For the best efficiency and performance in a data centre, the goal is to have voltage and current waveforms primarily consisting of the fundamental frequency, with minimal higher-order frequencies. Most utilities provide power with a reasonably smooth waveform, however, non-linear loads such as servers, variable frequency drives (VFDs) and other electrical devices can distort current and voltage waveforms.
These distortions are known as harmonics. These changes appear as supplemental frequencies higher than the fundamental frequency, which is 60 Hz in the U.S. and 50Hz in most other countries.
Harmonic currents are often exaggerated by the power supply units (PSUs) within the IT equipment itself, or by VFDs in cooling and ventilation equipment. Harmonic currents can then distort the voltage that is consumed by IT equipment, as the devices pull current distortion through an impedance.
Why harmonics matter
Minimising energy costs and maximising reliability are top priorities for most data centre operators, but harmonics make both these goals harder to achieve.
Arguably the most prolific concern about harmonics is the increased losses on the power system conductors and transformers, adding heat to the power chain that drives up power and cooling costs. This has a knock-on effect on capital expenses, as harmonics reduce the lifespan of electrical equipment. Because of this, businesses may need to purchase replacement devices prematurely.
Harmonics can also result in costly utility penalties, as energy companies must compensate for harmonic-related waste by deploying additional generating capacity. For this reason, and to discourage harmonic pollution, many utility providers penalise customers that exceed distortion limits as defined by the IEEE-519 standard.
While these power quality issues are generally understood in the technical community, less appreciated is the effect of harmonic currents on the overall efficiency of a data centre. Ultimately, harmonic currents are wasted energy that appear as heat. This means the amount of heat that must be removed from the data centre is increased — a common and highly-publicised issue for data centres.