Y represents star connection and admittance in the circuit.
Star connection is a wiring method for three-phase AC power and three-phase electrical appliances. Connect the ends, X, Y, and Z of the three windings of the three-phase power supply together to form a common point O, and draw three terminal lines from the starting ends A, B, and C. It is a system in which three sinusoidal power supplies with the same frequency, equal amplitude and 120° phase difference are connected in a certain way to supply power to the outside.
Admittance is the collective name for conductance and susceptance. In power electronics, admittance is defined as the reciprocal of impedance, symbol Y, and the unit is Siemens, referred to as West. Like impedance, admittance is a complex number consisting of a real part and an imaginary part.