How to keep the magnitude in all time in a line ?

Hi

im trying to simulate a transmission line which is about 300 Km and has a 500kV 8500 MVA with a load 300 MW in one side and 500kV 9000 MVA in another side . all the time i simulate the current and voltage shape is not in a correct form i mean its seams that i face the swing problem in my simulation cause magnitude in both voltage and current is not in a straight line here are the picture of voltage and current :

Voltage :

Current :

P.S : what is the best block as a voltage source ??

Thanks