The excitation field circuits of synchronous generators are typically isolated under normal operating conditions. The field winding is subjected to mechanical and thermal stress cycles due to the increase in rotation speed and temperature. In addition to the normal stress, the field winding can be exposed to abnormal mechanical or thermal stress due to overspeed, vibrations, excessive field currents, poor cooling or stator negative sequence currents. This may result in the breakdown of insulation between the field winding and the rotor core at the points where the stress has the highest value.
When the excitation system is isolated, a single ground fault in the field winding or its associated circuits, causes a negligible fault current, which does not lead to any immediate danger. However, if a second ground fault occurs, high fault currents and severe mechanical unbalances may quickly arise, leading to serious damage. In some cases, the field current, flowing through the rotor core, could generate enough heat to melt it.
It is essential, therefore, that the first insulation failure has to be detected, and the generator has to be removed from service to check the insulation health.
Jong Chol Min, a section head at the Faculty of Electrical Engineering, has proposed an on-line rotor ground fault location method for synchronous machines with static excitation, and conducted computer simulations to validate its effectiveness. In addition, he has proposed a new algorithm for estimating the ground fault resistance value in rotor windings in order to improve the accuracy of location.
This novel technique has two important advantages. First, it does not need any additional injection source to detect faults. Second, the new algorithm locates the fault position under normal operating conditions.