While operating a diesel power plant in grid mode, if the grid voltage is much less that the expected what adverse impacts can it cause on generators?

عمليات توليد الطاقه بواسطه البخار عند طغوط مرتفعه وحرارة عالية

it wil icrease the capacitive lead to the generator tend to over heated to the generator winding /it couled be damaged ater time

When your DG set is parallel with Grid. And Grid Voltage is low, that means you need more excitation voltage.For that you need MRN2( Mains Decoupling Relay,with Under Voltage/Frequency,Over Volage/frequency) & MRG2( Vector Surge relay).If the vector angle between DG Voltage & Grid Voltage is more than7 Degree/more & your DG Set is in Outof-synchronism Condition but still connected with Grid.Dg rotor angle will increase,rotor will be heated/vibration occured,etc.You need to be Isolated if that type condition will occur.

Your operating Gen sets will be running in lower line voltage and in too much lagging power factor and if you will compensate on higher excitaton voltage, the more lagging power factor will come to your entire system and might affect your own station loads.

You should read about circulation current that will flow throw the machine..before its out of synchronism