Electro-mechanical interaction in the torsional dynamics of LNG compression trains with LCI variable speed drive

摘要

The aim of this paper is to describe the development of an electro-mechanical model to reproduce the torsional dynamics of Liquefied Natural Gas (LNG) compression trains including large Variable Speed Drives (VSDs). VSDs are used to maximize the efficiency of the compression train by operating the compressors at a speed where efficiency is maximized, considering the process parameters (mass flow, input pressure, etc.) [1]. The use of VSDs however is not free of concerns, in particular for high shaft-power applications. It has been observed that VSDs can apply pulsating torques to the shaft line, leading to high amplitude torsional vibrations and shaft line stresses. Typical consequences of these phenomena are coupling failures, broken shafts, worn gears, fractured gear teeth and thus undesired plant shutdowns and lacks in operability [2] [3] [4]. The activity has been focused to prevent shaft line vibrations excited by the operation of the Variable Speed Drive, potentially resulting in shaft line damages. An electro-mechanical model, including both the descriptions of the electrical and the mechanical system, has been implemented. The model has been specialized for a VSD composed by a synchronous machine fed by a load commutated inverter (LCI). The reason for this choice is that this kind of drive represents a common solution for the highest power drives (power can be higher than 35 MW). However, torsional issues have been observed in compression trains that included this kind of drive [5] [6]. The simulation model has been set up in order to describe the behavior of a real compression string tested in GE Oil & Gas facilities. The model electrical part requires as input a limited set of data that are always included in the VSD data sheet. Results obtained in simulation have been finally compared with the measurements collected during the compression string tests. The outcome of the research activity is that the simulation model is able to describe the behavior of the Variable Speed Drive, confirming the satisfactory performances of the tested compression string. A second comparison considered a situation where the torque set point changes, showing that the simulation model dynamics are usually slower than the actual system. In general comparisons show that the simulation model is able to describe the behavior of the variable speed drive, confirming the satisfactory behavior of the tested compression string. A general limitation that must be highlighted is that the simulation model does not include some secondary features of VSD control that could be critical for the vibrations induced in the mechanical system, as demonstrated by compression string tests and other tests conducted by GE in conjunction with the VSD vendor. Thus by using this model it is not possible to analyze the influence on torsional dynamics of these secondary control features, which must be adjusted using other techniques. Nevertheless, the simulation model can be used to adjust the control settings for the current regulator before the test and the installation of the compression string, with the aim to reduce any potential risk concerning the pulsating torques on in the actual train: thus, the simulation model can be useful for a virtual pre-tuning of the VSD system. In addition, by adjusting the values of a limited set of parameters, it is possible to adapt the simulation model to describe different drives with LCI architecture. The research activity was supported within GE Oil & Gas by Engineering Shaft Line Integrators Team, by the electrical designers of ENG PA EDES and by the Advanced Technology Laboratory, Test Data Analysis (TDA) team. The research created the basis for deeper insight into the dynamics of electro-mechanical systems, thus improving the ability to design compression strings with Variable Speed Drive systems.