New algorithm for the loss of excitation function

Loss of field is one of the incidents that threats synchronous generators and power systems. After the excitation is lost, rotor current decays at a rate determined by the field circuit time constant. The internal generator voltage will decay at the same rate. Then the machine behaves as an asynchronous generator and draws a high quantity of reactive power from the network. The active power output will intensely oscillate and excessive current will traverse through stator windings. In this paper two protection methods are considered. The first one is based on conventional impedance method which consists of offset Mho relay for the fast detection of loss of field in synchronous generators. The second one is based on the measurement of the three phase reactive power output of the machine, and monitors the direction and magnitude of leading reactive power at the generator output terminal. It mainly aims to develop algorithms for the conventional impedance method and the reactive power method. The comparison of these two methods is also carried out to find out the method in which quick tripping results can be obtained. A two machine system is used in case studies and is modeled using MATLAB simulink.