Thursday, March 13, 2014

Data Center Power System Design

Engineers should take a closer look at the different power strategies being used to distribute power, and how they impact the data center.

Alternating Current (AC) versus Direct Current (DC) is a battle that has been going on for more than a century and continues today in the data center industry. Although AC power is the standard, based on its potential for eliminating conversion losses and improving efficiency, many believe that DC power is the future of data center distribution. Still others believe that the same level of efficiency can be achieved with AC by using more efficient equipment with higher voltage distribution.

Electrical systems usually waste energy in the form of losses due to inefficiencies in the electrical equipment and distribution system. On average, the electrical distribution system losses account for 12% of the total energy consumed by the data center. For a data center with 2000 kW of IT load (2700 kW total load), that equates to an annual cost of USD$280,000

Power System Design Tips

So please review these six key items when planning a data center power distribution system:

  • Install or replace existing power and IT equipment with energy-efficient equipment
  • Review the proposed IT equipment to determine if the systems can operate on 240 V AC or 380 V DC
  • Review all the advantages and challenges of the different power systems
  • Determine how much of the existing infrastructure would need to be replaced to change power systems
  • Design flexibility into the power system that will allow the data center to adapt in the future
  • Design a power system that is modular to eliminate partial loading

Similar to the mechanical systems, modifications can be made to the electrical system to make it more efficient and save energy. The key to a good mission critical facility design is not to degrade the reliability of the facility in the process.

Typical Electrical Distribution Systems

Typical legacy data center electrical distribution system is made up of five major components:

Power is supplied to the data center at medium voltage from a utility/generator power source. The power is stepped down from medium voltage to distribution voltage by a substation transformer. The power then goes through an Uninterrupted Power Supply (UPS) system that conditions the power and provides ride-through capability during an outage until the generator starts. The power is then stepped down to substation voltage by a Power Distribution Unit (PDU). The PDU supplies power to the IT power supply where it is rectified and stepped down DC power, which is the internal operating voltage of the IT equipment.

Utility / Generator --MV AC--> MV/LV Transformers --480V AC--> Switchgear --480V AC-->
UPS --480V AC--> PDU --208/120V AC--> IT Power Supply --12V DC--> Servers

Four components in the legacy electrical distribution system with the highest losses are:

  • Substation transformer: Transformer no-load and core losses
  • UPS: Rectifier and inverter losses
  • PDU transformer: Transformer no-load and core losses
  • IT power supply: Rectifier and transformer losses

One method for increasing efficiency is to replace those pieces of equipment with more efficient equipment. Today with ultra-high-efficient transformers that efficiency is above 99.5%. Conventional double conversion UPS systems range from 84% efficient at 25% load to 94% at 100% load. Using flywheel or passive standby UPS topology can increase that range to 94% efficient at 25% load and 99% at 100% load.

Another method for increasing efficiency is to eliminate partial loading of the data center. Eliminating partial loading reduces losses by allowing the equipment to operate at its peak operating efficiency. This can be performed by designing a power system that is modular, grows with the load, or by designing a power system that uses flexible tiers, and matches the reliability and redundancy to the different programs within the data center.

A third method is to eliminate the inefficient electrical equipment altogether. Increasing efficiency by eliminating the equipment that has the most losses is the reason why different power strategies are being investigated for data center distribution.

About the Blogger

Strategic Media Asia (SMA) is one of the approved CPD course providers of the Chartered Institution of Building Services Engineers (CIBSE).

SMA, a critical infrastructure training and event organizer based in Hong Kong, provides an interactive environment and opportunities for members of IDC industry and engineers to exchange professional views and experience on critical infrastructure and E&M facilities.

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