The term ground has a very important and specific meaning in the context of electric circuits: it is an electrically neutral place, meaning that it has zero voltage or potential, which moreover can absorb excesses of either positive or negative charge and disperse them so as to remain neutral regardless of what might be electrically connected to it. The literal ground outdoors has this ability because the Earth as a whole acts as a vast reservoir of charge and is electrically neutral, and because most soils are sufficiently conductive to allow charge to move away from any local accumulation. The term earth is synonymous with ground, especially in British usage. A circuit “ground” is constructed simply by creating a pathway for charge into the earth. In the home, this is often done by attaching a wire to metal water pipes. In power systems, ground wires, capable of carrying large currents if necessary, are specifically dug into the earth.
Power system stability is a single problem, however, it is impractical to deal with it as such. Instability of the power system can take different forms and is influenced by a wide range of factors. Analysis of stability problems, including identifying essential factors that contribute to instability and devising methods of improving stable operation is greatly facilitated by classification of stability into appropriate categories. These are based on the following considerations: Ø The physical nature of the resulting instability related to the main system parameter in which instability can be observed. Ø The size of the disturbance considered indicates the most appropriate method of calculation and prediction of stability. Ø The devices, processes, and the time span that must be taken into consideration in order to determine stability. Figure 7.1 Possible classification of power system stability into various categories and subcategories. 1) ROTOR ANGLE STABILITY: Ro...
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