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A BC Hydro regional system experienced a blackout after multiple 500 kV lines, sharing the same transmission corridor, were subjected to lightning strikes in the time span of about three minutes. These lines connect two large generating stations, regional loads, and five nonutility generators to the rest of the BC Hydro system. The protection scheme on the first line tripped three-phase and initiated autoreclose as designed. However immediately after three-phase opening, high magnitudes of low-frequency (about 9 Hz) currents appeared in the line shunt reactor, causing a protection trip, and aborting the autoreclose. Subsequent lightning strikes coupled with a pre-existing line outage disconnected both generating stations from each other as well as from the integrated system. The larger of two generating stations and almost all regional load formed an islanded subsystem, which eventually collapsed and blacked out. During the restoration of the subsystem, the system operator inadvertently picked up the entire regional system without load, causing steady-state resonant overvoltages higher than 145% and damaging a transmission customer. This paper shares lessons learned from forensic analysis of disturbances leading to regional blackout and describes the risk of unsafe overvoltages from near power frequency resonant condition during the restoration process. [ABSTRACT FROM PUBLISHER]

A single-phase trip and reclose scheme applied to a long transmission circuit, interconnecting a remote subnetwork to the grid, improves the interconnection reliability. However, the entire subnetwork loses supply in one phase should there be no generation connected in it during the open-phase condition. Under these conditions, BC Hydro experienced a hazardous temporary overvoltage on the open phase in a 287 kV subnetwork. Highly shunt compensated lines have experienced high open phase overvoltage but this incident revealed that transformers in the subnetwork having delta-connected secondary or tertiary windings can also cause this concern. Though it was not designed to do so, the transformer differential protection operated on excessive magnetizing current after the 5th harmonic blocking current dropped below the setting threshold (35%) and prevented damage to the surge arresters. This paper will use disturbance records to recreate the sequence of events and present a detailed waveform analysis. Expected differential relay response when the transformer is severely overexcited will be discussed. A steady-state analysis of the simplified system will be presented, followed by detailed electromagnetic transients simulations which successfully duplicated the disturbance. Based on further simulations, a mitigation plan was developed to avoid a reoccurrence of the problem. [ABSTRACT FROM PUBLISHER]