Investigation of current harmonics in thruster AC drives in oil-rig platforms

The paper investigates the effect of current harmonics in oil-rig's thruster drives, which are based on variable frequency AC induction motor drives. The electrical system in the oil rig consists of two 12 pulse phase shifting transformers having a relative phase shift of 15o, a frequency converter and an induction motor. The electrical system exhibits clean energy characteristics according to IEEE.Std 519.1992 with I-THD of 2.48% and harmonics of the order of 24n±1 at the busbar when both transformers are equally loaded. Due to the nonlinear nature of diode bridge rectifier and dc link capacitors, the input line currents are highly distorted and contain significant amount of low order harmonics causing overheating of the transformer. To investigate the effect of harmonics on the transformer, the mathematical model of the electrical system of oil rig and thermal model of the three windings dry transformer is developed in MATLAB. The temperature rise in the transformer is calculated using IEEE.Std C57.12.91.2001. The test results are investigated and to increase the thermal capability of the transformer, K-factor is calculated and transformer is derated accordingly. The temperature rise decreases with the derating of the transformer and the transformer can now be loaded beyond the nameplate rating upto 1.2 pu load without overheating.