A Novel Loss-of-Excitation Protection Scheme based on Load Angle Variation of Parallel Synchronous Generators

The main weaknesses of the impedance-based loss of excitation (LOE) relay are its mal-operation during the power system disturbances (PSD) and its long detection time during LOE events. In this paper, to rectify the mentioned problems, a new LOE protection scheme is presented based on the load angle ($\delta$) variation of the parallel synchronous generators (SGs) connected to a point of connection (POC). It will be shown that during a PSD, the variation of $\delta$ for all parallel SGs is similar such that their $\delta$ increases or decreases simultaneously. However, it will be shown that when the LOE happens in one of the parallel SGs, the $\delta$ of faulty SG increases while other SGs will experience reduction in their $\delta$. Hence, when more than one SG is connected to a POC, the LOE condition can be discriminated from PSDs by the proposed LOE relay. However, when one SG is connected to the POC, the conventional impedance-based relay is employed to detect LOE/PSD. Since in real condition, the outage rate of the parallel connected SGs in a power plant is very low on a yearly basis, therefore, the contribution of the conventional impedance relay in detection of LOE/PSD is quite limited to some special instances. Simulation studies on a typical power prove high efficiency of the proposed algorithm.