Key Technology For Quasi-distributed Temperature Measurement Of Stator Windings In Large Generator Using Fiber Bragg Grating

Operating temperatures of stator windings are closely related to safety and life of large generator.If the temperature distribution inside stator windings can be directly measured when large generator is running,it is very useful for monitoring the security of operation,regulating and controlling operation condition,and extracting the state maintenance information of generator.Due to high potential and strong electromagnetic interference inside generator,traditional electrical sensors can't be integrated in conductors and reliably report actual working status.FBG(fiber Bragg grating)can solve the problem of temperature measurement at high potential with its characteristics of electrical insulating and immunity to electromagnetic interference,and FBG is suitable for multi-point temperature measurement of large generator.In the dissertation,FBG-based quasi-distributed temperature sensors are applied to realize the direct measurement of conductors and main inslation where there is a latent electrical fault.Through numerical simulation and analysing framework and manufacturing process of stator bars,the positions to integrate FBG sensor array into stator bars were determined.FBG sensors should be embedded into solid strands so as to measure running temperature of conductior and evaluate the load of large generator.In order to detect abnormal temperature rise arised from partial dischare,FBG sensors should be integrated into main insulation or the interface between main insulation and iron core.The sensing characteristics of FBG are influenced by physical characteristics of insulation material,when it is embedded into main insulation of staro bars.Without considerating external stress put on FBG-epoxy resin system,the wavelength-temperature curve the system had been obtained by experimental research.The experimental results show that the temperature sensitivity of the bare FBG is increased,and there is a turning point near 70?.The dynamic mechanics analysis experiment demonstrates the turning point is the glass transition temperature of epoxy resin.By numerical simulation,the influence on the wavelength-temperature feaure of FBG-epoxy resin system was analyzed from varieties of modulus of epoxy resin with the temperature.The modulus of epoxy resin rapidly drops with temperature rising when temperature is higer than glass transition temperature of epoxy resin.It results in the obvious fall of wavelength-temperature sensitivity of FBG.The theoretical simulation results are in agreement with the experimental outcomes.Although the sensing characteristic of FBG is influenced by epoxy resin,it can be used to detect latent electrical fault.The process of coating grid region with polyimide was studied so as to implant FBG sensors into stator windings of large generators which satisfied highest insulation class.After being coated with polyimide,the temperature sensitivity of FBG sensor is slightly increased,and the mechanical strength of grating is improved.The monolithic encapsulation technology was also investigated to package FBG sensors and fiber with thin metal tubes.Experiments prove that the package method is feasible.After being packaged,the FBG sensors were successfully integrated into solid strands of stator bars.These FBG sensors have good linearity.The largest temperature deviation is 1.2?between FBGs and the thermocouples in the same locations,so FBG temperature sensors satisfy the precison requirements.FBG wavelength demodulation system based on tunable F-P(Fabry-Perot)filter had been developed.To reduce measurement errors resulted from the driving element of tunable F-P filter,which had the characters of temperature drift and nonlinear,the dynamic real-time correction technology was studied.Two kinds of real-time correction methods respectively with series and parallel structure were analyzed and compared.Then the real-time correction system with improved series structure was choosen.The reference grating module was designed to have good thermal stability,thus the wavelengths of four reference FBGs were held constant in a long time.An accurate RTD(resistance temperature detector)was integrated into the model and clung to the reference FBGs.Before measurement,the relationship between the wavelength of four reference FBGs and temperature measured by RTD had been calibrated.Through the temperature acquired by RTD,the accurate wavelengths of four FBGS could be gained.By reference points provided by referenced FBGs,the functional relation between transimission wavelength and driving voltage of tunable F-P filter was adjusted in real time during every scanning process.Thus measurement precision of the system was improved.The signal processing unit was designed based on FPGA(field programmable gate array).All digital circuits,such as A/D and D/A controlling unit,triangle wave driving signal generating unit,filter unit,calculation unit,cutting out amplitude and dividing peak unit were all integrated in the FPGA,so the real-time performance of the system was guaranteed.