Harmonic Analysis of Shipboard Power Systems Considering VFD Loads

In recent shipboard power systems, variable-frequency drive (VFD) loads are increasingly used not only for propulsion but also for operating various auxiliary devices such as fans, pumps, and compressors. Given that most VFDs employ diode rectifiers, the inclusion of multiple VFD loads leads to significant degradation of voltage and current waveforms in the power system. According to classification society regulations, the voltage total harmonic distortion (THD) must generally remain below 8%, with individual harmonic components not exceeding 5%. However, these limits are often exceeded as the proportion of VFD loads increases. This paper presents a detailed harmonic analysis of shipboard power system under varying VFD load ratios, filter installation conditions, and system inductance levels. In particular this study identifies the critical VFD load ratio beyond which the harmonic distortion exceeds classification limits and proposes the appropriate sizing of passive filters to mitigate this issue. Furthermore, it investigates the nonlinear relationship between system inductance and VFD load ratio, offering predictive insights into THD behavior under different system conditions.