Hydrogen gas generation due to moderately overheated transformer cores
This paper first presents an overview of this phenomenon from its initial discovery to the stage of the factory/laboratory investigations performed to confirm this new mechanism of gas generat ion in power transformers. This is followed by an overview of the increased significance of developing accurate calculation of the core hot spot temperature. The paper then presents the detailed accurate diagnosis performed on a 600 MVA transformer that had this issue in the field, the improvement made, and the calculation of the predicted gas generation performance of its loading cycle throughout a year of operation. The paper then presents the basis for the additions/changes proposed to be implemented in the IEEE Standards as a result of the discovery of this new gas generation mechanism. The paper also provides the basis for recommending to the IEEE Standards the selection of 130°C as the allowed maximum core hot spot temperature under sustained worst conditions of load, core — excitation and ambient temperature.
Production mechanism of single excessive hydrogen in current transformers: A reactive molecular dynamics simulation study
Abstract The present work aims at investigating the single excessive hydrogen (SEH) mechanism in the current transformers (CTs) by reactive force field molecular dynamics (ReaxFF MD) simulations. The chemical components in the CTs, i.e. mineral insulating oil, insulating paper, nickel crystal and moisture were established. Temporal evolutions of dissolved-gas-analysis (DGA) characteristic gases were captured reasonably in the mono-component, dual-component and multi-component simulation systems under electrical stress. And the sources of H2 molecules were traced as well as their dehydrogenation reactions. In addition, the oil chromatogram and dismantle experiments were performed to verify the dynamics simulations. The relative contents of the gas productions in the all-component composite simulation system were well agreed to that from oil chromatogram measurement. The results identified that the nickel-catalyse dehydrogenation from insulating oil played the dominant role in SEH phenomenon. The Nickel promoted the generation of H2 by seizing H atoms from oil molecules under the imposed electric field. In the process of dehydrogenation, the insulating oil molecules reacted to produce macromolecule wax-like solid unsaturated hydrocarbons, which were found in the insulating paper of a dismantled CT.
Magnetic flux distributions in transformer core joints
This paper presents results of a rigorous analytical study performed in order to understand the role which joints play in determining the performance of cores of power and distribution transformers. The paper presents the details of the magnetic flux distribution and redistribution in the core joint region along the core stack height direction as a function of the operating flux density, core joint material, joint configuration, and joint design parameters. The results are presented for both conventional and step-lap joints. The results illustrate the benefits of the step-lap joint in reducing core losses, noise and excitation current. The results also explain the impact of overlap length, the number of laminations per step, and the contribution of the joint region to the whole core performance. The calculated flux density wave shapes in the joint region were found to be in good agreement with measured wave shapes. The results presented in this paper have been used in developing an accurate calculation of core losses for both power and distribution transformers. They have also been used to arrive at the optimum core joint configuration for a transformer core.