What are the causes of High T5 in solar gas turbine engine?

Your question is a bit to vague to cover everything. You'd need to be specific about the running conditions. High T5 can be caused by overload of the unit as well as a variety of fault conditions. If the unit is running in T5 control mode (ie: full load) and experiences alarms or trips, it may be that T5 gain and integral values need to be adjusted to smooth out the over-shoot. Make sure that the fuel supply pressure to the unit is stable as the fuel system will have to counteract any changes in flow due to oscillations in supply pressure. This can introduce instability in the loop leading to high T5. If the unit is running below T5 in speed control (NGP) and suddenly trips on a very fast rise in T5, it's worth checking the speed probe and connections all the way back to the card. If there's a downward spike in the sensed speed due to loose connection or poor probe gap adjustment, the control system will react by putting in extra fuel to try to get back to the set-point, causing high T5 trip. This will very often be too fast to pick up on HMI or trigger log. There can also be problems with fuel valves that can cause poor temperature stability. Injector fouling is another possibility if your fuel quality is poor. Have a look at the T5 spread to see if they are all reading close to the same value. In very hot climates where the air inlet temperature go very high in the Summer months, you will hit the T5 control mode limit earlier than when the T1 air inlet temperatures are lower.

Hope that this helps.