MEGAC and Fuel Gas Valve Control

in our plant we have a turbo-generator MITSUBISHI from JGC. in 2013, it has undertaken a major inspection (MI overhaul), but after that it doesn't operate well. we have done many inspection and more than 10 tests. the last tests it shows this parameters:

turbine signal CSO = 65%, normal operation 65%

combustion shell pressure = 4.5 bar, normal operation 5.5-6 bar

maximum speed 5900 tpm, normal operation 6000 tpm.

but the breaker closed and the frequency not reaches 50 HZ so we can't synchronize it. (we have another 2 supply lines, this are the main source of electrical power).

we have done another inspected for combustion chamber with a fuel gas valve, and change all injectors but the resultant is the same.

the control engineer decide that the problem is in MEGAC control, but unfortunately he don't know a lot about MEGAC. you might tell me where is the problem in MEGAC, instrumentations, or operation?

where is the VCE or FSR in this old control system MEGAC?
you could compare between MARK 02, MARK05/06 and MEGAC?

you might need this information:<pre>
TURBINE
nominal speed 6047 tpm
nominal output 6220 KW

GENERATOR
nominal speed 3000 tpm
nominal output 7775 KVA, 0.8 PF
5500 V, 50HZ ,816 AMP

GENERATOR 3 phases 2 poles.</pre>
KIND REGARDS