Comparison between Electrical Drives in Lng Plant for Subsynchronous Torsional Interactions

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 a diode rectifier and VSIs technology comparing with LCIs and VSIs using the PWM rectifier (active front end) for subsynchronous torsional interactions (SSTI). Electrical damping is investigated by transient simulation using PSCAD/EMTDC. It is found that the proposed VSDS has positive electrical damping for wide range of torsional resonant frequency. The electrical damping is kept positive values even in case of a higher impedance of generator connection. On the other hand, negative electrical damping characteristics are observed for the LCI and the VSI with PWM rectifier. This study shows that the SSTI is less concerned with the proposed VSDS that would be suitable to apply large capacity drive applications in LNG plant.