Negative Damping Mechanism Research Articles

Revealing the mechanism of subsynchronous torsional interaction caused by LCC-HVDC from the perspective of vector synchronization

Line-commutated converter-based high voltage direct current transmission (LCC-HVDC) has been widely applied worldwide due to its great advantages in realizing long-distance and large-capacity power transmission. However, it may also lead to instability problems, including subsynchronous torsional interaction (SSTI). This destructive phenomenon greatly threatens the safe and stable operation of power systems and has been widely concerned since the 1970 s. Existing literature has found that SSTI is caused by DC current control of rectifier station. However, the physical mechanism of this phenomenon has not been clarified clearly, which hampers further understanding of how negative damping is generated and whether it is evitable. To fill this gap, the physical process of SSTI caused by LCC-HVDC is clarified through the perspective of vector synchronization. Based on this, the negative damping mechanism is revealed. The influence of control on damping is also studied with the contribution of different control loops quantified. All results are verified through time-domain simulations and damping torque analysis.

Negative Damping Mechanism Analysis of Power System Stabilizer Based on Damping Torque

Power system stabilizer plays an important role in suppressing the low frequency oscillations, but a PSS with poorly tuned parameters may produce negative damping and aggravate the system oscillation. The purpose of this paper is to study the mechanism of damping of a PSS in multi machine system and to analyze whether the PSS has produced negative damping. This paper adopts Park model for analysis. According to the concepts of complex frequency and generalized phasor, the correlation between the damping torque phasor and the real part of the complex eigenvalue is studied. With the auxiliary of Park equivalent circuits, the damping torque provided by the excitation system is separated from the electrical torque phasor. Then, the damping torque coefficient of the PSS is defined as an indicator of the damp effect of the stabilizer. Finally, the simulation results of single machine infinite bus system and 4-machine system demonstrate the validation of the proposed method for evaluating the damping effect of PSS on the system.

A wind-induced negative damping method to achieve high-energy orbit of a nonlinear vibration energy harvester

Maintaining high-energy orbit oscillation of a nonlinear vibration energy harvester (VEH) is the key to achieve high-performance, broadband energy harvesting. Conventional orbit-jump strategies, such as mechanical modulation or electrical control methods, need to consume the limited harvested energy and may unfavourably reduce the energy harvesting efficiency. To avoid the undesired energy consumption, we focus on utilizing the overlooked wind energy to assist a nonlinear VEH to attain the preferred high-energy orbit. The novel orbit-jump method proposed in this letter is based on the wind-induced negative damping mechanism and the resultant self-excited behaviour. Both numerical simulation and experimental results validate the feasibility of the proposed method to efficiently trigger the high-energy orbit oscillations of a nonlinear VEH. Moreover, the required wind energy to achieve self-excited oscillation for different excitation frequencies and acceleration levels, is quite stable and can be easily satisfied, which demonstrates good robustness for practical applications.

Experimental identification of the fluid-elastic coupling forces on a flexible tube within a rigid square bundle subjected to single-phase cross-flow

The importance of fluid-elastic coupling forces in tube bundle vibrations is well documented and can hardly be over-emphasized, in view of their damaging potential. Even when adequate tube supports are provided to suppress fluid-elastic instabilities, the flow-coupling forces still affect the dynamical tube responses and remain a significant issue, in particular concerning the vibro-impact motions of tubes assembled using clearance supports. Therefore, the need remains for more advanced models of fluid-elastic coupling, as well as for experimental flow-coupling coefficients to feed and validate such models. In this work, we report an extensive series of experiments performed at CEA-Saclay leading to the identification of stiffness and damping fluid-elastic coefficients, for a 3×5 square tube bundle (D = 30 mm, P/D = 1.5) subjected to single-phase transverse flow, in a water test-loop. The bundle is rigid, except for the central tube which is mounted on a flexible suspension (two parallel steel blades) allowing for translation motions of the tube in the lift direction. The system is thus single-degree of freedom, allowing fluid-elastic instability to arise through a negative damping mechanism. The flow-coupling stiffness and damping coefficients, Kf(Vr) and Cf(Vr), are experimentally identified as functions of the reduced velocity Vr. The maximum value of the Reynolds number ranged from 105 to 2.16 105 (based on the maximum pitch velocity), according to the tested configuration. Identification is achieved on the basis of changes in tube vibration frequency and reduced damping as a function of flow velocity, assuming a constant fluid added mass. In the present experiments, coefficient identification is performed well beyond the instability boundary, by using active control, thereafter allowing exploration of a significant range of flow velocity. The modal frequency and the modal mass of the system are respectively modified by changing the tube suspension stiffness, and/or by adding a mass to the system. We can thus assert how the fluid-elastic coefficients change, for this configuration, with these two system parameters, all other parameters being kept constant. The results obtained from the configurations tested suggest that formulations for coefficient reduction may be improved, in order to better collapse the identified data.

Effect of Reynolds number on the stability of a single flexible tube predicted by the quasi-steady model in tube bundles

Fluidelastic instability is the most destructive phenomenon in steam generators and boilers. This phenomenon can be reproduced in laboratories by considering cylinder arrays subjected to cross-flow. We use the quasi-steady model to study the stability of the central tube in a parallel triangular tube bundle. The model relies strongly on the derivative of the lift coefficient to predict the onset of the negative damping mechanism and static divergence. In this work, we experimentally study the effect of the Reynolds number on the lift coefficient derivative on the central tube, for a parallel triangular array with P/D=1.5 and low Reynolds number between 20 and 700. Direct numerical simulation (DNS) results are also presented for low values of the Reynolds number, between 20 and 400, and different tube array geometries. The effect of higher Reynolds number was considered using results from the literature for Reynolds numbers in the range [2×104–2×106] and also the potential flow theory. It was found that the lift coefficient derivative strongly depends on the Reynolds number for the lower range of Reynolds number [0,1000]. Additionally, numerical methods (DNS and potential) allowed one to investigate the effect of pitch ratio and orientation of the array. The sensitivity of the lift coefficient derivative on these parameters provides some perspective on the scatter in the data found in the literature. As a result of this study, the effect of the Reynolds number on the stability of a single flexible tube is discussed. The quasi-steady model seems unable to predict the experimental results for the critical velocity, for high values of the Reynolds number.

The Analysis on the Effect of Speed Regulating System on Low Frequency Oscillation

With the strengthening of interconnected power grid connection, interval oscillation mode damping continue to improve, interval power oscillation accidents still occur, however, the negative damping mechanism of the classic cannot be satisfactorily explain these phenomena, this paper provides the researches forced oscillation mechanism of the low frequency oscillation, the speed control system is likely to form the forced oscillation source, speed regulation system and low frequency oscillation formed an adverse coupling effect. In this paper, based on the mechanism of their method, speed control system are analyzed, the effect of various parameters on the low frequency oscillation in order to avoid speed regulation system to participate in the low frequency oscillation provides technical support.

Stability Analysis of Chatter in Tandem Rolling Mills—Part 1: Single- and Multi-Stand Negative Damping Effect

Many different modes of chatter in rolling and their possible causes have been identified after years of research, yet no clear and definite theory of their mechanics has been fully established and accepted. In this two-part paper, stability of tandem mills is investigated. In Part 1, state-space models of single- and multi-stand chatter are formulated in a rigorous and comprehensive mathematical form. Then, the stability of the rolling system is investigated in the sense of the single- and multi-stand negative damping effects. First, a single-stand chatter model in state-space representation is proposed by coupling a dynamic rolling process model with a structural model for the mill stand. Subsequently, a multi-stand chatter model is developed by incorporating the inter-stand tension variations and the time delay effect of the strip transportation based on the single-stand chatter model. Stability criteria are proposed and stability analyses are performed to create corresponding stability charts in terms of the single- and multi-stand negative damping mechanism through numerical simulations. Particularly, the effect of friction conditions on chatter is examined and an explanation is given for the existence of an optimum friction condition. In Part 2, the regenerative effect and resulting instabilities are examined. Suitable stability criteria for each mechanism are established and stability charts are demonstrated in terms of relevant rolling process parameters.

On the difference between negative damping and eigenmode synchronization as two phonation onset mechanisms

Negative damping and eigenmode synchronization as two different mechanisms of phonation onset are distinguished. Although both mechanisms lead to a favorable phase relationship between the flow pressure and the vocal fold motion as required for a net energy transfer into the vocal folds, the underlying mechanisms for this favorable phase relationship are different. The negative damping mechanism relies on glottal aerodynamics or acoustics to establish before onset and maintain after onset the favorable phase relationship, and therefore has minimum requirements on vocal fold geometry and biomechanics. A single degree-of-freedom vocal fold model is all that is needed for self-oscillation in the presence of a negative damping mechanism. In contrast, the mechanism of eigenmode synchronization critically depends on the geometrical and biomechanical properties of the vocal folds (at least 2-degrees-of-freedom are required), and has little requirement on the glottal aerodynamics other than flow separation. The favorable phase relation is established once synchronization occurs, regardless of the phase relationship imposed by glottal aerodynamics before onset. Unlike that of the negative damping mechanism, initiation of eigenmode synchronization requires neither a velocity-dependent flow pressure nor an alternating convergent-divergent glottis. The clinical implications of the distinctions between these two mechanisms are discussed.

A generalization of the van-der-Pol oscillator underlies active signal amplification in Drosophila hearing

The antennal hearing organs of the fruit fly Drosophila melanogaster boost their sensitivity by an active mechanical process that, analogous to the cochlear amplifier of vertebrates, resides in the motility of mechanosensory cells. This process nonlinearly improves the sensitivity of hearing and occasionally gives rise to self-sustained oscillations in the absence of sound. Time series analysis of self-sustained oscillations now unveils that the underlying dynamical system is well described by a generalization of the van-der-Pol oscillator. From the dynamic equations, the underlying amplification dynamics can explicitly be derived. According to the model, oscillations emerge from a combination of negative damping, which reflects active amplification, and a nonlinear restoring force that dictates the amplitude of the oscillations. Hence, active amplification in fly hearing seems to rely on the negative damping mechanism initially proposed for the cochlear amplifier of vertebrates.