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An uninterruptible power supply is the linchpin of the data center backup chain, providing vital protection against power disruptions that would interfere with workloads or even cripple server hardware. Selecting the right uninterruptible power supply (UPS) for deployment can be a tricky proposition. One size does not fit all, and the UPS unit’s characteristics and performance must meet the demands of modern computing equipment. Management isn’t as simple as just matching the UPS to a load or achieving an acceptable runtime.
Today’s UPS systems offer a wealth of maintenance and management features that ensure long, reliable operation. UPS choices are vast, with a bewildering spectrum of features to choose from, but a UPS must deliver adequate power to run the load long enough to restore utility power, also called runtime, or to fully shut down. Load power is Hp PB991A battery measured in watts (W) or, more accurately, volt-amperes (VA); UPS systems range from just a few hundred VA up to massive 800 kVA behemoths.
Getting started
This buying guide outlines the most important features of enterprise-class UPS systems in the 10 kVA to 50 kVA range, which is ideal for small to midsized data center deployments, and such UPSes Hp PB994A battery are quite quite common in many businesses and branch-office scenarios. Table 1 (shown at the end of this guide) shows a variety of midsized UPSes. The vendors in these two tables offer products in the 10 kVA to 50 kVA range with some outside that range.
The goal of this guide is not to recommend specific products, but to provide a representative cross-section of products that you can use as a starting point for your own evaluation process. As part of that process, you should talk with each manufacturer, see which optional features best address your particular needs, and find actual UPS users who can give first-hand insight into the systems and how they’re supported. Only then can you make an informed buying recommendation for your organization.
Choosing the right UPS type
There are basically three approaches to Hp PB992A battery UPS design: standby, line-interactive and online. Enterprises rarely use standby and line-interactive UPSes because of long switch-over times and load problems. Online UPSes can be quite large and are preferred for use in most enterprise data centers.
Rather than switching a battery stack in or out, an online uninterruptible power supply always uses batteries to operate the load and continually charges the batteries from line voltage. Thus, it is constantly converting AC into battery DC and then converting battery DC into AC for the load, earning it the moniker “double-conversion UPS.”When input voltage fails, it simply drops out, and batteries continue to power the system without disruption—there is no switching delay to Hp 337607-003 battery interrupt vital servers.The other benefit for a data center is that load servers are completely isolated from the line AC, so “dirty” power—power from a backup data center generator—is effectively filtered before powering the load.
It’s also important to consider transformer and transformerless UPS designs. Traditional online UPS systems create AC from the DC battery, but they use step-up transformers to multiply low-voltage AC from the inverter to a higher voltage level that is suitable for the load (such as 120 volts AC to power a 1U server). Transformer-based UPS Hp 337607-001 battery systems can range from modest 8 kVA to huge 800 kVA systems. They also provide a level of electrical isolation between the UPS and the load, further protecting the output from spikes and transients.
By comparison, transformerless UPS designs replace the bulk and expense of a traditional transformer with high-power transistor circuits that create a high-voltage DC output first (perhaps as high as Hp Pavilion ZT3000 battery 700 Vdc to 800 Vdc), which is then inverted to an AC output for the load. A transformerless UPS can generally support a sizable midrange UPS of up to 120 kVA. Eliminating a large transformer also increases power efficiency. In Table 1, the Liebert APMUPS is a transformerless online UPS.
Two additional options
A typical standby UPS runs the load directly from incoming utility Hp EX942AA battery power. When the system detects a power disruption, the standby UPS takes over the load and provides power froman internal battery stack.
AC power is created from a DC/AC inverter circuit. The switchover time, however, can be as long as 25milliseconds.Under the heavy load of a server rack or blade chassis, that’s simply not fast enough to prevent power disruptions. A standby UPS is often a point solution used to protect single PCs or servers at an administrator’s desk or auxiliary server closet.
The line-interactive UPS switches similarly to the way it switches in the standby design, but it includes a multi-tap, variable-voltage transformer that can buck or boost the output voltage in response to fluctuations in input voltage. For example, if the line voltage sags below an acceptable level, also called a brownout, the UPS automatically switches transformer taps to compensate and preserve AC voltage
levels to the load. This prevents the UPS from switching to the battery stack unnecessarily. As with a standby UPS, a line-interactive UPS is rare in enterprise data centers, though it may also be used as Hp EV087AA battery point protection for almost any single PC or server.
Flywheels vs. batteries
Flywheel backups are increasingly popular as a data center backup power option, but there are important differences between flywheels and battery backups. Flywheel backups work by spinning a physical flywheel at a high rate of speed.When power fails, the flywheel continues to spin—effectively becoming a generator that produces voltage to power loads.
However, a flywheel will start slowing as soon as Hp Pavilion dv9000 battery
(Hp dv9000 battery) power is lost, so it only provides power for a short period of time compared with batteries. Flywheels are most often used during brief power disruptions. Since flywheels are generally inadequate backup power sources on their own, they may be used with backup generators or conventional UPS systems.
“The flywheel will keep the batteries from being exercised during short power anomalies, thereby extending their life, mainly on VRLA installations,” said Robert McFarlane, an analyst at Shen Milsom & Wilke. McFarlane also suggests using flywheels to start backup generators, adding that dead batteries are to blame for many stalled generators.
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