances is the outage/failure of one or more major production units, resulting in unbalance of active power in the system, that is, frequency decrease. Many factors influence whether or not the severity of frequency decrease will trigger under-frequency protection. Today, under-frequency protection is based on local measurements of state variables and provides only limited results. Their operation is frequently unselective and affects the whole system.

This chapter illustrated the estimate of unbalance of active power in the power system with DTW application, provided WAMS is available. Estimate of *df/dt* value is a genuine indicator of active power unbalance, and given the oscillatory nature of signal frequency, its estimate is rather difficult. Taking into account its advantages in signal processing when compared to other techniques, WT enables direct estimate of medium value of the change of frequency of the centre of inertia, providing a complete picture about the system's operation as a whole. In this way, and provided with the complete inertia of the system, we obtain very important information about a complete unbalance of active power in the system, in a rather simple manner. In addition to this particularly important piece of information obtained from the low-frequency component of the signal angle or frequency, other levels of signal decomposition in frequency range encompassing low-frequency electromechanic oscillations provide information about the onset of some dynamic occurrence in the system, localize system disturbance, identify and define the character of intra-area and inter-area oscillations and provide insight into the system's operation after the disturbance. All of this points to a possible development of such under-frequency protective measures which will operate locally, that is, whose operation will be at (or in the vicinity of) the disturbance, in order to reduce the effect of disturbance, and adjust the operation of effective measures to identified unbalance of active power.
