Marcelo E. Valdes

Zone Based Protection for Low Voltage Systems; Zone Selective Interlocking, Bus Differential and the Single Processor Concept

Time based coordination and protection is the normal basis for coordinating low voltage power distribution systems. Enhancements, such as zone selective interlocking and bus differential protection, may be used to accelerate protective devices. However, these improvements may be costly, difficult to implement, and may not function as expected using commonly available technology. Nevertheless, the potential benefit of fault clearing speed and selectivity are more valued in todaypsilas arc-flash and reliability conscious environment than ever before. The writers shall discuss some issues associated with these traditional improvements, pitfalls to avoid and, more effective ways to implement zone-based protection to achieve fast fault protection while maintaining complete selectivity for a broad range of fault magnitudes, system configurations and load types.

Arc-flash energy mitigation by fast energy capture

The predominant technologies for reducing arc-flash incident energy today rely on the speed of protective devices, remote operation, encapsulating arc-flash energy in arc-resistant enclosures, which channel energy where it is less dangerous, and on crowbars to divert the arc energy into a bolted fault. Though more thoughtfully applied than they may have been in the past, none of these methods has provided a solution for all situations, particularly in existing installations. This paper will describe a method and include test results of an arc-flash energy sequestration that is able to divert an arcing faults energy into a specific environment within a half-cycle after initiation of the arc. This is achieved without the need to introduce bolted fault current like a crowbar or for fast current interruption, such as a current-limiting fuse. The system protection provided has the benefits of arc-resistant switchgear without reliance on equipment sheathing and can be added after normal equipment is installed. Further advantages include the protection system and switchgears ability for reuse after an arcing event, as well as the ability to easily test without the need for cumbersome high-current test equipment.

Optimized Instantaneous Protection Settings: Improving Selectivity and Arc-Flash Protection

In todays power distribution systems, the protection methods do not provide full selectivity and instantaneous fault clearing for expected fault current, including lower-magnitude arcing currents simultaneously. This article discusses two methods that may change that. The first method is a selectivity analytical technique useful with many circuit breaker (CB) trips currently available in the industry. The second method is a new CB trip technology. Both methods allow switch gear CBs to use sensitive instantaneous settings and maintain selectivity when used in upstream current-limiting (CL) molded case CBs (MCCBs), CL motor circuit protectors (MCPs), or CL fuses in downstream equipment such as motor control centers (MCCs).

Micro-Electromechanical-System (MEMS) based switches for power applications

A new system for switching electrical power using Micro-Electromechanical-Systems (MEMS) is presented. The heart of the system utilizes custom designed MEMS switching device arrays that are able to conduct current more efficiently and can open orders of magnitude faster than traditional macroscopic mechanical relays. Up to now, MEMS switches have been recognized for their ability to switch very quickly due to their low mass, but have only been used to carry and switch very low currents at extremely low voltage. However, recent developments have enabled suppression of the arc that normally occurs when the MEMS switch is opened while current is flowing. The combination of the arc suppression with the MEMS switch arrays designed for this purpose enables a breakthrough increase in current and voltage handling capability. The resultant technology has been scaled to handle many Amps of current and switch 100s of volts. Such current and voltage handling capability delivers improved energy efficiency and the capacity to handle fault current levels that are encountered in typical AC or DC power systems. Fault current interruption takes place in less than 10 microseconds, almost regardless of the prospective fault current magnitude. The properties of the MEMS switch arrays allow the switching mechanism to operate at temperatures in excess of 200 deg. C. The switches also have a vibration tolerance in excess of 1000G. The combination of fast MEMS switching speed, optimized current and voltage handling capacity of the switch arrays, the arc suppression circuitry and optimized sensing and control enable a single sensing, control and switching system to operate in a small fraction of a millisecond. This paper will present the basic physics of the MEMS switches together with recent advances that enable the technology. Some illustrative examples of the ways the devices may be used to provide protection and control within electrical systems will also be presented.

The single-processor concept for protection and control of circuit breakers in low-voltage switchgear

Protection of low-voltage circuits has been a key area of research and continuous improvement since the invention of electricity. In the current state of the art, fuses and circuit breakers protect individual circuits. These devices, in turn, use many different kinds of trip mechanisms, operating in various modes that require sensing the current that travels through the circuits. Communication networks and external relays are sometimes used to enhance selectivity and provide better protection. In this paper, the authors explore a protection-and-control architecture based on a single processor that provides all the protection and control functions for a lineup of low-voltage switchgear. We demonstrate how this method of protection changes the paradigm from individual circuit protection to system protection, while cost-effectively and significantly increasing system reliability and protection.

Methods for arc-flash detection in electrical equipment

Arc-flash protection systems have evolved over the past decade with a range of solutions from arc-resistant gear to circuit breaker temporary instantaneous settings. One technology that has gained acceptance, especially in medium voltage applications is light sensing as applied in arc-flash relays. Recently, arc flash relays have moved into low voltage applications. While light based arc flash relays work well in medium voltage applications, low voltage applications pose reliability challenges. Additionally, in all applications it is desirable to minimize wiring and installation complexity. The authors will describe arc flash detection research and development to address some of these concerns.

Optimizing circuit breaker instantaneous trip settings for selectivity and arc flash performance simultaneously

This paper and presentation will discus analytical techniques and new technology that allow switchgear CBs to use sensitive instantaneous settings and maintain selectivity when using traditional electronic trips in switchgear and current limiting molded case circuit breakers, motor circuit protectors or fuses in downstream equipment. New developments in trip technology that allow systems designed with instantaneous protection while maintaining high levels of selectivity and arc flash protection are also discussed.

Predicting Let-Through Arc-Flash Energy for Current-Limiting Circuit Breakers

IEEE 1584, “Guide For Performing Arc Flash Incident Energy Calculations,” provides a method to conservatively predict the incident energy let-through by over-current devices based on the devices time-current-curve or tested transfer functions. Additional testing by various manufacturers shows that current-limiting circuit breakers operating in their instantaneous current-limiting range perform significantly better than the IEEE Time-Current Curve-based model predicts. The authors will present an analytical method to estimate incident energy from published circuit breaker let-through curves. The method will provide a conservative, yet better estimate of the actual performance compared to test data. This method can be used as part of hazard risk planning rather than the method described in IEEE 1584 or when the manufacturer does not provide an incident-energy transfer function based on tests.

Method for determining selective capability of current-limiting overcurrent devices using peak-let-through current

Time-Current-Curves are the accepted industry standard for predicting overcurrent device operation and analyzing selective behavior under overload or fault conditions. A conservative interpretation of drawn curves has sufficed for many years and provided acceptable performance. However, recent emphasis on better selectivity while still trying to provide optimal protection increases the demand for more accurate selectivity predictions. Currently, manufacturers are publishing tables and other guidelines to facilitate the selection of optimally coordinated devices by systems designers. However, no standard or single methodology exists for the creation of these tables. The writers shall present three methods of device interaction analysis based on peak let-through current, suitable for predicting selective behavior of protective devices above what time current curves may indicate. The method may be applied with published information or manufacturers internal test information.

Method for determining selective capability of current-limiting overcurrent devices using peak-let-through current “What traditional time current curves will not tell you”

Time-Current-Curves are the accepted industry standard for predicting overcurrent device operation and analyzing selective behavior under overload or fault conditions. A conservative interpretation of drawn curves has sufficed for many years and provided acceptable performance. However, recent emphasis on better selectivity while still trying to provide optimal protection increases the demand for more accurate selectivity predictions. Currently, manufacturers are publishing tables and other guidelines to facilitate the selection of optimally coordinated devices by systems designers. However, no standard or single methodology exists for the creation of these tables. The writers shall present three methods of device interaction analysis based on peak let-through current, suitable for predicting selective behavior of protective devices above what time current curves may indicate. The method may be applied with published information or manufacturers internal test information.