Experience in transmission systems

Abstract

The International Standard IEC 909 Short-circuit current calculation in three phase AC was issued, the UK supply industry had no standard or uniform methodology for fault level assessment. The hand calculation methodology of IEC-909 was considered conservative (pessimistic) by a UK working group that was established in 1990 to define good industry practice for the calculation of short circuit currents. The resulting document, ER G74, defines a computer-based method for the calculation of short circuit currents that is more accurate (less pessimistic) than the methodology of IEC-909 and as a consequence, potential capital investment in new and replacement switchgear can be more accurately identified. The main principles of the computer-based ER G74 methodology are: infeeds from all rotating machines must be included in the calculations of short-circuit currents; and the network pre-fault initial conditions must be established using an AC load flow calculation. These initial conditions are then used in the subsequent short-circuit current calculations. ER G74 methodology also describes the calculation of circuit-breaker symmetrical and asymmetrical short-circuit duties. The author outlines short circuit analysis techniques and the discusses the implementation of ER G74 an the NGC short circuit program. The technical issues in short circuit calculations on transmission systems are then discussed.