The Protection Engineer is concerned with limiting the effects of disturbances in a power system. These disturbances, if allowed to persist, may damage plant and interrupt the supply of electric energy. They are described as faults (short and open circuits) or power swings, and result from natural hazards (for instance lightning), plant failure or human error.
To facilitate rapid removal of a disturbance from a power system, the system is divided into 'protection zones'. Relays monitor the system quantities (current, voltage)
appearing in these zones; if a fault occurs inside a zone, the relays operate to isolate the zone from the remainder of the power system.
The operating characteristic of a relay depends on the energizing quantities fed to it such as current or voltage, or various combinations of these two quantities, and on
the manner in which the relay is designed to respond to this information. For example, a directional relay characteristic would be obtained by designing the relay
to compare the phase angle between voltage and current at the relaying point. An impedance-measuring characteristic, on the other hand, would be obtained by
designing the relay to divide voltage by current. Many other more complex relay characteristics may be obtained by supplying various combinations of current and voltage to the relay. Relays may also be designed to
respond to other system quantities such as frequency, power, etc.
In order to apply protection relays, it is usually necessary to know the limiting values of current and voltage, and their relative phase displacement at the relay location, for various types of short circuit and their position in the system. This normally requires some system analysis for faults occurring at various points in the system.
The main components that make up a power system are generating sources, transmission and distribution networks, and loads. Many transmission and distribution circuits radiate from key points in the system and these circuits are controlled by circuit breakers. For the purpose of analysis, the power system is treated as a network of circuit elements contained in branches radiating from nodes to form closed loops or meshes.
The system variables are current and voltage, and insteady state analysis, they are regarded as time varying quantities at a single and constant frequency. The network parameters are impedance and admittance; these are assumed to be linear, bilateral (independent of current direction) and constant for a constant frequency
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