Consulting-Specifying Engineer - November 2007 - (Page 20)

At this point in the design, the emergency system is starting to take shape. We have clearly defined how long we may be without power, and what additional equipment, beyond code-required equipment, the owner would like to include in the emergency system. It is time to start the emergency system design. Design of the emergency system The third stage of the essential system design is to determine the emergency power source, elements required for the essential system, and fuel storage requirements. State and local AHJs have requirements. For instance, Florida requires a diesel generator with enough fuel for 64 h of operation with the generator loaded 100%. However, NFPA 99 only requires sufficient fuel to operate the emergency system for 90 min. This also brings us back to the first question: How long can the hospital operate without normal power? An anticipated outage time of a few hours would require a limited amount of on-site generation. If the anticipated outage is a day or more, the size and scope of the emergency power system must provide emergency power for even more of the facility. Some designs use UPS systems, but this solution is limited because it requires the normal service to recharge the batteries. A fuel truck can deliver more fuel just before fuel runs out, but UPS systems are limited. A UPS system should be used for equipment that cannot tolerate interruption of service. The UPS system should be backed by the emergency generator system. Typically, diesel generators are used for emergency systems. While you determine your loads in step 2, determine which loads are priority 1, non-shedable loads. These loads are required to have power restored in 10 s. What is the largest generator that can accomplish this? Engineers must consult with the engine manufacturers to determine the appropriate solution. The fuel source will affect the size; for Sometimes the project itself will determine the number of generators needed, but it’s not always so straightforward—engineers must use their judgment. instance, a 1,500-kW diesel generator will be able to restore power in 10 s, while a 1,500-kW propane generator may not. Sometimes the project itself will determine the number of generators needed. For example, a hospital in Florida has a parallel system with three 600-kW gensets, and space for a fourth 600-kW genset. The existing generators are operating at approximately 95% capacity, but the new loads to be added as part of an expansion are estimated at 1,000 kW. The new load is larger than the potential emergency system expansion, reducing the available options for the expansion. The design can either include new parallel switchgear rated for five 600-kW generators and two new 600-kW generators or a stand-alone generator unit rated for 1,000 kW. The cost-effective solution will be to provide a new stand-alone generator. The existing parallel switchgear has limited flexibility because the size of the future generator and future expansions are defined when the parallel switchgear is installed. It also is limited by the bus rating, or the space in the room that an expansion section would take up, or maybe the fault rating. In this example, the original design allowed the addition of a 600kW generator, but no greater. In order to increase this capacity, the parallel switchgear would have to be replaced. Usually it is not as straightforward as this example, so engineers must decide which direction to use based on their judgment. More often than not, the essential load will require the use of multiple generators in a parallel system. The parallel generator system is more reliable than a single generator system, allowing the use of multiple generators to provide emergency power. There are a few limitations to parallel generator systems, such as priority 1 loads and increased fault current levels. Priority 1 loads have to be on in 10 s, and therefore are limited in size. When an outage occurs, a signal is sent to all generators to start. The first generator to come up to full speed will be connected to the bus, then the remaining generators will sync to the first generator and connect to the bus. For priority 1 loads the first generator must be on in 10 s. The first generator on could be any of the generators, therefore the priority 1 loads are limited by the size of the smallest generator. Priority 1 loads will include life safety branch, critical branch, and some equipment branch loads such as generator equipment. Single or multiple gensets? The engineer must determine if the emergency power system should be from a single generator or from multiple generators paralleled together. The pros and cons for each strategy include the following: Single generator Pros Cost Less space required Simplicity Reduced maintenance Flexibility Multiple generators Pros Increased generation capacity Reliability Future growth Control schemes Tie breaker schemes Cons Cost Space Fault levels Limited flexibility Cons Single point of failure Future growth is limited 20 Consulting-Specifying Engineer • NOVEMBER 2007

Table of Contents Feed for the Digital Edition of Consulting-Specifying Engineer - November 2007

Consulting-Specifying Engineer Contents Editor's Viewpoint Letters In The News M/E Roundtable Emergency Power for Healthcare 2007 Products of the Year Industrial-strength Lighting Hazardous HVAC Codes and Standards Management Report Jobs/Classifieds Specifier's Notebook On-Peak Performance Contents Best Battery Selector in Engine Starting Applications NEC 708: Practical Impact on Backup Power Systems Protecting Power: Specifying Outdoor Generator Enclosures Investing in Backup Power Systems Coes, Consultants, Manufacturers, and Standby Power Systems

Consulting-Specifying Engineer - November 2007

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