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On March 31, 2019, a 500 kV air-core, shunt reactor at a large generating station in BC Hydro was damaged shortly after four unit transformers were simultaneously energized from a 500 kV series and shunt compensated line source. Forensic investigations revealed that the controlled switching system, controlling the instant at which each breaker phase closes during energization to minimize inrush current, had failed. Uncontrolled closing led to under-damped, high-magnitude and sub-synchronous currents. Protection tripped and isolated the four transformers, followed shortly by shunt reactor protection tripping. Before the reactor could be isolated by circuit breakers, a damaging internal fault developed. This paper presents forensic investigation describing sequence of events leading up to reactor damage and shares the lessons learned with the peer utilities having similar installations.    [ABSTRACT FROM AUTHOR]

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]

Traditional connection of a generating station to the transmission grid typically requires either a new transmission line to an existing substation or building a new substation to segment an existing transmission line. Both of these options are capital intensive which often hurts economic viability especially small- or medium-size nonconventional generating stations. The direct connection to an existing transmission line, without the switching station, is the most economical solution to interconnect the generating station but it desensitizes the line protection and poses a risk to the ability of the protection to detect faults, particularly ground faults involving fault resistances. Multifunction relays, combined with communication-assisted schemes, now offer innovative solutions to overcome protection desensitization concerns. This paper describes these solutions which are successfully implemented by a large Canadian electric utility over the last two decades to tap connect numerous generators to the transmission grid. [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]

Shunt reactors are applied to long extra high-voltage (HV) transmission lines to compensate for their natural capacitance which otherwise could cause overvoltages under light load conditions. However, when the level of compensation approaches around 65% or higher, these reactors themselves can, due to contingency, become the cause of hazardous overvoltages during unbalanced open-phase conditions on the healthy line. This paper reports on an overvoltage incident that occurred on a 500-kV 72% shunt-compensated line in the BC Hydro system. The overvoltage occurred when one of the breaker poles failed to interrupt the current following an inadvertent three-phase trip of the healthy line. The uninterrupted current kept one phase energized for an extended time. Temporary overvoltages during this condition and restrike transients eventually led to surge arrester failures at both terminals. Analysis of voltage and current recordings provided useful insight into the mechanism responsible for the damaging overvoltages as well as the protection and control responses during the disturbance. The intent of this paper is to share the knowledge gained from analyses of the recorded waveforms with other utilities either having or planning for highly compensated lines. [ABSTRACT FROM PUBLISHER]

Interconnection of generation to an electric distribution system, which is primarily designed to serve radial loads, must not compromise reliability and quality of supply to customers or safety of public and equipment. This paper presents criteria used by BC Hydro, the third largest utility in Canada, to determine the requirements for interconnecting a generating source to its distribution system. The presented criteria discuss tradeoff involved in specifying the grounding connection of interconnecting transformer or how the size of a distributed generator impacts protection upgrades within the utility system, out-of-step tripping duty on the feeder breaker, and requirement of transfer trip to avoid temporary overvoltages. These criteria are applied consistently and uniformly to minimize cost of interconnection engineering. They are based on sound engineering principles to ensure that interconnection upgrades specified are justifiable and defensible, if necessary, to the utility's regulator to ensure open access of the grid. [ABSTRACT FROM AUTHOR]

Transformers are essential elements, which facilitate the transmission of electric power at high voltages over long distances and transformer energization is a common occurrence. The energization can lead to excessive transient inrush current, especially when the transformer core has remnant flux that adds to the flux build-up after switching. Inrush current sags the system voltage, thereby affecting the power quality of the network in proximity of the transformer. The extent to which power quality is degraded depends on short circuit MVA at the source bus, and the magnitude and decay time constant of the transient current. Present day thyristor-controlled machinery used in mining, pulp and paper industries and semiconductor manufacturing require high quality power. Some of these industries even pay a premium price for high quality power. It is, therefore, necessary to first assess the impact of energizing a large transformer on power quality and then develop a technique to limit this impact. This paper presents results from field tests and simulations using Electromagnetic Transients Program a 138 kV 315 MVA transformer energization in the BC Hydro system. The transformer is situated close to a pulp and paper mill with voltage sensitive loads. Methods to assess and to limit the voltage sags during energization are discussed. [ABSTRACT FROM AUTHOR]