Analysis of Synchronous Generators’ Local Mode Eigenvalues in Modern Power Systems

New energy sources, storage facilities, power electronics devices, advanced and complex control concepts, economic operating doctrines, and cost-optimized construction and production of machines and equipment in power systems adversely affect small-signal stability associated with local oscillations. The objective of the article is to analyze local oscillations and the causes that affect them in order to reduce their negative impact. There are no recognized analyses of the oscillations of modern operating synchronous generators exposed to new conditions in power systems. The basic idea is to perform a numerical analysis of local oscillations of a large number of synchronous generators in the power system. The paper represents the local mode data obtained from a systematic analysis of synchronous generators in the Slovenian power system. Analyzed were 74 synchronous generators of the Slovenian power system, plus many additional synchronous generators for which data were accessible in references. The mathematical models convenient for the study of local oscillations are described first in the paper. Next, the influences of transmission lines, size of the synchronous generators, operating conditions, and control systems were investigated. The paper’s merit is the applicable rules that have been defined to help power plant operators avoid stability-problematic situations. Consequently, boundaries were estimated of the eigenvalues of local modes. Finally, experiments were performed with a laboratory-size synchronous generator to assess the regularity of the numerically obtained conclusions. The obtained results enable the prediction of local oscillations’ frequencies and dampings and will be useful in PSS planning.

The Band Gap Formation in Rotors with Longitudinal Periodicity and Quasi-Periodicity

Modal spacing (band gaps) in the frequency spectrum of rotating machines can be imposed by geometric periodicity. By designing the rotor with a geometry that repeats periodically, we can impose to the vibration response of the rotor a modal "gap" considerably large, where no resonances appear. In this work, we consider that the rotating elements of the machine (e.g. the stages or the impellers) are the periodic elements of the rotor. In this disk-like configuration of the rotor, the system can present band gaps due to two different reasons: due to matching between the number of disks and the eigenmode wavenumber (usually in slender rotors); due to the presence of local-mode shapes (usually in large rotors). Analytical modeling of the system is presented, whose approximated solution can be used to predict the start and stop frequencies of the band gaps. It is also shown the limitations in band gap formation when the rotor is not perfectly periodic (quasi-periodic geometry). In this case, disk positioning plays an important role in the band gap formation.

Identification of Damage on Sluice Hoist Beams Using Local Mode Evoked by Swept Frequency Excitation

As a global vibration characteristic, natural frequency often suffers from insufficient sensitivity to structural damage, which is associated with local variations of structural material or geometric properties. Such a drawback is particularly significant when dealing with the large scale and complexity of sluice structural systems. To this end, a damage detection method in sluice hoist beams is proposed that relies on the utilization of the local primary frequency (LPF), which is obtained based on the swept frequency excitation (SFE) technique and local resonance response band (LRRB) selection. Using this method, the local mode of the target sluice hoist beam can be effectively excited, while the vibrations of other components in the system are suppressed. As a result, the damage will cause a significant shift in the LPF of the sluice hoist beam at the local mode. A damage index was constructed to quantitatively reflect the damage degree of the sluice hoist beam. The accuracy and reliability of the proposed method were verified on a three-dimensional finite element model of a sluice system, with the noise resistance increased from 0.05 to 0.2 based on the hammer impact method. The proposed method exhibits promising potential for damage detection in complex structural systems.

CLASSIFICATION OF BUCKLING MODES IN STIFFENED FUNCTIONALLY GRADED COMPOSITE TUBES SUBJECT TO BENDING

Bamboo poles, and other one-dimensional thin-walled structures are usually loaded under compression, which may also be subject to bending arising from eccentric loading. Many of these structures contain diaphragms or circumferential stiffeners to prevent cross-sectional distortions and so enhance overall load-carrying response. Such hierarchical structures can compartmentalize buckling to local regions in addition to withstanding global buckling phenomena. Predicting the buckling mode shapes of such structures for a range of geometric parameters is challenging due to the interaction of these global and local modes. Abaqus finite element software is used to model thousands of circular hollow tubes with random geometric parameters such that the ratios of radius to periodic length range from 1/3-1/7, the ratio of wall thickness to radius varies from 1/4-1/10. The material used in this study is a type of bamboo, where the Young’s and shear moduli are point-wise orthotropic and gradually increase in magnitude in the radial direction. Under eccentric loads with varying eccentricity, the structures can buckle into a global mode or local modes within an internode, i.e. periodic unit. Moreover, the local modes may contain only one wave or multiple waves in the circumferential direction. As expected, numerical results show that the global mode is more likely to occur in small and thick tubes, whereas the local modes are observed in larger tubes with a smaller number of circumferential waves present in thicker walls. Also, greater eccentricity pushes the local mode domains towards smaller tubes. An efficient classification method is developed herein to identify the domains of each mode shape in terms of radius, wall thickness and eccentricity. Based on linear discriminant analysis, explicit boundary surfaces for the three domains are defined for the obtained data, which can help designers in predicting the mode shapes of tubular structures under axial bending.

Evaluation of Operating Schemes on Municipal Rail Transit with Express/Local Mode

The express/local train mode has made municipal rail more competitive in connecting the city center with its suburbs. Therefore, how to design operating schemes is of concern to decision makers. Unlike earlier line planning optimization, this paper aims to provide a quick and comprehensive evaluation of operating performance in all feasible options through reasonable reduction of the scope of the operating schemes, rather than optimization for a single objective. To achieve this, the paper constructs a travel topology network structure based on the separation of trains, which regards express and local trains running on the common line as independent competitors. Focusing on the problems of uneven departure intervals and inconsistent dwell times caused by overtaking, a generalized cost model suitable for the express/local train mode is established. To solve the express/local line problems of multi-routes and high-overlapping, the improved Multinominal Logit (MNL) Model is used to construct a passenger flow assignment model, with the solution achieved by the Method of Successive Averages algorithm. Taking a municipal rail line in Guangzhou as a case study, this paper analyzes the advantages and disadvantages of feasible operation schemes with different characteristics in the express/local train ratio, the location and number of overtaking stations based on passenger flow assignment results and also the impacts of crowdedness on passenger flow dynamics. The results verified the effectiveness of our approach in the comprehensive evaluation of the express/local train mode and shed light on potential practical applications in determining a reasonable operating scheme.

Towards Computational Screening for New Energetic Molecules: Calculation of Heat of Formation and Determination of Bond Strengths by Local Mode Analysis

The reliable determination of gas-phase and solid-state heats of formation are important considerations in energetic materials research. Herein, the ability of PM7 to calculate the gas-phase heats of formation for CNHO-only and inorganic compounds has been critically evaluated, and for the former, comparisons drawn with isodesmic equations and atom equivalence methods. Routes to obtain solid-state heats of formation for a range of single-component molecular solids, salts, and co-crystals were also evaluated. Finally, local vibrational mode analysis has been used to calculate bond length/force constant curves for seven different chemical bonds occurring in CHNO-containing molecules, which allow for rapid identification of the weakest bond, opening up great potential to rationalise decomposition pathways. Both metrics are important tools in rationalising the design of new energetic materials through computational screening processes.

Vibrational Analysis of Benziodoxoles and Benziodazolotetrazoles

Tetrazoles are well known for their high positive enthalpy of formation which makes them attractive as propellants, explosives, and energetic materials. As a step towards a deeper understanding of the stability of benziodazolotetrazole (BIAT)-based materials compared to their benziodoxole (BIO) counterparts, we investigated in this work electronic structure features and bonding properties of two monovalent iodine precursors: 2-iodobenzoic acid and 5-(2-iodophenyl)tetrazole and eight hypervalent iodine (III) compounds: I-hydroxybenzidoxolone, I-methoxybenziodoxolone, I-ethoxybenziodoxolone, I-iso-propoxybenziodoxolone and the corresponding I-hydroxyben ziodazolotetrazole, I-methoxybenziodazolotetrazole, I-ethoxybenziodazolotetrazole and I-iso- propoxybenziodazolotetrazole. As an efficient tool for the interpretation of the experimental IR spectra and for the quantitative assessment of the I−C, I−N, and I−O bond strengths in these compounds reflecting substituent effects, we used the local vibrational mode analysis, originally introduced by Konkoli and Cremer, complemented by electron density and natural bond orbital analyses. Based on the hypothesis that stronger bonds correlate with increased stability, we predict that, for both series, i.e., substituted benziodoxoles and benziodazolotetrazoles, the stability increases as follows: I-iso-propoxy < I-ethoxy < I-methoxy < I-hydroxy. In particular, the I−N bonds in the benziodazolotetrazoles could be identified as the so-called trigger bonds being responsible for the initiation of explosive decomposition in benziodazolotetrazoles. The new insight gained by this work will allow for the design of new benziodazolotetrazole materials with controlled performance or stability based on the modulation of the iodine bonds with its three ligands. The local mode analysis can serve as an effective tool to monitor the bond strengths, in particular to identify potential trigger bonds. We hope that this article will foster future collaboration between the experimental and computational community being engaged in vibrational spectroscopy.