400/230 Volt 60Hz UPS Power

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Using Dual Voltage standby generation and UPS in one

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Contents

1 Abstract...... 3

2 Introduction ...... 4

3 Alternative Power Distribution ...... 6

4 Integrating UPS in a 400 V 60 Hz distribution...... 8

5 Supplying Essential Loads in a 400 V distribution ...... 10

6 Integrated power supply of Critical and Essential loads...... 13

7 Summary...... 15

8 References...... 16

1 Abstract

Within data centers the power used for operating the facility, running IT loads and cooling is the largest expense. With power densities continuing to rise, overall efficiency is still a major issue especially as power cost increases as well. The more the power density per rack raises the more floor space needs to be allocated for power supply components like PDUs, breakers and cabling.

The power distribution through the building to the IT loads contains several power lines, converters and transformers. Each components losses require equivalent cooling which consumes additional power. Reducing the number of components and operating IT equipment at 400 V will save floor space and will result in greater efficiency and reduced electrical costs. The resulting alternative power distribution allows different ways of integrating UPS modules including the power distribution for the short break loads. A special kind of Dual Output Diesel Rotary UPS with two output voltages integrates the power supply of critical and essential loads in one unit and additionally reduces upfront capital costs, infrastructure and floor space.

2 Introduction

To understand this new approach to power distribution inside United States data centers, it helps to look at the current standard distribution systems first.

The power delivered to most large commercial buildings and IT facilities is either 480/277 V 3-phase or medium voltage 3-phase. In case of a medium voltage feeder the voltage needs to be transformed down to 480/277 V which is the typical voltage level for the facilities internal power distribution. The voltage is 480 V line-to-line and 277 V line-to-neutral with a frequency of 60 Hz, like it is shown in Figure 1.

Line 1 277 V 480 V 480 V Line 2 277 V 480 V N Phase 1 Phase 2 Line 3 277 V

Phase 3 Neutral

Figure 1: 480/277 V used for power distribution inside a facility

Switch-mode power supplies inside IT equipment typically operate within a voltage range of 100 V to 240 V single-phase, so the voltage level of the power distributed inside the facility is not suitable for this type of equipment. The voltage must be stepped down before it can be fed to the power supplies inside the computer racks. This is achieved by routing the power through an isolation transformer located inside a Power Distribution Unit (PDU) where it is transformed from 480/277 V to 208/120 V 3-phase, as outlined in Figure 2.

MV-Transformer PDU-Transformer

MV Feeder Facility Distribution IT Distribution IT-Loads MV 3x 480/277 VAC 3x 208/120 VAC

Line X 208 V 120 V 208 V IT-Loads 208 V Line Y 120 V 208 V N Phase X Phase Y Line Z 120 V 120 V IT-Loads Phase Z Neutral

Figure 2: 208/120 V standard power distribution for IT equipment

From the PDU, power is typically distributed in three ways:

1. 120 VAC single-phase (line-to-neutral)

2. 208 VAC single-phase (line-to-line)

3. 208 VAC 3-phase (for further distribution before being split into single phases)

In the past it was not practical to consider alternative voltage levels for the power distribution because a significant fraction of IT equipment operated from 120 V. But in modern high density data centers most switch-mode power supplies for IT devices are designed for worldwide compatibility and do accept both low-line voltages 100-120 V and high-line voltages 200-240 V. So having 120 V available in the rack might be useful to power some legacy devices but should be of no importance in modern data centers.

Considering the fact that running the devices at the higher voltages will increase efficiencies by approximately 2 to 3.5% [1], feeding the power supplies with 208 V or higher should be the preferred solution.

3 Alternative Power Distribution

As mentioned before virtually all IT equipment manufactured today is designed for worldwide compatibility. This means that it can operate with the North American 208/120 V voltage system, which is also used in Japan, as well as with the European 400/230 V voltage system. Mentioning only two voltage systems takes into account that many derivates like 380/220 V or 415/240 V are already included within the typical tolerance of ±10% related to these standard voltage levels.

It is obvious that data centers using the European voltage level of 400/230 V do not need any PDU transformers because they can feed the 230 V line-to-neutral directly to the IT devices, like it is shown in Figure 3.

MV-Transformer

MV Feeder Facility + IT Distribution IT-Loads MV 3x 400/230 VAC

Line 1 230 V 400 V 400 V Line 2 230 V 230 V 400 V N IT-Loads Phase 1 Phase 2 Line 3 230 V

Phase 3 Neutral

Figure 3: 400/230 V distribution directly feeding the IT equipment

Since all kind of IT equipment uses switch-mode power supplies it operates independently of the frequency of the supply voltage and can be used in 50 Hz and 60 Hz voltage systems as well. Looking at the data sheets of power supplies like the HP ProLiant DL380 G5 shows, that operating rack-level equipment at 230 V vs. 208 V will result in up to 1% efficiency gain, and the efficiency gain will be up to 3% compared to equipment that is still running at 120 V.

The difference between 400/230 V and the typical US Baseline System becomes readily apparent when the power capacity for a three-phase branch circuit is calculated. Assuming that the feeding circuit is designed for 20 A, the power capacity of a 208/120 V circuit is 7.2 kVA, while the power capacity of a 400/230 V circuit is 13.8 kVA. So based on the same circuit current rating, the 400/230 V distribution is able to provide 92% more power than the 208/120 V distribution. This almost doubles the power density capability per rack without the need of changing the diameter of the cabling of the distribution network.

Regarding the overall cabling losses it also needs to be considered that the currents in the main distribution network rise by 20% changing the voltage from 480 V to 400 V. This slightly reduces the positive effect of the above mentioned raised power capability in the branch circuits.

Even more important than the cost, size and weight savings in wiring is the elimination of the PDU transformers. In a North American data center utilizing high density racks approximately 20-30% of the floor space and the total weight on the high raised floor is consumed by the PDUs. Realizing that this impact is lower using racks with less density, the new power distribution system with higher voltages and no PDU transformers becomes even more important as the power density of the racks increases.

As important as the savings in space and weight are the savings of energy by leaving out the PDU transformers. This results in an additional efficiency gain of 1.5% to 2%, based on the typical Energy Star® efficiency guideline for PDU transformers.

Eliminating the PDU transformers also leads to increased fault current levels down to the branch breaker level. The positive effect of having this high short circuit current capability is the ability to clear any fault in the branch circuits within a few milliseconds and therefore without effecting the remaining loads. But the increased short circuit currents also need to be considered regarding breaker selection and tuning to guarantee proper electrical discrimination.

So considering a modern data center which devices already run at 208 V and which utilizes highly efficient PDU transformers the energy savings by changing the distribution voltage to 400/230 V can be expected to be between 2.5% and 3%, but can raise up to 7% if devices are still running at 120 V and less efficient equipment is used. Savings in space, cabling, material and cooling need to be considered additionally and show the high potential of overall savings coming with the change to a 400/230 V distribution.

4 Integrating UPS in a 400 V 60 Hz distribution

Even though the 400/230 V system is very common outside Northern America, standard 400 V UPS units can not be used in the United States. The reason for this is that the standard 400/230 V comes with a frequency of 50 Hz, which is significantly different to the 60 Hz mains frequency in the US.

So the distribution voltage needed for North American data centers is 400/230 V 60 Hz, a combination of voltage and frequency which is not a standard, but which easily can be generated from the existing 60 Hz mains by using transformers.

There are two basic ways to implement a UPS in such a distribution system:

1. Using a standard 480 V 60 Hz UPS and add an autotransformer to its output to convert the voltage to 400 V.

2. Using a special 400 V 60 Hz UPS with no need for an additional transformer downstream.

On the first sight solution #1 seems to be the most convenient because standard 480 V 60 Hz UPS units can be used. But it adds another transformer to the distribution network, which has just been removed. Nevertheless this can still be considered to be an advantage because of the low voltage difference that allows the utilization of autotransformers. This kind of transformer has a much better efficiency and requires less space than the isolation transformers required for the PDUs.

But the chance to get a distribution network without additional transformers and to avoid their losses, costs and space requirements is still worth being considered. First of all this requires UPS modules that are designed to operate with 400 V 60 Hz. This should not be a serious problem for the UPS manufacturers and so the remaining question is where to get the feeding 400 V 60 Hz from.

In case of a 480 V feeder from the utility there again an extra autotransformer is needed to convert the 480 V to 400 V. But, in contrast to place a transformer downstream the UPS, an upstream transformer, like it is shown in Figure 4, can be designed much bigger to feed multiple UPS units, which comes along with a much better efficiency than an autotransformer for a single unit would have. Additionally an upstream transformer has no influence on the UPSs short circuit capability and simplifies the design of the downstream distribution network.

MV MV

480/277 V 480/277 V 60 Hz 60 Hz

UPS UPS UPS UPS

400/230 V 60 Hz IT-Loads IT-Loads Figure 4: Moving the transformer upstream the UPS can increase the overall efficiency and simplifies the downstream distribution

A medium voltage feeder, like it is common for large data centers with a power consumption above 5 MVA, allows the generation of 400 V 60Hz without any additional transformers by just using an MV transformer with 400 V secondary voltage. Figure 5 shows the design simplification as a result of this.

MV MV

480/277 V 400/230 V 60 Hz 60 Hz

400/230 V 60 Hz

UPS UPS UPS UPS

IT-Loads IT-Loads Figure 5: A 2nd transformer becomes unnecessary in case of a 400V feeder

So this would be the optimal condition to realize a highly efficient and reliable 400/230 V power distribution for data centers in the 60 Hz world.

5 Supplying Essential Loads in a 400 V distribution

The term Essential Loads describes the type of equipment that is essentially necessary to run a data center but which is less sensitive to power outages than the critical IT loads. In case of a mains outage these short-break or mechanical loads like chillers and pumps for cooling purposes will typically be supplied by Diesel generators after a short power interruption of approximately 15 seconds. They are normally directly fed from the common 480 V power distribution and need no additional transformers, as it is outlined in Figure 6.

D MV G 480/277 V 60 Hz

UPS

PDU- Transformer 208/120 V 480/277 V 60 Hz 60 Hz IT-Loads Short-Break-Loads Figure 6: Conventional Distribution including IT- and Short-Break Loads

It is also quite common that these loads are fed by Diesel Rotary UPS (DRUPS) with a so called Dual-Output, like it is shown in Figure 7.

480/277 V 60 Hz

DRUPS

PDU- Transformer 208/120 V 480/277 V 60 Hz 60 Hz IT-Loads Short-Break-Loads Figure 7: Conventional Distribution Scheme utilizing Dual Output DRUPS

This type of DRUPS utilizes an energy storage which is designed to support the critical loads only, but the Diesel engine is strong enough to additionally supply the short break loads once it has been started. The 2nd output of this kind of DRUPS is either taken directly from the UPS output or from an isolated 2nd winding of the generator. The breaker pair feeding the short-break loads is normally controlled by the DRUPS and can be integrated in the DRUPS switchgear cabinet as well as in the main distribution switchgear.

Most mechanical loads are designed to be supplied by a standard combination of voltage and frequency (i.e. 480 V 60 Hz or 400 V 50 Hz) and are therefore not suited to operate with the unusual combination of 400 V and 60 Hz. So in case of a 400/230 V IT distribution the 480 V for the short-break loads needs to be taken from the line which feeds the UPS or, in case of high power applications, from a separated MV transformer.

An example of a standard 480 V distribution including a 400/230 V supply line for the IT loads is shown in Figure 8. The power for the critical and the essential loads is taken from a common 480 V bus which can be fed by standard 480 V Diesel generators to supply both the critical and the short break loads in case of a mains outage. To take the advantage to supply multiple UPS with one transformer the auto-transformers feeding the 400 V IT distribution are placed upstream the UPS. There is no difference in the supply for the essential loads compared to the conventional 208/120 V distribution.

D MV G 480/277 V 60 Hz

UPS

400/230 V 480/277 V 60 Hz 60 Hz IT-Loads Short-Break-Loads Figure 8: Common 480 V distribution line for IT-Loads and Short-Break-Loads

Figure 9 shows a solution that does not utilize additional auto-transformers. This scheme uses 2 separate MV-transformers, each transforming the voltage right to the level needed for the loads. Considering that the power consumption of a big data center requires more than one MV transformer feeding the loads, this can simply be realized without creating additional costs by splitting the necessary transformers into two groups, each supplying its dedicated distribution network.

MV MV

400/230 V 480/277 V 60 Hz 60 Hz D D G G

UPS