Flux measurements with AC and DC components of current present show transformer equivalent circuit models need core joint details

Banks of three single-phase transformers are easier to transport than large three-phase transformers, and single-phase four-limb (1P-4L) transformers have recently been installed in a power station in South Africa. Predicting the response of this core structure is important for network studies of the effects of geo-magnetically induced and dc leakage currents. A topological, equivalent circuit model of 1P-4L transformers was developed and tests carried out on model transformers to determine the equivalent parameters. The experiments used scaled down 8.3 kVA, 1P-4L transformers, which resemble a power transformer in respect of high quality core material and winding configuration. FEM studies assisted with placing some search coils used to examine the core response. The results show the core joints have a significant effect in the presence of dc components. Neglecting the effects of core joints in deep saturation might have implications for existing transformer models.