Introduction Ferroresonance is a phenomenon that can occur in medium voltage transformers when certain conditions are met. In medium voltage transformers, ferroresonance can occur when the voltage on the primary side of the transformer changes rapidly, and the load on the secondary side is disconnected. This can cause the voltage on the secondary side to rise rapidly, producing high voltages and creating a dangerous situation. In extreme cases, ferroresonance can cause the transformer to explode, leading to catastrophic damage to the surrounding equipment. There are several techniques to mitigate ferroresonance in medium voltage systems, such as: to use a ferroresonance suppressor, which is a passive device that is connected in parallel with the transformer; Install a surge arrester or a metal oxide varistor (MOV) on the secondary side of the transformer to suppress voltage transients; Install a damping circuit to reduce the effect of ferroresonance. This can include a resistor, capacitor, or a combination of the two; Install a fuse on the primary side of the transformer to prevent overvoltage; etc. This paper reviews the results of the investigation resulting from the failure in 20kV/100V medium voltage transformer on the medium voltage side of the incoming transformer unit 3 at Pantai Indah Kapuk substation which is suspected to be due to ferroresonance along with the resulting impacts and mitigation recommendations.