Active high-speed switching can mitigate arc flash

Mitigating arcing-fault hazards in medium-voltage switchgear is an urgent concern, which is being addressed by safe work practices, operator training, and innovative solutions.

03/06/2012


Balancing the competing considerations of arc-flash mitigation methods is of particular interest to today’s electrical engineer. Considerations include compliance to National Fire Protection Association (NFPA) 70: National Electrical Code and NFPA 70E: Standard for Electrical Safety in the Workplace, design impacts to reduce required personal protective equipment (PPE) levels, cost of mitigation means, and impact on other disciplines such as architectural considerations such as ducting and/or increased ceiling height required by using passive-arc switchgear.

One method of mitigating arc-flash hazards associated with medium-voltage switchgear is the installation of active, high-speed-switch (HSS) systems. These systems are designed to detect and quench a burning internal arc in less than one third of one electrical cycle. The HSS extinguishes the internal arc by redirecting the fault-current path from an open-air arc back to the switchgear bus, which is the intended current path. The HSS operation provides a new low-impedance current path that collapses the voltage at the fault to near zero, so that the arc is no longer sustainable.

The system’s high operating speed—compared to arc quenching via circuit breaker tripping—translates directly to lower arc-flash incident energy (and associated PPE), greatly reduced personnel exposure to nonburn hazards, and minimal equipment damage. Another benefit of active HSS systems could include switchgear compliance to the IEEE C37.20.7 guide for testing arc-resistant metal-enclosed switchgear without any arc by-product venting requirements.

HSS operation

The HSS system is one of the options for mitigating arcing-fault hazards in medium-voltage switchgear. The HSS concept is simple and effective, but often viewed skeptically as a radical approach that places too much stress on the power system when it operates. In fact, it does transfer an internal arcing fault to the switchgear bus, which creates a bolted fault on the system.

The HSS control system determines that an arc-flash event has begun when it detects illumination with characteristics of an internal arc combined with a simultaneous rate of change of current. Upon determining the arc-flash initiation, the normally open HSS closes very rapidly to create a 3-phase bolted fault, which extinguishes the higher- impedance internal arc (see Figure 1). While the HSS technology described in this article is based on the light and current characteristics unique to internal arcing, some HSS designs may be based on other parameters, such as temperature, pressure, sound, or harmonics.

Figure 1: This drawing shows a schematic of a typical HSS connected to the switchgear bus. Courtesy: Schneider Electric

The HSS stays closed, keeping the bolted-fault condition on the system until the source overcurrent device clears the system current path. Although the stress of a bolted fault is certainly a valid concern, the HSS technology should not be dismissed without carefully considering these benefits:

  • Speed of operation, which provides personnel with increased protection from incident energy and nonburn injuries, reduces equipment damage, minimizes downtime, and reduces motor contribution to an internal arcing event
  • Effective personnel protection, even with exposed live parts
  • Simplification of overcurrent coordination and arcing-fault current variations
  • Elimination of impact on switchgear room.

 

These benefits translate to improved worker safety, procedural simplicity, power system reliability, improved system availability, and, in some cases, reduced installed cost.

Speed benefits

When commercially available HSS systems detect an arc, they typically close in approximately 4-6 msec (0.24-0.36 cycles at 60 Hz). In contrast, the time required for modern vacuum circuit breakers to detect and clear an arcing fault typically is at least 50 msec (3 cycles at 60 Hz), allowing for overcurrent or flash-detection relay-trip contact-closure time, plus circuit-breaker clearing time.

In many cases, the operating time is greater than 50 msec, depending on the use of lockout relays, relay and circuit breaker vintage, and vendor type. Lockout relays add 1 cycle. In retrofit scenarios, older circuit breakers may be 5-cycle or 8-cycle rated. By these criteria, a typical HSS system is about 10 times faster than the circuit-breaker-based arc detection and quenching designs, which leads to many benefits.

Arc-flash incident-energy reduction: Arc-flash incident energy (AFIE) is directly proportional to the time required to extinguish the arc, according to IEEE Standard 1584, Guide for Performing Arc-Flash Hazard Calculations. For example, consider a solidly grounded, 13.8-kV system with 50 kA of available fault current, using a standard 36-in. working distance and a 6-in. bus gap, per IEEE 1584 (see Figure 2).

Figure 2: This graph shows the accumulated AFIE versus time for a hypothetical solidly grounded 13.8-kV system with 50 kA available fault current. In this example, AFIE calculations for switchgear with HSS result in less than 1.2 cal/cm2. Courtesy: Schnei

In this example, AFIE calculations for switchgear with HSS result in less than 1.2 cal/cm2, which is the industry-referenced second-degree burn threshold. At this calculated AFIE, nonmelting or untreated natural fiber clothing may be worn along with hearing protection, eye protection, and leather gloves as needed. For the best-case circuit-breaker tripping times, heavier PPE with arc-rated clothing is required for higher energy exposures. PPE for increasingly higher energy exposures is progressively more bulky, hot, and difficult to work in due to loss of visibility and dexterity.

Nonburn injury hazard reduction: PPE helps protect personnel from severe burns and electrical shock. However, according to IEEE Standard C37.20.7-2007, Guide for Testing Metal-Enclosed Switchgear Rated Up to 38 kV for Internal Arcing Faults, arcing faults are “closer to a detonation than to a wind load or a deflagration.” Standard electrical PPE cannot entirely mitigate nonburn injuries that can kill and impose severe impact on quality of life for arc-flash survivors. The pressure wave, shrapnel, sound blast, blinding light, and poisonous gases are well known and documented. Effects on the human body are known all too well.

Fast speed of operation is essential to reducing the effects of nonburn injuries. The exact speed required is difficult to mathematically quantify for a particular location due to the many variables involved. However, it is generally known that peak arc pressures track expended I2t, and that the overpressure rise time is faster than the opening time of the interrupting device.

HSS system manufacturers cannot claim to eliminate all personnel danger. However, it is valid to claim that the danger is greatly reduced due to reduced time of exposure, as verified by documented test results and videotapes.

Equipment damage reduction: Arc-blast effects can destroy equipment with the same phenomena that kill and injure people. IEEE Standard C37.20.7 does not include internal equipment destruction as a failure criterion. Rework or replacement is expected.

HSS manufacturer tests and actual field events, however, illustrate that the fast arc-quenching feature of typical HSS systems limits the damage to the point of the arc occurrence, with minimal additional damage. As a result, troubleshooting, repair, testing, and return-to-service are simplified and minimized. “As a general rule, removing the fault quickly will minimize the damage; however, the overpressure event typically occurs in a time frame of less than 1 electrical cycle,” according to IEEE Standard C37.20.7. For medium-voltage switchgear, HSS systems are the only available devices to date that can compete with the speed of overpressure and equipment destruction.

Figure 3: This photo of a healthcare facility switchgear rear cable compartment shows a close-up view of the phase run-back with the bus cover panel resting on the phase bus. Courtesy: Schneider Electric

An HSS system minimized equipment damage during an actual internal arcing event at a large U.S. healthcare facility (see Figure 3). An incorrectly placed main bus access cover inside a medium-voltage metal-clad switchgear unit caused the arcing event. The switchgear was energized with the metal panel lying on top of two of the phase buses while simultaneously resting against the side sheet, which was at ground potential. Damage was minimal. The switchgear was returned to service without further problems the same day, after replacing the affected phase run-back bus bars.

Reduction of motor contribution to AFIE: Large induction and synchronous motors can contribute significantly to AFIE in some industrial settings. The medium-voltage feeder breakers supplying motor loads will not trip for motor contribution levels in many cases, so the full motor contribution can persist for several cycles regardless of main circuit-breaker tripping time. In the case of bus differential relay application, the motor contribution will persist until it decays to zero, or until the associated bus lockout relay and feeder breaker trip, whichever comes first.

HSS systems address this issue in large measure due to the fast arc-quenching time.

Personnel protection benefits

IEEE Guide C37.20.7 states: “The use of equipment qualified to this guide is intended to provide an additional degree of protection to the personnel performing normal operating duties in close proximity to the equipment while the equipment is operating under normal conditions.” The standard excludes alteration of the equipment from normal operating condition, and from activities above or below the equipment, such as catwalks, installations on open grating, and cable vaults. Any opening in the equipment invalidates the arc-resistant category and can expose personnel to the full effects of an arcing event. Arc-resistant switchgear is arc resistant only when all covers are secured in place.

HSS systems, however, operate effectively regardless of exposed live parts, or personnel performing work above or below the equipment. Working around exposed live parts should normally be prohibited, but situations may arise where the risks associated with equipment shutdown exceed the risks of working with the equipment opened.

Examples of relatively commonplace actions that should be considered abnormal are verifying a circuit is de-energized and applying grounds. The circuit should be considered live until verification that it has been de-energized is obtained. Until then, appropriate PPE must be worn. Due to potential backfeed scenarios, operator errors, or documentation errors, an arc flash is possible. However, in this situation the HSS can effectively reduce the risks to the worker.

Overcurrent coordination simplification benefits

The bolted-fault current resulting from HSS actuation must be cleared by the source overcurrent device within the withstand ratings of the switchgear and HSS system, but the protection of the worker is effective even if the relays in the system are improperly coordinated or if the arcing-fault magnitude is not as anticipated.

Selective coordination of overcurrent devices is often in direct conflict with the need to trip source circuit breakers as fast as possible for arc-flash hazard mitigation purposes. For example, if the instantaneous trip level is above the lowest arcing-fault current magnitude, the relay may not trip instantaneously, resulting in high AFIE. On the other hand, if the instantaneous trip level is set low enough to trip quickly for all possible values of arcing current, selective coordination with downstream devices is frequently compromised.

Therefore all values of arcing-fault current magnitude must be considered from highest to lowest. This requires careful judgment and frequent trade-offs between selective coordination and AFIE mitigation. There are many reasons that arcing-fault current levels can change, including utility system upgrades; system switching; plant switching between main, tie, and in-plant generator circuit breakers; and varying quantities of running motors. Additionally, many times, the utility system changes without the customer being made aware.

Bus differential relays: Bus differential relays are fast yet inherently selective, but cannot detect arcing faults outside the protected zone current transformers, which in most metal-clad switchgear does not encompass the cable compartments. For example, the bus differential relays installed at the healthcare facility cited in this article did not detect the arcing-fault condition because the bus differential current transformers are inside the breaker cell while the fault occurred in the cable compartment. Another example is that while a worker may have an additional degree of protection when racking a breaker from the front of the equipment, he or she may not be protected if the cable compartment is opened.

Zone-selective interlocking: Zone-selective interlocking schemes achieve selective coordination via restraint signals from the feeder circuit breakers back to the main. If the main detects a fault but receives a restraint signal from the feeder, the main breaker relay times out normally per its time-current curve, allowing the feeder to selectively clear the downstream fault. If the main detects a fault without a restraint signal, the main trips instantaneously because the fault logically must be on the switchgear bus. The main circuit breaker relay must detect the fault at arcing-current level, and it must still coordinate with the feeder. Feeder breakers cannot be permitted to trip on motor contribution; otherwise, they will restrain the source breakers and defeat the scheme entirely. Multi-source lineups such as main-tie-main add even more complexity.

Arc-flash-reduction maintenance switches: Arc-flash-reduction maintenance switches are used to lower relay pickup levels and sacrifice coordination only when personnel are present or maintenance is being performed. Again, the main circuit breaker relay must detect the fault at arcing-current level. Also, careful procedural rules must be implemented to ensure that personnel do not forget to turn on the maintenance trip settings when beginning the work, or forget to turn it off when done. Occupancy sensors have been used to turn on inputs to electronic relays that automatically lower the relay instantaneous settings, then restore them when personnel leave. While the reduced settings are in effect, there is a possibility of nuisance nonselective tripping.

Arc-flash detection relays: Arc-flash detection relays that combine light sensors, current sensing, and high-speed relay outputs are immune to overcurrent coordination and arcing-fault current magnitude but rely on the relatively slow circuit-breaker tripping to quench the arc.

HSS systems: HSS systems are likewise immune to the downstream coordination and arcing-fault current- magnitude considerations, but with the advantage of speed. The short-circuit withstand rating of the HSS itself is all that needs to be considered in setting the source circuit-breaker relays with regard to arc-flash protection. For circuit-breaker-based arc quenching (other than arc-flash detection relays), relay settings are critical and must consider all possible arcing-fault current magnitudes.

Power system short circuit, coordination, and arc-flash studies must be kept up-to-date and relay changes implemented as necessary. This statement is always true, but for HSS installations it is less critical because the arc-flash hazard protection is unaffected. A related benefit is that selective coordination for critical systems is greatly simplified.

Switchgear room accommodation benefit

Arc-resistant switchgear that relies on circuit-breaker tripping must have a safe path to vent the arc by-products. Ceiling and wall clearances, overhead equipment, doors, windows, the building’s capability to absorb the pressure wave, fireproofing, weather, and vermin ingress are among the considerations. Additionally, the arc-resistant switchgear may be larger and heavier than standard switchgear. These factors can grow the size, cost, and complexity of the switchgear room. Some HSS systems have been tested and comply with IEEE C37.20.7 (all accessibility types: 2, 2B, 2BC) and negate the need to purchase passive arc containment.

HSS system installations may require an additional section to accommodate the HSS; otherwise, the installation is identical to standard nonarc-resistant switchgear, as no arc by-products need to be accommodated.

Bolted-fault stress considerations

Medium-voltage arcing faults are high magnitude, as determined by the following IEEE 1584 calculations (see Figure 4):

Equation No. 1: log Ia = 0.00402 + 0.983 log Ibf

Equation No. 2: Ia = 10log Ia

Where Ibf is bolted-fault current (kA), Ia is arcing current (kA)

Figure 4 stops at 63 kA, which is typically the highest currently available interrupting rating for medium-voltage metal-clad switchgear. Arcing-fault magnitudes are calculated at greater than 94% of bolted-fault magnitude.

Figure 4: This graph shows RMS symmetrical arcing-fault magnitude comparisons as a percentage of bolted-fault current. Courtesy: Schneider Electric

However, these RMS symmetrical magnitude comparisons do not yield an accurate picture of the HSS applying a bolted fault in approximately 0.25 cycles after the arcing-fault occurrence. The arcing-fault circuit X/R in most medium-voltage systems will be in the range of 2.5 to 4.0 when calculated, assuming the arc is purely resistive (see “Arcing fault X/R ratio calculations” in the online version of this article).

The bolted-fault circuit X/R will typically be a much higher value than this for most medium-voltage systems. Therefore the dc-offset component of fault current will suddenly increase when the HSS actuates. The peak current will be nearly as high as it would be for an initial closure directly into a bolted fault, even though the RMS symmetrical ac component will be only a few percent higher. Note, however, that the worst-case peak currents following HSS actuation cannot exceed bolted-fault levels, although they are quite close (see “Fault-current calculations” in the online version of this article).

Therefore momentary and interrupting duty on the source overcurrent device, switchgear bus, and other system equipment should be considered to be equal to bolted-fault levels, regardless of whether an HSS system is installed.

This is not a new concern for system design or equipment ratings, although it might be considered unwise by some to apply the HSS and intentionally subject the system to the possibility of a bolted fault. This is a judgment call that must be weighed against the significant advantages of HSS system installation.

Transformers and large medium-voltage induction and synchronous motors are expensive and have considerably long lead times. Design standards require that transformers withstand bolted through-faults. For the first few cycles after a fault at the motor terminals, induction and synchronous motors initially supply the ac component current to a fault based on motor subtransient reactance. These issues are of particular concern when an HSS application is being considered.

Arcing ground-fault considerations

Arcing ground faults on solidly grounded medium-voltage systems are expected to escalate to 3-phase faults. HSS operation is the same as for a 3-phase fault.

HSS system manufacturers are aware that impedance-grounded systems are often used on medium-voltage systems, that the resulting low ground-fault current magnitudes reduce the probability of an arc developing, and that the fault should be cleared or annunciated by system ground-fault relays, depending on system design. Minimum-current HSS actuation levels are built into the electronic detection logic to prevent unnecessary closing of the HSS.

Ungrounded medium-voltage systems should not be used regardless of whether an HSS system is present. The HSS should function as intended with any type of system grounding other than an ungrounded system.

System stability considerations

For sophisticated power systems with multiple utility sources, in-plant standby generation, and co-generation, network stability simulation analyzes the effect of network disturbances on the system. Disturbance types include utility isolation, fast transfer, motor starting, fault study, loss of generation, and relay operation.

The concept that a bolted fault from an HSS actuation would cause system instability—while an internal arcing fault would not—is questionable. Stability analysis could prove scenarios for arcing faults of 94% or more of bolted-fault current. The user would be lucky if a bolted fault never occurs. Stability studies are most often based on worst-case 3-phase bolted-fault conditions. HSS system application should have no bearing on the stability issue.

Conclusion

HSS systems should be seriously considered for installation in medium-voltage switchgear. Switchgear size, importance, cost, complexity, growth needs, and architectural considerations should be considered, along with plant safe-work practices, procedures, and other available arc-flash mitigating features.

HSS systems can provide substantial rewards without exposing the power system to undue risks beyond the unavoidable.

Divinnie is a staff electrical engineer with Schneider Electric’s Engineering Services Division and is a member of IEEE. Stacy is a strategic marketing manager with Schneider Electric’s Infrastructure Business and is a senior member of IEEE.



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