Brazilian Blackout 2011

Author: Antonio Carlos da Rocha Duarte, Brazil

Blackout Watch

Reference:  Report ONS RE 3/0032/2011 “Análise da Perturbação do Dia 04/02/2011 à 00h21min Envolvendo os Estados da Região Nordeste”

This article presents the summary of the analysis on the disturbance started on February 4th, 2011 at 00:08 am, as per Report ONS RE 3/0032/2011.

The disturbance commenced at the 500 kV substation of L. Gonzaga hydro plant, islanding a large part of the Northeast system from the National Interconnected Power System (BIPS) by tripping SE/NE and N/NE interconnections and culminating with the collapse of a large part of the Northeast System load, except for Piauí and Maranhão states, and a part of the Southwestern Bahia state. Before the disturbance, the NE system load was 8,884 MW (an average load condition.)

The disturbance began when the BF protection of circuit breaker 1 (CB1) accidentally tripped the #1 transmission line (TL) to Sobradinho and the busbar section 1 of L. Gonzaga substation. The BF protection misoperation was caused by a spurious signal (exclusive for BF initialization) issued by a defective line static protection.

Thus, two L. Gonzaga generators remained connected to the TL to P. Afonso IV, once CB3 was isolated for maintenance. Despite the TL between S.J. Piauí and Milagres being out of service for maintenance, up to then, the CB openings led to no consequence at all.

At 00:21 am, during the manual reclosing attempt of #1 TL to Sobradinho, L. Gonzaga substation busbar section 2 was accidentally tripped by CB2 BF protection for the same reason as above. Once busbar section 1 was still out, #2 TL to Sobradinho and TL to Milagres were disconnected.

Thus, the L. Gonzaga generators remained connected as follows:

  • One unit to TL to Angelim II
  • Two units to TL to P. Afonso IV
  • Two units connected to TL to Olindina

The 500kV substation of L. Gonzaga hydro plant configurations at 00:08 & 00:21am are described in Figure 1.

Then, power oscillations of NE system against N and SE systems led to synchronism loss among them and 68OST protections, tripping the respective 500 kV interconnections. In consequence, there was a voltage sag in two 230 kV transmission trunks, definitely islanding NE system from National Interconnected Power System (NIPS) by disconnecting 230 kV lines due to 21-1 tripping actions. These disconnections prevented the disturbance to spread out to other parts of NIPS. Figure 2 shows the main disconnections between NE system and NIPS.

On the other hand, there was a high generation deficit in NE system, which was importing 3,237 MW when the disturbance took place.  Such a lack of generation imposed under-frequencies as low as 56.44 Hz to the islanded NE system, causing 3,297 MW (41.21% of total) automatic load shedding in five stages.  At the same time, the Northern area of the NE system was subjected to undervoltage, because after the loss of the 500 kV network, the transmission path became restricted to the 230 kV network, which was too weak to keep the voltage profile. This caused the operation of the undervoltage load shedding scheme and natural load rejection, resulting in a load loss around 1,300 MW, thus remaining only 200 MW of load in the N area of NE system.

After automatic load shedding by under-frequency and undervoltage, plus natural load rejection, the load x generation balance was recovered, stabilizing voltage and frequency in the NE system.  As a result of the load rejection across all areas, dynamic overvoltage occurred in the NE system, leading to automatic disconnections of capacitor banks, static and synchronous compensators, as well as tripping of three 500 kV TLs.

After about 40 seconds, five generators (1,768 MW) of the Xingó hydro plant were tripped by Loss of auxiliary services and after 10 more seconds, three generators (812 MW) of the P. Afonso IV hydro plant were tripped by overcurrent on excitation transformers, leaving only one generator in service in each of these plants. After about 1 or 2 minutes, there were also outages of one generator in P. Afonso I, P. Afonso III and Apolônio Sales hydro plants, totaling 193 MW generation loss. Due to these shutdowns, the islanded NE system was submitted to under-frequency and undervoltage, causing the actuation of automatic undervoltage load shedding and natural load rejection in the Eastern and Southern areas of the NE system.

After these events, the islanded NE system remained in service with degraded voltage and frequency levels for about 7 minutes until 00:29 am, when the islanded system collapse took place, rejecting the remaining load of 2,316 MW.

NIPS remained supplying 473 MW of Piauí state load and 340 MW of SW Bahia state load, as well as Maranhão state load. The NE system restoration average time was 194 minutes.

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