Yong-June Shin

Application of time-frequency domain reflectometry for detection and localization of a fault on a coaxial cable

In this paper, we introduce a new high-resolution reflectometry technique that operates simultaneously in both the time and frequency domains. The approach rests upon time-frequency signal analysis and utilizes a chirp signal multiplied by a Gaussian time envelope. The Gaussian envelope provides time localization, while the chirp allows one to excite the system under test with a swept sinewave covering a frequency band of interest. This latter capability is of particular interest when testing communication cables and systems. Sensitivity in detecting the reflected signal is provided by a time-frequency cross-correlation function. The approach is verified by experimentally locating various types of faults, located at various distances, in RG 142 and RG 400 coaxial cables.

Power quality indices for transient disturbances

For reasonable power quality assessment of transient disturbances in electric power systems, new transient power quality indices are developed based on a signal processing technique, time-frequency analysis. Based on the time-frequency distribution of a transient disturbance, a set of time-frequency based power quality indices are developed. In this paper, the instantaneous disturbance energy ratio, normalized instantaneous disturbance energy ratio, instantaneous frequency, and instantaneous K-factor are suggested for transient power quality assessment. Time-frequency based power quality indices allow one to quantify the effects of transient disturbances with high-resolution and accuracy.

A wavelet-based approach to detect shared congestion

Per-flow congestion control helps endpoints fairly and efficiently share network resources. Better utilization of network resources can be achieved, however, if congestion management algorithms can determine when two different flows share a congested link. Such knowledge can be used to implement cooperative congestion control or improve the overlay topology of a P2P system. Previous techniques to detect shared congestion either assume a common source or destination node, drop-tail queueing, or a single point of congestion. We propose in this paper a novel technique, applicable to any pair of paths on the Internet, without such limitations. Our technique employs a signal processing method, wavelet denoising, to separate queueing delay caused by network congestion from various other delay variations. Our wavelet-based technique is evaluated through both simulations and Internet experiments. We show that, when detecting shared congestion of paths with a common endpoint, our technique provides faster convergence and higher accuracy while using fewer packets than previous techniques, and that it also accurately determines when there is no shared congestion. Furthermore, we show that our technique is robust and accurate for paths without a common endpoint or synchronized clocks; more specifically, it can tolerate a synchronization offset of up to one second between two packet flows.

Health Monitoring of Power Cable via Joint Time-Frequency Domain Reflectometry

Utilities are experiencing premature failures of power cables. In order to prevent electrical outages and to save on repair expenses, a nondestructive and nonintrusive condition assessment technique is highly desirable to evaluate the cable status and to predict the remaining life of a cable. In this paper, the capability of joint time-frequency domain reflectometry (JTFDR) as such a condition assessment technique is studied. The health status of three popular insulations in power system cables - cross-linked polyethylene, ethylene propylene rubber, and silicone rubber - is monitored using the JTFDR in a thermal accelerated aging test. The experimental results show that the JTFDR can successfully monitor the aging process of all three insulations. Then, the results from the JTFDR are compared with the results from the elongation at break (EAB); the results show that the JTFDR technique is comparable with the EAB and has a great potential as a nondestructive and nonintrusive condition assessment technique.

Diagnostics and Prognostics of Electric Cables in Ship Power Systems via Joint Time-Frequency Domain Reflectometry

The integrity of the wiring in the electric power system of a ship is vital to its safe operation. To ensure the wiring integrity, it must be tested to determine if any incipient defects exist. Due to this problem, a non-destructive, non-intrusive condition assessment technique is highly desirable. Joint time-frequency domain reflectometry (JTFDR) is proposed and the theory behind JTFDR is also discussed. The experimental results demonstrate and verify the ability of JTFDR to be effective for polytetrafluoroethylene (PTFE) coaxial cable, which has been widely adopted for military applications in ship power systems. It is shown that JTFDR has the ability to detect and locate incipient defects with high accuracy and monitor the aging process of the cables to predict both future defects and the remaining service life of the cables.

Scalable Clustering of Internet Paths by Shared Congestion

Internet paths sharing the same bottleneck can be identified using several shared congestion detection techniques. However, all of these techniques have been designed to detect shared congestion between a pair of paths. To cluster N paths by shared congestion, a straightforward approach of using pairwise tests would require O(N) time complexity. In this paper, we present a scalable approach to cluster Internet paths based on DCW (Delay Correlation with Wavelet denoising) which does not require a common end point between paths. We present a function to map each path’s measurement data into a point in a multidimensional space such that points are close to each other if and only if the corresponding paths share congestion. Because points in the space are indexed using a tree-like structure, the computational complexity of clustering N paths can be reduced to O(N log N). The indexing overhead can be further improved by reducing dimensionality of the space through wavelet transform. Computation cost is kept low by reusing for dimensionality reduction the same wavelet coefficients obtained in DCW. Our approach is evaluated by simulations and found to be effective for a large N . The tradeoff between dimensionality and clustering accuracy is shown empirically.

Signal Processing-Based Direction Finder for Transient Capacitor Switching Disturbances

This paper develops a signal processing, time-frequency analysis-based analytic solution to locate transient capacitor switching disturbances. The flow of transient disturbance energy caused by the capacitor switching is determined by the time and frequency localized phase difference. Cross time-frequency analysis provides time- and frequency-localized phase difference between the transient voltage and current disturbance waveforms which determine the direction of transient disturbance energy flow. The time and frequency localization properties of the proposed scheme allows one to expand the application to complicated power distribution systems without knowledge of system parameters, capacitor size, and configuration. The proposed scheme has been verified by the electromagnetic transients program simulation for all possible spatial locations of the capacitor switching.

Diagnostics and Prognostics of Electric Cables in Nuclear Power Plants via Joint Time-Frequency Domain Reflectometry

Defective cables in the electric power systems of nuclear power plants can cause a component to fail, resulting in potential safety concerns. Due to this problem, a non-destructive, non-intrusive condition assessment technique is highly desirable. Joint time-frequency domain reflectometry (JTFDR) is proposed and verified to be effective for cross-linked polyethylene (XLPE) cable, which serves critical instrumentation and control operations in nuclear power plants. The experimental results demonstrate and verify the ability of JTFDR to effectively detect and locate incipient defects with high accuracy and to monitor the aging process of the cables to predict both future defects and the remaining service life of the cables.

Evaluation of the load impedance in coaxial cable via time-frequency domain reflectometry

A new impedance measurement methodology based on time-frequency domain reflectometry (TFDR) is proposed. For the evaluation of the reflection coefficient in time-frequency domain reflectometry, the distortion of the reflected wave by the frequency-dependent attenuation is compensated which otherwise results in inaccurate impedance measurement. Also, the phase difference between the incident and reflected waveforms caused by the state of the load impedance is evaluated by the cross time-frequency distribution which provides time-frequency localized phase difference information. The proposed methodology is verified by a set of numerical electromagnectic simulation experiments and the results are compared with classical time domain reflectometry (TDR). Impedance measurement via time-frequency domain reflectometry is more accurate over a wider range of impedances than TDR.

Cross time-frequency distribution function

In this paper, we consider cross time-frequency distributions, with particular emphasis on their ability to preserve phase difference (between two signals) information as a function of time and frequency. In addition, some properties of cross time-frequency distributions are examined and compared with the traditional Cohens class of time-frequency distributions. Finally, the phase preserving properties of appropriately defined cross time-frequency distributions are illustrated with a pair of Gabor logons.