The set up is a mini hydro with two units rated at 500kW generating at 400V which is stepped up to 33kV. Then one outgoing 33kV line comes out from the station. Once you try to synchronyse using unit 2 the unite voltage keeps rising. When you try to stabilise it by de-excitation, it does respond. but immediately you stop pressing the de-excitation button, the voltage begins to rise again. what could be the problem? When you try synchronising using unit 1 while unit two is working they is no problem.