The Mumbai region in India experienced a massive outage on October 12, 2020, due to cascade failure. The event taught some serious lessons to the grid operators and highlighted the need for energy security and reliability. In this incident, the cascade tripping of the external transmission network resulted in an unexpected island containing Mumbai city isolating it from the rest of Indian power grid. Although the Mumbai islanding scheme was operational, it failed to survive due to high rate of change of frequency (ROCOF). Globally, such blackouts occur due to low probability, high impact events. Major blackouts are caused by a set of triggering events that are usually preceded by some incidents that weaken the power system. Further, the failure of important security mechanisms and protections can compound the effect of disturbances. In view of this, the paper focuses on the consequences of cascading events that led to several blackouts in history. This paper analyses the October 12, 2020, Mumbai power grid failure by recreating various scenarios that resulted in the blackout through dynamic modelling on the PSS/E platform. The results were validated using data collected from phasor measurement units (PMUs) and SCADA. In this article various challenges faced during unfolding of the event are presented. Lessons learnt such as appropriate settings and tuning were identified to ensure the survival of islanding scheme. Assessment of Mumbai power system's transfer capability is done to facilitate the optimal mix of imported and embedded generation.INDEX TERMS Blackout, cascade failure, critical infrastructure, islanding, power system modelling and simulator for engineering (PSS/E), rate of change of frequency (ROCOF), smart grid, under-frequency load shedding.

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