Abstract
In electrified liquefied natural gas (LNG) plants variable frequency drives (VFDs) interact with turbine-generator (TG) units creating torsional vibrations known as sub-synchronous torsional interactions (SSTIs). Torsional vibrations can be dangerous for an LNG plant when they involve torsional instability. The stability of an LNG plant depends on the plant configuration and on the number of TG units and of VFDs. In such peculiar configurations stability cannot be achieved acting of the VFDs control system. Alternatively, dedicated equipment is needed to damp the torsional vibrations. In this paper, a sub-synchronous damper (SSD) converter is used to mitigate the SSTI phenomena. The SSD converter consists of a thyristor H-bridge regulating the phase of the additional torque provided at the TG unit air-gap. A phase control system is proposed and is based on the torsional torque oscillations measurement. An adaptive reference signal is employed, also guaranteeing high performance in island-mode operation. The proposed solution increases the damping of the LNG plant in all the considered configurations. The LNG plant successful operation is validated by comprehensive results.
Highlights
The torsional stability of an liquefied natural gas (LNG) plant depends on the plant configurations
Starting from a condition of torsional instability, sometimes the torsional stability can be achieved by acting on the control system parameters of the LNG plant power conversion stages
The same additional electrical torque is applied for all the other TG units but with other phase differences
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The damping increases or decreases according to the phase difference between the additional torque and the relative torsional speed of the TG unit It is common practice [13,17,21] to operate the SSD with a constant phase. The rest of the paper is organized as follows: in Section 2 the case study is presented; in Section 3 the theoretical relationship between the electromechanical torque variation and electrical damping is investigated; in Section 4 the overall SSD control system is analyzed; Section 5 provides the results considering four different configurations of the plant; and Section 6 is focused on final remarks
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