February 11, 2002
Determine the Effects of Harmonic Currents on the Loadability of Medium-Voltage Power Fuses.
The impact of harmonic currents on power fuses historically has not been much of a concern. It was generally understood that the presence of harmonics in no way influenced the interrupting performance of the fuse, and the additional heating effect of harmonic currents on the fuse’s continuous peak-load capability was thought to be inconsequential. Today, however, harmonic currents in general, and their influences on the continuous loadability of power fuses and other medium-voltage switchgear components in particular, is the subject of much discussion. This is due in large part to the increasing presence of rectifier-based adjustable-speed drives used to control motors in industrial processes. These drives produce a variety of harmonic currents that can be “seen” by medium-voltage power fuses applied in in-plant distribution switchgear. In some cases the harmonic currents result in a given fuse being loaded to values only slightly in excess of that planned. In other cases, it is possible for a fuse that is already loaded to a high percentage of its published peak-load capability, to be loaded to currents in excess of its capability in the presence of harmonic currents. While such overloading will not cause the fuse to operate (since medium- and high-voltage fuses are designed to melt at currents approximately twice their ampere ratings), it may cause long-term degradation of the fuse’s interrupting mechanism that could impact the ability of the fuse to clear faults in the future.
A periodic current can be written as a sum of its harmonic components, as indicated in Equation 1.
The RMS heating equivalent of harmonic currents can be determined by simply taking the square root of the sum of the squares of the coefficients of the various harmonics as shown in Equation 2.
Equation 2.
Consider, for example, a six-pulse PWM adjustable-speed drive, which produces the following harmonics:
| Harmonic | Frequency, Hz | Magnitude, Percent | Coefficient, Ck |
|---|---|---|---|
| Fundamental 5th 7th 11th 13th |
60 300 420 660 780 |
100.0% 17.5% 11.0% 4.5% 2.9% |
1.000 0.175 0.110 0.045 0.029 |
The square root of the sum of the squares of the harmonics associated with the six-pulse PWM drive noted above is 1.0225. In other words, the heating equivalent of the harmonic currents on the medium-voltage fuse amounts to approximately 2.25%. A fuse loaded to 50 amperes RMS (60 Hz) would, in effect, be loaded to slightly more than 51 amperes. A fuse loaded to 100 amperes RMS (60 Hz) would be loaded to slightly more than 102 amperes. It is generally the case that medium-voltage power fuses applied in industrial, commercial and institutional power systems — in particular, those protecting small power transformers — are not loaded to values approaching their published continuous peak-load capabilities. Thus, the affects of harmonic currents produced by typical PWM motor drives on these fuses would not likely have any deleterious affects on the long-term performance of these fuses.
On the other hand, medium-voltage fuses applied in the feeder position of an in-plant switchgear assembly protecting cables feeding one or more individually protected transformers might conceivably be loaded to values approaching their published continuous peak-load capabilities. In these instances, the affects of harmonic currents should be determined, and the fuse ampere rating increased, if necessary, to ensure that the fuse will not be overloaded.