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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. more...

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. more...

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. more...

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. more...

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. more...

This paper covers principles of transmission-to-generation interconnection protection and summarizes the IEEE C37.246-2017, Guide for Protection Systems of Transmission-to-Generation Interconnections. The scope of the guide is to document accepted protection practices for transmission-to-generation interconnections and is intended to cover the protection system applications at the interconnections between transmission systems and generation facilities. Consideration is given to technical data needed to design a reliable transmission-to-generation interconnection and data exchange between the generator and transmission owners. All types of generation are discussed such as conventional and inverter-based. Typical transmission-to-generation interconnections are presented; their adequacy, complexity, strengths and limitations with respect to a variety of bus arrangements are discussed. System studies necessary for designing reliable interconnections are described. Protection system functions that are typically applied at transmission-to-generation interconnections and their settings considerations are thoroughly presented and discussed. Specific application guidelines are provided for tapped connections as they are the most complicated relative to interconnection protection. more...

Blackouts usually occur when a heavilyloaded transmission interconnection is subjected to a disturbance that pushes it over the stability limit. In this case, a blackout was caused by multiple lightning strikes to a transmission link consisting of three parallel 500 kV power lines carrying a small fraction of their rated load. The incident caused the islanding of seven power stations, the shedding of 663 MW of generation, a regional blackout in British Columbia, Canada that affected 34,500 customers and damage to the equipment of a major customer. One of the subject lines experienced a double line-to-ground fault, and the other two lines experienced single line-to-ground faults. It took about 3.25 hours to end the blackout and totally restore the system to its pre-outage condition. The following unusual phenomena were encountered during this incident: a) 9 Hz ferroresonance currents that triggered the relay of the shunt reactor bank of the 500 kV line which experienced the double line-to-ground fault, and; b) Ferranti Effect overvoltages that reached 148% at the remote end of a chain of 138 kV & 230 kV lightly-loaded lines that became electrically connected in series, with no voltage control at the intermediate points. This paper gives a review of both this incident and a prior unreported one which also caused a regional blackout, and presents the lessons learned from them. more...

As microgrids evolve and proliferate, with sharp increase in renewable-fed distributed generation, islanded operation of microgrids that are fed entirely by inverter-interfaced renewables has become an imminent reality. Commercially available grid-forming inverters have made it technically possible to form a stable microgrid island with other inverters following in a coordinated controlled operation. For reliable operation of islands, inverters participating in such islands may need to stay connected during faulted conditions until the fault is isolated. Though ample literature exists on protection of microgrids, protection of islands entirely fed by inverters has not been addressed well. This paper develops a simplified, yet accurate model for an inverter-interfaced distributed energy resource, and uses it to explore operation and protection issues in islanded microgrids that arise due to the control and design of commercial inverters. more...

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. more...

Large induction motors, when they become disconnected from their synchronizing source and isolate with part of the high-voltage system, become self-excited as long as their inertia keeps them spinning and there is enough capacitance from the system to provide the magnetizing current for the machine and maintain air-gap flux. These induction motors, operating as self-excited induction generators, temporarily backfeed the islanded subnetwork. Under these conditions, they act as a weak inductive source and can excite near fundamental frequency series resonance due to interaction with the high voltage system shunt capacitance, such as from long lines or shunt capacitors used for network voltage support. This paper uses an actual case study to provide insight into the phenomenon of self-excitation with dynamic overvoltage with induction motors. A 60-Hz analysis of equivalent sequence circuits was able to predict the occurrence of overvoltage problems and this was then confirmed by detailed studies using an electromagnetic transient program. A protection solution to avoid backfeed was implemented to safeguard the system against these overvoltages from occurring. more...

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. more...

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. more...

The cost of interconnection is an important factor in Non Utility Generator (NUG) overall project costs. This cost has to be minimized without compromising the safety or reliability of the transmission system. The lowest cost interconnection is a transmission line tap. However, low cost taps may negatively impact transmission system performance and protection system performance. The negative impacts can often be mitigated with increasing complexity (and cost) of the interconnection. This paper describes BC Hydro's experience with studying interconnection proposals, using internally developed protection criteria to minimize costs while retaining acceptable protection performance. more...

This paper concerns the justification of the use of pilot protection on transmission lines.

The BC Hydro transmission network includes 33 wholly owned 500 kV circuits and three 500 kV interconnections with other utilities. Eleven of these lines include series compensation, and many were upgraded to provide single-pole tripping (SPT) capability to improve power transfer capability and stability margins. To reduce construction costs, the majority of these lines are not shielded, which increases the probability of low-grade, single-line-to-ground faults due to high tower grounding resistance. Over the years, BC Hydro has developed strict performance specifications for both sensitivity and speed of protection systems based upon fault location and fault type. We also have a well-developed process for model power system testing of protection systems to verify conformance to these performance specifications. In this paper, we discuss the lessons learned in applying protection systems and optimizing protection sensitivity and speed for these challenging applications. We also discuss a number of problems and challenges discovered in real-world experiences and during simulation, many of which are associated with discerning the difference between normal system unbalances and/or unbalances caused by operation of series capacitor protection and resistive ground faults up to 300 Ω. The paper includes a discussion of the impact of reclosing control and automatic reinsertion control of series capacitors on protection systems. The paper also includes a number of digital fault recorder (DFR) records for actual power system faults on series-compensated lines. more...

The North American western grid experienced two major power disturbances on July 2 and August 10, 1996. The postmortem studies of the incidents indicated that some of the generating units did not respond as predicted by system studies. Consequently, the Western System Coordinating Council (WSCC) mandated that all units (generator, excitation, governor and turbine systems) in the western grid greater than 10 MVA be tested to verify the generator reactive power limits as well as the dynamic model data being used for system studies. This paper presents field experiences of the authors in testing and modeling of gas turbines and their associated governors during the WSCC recommended test program. more...