Differential protection issues for combined autotransformer - phase shifting transformer

In the Zerjavinec substation, a Croatian power company is going to install a special type of power transformer, which has two different modes of operation. The power transformer can be used either as a conventional 400 MVA, 400/220kV autotransformer with on-load tapchanger or as phase shifting transformer, interconnecting 400 kV and 220 kV networks. When the power transformer is deenergized, its operation mode can be changed. This change can be done even remotely, from the network control centre in order to maximize utilization of the electrical network infrastructure. Two different operating modes of the power transformer, as well as remote selection facility, complicate the selection of a power transformer protection scheme. Especially, the correct operation of the power transformer differential protection must be assured during all operating conditions. Necessary calculations for proper selection of transformer differential relay and a final solution for overall power transformer protection scheme are presented in this paper.