Designing electrical systems for higher education

By accommodating diverse functional requirements while following safety codes and standards, engineers can design reliable and durable electrical systems for colleges and universities and their high-performing buildings.

06/10/2013


Figure 1: The main reading room of North Carolina State University’s new James B. Hunt Jr. library features the use of energy efficient design strategies such as integrating daylighting and lighting aesthetics. Inset: The James B. Hunt Jr. library is theCollege and university campuses depend on reliable, readily changeable, and easily maintainable electrical system networks to fulfill their academic and research missions. Regardless of cause, power disturbances can compromise and even invalidate scientific investigations, as well as disrupt and inconvenience the routine classroom functions of an institution.

To design such systems, the electrical professional must weigh a series of diverse immediate and long-term functional requirements—in addition to safety codes and standards—to design a reliable and durable electrical system for the entire campus and its individual exceptional high-performing buildings. Indeed, thorough consideration of infrastructure, reliability, backup systems, metering, changeability, and maintainability illustrates the extent to which the complex design requirements of higher education facilities exceed code minimum guidelines. 

Infrastructure and reliability

The electrical network infrastructure supplying the higher education campus must provide reliable and safe power to its individual components. To do so, the incoming electric utility service must be distributed in such a way as to facilitate restoration of power during an outage in a safe and expeditious fashion. Table 1: This table compares the reliability of a simple radial system, a primary selective system with manual throw over, and a primary selective system with automatic selective throw over. Courtesy: Affiliated Engineers Inc.; source: IEEE 493-2007, TablWhen requesting utility services, facility owners must weigh various factors in selecting the most suitable electric service to bring into the campus. Solutions vary in terms of geography. Campuses at the heart of large cities can rely on the available utility network to serve their buildings directly, while a more remote campus may have to manage its electrical infrastructure and distribute its own services internally. For the latter, the challenge lies in selecting the proper utility services and determining how to most effectively distribute them. 

To analyze the reliability of the electric utility service, which, according to IEEE, is the largest contributor to both the failure rate and the forced-hours downtime per year at the 480 V point of use, engineers should refer to IEEE Standard 493-2007: Recommended Practice for the Design of Reliable Industrial and Commercial Power Systems. This standard provides valuable examples that prove useful in determining the reliability of single- and dual-utility sources, and in comparing various campus distribution techniques. The examples given conclude that increased reliability is obtained with a dual utility source arranged in a primary selective configuration. 

IEEE 493-2007 compares the reliability of a simple radial system (one source), a primary selective system with manual throw over (9 min switchover), and a primary selective system with automatic selective throw over (5 sec switchover) (see Table 1). Figure 2: This diagram illustrates two primary selective electrical systems with secondary radial distribution. Courtesy: Affiliated Engineers Inc.With automatic throw-over equipment at the primary switches, the number of failures per year is reduced by a factor of 6. IEEE 493-2007, Section 3.3.5.3 states, “The use of automatic transfer equipment that could sense a failure of one 13.8-kV utility supply, and switch over to the second supply in less than 5 sec would give a 6-to-1 improvement in the failure rate at the 480 V point of use.”

Based on these data, university campuses should request two utility sources and establish a primary selective system with automatic throw-over equipment for increased reliability at the 480 V building point of use (see Figure 2). 

The transformer in secondary distribution systems is a very reliable component (at a low failure rate λ of 0.0062) but possesses the second highest outage time after the utility company (at 132 hr, resulting in larger forced-hours downtime/yr, λr). This implies that while the transformer is quite reliable, means must be accounted for to deal with the long outage experienced to replace it where high overall system availability is required. A secondary selective system using double-ended substations allows additional protection for transformer failures or maintenance (see Figure 3). 

However, not all buildings on campus may necessitate this additional level of reliability or costs associated with the double-ended substation concept. Figure 3: This diagram illustrates a secondary selective system consisting of multiple double-ended substations fed by a two-source primary utility distribution system. Courtesy: Affiliated Engineers Inc.It then becomes a programmatic decision to select which buildings on campus are classified as “vital” in operability and the costs associated with the additional redundant components can be used to weigh its benefits, such as for computerized data centers and research buildings. 

An alternative to the double-ended substation concept implemented at the recently completed Wisconsin Institutes for Discovery building at the University of Wisconsin is the sparing transformer system (see Figure 4). Having established that transformers are highly reliable and rarely fail, the outage time reduced by the double-ended substation may be replicated by installing a spare transformer, which is essentially interconnected initially with a main circuit breaker and several interlocked tie circuit breakers. A true double-ended redundant system implies that transformers are sized at 50% of their ratings. An advantage is that in this sparing scheme, each transformer carries only its own load (single-ended), and transformer kVA ratings can effectively match building load and would not need to rely on fan ratings for the transfer events. This sparing system also has the advantage of a reduced footprint compared to double-ended substations. 

At a Midwestern university, a primary selective system composed of a main utility line, with a secondary reserve line, is distributed to buildings throughout the campus, creating a loop system (see Figure 5). The sectionalizing switches are used to create the main outer loop that is open at one location in campus to allow for the utility service to come from two directions. The sectionalizing switches are used to isolate any feeder faults at this level. These switches are then used to create a second inner loop, which interconnects sections of campus in terms of geography via pad-mounted transformers with integral oil-immersed sectionalizing loop switches and draw-out current-limiting fuses. This system allows for any section of the loop system to be isolated, and also for the removal of any local transformer from the loop without disruption to other buildings. Figure 5 shows radial secondary systems at each building (single-ended), but increased reliability can be added by introducing double-ended equipment at the “vital” facilities on campus. This system is very common and adequate for a large university campus. 


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