Study Of Wetness Measurement System For Wet Steam Based On Fourier Transformation

The quantity and distribution of steam’s wetness in steam turbines directly affect the economy and security of steam turbines’operation. So the on-line, real-time, accurate monitoring for the steam’s wetness has important meanings. In the paper,firstly it chooses the resonator’s perturbation method to measure wetness, which has characters containing continuation and without contact, after comparing lots of methods to measure wetness, and then analyze the relation theory between the wetness of steam in the resonator and the resonate frequency shift, the principle of resonator’s perturbation method to measure the wetness, the interference factors of resonator’s resonate frequency.Secondly, according to the need of speediness and easiness to be implemented, it designs the wetness measurement system for wet steam based on Fourier transformation. In this system, through studying the special problems of sinusoidal signal’s Fourier transformation, it finds the conclusion that the sampling rate should be an integral multiple(≥4) of the signal’s frequency, the quantity of sampling nodes should be some integral periods of sinusoidal signal.Thirdly, with the application of design-software FILTERCAD, MAX274and the chips of LTC1562, MAX274, it makes an new style resonator, which has a high precision, to simulate the resonator filled with wet steam. According the requirements for sinusoidal signal’s sampling, it complete the data collection using the NI data sampling card, and then with the application of LABVIEW and MATLAB to make the Fourier analysis and find the peak to calculate the wetness. For improving the accuracy of measuring, it analyzes the problems of leakage of frequency to provide two improved methods, the mutative sampling-rate and sampling-quantity method and the system function method. It chooses the latter one after the theoretical derivationand comparing the performance.Lastly, according the improved simulative system, under the available condition, it completes the wetness measurement real system for wet steam based on Fourier transformation. Due to the laboratory environmental limit, it completes the measurement with adjusting manually the resonator’s resonate frequency to set the sampled signal’s frequency in the data-sampling card’s ability. It proves that this measurement system has characters of real-time, easiness to be implemented and high accuracy. For the every ports of this system could be more modular to be used in Thermal Power Plant, it studys and chooses the designing method of A/D module, digital down converter module and DSP chip, to provide a direction for the next research work.