Data Center Design Consideration: Transformers and Harmonic Distortion

Data center managers and information technology (IT) engineers in today's critical facilities are in search of reliable and energy-efficient equipment with low total cost of ownership. But after equipment investments are made, it's important to pay attention to possible threats to the operational efficiency.

One threat that is often overlooked is harmonic currents, which can have a significant impact on electrical distribution systems and the facilities they feed. Wasted power and temperature fluctuations caused by these currents can prevent facilities from achieving maximum efficiency, so it's more important than ever for data center managers to evaluate their facilities and to take the time to develop a strategy mitigating harmonic current.

Harmonic Distortion

Harmonics are distortions of the normal electrical current waveform, generally transmitted by nonlinear loads. Switch-mode power supplies (SMPS), variable speed motors and drives, photocopiers, personal computers, laser printers, fax machines, battery chargers, and UPSs are examples of nonlinear loads. Single-phase nonlinear loads are prevalent in modern office buildings, while 3-phase nonlinear loads are common in factories and industrial plants.

A large portion of the nonlinear electrical loads in most electrical distribution systems comes from SMPS equipment. For example, all computer systems use SMPS that convert utility ac voltage to regulated low-voltage dc for internal electronics. These nonlinear power supplies draw current in high-amplitude short pulses that create significant distortion in the electrical current and voltage wave shape (Figure 1). This harmonic distortion, measured as total harmonic distortion (THD), travels back into the power source and can affect other equipment connected to the same source.

All periodic waves can be generated with sine waves of various frequencies. The Fourier theorem breaks down a periodic wave into its component frequencies.

Harmonic currents generated by nonlinear loads increase power-system heat losses and power bills for end users. These harmonic-related losses reduce system efficiency, cause apparatus overheating, and increase power and air conditioning costs. As the number of harmonics-producing loads has increased over the years, it has become increasingly necessary to address harmonics when making any additions or changes to a facility. 

Most power systems can accommodate a certain level of harmonic currents but will experience problems when harmonics become a significant percentage of the overall load. As these higher frequency harmonic currents flow through the power system, they can cause communication errors, overheating, and hardware damage. 

Reducing Harmonics

To determine if harmonic mitigation is necessary, facilities managers should conduct an assessment to precisely measure the harmonics affecting the data center and identify their origin. Solutions for harmonic mitigation vary in complexity and cost and can be deployed individually or in combination. The strategy that makes the most sense for a facility will vary based on the loads it supports, its budget, and the nature of the harmonic-related problems it is experiencing.


Solution 1 - K-rated Transformers in Power Distribution Components


A standard transformer is not designed for high harmonic currents produced by nonlinear loads. It will overheat and fail prematurely when connected to these loads. Therefore, when harmonics were first introduced into electrical systems at levels that showed detrimental effects (circa 1980), the industry responded by developing the K-rated transformer. K-rated transformers are not used to eliminate harmonics, but to manage the heat generated by harmonic currents.

K factor ratings range between 1 and 50. A standard transformer designed for linear loads is designated with a K-factor of 1. The higher the K-factor, the more heat from harmonic currents the transformer is able to withstand. When selecting a K rating, managers should consider the trade-offs between size, efficiency, and heat tolerance. For example, transformers with higher K factors are typically larger than those with lower K factors. The table shows appropriate K ratings to use for different percentages of nonlinear current in the electrical system.

Power distribution units (PDUs) with a K 13-rated transformer are readily available to efficiently handle harmonic currents. Units with K 20 transformers are common, but are typically overkill for most modern data centers.




Units with K 20 transformers are common, but are typically overkill for most modern data centers. The K-rated, dry-type transformer is widely used in electrical environments - either in a PDU or as a stand-alone unit. However, recent advances in transformer design are changing the way facilities managers reduce voltage distortion and power loss caused by harmonic currents. 

Solution 2 - Harmonic Mitigating Transformer


A harmonic-mitigating transformer (HMT) is designed to handle the nonlinear loads of today's electrical infrastructures. This transformer uses electromagnetic mitigation to deal specifically with the triplen (3rd, 9th, 15th, and so on) harmonics. Secondary windings of the transformer are arranged to cancel zero sequence fluxes and eliminate primary winding circulating currents. This transformer also addresses the 5th and 7th harmonics by using phase shifting.

Using these two electromagnetic techniques, an HMT allows loads to operate as they were intended, while minimizing the energy loss and distortion caused by harmonics. Most HMTs exceed NEMA TP-1 efficiency standards, even when tested with 100% nonlinear loads. Wherever a K-rated transformer is specified, an equivalent HMT is a direct substitute.



Solution 3 - Harmonic Mitigating UPS


Much like an active filter, harmonic-mitigating UPS eliminates harmonic distortion by inserting equal and opposite current into the line. They also compensate for reactive power from low power-factor loads and balance loads across 3-phase systems to avoid stranded capacity, as well as to provide clean and continuous power during utility outages or in response to electrical disturbances.

Going Further


Data center managers are increasingly deploying UPSs with energy-saver operating modes to boost efficiency and lower power costs. Recently, harmonic-mitigating UPSs capable of keeping distortion within predetermined and adjustable limits, correcting power factor, and balancing loads while in energy-saver mode have begun to reach the market.

These new systems typically remain within 1% of energy-saver levels while performing these functions, a significant improvement over past technologies. The harmonic mitigation technology in the latest energy-saver UPSs is a built-in feature that requires no additional footprint, saving valuable data center floor space and reducing installation and maintenance costs.

Harmonics continue to be costly for data centers, preventing data center managers and engineers from achieving maximum reliability and efficiency. Fortunately, the latest enhancements in UPS technology offer next-generation harmonic-mitigation capabilities. Though not always required, such systems enable data centers to achieve the highest efficiency possible by actively correcting for harmonic currents as they occur.

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SMA connects IT, Facilities and Design. For the Data Center Design Consideration, please visit 

(1) Site Selection,
(2) Space Planning,
(3) Cooling,
(4) Redundancy,
(5) Fire Suppression,
(6) Meet Me Rooms,
(7) UPS Selection, and
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All topics focus on key components and provide technical advice and recommendations for designing a data center and critical facilities.