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Dense smoke from wildfires within a 500-kV line corridor led to several line protection trips over a three-day period. A switchable line-connected reactor, provided to control Ferranti voltages, tripped and locked out twice by its protection shortly after the line was de-energized. Lockout operation prevented quick reactor reinsertion and impeded operational flexibility until visual inspections of the reactor were conducted on both occasions. Forensic investigation of the event concluded that the smoke increased the line capacitances. This created a near 60-Hz zero sequence resonant circuit when the line tripped and became de-energized along with the reactor. The resonant circuit amplified the capacitively coupled zero sequence 60-Hz voltage induced from circuits which run adjacent to the line for a short distance. The voltage amplification drove enough current through the reactor to trip the sensitive ground overcurrent protection. A holistic mitigation is designed to avoid reoccurrence of the incident and improve overall line reliability. The intent of this paper is to share lessons learned from the event investigation with peer utilities because this problem of unexpected operation of line shunt reactor protection has not previously been reported. [ABSTRACT FROM AUTHOR]

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]

Power-electronic sources, such as Type 3 wind turbine generators and static synchronous compensators (STATCOM), interface to the grid through, partial- or full-scale, and power converters that have inherently fast switching capability to control their output current during short circuits. The short-circuit current is a function of the specific converter control algorithm and differs significantly from the conventional rotating machine sources without converter interfaces. Therefore, if a transmission-line protection scheme is designed for conventional sources, (not taking into account these differences in short-circuit current characteristics), reliability can be at risk. Using real-life short-circuit currents on lines supplied by sources having a power converter interface, this paper illustrates the reliability risk to conventional line protection schemes, in particular, to those which use negative-sequence quantities for the detection of unbalanced faults. This paper discusses the protection schemes, adopted by BC Hydro–a large Canadian Electric Utility, for transmission lines interconnecting Type 3 wind turbine and STATCOM sources. Their application for ground faults is independent of the converter control algorithm as long as the source is interconnected to the grid via a transformer which is a source of zero-sequence current. [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]

Single-phase trip and high-speed reclose (SPTR) schemes applied on extra-high-voltage (EHV) lines offer significant benefits over three-phase schemes as they help maintain synchronism and stability in sparse transmission networks. However, a secondary arc may persist and prevent timely recovery of the air insulation. In this situation, high-speed reclosing will attempt to re-energize the still-faulted line, adding stress to the system instead of benefiting the system from the SPTR scheme. Using staged fault and fault disturbance data recorded from several 500 kV lines, this paper presents validation of an enhanced hybrid scheme incorporating secondary arc extinction detection logic designed to avoid high-speed reclosing onto faults while also allowing single-phase reclose in cases where it would be successful. The new logic is now being applied to several 500 kV lines in BC Hydro to improve transmission system reliability. [ABSTRACT FROM PUBLISHER]