The active part of the transformer is created by mutual connecting of core and low voltage and medium voltage windings, see Fig. 1.
Fig. 1 The active part from the LV side of transformer
Fig. 2 The active part from the lateral side
Fig. 3 The active part from the MV side and bringing out the taping leads
Source: MRAJCA, Miroslav. Design of Oil Distribution Transformer. Brno, 2021. Also available from: https://www.vutbr.cz/en/studenti/zav-prace/detail/131597. Semestral thesis. Brno University of Technology, Faculty of Electrical Engineering and Communication Technologies, Department of Power Electrical Engineering and Electronics. Thesis supervisor Čestmír Ondrůšek.