New standard for powering data centres



 

Both IT server installations and telecom sites are becoming increasingly power hungry, and the quest for greater efficiency is driving a move away from the classic AC UPS and 48 V DC configurations to systems which offer fewer losses and reduce the amount of equipment to be installed. Systems based on 400 V DC are an emerging standard and are available from several suppliers.

Data and telecomm centres are major energy users, with energy consumption estimated at 40 TWh in 2005 in the US alone, and 120 TWh worldwide [1]. A very large data centre requires in the order of 10 MW of power to support the computing infrastructure and this is expected to increase to 50 MW in the future.

In a typical data centre, less than half of this power is delivered to the information technology (IT) loads, which includes microprocessors, memory and disk drives. The rest of the power is lost in power conversion, distribution, and cooling, resulting in high utility bills, a large environmental footprint and the inability to fill equipment racks. For this reason, many organisations around the world are looking into ways to improve the efficiency of these systems and equipment [1].

In a typical telecomm centre, current distribution at 48 V DC is commonly used, but high loads and the high power density of modern equipment are making the use of this voltage impractical.

Fig. 1: Typical data centre AC supply system.

As more and more equipment (already a majority of demand from utilities) is transitioning to electronic loads, which are natively DC, it makes sense to eliminate costly and inefficient AC/DC and DC/AC conversions required to operate electronic equipment. The majority of renewable resource power generation systems inherently produce DC output. In present AC applications, this requires additional conversion stages to couple with existing electrical distribution [2]. A major concern is the percentage of power that is used for the application. In older telecomm buildings, more than half the power supplied is consumed by conversion and distribution losses, cooling and lighting.

A solution to the problem is to use DC distribution at a voltage between 260 and 400 V. The solution, which is referred to as high voltage DC (HVDC) in the IT industry, offers energy savings by reducing the number of conversion stages, resulting in fewer losses, less equipment, and a lower cooling load.

There are a number of systems available on the market which offer this approach and industry has not been far behind in developing standards to cover the system.

Standards

Standards are emerging which cover HVDC distribution in ITC installations. Typical would be the European Telecommunications Standards Institute EN 300 132-3-1 standard for 400 V DC, which sets the stage for applications in this area and for the development of more detailed standards. The standard covers the requirements for the interface between telecommunications and ICT equipment and its power supply and also includes requirements relating to power supply stability and measurement. The standard covers a voltage range of 260 to 400 V DC. Other industry standards covering this application have also been developed and are being applied.

Comparison of AC and DC systems

AC distribution system

In the AC system, protection against loss of power in the case of mains failure is provided by the UPS, usually a large centralised unit providing reliable power to all equipment in the data centre.

The output of the UPS is AC at a distribution voltage, which could be 380 or 600 V 3-phase. Each item of IT equipment is equipped with its own AC/DC converter which generally incorporates power factor correction equipment. The system typically has at least six stages of conversion or loss points, each of which contributes to the total losses of the system:

  • Incoming transformer
  • UPS system with two conversion stages AC/DC and then DC/AC
  • AC distribution system
  • IT equipment containing an AC/DC converter, with a typical efficiency of 65% and with PFC equipment

Fig. 2: HVDC system for data centre application.

Losses occur at each stage. The power factor correction system is essential to reduce extra losses due to low power factor on AC/DC converters contained in the equipment, and to reduce harmonics.

HVDC distribution system

In the DC system, a 400 V DC bus follows directly from the AC/DC conversion unit, and the battery back-up is connected directly to the bus. Each item of ITC equipment is provided with its own DC/DC converter. The system typically has four stages of conversion, or loss points each of which contributes to the total loss:

  • Incoming transformer
  • AC/DC converter
  • DC distribution
  • DC/DC converters with a typical efficiency of 90%

The HVDC system typically has at least two stages of conversion less than the AC system, and may have even more savings depending on the actual configuration. With DC power, operational issues, such as harmonics and phase/voltage balancing, are eliminated. Local energy generation and storage are easy to integrate on a common DC bus.

Savings on DC compared to AC distribution

Energy savings

The savings claimed by various suppliers vary. One study found that using 400 V DC power delivery would result in an energy savings of approximately 7%, as well as a 33% space savings, a 200% reliability improvement, and a 15% electrical facility capital cost savings.

Reliability is dramatically improved with the 400 V DC bus connected directly to the batteries. Claims of energy savings as high as 10 to 20% have been made, but are dependent on the existing configurations, and a claim of 28% savings vs. the worst AC case has also been made [3]. A figure of 7%, when compared to the most efficient AC distribution system, seems to be a reasonable assumption.

Energy reduction also adds to a reduction in cooling requirements, an additional spin off which needs to be taken into account. Using DC driven fans would also improve the efficiency of the cooling system.

Space savings

The lower number of converters provides a space savings in terms of equipment cabinets, and the HVDC bus concept allows location of the battery separately from the converter stage, in contrast to the AC UPS, where the battery is usually housed within the UPS.

Fig. 3: Difference in cable required for 50 V DC and 400 V DC systems, based on
transmitting 200 kW over 80 m [4].

Synchronisation of redundant multi source systems

The adoption of HVDC distribution eliminates the need for complex synchronisation circuits associated with multi-source AC distribution. Where redundancy is built in to ensure reliability, parallel paths need to be closely synchronised to avoid phase jumps when changing sources, and circulating currents when sources are connected in parallel. DC systems do not have this problem.

Balancing the AC supply

AC power is provided to a building from the utility in three phases. Utilities and much of the data centre equipment (like UPSs and generators) expect the load on the phases to be nearly balanced. Phase balancing creates a significant challenge for data centre operations. Many of the devices in the data centre have single-phase power supplies.

To support single phase power supplies, the three-phase power is split into single phases. It is up to the installtion teams to make sure that power consumption is spread evenly among the different phases. This effort is complicated by the fact that power consumption is highly dependent both on hardware configuration and on workload; as workloads change, a hardware distribution that is balanced today may not be balanced tomorrow. With 400 VDC power, the notion of phase goes away completely.

AC power factor, phase angle and many other complicating factors are no longer relevant. From an operational standpoint, this relieves the end user burden of balancing the load. In addition DC distribution does not suffer from losses related to skin effect and proximity effect present in ac circuits due to fundamental frequency and harmonics content and eliminates the need for components derating and additional filtering.

Telecom power systems

50 V DC was originally chosen as the most suitable voltage to directly drive electromechanical switches. Digital telecomm equipment uses lower voltages and DC/DC converters are used in most equipment fed from the 50 V supply today. The move to digital switching and transmission meant a substantial increase in the power density requirements at telecomm centres, which led to large conductor sizes being required to carry current at 50 V DC, and centres originally equipped with 50 V DC systems are struggling to meet the new demands.

The most obvious savings for telecomm systems will be in cabling and losses. Use of 400 V DC compared to 50 V DC results in an eight-times reduction of current, and an associated eight-times reduction in conductor size required. In addition a reduction in protection device sizing and rating will be achieved.

Fig. 4: Options for additional DC sources and battery placement.

Safety and certification

The IT environment uses low voltage AC distribution at the moment, and thus no additional certification would be required. In the telecomm environment, the adoption of 400 V DC distribution poses a regulatory problem, as the voltage levels moves from the “extra low” voltage of 48 V to the low voltage sector, as defined by SANS 0142, which is subject to a more strict set of regulations and requires an accredited person to do any work.

Although, according to regulations, 48 V DC also requires accreditation, there is no industry qualification in South Africa, other the full electrician accreditation, that meets this requirement, and work on such systems is regularly carried out by telecom trained technicians without incident. 400 V DC is just as dangerous as 400 V AC, and can cause electrocution, so addition insulation and protection of conductors is required, as well as additional training for technical staff. 400 V DC installations will be required to be performed by accredited persons and will require certification.

Bare busbars are a common feature of 48 V DC systems, but 400 V DC will require a totally different distribution system from 48 V DC. Such systems have been developed and are available on the market.

Additional advantages

The adoption of LED lighting, which is DC driven, can also make use of 400 V DC supplies in a data or telecom centre. LED lighting can result in energy savings of up to 80% on lighting and use of the 400 V DC system eliminates the need for converters.

DC backup advantages

The use of a DC bus allows backup power, in the form of batteries, to be located anywhere in the distribution network. In a classical AC UPS based system, the battery is located with the UPS system. With a DC distribution system, the back-up power can be located as close to the equipment cabinet as required. Locating the battery remotely from the load increases the chance of failure due to failure of the distribution system, and the more complex the distribution system is, the greater the chance of failure. Locating the back-up battery at the end of the distribution system reduces the chance of failure.

Fault tolerant configurations

Where fault tolerant redundant supply systems are required, DC distribution offers advantages over AC, the main being that there is no need to synchronise duplicated and redundant (N+1) sources and interconnection between duplicated supply paths is far simpler.

Supplementary DC sources

Supplementary power from DC sources such as solar PV and fuel cells could be connected directly to the DC bus (Fig. 4).

Distribution systems hardware

Systems on the market are based on plug-and-play overhead busbar systems, with correctly insulated connectors that allow rapid assembly and dismantling of systems. DC operates at a PF of unity, compared to a possible 0,9 for AC, so smaller busbars can be used to carry the same amount of energy. A full range of connection and protection devices suitable for 400 V DC is available.

References

[1] T Aldridge, et al: “Evaluating 400V Direct-Current for Data Centers”, Intel labs.
[2] Emerge Alliance: “380 VDC Architectures for the Modern Data Center”, www.emergealliance.org
[3] E de Jong: “DC power distribution for server farms”, www.leonardo-energy.org
[4] Vertiv: “Netsure 400 VDC power solutions”, Vertivco brochure.

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