Even though more than 95 FFPOWER of all outages last less than five minutes, and nearly 99 percent are no longer than 20 minutes, the facility executive must consider the impact outages will have on the facility’s operation and occupants.
Moreover, facility executives may now expect on-site power generation systems to provide more than standby or emergency power. One example: reducing the cost of purchased electricity by performing such tasks as peak shaving.
Understanding the system
Selection of a system requires that facility executives understand system capabilities and the power requirements of the loads they are trying to meet. Three basic types of systems meet on-site power requirements: battery systems, generators and uninterruptible power supply (UPS) systems. Selection depends on the loads being served. In many cases, facility executives will use more than one power generation option. More complex facilities often use all three.
There are two types of loads to consider when looking to provide standby power: critical and essential. Critical loads must remain online to prevent major losses and avoid creating safety risks. Critical loads must be met even if the facility is not going to continue operating. Critical loads include fire alarms, exit lights, emergency lighting, security systems and select mechanical equipment. Most critical loads can withstand power interruptions of only a few seconds. Some, such as security systems, may require continuous power.
Essential loads must be powered if the facility is to continue to operate. Some essential loads, such as lighting systems, can be interrupted for several minutes, particularly if there is an emergency lighting system installed. Other loads, such as telecommunications systems and computers, cannot survive even a momentary outage without loss of data and a lengthy restart process.
Selection of a particular type of power generation system requires an understanding of the type of critical and essential loads that must be powered during an interruption of electrical service. Each type of system has its own capabilities and limitations.
Battery systems have been used to provide power to specific loads, such as telecommunications equipment and security systems. However, with the development of small, low-cost, self-contained UPS systems sized to handle these relatively small loads, the use of battery systems is limited to self-contained emergency lights.
Low in first costs and maintenance requirements, battery systems are popular where the main concern is providing sufficient lighting for people to exit the building during a power outage. Units in hallways, stairwells and other locations provide light for between 15 and 60 minutes — sufficient time to evacuate most facilities.
Most units are self-contained, including the light source, battery and charging system, in a single housing. When the unit senses a loss of power, the lamp is automatically switched to battery power and turns on. When power is restored, the unit turns the lamp off and recharges the battery. Maintenance is limited to testing operation of the unit, and periodically inspecting and testing the battery.
A drawback of battery-powered systems is that they provide power only for lighting or specialized systems. Powering other equipment requires one of the other two options.
Standby generators have long been the primary source of emergency power. Properly sized, they can serve nearly any load for as long as fuel is supplied.
Generators are available with a number of different driving engines, the most common being gasoline, natural gas and diesel.
Selection of a particular type of drive engine depends on the application, size of the load and availability of fuel.
Gasoline-driven generators are lowest in first cost and available in sizes from a few kilowatts to approximately 100 kW. The units are easy starting, particularly in cold climates.
Their main disadvantage is that gasoline has a relatively short tank life and requires special handling precautions.
Natural-gas-driven generators are slightly more expensive than gasoline units and available in sizes up to about 100 kW. Natural-gas-driven generators eliminate fuel storage problems, but the operation of the system depends on the natural-gas supplier.
Diesel generators have proven to be rugged and reliable. Units are available from a few kilowatts to more than two megawatts.
They are lower than gasoline and natural-gas units in first cost for sizes above 75 kW. Although diesel units do not have the fuel storage problems associated with gasoline, their fuel does require periodic testing and treatment. They are more difficult to start than gasoline or natural-gas generators, particularly during cold weather.
Regardless of the type of drive engine, units must be exercised under load if they are to perform during a power outage. Running the generator for a few minutes with no load connected does not fully test the generator, its cooling system or the transfer switch.
Reducing peak demand
Although standby and emergency power have long been the forces behind the use of generators, facility managers today are using the system to help cut the cost of purchased electricity. Most facilities are faced with demand charges as part of their electrical rate structure. By monitoring the electrical demand, managers can see when they are approaching a peak for the month. If the generators are brought online before the peak is reached, the demand charges can be significantly reduced.
The savings from using emergency generators to reduce peak demand are sufficient to pay for the monitoring and automatic starting equipment in less than one season.
The growth of the use of electronic equipment that requires a clean, stable source of electricity has resulted in the development of uninterruptible power supplies (UPS). The systems are designed to clean up incoming power and provide a continuous, stable source of electricity with the building’s service. Units can power loads from individual PCs to entire mainframes.
Although several designs are used in constructing UPS systems, the most common is the online system. Here, electricity from the utility is filtered and converted to direct current, then inverted back to alternating current.
A bank of batteries is connected to the direct current portion of the system. Power transients, voltage spikes, voltage dips and system harmonics are filtered out by the conversion/inversion process so the connected load is protected. In the event of a loss of power, the batteries automatically supply power to the inverter, thus preventing an interruption of service to the connected load. Typically, the batteries are sized to provide between 15 and 30 minutes of service. If additional time is required, a generator can be connected to the system on the incoming line side of the converter.