So, how do we improve the efficiency downstream of the UPS? We will focus on three key areas that offer opportunities: electrical distribution architecture, transformers and voltage.

Electrical Distribution Architecture

You should eliminate Static Bus Transfer Switches (STSs or SBTSs) at Power Distribution Units (PDUs) and Remote Power Panelboards (RPPs) downstream of the UPS wherever possible. In addition to having some inherent electrical losses, STSs add cost, reduce available floor space, can reduce availability (if they become single points of failure), and increase operational complexity. If STSs are necessary, they should be mounted in the computer cabinets and supply devices in those cabinets.

In Tiers I, II and III facilities, you should consider whether redundant PDUs (in effect, 2N or System + System distribution) make sense. In general, PDU transformer efficiency increases with percentage of load, so two PDUs sharing the same load will have a lower efficiency than one PDU supplying the same load. Also, single-cord servers are more electrical energy efficient than dual-cord servers. Stay tuned for more about transformer efficiency later in this article.

Just two thoughts about UPS architecture. If 2N or System + System architecture is required, you should consider 3N/2 or 4N/3 architecture as an alternate. Assuming that the critical load is 2000kW, a 2N arrangement would require two UPS systems, each of 2000kW capacity. In a 3N/2 arrangement you would need three UPS systems, each of 1000kW capacity. The advantages of the 3N/2 arrangement would be:
1. Lower initial cost since you are buying a total of 3000kW of UPS capacity versus 4000kW with the 2N arrangement.
2. More efficient operation since each 3N/2 system would operate at a higher percentage of rating than each 2N system. At full load each 3N/2 system would operate at 67% of rating while the 2N systems would operate at 50% of rating.
The disadvantage of 3N/2 or 4N/3 versus 2N is that either 3N/2 or 4N/3 requires more attention to load management than 2N.

The second thought about UPS architecture is to consider whether redundant modules in UPS systems make sense for the project. Redundant modules increase initial cost (CAPEX) and operating costs (CAPEX). The major component of increased CAPEX is the reduced operating efficiency resulting from redundant modules.

Transformers

Transformers downstream of the UPS are typically contained in PDUs. You should understand that every time we transform voltage we lose 1.5%-3% of the energy in the process. Eliminating transformers wherever possible is a good technique for increasing energy efficiency.

If you need transformers, think of using fewer larger transformers instead of smaller transformers. A larger capacity transformer is generally more efficient than a smaller capacity transformer. So, it makes sense to specify a smaller number of larger capacity transformers than the reverse. For example, the minimum Energy Star® efficiency is 98% for a 75kVA transformer and 98.6% for a 300kVA.

You should specify transformer efficiency at its intended normal operating load. Consider a generic Tier II arrangement where each transformer may operate at up to 90% of rating: peak efficiency should be specified at 50%-90% of rating. Contrast that to a generic Tier IV arrangement where peak transformer efficiency should be attained at 20%-45% of rating.

Voltage Considerations

Most present-day computer power supplies are rated for 100-240 volts, so they don’t care if the input voltage is 120 volts, 208 volts or 230 volts. Likewise, they don’t care if the input frequency is 50Hz or 60Hz.

In a traditional US design the UPS supplies 480 volts to downstream transformers that reduce the voltage to 208 volts for distribution to computer equipment. A 30 amp, 208 volt, 3-pole, 4-wire branch circuit can supply about 8kW of load.

400Y230 volt distribution is the standard in much of the world outside the US. Computer equipment is supplied at 230 volts, line-to-neutral. A 30 amp, 400Y230 volt, 3-pole, 4-wire branch circuit can supply about 15kW of load, roughly double what the traditional 208Y120 volts system can supply. There are seldom any transformers downstream of the UPS, so 1.5%-3% of efficiency is gained over the US practice.

To apply 400Y230 volts in the US in a 480 volts distribution system, you can have a large, very efficient transformer upstream of the UPS and eliminate the PDU transformers downstream of the UPS. The upstream transformer cost and energy losses can be much lower than those of the downstream transformers.

To apply 400Y230 volts in the US in a medium voltage distribution system is much simpler - you simply specify the substation transformer secondary voltage as 400Y230 volts. This substation transformer should be as efficient at 400Y230 volts as a 480 volts transformer of the same size. You will have specified a physically larger capacity UPS since you are operating the UPS at 400Y230 volts rather than 480 volts, but you have eliminated the PDU transformers downstream of the UPS and increased the efficiency. In a nutshell, you are balancing reduced transformer losses and branch circuit wiring costs against increased distribution wiring costs.

Here is a forecast for the future as long as we are discussing 400Y230 volts distribution. Once the computer power supply manufacturers increase the acceptable input voltage from 240 volts to 277 volts, all advantages for 400Y230 volts distribution disappear in the US. At that time, 480Y277 volts will be the standard UPS output voltage and the distribution voltage for new data centers. A 30 amp, 480Y277 volts, 3-phase, 4-wire branch circuit will supply 18kW that is 1.2 times the capacity of a similar size 400Y230 volts circuit and 2.3 times the capacity of a similar size 208Y120 volts circuit.

Another voltage consideration is using 575 volts rather than 480 volts for bulk low voltage power distribution. 575 volts is a common voltage in Canada but is not in the US. It is finding increasing use in large US data centers because it can offer economies in initial costs and operating expenses.

Initial costs can be reduced because the same size conductor, bus or circuit breaker can carry 20% more power at 575 volts than it can at 480 volts. For example, a 3750kVA substation can have a 4000 amp secondary bus at 575 volts while a 5000 amp bus is required at 480 volts.

Operating costs can reduced because of reduced conductor losses.

My forecast for 575 volts is that it will sadly fall out of favor once 277 volts computer power supplies become available. I have been told by computer power supply manufacturers that they have no hopes of being able to increase the acceptable input voltage to 347 volts (line-to-neutral voltage in a 575 volts system). So, distributing 575 volts bulk low voltage power and then transforming to 480 volts for UPS would not make sense.

In summary, we have discussed three readily available tools to increase electrical efficiency downstream of the UPS. Keep them at hand and use them to reduce operating costs and increase efficiency. Best wishes!

About the Author: Christopher M. Johnston, PE is the National Critical Facilities Chief Engineer for the Syska Hennessy Group.

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