الفهرس 
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Abstract
Generator is the core of electrical power system; once it is defected the network cannot continue working properly. Many types of faults will be occurring in the power system and on the generator itself. So, there is a necessity to protect the generator from those faults to limit the possible damage. Generator protection has dual protection objectives. Generator faults may occur due to stator earth and phase faults, interturn faults, unbalanced stator currents, overheating, overvoltage, undervoltage, loss-of-excitation, overexcitation, overspeed, generator motoring, rotor earth faults and other abnormal conditions,
such as out-of-step.
The conventional protection schemes for protecting large synchronous generators have many drawbacks, faults that might take place near the neutral point of stator windings are not detected in case of high neutral grounding impedance for generator, besides inter-turn faults are not usually discovered. In addition, the advent of large power stations and highly interconnected power systems makes rapid fault isolation to maintain system stability.
The thesis presents a new proposed algorithm of multifunction digital relay for large synchronous generators protection. The protection scheme is based on measuring the ten input signals, which are 3-phase terminal voltages, 3-phase terminal-side currents, 3-phase neutral-side currents and neutral point voltage/or current. The main concept of the suggested technique is based on calculation of correlation and determination coefficients between any two variables. The coefficient of correlation has upper and lower limits to determine the strength of association for the two variables, its value can vary from positive one, through zero, to negative one. There are two famous functions of correlation theory, i.e. auto-correlation and cross-correlation.
Auto/cross-correlation coefficient is considered a proper method for determining any distortion, transient and difference in current or/and voltage quantities. Various types of protective-relaying functions are available, each of which is designed to recognize a particular difference and to operate in response to it. The protection functions included in the digital relay are differential, restricted earth fault, unbalance current, voltage dependent over-current, neutral overvoltage, over/undervoltage, over/underfrequency, out-of-step and loss-of- excitation.
Simulation studies have been performed on a typical power system model using ATP-EMTP program. The input file of ATP representing the studied power system is given in Appendix A
The proposed protection algorithm is operated by using MATLAB package and is examined under different operating conditions for various types of internal, external faults and other abnormal conditions that may occur in the simulated power system. The simulation results obtained indicate that the proposed algorithm has the ability to differentiate between internal and external faults within half cycle of the fundamental frequency cycle time and consequently takes the accurate decision by producing trip signal in case of internal faults and blocking signal in case of external faults. Thus, the algorithm is well suited for implementation in a digital generator protection scheme, in addition, it is able to detect internal faults on generator stator windings near the neutral point which is considered a drawback for conventional differential protection.
Performance of the proposed algorithm for protecting synchronous generators is investigated on a physical model of a power system. The physical model is able to emulate the behavior of the actual power plant in the laboratory environment. The multifunction digital relay has been implemented on a PC computer. Data acquisition card is used to feed the digital relay by voltage/current signals. The proposed algorithm has been examined experimentally under different operating conditions for various types of internal and external faults. The test results obtained from the experimental tests verify the results obtained from the simulation studies.
Two papers based on the research carried out in this thesis are accepted for a Journal of Engineering and Applied Science-Faculty of Engineering-Caire University. Details of these papers are given in EE-6/2012, EE-7/2012 and attached in Appendix B.