Assessment and Mitigation Studies of Voltage Sags during Sympathetic Inrush Current

The system voltage may be negatively impacted by the transformer’s energisation which draws a large value of magnetising inrush current for a few seconds. During the transformer’s energisation process, it experiences an unexpected saturation. When the transformer is saturated, the growth of magnetising inrush current, sympathetic inrush current and voltage sags are a few issues that are appear. Numerous variables that affect the inrush current and the resulting voltage sags were taken into consideration. This is due to the fact that the energisation of a large power transformer may result in a high voltage sag which relies on a value of parameters, including the duration of the circuit breaker closing time. One of the most essential objectives for power quality is the research of voltage sags that appear in a network. The goal of this project is to investigate sympathetic and inrush currents as well as to assess the voltage sag that occurs during transformer energisation and how severe it is. In this paper, the voltage sags are analysed by modelling 100kVA, 11kV/415V of wye-delta of single line (circles) transformer and using PSCAD software. It is expected to observe the magnitude, the duration and the percentage of voltage sags in order to make a comparison result between single connected transformer, two parallel-connected transformers and three parallel-connected transformers. To predict the severity of the sympathetic inrush current, the effects of voltage sags and their assessment is developed. As a result, it has been shown that the more energisation of the transformers, the peak of inrush current increases causing the higher magnitude and the longer duration of voltage sags. In conclusion, the voltage sags also become severe and persists for a longer duration when the number of transformers in operation are increased.