Choosing UPS systems for the data center

Selecting the right UPS for data centers is complex. This buying guide can help you sort through the features offered by today's UPS systems.

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 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.

TABLE OF CONTENTS:

Getting started
Choosing the right UPS type
Factors to consider when buying a UPS

 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 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 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 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 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 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 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 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 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.

 Factors to consider when buying a UPS
As you select a UPS, consider functionality and features as well as long-term durability. Resilience, management and monitoring, bypass control, and maintenance and output are the key components to compare for your organization's particular needs.

Resilience. A UPS is typically a reliable device, but it can become a single point of failure that affects multiple servers connected to it. If each server is virtualized and hosts multiple virtual machines (VMs), the potential for workload disruption can be enormous. So any evaluation of data center UPS systems should take resilience options into account.

One option is to select a UPS that supports multiple power inputs, allowing two different sources to power the UPS unit. If one utility line fails, power from the other line can run the load and keep the UPS charged.

Another approach is to select UPS systems that support parallel operation. Not only does this allow for greater runtime, but if other batteries in parallel fail, UPS power will still be available. This is termed +1 redundant because two UPS systems are running in parallel. If multiple UPS systems can be configured in parallel, it’s generally called an N+1 architecture, where N devices supply the required load and +1 ensures that load demands are met if one UPS should fail. For example, the Tripp Lite SU40KX UPS supports both 1+1 and N+1 configurations.

Still, experts recommend a careful assessment of UPS stability as load conditions change. “Particularly if you are installing a redundant UPS, step-function stability would be another important criterion to examine,” said Robert McFarlane, an analyst at Shen Milsom & Wilke, a technology consulting and design firm in New York. “When the entire load is suddenly thrown onto the remaining UPS, it has to reach stability very quickly.” 

Many data center managers prefer to use parallel operation to scale up UPS capacity. McFarlane advises against purchasing an oversized UPS to accommodate future growth, citing that this method is inefficient (if all of the UPS unit’s capacity isn‘t needed), can waste capital and is not environmentally friendly.

Many servers are built with redundant power supplies. Using a strategy sometimes called 2N redundancy can give you additional resilience if you connect each of the server’s redundant power supplies to a different UPS. Budget-conscious organizations may connect one supply directly to line power and a second supply to a UPS.

Management and monitoring. Don’t forget the importance of UPS manageability. Although USB and RS-232 (serial port) connectivity may be suitable for small installations, data center UPS systems should include IP connectivity to allow remote management across an Ethernet LAN—the same mechanism that supports remote server, storage and other systems management in the data center. The HV Series from Controlled Power Co., for example, supplies three IP-type controllers offering a variety of communication features.

Good management tools allow you to monitor your equipment from anywhere and receive alerts if something goes wrong. “You should also be able to direct critical information to your [building management system],”McFarlane said.

Facilities personnel are also responsible for power distribution, which includes UPSes, so integrating the building management system wherever possible is important. If a UPS problem arises, the system will alert both IT and the facilities team.

An application or a Web/SNMP interface typically monitors and manages the UPS, and allows the administrator to track factors like UPS temperature, power, available runtime, current load (in kVA or watts) and battery condition. The admin should then be able to remotely control operations such as controlled shutdowns and restarts.

Experts note that the level of information and control that UPS management software allows can be a serious security flaw for careless organizations. “It should be possible to disable remote control of anything in the data center and just get operational readouts,” McFarlane said. “Anything you can access and control remotely could also be accessed and controlled by a hacker or disgruntled former employee.”

Bypass control capabilities. The choice of which UPS unit works best for your data center must extend beyond basic design and transformer use. A bypass control capability that allows the UPS to be completely removed from the power-distribution system is a feature that’s often overlooked.

This may seem counterintuitive for an online UPS design, but the feature can prevent a defective UPS from causing a power outage that extends downtime. In addition, the ability to “bypass” a UPS allows field maintenance and repair without disruption to the load. For example, the 30 kVA APC Smart-UPS VT on page 8 is one system that supports automatic internal bypass.

Without such a bypass, McFarlane said, you could find yourself “in the embarrassing position of having to explain why utility power is still on, but your expensive UPS has become a roadblock between [utility power] and your data center, causing everything to do a hard crash.”

Maintenance and output. Maintenance and repair are certainly major concerns for UPS deployments, particularly for batteries that eventually wear and fail. When shopping for a UPS, think about what will happen once its batteries die. For example, field-replaceable batteries allow technicians to easily upgrade or replace batteries (often with no tools). Selecting a UPS with hot-swappable batteries allows technicians to replace old or damaged batteries without having to take the entire UPS out of service first.

As UPS systems evolve and gain intelligence, they are getting more adept at controlling how output power is delivered to the load. For example, a UPS with controlled output groups lets the UPS first shut down less-important loads such as print servers, allowing more runtime for important loads like a SQL Server.

Output groups also allow administrators to cycle power to one group to reboot a troublesome server without having to reboot everything attached to the UPS. Conversely, the UPS can use controlled output groups to sequence the startup of servers so that more important equipment starts first.

Another feature appearing in some UPS systems allows the unit to turn off power to a load that is in standby or otherwise “sleeping.” This feature might be useful in a virtual server environment where you can migrate active loads to fewer servers, and the UPS can shut down those idle servers to save even more power.

Staying ahead of the game
Batteries are the heart of every conventional UPS system, and a UPS is useless if the batteries fail when you need them most. It’s important to stay one step ahead of UPS needs and vulnerabilities. Most modern systems employ valve-regulated lead-acid (VRLA) batteries. These are often called “sealed cells” or “maintenance-free” batteries, but their lifespan is relatively short. Typical VRLA batteries must be replaced every three to five years.

To ensure reliable UPS performance, continuously monitor the battery stack. This is particularly important with online UPS systems where batteries are constantly providing load power and being recharged simultaneously. In addition to the internal batteries, be sure to monitor any external battery stacks. Configure multiple battery sets in parallel wherever possible. This may occur when placing multiple UPS systems in parallel or when setting up multiple sets of batteries within the same UPS system.

The goal is to provide resilience in the event of a battery fault, and even though the resulting runtime might be shorter, UPS service will continue long enough to orchestrate a graceful shutdown of the load. Be practical when specifying UPS runtime.

Remember that a utility power failure will almost certainly disable computer room air conditioning (CRAC), causing a serious overheating situation in relatively little time. Data center experts point out that without a backup generator to run CRAC systems, a thermal shutdown may occur within as few as 30 minutes. Using a UPS with four hours of runtime would simply be a waste of money. Instead, experts advise using the UPS automatic shutdown feature to manage a graceful shutdown when battery power gets low.

Table 1: Midrange UPS options and specs
Below, we compare six UPS units in the 10 kVA to 50 kVA load range.

FEATURES

PRODUCTS

APC Smart-UPS VT rack mounted

Controlled Power Co. HV series

Eaton 9390IT

Output Power Capacity (kVA)

30 kVA

20 to 25 kVA

40 kVA

Nominal Output (volts AC)

120 V, 208 V

208 V, 240 V/120 V, 277 V, 347 V, 600 V

480 V, 400 V, 208 V

Output Connections (number and styles of outlets)

Hardwire 5-wire (3PH + N + G), NEMA L21-20

N/A

Hardwired, various options: L6-30, L21-30, L22-30, IEC309-60

Input (Volts AC)

Nominal: 208 V 3PH, other input voltage: 220 V

208 V, 240 V, 277 V, 347 V, 600 V

480 V, 400 V, 208 V

Input Connections (style of inputs, such as hardwiring)

Hardwire 5-wire (3PH + N + G)

N/A

Hardwire 5-wire (3PH + N + G)

Battery Type

VRLA

Sealed, maintenance-free

VRLA

Maximum Efficiency Under Load

93%

91%

92% to 94%

Interface Port(s) or Management Card(s)

Yes. Interface Port DB-9 (RS-232)

RS-232

Yes. RS-232; four slots available for management cards: HotSync CAN Bridge card (for paralleling), ConnectUPS Web/SNMP with Environmental Monitoring Probe, Modbus, Standard Relay Card, Industrial Relay Card, Modem Card, Power Xpert Gateway 2000 cards

Emergency Power Off (EPO)

Yes

Yes

Yes

Management Software

Yes. CD w/ software

No. Optional NetMinder Ethernet/Modbus adapter for network monitoring; optional Dataguard communications software

Yes. Intelligent Power Manager: Free for up to 10 units, fully VMware vSphere-integrated; manage all UPS and distribution from one interface

Front-Panel Controls

Yes. Multifunction LCD display shows system parameters and alarms

Yes. Monitor electrical operating parameters, monitor alarm conditions, UPS system control settings and log functions

Yes. Visual power diagram, all input and output ratings viewable, all configurations can be accessed through interface

Hot-Swappable Batteries

Yes

N/A

Yes

Controlled Output Groups

No

N/A

No

Automatic Voltage Regulation

Yes

Yes

Yes

Parallel Operation-Compatible

No

N/A

Yes

Network-Manageable

Yes

N/A

Yes


 

Table 1: Midrange UPS options and specs (continued)

FEATURES

PRODUCTS

Liebert APM
Online

Minuteman Power Technologies Endeavor 6-10 kVA Tower Online ED10000T

Tripp Lite SU40KX

Output Power Capacity (kVA)

Two frame sizes: 15 to 45 kW and 15 to 90 kW

6 kVA to 10 kVA

40 kVA

Nominal Output (Volts AC)

208 V/120 V

120 V and 208 V/240 V

220 V/380 V, 230 V/400 V, 240 V/415 V

Output Connections (number and styles of outlets)

Uses breaker or panelboards, no outlets

Hardwired

3-phase, 4-wire + ground

Input (Volts AC)

408 V or 208 V

208 V/220 V, 230 V/240 V

220 V/380 V, 230 V/400 V, 240 V/415 V

Input Connections (style of inputs, such as hard wiring, etc.)

Hardwired

Hardwired

3-phase, 4-wire + ground

Battery Type

VRLA

Sealed, nonspillable, maintenance-free, value-regulated, lead-acid

N/A

Maximum Efficiency Under Load

94%

87%

Up to 97%

Interface Port(s) or Management Card(s)

Yes. Web/SNMP, OpenComms 485 Card (Modbus RTU), and Relay Card

Yes. RS-232; optional interface cards are ED6/10 kVA Programmable Relay Card (dry contact relay) and the SNMP card

Yes. RS-232; optional internal SNMP card

Emergency Power Off (EPO)

Yes

Yes

Yes

Management Software

No. Web card has built-in GUI-driven management application

Yes. SentryPlus software for power monitoring

Yes. PowerAlert software

Front-Panel Controls

Yes

Yes. LCD display, indicators for abnormal conditions or internal fault and when operating in the Parallel/Redundant (N+1) mode

Yes. Front-panel LCD/LED interface offers complete status information, includes a real-time event log screen

Hot-Swappable Batteries

No

No

Yes

Controlled Output Groups

No

No

N/A

Automatic Voltage Regulation

Yes

Yes

Yes

Parallel Operation-Compatible

Yes

Yes

Yes. 1+1 parallel capability

Network-Manageable

Yes

Yes

Yes

This was first published in July 2011

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