Table 61. THD Limits from IEC 6100032 and 6100034
Wattage

THD Limit (%)

< 600

90

1000 – 1500

60

> 3500

26

Fig. 2 show the same information contained in this table, but also shows the linear decline between intervals (i.e., between 600 W & 1000 W, and between 1500 W & 3500 W).
Figure 63. IEC Harmonic Limits as a Function of Wattage
By comparison with the limits suggested by IEEE 519, the IEC limits permit higher levels of harmonic distortion.

At this point, we can either develop our own limits or we can adopt limits of others, like the IEC. We are fortunate in that a number of studies in the U.S. that have already been undertaken. The reports on these efforts cover the spectrum of low power devices like compact fluorescent lamps [Pileggi & Emanuel], larger loads like variable speed heat pumps and central air conditioning [Grady] and large electrical loads like battery chargers for electric vehicles [Grady]. These studies determine how much of a given nonlinear load a distribution system can tolerate before one exceeds 5% voltage THD. This is probably a more meaningful starting point since these studies involve system models conforming to the distribution systems found in the U.S. and the harmonic spectrum of modern singlephase converters.
The other option is to adopt the proposed IEC limits (i.e., 6100032 and 6100034). If the distribution systems were similar enough around the world, and if the use of such a standard was validated by field experience, this would make sense. However, there are two basic distribution systems found around the world and the two are different enough – in terms of harmonic implications.
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