Effects of harmonics on the current carrying capacity of insulated power cables used in three phase electrical power distribution systems

Today non-linear loads are becoming a bigger part of the electrical load in industrial and commercial power system-such as solid-state energy conversion devices, traction drives industrial variable speed drives, andarc welding equipment. Non-linear loads generate harmonics in line currents resulting in additional conductor heating, This heating results in a higher temperature rise of the cable if it is not sized to cany harmonic currents. To include the effects of harmonics the frequency spectrum of the load has to be known. In three phase wye connected electrical power distribution systems with neutrals, sinusoidal line currents are symmetrical resulting in zero neutral current whereas line currents with harmonics such as those supplying non-linear loads result in non-zero neutral currents due to the summation of zero sequence currents in the neutral conductor. Present standards by NEQNational Electric Code) [5] and IEEE[8]define cable current ratings for sinusoidal (fundamental) currents only and do not provide a suitable transformation for cable current ratings for non-linear loads. This paper defines a suitable transformation to convert standard sinusoidal current ratings to those suitable for non-linear loads by use of harmonic derating factors based on the harmonic spectrum of the load which can be used to size line and neutral conductors of three phase electrical power distribution system[l]. Harmonics generated by non-linear loads decrease the quality of power supplied.