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This letter presents a comprehensive model of on-load
tap-changer (OLTC) transformers that connect 3-wire medium voltage (MV) with
4-wire multigrounded low voltage (LV) networks. The proposed model enables the inclusion
of the 3-wire MV network and the 4-wire multigrounded LV network into a single
YBUS matrix without any assumption or simplification. Its distinct
feature is that the tap changer of the transformer is simulated outside the YBUS
matrix, thus a refactorization of the YBUS matrix is not required in
every tap change. The proposed transformer model has been validated in a 4-Bus
network, while its performance has been tested in the IEEE 8500-Node and IEEE
906-Bus test networks.