The book describes a novel approach to define, calculate and estimate the frequency in electrical energy systems. As it is well known, electric power is generated, transmitted and distributed using mostly alternate current (AC) three-phase systems. The variations of the frequency with respect to its nominal value (in Europe, 50 Hz) are a symptom of the power unbalance in the system between generation and demand. The link between this power unbalance and frequency variations is due to the conventional synchronous generators that are connected to the grid, i.e., the generators of thermal, nuclear and hydro power plants. A common assumption is that, in an interconnected AC system, the frequency can vary in a range defined by the system operator but is the same everywhere in the grid. This, however, is not true during the transients (e.g., short-circuits), where the frequency is different from point to point. The whole book is a thorough discussion on the several implications and consequences of what happens if this assumption is removed and local variations of the frequency are considered. The authors first define the meaning of frequency of an electrical quantity (e.g., voltage and current) in non-stationary conditions (i.e., the frequency is not equal to the nominal one) and pose the foundation of the frequency divider formula (FDF). They then proceed to describe the consequences of using a variable frequency in power system modelling and simulations, in state estimation and frequency control applications. Finally, they discuss the applications of the frequency divider in systems where part of the generation is not based on synchronous machines, but rather on converter-interfaced energy resources, such a wind and solar power plants.

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