important to devise a measuring procedure where the (X - i) character- istics of the three limbs ... Abstract: Whenan electric arc is generated in transformer oil, a.
Abstract: Modeling of three-phase transformers with three-limb cores is important. Such units are very ubiquitous and are asymmetric due to the different lengths of the three limbs of the core. It is, therefore, important to devise a measuring procedure where the (X - i) characteristics of the three limbs and the zero-sequence (X0 -io) characteristic due to the tank can be determined. A simulation program is developed so that currents at no-load and symmetric or asymmetric loads can be computed. Measured currents are compared with computed results for 2.3 kVA and 45 kVA units. Keywords: Transformers with three limbs, nonlinear (k - i) characteristic for each limb, (X0 -io) characteristic for tank. Preprint Order Number: PE-609PRD (05-2002) Discussion Deadline: October 2002
Transfer Function Method To Diagnose Axial Displacement and Radial Deformation of Transformer Winding Rahimpour, E.; Christian, J.; Feser, K.; Mohseni, H. Author Affiliation: Stuttgart University; University of Tehran. Abstract: Short circuit currents or forces during transport can cause mechanical displacements of transformer windings. The transfer function method is presented as a tool to detect these displacements. In order to be able to evaluate the measurements, the correlation between the characteristics of transfer functions and possible damages must be known. Axial displacement and radial deformation of transformer windings have been studied-in this research work using two test transformers. The primary winding of the first transformer for axial displacement has 31 double disk coils (1.3 MVA, 10 kV) and the secondary winding is a four-layer winding. The second transformer for the study of radial deformation has 30 double disk coils (1.2 MVA, 10 kV) as primary winding and a one-layer winding as secondary winding. The detailed mathematical models were developed for the test objects and a comparison was carried out between measured and calculated results. It is shown that this model can present the behavior of the transformer windings in the frequency domain in the case of sound and displaced conditions. Keywords: Transfer function, modeling, transformer windings, mechanical displacements, diagnose. Preprint Order Number: PE-807PRD (05-2002) Discussion Deadline: October 2002
Estimating Overpressures in Pole-Type Distribution Transformers, Part I: Tank Withstand Evaluation Hamel, A.; Dastous, J.B.; Foata, M. Author Affiliation: Hydro-Quebec; Hydro-Quebec/IREQ. Abstract: When an electric arc is generated in transformer oil, a strong overpressure may rapidly develop in the tank, especially for low-impedance faults. This paper presents the test method and results obtained in a project aimed at assessing the human and environmental risks associated with this type of fault. Limit energy and pressure values were defined in terms of different safety levels. The best means of limiting the energy and pressure at a safety level in the transformer tank during intemal faults greater than 1.7 kA is to protect the transformer with a current-limiting fuse. Other means to increase the mechanical withstand of transformer tanks under such faults are also discussed. Keywords: Transformers, fault current, arc discharge, test pressure, safety, protection, fuse. Preprint Order Number: PE-178PRD (05-2002) Discussion Deadline: October 2002
Estimating Overpressures in Pole-Type Distribution Transformers, Part II: Prediction Tools Dastous, J.B.; Hamel, A.; Foata, M. Author Affiliation: Hydro-Quebec/IREQ; Hydro-Quebec. 70
Abstract: Low-impedance faults are difficult to contain from the point of view of cover retention and tank rupture in pole-type distribution transformers. Evaluation of a transformer's containment capabilities needs to consider two aspects: the overpressure caused by an arc of given parameters and the pressure withstand of the tank. This paper presents two methods to calculate overpressures as a function of the arc and all the relevant geometrical parameters: tank diameter, air space height, and the depth of the arc below the oil. Both methods have been validated with experiments. Results of tests to determine the pressure withstand of typical tanks are presented. It was found that an average value of the overpressure withstand capability is around 100 kPa. For a given tank geometry and fault parameters, a current-limiting fuse would be reqwred if the methods proposed here predict an overpressure over 100 kPa. Keywords: Arc discharges, explosions, finite difference methods, prediction methods, pressure effects, transformers. Preprint Order Number: PE-182PRD (05-2002) Discussion Deadline: October 2002
A Harmonic Model for the
Nonlinearities of Single-Phase Transformer with Describing Functions Huang, S.R.; Chung, S.C.; Chen, B.N.; Chen, YH. Author Affiliation: Feng Chia University. Abstract: In a transformer, the harmonic model is essential for the filter design and analysis of harmonic power flow. Among all the harmonic analyses of nonlinear devices, the core nonlinearity caused by the transformer hysteresis has received the most interest. Although many harmonic models have been proposed to interpret the nonlinearity of the transformer core, most of them require complex calculations. In this study, we derive the mathematical formulas of the exciting current for a single-phase transformer by the describing function methods. The mathematical expression allows us to obtain the Fourier series for the exciting current; and, then, it provides the various order harmonic magnitudes and phase angles of the exciting current. This work also presents a mathematical model characterized by simple calculation and with high precision for the harmonics of exciting current. The transformer is under no load condition. Actual measuring verifies the effectiveness of the proposed model. Keywords: Hysteresis loop, describing function, transformer, harmonics. Preprint Order Number: PE-274PRD (05-2002) Discussion Deadline: October 2002
Experience with Return Voltage Measurements for Assessing Insulation Conditions in Service Aged Transformers Saha, T.K.; Yao, Z.T. Author Affiliation: University of Queensland.
Abstract: This paper describes the investigation of the insulation condition of a number of power and distribution type transformers of different manufacturing dates and with different operating histories. Retum voltage measurements were conducted for these transformers. Effects of moisture and aging on the oilpaper insulation of these transformers were investigated, and results are presented. Moisture in oil samples at known temperature was measured for these transformers. Previously accelerated aging experiments were performed on paper-wrapped insulated conductors in different environments over a temperature range of 115 to 145 'C. Results from the measurements on transformers are compared with those of the accelerated aged samples and are described. Keywords: Power transformer, insulation aging, transformer insulation, polarization, dielectric measurement, insulation life, molecular weight, condition monitoring. Preprint Order Number: PE-282PRD (05-2002) Discussion Deadline: October 2002 IEEE Power Engineering Review, August 2002
