The design of electric power systems in generating stations requires the consideration of many factors. One recent area of attention is mitigating arc-flash hazards with updated design philosophies. Protection systems have traditionally been designed with an undesirable compromise between safety and reliability. In this arrangement, inherent time delays in relay protection allow for a considerable amount of energy to be available at distribution equipment during electrical faults. This can make short-circuit and arc-flash incidents exceptionally dangerous to operators and service personnel.
Arc-flash hazards have been a point of focus for several decades. A significant amount of research provides methods of accurately calculating available arc-flash incident energy. These calculations have paved the way for new technologies to reduce energy levels. Additionally, the information has been used to label equipment with energy levels, helping workers choose the appropriate personal protective equipment (PPE) for the job.
More recently, improvements in the understanding of arc-flash hazards and advances in technology are allowing power system designers to significantly improve the safety of arc flash-susceptible electrical equipment without reducing reliability. This has made it possible to take advantage of these improvements by implementing several design philosophies and methods of mitigating arc-flash risk during the design phase of power station development.
Example Methods of Mitigation
Understanding the factors that affect arc-flash incident energy is critical when designing a new system. Decisions need to be made on equipment ratings, current transformer locations and protective device models. In any station design, careful consideration must be made in choosing the best arc-flash mitigating methods and technologies to use for each piece of equipment. Then, an electrical system model can be developed to create protective device settings and identify arc-flash energy levels.
Several design options can be used in new electric power systems to reduce arc-flash risks. A sampling of those technologies:
- Arc-Resistant Switchgear
Arc-resistant switchgear can be selected for all medium- and low-voltage switchgear buses. Keep in mind, this can add approximately 5% to 20% cost to equipment and limit available vendors, but those impacts are minimal considering the significant improvement in safety.
- Remote Control and Remote Racking
All medium- and low-voltage switchgear breakers and contactors, as well as low-voltage motor control center (MCC) contactors, can be designed to be controlled remotely from the distributed control systems (DCS). In this way, local operation is only allowed with breakers in test position. Remote racking also can be incorporated into the switchgear designs to allow breakers to be racked in or out from a safe distance.
- Protective Relay Coordination
Emphasis should be placed on calculating and programming protective relays to be coordinated as efficiently as possible. Software can assist in this process. The biggest challenge: time delays that are introduced as the coordination progresses up the hierarchy of the system.
- Maintenance Mode
To reduce the available incident energy on the line side of the 480-V main breakers, maintenance mode instantaneous phase overcurrent elements can be enabled in the upstream medium-voltage feeder breakers to provide fast clearing in the event of a fault. The criterion for setting this element is to reduce the available incident energy without consideration of protective device coordination.
- Differential Protection
High-impedance differential protective relays can be incorporated into all medium-voltage switchgear buses. A dedicated current transformer can be installed on all breakers and contactors on the side opposite the bus and wired into a protective relay. This provides instantaneous protection for a fault at the switchgear and allows for overlap between the differential zones to provide full protection at all points on the main breakers and incoming cables.
- Zone Selective Interlock
A zone selective interlock (ZSI) scheme can be incorporated in all low-voltage switchgear buses, extending from the main breakers to the feeder breakers and down to MCC main breakers. This allows the low-voltage switchgear to have significantly reduced clearing times from the coordinated protective curves.
- Light Detection
Schemes can be implemented using specialized relays and optical fibers to detect light created from an arc flash. This allows for very fast relay operation to clear an arc-flash event. Light detection schemes are often supervised by signals from current transformers to avoid nuisance operations.
- High Resistance Grounding
Systems within a certain voltage rating can be designed as high resistance grounded. In this configuration, a high impedance grounding resistor is connected to the neutral point of all station service transformers with a certain rating which limits the ground fault current to less than 10 amps.
- Location of Breakers
Main breakers are beneficial to be included in the design of all medium- and low-voltage switchgear, as well as low-voltage MCCs. Main breakers on low-voltage MCCs are often not desired because they add a level of coordination to the protective curves. However, this can allow the ZSI scheme to be extended down to the MCCs and improve the trip times for this equipment.
- Generator Circuit Breakers
Low-side generator circuit breakers can be included in the design for gas turbine generators. As discussed in previous sections, this allows for generator fault current contributions to be eliminated quickly for faults between the generators and the medium-voltage switchgear main breakers. Without these breakers, the effectiveness of high-speed differential protection is limited.
- Earthing Switch
An arc can be quickly eliminated if the voltage across the air gap is reduced to zero by intentionally introducing a bolted three-phase ground fault. Many manufacturers have developed high-speed earthing switches for the purpose of reducing arc-flash incident energy by using this design concept.
These arc-flash risk mitigation techniques are not the only solutions available, but they can provide great reductions in the incident energy available at equipment in electric power systems. These and other reductions can be achieved with small changes to the system design without compromising the reliability and proper coordination of the system.
Even with these design considerations, all operations and maintenance personnel must understand the risks associated with arc-flash hazards and be trained to use the equipment features properly and wear appropriate PPE for the task. By mitigating risks through aggressive design and proper training, injuries related to arc-flash incidents can be greatly reduced during the operating life of an electric power system.
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