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. Regardless, having the option is desirable. According to the Uptime Institute’s 2012 Data Center Industry Survey, a third of large companies and nearly a quarter of small companies will deploy smaller floor plans and modular components in data centers in the next 12 months. If so, more flexible, smaller footprint UPS units will be desirable.
In general, transformer-based UPS systems excel at providing the highest capacities and availability while simplifying external and internal voltage management and fault current control. The latest transformer-free designs 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.
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.
However, if operational efficiency, expansion flexibility, or limiting the UPS footprint are of paramount importance, and other appropriate measures are instituted to provide an acceptable level of availability, transformer-free technology may be the optimal choice.
“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,” said Peter Panfil, vice president global power sales, Emerson Network Power. “These 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 1) 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 2 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 Figure 3.
• 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 example in Figure 2) 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,” said Panfil. “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, while achieving system output availability values that approach those achieved by transformer-based designs, 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
• DC energy storage system isolation
• Engine-generator interface
• UPS output interface considerations
• High resistance grounding
• Fault current management
• Arc flash energy
• 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.