6RA24 - 24MW Generator excitation problem

Message

24MW hydro unit is tripping on excitation protection. In last one month 6 times it has tripped on trip by excitation from PT10 board. In general it is observed that filed voltage is reducing to zero and after one second machine is tripping. There is no fault message appearing on simoreg drive. Excitation Drive is thoroughly checked but nothing abnormal observed. So far we have replaced Rotor Over Voltage Card with spare card, MW Feedback Transducer and Excitation Voltage Feedback Transducer. PT10 card, power supply module, drive is thoroughly checked. Help is required for troubleshooting and guidance on replacement of PT10.