Harmonic is a 9.6V 1.2A
nicd battery charger in 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.
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.
Mitigation of total harmonics distortion. Harmonics issues need to be addressed at the design stage of any power continuity plan. Not least, because consumers are responsible for the harmonic levels introduced into their three-phase mains power supply.
A UPS can sometimes be fitted with a harmonic filter (post installation) but this can be a costly and inelegant solution as extensive internal wiring changes may be required. For a transformer-based UPS, using a 12-pulse rectifier in place of a 6-pulse set will reduce the levels of THDi (total harmonic distortion). Coupling this with a passive filter will provide further reduction to around 4%.
For a transformerless uninterruptible power supply, THDi levels of less than 4% can be achieved by installing an active harmonic filter. However, levels as low as 3% can now be achieved by some designs whose rectifiers are IGBT (Insulated Gate Bipolar Transistor) based. This can remove the need for an additional active harmonic filter and simplify the UPS design process. Such designs are expected to become the norm: not only do they reduce initial costs, but they allow a smaller UPS system footprint whilst increasing input power factors.
A popular approach to reduce the effect of leading power
battery charger factors on a UPS installation is to use an active harmonic filter with power factor correction on the UPS output. This presents the UPS with a more acceptable load, but results in higher capital and installation costs, lower efficiency and a greater footprint.
Familiar territory for UPS manufacturers. Although many aspects of harmonics must be considered when specifying a UPS system, reassurance can be gained from the fact that this is familiar territory for UPS manufacturers such as Riello UPS. End users and their professional advisers can certainly be confident that this specialised aspect of UPS application will be thoroughly addressed during the modern consultative sales and specification process.
