On September 22, at 3:49 pm, a disturbance with origin in a monophasic short circuit in the span of Reactor No. 11 connected to the 500 kV busbar at Imperatriz substation involved the automatic shutdown of various equipment and transmission lines in the North and Northeast. The disturbance was extinguished by the tripping of protections of remote terminals, due to non-performance of the protections associated with the span of the reactor, resulting in the loss of synchronism of systems North and Northeast over the rest of the BIPS. In the North and Northeast regions there was a decrease in frequency, with consequent performance of all stages of the Regional Scheme for Load Relief - ERAC of these regions, resulting in the shutdown of 3,918 MW of load.

Main actions and measures implemented:

• Review of ERAC of North and Northeast regions, considering the feasibility of adjusting it to avoid that the minimum frequency value corresponding to the shutdown of power plants are met in order to keep them connected to the system and thereby ensure continuity of the supply of a greater amount of charges.
• Adequacy of bar 1 differential protection system of 500 kV substation Imperatriz and reactor No. 11, for compliance with established in item 6.1.4 of Submodule 2.6 of the Grid Procedures, regarding the overlapping of the zones of protection.