Anyone who can help me to figure it out. why HP drum level swing too much and it is gone completely haywire reaches to a point of GT/HRSG protection trip (Low-low or sometimes high-high) during cold start up. Ours is Siemens 2 GTs and a ST.

 

You're going to need to provide some additional information for anybody to try to assist you.

Some SAMA's or logic diagrams
Trends of the event

is this occurring during single element control or 3 element control?

when was the last time the level transmitters were calibrated along with outlet steam flow and make up flow?

When the last time the control valve was calibrated, was it accurate and traveling smoothly? normally if the position vs demand is off, it wouldn't have that big of impact on the control unless it was linearized with flow.