The rise of a new generation of transformerless uninterruptible powers supplies (UPSes) calls into question the advantages and disadvantages of these two types of UPSes. Note that this tip compares transformerless UPSes with three-phase dual conversion units that use an internal transformer as an integral part of the inverter system, not just an input or output transformer solely for voltage conversion.
Over the past five to seven years, the transformerless UPS has come to dominate the smaller three-phase (30 kVA and under) marketplace. These units are much smaller, lighter and lower in cost than the previous generation of transformer-based units. This type of design has rapidly moved up to the 100 kVA range and established a solid foothold up to 300 kVA units -- and, when utilized as part of multi-module systems, to 1000 kVA or more.
IGBTs and the transformerless UPS
A little history first: Older UPSes were based on SCR inverter technology, which were either on or off, and required internal transformers to operate, so virtually all UPSes were once designed with transformers. This changed with the advent of insulated-gate bipolar transistor (IGBT) technology. IGBTs are the core technology underpinning the existence of the transformerless UPS. A modern IGBT-based UPS inverter uses high-frequency pulse-width modulation to re-create a nearly pure sinusoidal waveform and eliminates the need for bulky output transformers or large iron-core output filters. (IGBTs are also used in transformer-based UPSes.)
Transformerless designs began to appear in smaller UPSes in the mid-1990s and became the mainstream design by 2000. Earlier UPS designs relied on an input transformer to boost low incoming line voltages without forcing the UPS on to battery during low-line or "brownout" conditions. The newer systems also use IGBTs for more efficient AC-to-DC input conversion, which allows the DC bus and the inverter to hold a steady output voltage over a broader range of input voltage and frequency variations without going on battery.
Transformerless vs. transformer-based: What's more effective?
In the data center world, traditionally only two things have mattered: reliability/availability and the proverbial five 9s. One of the primary claims made by vendors and some customers is that a transformer-based UPS is more robust and therefore more reliable.
In today's energy-conscious world, we all want a more efficient UPS, and a transformer, by its very nature, will introduce some additional losses to the system. One of the arguments in favor of the transformerless system is that transformers reduce energy efficiency. Older transformers lost 2-3% and sometimes more to the non-linear IT loads. More recently, since the advent of the TP-1-rated transformer (as well as using a high K rating, i.e., K20), this has improved to only 1.5-2%. However, in the 24/7 mission-critical world of the data center, efficiency is still -- and perhaps always will be -- a distant second to reliability and availability.
Of course, a theoretical analysis of technical differences is nearly meaningless without looking at the actual available products and market acceptance. Besides the technical arguments presented by both camps, it sometimes boils down to personal preference. Many times, the choice is made based on the preference of the specifying engineer or those who make the final purchasing decision. The two camps seem to closely align themselves and their choices to the different manufacturers of these systems.
So are you a transformerless "liberal" or a transformer-demanding "conservative"? Is this just "old school versus new school" thinking or are there solid differences and benefits that each design offers?
Vendor UPS offerings
One manufacturer in particular, Emerson-Liebert, strongly favors the transformer-based design for its flagship line of larger UPSes, which are available up to 750 kVA as a single module. The vendor also offers a full line of transformerless systems in the lower power ranges.
"The dividing line seems to fall in the 200-300 kVA range," said Alan French, Manager of Technical Relations at Emerson-Liebert. "Below that, a substantial number of our sales of new systems are transformerless, while the larger units are mostly transformer-based. We believe that our typical large-enterprise customer wants the extra measure of reliability that a transformer-based UPS provides."
Other brands, such as Schneider Electric's APC and MGE, offer both types systems. APC offers both modular and transformerless, while the MGE division offers transformer-based systems in some of the larger systems and transformerless units in the lower (150 kVA or less) ranges.
"APC manufactures both transformer-based and transformerless UPS topologies, but we see an ongoing shift toward transformerless designs," said APC's John Collins, director of 3 Phase UPS Product Management. "Note that a transformer-based UPS should not be considered lower or higher reliability or lower or higher performance just because there happens to be a transformer in the UPS. Our advice to customers is, 'Don't worry about the topology.' Be clear with your potential vendors regarding your intended electrical performance, your application and your financial goal of total cost of ownership or lowest first cost."
Eaton-Powerware has gone transformerless across virtually the entire product line, up to and including systems of 1100 kVA (composed of multiple modules).
"We utilize a transformerless design primarily because it provides reductions in size, weight, audible noise and output impedance (better transient response)," said product manager Ed Spears. "Additional advantages include an improvement in UPS system efficiency of 1-4% and, of course, a lower BTUH rating. In our newer designs, the absence of the output transformer allows us to instantly (within 2 ms) transition our UPS from 'ready state' to full power-processing operation, since we do not need to magnetize a transformer. This is useful in our Energy Saver System and Variable Module Managements System, which improves efficiency significantly (2-10%) over previous and conventional designs."
According to Chuck Heller, product manager of Three Phase Power at Chloride, the company offers both types of UPSes but is transitioning to a transformerless product range. "We see a general shift toward transformerless UPS designs for data center applications, with the use of internal transformers being driven by specific application requirements," he said. "Interest in using 415/230 V distribution systems as a way to further improve overall data center efficiency is growing."
The rise of the transformerless UPS
Besides efficiency, these two types of systems are significantly different in size, weight and cost. One of the driving forces behind this new breed of UPS is the exponentially increasing demand for overall power and power density in the data center. When power requirements were only 1-2 kW per rack, the UPS footprint was fairly small in relation to the total white space and did not require an inordinate amount of space within the overall data center envelope. As power requirements jumped to 5, 10, 20 kW or more per rack, the ratio of space required by the UPS changed significantly.
Not that many years ago, a 30-50 kVA UPS was ample for a typical small data center, but now a single rack of four blade servers can require 20-30 kVA, so that the "small" UPS is now 100-250 kVA for a small data center. To meet customer demand for more power that had to fit within limited space, manufacturers started adopting the transformerless design for larger UPS systems.
Also, by eliminating the transformer cost, the UPS price was significantly reduced -- always a market driver. Even for a data center that believes a transformer-based UPSes may be more reliable, lower UPS costs may now allow it to budget for an N+1, a full 2N or even a 2(N+1) modular redundant design. With this added redundancy, even if there were a UPS failure, the other UPS (and/or power path) would be able to carry the load. And since transformerless units are smaller, more efficient and cheaper, the data center could better afford N+1 or 2N redundancy in a smaller site. This helped overcome the reliability and availability issue, which makers of transformer-based systems claim as a primary advantage.
The advantages of a transformer-based system
So are transformer-based systems passé? Transformers are not without merit and, in fact, are inherently part of many power systems, whether they are contained in the UPS or located upstream or downstream from the UPS. One primary function of a transformer is to transform the voltage. In a typical power chain, they are sometimes external and used upstream at 13 kV (or higher on larger installations), stepped down to 480 V (or 208 V for some smaller systems) to feed the UPS. In North America, downstream from a 480 V UPS output (some Canadian systems use 600 V), they are required to step down to 208/120 V. Transformers can be incorporated in an adjacent cabinet or in external PDUs.
Chuck HellerProduct manager of Three Phase PowerChloride
In Europe, the voltage and distribution scheme is based on 400/230 V (stated generically to include 380/220 V through 415/240 V). Transformers are used upstream of the UPS to convert the high-transmission voltage to 400/230 V. This somewhat changes the issues downstream, since virtually all the IT loads are single-phase 230 V (line to neutral). The UPS inputs and outputs 400/230 V, and there is no voltage conversion or transformer required. In this case, the inclusion of a transformer could play a potentially beneficial role by providing isolation and acting as a buffer for phase imbalance as well as fault current limitation via its impedance. Yet many European UPS manufacturers are transformerless or moving in that direction. The use of 400/230 V systems is being considered in the U.S., and will be discussed in part 2 of this series.
There are some instances where input and output transformers are necessary, such as for medical equipment, where total ground and neutral isolation and avoiding any leakage currents are required.
What are the advantages of transformers in the UPS in a typical data center application? They offer greater tolerance to phase imbalance from single-phase loads, which are very typical of most IT loads, both for 120 V to neutral and 208 V line-to-line. This is especially true for a 208/208 V transformerless UPS system where the inverter output goes directly to the power distribution system. In that case, a transformer (either internal to the UPS or in a separate PDU cabinet) will partially mitigate phase imbalance and prevent the UPS from overloading from an imbalance.
It is important to note that in a typical 208 V transformerless UPS system, there are essentially three inverters tied to a common neutral, each facing the load (L1, L2, L3 + Neutral + Ground). In a typical small to mid-size installation, the unbalanced loads may sometimes look like this:
Overall, the UPS is only delivering approximately 70% of its total rating; however, the L2 circuit is at 90% and only has 10% of headroom before reaching its maximum, and the UPS would report an overload if L2 experiences any additional load. While a transformer would not be happy with this imbalance, it would normally tolerate this L2 imbalance better. The load presented to the UPS by the primary side of the transformer would be very close to 70% and balanced across all three phases. Obviously this is a somewhat extreme example, and a well-managed power distribution system would not normally allow this level of imbalance. However, when installing equipment, IT personnel are prone to using whichever circuits are available and may not have any branch-level power metering, a very common scenario.
A transformer's inherent impedance can also limit the instantaneous over-current from a circuit fault. This is especially true for an all-208 V transformerless power distribution system. However, a typical mid-size or larger data center commonly uses a 480 V UPS, and the inverter output will wind up facing an external stepdown transformer, normally located in one or more PDU units.
The transformer also helps mitigate harmonics caused by a less-than-perfect sine wave from the inverter output and also by the non-linear loads cause by the IT equipment's switching power supply. However, if the PDU has a transformer, as in the case of a 480 V UPS, it mostly negates the argument of the benefit that an internal (inverter) transformer would offer.
In some cases, an inverter transformer-based system provides somewhat better neutral isolation from poor-quality utility power to the IT load; however, this does not protect the UPS input. In cases of extremely poor-quality mains power, the addition of an input transformer will help to limit the energy intensity of some spikes and surges to the UPS; however, if the utility power quality is that poor, dedicated power conditioning (which usually contains inductors) and/or Transient Voltage Surge Suppression is recommended -- although if the power quality is that poor, you may want to rethink placing a high-value data center in that location. So the logic of an inverter transformer is rendered as somewhat of a moot point.
The market will decide
Either type of modern UPS with an IGBT-based input section can also control and shape its input power factor to approach unity (typically over 0.95) over a much broader range of loads. This lowers the strain and improves the efficiency of the upstream power path and especially the back-up generator, which no longer needs to be significantly oversized to support the UPS.
The main input voltage may also influence the decision of UPS type. In the lower power arena (i.e., 100 KVA and under), 208 V input systems are fairly common. The choice is then dictated by the available utility power and the size of the installation. The transformerless UPS is solidly entrenched with the majority of sales in the below-100 KVA market and holds approximately 50% of the 100-250 KVA space for new units.
In the end, vendors will always move toward what customers demand. As long as customers or their engineers want to use a transformer-based UPS, vendors will build them. As time goes on and the transformerless UPS eventually establishes a record of reliability, the market will decide which will be become the favored choice in the mid-market space.
Like the early light beer commercials (real beer versus light beer -- i.e., "great taste, less filling"), the debate over the transformer-based versus transformerless UPS sometimes becomes a matter of political or religious beliefs. Ultimately, however, unless there proves to be a rash of failures of transformerless UPSes, they will continue to increase their market penetration into the larger spaces.
ABOUT THE AUTHOR: Julius Neudorfer has been CTO and a founding principal of NAAT since its inception in 1987. He has designed and managed communications and data systems projects for both commercial clients and government customers.
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