Phase-Controlled Thyristor Sub-Synchronous Damper Converter for a Liquefied Natural Gas Plant

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.

A Nonlinear Analysis of the Wahl–Fischer Torsional Instability

This is an extension to a previous study of the Wahl–Fischer torsional instability problem (Infante and Doughty, “An Old Problem Reconsidered: The Wahl–Fischer Torsional Instability Problem”, J. Appl. Mech. Trans. ASME, 2020, 87(10), p. 101004). There, we provided a mathematical explanation of the reasons for the existence of torsional oscillations observed in numerical simulations and in actual mechanical devices such as the exhaust fan system studied by Wahl and Fischer. That explanation was mostly based on linear analysis. This paper presents an additional mathematical explanation of the nature and form of the large self-excited oscillations, due to the strongly nonlinear nature of the system and the large amplitude of these oscillations. Because the oscillations are large, their study requires the use of nonlinear methods.

An Old Problem Reconsidered: The Wahl–Fischer Torsional Instability Problem

The phenomenon of torsional instabilities found in rotating mechanical systems powered by induction motors was encountered in the late 1930s by Wahl and Fisher in a turnpike exhaust system. Subsequently, this phenomenon has been repeatedly found to affect a number of similar physical systems and has led to a number of studies and analysis that have clarified the reasons for these instabilities, associated with the torque characteristics of induction motors. Surprisingly, none of these studies have presented a mathematical qualitative analysis of the eigenvalues of the differential equations that describe such electromechanical systems. This is the central purpose of this paper. It depends on the identification and exploitation of a particular “central” solution to the differential equations that describe the system, and of its relationship to a simpler formulation of the same system. This turns out to be a rather modest mathematical endeavor which, however, yields result that illuminate the nature of the instabilities encountered and provides a designer of such systems with the tools to abate or avoid these instabilities.

The experimental study and finite element analysis of wheel - buckle scaffold

With the rapid development of the construction industry, the country has a higher demand for scaffolding engineering, so it is very necessary to develop and promote the application of wheel buckle scaffolding. Steel tube scaffold with wheel buckle has the characteristics of clear transmission and good mechanical performance. In order to study the structural performance of steel tubular scaffolding with wheel buckle, the single span three-step element frame was tested. The failure mode and ultimate bearing capacity of the frame are obtained. The finite element software Sap2000 was used to conduct 3d modeling and linear buckling analysis of scaffolds in the test. The results of experiments and finite element analysis show that the failure type of steel tubular scaffolding is the overall torsional instability failure. The connection stiffness at the joint of the diagonal brace fastener has a great influence on the wheel-buckle scaffold. The diagonal brace has obvious influence on the bearing capacity of steel tubular scaffolding body with buckles.

The Third Bosporus Bridge — the Aerodynamic Stability of the Steel Segments During the Lifting

<p>The Third Bosporus Bridge is a suspendion bridge with a main span length of 1 408 m and a total length of 2 408 m located at the north of Istanbul near the Black Sea.</p><p>The main span is partially suspended at the pylons by stiffening cables and at the main cables with vertical hangers (Fig.1‐2). The deck is 58.8 m wide. But contrary to a classical arrangement, the transversal distance between the vertical hangers, in the suspended zone, is only 13.50 m. Due to this geometrical configuration of the vertical hangers, it was necessary to verify the risk of aeroelastic instabilities of steel segments of the deck during its lifting: risk of a torsional instability around the longitudinal axis but also around the vertical axis. Countermeasures have been proposed and adopted to suppress these risks.</p>