Marty Trivette 2016-08-02 15:21:29
Arc flash events in electrical distribution systems can be devastating, costing up to $15 million in direct damages and indirect costs, such as health care, workers’ compensation, and others (EPRI 1999). An arc flash can generate temperatures over 35,000°F and projectile-producing pressures equivalent to 700 miles per hour, and is able to throw a person across the room (GE Industrial Solutions). Despite this, effectively dealing with arc flashes in the past has inevitably involved compromise. That is, maximizing protection against arc flashes could increase the number of nuisance service outages, but reducing these unnecessary disruptions could place equipment and personnel safety at risk. Fortunately, today’s methods for containing, mitigating, and preventing arc flash events have progressed significantly. Now, compromise can be a thing of the past. Arc Containment The IEEE (Institute of Electrical and Electronics Engineers), IEC (International Electromechanical Commission), and other standards bodies may have different words for it, but they all agree that at its most fundamental level, arc resistance in switchgear is intended to contain an arc flash event. Typically, this involves passive physical features and capabilities, such as reinforced metal containers or vaults with heavy doors that remain sealed most of the time. Additionally, ductwork, flues, and chimneys are installed to carry the high-temperature gases that an arc flash will produce, away from equipment and workers in the area. Unfortunately, these containment techniques are only effective as long as the containment vessel is tightly sealed. Regular inspection and maintenance is typically limited to just the low-voltage control compartment. In order to maintain arc containment capabilities, the primary equipment must be de-energized to open the primary panels. Arc containment techniques are passive insofar as they only react to an arc flash when it occurs. They do not actively affect the scale, size, or duration of an arc, nor do they in any way help eliminate at least one of the causes of arc flash events, namely, failures in the bus or circuit breakers, drives, transformers, or other equipment. That would require active arc resistance, or what is commonly referred to as arc flash mitigation. Arc Flash Mitigation Through several technological advancements, arc flash mitigation techniques are able to reduce the size and physical force that result from an arc flash. These mitigation techniques can limit the scale and duration of an arc flash by quickly reducing or eliminating the energy that an arc feeds off of. Plus, mitigation methods react rapidly, (some equipment can react in a matter of milliseconds) so an arc flash can be quenched well before it is out of control. In addition, arc containment and mitigation methods are not mutually exclusive of each other. They are often deployed together in the same system, although mitigation technologies can reduce the size and extent of the physical containment features in the system. Since mitigation technologies will limit the scale of arc flash events when they occur, containment vessels can be less massive, and the venting mechanisms for hazardous gasses simpler and smaller. Instead of ductwork that vents through the exterior of the building, for example, the gases might just be vented out the top of the containment vessel. In some cases, mitigation technologies are able to eliminate the need for containment features altogether. Moreover, already deployed containment equipment can be retrofitted with mitigation technologies to more effectively control and limit arc flash events. Several different arc flash mitigation technologies contribute to the effectiveness of an arc flash mitigation technology like GE’s ArcWatch. For example, zone selective interlocking (ZSI) has been recommended in various electrical codes, such as the National Electrical Code (NEC), for some time now. In general, zone selectivity is a concept whereby components such as circuit breakers in an electrical distribution system communicate with each other in order to enable hierarchical coordination. The intent of this coordination is to compartmentalize and minimize a service interruption to the smallest possible zone in the overall distribution system, and still protect the rest of the system. For example, a high-current molded case circuit breaker (MCCB) might detect an overcurrent condition at the same time as a low-current circuit breaker that controls the power supplied to a smaller section of the system. Rather than the higher current circuit breaker tripping and shutting down service to the larger section, the higher-order circuit breaker would cede responsibility for the condition to the lower-order MCCB which is closer to the condition. Thus, through selective coordination, the service interruption would be limited to a smaller portion of the system and technicians would know more precisely where to look for the cause of the problem. Additional innovations have enhanced the capabilities of ZSI even further. One example is a technique known as instantaneous zone selective interlocking (I-ZSI), which has a much faster reaction time than traditional ZSI. Circuit breakers are practically able to coordinate in real-time. The algorithms that enable the much faster I-ZSI make use of several advanced analytic techniques, including wave form recognition (WFR). Rather than just monitoring for peaks or spikes in the current as it passes through a circuit breaker, WFR performs a more thorough analysis by plotting and taking into consideration the implications of a power curve over time. As a result, circuit breakers can have more sensitive settings to increase the system’s protection against arc flash events without degrading the ability of the circuit breakers in the system to selectively coordinate with each other. Another arc flash mitigation technique involves lowering the energy available in a section of the distribution system while technicians are working on energized equipment in the area. By doing so, the amount of energy available to an arc flash, should one occur, is less than it normally would be. This is accomplished by increasing the current sensitivity of circuit breakers so that they trip sooner. This reduces the amount of energy an arc flash would feed off of by shortening the trip time. An example of this capability is the reduced energy let through (RELT) feature in many of GE’s electrical equipment and circuit breaker offerings. This lower power mode can be activated manually by technicians through a switch of some sort located on the overcurrent device itself or, if greater safety is required, the switch may be located some distance from the device so that the worker is outside the arc flash boundary when the low power mode is activated. If the circuit breaker is equipped with serial communications, the lower power mode could be engaged through the facility’s power monitoring or supervisory control and data acquisition (SCADA) system. Motion sensors that detect personnel in the area could also switch the circuit breakers to a lower power mode for the safety of the workers and automatically return the circuit breakers to their normal mode when the workers have left the area. Of course, once power devices like circuit breakers are able to communicate with themselves and with other types of devices, such as various types of sensors, the electrical distribution system begins to resemble a sophisticated Industrial Internet of Things (IIoT), where systemwide software strategies, such as GE’s Envisage, can be adopted to achieve a number of long-term benefits, including better control of arc flash incidents. The Industrial Internet of Things The critical aspects of an IIoT are connectivity, analytics, and a systemwide perspective. Over the last decade, small logic modules or processor-like capabilities have been incorporated into practically every aspect of everyday life, including the electrical grid and individual electrical distribution systems. Applying system-wide analytics to the data collected by all of these logic nodes geometrically enhances the value of data collected by any one mode. Integrating electrical distribution systems into an IIoT by connecting electronic trip units, relays, meters, transformers, and other devices creates the opportunity for improved power reliability, more efficient energy consumption, predictive maintenance programs, and much more. For example, software management systems like GE’s Predix can gather performance and operational data on circuit breakers over time to determine any unexpected degradation in performance. As a result, the failure of individual circuit breakers can be predicted before a failure actually occurs, improving the organization’s service planning and response. Replacing soon-to-fail protection devices can eliminate at least one cause of arc flash events. In addition, the converse is also true. If circuit breakers and other protection devices perform longer than expected and replacement can be postponed, then maintenance costs can be reduced. The first step toward an IIoT is connecting all of the “things” that make it up. The devices in older electrical systems may not have data communications capabilities, but these devices can be replaced over the course of a regular maintenance replacement program, so eventually a modern IIoT has been retrofitted onto the old system. Then, the electrical IIoT can be grafted onto other industrial networks, like factory or building automation systems. In a factory’s machine control system the electrical IIoT might employ electrical signature analysis to more reliably detect a failure in an expensive machine’s gearbox bearings than other sensing technologies, like vibration sensors, might be able to detect. No Flash in the Pan The innovative technologies and methodologies that have been brought to bear on arc resistant switchgear in recent years have been significant. And the improvements considerable. Now, compromising arc flash protection in favor of power supply reliability, or vice versa, is a thing of the past. The two are no longer mutually exclusive. And the vast potential of the IIoT promises even greater benefits moving forward. Resources A 1999 Electric Power Research Institute (EPRI) study of pegged total direct and indirect costs of an arc flash incident. GE Industrial Solutions. “Arc Flash: The Real Danger of Conducting Business” factsheet. BE Marty Trivette is the North American product marketing manager for GE’s Industrial Solutions business.
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