Effect Of Generator Frequency On Generator Efficiency And Voltage Output

It sounds as though it is slightly overloaded.

but does it has any impact on generator performance?

Of course. If it is slightly overloaded then it will not be able to keep up with demand!

imagine you carry out while running then speed drop....what could it be ::: :)

1. To keep same output voltage, rotor field current must increase ratio 50/49 since speed is reduced. Power factors other than unity will take more because e.m.f. > terminal voltage. Assumes generator has good voltage regulator.
2. Low system voltage usually occurs with low frequency [reducing voltage is a way to reduce system overload] but same power at lower voltage means more current & I²R losses in generator.
3. Lower speed means higher stator & rotor temperatures due to lower speed of shaft cooling fan. However, I think IEC34 for generators requires full output continuous at 49 Hz.
4. Higher temperature rises mean more electrical losses, reducing efficiency. However, fan and bearing losses will reduce with speed & magnetic core losses fall with frequency.
5. If the prime mover is a diesel or single shaft gas turbine its rating may be reduced or life penalty incurred. Free turbine, gas or steam, may be less affected.
6. Auxiliary fans & pumps will deliver less, contributing to effects.
7. In the long term, plant efficiency, life & maintenance will be adversely affected