Both emergency generators and uninterrupted power supplies (UPS) are designed to back up a system when the original power source fails; however, there is a major distinction between the two. Generators installed at private corporations and public facilities can generate power for long periods of time and supply power to the facility. The only caveat is that it takes about one minute from the time of the outage to the time power is supplied. Also, unlike grid power supplied by the utility, the quality of power from generators is unreliable, characterized by unstable voltage and frequencies.
Emergency generators are suitable for long-term power outages, but they are not appropriate for handling the types of power outages like dips and interruptions nor the consumer-end disturbances like harmonics that we learned about in the previous sessions.
So now, what can a UPS do exactly? For starters, the energy storage device (i.e. the battery) in a UPS ensures that clean, high-quality power is supplied. For this reason, UPSs are an ideal countermeasure for a variety of power disturbances including dips, interruptions, and frequency variations.
|Generator||UPS (uninterruptible power supply)|
|Power source||Mechanical energy||Energy storage device (battery)|
|Time needed to power equipment after an outage||40 to 60 seconds||None (0 seconds)|
|Purpose||・Supply electricity||・Prevention of interruptions/dips
・Safe shut down of equipment
An energy storage device, in most cases a battery, is the source of electricity supplied by UPS. The type of battery used in the UPS has a dramatic effect on its functions and characteristics.
There are four types of energy storage devices: lead-acid batteries, nickel-hydrogen batteries, lithium-ion batteries, and electric double-layer capacitors. Lead-acid batteries and electric double-layer capacitors are the most common storage methods employed in UPSs.
|Lead-acid battery||Nickel-hydrogen battery||Lithium-ion battery||Electric double-layer capacitor|
|Backup time||Over 5 min||Over 5 min||Over 5 min||Over 5 min|
|Number of charge/discharge cycles per year||Under 20||Over 300||Over 800||Theoretically unlimited|
|Expected service life||3 to 5 years / 7 to 8 years||About 10 years||About 10 years||About 10 years|
|Energy density||Good||Very good||Very good||Good|
Their attractive price point has made these batteries a standard configuration in UPSs. Although they are useful when a relatively long backup time of more than 5 minutes is required, frequent charging/discharging shortens their service life.
Not only does cycle life and energy density exceed that of lead-acid batteries, but durability is not affected by frequent cycling. However, the initial high costs of these batteries see their employment in UPS fairly low.
This type of device is suitable for counteracting dips and interruptions thanks to its ability to store and release large bursts of power instantaneously. In addition, EDLC do not rely on chemical reactions for their energy discharge, unlike their battery counterparts. This means slower deterioration for a longer service life. The trade-off is that they are not suitable for storing large volumes of power.
The maintenance costs also vary depending on the type of storage device. Those that do not mind the initially higher capital investment in favor of reducing maintenance costs may prefer storage devices with a longer cycle life. UPS can respond to a variety of power problems depending on the combination of battery type and the UPS topology—that is, the power supply system and internal design of the UPS. Next, we’ll learn about and compare the different types of UPS topologies.