Upgrade of 4.9MW high speed helper motor drive system of LNG hydrocarbon gas compressor train; a comparison of Voltage Source Inverter and Load Commutated Inverter topologies

This paper presents a comparison between Voltage Source Inverter (VSI) and Load Commutated Inverter (LCI) technologies based on a project to replace an LCI based motor drive system by a VSI based motor drive system for use on a hydrocarbon compression train. The compressor is driven by two mechanical power sources on a common shaft. One power source is a 20MW gas turbine and the other is the 4.9MW, 4905rev/min high speed electric helper motor drive system. Unlike most motor drives, the helper motor drive does not control the speed of the compressor train´s common shaft. The speed of the train is set by the gas turbine. The inverter synchronises with (i.e. captures) the spinning helper motor and controls the motor´s mechanical power contribution to the train´s common shaft. The paper discusses the specific application engineering challenges and solutions in order to achieve the seamless change out of equipment, minimising downtime and installation cost. The details of new VSI and existing LCI based motor drive systems are presented, including the benefits of the new VSI system. In addition, a performance comparison between VSI and LCI systems is presented, based on the comprehensive motor drive system tests conducted in the factory. This comparison includes: harmonics, power factor, efficiency and torque ripple characteristics of each topology.