Performance of under excitation limiter of synchronous machines for system critical disturbances

This paper presents actual cases of machine undamped oscillations caused by unstable under excitation limiter (UEL), when the machine is forced to absorb an excessive amount of reactive power during system disturbances. A different and simpler UEL structure has shown not to be susceptible to this problem. An actual system situation is also shown, where the use of PSS based on electric power in machines electrically close to others making use of accelerating power PSS will result in inadequate interchange of reactive power between machines. This may lead to unnecessary machine tripping by the loss of excitation protection. The employment of the same type of PSS in electrically close machines may be required in cases where reactive power absorption is critical.