Boost Your Data Center Efficiency with Multi-mode UPS Systems

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 efficiency can add up to significant lifecycle operational cost savings.

Figure 1: Typical Data Center AC-Power Configuration

Traditional double conversion UPS units (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.


Multi-mode Transfer Speed


So what’s that optimum switching or transfer time? 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.


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?

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


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 (Capital Expenditure) are fixed, a Total Cost of Ownership (TCO) evaluation of the OpEx (Operating Expense) 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.


About us

Strategic Media Asia (SMA) is one of the approved CPD course providers of the Chartered Institution of Building Services Engineers (CIBSE) UK. The team exists to provide an interactive environment and opportunities for members of ICT industry and facilities' engineers to exchange professional views and experience.

SMA connects IT, Facilities and Design. For the Data Center Design Consideration, please visit 

(1) Site Selection,
(2) Space Planning,
(3) Cooling,
(4) Redundancy,
(5) Fire Suppression,
(6) Meet Me Rooms,
(7) UPS Selection,
(8) Raised Floor,
(9) Code & Standards,
(10) Transformers and Harmonic Distortion, and

(11) Multi-mode UPS Systems

All topics focus on key components and provide technical advice and recommendations for designing a data center and critical facilities.

A Data Center Nightmare: Single Point of Failure (2)

Refer to "A Data Center Nightmare: Single Point of Failure (1)"


Data center failures can be rooted in several sources - design, construction, maintenance, quality of material, quality of equipment, commissioning and direct human intervention. For the most part, data centers, even ones that fail, have the benefits of good design practice and intention, professional construction oversight, and high-quality craftsmanship. They are maintained according to data center quality guidelines. But a single overlooked mistake can quickly become significant issues - power and air conditioning failure - that can bring down a data center.

Another story is a high-profile government data center, with a busduct-panelboard connection exploded, effectively shutting off power to approximately 15,000 square feet of the most critical computing in the facility.

In this incident, the design relied on an isolated redundant uninterruptible power supply (UPS) back-up. When a UPS system failed, a static automatic transfer switch was to shift to the already-operating isolated redundant UPS and transfer the load within a quarter cycle. The system worked well and the client was satisfied with the transfer scheme and the rotary concept.


Source of the Problem


Where this system failed was downstream from the automatic transfer switch. Each of the switches fed one busduct riser and terminated directly into a main distribution panel located on each floor of the facility - one busduct per panel. A single fault on any busduct or main distribution panel compromised the critical load.

As it occurred, the electrical connection between the busduct and the distribution panelboard failed and the load was lost. A single point of failure succeeded in bringing down the floor. Not until the facility’s electricians ran jumper cables from one of the intact risers and back-fed the main distribution panel did the floor have power.

Why did this failure occur? The building had been designed in tight coordination between the government representative and the designer; the entire system had been commissioned and had been running with tight oversight for more than two years. What happened?

The cause of the problem was the failure of a manufactured busduct connector, one of hundreds in the building. The connector joined lengths of feeder busduct via a sliding piece - designed to slide approximately one-quarter of an inch to make installation easier - and a break-away torque bolt designed to ensure that the installer did not over-torque the bolt.

Although the investigation team was not asked to explain exactly why the joint exploded, it determined that the quarter-inch of play designed into the connector had actually allowed for a portion of uninsulated section of the copper busduct to be exposed to the atmosphere without insulation. The team surmised that the perfect combination of air borne dust, humidity and possibly other contaminants led to an arc that became a fault and exploded.

During the analysis, the investigation team isolated each busduct riser from the static automatic transfer switch at the source and from the main distribution panel at the termination. During the megger test, the electrical forensic team discovered two additional joints that didn’t pass, clearly more candidates for potential failure. Not only did the joints not pass the megger test, two of them visibly and audibly arced while the voltage was ramped up during the testing. The joints had shown themselves to be the weak link in the system. The installed busduct technology was vulnerable to catastrophic failure.



Continue - A Data Center Nightmare: Single Point of Failure (3)

About the Blog

Strategic Media Asia (SMA) is one of the approved CPD course providers of the Chartered Institution of Building Services Engineers (CIBSE) UK. The team exists to provide an interactive environment and opportunities for members of ICT industry and facilities' engineers to exchange professional views and experience.

SMA connects IT, Facilities and Design. For Data Center Design Consideration, please visit 

(1) Site Selection,
(2) Space Planning,
(3) Cooling,
(4) Redundancy,
(5) Fire Suppression,
(6) Meet Me Rooms,
(7) UPS Selection, and
(8) Raised Floor

All topics focus on key components and provide technical advice and recommendations for designing a data center and critical facilities.

Data Center Design Consideration: UPS

Uninterruptible Power Supply (UPS) is one of the key component of a data center. Understanding of the UPS technology is critical and important for the data center design.


Transformer-based or Transformer-free UPS?


There is growing interest in using transformer-free UPS modules in higher power, three-phase, mission-critical power backup applications (e.g., 200 kW to 5 MW). However, many organizations are unclear about which architecture — transformer-based or transformer-free — is best suited for a particular application.


With limited floor space and modular components in data centers, more flexible, smaller footprint UPS units are desirable by companies in the near future. On the one hand, the latest transformer-free systems offer better efficiency, a smaller footprint, and improved flexibility while providing high levels of availability. Driven by data center designer demand, most leading UPS suppliers offer both topologies. On the other hand, transformer-based UPS systems excel at providing the highest capacities and availability while simplifying external and internal voltage management and fault current control.





Currently, large transformer-free systems are constructed using modular building blocks that deliver high power in a lightweight, compact package. This modular design offers advantages when the timing of future load requirements is uncertain by allowing capacity to be more easily added as needed, either physically or via control settings. On the other hand, a modular design means higher component counts, which may result in lower unit mean time between failure (MTBF) and higher unit service rates.





For high-power enterprise data centers and other critical applications, a state-of-the-art transformer-based UPS still provides an edge in availability. Transformers within the UPS provide integrated fault management and galvanic isolation as well as greater compatibility with critical power distribution system requirements that should be considered when designing a high availability UPS system. Technology developments and configuration options allow the latest transformer-based designs to operate at higher efficiencies compared to previous designs, making them more comparable to the transformer-free models in terms of efficiency.


In general, 200 kW is a threshold below which the space, weight, and cost advantages of transformer-free UPS systems outweigh the robustness and higher capacity capabilities of transformer-based systems. The under-200 kW applications can benefit from the high efficiency and excellent input power conditioning through active components offered by transformer-free designs. In addition, the scalability of a modular transformer-free UPS can help avoid over-provisioning while maintaining operational efficiency.



FACTORS TO CONSIDER





Both approaches use a double-conversion process (Figure Above) to provide power protection for mission-critical applications. The primary difference between the two technologies is in their respective use of transformers.

A transformer-based UPS may use a transformer before the rectifier and requires an isolation transformer after the inverter to derive the voltage being delivered to the critical load.

Transformer-free UPS designs use power and control electronics technologies to eliminate the need for an isolation transformer as an integral part of the inverter output section.



TRANSFORMER-BASED UPS DESIGN

Large systems are typically manufactured based on serviceable sub-assemblies and are available in discrete units rated up to 1,100 kVA. Key components of this design include:

• A passive filter (inductors and capacitors) on the rectifier input to reduce input current distortion and improve the power factor.

• A six-pulse (or optional twelve-pulse), semiconductor (SCR)-based rectifier on the input. Optionally, an additional transformer (Xfmr) provides AC-DC isolation for the DC bus and the battery.

• A DC energy storage system (typically a battery) connected directly to the DC bus between the rectifier and the inverter to provide AC output power ride-thru capability during a loss of AC input power. This example uses 540 VDC.

• An insulated gate bipolar transistor- (IGBT) based, pulse-width modulation (PWM) inverter on the output.

• An isolation Xfmr on the inverter output to derive the appropriate output voltage. This also provides a convenient and solid point for referencing the AC output neutral to ground. This neutral ground connection provides excellent common mode noise rejection.

• A passive filter on the inverter output to provide a very low distortion AC voltage supply.

• An automatic bypass switch (static switch) using power SCRs provides instantaneous switchover to an alternate source if a UPS output disturbance occurs.

TRANSFORMER-FREE UPS DESIGN





Transformer-free UPS topologies replace simple passive magnetic-voltage transformation functions with solid-state power electronics circuitry. Figure above shows a simplified block diagram of a transformer-free UPS design. There are a number of key differences listed below between this circuit and the unit depicted in the figure.

• By replacing passive power components (transformers, capacitors, inductors) with power circuit assemblies utilizing PWM power conversion techniques, transformer-free UPS rectifiers are physically smaller and produce low input current harmonics with near unity input power factor.

• Typically, the UPS battery in transformer-free applications is connected to the internal DC bus (about 800 VDC in the previous example) through an integrated bi-directional DC-DC converter. This puts an additional power conversion element in series with the battery.

• Using similar PWM power conversion techniques, transformer-free UPS inverters are physically smaller as well, and produce low output voltage harmonics over a wider range of connected load characteristics.

• The bypass function (static bypass switch) is similar to the transformer-based design. However, without external transformers added, the bypass AC input must be the same voltage as the inverter AC output.

• Transformer-free UPS are typically designed and styled for both computer room in-row lineups and equipment room installations. Complete transformer-free UPS units are typically an assembly of standard frames plus functional control and power modules.

• A transformer-free UPS is lighter and smaller than the power-equivalent transformer-based design with both physical volume and footprint being less. And, according to the fall 2012 Data Center Users Group (DCUG) survey, data center energy costs and equipment efficiency are the top-of-mind issue for DCUG members, with nearly half of the respondents listing it as one of their top facility/network concerns.

However, other external transformers may be required for AC-DC isolation purposes, safety reasons, AC voltage changes, or to provide power distribution flexibility. With the addition of external transformers, the overall facility weight and footprint totals may be higher than with a transformer-based UPS design with implications for end-to-end system efficiency. If transformers need to be added to a transformer-free unit to make it compatible with a facility, a transformer-based unit may be a better solution.

Transformer-free UPS topologies have emerged to meet the demand for more efficient, flexible, smaller footprint, lighter weight UPS systems. The price of these performance feature improvements has been the replacement of a few robust but physically large, passive components, such as transformers, inductors, and capacitors with functional power electronic equivalents packaged in field replaceable, modular sub-assemblies. It is reasonable to expect that the transformer-free units will have service call rates somewhat higher than their transformer-based counterparts.



TECHNICAL FEATURES AND PERFORMANCE DIFFERENCES


In choosing between transformer-based and transformer-free UPS solutions, a system designer should determine where transformers are best utilized and whether they should be internal and/or external to the UPS in view of physical and electrical distribution requirements and tradeoffs. It’s important to review the techniques and tradeoffs utilized in the various rectifier, DC energy storage, inverter, and static bypass functions of these two UPS designs for various UPS system performance functions including:


• Site planning and adaptability to change

• Reliability and availability

• Robustness

• DC energy storage system isolation

• Engine-generator interface

• UPS output interface considerations

• High resistance grounding

• Fault current management

• Arc flash energy

• Isolation

• Maintainability

• External components needed to complete the system design

• Total cost of ownership

• Capital expenses (CAPEX) and operating expenses (OPEX)


When considering the total cost of ownership for these two architectures, it is important to include both the initial upfront or CAPEX as well as the ongoing or OPEX to power, maintain and service various options.


Technological evolution is constantly impacting the relative efficiency of transformer-based and transformer-free solutions. After years of optimizing performance, transformer-based UPS systems have achieved a relatively flat efficiency curve from 30% to 80% loading where typical tier 3 and tier 4 data centers operate. The latest transformer-free designs also have very flat curves down to as low as 20% of capacity, and have efficiencies in the 95% to 96% range in double-conversion mode. A study of the whole system design is necessary to determine the relative efficiencies as the addition of transformers, and the efficiency of those transformers, will have an impact.


In summary, transformers, whether internal or external to the UPS, are necessary to establish circuit isolation and local neutral and grounding points, as well as to provide voltage transformation points. This facilitates, for example, the implementation of very high power density installations based on 600V distribution sources, subsequently stepped down to 208/120V for IT load applications. When transformers are utilized in conjunction with the UPS internal DC link, DC-to-AC output, and AC-to-DC input isolation can be provided, reducing or eliminating the risk of DC faults propagating upstream or downstream of the UPS.



About the Blogger

Strategic Media Asia (SMA) is one of the approved CPD course providers of the Chartered Institution of Building Services Engineers (CIBSE) UK. The team exits provide an interactive environment and opportunities for members of ICT industry and facilities' engineers to exchange professional views and experience.

SMA connects IT, Facilities and Design. For the Data Center Consideration Series, please visit 

(1) Site Selection,
(2) Space Planning,
(3) Cooling,
(4) Redundancy,
(5) Fire Suppression, and
(6) Meet Me Rooms