Line Voltage Increases While Two Plant are Sync Condition

Do you have power factor correction in the plants? Capacitors can cause voltage rise.

Chemical plant also running with 3000 KVA and 2000 KVA PFI plants. pls see below mentioned parameters.

If I read the columns in post #2 right, kVAr relative to load is normal at synchronisation, but climbs out of normal over minutes.

Is your kVAr inductive or capacitative, we do not know your convention?

Are the Load and kVAr numbers the sum/difference of generator outputs or loads on the line?

It is not clear whether the two plants are 1 KM apart.Before synchronizing, both plant voltages are adjusted to be equal. But after synchronizing, may be the plant B has to make up for the line voltage drop and hence maintains higher voltage at the generator end. It is also possible that the loads shared( both MW and RMVA) are not proportional to the plant capacities. This rise in voltage can also happen because of the difference in AVR response.

More information is needed, a single-line diagram would be useful and would help in answering the following questions. Without engaging in "twenty questions" it sounds like you have two power stations and an overhead line between them, and loads which are tapped off of this line. Is this a new problem or one caused by recent changes? Is this system truly isolated or is there another connection to a local utility? Are there any transformers with or without Load Tap Changers (LTC)? What is the load and does it vary? Is there any mechanism for controlling the real and reactive power out of each generator bus?

It is unrealistic to expect four independent controllers (2 governors and 2 voltage regulators) to be able to control one point on a system unless they are coordinated. Over time the normal drift due to thermal effects, small variations in loading, etc., will cause one regulator to dominate. This has the effect of causing the other one to back-off; i.e., if A's VR sees a slight drop in voltage at its sensing point, it will cause A's voltage to rise. B's VR then sees this rise (from A) and then responds by lowering its voltage, before long A is trying to maintain the voltage by increasing its excitation and subsequent VAR output, and B is trying to maintain its voltage by lowering the excitation and absorbing VARS. In a small system such as this the drifting/hunting will eventually lead to the trip of the more heavily loaded machine.