In today’s data center landscape, facility managers seek to operate with reliable and energy efficient equipment that produces a low total cost of ownership. In many cases, wasted power and interruptions from harmonic currents can serve as a “fly in the ointment” for these IT professionals striving to achieve cost-effective operational goals.
Innovations like faster switching devices and power factor corrected power supplies have led many data center operators to mistakenly think of harmonics as a thing of the past. In actuality, fluctuating loads, equipment, and operating conditions can still produce harmonics, power factor problems, and load balancing issues even at facilities that have adopted the latest technologies.
Harmonic-related losses can reduce system efficiency, cause overheating problems, and reduce equipment life. It is important to consider the impact of harmonics when planning additions or changes to a system, analyzing potential issues, and identifying options for mitigation that incorporate the latest harmonic mitigating technology.
WHAT CAUSES HARMONICS?
Most utilities provide power with a reasonably smooth waveform. However, non-linear loads such as servers, variable-frequency drives (VFDs), florescent lighting, and other electronic devices can distort current and voltage waveforms. Known as harmonics, those distortions appear as supplemental frequencies higher than the fundamental frequency, which is 60 Hz in the U.S. and 50 Hz in most other countries.
Harmonics come in two basic varieties, current distortion and voltage distortion. Non-linear loads are the most common source of current distortion. Voltage distortion is most commonly produced when an electrical device pulls current distortion through an impedance. Additionally, poor harmonics at facilities that share a point of common utility coupling can impact neighboring structures as well, so “harmonic pollution” from nearby buildings can also cause current distortion, voltage distortion, or both.
SOLUTIONS TO COMPENSATE FOR AND REDUCE HARMONICS
Data center operators have a range of options of varying complexity and cost for mitigating harmonics, and these can be employed individually or in combination. The strategy that makes the most sense for a given facility will vary based on the loads it supports, its budget, and the specific harmonic-related problems it’s experiencing.
Active filters provide excellent cancellation but they are typically the most expensive option for mitigating harmonics. Active filters sense harmonics and inject an equal and opposite harmonic current into the line and provide a fast response for dynamic loads. Much as noise-canceling headphones block out sonic disturbances, active filters cancel the entire spectrum of harmonics, while also lowering total harmonic distortion to less than 5% and correcting for power factor by adding phase-displaced fundamental current to the line.
HARMONIC MITIGATION IN UPS’S
Much like active filters, harmonic mitigating uninterruptible power systems (UPSs) eliminate harmonic distortion by inserting equal and opposite current into the line. They also compensate for reactive low power factor in non-linear loads and balance loads across three phases to avoid stranded capacity, while providing clean and continuous power during utility outages or in response to electrical disturbances.
In their quest to boost efficiency, data centers are increasingly deploying UPSs with energy saver operating modes. Such products save money and enhance sustainability by reducing data center energy waste up to 10% under typical loading conditions.
The most recent harmonic mitigating UPSs are capable of keeping distortion within pre-determined and adjustable limits, correcting power factor, and balancing loads while in energy saver mode. These new systems typically remain within 1% of energy saver levels while performing these functions, providing a significant improvement over double-conversion efficiency levels.
Furthermore, the harmonic mitigation technology in the latest energy saver UPSs is a built-in feature that requires no additional footprint, so it doesn’t consume valuable data center floor space or inflate installation and maintenance costs.
The K-rated, dry-type transformer is widely used in electrical environments — either in a power distribution unit or as a standalone unit. But, there have been more recent advances in transformer design that offer even better performance in reducing voltage distortion and power losses due to current harmonics.
A harmonic-mitigating transformer (HMT) is designed to handle the non-linear loads of today’s electrical infrastructures. This transformer uses electromagnetic mitigation to deal with triplen harmonics, while also incorporating secondary windings to cancel zero sequence fluxes and eliminate primary winding circulating currents.
Using these two electromagnetic techniques, an HMT allows loads to operate the way their manufacturers designed them, while minimizing the impact of the harmonics to energy losses and distortion. Most HMTs exceed NEMA TP-1 efficiency standards, even when tested with 100% non-linear loads. Wherever a K-rated transformer is specified, an equivalent HMT is a direct substitute.
Generators have two to three times the impedance of a typical transformer, making them especially sensitive to voltage distortion. Using larger generators than electrical loads would otherwise require stabilizing their output and helps compensate for the increased heating that distortion can cause, improving data center reliability and lengthening the generator’s lifespan. Oversized generators also cost more, however, and therefore drive up capital expenses and cost per watt of power delivered through the generator.
LOW DISTORTION ELECTRONIC BALLASTS
Like harmonic mitigating transformers, low distortion electronic ballasts use passive filtering technology to ease line harmonics. However, they’re usually more expensive than the standard magnetic ballasts they typically replace.
Harmonics continue to be a costly issue for data centers, with implications for both reliability and efficiency. Fortunately, there is a wide array of harmonic mitigation technologies. IT and facilities managers today should evaluate each option in conjunction with their individual infrastructure and power quality needs. Though not all data centers require them, such systems enable companies to achieve the highest efficiency possible while actively correcting for harmonics as they occur.