In order to reveal the internal characteristics of the Sen transformer（ST）

we propose an electromagnetic analytical model in which the multi-winding coupling in the ST with three-phase three-limb structure is taken into consideration. On the one hand

according to the electromagnetic coupling relationship of ST

the electromagnetic coupling model for ST composed of self-inductance coefficient and mutual inductance is derived from unified magnetic equivalent circuit. On the other hand

an electrical analytical model involving the internal voltage and current for the ST is deduced from the electrical connection relationship between the ST and the external equivalent system. Subsequently

an electromagnetic analytical model of ST considering multi-winding coupling can be acquired

which is composed of the electromagnetic coupling model and the electrical analytical model mentioned above. Finally

with the help of MATLAB software

by taking a three-phase three-limb model of ST as an example

the electromagnetic analytical results are compared with the existing results from time-domain simulation about the series compensating voltage of ST. While the most obvious changes are found in the amplitude and phase angle of individual winding voltage and branch current

the change in the amplitude does not exceed 6%

and the change in the angle does not exceed 7°. The results show that the multi-winding coupling effect has a slight effect on the analytical calculation results

but it should not be ignored. It also implies that the proposed model can more accurately reflect its internal electromagnetic characteristics through the asymmetry characterization of its magnetic circuit.