Contents 1. Introduction
Every power system is subjected to transient disturbances primarily due to faults and/or switching of major loads. Normally the systems adapts to a new steady state condition with the help of power system control schemes such as generator excitation, turbine governor control systems and so on. For this reason, a synchronous generator is one of the most important elements in the power system and any faults in generators will hamper the available power to a system. Therefore, protecting these vital units against abnormal operating conditions and faults, while at the same time keeping protection schemes simple, reliable and fast in operation, has always posed a challenge to the power system protection engineer. Large system disturbances are typically caused by short-circuits of different types. The opening of appropriate high-speed breakers isolates the fault. During the fault, the terminal voltage dips and presents a non-stationary nature. In response, the exciter increases its output voltage to ceiling, which causes the excitation current into the field to increase at a rate determined, by the voltage divided by the inductance of the field [1]–[3]. Most of the research conducted in past years for the protection of the salient pole synchronous generators or turbo generators has focused on faults in the four major components of any synchronous generators, which are stator windings, field windings/dampers, and magnetic cores. However, stator-winding faults are diagnosed as the most frequent faults, which can cause severe damage in the generator itself, and consequently require a lot of time and high cost of maintenance [4]. The most widely used method for the protection of stator winding of the synchronous generator is the current differential technique. An artificial neural network (ANN) based digital differential protection scheme was developed in [5] to provide protection for generator stator windings. The computer simulation based digital differential relay of the preceding work is implemented in the laboratory environment in [6] for detecting and classifying internal faults in the stator winding of synchronous generators. The exciting current harmonics analysis method was used in references [7]–[8] to study the effect of internal faults, including short circuit between turns of the stator windings and rotor field coils. An additional harmonic current is induced in the rotor winding when the stator winding experiences inter-turn short circuit fault [8]. The spectra of two bands of frequencies of fault generated transient signals were compared with a predefined threshold in [9] for discriminating between internal and external faults in the stator of a generator unit connected directly to the distribution system.
