For dry type transformers, to determine what amount of harmonic content is present, a "K" factor calculation is made instead of using the THD formula.
The total amount of harmonics will determine the percentage of non-linear load, which can be specified with the following typical examples:
Transformers shall be sized to account for harmonic non-linear loads of 50% minimum (K-4), 100% (K-13), 125% (K-20), 150% (K-30).
The neutral connection shall be sized at 200% of the current rating of the phase connections.
The conductors of the transformer winding shall be sized to handle circulation of 3rd harmonic current and not exceed the rated temperature rise.
Transformers shall be capable of operating within the specified temperature rise while supplying 100% of the 60 Hertz fundamental rated current values plus the following harmonics as calculated by ANSI/IEEE 57.110-1986.
16.7% of the rated current at the 3rd Harmonic
10.0% of the rated current at the 5th Harmonic
7.1% of the rated current at the 7th Harmonic
5.6% of the rated current at the 9th Harmonic
Beyond the 9th Harmonic the percentages of the fundamental current through the 25th Harmonic shall be equal to the reciprocal of the odd harmonic number involved times 0.5
The FPT Type FHK4 series transformer is designed for 100% linear load plus 50% non-linear load which can operate at a total Ih (pu)2 h2 K-factor load value of 4.0. See Table 1.
(B) 100% Non-Linear Load (K-13 Rating)
33.3% of the rated current at the 3rd harmonic
20.0% of the rated current at the 5th harmonic
14.3% of the rated current at the 7th harmonic
11.1% of the rated current at the 9th harmonic
Beyond the 9th Harmonic the percentages of the fundamental current through the 25th Harmonic shall be equal to the reciprocal of the odd harmonic number involved times 1.0
The FPT Type FHK13 series transformer is designed for 100% linear load plus 100% non-linear load which can operate at a total (Ih (pu)2h2) K-factor load value of 13.0. See Table 1.
(C) 125% Non-Linear Load (K-20 Rating)
41.7% of the rated current at the 3rd harmonic
25.0% of the rated current at the 5th harmonic
17.9% of the rated current at the 7th harmonic
13.9% of the rated current at the 9th harmonic
Beyond the 9th Harmonic the percentages of the fundamental current through the 25th Harmonic shall be equal to the reciprocal of the odd harmonic number involved times 1.25
The FPT Type FHK20 series transformer is designed for 100%linear load plus 125% non-linear load which can operate at a total (Ih(pu)2h2) K-factor load value of 20. See Table 1.
(D) 150% Non-Linear Load (K-30 Rating)
50.0% of the rated current at the 3rd harmonic
30.0% of the rated current at the 5th harmonic
21.4% of the rated current at the 7th harmonic
16.7% of the rated current at the 9th harmonic
Beyond the 9th Harmonic the percentages of the fundamental current through the 25th Harmonic shall be equal to the reciprocal of the odd harmonic number involved times 1.50
The FPT Type FHK30 series transformer is designed for 100%linear load plus 150% non-linear load which can operate at a total (Ih(pu)2h2) K-factor load value of 30. See Table 1.
