Distortion and power factor of nonlinear loads

When large nonlinear loads, in the order of tens of megawatts, are connected to utility systems, significant harmonic voltages and currents are produced. These loads include static frequency converters serving 25 Hz and 16 2/3 Hz intercity and commuter rail systems. The converter harmonics cause increased heating of the utility and other customer equipment and can lead to system resonance. Therefore, the harmonics need to be taken into account in various system evaluations. The difficulties in dealing with nonlinear loads encountered by the author include the following: calculation of the harmonic distortion; and calculation of power factor. The IEEE method of harmonic distortion calculation is compared with an alternative method proposed in technical literature. The alternative method resolves the intuitive difficulty of visualizing harmonic distortion of over 100% when one or more of the frequencies in the harmonic spectrum is higher than the fundamental frequency, but introduces other difficulties, such as understating the magnitude of distortion. The complexity of determination of power factor for distorted voltage and current increases as more harmonics are included in the calculation. Due to distorted waveforms, the true power factor is always lower than the power factor calculated when, as often is the case, the harmonics are ignored. In this paper the concepts of nonlinear load and harmonics are reviewed first. Subsequently, calculation of the harmonic distortion and power factor are discussed in mathematical terms and supplemented with practical examples.