How generator get reverse power when working in leading power factor?

kVAr is reactive power. That is, it is power generated but not used due to low power factor. Otherwise known as watt-less current.A generator can go into reverse power whether leading or lagging power factor. This occurs when drive to the generator is lost.If diesel driven, then loss of fuel to engine.As AC. generators are synchronous and when operating in parallel, then if one loses the prime mover, then the other generators will attempt to keep it running by supplying power to it. 

The generator with problems will then run as a motor, putting extra load onto the remaining generators, which could very well lead to losing all generators due to overload, as well as driving the prime mover, which may have tripped due to low lubricating oil and therefore destroy the engine.

A reverse power trip relay will then detect this change and trip the circuit breaker for the generator with a problem.If you draw a right-angle triangle as the power triangle,
Call the hypotenuse - kVAthe vertical perpendicular - kW the horizontal base - kVArand the angle between kVA and kW - angle 'a'then the power factor of the circuit is cos 'a'.

If you now make angle 'a' 0 then cos 0 = 1, or unity and kVA = kW,(a straight vertical line and no kVAr component).Now all that is generated is what is being used.(Neglecting other losses).
If you now make angle 'a' 90 then cos 90 = 0 and kVA = kVAr, (a straight horizontal line and no kW component).Now all that is generated is not being used, it is simply circulating between generators.

This is the reason why electricity suppliers impose penalties for consumers with low power factors.The supplier has to pay for the generated kVA, but the consumer is only paying for the kW used.