What happened

A connector on one phase of the 500 kV potential transformer associated with Yguazú Substation's 500/220 kV T01 transformer detached during an afternoon of extreme heat. According to ANDE's later technical account, that initial fault caused protection systems to disconnect both 220 kV Yguazú–Coronel Oviedo transmission lines. With insufficient redundant transmission capacity, voltage fell, sensitive loads disconnected and the generation-demand imbalance separated the systems supplied by Itaipu, Yacyretá and Acaray. The reduced load then produced an overvoltage that tripped protection at Itaipu's Margen Derecha substation, leaving the Itaipu- and Acaray-supplied system without service. Itaipu said its generating plant and substation had operated normally and that the initiating event was external to the utility. The outage affected about five million people amid heat indices reported around 45–49°C, disrupted traffic signals and threatened water-pumping operations. This is the power-grid companion to the communications-layer record for the same 18 February blackout; it is not a second event. Early coverage published on 19 February described two 500 kV lines, but ANDE's subsequent technical report identified one detached 500 kV connector followed by the loss of two 220 kV lines.

When cited sources give different casualty, damage or spill figures for this event, they’re reported side by side above rather than merged into one number. See how confidence levels work.