Fonte (Source): Consulting – Specifying Engineer

Por (By): David Chesley, PE, LEED AP, RCDD, Interface Engineering, Portland, Ore.

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By using the building automation system (BAS) to its fullest extent, a school district was able to control its HVAC, electrical, and lighting systems.

Learning objectives

• Understand how to coordinate a building automation system so that facility managers can use it wisely.
• Learn how to monitor various building systems to achieve energy goals.
• Use the strengths of the BAS to optimally size the standby power system.

In planning for most buildings together with an owner’s maintenance staff, engineers find that facility managers enjoy the ability to use the building automation system (BAS) to view the operating status of the HVAC system and adjust setpoints with the click of the mouse. With the continuing improvements to the graphic interface of the BAS, facility managers increasingly want the monitoring and control of the electrical systems integrated into the BAS rather than displayed through stand-alone software that is proprietary to a particular electrical subsystem.

When designing for a new high school for the Oregon Trail School District, Interface Engineering leveraged the BAS to provide multiple benefits to the design, monitoring, and maintenance of the electrical system. These benefits started with the design for the backup generator, continued with master lighting controls of the campus for improved energy efficiency, included the monitoring of energy consumption and production on-site, and featured supervision of large electrical equipment such as building uninterruptible power supplies (UPS) and surge suppressors to alert when repairs and maintenance are required.

Right-sizing the generator

Sandy High School is located by the foothills of the Cascade Mountains, and so is subject to significantly more snow and freezing rain than nearby Portland, Ore. For this reason, winter outages are a more frequent occurrence. While the district wanted to provide emergency lighting and power backup to phones, fire alarm, and other mission critical systems, it also wanted the ability to provide backup power to the administrative wing, gymnasium, commons area, and kitchen, in the event of an outage that lasted for more than a few hours.

This design need by the district needed to be balanced with a fixed budget and limited space for the backup diesel generator. While the sum of connected load would have required at minimum one 2000 kW generator, two strategies were employed to allow the use of a smaller 1250 kW generator. First, the electrical service to which the generator is connected was divided into essential and non-essential branches.

Essential loads included lighting and power outlets in the administrative wing, gym, commons area, and kitchen, as well ventilation fans for the same areas; the central UPS for the district data center; pumps associated with the geo-exchange and boilers for heating; building elevators; and the kitchen freezers and coolers. These essential loads are in addition to emergency lighting, fire alarm, and communications, which are on the life safety branch covered by NFPA 70: National Electrical Code (NEC) Article 700.

Non-essential loads include the building multi-stage chiller, power for the theater and the counseling services wing, and the laundry room associated with the athletic department. The ability to use a master “load shed” signal to reduce power consumption in the event that the generator is more than 90% loaded allowed the 1250 kW generator to be used for a larger connected load, and in effect gave two steps to reduce the load below the targeted maximum: first to reduce the power draw of the building’s multi-stage chiller, and second to shed the non-essential branch altogether. The load shed signal originates from the generator control panel, which is tied to the BAS through a set of dry contacts.

Adding to these benefits the local utility, Portland General Electric, entered the school into its dispatching standby generation program, where it provides funding to the owner in exchange for the owner installing a paralleling switchgear and supervisory control and data acquisition system for the generator. These added features allow the utility to run the generator for up to 100 hours per year for power production at times of peak demand. By adding motorized breakers in the switchgear for the essential and non-essential branches, load can be shed when necessary.

It’s important to note that so far, peak load on the switchgear has remained under 40% of the connected load, which has translated to not needing to use the load shed feature, even during summer power outages. This is a lesson for future projects; while code necessitates designing the generator for connected load, the common occurrence that commercial and institutional buildings typically draw a peak load of 30% to 50% of design load gives room to use a load shed to power.