Trend Window History always reset
How can i solve this problem?
A Trend object in a FGR collects Data only as long as the FGR is active as tr10091969 already said.
You would need to configure a Trend Display with an archive function to keep a history. But that history can only be shown in that Trend display.
You can either use Trend acquisition function blocks running in the controller or use the Trendserver of Freelance to create Trend-Displays based on variables not collected by the controller. The latter can also be used for user defined trend displays (with a limited history storage) to be created by an operator.