Combined Torsional And Electromechanical Analysis Of An Lng Compression Train With Variable Speed Drive System.

Abstract

Nowadays variable speed drive systems (VSDS) are preferred to constant speed drives in the oil and gas industry because they can improve the efficiency of the process while avoiding the use of complex mechanical mechanisms (e.g., guide vanes) or plant recycling and throttling. However, the reputation of VSDS has not always been favorable since they can be a cause of torsional vibration problems. This is mainly due to the intrinsic nature of switching-based electrical systems to produce pulsating torque ripple on the shaft. In addition, closed-loop electromechanical interactions could lead to system instability. The aim of this paper is to introduce a new combined approach to deal with torsional phenomena, especially in high shaft-power applications such as liquefied natural gas (LNG) trains. Besides the conventional shaft-line modal analysis, a precise understanding of the electromechanical interactions in achieved through combined simulations between the shaft-line designer (compressor manufacturer) and the VSDS supplier together with a full-load-full-speed (FLFS) train string test. Simulations and string test results confirmed satisfactory torsional behavior and electromechanical stability of the overall system.