Study Of Real-time Analyzing For Large Generators Under Lightning Overvoltage

Large generator unit is the major equipment in larger power station, and also the heart of power system. The growing demand of electrical energy in modern society and the rapid development of power industry have resulted in a higher requisition for the safe generating operation of generator units. In the front of generators have some protective arrester, but the damage of generators made by lightning can't be ignored. Particularly, the problem about lightning surge of large generator in high voltage power line, which may lead to serious insulative accident finally. Simultaneously, if generator's insulation is damaged, it will cost very long time to repair or replace, which will bring about great influence. Therefore, the study of real-time analyzing for large generators under lightning overvoltage has very important practical meanings to safe generating operation of power systems.Based on the investigation of analysis methods for electric machines lightning overvoltage and its research status at home and abroad, this paper has made theoretical analysis on lightning overvoltage, researched an lightning overvoltage on-line monitoring method of large generator and presented the method which combines Fast Fourier Transformation (FFT) with multiconductor transmission line model to calculate voltage distribution in stator windings of large generator. For this subject, the main contents are described as follows: 1 Make the theoretical analysis and calculation on lightning overvoltage. Expound the generation, formation and parameters of lightning overvoltage.Respectively discuss lightning overvoltage of transmission line, electric substation and power station. Calculate lightning overvoltage of generator and the analyze insulation fault.2 Research a real-time lightning overvoltage monitoring method of generator, which used LiNbO3 as sensing element, thus it has many advantages, such as nice transient function, rapid respond, wide frequency, simple structure,good insulation, electric isolation, no electromagnetic field interference, and so on. This method can accurately get lightning overvoltage signal in ns grade, which ensures the accuracy of using computer model to calculate voltage distribution in windings.3 Build the model of multiconductor transmission line of generator windings and analyze generator windings. Give the generator single coih single phases three phase winding models and multiconductor transmission line model. Propose the method that DIT to deal with complex excitation. Then decompose lightning overvoltage signal to a number of harmonic waves and put these harmonic waves into the multiconductor transmission line model respectively to calculate voltages in the windings and then added these response of the harmonic waves in time domain, the real voltages of stator winding is obtained.4 Use MATLAB software to simulate all lightning wave lightning wave cut off in tail and lightning wave cut off in the head. Firstly, FFT decompose these waves into multiple harmonic waves; then according to the amplitude to take out of larger harmonic wave. For the generator with 540 slots of the Yangtze Gorges example, use the multiconductor transmission line model to calculate all wave lightning overvoltage in the stator windings and give the simulation result, which verified the accuracy, feasibility and advantage of this theory.This paper proposed the method of lightning overvoltage real-time monitoring of generators, which has combined the multiconductor transmission line theory and FFT. It has certain meanings in practices.