This paper presents an analysis of impacts of various non-linearities on communication delay margin for stability of Automatic Generation Control (AGC) of hydro power systems using open channel communication which introduces time delay in the AGC loop. This delay may lead to system instability. The delay margin depends not only on the secondary controller gains but also on the various non-linearities inherently presently in the AGC loop. To study their impact, these non-linearities are one by one introduced in the AGC of a hydro system and their impacts on critical delay (delay margin) is studied by simulating the system in MATLAB / Simulink by keeping the secondary controller gains constant at a random value to observe the effect only of communication delay and the non-linearities. The non-linearities considered are frequency of AGC execution and governor intentional dead band and governor backlash. © 2018 IEEE.