AVR problem in PRISMIC A30 Brush make AVR.

20MW Brush make generator is having Brush make PRISMIC A30 AVR. This AVR is having two chananels, one is main and the other is hot standby. Recently we faced a peculiar problem that the AVR switched to standby chanel from main. Suspected control card may be fauly and replaced the AVR module with a new one. The machine runs smoothly for 12 hrs in main mode and again switched to standby mode. Checked all the fuses and tightness of wiring. PMG supply value is also getting correct.The machine is running smoothly in standby chanel. What are the things to be checked more or else what may be the reason. wheather it is permissible to run the machine in standby mode.