Shopping for the Right Size

Capacity planning is one of the trickiest challenges that designers of mission-critical facilities face. Many data centers go to extremes to assure reliability, installing several layers of costly backup and redundancy with heavy investments in equipment—switchgear, switchboards, UPS and parallel gensets.


Capacity planning is one of the trickiest challenges that designers of mission-critical facilities face. Many data centers go to extremes to assure reliability, installing several layers of costly backup and redundancy with heavy investments in equipment—switchgear, switchboards, UPS and parallel gensets.

Assuring the reliability of the incoming power source means installing multiple sources of incoming power with dual utility feeds, usually from different substations or power utility grids, that are interconnected to the facility's main load via breakers. Dual unit substations (transformers) are then stepped down to deliver power to the server cages and cabinets. So, if one power source or high voltage line goes down, the system can switch to the redundant feed in a matter of milliseconds without losing any electrical load. Likewise, if a transformer fails, the backup system picks up the load instantaneously.

One of the biggest costs of delivering redundant power is installing oversized transformers rated to accommodate high-capacity loads, redundancy and harmonic current. To support future expansion, most data centers install transformers with ratings of between 1000 kVA and 3,750 kVA, based on calculations that factor in the current load and the potential maximum load anticipated for five to seven years down the road. The downside of super-sizing is that when trans-formers carry light loads relative to their nameplate sizes, the considerable losses that result from energizing the magnetic core drive efficiency down.

Compounding the issue, the high purchase and installation costs are further inflated by the life-cycle costs of running at lower efficiency levels, wasted electricity and load losses that occur when power passes through the transformer.

Can designers assure capacity and re-liability while managing losses and controlling costs? Yes. The key is to optimize the costs of energy, reliability, maintenance and installation. To achieve a better total cost of ownership, many experts advocate sizing transformers to step down voltages as close to load as possible by optimizing rather than oversizing the transformer.

Smaller, optimized transformers compensate for harmonics and minimize the losses incurred by energizing the transformers. Consequently, they can be sized more efficiently and can be implemented as part of a modular data center design that assures scalability, enabling highly complex systems to be built from smaller, more manageable building blocks.

Clearly, in the transformer equation, optimization equals efficiency, and the most efficient way to deliver energy to critical power facilities is to buy what you need today and add capacity incrementally as the data center fills out. This extensible approach contradicts the conventional method of determining power capacity based on watts per sq. ft. However, it makes more economic sense on several counts. First, the initial costs of purchasing and installing smaller transformers are lower than that of larger ones. Second, lowering the initial cost enables you to recoup the time value of money. Third, you can eventually buy the capacity you need by purchasing a second transformer when you need it—four, five or six years down the road. So, instead of running a large transformer at 20% or 30% capacity or less, running a right-sized trans-former at between 50% and 75% capacity can result in dramatic savings.

But what about capacity and reliability? When you shift the focus to efficiency and cost savings, are you not reversing the previous equation at the expense of reliability? Not if you specify transformers optimized to operate at low temperature rises to prevent overheating, support higher loads, carry and dissipate harmonic current, and manage the magnetic core current. This minimizes the losses that contribute most to low efficiency for lower loads. Because optimized transformers are sized more appropriately for the loads they carry, incur smaller core losses and use less energy, they perform more efficiently than larger transformers. As for redundancy, they can be used in redundant con-figurations to deliver the reliability that critical power applications require just as easily as larger transformers.

The current trend is to do whatever it takes to deliver reliability. However, transforming a competitive foothold into a competitive edge means delivering reliability in the most cost-effective manner.

With an intelligent, integrated power solution, usage and costs can be tracked and measured throughout the data center by tenant, simplifying the process of identifying losses and their causes. Access to real-time performance data allows data center owners and managers to balance all of the components of total cost of ownership, making whatever modifications are required to return efficiency to peak levels whenever necessary.

In the end, balancing reliability and redundancy with cost savings comes down to numbers. With the proper calculations, a modular approach to right-sizing transformers, along with the ability to keep a finger on the electrical pulse of the power system, will simply add up to a much better looking bottom line.

Transformers Affect Power Quality

As an integral part of the electrical system, transformers play an important role in power quality. In some transformer configurations, a transformer can be an effective tool used to:

mitigate triplen harmonics (3rd, 6th, 9th, etc.);

reduce the magnitude of single-phase voltage sags;

provide voltage disturbance isolation.

However, other transformer configurations can result in problems: increased potential for damaging transient overvoltages; greater likelihood of unbalanced voltages and currents; and exacerbation of existing harmonic problems.

An end user must understand these and other ramifications when determining the transformer configuration. Two transformer configurations that are prone to power quality problems are the ungrounded and "open" secondary configurations. These two configurations are used by some utilities and end users for a variety of reasons (some valid, some not).

It is important to consider more than just the short-term economic advantages of the design. Other factors must be considered-for example, the type of load(s), amount of required maintenance and difficulty of troubleshooting.

Also, the total cost of ownership is an important factor when making any business investment. Although it is not easy to initially quantify, poor power quality can have a large cost. Power quality can affect the reliability, productivity and service life of electrical equipment. Given the global market that businesses must compete in, the consequences of production losses or increased production costs due to poor power quality can be severe. Transformer configuration can play an important role in the quality of the electric service to a facility and should be given careful consideration.

(For the complete story on the advantages and disadvantages of ungrounded delta and open-delta transformer secondary schemes, go to and click on the green "Electrical" button on the left.)

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