Mitigation of power quality issues in all-electric ship power system

Abstract

The proliferation of nonlinear loads on ship power systems is leading to power quality issues thus necessitating some form of mitigation. The main objectives of this project are to enhance the simulation model to represent a more realistic ship power system; examination of dominant power quality issues under typical operating conditions and implementing solutions to mitigate their negative effects. The project focused on harmonics arising from the operation of the main traction motors. The harmonic content was noted under various operating conditions both at the load and generation ends. Afterwards, methods to mitigate these harmonics were considered. These include altering the rectification stage of the main traction drive from a six-pulse rectifier to a twelve-pulse rectifier through the introduction of a three winding transformer and an additional six-pulse rectifier connected in series to the original one and the addition of single-tuned passive filters. Two filter designs were considered and the most feasible one in terms of space and weight was chosen and implemented. From the results obtained at different loading and speed scenarios, it could be observed that the most dominant harmonics in the original system were the 5th and 7th and, as the cable impedance was introduced, the 11th and 13th harmonics also became more significant. However, with the improvement in the rectification stage to a twelve-pulse rectifier, the 5th and 7th harmonics were practically mitigated. After the addition of the passive filters, the 11th and 13th harmonics’ magnitudes were effectively reduced, hence the total harmonic distortion (THD) for both voltage and current was improved. From the harmonic spectra, it was observed that the harmonics did not occur at the system frequency, but slightly below. This is affected by the droop settings of the generators and as the load is increased, this drop in frequency increases. The system including the bow thruster was tested in order to ascertain that no parallel resonance occurred after the addition of the passive filters.