The Association of Electrical Equipment and Medical Imaging Manufacturers
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Backup Power Systems


Onsite backup power provides a reliable and cost-effective way to mitigate the risk of economic loss and societal hardship from power outages. Many businesses suffer economic losses due to disruptions of electric power supply during a natural disaster. For businesses with highly sensitive loads such as data centers and financial institutions, the risk of economic losses from downtime is high. For many facilities, such as assisted living facilities and nursing homes, there is a life safety aspect to consider. Other facilities, such as cell tower sites, emergency call centers, and gas stations, have far-reaching social impact and their availability is critical. Investment in onsite backup power equipment can ensure reliability, safety, and productivity.

Backup Power Systems: Brief Overview

Onsite backup systems use local generation at the facility site to provide power when the utility is not available. The backup power system may or not be interconnected with the utility grid. Onsite electrical power generating systems are readily available in a wide variety of designs for specific uses and customer applications. This type of power system consists of a power source and a means to transfer power from that source to the load when an outage occurs. Remote monitoring and control systems that allow an operator to check the system status and operate the system remotely are becoming more commonplace. The generator’s primary fuel source can be natural gas (NG), propane, or diesel.

Fuel selection : The selection of NG, diesel, or liquefied petroleum gas (LPG) should be made based on the application’s characteristics and requirements. Considerations for choosing among the different types include:

  • equipment costs (initial and installation)
  • fuel costs
  • fuel availability
  • equipment start up time

Design Considerations for Diesel Onsite Generator Systems

Designing a generator set installation requires consideration of equipment and application requirements. These vary depending on the reasons for having the generator set and used. Reviewing and understanding these reasons is an appropriate starting point for system design and equipment choices. No single solution meets all needs. Before configuring a system, facility managers should consider the intended use of the generator set and a number of other factors, as follows:

General requirements: Consider code requirements for emergency power and voluntary installations of standby power to mitigate the risk of loss of services, data, or other valuable assets. One system may be used for both of these general needs provided that life safety needs have priority.

Load-specific requirements: A wide range of specific requirements will result in the need for onsite electric generation systems which tend to vary by application type. Some common installations are:

  • Healthcare: Standby power is required for all life-safety systems which include evacuation/egress lighting, HVAC systems for patient care and operating rooms, critical process equipment such as medical imaging devices, and fire suppression equipment to aid response teams in the event of an emergency.
  • Data centers: The servers housed in data centers drive our economies and the financial health of businesses and households. Without a backup power system for these loads, the loss of data could cause a global catastrophe. Apart from the data held in these facilities, the cooling equipment required to maintain their operation must be kept online in order for the digital equipment to work properly.
  • Communications: From cellular towers to 911 call centers, an efficient emergency response requires communication. Without power to transmitters and receivers, the storm recovery process is significantly delayed.
  • Commercial/Residential properties: Ambient lighting, temperature control, and the computers that most of us rely on every day need sustained power in order to operate. Without electricity, even the most mundane tasks become points of concern, from sending an email to keeping food cold in the refrigerator.

Location: One of the first design decisions will be to determine whether the location of the generator set will be inside a building or outside in a shelter or housing. The overall cost and ease of installation of the power system depend upon the layout and physical location of all elements of the system—generator set, fuel tanks, ventilation ducts and louvers, accessories, etc. For indoor and outdoor locations, key considerations include:

  • generator set mounting
  • noise and emissions regulations
  • location of distribution switchboard and transfer switches
  • containment of accidentally spilled or leaked fuel and coolant
  • service access for general maintenance and inspections
  • access and working space for overhauls or component removal/replacement
  • access for load bank testing when required for maintenance or scheduled exercise

It is critical to recognize and take into account all these factors while designing the system and think through possible disruptions of an emergency event or natural disaster. The systems components need to be designed for security from flooding, fire, icing, wind, and snow. For example, during Superstorm Sandy, some facilities experienced disruption in backup power because diesel fuel pumps flooded. This could have been avoided by placing the pumps in a different location.

Generator ratings: Onsite power generation systems can be classified by type and generating equipment rating. The generating equipment is rated using standby, prime, and continuous ratings. The ratings definitions are important to understand when applying the equipment and depend on the intended use of the equipment. Power ratings for diesel generator sets are published by the manufacturers in accordance with ISO 8528. These ratings describe maximum allowable loading conditions on a generator set.

It is important to operate generator sets according to published ratings and at a sufficient minimum load to achieve normal temperatures and properly burn fuel.

Environmental considerations: The most critical environmental issues are those related to noise, exhaust emissions, and fuel storage. Emissions are a complex topic and should be taken into consideration at the early stages of backup power decision making. The Environmental Protection Agency (EPA) defines “stationary emergency applications” as those in which the generator set operates only during periods of an outage of the normal utility power supply (with the exception of limited-duration operation for testing and maintenance). All other uses, such as prime power, rate curtailment, and storm avoidance, constitute non-emergency use. While the EPA does not impose a limit on the number of hours that a generator may operate in emergency situations, the EPA does limit operators to 100 hours per year.

Maintenance and Readiness Recommendations

Preventive maintenance for diesel engine generators plays a critical role in maximizing reliability, minimizing repairs, and reducing long-term costs. Because of the durability of diesel engines, most maintenance is preventive in nature. By following generally recognized generator maintenance procedures and specific manufacturer recommendations for the application, facilities will be assured that the backup power system will start and run when needed most. It is generally a good idea to establish and adhere to a schedule of maintenance and service based on the specific power application and the severity of the environment. The following areas should be inspected frequently to maintain safe and reliable operation:

  • exhaust system
  • fuel system
  • dc electrical system
  • engine

Lack of adherence to a preventative maintenance schedule is one of the leading causes of failure of a backup power system. When preparing for an emergency, one should pay particular attention to the starting batteries. Weak or undercharged starting batteries are the most common cause of standby power system failures. Even when kept fully charged and maintained, lead-acid starting batteries are subject to deterioration over time and must be periodically replaced when they no longer hold a proper charge. Only a regular schedule of inspection and testing under load can prevent generator starting problems. Merely checking the output voltage of the batteries is not indicative of their ability to deliver adequate starting power. As batteries age, their internal resistance to current flow increases, and the only accurate measure of terminal voltage must be done under load.

Generator sets on standby must be able to go from a cold start to fully operational in a matter of seconds. This can impose a severe burden on engine parts; however, regular exercising keeps engine parts lubricated, prevents oxidation of electrical contacts, uses up fuel before it deteriorates, and helps provide reliable engine starting. Periods of no-load operation should be held to a minimum because unburned fuel tends to accumulate in the exhaust system.

Roadmap Recommendations

To ensure continuity of critical services and protect crucial facilities from power outages, facility owners, and operators should follow these recommendations:

  • Evaluate and mitigate the risk : Identifying the facility's critical loads is an important first step. Understand the social risks and costs of a facility shutdown and invest accordingly in a backup power system or make arrangements for temporary rental power.
  • Design for emergencies : Work with a power generation firm that can help you understand what your backup power needs would be to ensure optimal selection of a backup power system. Depending on needs, develop a plan that includes a rental agreement with that company before or after a disaster.
  • Ensure sufficient fuel storage and supply : Have emergency generator fuel on hand to allow at least 48 hours of operation, or as required by code (for example, some healthcare facilities require 96 hours), and develop contracts with fuel operators for restocking.
  • Ensure scheduled exercise and maintenance : Generators should be exercised periodically to ensure they will operate as designed in the event of an emergency. Preventive maintenance plays a critical role in maximizing reliability, minimizing repairs, and reducing long-term costs. Follow generally recognized diesel maintenance procedures and specific manufacturer recommendations for your application.
  • Ensure trained personnel : Staff need to be trained to maintain and operate the generator unit and should be ready for deployment.