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K-Factor vs. H-Factor in Drive Applications

H-factor is defined under ANSI/IEEE C57.18.10 and also referenced under C57.110. K-factor is a UL classification and is not appropriate, nor covered by ANSI or IEEE, for transformer construction based on drive isolation transformer applications. To be in full compliance with ANSI and IEEE standards, a manufacturer must be able to provide an H-factor classification for the transformer.

K-factor is formulated based on harmonic conditions caused by a mix of single and three phase non-linear loads. Non-linear loads are often associated with DC power supplies and can include: VFD’s, DC chargers, UPS systems, computers, electronic ballasts and other devices that utilize power supplies. The harmonic content of these “typical” distribution circuits is significant. This harmonic load decreases the efficiency and increases the operating temperatures of transformers, thereby decreasing their life expectancy and load carrying capability.

It should be noted that the majority of harmonic loads in many electrical systems can be single-phase loads, not three-phase which means the primary harmonic is the 3rd harmonic. Large 3rd harmonic content has the additional difficulty of having these harmonic currents ground to the neutral thereby causing a theoretical maximum neutral current of 1.73 times the fundamental. Another important factor is line side voltage distortion was never anticipated within a K-factor circuit. The entire K-factor principal is based on load current harmonics with no consideration for line voltage distortion.

In the case of industrial loads, the loads on these transformers are functionally designed as delta circuits with no direct load placed on the neutrals. Three-phase non-linear loads such as VFD’s produce their major harmonic at the 5th, there is no 3rd harmonic component. With little to no 3rd harmonic and balanced loads, there is little current on the neutral. This then shifts the harmonic thermal contribution directly to the core and windings of the transformer. Any neutral currents that may develop will be induced versus direct injected. Also, within industrial loads, the harmonic spectrum will not be the same as within traditional K-factor single-phase commercial loads. This is the reason K-factor should not be used to evaluate high kVA industrial three-phase non-linear VFD loads. H-factor takes these significant differences into account when evaluating the design of the transformer relative to its potential harmonic.

Transformers designed with a K-Factor must be:

  • U.L. only allows K-ratings of K=4, 9, 13 and 20. 
  • K=1 would be referred to a perfectly sinusoidal load with no harmonics.
  • 200% Rated Neutral
  • Electro-Static Shield Required
  • Built to withstand extra heating from harmonics for equivalent K-Factor.

Transformers designed with a H-Factor must be:

  • 125% Rated Neutral (balanced loads and no 3rd harmonic adding to neutral currents)
  • Electro-Static Shield Optional.
  • Built to withstand extra heating from harmonics for equivalent K-Factor.

As previously mentioned, the circuit connections relative to a VFD load are significantly different than a standard commercial load. We have anticipated that the neutral will either float or be brought out for a zero voltage ground reference and not hooked directly into the load. Any harmonic currents present on the neutral would then be induced and not direct injected. The transformer manufacturer should concentrate efforts on designing the windings and the core to withstand the direct effect of the harmonic currents, and focus our design of the neutral based on induced currents. The induced currents will be less due to the impedance of the windings. H-factor designs focus attention on the windings and core since this is where the real harmonic design issues lie. The neutral will be beefed up but it will not be the center of an H-factor harmonic current issue.

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