Uninterruptible Power Supplies and Harmonics

Harmonic pollution is a growing goldshell Miners problem in Europe and one that designers of power continuity programmes and manufacturers of UPS (uninterruptible power supplies) cannot ignore. Typical harmonic problems include the distortion of mains power supply voltage, overheating of wiring, neutral conductors, supply transformers and switchgear and nuisance tripping of breakers. Harmonics can also cause disruption to equipment on the same supply and lead to random failures.

Harmonics are caused by voltage or current waveforms with frequencies that are multiples of the fundamental frequency – in Europe, 50Hz (50 cycles per second). The multiples are always ordered in a specific sequence: for example, the 2nd harmonic is 100Hz (2x50Hz), the third 150Hz and the fourth 200Hz and so on.

The particular problem of Triplens (third order) harmonics. Harmonics are a particular issue for power continuity management due to the large number of switch mode power supply (SMPS) loads being connected to modern electrical distribution networks – and their associated UPS systems. These are the most common form of power supply unit (PSU) in use today. As a non-linear load, they draw their power in regular modulated pulses of current from a mains power supply rather than as a continuous linear supply. This can lead to SMPSs generating high levels of harmonics, especially when many are being supplied from a single three-phase mains power supply.

In particular, system designers must be aware of the potentially damaging Triple-Ns (or Triplens) whose harmonic order numbers are multiples of three and include the notorious third-harmonics as well as ninths and fifteenths. Thirds are probably the most challenging harmonic in terms of neutral conductor loading within a three-phase system. Whereas other harmonics cancel each other out, third-harmonics are in phase with each other and exhibit a summing effect which greatly increases the current – potentially overloading conductors and switchgear.

Harmonics and total power factor – implications for UPS sizing. Harmonics are also closely related to power factor management – and another key aspect of uninterruptible power supply system design and implementation. The displacement power factor is only applicable to the fundamental frequency (50Hz in Europe) and therefore does not take into account the power factor.

Generated by any harmonics induced into the mains power supply by the load itself (referred to as the distortion power factor and produced by the harmonics produced by non-linear loads). The combination of the displacement power factor and the distortion power factor gives what is known to UPS systems experts as the true power factor. When correctly sizing a UPS, an understanding of this is critical.

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