woodward EGCP_2 control panel

i have a problem as there is "reverse KVAR" alarm apears at display what can i do and what the reason of this case?

your generator wont's to be a motor

In other words the generators that are working correctly are putting power back into the generator that is in reverse KVAR's ( normally because this generators prime mover isn't supplying the right amount of grunt) as well as having to supply all the power for the plant as well as trying to drive the generator that is in reverse KVAR. Depending on surplus KW of the generation, you will end up overloading the correctly running gens and trip out the power station. Have you got plenty of torches and batteries?.

Kvar is controlled by the voltage that is supplied to the exciter

By raising or lowering that voltage you alter the power factor to more leading or lagging.

It would appear to me that one generator has a leading power factor,
whilst the other has a lagging power factor, and the fault relay is telling
you to correct this situation to stop risk of overloading

It can happen at startup, as the rpm is too high,
it can happen when motor is hunting up and down, it can happen when speed is too low of regulation, and a big load is switched from the net.
etc
and some faults in controller or governor.