code of load flow equation four buses

clc
clear all
cg   =[0.86 0 0 0.6];
Pg = [0 0 0 318];
Pd = [50 170 200 80];
Qd = [30.9 105.35 123.94 49.58];
Y  = [9.934-1i*44.925, -3.815+1i*19.078, -5.169+1i*25.847, 0
    -3.815+1i*19.078, 8.984 - 1i*44.925, 0, -5.169+1i*25.847
    -5.169+1i*25.847, 0, 8.192 - 1i*40.965, -3.023 + 1i*15.118
    0, -5.169+1i*25.847, -3.023 + 1i*15.118, 8.192 - 1i*40.965];
Pg=sdpvar(1,4);
Qg = sdpvar(1,4);
objective= sum(cg.*Qg)
Qgmin=[-100 0 0 -100];
Qgmax =[100 0 0 100];
Pgmin =[0 0 0 0];
Pgmax =[318 0 0 318];
vang = sdpvar(1,4);
vmag = sdpvar(1,4);
vmin =0.9;
vmax =1.1
Constraints = [];

[Costraints,Pg(1,1)-Pd(1,1)==vmag(1,1)*[vmag(1,1)*Ymag(1,1)*cos(Yang(1,1))+...
    vmag(1,2)*Ymag(2,1)*cos(Yang(2,1) + vang(1,1) - vang(1,2))+...
    vmag(1,3)*Ymag(3,1)*cos(Yang(3,1) + vang(1,1) - vang(1,3))+...
    vmag(1,4)*Ymag(4,1)*cos(Yang(4,1) + vang(1,1) - vang(4))];
    [Costraints,Qg(1,1)-Qd(1,1)==(-vmag)(1,1)*[vmag(1,1)*Ymag(1,1)*sin(Yang(1,1))+...
    vmag(1,2)*Ymag(2,1)*sin(Yang(2,1) + vang(1,1) - vang(1,2))+...
    vmag(1,3)*Ymag(3,1)*sin(Yang(3,1) + vang(1,1) - vang(1,3))+...
    vmag(1,4)*Ymag(4,1)*sin(Yang(4,1) + vang(1,1) - vang(4))];
    
    
    % Constraints = [Constraints,sum(Pg)==sum(Pd)];
    % Constraints=[Constraints,Pg(i)-Pd(i)=P(i)]
    % Constraints=[Constraints,Qg(i)-Qd(i)=Q(i)]
    Constraints = [Constraints,0<=Pg<=318];
    Constraints = [Constraints,-100<=Qg<=100];
    Constraints = [Constraints, vmin <= vmag]; 
    Constraints = [Constraints, vmag <= vmax]; 
    options = sdpsettings('solver','gurobi');
    sol = optimize(Constraints,objective,options);
    value(objective)
    reactive_power=value(Qg)
    active_power=value(Pg)
    voltage_mag=value(vmag)
    voltage_ang=value(vang)

[Torsten]

YALMIP is not an official MATLAB tool. So it's not possible to test your code here and find possible errors.

What we can see is that you don't define Ymag and Yang and that you use "Costraints" instead of "Constraints" two times in the above code.

If you formulate your problem for "fmincon", e.g., maybe somebody will be able to help.