Excitation Of Torsional Oscillations Research Articles

Normal Form Analysis of the Subsynchronous Torsional Interaction with SVC in the Complex Power Systems

Due to the importance of nonlinear interactions in the complex power systems, in this paper the subsynchronous torsional interactions (SSTI) between turbine-generator shafts and flexible AC transmission system (FACTS) are studied using the nonlinear modal analysis method. Normal Form method is applied to study the nonlinear effects on the SSTI phenomenon in the presence of Static VAr Compensator (SVC) as a commonly used FACTS device. The results show that under stressed conditions the SVC voltage controller gains have an adverse effect on the nonlinear interactions of torsional modes. These effects can lead to severe excitation of torsional oscillations. The study also illustrates that the type of SVC control system model will play an important role in the nonlinear analysis of SSTI, and using a simple PI model is not suitable for nonlinear analysis of SSTI phenomenon. Moreover, in a multi-area power system, the location of SVC and its capacity have significant effects on the severity of nonlinear interactions of torsional modes.

Investigation of conditions for the generation and propagation of low-frequency disturbances in the troposphere

Low-frequency disturbances responsible for the excitation of torsional oscillations—variations in the zonal mean flow intensity with a characteristic scale of 15–20 days—propagating along the meridian at mid and low latitudes of both hemispheres are investigated [1]. As data observed over the eastern parts of continents and the western parts of oceans are processed with the lag correlation statistics, traveling waves intersecting the eastern parts of continents from northwest to southeast and then returning to the north along the ocean coasts are identified. In this case, trains of anomalies oriented in the zonal direction periodically appear and are destructed in the western parts of continents. The simulation of the propagation of disturbances in the quasi-geostrophic approximation made it possible to explain the specific features of lag correlation statistics over continents by the dispersion of two-dimensional Rossby waves from traveling sources. The turnover of disturbances over Asia and wave trains to the west from the pole were reproduced. Torsional oscillations caused by the dispersion of two-dimensional Rossby waves have a characteristic form of inclined bands in the latitude-time diagram, whose steepness is controlled by the velocity of displacement of the vorticity source along the meridian.

Resonance method of measuring shear viscoelastic properties of liquid media based on excitation of torsional oscillations in tubes

Possibilities of using torsional oscillations for measuring viscoelastic properties of liquids are discussed. The theory of torsional oscillations of an elastic tube filled with the media to be investigated possessing viscosity and shear elasticity is developed. It is shown that to determine a complex shear modulus it is sufficient to determine the resonance frequency and Q-factor of torsional oscillations. An experimental installation and the results of measurements of viscoelastic modulus of glycerin and oil of one oilfield within the temperature range from −10° to 60°C are given. The experimental installation allows measuring a viscoelastic modulus within the range of acoustic logging frequencies (10–20 kHz). The obtained results are compared with the results of rheometric measurements.

On the Convective Excitation of Torsional Oscillations in Rotating Systems

An asymptotic theory is presented to describe the possible mechanism for the excitation and maintenance of torsional oscillations in rotating convective stars and planets. It is demonstrated analytically that the torsional oscillations can be excited and sustained by global thermal convection.

Excitation of torsional oscillations in generator shaft lines by plasma feedback control

The ASDEX Upgrade (AUG) tokamak requires an electrical power up to a few hundred MVA for a time period of 10–20 s. The power and energy is provided by three separate networks based on flywheel generators. In 1999, during a routine check performed on generator EZ3, it was discovered that coupling bolts of the flywheel generator shaft were deformed. Given that the active load of the generator (∼100 MW) in service is well below the design value of the shaft (∼800 MW), the damages may only be explained by a torsional resonance of the shaft line, itself excited by active power transients from the converter loads. A value of 23.6 Hz was calculated for the first eigen-frequency of the shaft line. Frequencies between 10 and 30 Hz have been identified in the spectrum of the load curves. Since torsional shaft oscillations are characterised by very low damping, torsional resonance can become dangerous even for over-dimensioned generator shafts. Therefore, a novel ‘torque’ measurement system was installed. The paper presents results from calculations and measurements showing that devices capable to measure the stress in the shaft line are essential for generator protection in feedback controlled fusion experiments.

Excitation of torsional oscillations of the Earth: Higher modes

Spherical models of the earth, with physical properties varying continuously with depth, are considered. The periods and spectral amplitudes of the torsional free oscillations lTn are calculated for several focal depths, for higher modes l = 1, 2, and 3, and values of n up to several hundreds. The displacement caused by a torque source at various depths (the initial motion being a unit step-function in time) is next calculated by means of a series of spherical functions; at least 800 terms in n must be included in it in order to include the spectral range in which the group velocity reaches a maximum associated with Sa waves. Three continental models are considered: the Lehmann and Gutenberg models, both with a low-velocity layer at 150-km depth, and a model without a low-velocity layer in the upper mantle. For all of them the displacement caused by the higher modes is weak relative to that produced by the fundamental mode, if the focus is at the surface. But the higher modes predominate as soon as the depth exceeds 100 km, the fundamental mode being then little excited. Because of the strong discontinuities in the first model, the dispersion curves and resulting seismograms are very complicated. There is no striking difference between the results for the second and third models, but for the Gutenberg model the features of the ‘guided’ group of waves resemble more closely those of the experimental Sa waves. At epicentral distance θ = 45°, on the seismograms obtained by superimposing the displacements for the three first higher modes, there appear, clearly separated from the main group of waves, long-period body waves: S, sS, and SS. Other body waves may be identified inside the main group.