Investigation of long-time stability of electrical contacts

Abstract

In today’s world, it is an undeniable fact that surplus amounts of power are being utilized for almost every domestic and industrial need. Load variation is one thing that is quite common in our scenario, wherein there is a sudden change in the required amount of power supply. To meet with such load variations, tap-changers are used in transformers which help in the regulation of the output voltage to the required levels. There are different kinds of tap-changers used depending on ratings as well as applications. These tap-changers operate by selecting different taps in a regulation winding to produce different output voltages. So they have a lot of fixed and moving contacts in them and these are the most important parts of a tap-changer that help keep them in operation. This thesis aims to bring about a design guideline which focuses on the various parameters that affect the contacts and also the most important ones to be considered while designing a contact. An accelerated ageing test called the Hopkinson test was conducted on a tap-changer to evaluate its operation for the entire life time in the field. The wear of the contacts in the test was noted and the resistance was measured by passing a DC current through them. A list of all the possible parameters that could affect the contacts was put up and interdependency between them was found. Visual aids were formed taking three parameters at a time. COMSOL simulations were also done to find the current distribution in the contacts. Simulations were done at different rated currents and with various contact resistance values and their corresponding temperature and current density plots are shown in the report. It was determined that it was a really complicated relation to bring about between the parameters and display it as a single dimensional table.