Short circuit force in an AIS with special reference to L-L and L-L-L faults

Short circuit force (SCF) in an air insulted substation is a feature which is of importance in its electromechanical design. SCF which are generated due to short circuit current is transferred from conductors (flexible or rigid) to end connections where the conductors are being held. The end connections and the conductor are supported by means of insulator, steel structure, and foundation. Thus, SCF is transferred from conductors to end connections and then to steel structures via insulator. If estimation of SCF is erroneous, then the end result will be a disaster (as the structural designs are based on this). On the other hand, too much conservative structural designs based on the estimated SCF results in increased size and cost of insulators, steel structure, and foundation requirements. Hence, the computation of SCF becomes mandatory in design of the substation from the reliability and economic considerations. In this paper, SCF in a 400 kV air insulated (upcoming) substation (in Africa) due to line-to-line short circuit currents and three phase short circuit currents on flexible conductor are computed using basic electromagnetic laws and are discussed. The computed values are compared with the values of SCF obtained as per IEC 60865 standard.