This paper presents a new approach for Loss of Excitation (LOE) faults detection in

This paper presents a new advanced methodology as a solution for the problem due to the current setting of Relay (21) when it is set to provide thermal backup protection for the generator during two common system disturbances, namely a system fault and a sudden application of a large system load. Keywords: adaptive neuro fuzzy inference system, distance protection, phase backup protection, turbogenerator, dynamic performance, simulationCopyright © 2016 Institute of Advanced Engineering and Science. All rights reserved. IntroductionProtective relaying provides important defense for power grid operation. Conventional backup protection depends on local electrical information to make relevant decisions, and cannot securely distinguish an internal fault from heavy load during flow transfer [1]. This may cause cascading trip events and accelerate power system failure. In addition, the complex setting principle of conventional backup protection may induce hidden failures caused by setting mistakes [2], which would increase the risk of system instability during a disturbance [3,4].Moreover, some of the researchers have proved the existence of a potential problem due to the current setting of Relay (21) when it is set according to the present standards and recent related publications for generator thermal backup protection against transmission line uncleared faults, it is found that the current setting of the Relay (21) for generator thermal backup protection restricts the over excitation thermal capability of the generator [5]. Such a restriction does not allow the generator to supply its maximum reactive power during such events. Thus, the necessity of revising the Relay (21) reach to ensure a secure performance for the relay during major system disturbances was initiated, which would allow the generator to fulfill the system requirements during such events to ensure adequate level of voltage stability.The Relay (21) element is typically set at the smallest of the following three criteria [6]: 1. 120% of the longest line with in-feeds. 2. 50% to 67% of the generator load impedance (Z load ) at the rated power factor angle (RPFA) of the generator. This provides a 150% to 200% margin over generator full load.3. 80% to 90% of the generator load impedance at the maximum torque angle (MTA) of the relay setting (typically 85°) (Z GCC ).In particular situations when the generator distance phase backup protection relay is mainly required to provide thermal backup protection for the generator against transmission line faults, which are not cleared by transmission line relays, Relay (21) is set directly according to the second criterion (which is the setting considered during the investigations of this paper). The