Cumulative Impact of Frequent Through Faults on Conventional Substation Switchyard Equipment: Diagnostics and Long-Term Mitigation Strategies

The 400/220 kV Mandola substation of the transmission system operator(TSO), commissioned in the early 1990s, was designed for short-circuit levels prevailing at that time. Progressive transmission network strengthening and large-scale generation integration have since resulted in short-circuit levels exceeding original equipment fault-duty ratings. The substation is also subjected to a high incidence of faults—approximately 80–100 events annually— predominantly originating from the 220 kV network and downstream systems. These frequent through-faults impose severe thermal and electrodynamic stresses on switchyard equipment, particularly interconnecting transformers, leading to progressive insulation degradation as evidenced by Dissolved Gas Analysis.