Data center power strategies

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

11/17/2013


Learning objectives

  1. Understand the different strategies used to distribute power in a data center.
  2. Learn how to measure power efficiency in data centers.
  3. Know which distribution variation is most appropriate for the application.

Figure 1: This university data center has multiple data halls. This data hall is a Tier III design with fully redundant (2N) UPS systems. Power is distributed to the server cabinets at 415/240V using overhead plug-in type busway. Courtesy: JacobsNikola Tesla’s alternate current (ac) versus Thomas Edison’s 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 such as 415/230 V and 480/277 V.

So how do you know what power strategy is best for your data center application? What are the advantages and challenges of each type of power distribution technique? These are important questions that need to be evaluated when planning a data center. The goal of this article is to take a closer look at the different power strategies being used to distribute power and how they impact the data center.

Electrical efficiency

One of the most common metrics for measuring efficiency in data centers is power usage effectiveness (PUE) created by The Green Grid. It compares the total data center facility power to the power used to run the IT equipment. The optimum data center would have a PUE value of 1.0, where all the power going into the data center is being directly used to power the IT equipment. Any value above 1.0 means that a portion of the total facility power is being diverted to data center support systems such as cooling, lighting, and the power system. The higher the PUE number, the larger portion of power is consumed by the support systems relative to the IT equipment itself, resulting in a less efficient data center.

Figure 2: In this data center power consumption example, the IT load makes up the bulk of the electrical load. Courtesy: JacobsIn the recent past, the primary focus with lowering the PUE and increasing efficiency has been on the mechanical systems and the ability to use free cooling. As data center owners strive to further reduce cost, the focus has shifted toward electrical systems. Electrical systems 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 $280,000 (see Figure 2).

 

 

Power system design tips

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 Vac or 380 Vdc
  • 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 and scalable 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

The 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 (480 V) by a substation transformer. The power then goes through an uninterruptible 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 (208/120 V) by a power distribution unit (PDU). The PDU supplies power to the IT power supply where it is rectified and stepped down to 12 Vdc, which is the internal operating voltage of the IT equipment (see Figure 3).

Figure 3: The typical legacy data center electrical distribution system is made up of five major components. Courtesy: Jacobs

The 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. Prior to 2005, when the NEMA TP1 Guide for Determining Energy Efficiency for Distribution Transformers was adopted, transformer efficiencies were around 97%. 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 and scalable, one that 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.


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LEONID , MD, United States, 12/31/13 12:28 PM:

In my opinion, 120 volt power branch circuits should be eliminated in all industrial installations and large IT centers. Three phase 380/220 VAC and/or single phase 480/240 VAC circuits should be used instead.
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