Stability of Autonomous Power Systems on Ships and Offshore Installations

Abstract

The power quality problem such as voltage dips, swells and flickers cause the fluctuations in the grid voltage which in worse case may lead to blackout of the whole electric system. The non-linear loads in the ship power system produce harmonics in the source current, which produces fluctuations in the voltage. These problem can be handled if the converters on the ship power system works as active filters. A thorough review of these problems and their mitigation techniques are done in the theoretical part of this thesis. This rectifier not only supply the voltage to the propulsion motor but also regulates the voltage of the system by removing the harmonics from the source current and by injecting the reactive power into the system. In this thesis the vector control scheme is used for the control of both the rectifier and inverter. The harmonics and reactive power compensation is performed by the VSC converter, which increases the stability of the system by improving the voltage regulation.The simulations were carried out to improve the total harmonic distortion in the source current waveform. The harmonics and reactive power compensation was first performed in the system with ideal power supply to check the operation of VSC converter. The total harmonic distortion is kept down to 0.70 % which is with in the standards set by IEEE and IEC. The parallel operation of synchronous generators were performed to build the same model as on ship. The variable frequency drive was used, which had VSC converter for harmonics compensation. The simulations were carried out first with one and then with two variable frequency drives. The THD was reduced to 4.05 % with one VFD and 3.87 % when two VFDs were installed.The voltage support of the grid was also performed during the symmetrical three phase to ground fault with the help of VFD. The voltage was improved from 350V to 510V, which is with in the allowable limits for the ship power system.