A TWELVE-PULSE LOAD COMMUTATED CONVERTER DRIVE SYSTEM WITH VSI FOR STARTING UP AND ACTIVE POWER FILTERING IN AN LNG APPLICATION

Abstract

Variable Frequency Drives (VFDs) are an integral component of the industry in today’s age. VFDs provide a great range of control for electrical machines, and can be integrated in a variety of applications to meet the desired objectives of operation with improved reliability and efficiency. This thesis presents the Load-Commutated Converter (LCC) drive, which belongs to the Current Source Converter (CSC) based drive system family. Such drives are widely used in high power applications, due to power handling capabilities and the maturity of the drive system. The application under study is that of a helper/starter motor for a turbine compressor in a Liquefied Natural Gas (LNG) plant. Primarily, the thesis presents real-life scenarios of drive system operation such as constant/variable speed operation at constant/varying torque. The respective controllers for the LCC drive are presented alongside their results. In addition to simulating the drive system in this LNG application, current harmonic mitigation measures are presented in this study. The typical converter topology presented in this thesis is the 12-pulse type, however comparisons with different topologies (6, 18, and 24-pulse) have also been presented. Finally, a dual-purpose external Voltage Source Inverter (VSI) is used both as a starter and an Active Power Filter (APF), therefore addressing the issues of drive/load induced harmonics and LCC starting. As a conclusion, a controlled LCC drive model is simulated in SIMULINK to emulate the drive operation in actual plant conditions. The controlled drive is further studied for the presence of harmonics and their subsequent mitigation, by using passive as well as active power filters. The results obtained present the adequacy of the control system as well as the efficacy of the filters used for harmonics mitigation. Future work revolves around improving the efficiency of the APF, and the drive control system to be more robust and reliable. The system can further be investigated for enhancements as per operational requirements.