Brazilian Blackout - September 2012

Author: Alexandre Andrade Torres, ISO, Brazil

The disturbance was started by the incidence of a single-phase to ground short-circuit in the bay that connects Shunt Reactor #11 to the Busbar #1 of Imperatriz 500 kV substation. As shown in Figure 1, except for Shunt Reactor #11, the 500 kV substation configuration is a double-busbar scheme with four breaker-and-a-half bays and two double-breaker bays.

Figure 1 also highlights that the short-circuit occurred at a blind spot between the substation CTs and the bushing CTs of Reactor #11, which are connected to the busbar protection and the reactor protection, respectively. Under such conditions, no unit protection was able to detect the fault. Consequently, the short-circuit was cleared by time delayed remote backup protections related to the two 500/230 kV autotransformers and those located at the opposite terminals of all 500 kV transmission lines (Presidente Dutra #1 & #2, Colinas #1 & #2, Maraba#1 & #2, Estreito and Açailândia).

Along with the fault clearance process, the 500 kV transmission line between Bom Jesus da Lapa and Rio das Eguas was tripped by an unwanted operation of its teleprotection scheme based on permissive overreaching transfer-trip (POTT), due to the excessive sensitivity of the forward ground directional overcurrent units. Simultaneously with the fault clearance, the 500 kV #3 transmission line between Miracema and Colinas was tripped by the unwanted operation of main 1 and main 2 second zone units. Immediately after the disturbance the settings were properly modified in the protective schemes of these two transmission lines. Figure 2 shows the outages that took place so far.

After such a large number of automatic disconnections, the North and Northeast subsystems lost synchronism in relation to the Southeast and South subsystems. As a consequence, those subsystems were isolated by the tripping of the 500 kV #1 and #2 transmission lines between Miracema and Colinas. Line #1 was tripped by its out-of-step protection, while line #2 was tripped by second zone. North and Northeast regions became interconnected through the 500 kV transmission line between Acailandia and Presidente Dutra.

Before the beginning of the disturbance North/Northeast regions were importing 3,650 MW from Southeast/South, so their separation imposed a significant unbalance between load and generation, leading to a consequent underfrequency. Within Southeast and South subsystems, a slight overfrequency took place as a consequence of the temporary excess of generation. The underfrequency in North/Northeast subsystems was controlled by five stages of a load shedding scheme. The frequency of Southeast/South subsystems was restored to 60 Hz at 03:54 PM. The frequency of North/Northeast subsystems was restored to 60 Hz at 03:55 PM, showing the efficiency of the load shedding scheme.

The shunt reactor bay at Imperatriz 500 kV substation was immediately isolated, so the restoring process could be initiated at once. After one hour and eleven minutes all loads were fully restored, despite some problems in the manual reclosing of a few 500 kV transmission lines.


Alexandre Torres is a Professional electrical engineer. He graduated from UERJ, Rio de Janeiro in 1994 Received his M.Sc. Degreein 1998 and a Protection Specialization Degree in 2004. He worked for 13 years in AMPLA and at present he works for ONS (Brazilian ISO) dealing with disturbance analysis.

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