A Study on Inflammable Gas Generation according to Small Gap Discharge within Floating Electrodes by Induced Voltage in Power Transformer

摘要

With the increasing demand in power and reduction of transmission loss, electric power systems are being designed in high voltage and large capacity. Also with the competition among the power transformer manufacturers becoming fierce, complex internal structure design and insulation distance reduction for compact design of high voltage and large capacity transformer are the leading trends in the field. However, induced voltage and magnetic flux of conductors near the windings and leads have been increased due to such design features, leading to unexpected electromagnetic problems that occur occasionally. Especially, the conductor parts with a narrow gap below 1mm which are not connected to ground can become floating electrodes in a power transformer. A potential difference could occur in these two or more these floating electrodes due to induced voltage and magnetic flux from voltage and current source like windings and leads, and this potential difference between floating electrodes can cause discharge. A discharge between the floating electrodes with a narrow gap cannot lead dielectric breakdown because discharge point is far from the electric source such as windings or leads but maintain partial discharge or intermittent spark discharge. Therefore, it is difficult to observe the discharge phenomenon and measure the intensity of discharge from a narrow gap. Also, if the intensity of this micro discharge with a narrow gap is weak, the trace of discharge on electrode surface could be negligible and it is hard to find the position of discharge in large power transformer, but inflammable gas will be increasing slowly. In this study, the electrical characteristics of floating electrodes with a narrow gap were observed with the experimental electrode system designed to form a floating electrode in a narrow gap. In order to form a floating conductor in a narrow gap, several thin plate electrodes which are insulated each other with insulation papers were inserted in electrode system and this test jig was filled with insulating oil. Also, in order to analyse the electrical characteristics according to the magnitude of micro gap discharge, induced voltage between floating electrodes was controlled and the following occurrence of inflammable gas was verified through DGA (Dissolved Gas Analysis). As a result of this study, it was found that the induced voltage of the floating electrode under a certain magnitude caused partial discharge while the micro gap discharge which occurred at higher voltage level instantly caused an intermittent spark discharge between floating electrodes, producing inflammable gas.