On October 26, at 00:14 (GMT), there was a two-phase short circuit on disconnect switch insulating series capacitor bank from the circuit 2 of the transmission line at 500 kV Imperatriz-Colinas, at substation Colinas terminal. The contingency was atypical because, according to the oscillographic records and measuring the short circuit lasted approximately 12 seconds, due to the unavailability of the protections of all circuits 2 of the trunk 500 kV of North - East Interconnection.

Consequences and system restoration

Primary and alternative protections of 500 kV TL Imperatriz - Colinas C2 did not act because they were with their trip commands inhibited due to an incident unnoticed when running remote reconfiguration. Consequently, there was tripping of remote rear of transmission lines adjacent to the failure, except for the 500 kV TL Colinas - Miracema C2, which failed to act for the same reason. In order to completely eliminate the defect, it was necessary the tripping of other transmission lines farther from substation Colinas, as the transmission lines 500 kV Gurupi-Miracema, on the south side of Colinas, and the lines of interconnection North / Northeast from substation Imperatriz.

The delay in the elimination of the short-circuit electrically located next to the 500 kV busbar of substation Colinas caused a significant impact on the dynamic operation of the BIPS. The system suffered oscillations and subsequent loss of synchronism, which culminated in the separation of the North and Northeast regions among themselves and in relation to the Southeast, South and Midwest, which remained operating interconnected. The separation of the North and Northeast led performances of the respective Regional Schemes for Load Relief - ERAC, to balance load x generation, with interruption of 11,789 MW of load.

The process of restoration started immediately and was carried out in a gradual and coordinated way. The power failure lasted an average of 241 minutes in the states of Rio Grande do Norte, Paraíba, Pernambuco, Alagoas and Sergipe. Other states have had interruptions for periods less than 200 minutes: Pará, Tocantins, Maranhão, Piauí, Ceará, Bahia and Goiás.

Main actions and measures implemented:

The Disturbance Analysis Report (RAP), sent to CMSE and ANEEL presents a list of twenty-five actions, some already completed and several under implementation with deadlines and responsibilities of acting according to the three key goals pursued by ONS:

• work proactively, strengthening, wherever possible and practicable, the security of the system:
• minimize the domino effect when the problem occurs, and
• reduce the time of restoration.

The insulating disconnect switch of Series Capacitor Bank of substation Colinas terminal, circuit C2 of the transmission line 500 kV Imperatriz-Colinas, was submitted to a series of tests, conducted by Cepel and CESI - Centro Elettrotecnico Sperimentale Italiano – SpA.

Among the measures already taken by the company which owns the facilities, as a result of the disturbance and the test results, stands out a review of the procedures for the implementation of adjustments and remote settings and equipment maneuvers procedures.