Short-Circuit Currents (Power & Energy)

Jurgen Schlabach

Preference :

Electrical power systems have to be planned, projected, constructed, commissioned and operated in such a way to enable a safe, reliable and economic supply of the load. The knowledge of the loading of the equipment at the time of commissioning and as foreseeable in the future is necessary for the design and determination of the rating of the individual equipment and of the power system as a whole. Faults, i.e., short-circuits in the power system cannot be avoided despite careful planning and design, good maintenance and thorough operation of the system. This implies influences from outside the system, such as short-circuits following lightning strokes into phase-conductors of overhead lines and damages of cables due to earth construction works as well as internal faults, e.g., due to ageing of insulation materials.

Switchgear and fuses have to switch-off short-circuit currents in short time and in a safe way; switches and breakers have to be designed to allow even switch-on to an existing short-circuit followed by the normal switch-off operation. Short-circuit currents flowing through earth can induce impermissible voltages in neighbouring metallic pipelines, communication and power circuits. Unsymmetrical short-circuits cause displacement of the voltage neutral-to-earth and are one of the dominating criteria for the design of neutral handling. Short-circuits stimulate mechanical oscillations of generator units which will lead to oscillations of active and reactive power as well, thus causing problems of stability of the power transfer which can finally result in system black-out. Furthermore, equipment and installations must withstand the expected thermal and electromagnetic (mechanical) effects of short-circuit currents.