1. For how long period the trend shall be kept in history to be displayed on graph?
2. How to increase the trend graph history to be displayed for long time? for example, currently I could able to see the trend graph for past 3 days, but I would like to see for 10 days. How to change the settings?
3. Also, the history files gets deleted every three days. What could be the reason?
This problem does sound frustrating; however, I may be able to help. (And if any genuine Citect gurus are reading this, feel free to comment, too.)
Firstly, I wonder what version of Vijeo Citect you're using? I can talk a fair bit about Citect 2015, so hopefully that's relevant.
This may help with your questions: I think that it all depends on trend tag setup, which you can read about on this page at Schneider-Electric.
I'm puzzled by how it is limiting you to 3 days of trend history; that isn't very long at all. If you examine your trend tag editor, the form should show you the sample period, number of files, and period (different from "sample period").
With a sample period of 30, it would sample every 30 seconds.
The number of files may be left at 2 (the default).
The period can be specified as HH:MM:SS, for 14 days, 336:00:00 should work. (There are other ways to specify period.)
So this setup would give you 4 weeks of trend history, with samples occurring every 30 seconds.
To get 3 days of trend history and the history file deleted every 3 days, I think you'd need "number of files" set to 1, and the period set to 72:00:00. This is rather short. (30 second trends at my workplace have been set up to go back for about 2 years.)
Anyway, I hope you find this helpful.