Natural Frequencies of 3-Pkase Transformer Windings

Natural frequencies and surge response of the windings of 3-phase transformers have been determined in the past by neglecting the capacitive and inductive couplings between the phase windings. This paper shows that these assumptions are not valid and presents a new method of formulating equivalent networks of 3-phase transformer windings for the various winding connections and terminal conditions. By utilizing these equivalent networks the natural frequencies and surge response of the windings can be determined. Tests made on a model transformer showed good correlation with calculated results

Influence of surge impedance of the connecting lead in response time measurement of u.h.v. damped capacitive voltage dividers

In the determination of the response time of u.h.v. damped capacitive impulse voltage dividers using the CIGRE IMR-1MS group (1) method and the arrangement suggested by the International Electrotechnical Commission (the I EC square loop),the surge impedance of the connecting lead has been found to influence the accuracy of determination. To avoid this difficulty,a new graphical procedure is proposed. As this method uses only those data points which can be determined with good accuracy, errors in response-time area evaluation do not influence the result

Automatic detection and elimination of periodic pulse shaped interferences in partial discharge measurements

The interferences present in partial discharge (PD) measurement can be classified as narrow-band and broad-band, the latter being pulsed shaped. The pulse shaped interferences can be periodic or random with respect to power frequency, the former being very common and strong. The paper describes an algorithm for automatic detection and elimination of periodic pulse shaped interferences in PD measurements. The algorithm is developed on lines similar to that used in decomposing an electromyogram (EMG) into its individual motor unit potentials and is verified with practical and simulated PD measurements. In addition to interference pulse rejection, the algorithm can be used to process data from a digital PD detector for overcoming integration errors, obtaining pulse to pulse time interval histograms and distinguishing different PD pulses based on their waveshape

Evaluation of Digital Filters for Rejecting Discrete Spectral Interference in On-site PD Measurements

While Partial Discharge (PD) measurements are widely used in testing power apparatus after manufacture, there is now a trend to extend them to on-site measurements. The major problem encountered in the latter measurements is the strong coupling of external noises particularly from discrete spectral interferences (DSI) e.g. broadcasting stations as well as impulsive disturbances. A critical study of the performance of several digital filters for rejecting DSI is reported. The filters are evaluated with particular reference to distortion introduced on successive PD pulses. A new digital filtering method based on a cascaded 2nd order IIR lattice notch filter has been proposed. The study showed that, for similar performance, methods based on linear prediction required longer filter orders when compared to cascaded IIR notch filters. The non-adaptive methods were found to be stable against impulsive disturbances while adaptive methods were unstable. The new method proposed in this paper gave the best performance, low distortion to a sequence of PD pulses, good stability and fast filtering time. The performance at higher sampling rates, even with PD pulses of long widths was also found to be superior.Further, the method is implementable using fixed-point arithmetic leading to faster processing. Hence, the proposed method can be suggested for on-site PD measurements

Detection and Localization of Inter-turn Fault in the HV Winding of a Power Transformer using Wavelets- Comment

We discuss the paper by M.R. Rao and B.P. Singh (ibid. vol. 8, pp. 652-657, 2001) describing a wavelet approach for identification and localization of "minor" or "small" faults within HV transformer windings during lightning impulse tests on them. Although the authors' claim appear to be impressive, there are serious questions regarding the circuit models and computational procedures used, as well as, in the interpretation of the results obtained from the wavelet method. Therefore, the following is intended to obtain clarification from the authors and to help the reader with the usage of wavelets in addressing such problems

Partial discharge pattern classification using multilayer neural networks

Partial discharge measurement is an important means of assessing the condition and integrity of insulation systems in high voltage power apparatus. Commercially available partial discharge detectors display them as patterns by an elliptic time base. Over the years, experts have been interpreting and recognising the nature and cause of partial discharges by studying these patterns. A way to automate this process is reported by using the partial discharge patterns as input to a multilayer neural network with two hidden layers. The patterns are complex and can be further complicated by interference. Therefore the recognition process appropriately qualifies as a challenging neural network task. The simulation results, and those obtained when tested with actual patterns, indicate the suitability of neural nets for real world applications in this emerging domain. Some limitations of this method are also mentioned

Use of Hidden Markov Models for Partial Discharge Pattern Classification

The importance of partial discharge (PD) measurements for diagnosis of defects in insulation systems is well known. The image patterns obtained in these measurements contain features whose analysis leads to identification of the PD cause. These features are the phase position and amplitudes of PD pulses appearing on the image pattern (usually displayed on elliptic time base on conventional detectors). There is a close similarit y between PD signals and speech. Both are time-varying and similar in behavior. Hidden Markov models (HMM) have been very successful in modeling and recognizing speech. Hence, an attempt was made to employ them to classify PD image patterns. Basis for selection of model and training parameters and the obtained results are discussed. It is shown that successful recognition of PD image patterns using HMM is possible. The ability of HMM to classify some actual PD image patterns has also been ascertained

Design parameters for earth-wire power tapping

The paper deals with the calculation of the induced voltage on, and the equivalent capacitance of, an earth wire isolated for purposes of tapping small amounts of power from high-voltage lines. The influence of heights, diameters and spacings of conductors on these quantities have been studied and presented in the form of graphs

Wavelet Analysis for Estimation of Mean-Curve of Impulse Waveforms Superimposed by Noise, Oscillations and Overshoot

This paper describes a novel approach to estimate the mean-curve of impulse voltage waveforms that are recorded during impulse tests. These waveforms in practice are superposed by noise, oscillations, and overshoot. The approach is based on multiresolution signal decomposition (a kind of wavelet transform) and has many advantages over existing methods, since it does not assume any model for estimating the mean-curve, is interactive in nature, suitable for full and chopped impulses, does not introduce distortions due to its application, is easy to implement and does not call for changes to existing standards. Results presented show its applicability

Influence of Phase Connection and Terminal Conditions on Natural Frequencies of Three-Phase Transformer Windings

With the increasing use of extra high-voltage transmission in power system expansion, the manufacturers of power apparatus and the electric utilities are studying the nature of overvoltages in power systems due to lightning and, in particular, switching operations. For such analyses, knowledge of the natural frequencies of the windings of transformers under a wide variety of conditions is important. The work reported by the author in a previous paper is extended and equivalent circuits have been developed to represent several sets of terminal conditions. These equivalent circuits can be used to determine the natural frequencies and transient voltages in the windings. Comparison of the measured and the computed results obtained with a model transformer indicates that they are in good agreement. Hence, this method of analysis provides a satisfactory procedure for the estimation of natural frequencies and transient voltages in transformer windings