Specifying a multi-mode UPS in data centers

Engineers should consider lifecycle cost and energy efficiency models for evaluating data center uninterruptable power supply (UPS) systems.


 Learning objectives:

  1. Understand the impact of uninterruptible power supply (UPS) power conversion technologies on data center power efficiency.
  2. Examine the energy efficiency issues of traditional double conversion UPS systems versus higher efficiency, multi-mode UPS power conversion technologies. 
  3. Understand how to apply a lifecycle operating expense (OpEx) versus a capital expense (CapEx) model based on higher UPS power conversion efficiencies. 

When consulting-specifying engineers look at the hundreds of technology factors that go into a data center’s design, they know that even small variables, when multiplied by big numbers, add up quickly. That’s the case with seemingly small incremental increases in energy efficiency for uninterruptible power supply (UPS) systems used in data centers around the world. 

According to the Uptime Institute, traditional transformer-based UPS devices represent only 12% of a typical data center’s energy consumption, given power use and energy conversion inefficiencies and heat loss. Although they account for only a fraction of the total energy consumption in a data center, even small improvements in UPS energy conversion efficiencies can add up to significant lifecycle operational cost savings. A study by Frost and Sullivan found that the U.S. could reduce its yearly consumption of electricity by up to $3 billion by increasing the energy efficiency of UPS units in data centers from 90% to 98%.

Figure 1: A standard configuration for an ac-powered data center. Courtesy: GE Critical PowerTraditional double conversion UPS units (see Figure 1)—which protect the load during outages—use a rectifier to convert the alternating current (ac) power to direct current (dc) power, and an inverter to provide safe and clean ac power to the load using either the main or battery power. 

Unfortunately, in this scenario power efficiency is the price paid for protection. Transformer-based double conversion UPS systems have a typical power efficiency rating in the range of 88% to 92%. As a result, double conversion UPS systems place a steep toll on annual data center energy operating budgets. 

Newer three-level insulated gate bipolar transistor (IGBT) UPS technologies, which reduce switching and filtering power conversion losses, offer efficiency levels approaching 97% in double conversion mode, and up to 99% efficiency when operating in energy-saving multi-mode. These new, three-level UPS topologies create new OpEx rationales when designing data center power systems and specifying UPS technologies. 

Newer multi-mode UPS technologies, such as GE’s eBoost (which operates at 97% to 99% efficiency), employ precision disturbance analyzers and responsive monitoring technologies to provide more accurate and rapid seamless switching between utility power and UPS-conditioned power. This enables the UPS, depending on the quality of the utility power, to operate for a shorter time in the less efficient, double conversion mode. One of the keys to these efficiencies is the transfer speed. 

Multi-mode transfer speed 

So what’s that optimum switching or transfer time? Some earlier white papers and blogs suggest that anything over 8 to 10 milliseconds (ms) is problematic, given that not all data center sensitive equipment (e.g., servers) have tolerances at or above these levels. According to a Green Grid white paper on multi-mode (or eco-mode), “if, for example, a UPS has a transfer time of greater than 10 ms and is paired with information technology (IT) equipment that has ride-through capabilities of only 10 ms, the UPS may not be able to support the IT equipment.” 

That’s one of the reasons a few companies design their multi-mode UPS products with transfer speeds of less than 2 ms. The technologies that help achieve these speeds are seamless and represent a robust set of power disturbance detection, analysis, and control systems. 

When a multi-mode UPS unit’s responsive monitoring technologies detect any sort of deviation on the main or bypass power path, the inverter is immediately turned on to allow quality power to flow from the double conversion premium protection mode. In the same instant, the static switch on the bypass path from the utility is turned off to block the disturbance from reaching the load. 

A variety of disturbance analyzers and fast-switching technologies are employed in combination, including:

  • An instantaneous adaptive voltage error detector that monitors subtle changes in amplitude and duration 
  • A root mean square (RMS) voltage error detector that computes the RMS of all three UPS output voltages for variances 
  • An output short circuit detector that, after a breaker is tripped, will automatically increase line current to rapidly clear and reset the breaker 
  • A sophisticated transient inverter controller that quickly manages the transfer of the load to inverter power and back again to the bypass path. 

All of these advanced monitoring and control systems work in concert to anticipate and respond to a comprehensive set of possible power conditions, creating a transfer switch speed of less than 2 ms. This speed helps to maximize the intermittent transfer to double conversion protection, while maintaining higher multi-mode efficiency for the majority of the time when quality utility power is flowing.  

Figure 2: Small percentage improvements in power efficiency can yield significant OpEx savings over a 10-year period. Courtesy: GE Critical Power

Lifecycle costs

In evaluating efficiency and lifecycle costs for multi-mode UPS systems, some might ask: If our UPS running in double conversion already gets us to 93% efficiency, why take a “risk” for a few percentage points in efficiency? Can that extra energy efficiency provide a significant return?

If we look at a UPS deployment at a typical 10 MW data center realizing just a 1% gain in efficiency, we can see a significant impact over 10 years. As Figure 2 shows, while CapEx costs are fixed, a total cost of ownership (TCO) evaluation of the OpEx for running an UPS over 10 years creates an operational savings of $1.4 million when energy efficiency improves a single percent—from 93% to 94% efficiency. With newer multi-mode UPS technologies that provide up to 96.5% efficiency, that savings could jump to almost an additional $3.4 million. 

As both corporate and data center providers challenge their consulting-specifying partners to deliver projects that balance capital and lifecycle costs, as well as ensure the reliability and energy efficiency of their facilities, new multi-mode UPS efficiency models provide a compelling set of tools for data center designers and engineers. 

Harry Handlin is the director of critical power applications GE’s Critical Power business, with North American responsibility for providing technical and application support of electrical distribution products as well as integrating other GE businesses to serve mission critical customers. He is currently the technical committee chairman for The Green Grid; a nonprofit, open industry consortium of end users, policy makers, technology providers, facility architects, and utility companies. 

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