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A circular cylindrical shell piezoelectric transformer of radially polarized ceramics operating in thickness-stretch modes is described. The transforming ratio, input admittance and efficiency of the transformer are compared with circular cylindrical shell transformer of axially polarized ceramics operating in shear modes.

This investigation presents the measurement results of acoustic noise and vibration of a model three-phase three-limb transformer core under sinusoidal and PWM voltage excitation for assigned value of modulation index with switching frequency from 1 kHz to 3 kHz at no-load condition. Results show that the acoustic noise decreased around 2 (dBA) with increasing switching frequency. However, magneto-mechanical resonance phenomenon was observed at 3 kHz vibration frequency, which could cause excess acoustic noise under some excited conditions. Index Terms--Acoustic noise, magneto-mechanical resonance, PWM, vibration.

This paper describes the successful development of a 1-MV magnetically insulated Tesla transformer. Full details of the construction are provided, together with the basis of the design procedure. Preliminary results for the prototype transformer are presented and discussed, and future paper is outlined. Index Terms--High-voltage techniques, magnetic insulation, pulsed-power systems, transformers.

The 3-D open boundary eddy current field induced by heavy current flowing in the coils and winding leads in large power transformers is calculated. The eddy current loss and local overheating problem in the tank wall and in the ascending flange of bushings are analyzed and reported in this paper. The effectiveness of a magnetic bypass plate near the coil ends for reducing the eddy current loss and local overheating in the clamp plates is discussed. Index Terms--Eddy current field, loss, power transformer.

This paper proposes a control method for eliminating dc magnetic flux from the series transformer of a dynamic voltage restorer. The method is characterized by intentionally injecting no compensating voltage during a one-sixth line cycle (=3.3 ms at 50 Hz). The control method is independent of an initial phase angle of a voltage sag, thus requiring no flux detection. The period, during which no compensating voltage is injected, is one-third as short as that of an existing method. This paper confirms the effectiveness and viability of the proposed method. A 200-V 5-kW laboratory system verifies experimentally that the proposed method brings no dc magnetic flux to the series transformer. Moreover, the proposed method brings a smaller dc voltage drop to the dc load of a three-phase diode rectifier than the existing method does in the same conditions. Index Terms--Compensating voltage, dc magnetic flux, dynamic voltage restorers (DVRs), magnetic saturation, series transformers.

A practical method to measure transformer core loss in a high-frequency switching mode power converter is proposed. The method is also applicable when the transformer has de bias in the magnetizing current. Practical issues to minimize the measurement error are discussed. The measurement results are very close to simulation results obtained from a time-domain finite element analysis solver. Detailed error analysis for the proposed method provides useful guidelines on transformer core loss measurement for switching mode power supplies. Index Terms--Core loss, finite element analysis (FEA), pulse-width modulation (PWM), switching mode power supply (SMPS).

This paper presents and verifies a new mathematical model to explain the dynamic behavior of characteristic gases in the oil-cellulose insulation of high voltage devices like power transformers. The model is based on the diffusion process. Parameters of the model are already quantified from experiments and presented in previous publications. The mathematical model and assumptions are presented here. The model solution is obtained analytically and gas concentration change inside the paper insulation as a function of time is simulated. The results are converted to the concentration change in the oil using the principle of conservation of mass and validated with experimental measurements. The presented model can be used to reduce the error of Dissolved Gas Analysis due to the migration of characteristic gases inside a healthy power transformer. Index Terms--Transformers, diffusion, oil-paper system, Dissolved Gas Analysis.

To lower the insulation specifications (specifically, the lightning impulse withstand voltage) of oil-immersed transformers and thus reduce equipment cost while maintaining high insulation reliability, it is important to identify the insulation characteristics under non-standard lightning impulse waveforms that are associated with actual surge waveforms in the field besides the standard lightning impulse waveform and quantitatively compare them with the characteristics under the standard lightning impulse waveform. In a preceding paper, field overvoltages in the lightning surge time region were analyzed, and four typical non-standard lightning impulse waveforms were identified. Two of these including pulses were used to measure the breakdown voltages and the partial discharge inception voltages while changing the parameters, on three models of the winding insulation elements of oil-immersed transformers. This paper investigates the insulating properties of the other two types: 'damped-oscillation waveform' and 'rising-oscillation waveform', and evaluates the resultant average breakdown voltages in terms of the overvoltage duration, permitting their formulation in a unified way. In the examined range, the dielectric breakdown values under non-standard lightning impulse waveforms were up to 50% higher than those under standard lightning impulse waveforms in all cases. Further investigations will be expected for the reduction in insulation specifications. Index Terms--Oil-immersed transformer, non-standard lightning impulse voltage waveform, dielectric breakdown voltage, partial discharge inception voltage, oil-gap, turn-to-turn insulation model, section-to-section insulation model, lightning impulse withstand voltage.

This paper presents a new methodology for measuring integrated transformers. It gives a description of the transformer and its specifications and develops a measurement system. It also presents different transformer geometries and transformer terminal combinations. After the theoretical exposition of the measurement system, a differential-differential integrated transformer and a single-single integrated transformer are measured using this system in order to validate the methodology proposed. In the state-of-the-art, there is no fixed methodology for measuring the coupling factor k of a transformer. This paper presents a new methodology for obtaining this figure-of-merit systematically. As a result of this measurement, this paper presents the values obtained and corroborates the measurement system. Thus, this paper presents and explains a new systematic method for measuring transformers. Index Terms--Coupling factor, integrated transformer, measurement system, methodology.

An approximate nonlinear analysis of a simple plate thickness-shear mode transformer near resonance from the three-dimensional equations of nonlinear electroelasticity is presented. It was observed that on one side of the resonant frequency the input--output relation became nonlinear and on the other side there was a jump in the voltage output experience.

Although experts all over the world have investigated methods for partial discharge (PD) detection and classification for over 50 years, until now there is still no universal method available for this purpose. Even in the future, it would not be possible to define a universal method for unambiguous classification and localization of PD sources in complex insulating systems (e.g. transformers or generators) due to the unlimited variations of PD source type and its location. This paper deals with PD signals obtained by remote radiometric measurements performed on a power transformer. Extensive PD measurements were performed in the substation. Since fingerprints of the remote radiometric measurements are not available, the formation of clusters with similar features obtained from captured PD data is crucial. A data mining technique known as Self-organizing Map (SOM) for the analysis and interpretation of captured PD data is used. Since each signal contains a large number of samples, before applying the SOM, the dimensionality reduction based on principal component analysis is performed. As a result of a data mining process a clear separation of PDs emanating from a transformer and discharges emanating from its surrounding is achieved. Index Terms--Partial discharges, data mining, feature extraction, principal component analysis, self-organizing map.

A study is conducted to analyze the effects of a parallel resistor on the electrical transient characteristics, including maximum voltages, time constants, energy losses and average powers of a Rosen-type piezoelectric transformer (PT). The results show that the additional resistor is of a great help, as it leads to high energy losses and increased average powers in the PT and the oscilloscope due to the increased parallel resistance.

An innovative technique that produces a significant improvement of the power capability of piezoelectric transformers is presented. This technique consists of a nonlinear processing of the piezoelectric transformer output voltage, which results in strong enhancement when applied to the piezoelectric transformer.

Accurate diagnosis of faults in transformers can significantly enhance the safety, reliability, and economics of power systems. In the case of a fault, it has been established that the pattern of the fault currents contain a typical signature of the nature and location of the fault for a given winding. This paper describes a new approach using wavelet transform (WT) for extraction of features from the impulse test response of a transformer in time-frequency domain and support vector machine in regression mode to classify the patterns inherent in the features extracted through the WT of different fault currents. This paper also describes an approach to identify the type and location of the transformer faults accurately by analyzing experimental impulse responses that contain noise. Here, experimental impulse responses have been pre-processed with the help of wavelet-packet filters to remove the unwanted noise from the signal and thereby enhance the analyzing capability of continuous wavelet transform. Index Terms--Analog model, digital model, impulse test, support vector machine (SVM), time-frequency analysis, transformer insulation, wavelet packet (WP) transform, wavelet transform (WT).

This paper presents the measured and analyzed responses of the partial discharge (PD) when it propagates in a power network from its origin to the measuring nodes. On and offline measurements are done in a power network consisting of a 16.5-kV/250-MVA-rated power generator, busbar, and step-up transformer. The simultaneously measured PD at two ends of the busbar indicated that the intensity of PD decreased as it traveled a longer distance and the time interval of its occurrence had a finite delay indicating the dominant mode of PD propagation as a transmission line. Frequency-domain analysis indicated the presence of characteristic resonance frequencies and the number increased as the measuring node moved away from the PD origin. The offline measurement by injecting an exponential PD signal at the neutral indicated that the signal will be absorbed in the generator, and the signal at the measuring node will be attenuated by 50 times and will be distorted. Pspice simulation is used to develop the network model for the propagation path. The Pspice analysis indicated that the distributed models of generator, busbar, and lumped parameter models of a transformer can be used to match the offline measurement. The model showed that each model can have the characteristic resonant peaks depending on the selected parameters and it can distort the simultaneously observed PD at two different nodes. Index Terms--Busbar, generator, partial discharge (PD), propagation, simulation, transfer function, transformer.

In order to attain reduction in insulation test voltages, voltage-time (V-t) characteristics under AC voltage and voltage-number (V-N) characteristics under lightning impulse and switching impulse voltages were experimentally investigated on the turn-to-turn insulation model, section-to-section insulation model and barrier-oil-duct insulation model of core- and shell-type transformers in EHV and UHV Classes. First, V-t characteristics for AC voltage were obtained in a short time range of a few tens of milliseconds to a long time range of three to four months. The n-values (inclination) of V-t characteristics are distributed around 40 for the short time and several hundreds for the long time characteristics. These values are of essence in determining the AC withstand voltage test. Next, V-N characteristics for both lightning and switching impulse voltages were acquired for up to 1,000 times of application. The n-values (inclination) of V-N characteristics are in most cases distributed around 70 for both impulse waveforms and about 40 in only turn-to-turn insulation models. This result will be useful for evaluating the effect of frequent surges on apparatus insulation. Finally, insulation coordination studies were performed based on these experimental results. Index Terms--Transformer, oil-insulation, insulation testing, voltage-time characteristic, voltage-number characteristic.

Partial Discharge (PD) characteristics of four typical transformer oils, having respective viscosities of 3.5, 9.1, 13.0, 18.3 cSt at 40[degrees]C, were examined under ac conditions, using a needle-to-plane electrode system. Wide and narrow bandwidth measurements were carried out to determine simultaneously the apparent charge transfers associated with the overall PD pulse bursts in the oil as well as that of the individual discrete PD pulses within the PD pulse bursts themselves. Based on the apparent charge transfer value, the size or depth of the PD initiating microcavities in the field direction within the oils, producing the first detected discrete PD pulse, were estimated to be in the order of 2 [micro]m. The recurrence rate of the PD pulse bursts was found to increase with voltage above the PD inception voltage; this increase was accompanied by both an increase in number of discrete PD pulses and their amplitude within the pulse burst itself. These increases were reflected by a substantial rise in the apparent charge transfer per PD pulse burst. The charge transfer levels of the PD pulse bursts associated with the highly pressurized gas microcavities were of the same order of magnitude as those produced within the normally much larger macroscopic cavities that exist under atmospheric pressure in conventional oil-impregnated insulating systems of power transformers. Index Terms--Electric breakdown, oil insulation, partial discharges, standardization.

The measurement of real and apparent power derating of three-phase transformers is important for transformers feeding nonlinear loads. This paper presents a new digital data-acquisition method for measuring derating and reactive power demand of three-phase transformers under full or partial load conditions. The accuracy requirements of the instruments employed (potential, current transformers, shunts, voltage dividers, optocoupiers volt- and current meters) are addressed. Application examples demonstrate the usefulness of this new digital data-acquisition method. Index Terms--Harmonics, nonsinusoidal operation, reactive power demand, real and apparent power derating, transformer losses.