Study of subsynchronous torsional interaction with voltage source inverter drive for LNG plant

Electrical drives for large capacity compressors in a liquefied natural gas (LNG) plant have been investigated. There are mainly two topologies for such an electrical drive; load commutated inverters (LCIs) and voltage source inverters (VSIs). Due to commutation of the thyristors, LCIs produce current harmonics with variable frequencies, and their interharmonics are significant. Therefore, torsional interactions should strongly be concerned. The VSI systems have low harmonics of voltage and current, and become a matured and reliable technology. So this technology is getting attractive to the very large capacity motor drives. This paper shows characteristics of proposed variable speed drive system (VSDS) using VSIs technology comparing with LCIs at first for subsynchronous torsional interactions(SSTI). Second, interharmonics are considered. This study shows that the SSTI is less concerned with the proposed VSI-VSDS that would be suitable to apply large capacity drive applications in LNG plant.