Power System Studies
Understanding the importance of a short circuit protection and coordination study and an arc flash hazard assessment.
A typical power distribution system for a large facility or campus is comprised of multiple distribution voltages and corresponding equipment. An incoming electrical service is provided by the local utility, which may consist of one or more utility circuits, in either a split-bus arrangement (the facility load is shared between circuits) or a duty/standby arrangement (one circuit carries the entire load, under normal operating scenarios). The incoming electrical service is usually at a medium voltage, which ranges between 600V-69,000V (common voltages include: 4.16kV, 12.47kV and 13.8kV). The incoming service voltage can be stepped down to a lower medium voltage or it can be distributed around the facility to electrical service spaces. The medium voltage will subsequently stepped down to a utilization voltage – 600V or 480V for motor loads or equipment and 208/120V for receptacles and lighting. At each distribution voltage, major electrical equipment will include: switchgear/switchboards, feeders, transformers, distribution panels and lighting/receptacle panels.
Tasked with managing these electrical assets, facility managers should ask the following important questions. How will my electrical power system operate during abnormal operating conditions, such as a short-circuit event? Is equipment properly rated to prevent damage and failure during a short-circuit event? What level of personal protective equipment should operators wear, when performing routine switching operations or maintenance on electrical distribution equipment? Two important power system studies can provide answers to these questions, along with other essential information: a short-circuit protection and coordination study and an arc flash
At each voltage level in a power distribution system, protective devices, including fuses, circuit breakers and protective relays, are used to protect electrical distribution equipment and the loads served. Fundamental protection consists of protection from overload scenarios, where too many amps are drawn by loads and overheating becomes an issue, and protection from instantaneous overcurrent scenarios, where large magnitude currents can damage equipment in a fraction of a second (a short-circuit event). Protective devices have to be adequately rated for both scenarios.
In the event of a short-circuit or other abnormal event, a large magnitude fault current will flow through multiple protective devices and levels of distribution, before it reaches the point of failure. The flow of current in the faulted circuit will be interrupted by the melting of a fuse or the opening of a circuit breaker. In an ideal situation, the upstream protective device closest to the point of failure will open before a higher-level protective device opens. For example, a fault in a motor should trip the circuit breaker supplying the motor, without impacting the main breaker for the entire facility. When this occurs, protective devices are said to coordinate, power interruptions are localized and disruption to the rest of the facility is minimized.
A short-circuit protection and coordination study provides a complete evaluation of a power distribution system to ensure all protective devices are rated for the available fault level (at a particular voltage) and adequately protect downstream equipment. As part of the study, time current curves (TCC), which plot the interrupting time of an overcurrent device based on a given current level, are produced. TCC plots provide a graphical illustration of the coordination between multiple protective devices at an available fault level. In the event that devices do not coordinate, adjustable protection settings may be revised or devices may be replaced, to provide an optimal level of protection and coordination.
An arc flash hazard assessment takes information produced in a short-circuit protection and coordination study and produces a safety analysis for those who will be working on an electrical power system. The primary threat to electrical workers is the risk of an arcing ground fault and the associated blast. An arcing ground fault can cause thermal burn injuries and physical trauma, due to the force of the blast and flying projectiles, which may consist of partially melted components. Key elements to the assessment include: short-circuit levels at various points in the distribution system, the clearing time associated with upstream protective devices, the distance between the worker standing in front of the equipment to the arc source within the equipment, the incident energy available (cal/cm2) and the flash protection boundary. Once the incident energy available is calculated, the appropriate level of personal protective equipment can be identified.
The flash protection boundary will identify the minimum distance from live parts, that are uninsulated or exposed, within which a person could receive a second-degree burn. While one might expect higher short circuit levels to be associated with higher levels of incident energy, this is often not the case. Lower short circuit currents can often cause an arc to burn longer, before a protective device is tripped, resulting in a higher level of available incident energy. Time delays on protective devices may be increased to provide better levels of coordination, however this may also increase incident energy levels. Consideration should be given to both the coordination of protective devices and mitigation strategies for arc flash hazards. Temporary settings (maintenance settings) can be used to reduce incident energy levels, during routine maintenance and work on electrical systems.
Short-circuit protection and coordination studies and arc flash hazard analyses are typically performed with the use of industry standard power system software and computer modelling. A detailed model of a power system is created and information on the power system, including: the incoming utility service, equipment ratings, protective devices and settings, feeder lengths, transformer sizes and motor sizes are inputted. Information is typically collected from the facility’s electrical single line diagrams, electrical drawings with the location of equipment in plan, record shop drawings from construction and data gathering from site surveys. Software programs will have a large database of protective devices, with user-defined protective settings when adjustable. This will allow the modeler to select appropriate settings or suggest alternative protective devices, to achieve better levels of device coordination. Once a model is complete, a multitude of deliverables can be produced, such as reports, TCC plots, graphical representations of a various operating scenarios, arc flash labels and information on PPE requirements. Correct information must be inputted into the power system model, to ensure automated calculations and results are accurate.
Most new construction projects and projects that involve significant modifications to electrical equipment will include the requirements for power system studies in the project specifications. This will ensure that an electrical installation is optimized, properly integrated with any existing power distribution equipment and operators have the necessary information to operate new equipment. While new projects provide the opportunity for updated studies, many facility managers inherit complex power distribution systems, which have undergone a multitude of upgrades and modifications over the years, with minimal updates to record documentation.
Upper Canada College faced these challenges when they undertook a project to update record documentation on their power system, complete with an updated short-circuit protection and coordination study and arc flash hazard analysis.
Founded in 1829, Upper Canada College (UCC) is one of Canada’s leading independent schools and is located on a 16-hectare (40-acre) campus in midtown Toronto. The campus is home to a number of academic buildings, student and staff residences and facilities. The campus receives an incoming utility service at 13.8kV and distributes power to a number of campus buildings, via a 13.8kV distribution network. Major buildings have individual main electrical rooms, where the incoming medium voltage circuit is transformed down to 600/347V and 208/120V. Low voltage distribution systems provide power to building mechanical systems, lighting, equipment and academic facilities. Power distribution systems had been modified over the years along with campus re-development and renovations in various buildings.
Chris Martins, Senior Operations Manager, Angus Consulting Management Ltd. (UCC’s Facilities Management Group) said, “We recognized the need to update record information on electrical power systems throughout the campus and this provided an excellent opportunity to complete an updated coordination study and arc flash hazard analysis.”
C2C Enertec Inc. was selected to complete the electrical audit and provide updated power system studies. Detailed site investigation work was completed over the span of several months. As-built drawings and building records, spanning several decades, were reviewed in detail. Electrical equipment, protective devices and existing settings were reviewed on site and catalogued. Site work was completed after hours, to avoid disruption to building occupants. Updated electrical single line diagrams were created and information collected on site was used to produce a detailed power system model of UCC’s electrical power systems. A short-circuit study was completed and time current curves were produced for the power distribution system. An arc flash hazard analysis was completed and a report detailing arc flash hazard levels, along with recommended personal protective equipment, was produced.
Information was consolidated in a detailed report for UCC’s operations group, arc flash labels were installed on electrical equipment throughout the campus and updated electrical single line diagrams were mounted on walls, in main electrical rooms. The updated power system studies and electrical records provide operations staff with new insight into how their power distribution system can be expected to operate, along safety requirements when working on equipment.
Steve Thuringer, Executive Director of Facilities, UCC said, “UCC’s electrical power distribution system is an essential part of campus operations. Having updated record information will go a long way in helping our staff with future maintenance work and renovation projects.”
In today’s world of integrated systems, the requirements of a reliable power supply and the need for workplace safety are an integral part of facility management. It is recommended that every facility consider having an up to date arc flash hazard assessment and short circuit protection and coordination study, for its electrical power systems. These two important power system studies help ensure equipment is properly protected, can minimize the impact of an unexpected short-circuit event and promote operator safety when working with electrical equipment. By developing detailed requirements for technical experience and deliverables, such as compliance with industry standards and the associated methods for creating power system models, a facility manager can help ensure that their service provider produces meaningful results.
As demonstrated by the successful project at Upper Canada College, undertaking power system studies provide significant insight into an existing power distribution system, which has been modified and upgraded over time.
Michael Holdsworth, C.Tech., is a senior technical manager at C2C Enertec Inc. and was the project lead for the electrical audit and power system studies at Upper Canada College. Mike has significant experience with the operation and maintenance of electrical power systems. He can be reached at firstname.lastname@example.org.
Philip Chow, P.Eng., P.E., is a senior project manager and electrical engineer specializing in electrical infrastructure projects and construction in mission critical facilities. He can be reached at Philip.Chow@hhangus.com.