Sensitivity Of Modal Frequencies Research Articles

Harmonic impedance optimization scheme for multi‐resonance systems to suppress resonance

AbstractHarmonic resonance is caused by harmonic source matching resonance frequency and presents the characteristics of decentralization and whole network in modern grids. The current resonance suppression methods mainly focus on reducing the harmonic source, and are suitable for single‐site and small‐scale grids. This paper presents a new method for suppressing parallel resonance at system level. The holistic harmonic resonance is suppressed by reducing the harmonic impedance. In addition, new resonances are prevented by weakening the coupling between resonance and non‐resonance frequencies. The resonance modal analysis and resonance frequency shift (RFS) are employed to optimize harmonic impedance. Simulation results indicate that the capacitance parameter has higher sensitivity and is theoretically more suitable for RFS. Considering the accuracy of modal frequency sensitivity, a variable‐limit genetic algorithm for iteration is proposed. The validity of the proposed method was verified using an IEEE 14‐bus test network.

A novel double-hybrid learning method for modal frequency-based damage assessment of bridge structures under different environmental variation patterns

Monitoring of modal frequencies under an unsupervised learning framework is a practical strategy for damage assessment of civil structures, especially bridges. However, the key challenge is related to high sensitivity of modal frequencies to environmental and/or operational changes that may lead to economic and safety losses. The other challenge pertains to different environmental and/or operational variation patterns in modal frequencies due to differences in structural types, materials, and applications, measurement periods in terms of short and/or long monitoring programs, geographical locations of structures, weather conditions, and influences of single or multiple environmental and/or operational factors, which may cause barriers to employing state-of-the-art unsupervised learning approaches. To cope with these issues, this paper proposes a novel double-hybrid learning technique in an unsupervised manner. It contains two stages of data partitioning and anomaly detection, both of which comprise two hybrid algorithms. For the first stage, an improved hybrid clustering method based on a coupling of shared nearest neighbor searching and density peaks clustering is proposed to prepare local information for anomaly detection with the focus on mitigating environmental and/or operational effects. For the second stage, this paper proposes an innovative non-parametric hybrid anomaly detector based on local outlier factor. In both stages, the number of nearest neighbors is the key hyperparameter that is automatically determined by leveraging a self-adaptive neighbor searching algorithm. Modal frequencies of two full-scale bridges are utilized to validate the proposed technique with several comparisons. Results indicate that this technique is able to successfully eliminate different environmental and/or operational variations and correctly detect damage.

A modified modal sensitivity method for resonance analysis in power electronic systems

Modal sensitivity analysis (MSA) is an essential resonance influencing factor analysis tool in a power electronic system for quantifying the sensitivity of resonance modes to parameters. Unfortunately, in the conventional MSA, the calculation result of modal frequency sensitivity (MFS) is influenced by the frequency resolution, and the modal impedance sensitivity (MIS) could only be quantified under a specific frequency, posing damage to the MSA accuracy. In this paper, a modified MFS calculation method is proposed by solving the critical variable expression with non-homogeneous formula, which avoids the influence of frequency resolution and the accuracy is significantly improved. Furthermore, a new concept of modal resonance sensitivity (MRS) is proposed for quantifying the sensitivity of resonance peak impedance to parameters, which shows good adaptability for the resonance frequency changing and offers a supplement to MIS analysis. Both the proposed method provides precise guidance for designing resonance mitigation and suppression schemes in power electronic systems. Time domain simulations with multiple converters are carried out to verify the proposed method.

Modal Frequency Sensitivity Analysis and Application Using Complex Nodal Matrix

Modal impedance sensitivity and modal frequency sensitivity are two important characteristics of resonance modal analysis. Modal sensitivity is to calculate the sensitivity of a resonance mode against the parameters of network components. The eigendecomposition of a real matrix transformed from a complex matrix not only increases the dimension of nodal admittance matrix, but also creates two equal resonance frequencies in different modes. In this letter, a modal frequency sensitivity analysis using original complex nodal matrix (CMFS) is presented. The results obtained by this method are compared with those by realistic modal frequency sensitivity (RMFS) and the simulation verifies that this method possesses higher accuracy and applicability than that of RMFS. CMFS can determine each no-coupling resonance mode and investigate frequency sensitivity with respect to network component parameters. In addition, Newton's method is introduced to the shift of resonance frequency cooperating with the CMFS.

Modal Frequency Sensitivity for Power System Harmonic Resonance Analysis

Resonance-mode analysis is effective to determine the contributions of different network components to a resonance in power system harmonic studies. There are two forms of modal sensitivity in harmonic resonance mode analysis, that is, the modal impedance sensitivity and the modal frequency sensitivity. This paper presents a method to compute the modal frequency sensitivity index. The analysis results are compared with those obtained by using the modal impedance sensitivity index to clarify the differences and similarities between the two sensitivity indices. The comparison shows that these indices complement each other and should be used to jointly diagnose power system harmonic resonance problems.

Damage Detection and Vibration Control in Smart Plates: Towards Multifunctional Smart Structures

This paper demonstrates the concept of sensitivity enhancing control (SEC) to aid in damage detection in smart structures through both experimental and simulation evaluation. Methods of implementing state estimate feedback using point measurements of strain along the structure are described, and an initial proof-of-concept laboratory experiment demonstrating enhancement of modal frequency shifts due to tip mass damage in a cantilevered beam is reported. Simulation results focus on applying state feedback control to finite-element models of a cantilevered structure with slot, through-surface crack, or surface crack damage. The simulation analysis ascertains the ability to enhance sensitivity of modal frequency shifts due to realistic damage cases that are difficult to evaluate experimentally. The simulation also ascertains the potential for using the same sensors and actuators for implementing both sensitivity enhancing control laws and vibration damping control laws that are insensitive to damage. In the control model with which SEC laws are designed, damage consists of simple reductions in thickness over a small area of the structure. Finite-element models to which control laws are applied are developed using commercial software (ABAQUS TM ) that more accurately models stiffness damage by releasing element connections or by using line spring elements to model fatigue cracks. Experimental results show that enhancement in sensitivity of modal frequencies to damage can be achieved using a single piezoceramic actuator and multiple piezoelectric strain sensors along the beam. Simulation results indicate that feedback control laws can be designed for either sensitivity enhancement or vibration suppression using identical hardware, providing for multifunctional smart structures. In addition, analysis demonstrates that commercial finite-element software is useful for model-based simulation of damaged controlled structures.

DAMAGE DETECTION IN SMART STRUCTURES THROUGH SENSITIVITY ENHANCING FEEDBACK CONTROL

Non-destructive damage detection and isolation in structures using modal information is hindered by the sensitivity of modal frequencies to small changes in mass, stiffness, and damping parameters induced by damage. Here, a method of enhancing modal frequency sensitivity to damage using feedback control is introduced. Using state feedback, closed-loop modal frequencies are placed at locations in the complex plane that enhance sensitivity to particular types of damage. The method is intended for smart structures, which embody self-actuation and self-sensing capabilities. A simple example introduces the principle of sensitivity enhancing control for a single-degree-of-freedom structure. Then, the method is applied to finite-element models of a cantilevered beam to demonstrate the magnitude of sensitivity enhancement achievable for modest, local damage. Methods of implementing sensitivity enhancing full state or output feedback using point measurements of strain along the beam are described. Simulation results show that significant enhancement in sensitivity of modal frequencies of vibration to damage can be achieved using a single actuator and multiple strain sensors along the beam. The methodology enables a “dual use” smart structure—one that can be used for both vibration suppression and damage detection.