The "solution" you obtain with the code from above is just a repetition of your equations. Use instead
clc
clear
V1 = 480 ;
V2 = 587 * ( 1 / 1.36 ) ;
V3 = 400 * ( 1 / 0.85 ) ;
Ld = 30e-6 ;
w = 2 * pi * 100e3 ;
syms phi_grid phi_ev real
P12 = ( V1 * V2 / ( w * Ld ) ) * phi_grid * ( 1 - ( phi_grid / pi ) ) ;
P13 = ( 8 * V1 * V3 / ( pi^2 * w * Ld ) ) * sin( phi_ev ) ;
P32 = ( 8 * V2 * V3 / ( pi^2 * w * Ld ) ) * sin( phi_grid - phi_ev ) ;
P1 = 4000 ;
P2 = -4000 ;
P3 = 0 ;
P2_eq = -P12 - P32 == P2 ;
P3_eq = -P13 + P32 == P3 ;
%phi_grid_eq1 = phi_grid < pi / 2 ;
%phi_grid_eq2 = phi_grid > 0 ;
%phi_ev_eq1 = phi_ev < pi / 2 ;
%phi_ev_eq2 = phi_ev > 0 ;
%Equations = [ P2_eq P3_eq phi_grid_eq1 phi_grid_eq2 phi_ev_eq1 phi_ev_eq2 ] ;
%Solution = solve( Equations, [ phi_grid phi_ev ],'ReturnConditions',1 )
Solution = solve([P2_eq,P3_eq])
