Finding the exact value at which instability would occur, requires more information. However, in general, this value of leading power factor is likely to cause under-excitation of the generator which will lead to instability and tripping. You would need to determine the cause of the leading power factor. Are there capacitors that can be switched out? Is it another cause such as long unloaded powerlines or cables - in which case inductive reactors would usually need to be added when load is low.

Dear Rohit,

It seems that the information you have provided about the overall system condition is incomplete. It would be helpful to have a more comprehensive overview of the system, such as whether the negative reactive power is occurring in one generator only or in all of them. Additionally, is your system operating in island mode, or is it connected to the grid?

Nevertheless, assuming the negative reactive power is limited to one generator, this is typically caused by either the excitation system being unable to supply the required reactive power due to a voltage drop when providing more excitation current, or by the system not responding adequately to variations in reactive power. This results in other generators compensating for the reactive power demand of the system. In such cases, the unequal sharing of reactive power places additional stress on the other generators, potentially leading to damage and adversely affecting the overall system power factor. This issue needs further investigation to prevent damage to other components in the system.

For the generator operating with negative reactive power, the primary risk lies in pole slipping, instability, and possible tripping, as the generator is functioning in an unstable region within its capability curve. Tripping the machine is undesirable and should be avoided to maintain system reliability. Beyond these risks, other concerns are minimal.