The neglected issue of UPS downstream power efficiency

Increasing electrical efficiency downstream of the UPS was almost ignored in the past, but is getting more attention today. With the focus on improving PUE (or its inverse, DCiE), the industry is realizing that efficiency downstream of the UPS is just as important as the efficiency of the UPS itself and upstream of the UPS. Remember, every 1% loss downstream of the UPS increases PUE by 1.2% to 3%.

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

Power distribution structure

Whenever possible, you should eliminate static bus transfer switches (STS or SBTS) on power distribution boards (PDUs) and remote power panels (RPPs) downstream of the UPS. In addition to some inherent power losses, STSs increase cost, reduce floor space, reduce availability (if they become a single point of failure), and increase operational complexity. If STSs are necessary, they should be installed on the computer cabinets and the power supply equipment in the cabinets.

In Tier I, II, and III facilities, you should consider whether redundant PDUs (2N or system + system distribution in terms of effectiveness) make sense. In general, the efficiency of PDU transformers increases with load, so two PDUs sharing a load will be less efficient than one PDU with the same load. In addition, single-core servers are more power efficient than dual-core servers.

There are only two ideas about UPS configuration. If a 2N or system + system configuration is required, you should consider a 3N/2 or 4N/3 configuration as an alternative. Assuming the critical load is 2000kW, a 2N configuration requires two UPS systems, each with a capacity of 2000kW. In a 3N/2 configuration, you need three UPS systems, each with a capacity of 1000kW. The advantages of a 3N/2 configuration are as follows:

1. The initial cost is lower because you are purchasing a UPS with a total capacity of 3000kW instead of the 4000kW capacity required in a 2N assembly.

2. Operate more efficiently because each 3N/2 system operates at a higher percentage rate than each 2N system. At full load, each 3N/2 system will operate at 67% of the rated value, while the 2N system will operate at 50% of the rated value.

The disadvantage of a 3N/2 or 4N/3 system is that more attention needs to be paid to load management compared to a 2N system.

The second thought about UPS structure is to consider whether redundant modules in the UPS system make sense for the project. Redundant modules increase initial cost and operating cost. A significant part of the cost increase is that redundant modules reduce operating efficiency.

transformer

The transformer downstream of the UPS is usually mounted on the PDU. You should understand that every time we transform the voltage, we lose 1.5%-3% of energy in the process. Eliminating the transformer when possible is a good technique to improve energy efficiency.

If you need transformers, consider using fewer large transformers rather than smaller transformers. A larger transformer is generally more efficient than a smaller transformer. Therefore, it makes sense to specify a smaller number of larger transformers rather than the other way around. For example, the Energy Star® minimum efficiency for a 75kVA transformer is 98%, and the minimum efficiency for a 300kVA transformer is 98.6%.

You should specify the efficiency of the transformer at the load for which it is planned to operate normally. Consider a common Class II installation where each transformer can operate at up to 90% of rating: the peak efficiency should be specified at 50%-90% of rating. Conversely, for a Class IV installation, the peak transformer efficiency should be 20%-45% of rating.

Voltage considerations

Now, most 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 50 Hz or 60 Hz.

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

400Y230 volt distribution is standard in many parts of the world outside the United States. Computer equipment is powered at 230 volts (line to midpoint voltage). A 30 amp, 400Y230 volt, 3-pole, 4-wire branch circuit can supply about 15kW of load, about twice what a traditional 208Y120 volt system can supply. So, in the U.S. practice, there are few transformers downstream of the UPS, and 1.5%-3% efficiency is obtained.

In the US 480 volt distribution system, using 400Y230 volts, you can have a large, very efficient transformer upstream of the UPS and eliminate the PDU transformer downstream of the UPS. The cost and energy loss of the upstream transformer is significantly lower than that of the downstream transformer.

Applying 400Y230 volts is a relatively simple matter in the medium voltage distribution system in the United States. You simply specify the substation transformer secondary voltage value to be 400Y230 volts. This substation transformer should be as efficient as a 480 volt transformer of the same size at 400Y230 volts. You can specify a larger capacity UPS because you are running the UPS at 400Y230 volts instead of 480 volts, but you have eliminated the PDU transformer downstream of the UPS and improved efficiency. In summary, you are balancing the reduction of transformer losses and branch circuit wiring costs against the increase in wiring expenses.

Here's a prediction for the future, we are talking about 400Y230 volt power distribution. Once computer power supply manufacturers increase the acceptable input voltage from 240 volts to 277 volts, all the advantages of 400Y230 volt power distribution in the United States will disappear. At that time, 480Y277 volts will be the standard for UPS output voltage and power distribution voltage for new data centers. A 30 amp, 480Y277 volt, 3-phase, 4-wire branch circuit will provide 18kW, which is 1.2 times the capacity of a similar sized 400Y230 volt circuit and 2.3 times the capacity of a similar sized 208Y120 volt circuit.

Another voltage consideration is to use 575 volts instead of 480 volts for most low voltage distribution. 575 volts is a common voltage in Canada, but not in the U.S. Now, this voltage value is increasingly found in large data centers in the United States because it can provide economic initial cost and operating expenses.

Initial costs can be reduced because the same size conductor, busbar or circuit breaker can carry 20% more supply voltage at 575 V than at 480 V. For example, a 3750 kVA substation at 575 V can have a 4000 amp secondary busbar, whereas at 480 V it would require a 5000 amp busbar.

Operating costs are reduced because conductor losses are reduced.

My prediction for 575 volts is that once 277 volt computer power supplies are in place, 575 volts will sadly fall out of favor. Computer power supply manufacturers tell me that they have no hope of increasing the acceptable input voltage to 347 volts (the line-to-midpoint voltage on a 575 volt system). Therefore, it makes no sense to distribute 575 volts to the bulk of the low voltage power and then convert it to 480 volts for the UPS.

In summary, we have discussed three readily available tools to improve electrical efficiency downstream of the UPS. Keep them on hand and use them to reduce operating costs and improve efficiency.