Data center owners and operators have begun to exhibit interest in feeding the technical floor of a data center at an elevated voltage in order to reduce losses. The basic principle of this involves the utilization of IT servers that employ switch mode power supplies (SMPS). Many SMPS powered servers are capable of operating not only either at 50 or 60 cycles, but also over a wide single-phase voltage range (typically from 100 volts alternating current [Vac] up to 240 Vac) without modification.
Generally, higher operating voltage means greater server efficiency. That means that a 50-hertz (Hz) server having a nominal single-phase voltage of 230 V (400 V three phase) or 240 V (415 V three phase) will be more efficient than 60-Hz counterparts operating at either 120 or 208 V. The principles are identical at either voltage.
Data center owners and operators have begun to exhibit interest in feeding the technical floor of a data center at an elevated voltage in order to reduce losses. The basic principle of this involves the utilization of IT servers that employ switch mode power supplies (SMPS). Many SMPS powered servers are capable of operating not only either at 50 or 60 cycles, but also over a wide single-phase voltage range (typically from 100 volts alternating current [Vac] up to 240 Vac) without modification.
Generally, higher operating voltage means greater server efficiency. That means that a 50-hertz (Hz) server having a nominal single-phase voltage of 230 V (400 V three phase) or 240 V (415 V three phase) will be more efficient than 60-Hz counterparts operating at either 120 or 208 V. The principles are identical at either voltage.
Basic Distribution Options
The starting point for a large data center will invariably be at medium voltage. From here, the designer might decide to step down to 480 V for the main distribution, feed the essential loads directly, and then employ transformers to step down to the 400 V required for the IT loads (see figure 3). This somewhat defeats the purpose because the second transformer re-introduces losses, although these could be less than for the 480/208 V concept because autotransformers can be employed.
Alternatively the designer could use two distribution systems, one at 480 V and one at 400 V (see figure 4). This eliminates the additional series transformer but may increase capital expense for two standby generator systems at different voltages. In many cases, that would also mean wasted redundancy of engines, additional space, and higher maintenance costs. Finally, figure 5 shows a third option, if the UPS system could be made to cope with dissimilar input and output voltages.
Standby Generators with Static UPS
Conventional static UPS designs usually employ double conversion power techniques (ac to dc to ac). This double conversion can be employed to produce nominal output voltages of 400 V at 60 Hz, even though the source voltage might be 480 V. Such a unit is not standard, but the technology is fairly straightforward. There is a drawback however; the static UPS normally relies upon the use of a wraparound static transfer switch (bypass) for fault clearing and peak overload management.
When the input and output voltages are significantly different (480 V to 400 V respectively), this static bypass switch cannot be used. Accordingly, the static UPS that is used for 480/400 V conversion will be severely limited in its normal functionality and compromise the system as a whole.
Standby Generators with Hybrid Rotary UPS
The Hybrid Rotary UPS is an alternative to the static UPS. It combines a double-conversion circuit with a static switch path, both feeding into a synchronous motor-generator (MG) set. The wraparound bypass in this system (not shown) is not required for fault clearing or peak overload management. The normal operating path is directly via the static switch, through the MG set and the voltage differential are managed between the transformer action of the MG set and the winding excitation.
Furthermore, if desired, the designer can use the isolating transformer action of the MG set for harmonic isolation or local grounding, independent to the upstream transformers.
The Hybrid Rotary UPS has been employed in both voltage conversion and frequency conversion applications for many years.
Diesel Rotary UPS with Dual Voltage Isolated Windings
In normal operation, the essential loads are fed directly from the utility source. However, when in emergency operation, the MG set can be used as a dual synchronous generator, thereby providing separate electrical sources for the IT and the essential loads. Additionally, the independence of the windings means that the two circuits can operate at different voltages: 480 V for the essential loads and 400 V for the IT loads. By adopting this approach, there is no need for duplication of standby generation, no serial connection of transformers, and no compromise of UPS function. What’s more, the UPS can be used either in conjunction with conventional battery systems or with kinetic energy storage flywheels when space considerations are paramount.
Conclusion
The implementation of a higher efficiency 400 V distribution need not be a complex process and can generally be achieved with few modifications to typical one-line concepts. The barriers to entry are more about breaking the mold of convention than they are about technical difficulties. With most elements of the 400 V circuit being readily available, including the UPS products, the cost to implement a 400-V solution will be comparable with the 480-V system. Therefore, the data center operator can enjoy the returns of lower running costs right from the first day of operation.
For example, in a 50,000-sq-ft data center designed for 200 W/sq ft, a 3 percent efficiency gain produces an annual saving in electricity of around 2,650 MWh. That’s a saving in operating costs of up to $400,000 per year that will flow directly to the bottom line and significantly reduce the investment payback period.
Sidebar: Breaking with Convention
Q. Are the new fault levels a problem?
A. After elimination of the transformer the short circuit current, for the rest of the system downstream of the transformer, goes up. This will require slightly different short circuit, arc flash, and coordination studies. Due to the new short circuit currents, the breaker must be able to interrupt a higher short circuit current. In most cases, the cost of the transformer will outweigh the cost of any higher fault capacity breakers. That means saving money. The coordination will not be generally affected. The arc flash situation will vary case by case, and in fact, may be improved depending on the breaker settings.
Q. Is harmonic isolation still required?
A. The elimination of the transformer will let the harmonics flow into the system upstream of the PDU. Fortunately, with the newer generation of SMPS, the issue of load harmonics has been greatly improved. These new power factor corrected power supplies introduce a very low level of harmonics to the system. This allows the removal of the PDU transformer with little impact on the upstream system characteristics.
Q. Is there more distribution required with dual voltage systems?
A. The simple answer is no. In principle, the common 480-V data center usually has two distinct distribution circuits: one feeding the essential loads and one feeding the IT loads. In a large data center, these two circuits can not be handled by a single medium to low voltage transformer and accordingly, two or more transformers will be employed. In a dual voltage solution, one set of these transformers will have a 400-V secondary.
Q. What is the optimum solution for the 400-V UPS requirement?
A. From the analysis above, the preferred topology is to implement the UPS system with dissimilar input, and output voltages. There are then three possibilities for consideration: standby generators with either static or rotary UPS for voltage conversion or diesel rotary UPS with dual input/output voltage capability. The other solutions add unnecessary cost and/or reduce the efficiency gains.
