الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
Synchronous generators have been the main energy converters of mechanical energy in all commercial power systems since the early decades of the twelfth century. In a modern power system , synchronous generators perform the important stage of the aforementioned energy conversion process, following a long chain of primary transformation of thermal, nuclear, or hydro energy into mechanical. The last century has witnessed the vast expansions in electrical power systems all over the world with intense participation of all types of synchronous generators, driven by either high speed or low speed turbines. The electromechanical characteristics of synchronous generators are extensively studied and reported in electrical power and machines scientific researches.
In the last few decades, renewable energy resources are introduced into large and small electrical power systems, applying energy converters other than the conventional synchronous generators. However, the bulk electric power generation is still provided by the synchronous generators. More than 80% of all electric energy produced worldwide by synchronous generators, both in present time and the foreseen future.
Since the realization of the importance and reliability of the synchronous generators in the early in the nineteen hundreds by electrical engineers, these machines have been closely studied, regarding their electrical, electromechanical and electromagnetic characteristics. In addition to, their control systems, it is well known that synchronous generators have experienced a continuous process of modernization both in design and control. Due to recent studies and procedures in design, the power of a generator has been raised from a maximum of few mega watts in the early years of operation to more than one thousand mega watts in the early years of the twenty first century.
from a control point of view, synchronous generators have been to the target of basic and advanced developments of their control systems, mainly their automatic voltage regulators, speed governors, automatic load frequency controllers, power system stabilizers and advanced protection systems.
There are many studies about the natural oscillations of synchronous generators and their dynamics. In addition, they are concerned with the oscillatory nature of their response when connected in parallel in a power network. This oscillatory nature of the generator response stems from their operation theory in addition to their construction and design features. The strong magnetic fields established by the rotor windings, plus their magnetic coupling with rotating fields created by the stator windings produce huge mechanical torques. In addition, the damping effects of rotor structure and damping windings produce synchronizing forces getting into action at instants of disturbance, result in the oscillatory nature of synchronous generators to power impacts in power systems.
When a single generator or even several generators are connected to an infinite bus bar, a sudden disturbance of the operation regime may lead to a limited or large oscillations among these generators. The term ”infinite bus” means a power system with constant voltage and frequency supplying and receiving electrical powers of large magnitudes, incomparable with these of the coupled machine(s). A sudden disturbance means a fast variation of steady state conditions for either the loads or the generators. The frequency of the above mentioned electromechanical oscillations of these synchronous generators is usually of value ranging from 0.5 to 5 Hz. The frequencies of oscillations of the individual generators are selected by the generators themselves according to many factors such as their initial operating conditions. Natural frequencies are influenced by machines rotating masses. Disturbances, as previously mentioned, are caused by the sudden switching on, or off of an electrical load close to the generator or releasing a mechanical load coupled to the rotating masses. Such oscillations are usually referred to as natural frequency. Another type of disturbances which may lead to sustained oscillations to appear, is when the synchronous machines were coupled to reciprocating engines, whether as driving or loading mechanisms. If the band of the frequencies, associated with the mechanical parts of the system, contains the natural frequency of the synchronous generator-engine unit, large amplitude of oscillation will take place.
Mathematical formulation of natural frequency of the synchronous machines, during normal operation, both as generators and motors, experience three types of torques of electromagnetic origin , in addition to the mechanical one. Due to the constant speed characteristic of a synchronous machine, the same can be reported in reference to power. The types of power of the electro-magnetic origin are the electrical power delivered to the load or to the power network, the synchronizing power and the damping power generated by the induction machine principle at slight deflection of rotor speed from synchronous speed.
Under steady state operation, the mechanical power is balanced by the electrical power plus or minus the losses in the system. At transient conditions, which may be at a small deflection from the synchronous operation, the difference between the mechanical power and the electrical power, neglecting losses, operates as an accelerating power, or a decelerating one. This power, in turn, activates the two other types of synchronous machine powers, mainly the damping and synchronizing powers.
A laboratory set-up has been prepared to measure the natural frequency of a 5 KVA synchronous generator driven by a dc motor. The dc driving torque of the motor is distributed by injecting a low voltage but relatively high current source in series with the dc supply of motor. The frequency of oscillation is measured by an electromechanical clock mechanism. The natural frequency of a synchronous machine unit can be measured, in principle, by applying a sudden power impact to it and observing the resulting oscillations. In such a test, all the machine variables, such as, its terminal voltage, armature current, stator power, and the unit speed, will experience oscillations at the same natural frequency. One way to produce such a disturbance is to perform a synchronization process close, but not exact and correctly achieved to an infinite bus bar.
The machine will adhere to synchronization after some distinct oscillations, in armature current leading to some torque oscillations. The current oscillations are captured by a storage oscilloscope recorded by sensing voltage across .An alternative method of these measurements of the natural frequency is to disturb the steady state constant driving torque by a slow and limited-amplitude alternating mechanical torque. Varying the disturbing frequency but maintaining the disturbing torque amplitude at a small limited value not more than 5% of the driving torque will reveal the natural frequency as the frequency to which the rotating unit will respond most vigorously. This method of testing has the advantage of performing measurements under steady state conditions. However, it has the disadvantage of practicing the disturbance for a longer duration of time.
The oscillations in currents and voltages are easily measured in laboratory as well as the electrical parameters of the machines. However, the measurement of the moment of inertia of the rotating masses of a synchronous generator unit is complicated to some extent. A test for the measurements of the moment of inertia of the rotating masses is in fact, a way to determine the natural frequency.
Future Work:
Several tracks for the required future work. Some of them are mainly related to this thesis and the others are other aspects related to power system generation resources.
1- Nuclear Energy Technology
During future work will be about nuclear energy technology and science. Studying the reserve, cost and cycles of the nuclear fuel, processing, fission technology will be the main concern of this work. This have to include the optimum design of nuclear sources, the reactor physics, fluid dynamics, the analysis of the structure, the management of nuclear fuel and safety. Besides, this analysis will contain the physics of the neutron study with computer technology which will be associated with analysis of the thermal hydraulics of the nuclear reactor to have a simulation for the reactor behavior and dynamics. This has to come along with the integration of the power stations driven by nuclear reactors to the power grids. All of this work comes along with the increased importance of the nuclear energy generation nowadays.
2- Future Competence Power Systems:
There are a rapid increase in the electrical energy demand with a definite finite nature of the fossil power resources require a certain change in the excited PSs with the new integrations of nuclear energy sources with them. Centralized PSs, nowadays, with very large extended areas are going to change to de-centralized PSs consist of smaller generation powers stations forming many distributed PSs with lower demand density. Besides the dependable reliable structure of these distributed PSs, there is a certain need for a strong, sophisticated and optimized communication system links all of these systems and the associated control system to guarantee full control. Transmission systems and distribution systems have to be involved in the new structures to guarantee the solidness and robustness of all the elements affect power flow to the customer. Intelligent communication systems are already installed to the level of the transmission networks. They are going to be extended to distribution grids to assure stability. This is another interesting track to follow through the future work.