Before you replace the Y sustain you need to make sure the VS,VA ,Vscan and the low voltage(15V to 17V) provided to the Y sustain are normal.
If this is the case,then it means that the Y sustain is actually blown by something 'after' it ,loading it down.This can be one of the Y buffer boards.
You ca try to disconnect both buffer boards from the Y sustain(just leave them hanging onto th ribbon cables on the panel side).
Try and turn the tv on with these buffers disconnected.
If everything is fine,the tv will stay up nd it will run(with no picture ,of course,but it will not blow the y sustain again).
Now,you could try to connect the Y buffers ,one by one back into the circuit,but it is risky,because the defective Y buffer can blow the y sustain very fast!!
Samsung recommends in situations like this to replace all three parts at the same time:
Y sustain,upper buffer and lower buffer.
If ,after replacing all three,it still blows one or more from this 3 boards it is pretty sure that the plasma panel is shorted somewhere internally and it blows the buffers and the Y sustain over and over again.
At that point,the tv will be not economical to repair due to very high cot of the panel.
I hope this will bring you some closure,
Thank you and have very good New Year!