You must check to see if the phases wires to either or both of these are wired correctly to the instrument transformers (both phase wires are the correct phases and there is no phase shift created by the connections to the transformers).
There is a very high possibility that there exists a 30 degree difference between the 2 phase wires (one from each generator) when the instruments (auto sync or lights are indicating the generators are in the acceptable range to close the circuit breaker of the oncoming generator).
This can be checked (VERY CAREFULLY) by connecting the high voltage probes directly from the phase you are monitoring on each generator (assume phase A on each generator) to a high voltage voltmeter. Then you can watch the synch lights (or auto synchronizer) and the voltmeter simultaneously. Obviously, the voltmeter should read zero volts when the lights are completely dark. If this is not the case, there is a problem in the connections of the instrument transformers..
PLEASE NOTE THE PROPER AND CORRECT WAY TO CHECK THIS IS TO HAVE THE SWITCHGEAR VENDOR'S TECHNICAL PERSONNEL COME AND VERIFY THE WIRING. DO NOT ATTEMPT THIS UNLESS YOU ARE QUALIFIED TO DO THIS. YOU CAN BE KILLED.
Edit
In case one of the responders question was for me, I KNOW what I am talking about. Read what I said carefully. I had this happen to me many years ago when I was checking out a generator building with turbine driven generators (the aircraft turbine type).
The switchgear vendor's chief technician was there. When I attempted to parallel a generator to the switchgear voltage with only one other generator running there was a loud thump and vibration.
However with assurance the switchgear had been thoroughly checked out over the weekend, I attempted to parallel another generator with 2 other generators online (to my sorrow), it turned out the instrument transformers WERE wired incorrectly and when I closed the oncoming generator's circuit breaker, the oncoming generator was jerked with such force that the quill shaft in the turbine drive gear was broken. This caused me a week's delay.
Turned out that although the sync lights and auto sync unit indicated the speed and frequencies were within an acceptable range, in reality the oncoming generator wa 30 degrees out of phase with the 2 already on line.
TexMav
What are you talking? You can not connect two generators in parallel. They have to be in the same phase before you connect them and their speed should be controlled so that they will remain phase locked all the time.
Constructive interference.
Not sure.
