The Development Of Portable Gas Chromatography Detection System

1. IntroductionElectric transformer is not only the most important and most expensiveequipment, but also one of the most accident-prone equipments in electric system.The running reliability of electric transformer can ensure directly the safe runningof electric system. Oil is usually used to insulate and emit heats in the large electrictransformer. Under the condition of running voltage, the transformer oil and thesolid organic insulated material (paper and cardboard etc.) in the oil willdecompose because of the multiplicate factors such as electrics, heats, oxidationand partial arc. It will be decomposed to produce a few low molecularhydrocarbons such as firedamp (CH4), ethane (C2H6), ethene (C2H4), ethane (C2H2)and carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2). Most of thegases will be dissolved in the oil. The potential overheated and dischargingelectricity malfunction in the transformer will quicken the speed of gas producing.The decomposed gases in the oil form air bubble and increase continually becauseof the convection and pervasion. In a certain extent, the component and content ofdissolved gases in transformer indicate the insulation aging and malfunction degreeof transformer. In order to ensure safety and credibility of the transformer, it isimportant to detect the component, content and speed of gas producing in the oil oftransformer, to find out the potential malfunction in the transformer. The portablegas chromatography detection system can find out the potential malfunction in thetransformer as soon as possible and forecast the developing trend of malfunction. Itis an important instrumentality to monitor the running of electric transformer andensure its safe and economical running.Dissolved Gas Analysis (DGA) is one of the mainly technology methods todiagnose the internal malfunction in transformer. By request of DL/T722-2000, , thecomponent of H2, C2H2, C2H4, C2H6, CH4, CO and CO2 is used to judge andanalyze the internal malfunction in transformer. Because the method of gaschromatography to analyze the dissolved gas component has the advantages suchas high separation efficiency, rapid analysis speed, high detecting sensitivity etc. Itis applied extensively in electric power department at home and abroad. Of lateyears, with the development of chromatography analysis technology, thedevelopment of the on-line monitoring equipment and transformer malfunctiondiagnosing method and the improvement of Gas Supply Pipe, the level oftransformer running maintenance and malfunction detection is improved greatly.2. Research ContentThe aim of this paper is to design the portable gas chromatography detectionsystem. The carrier gas carries the measured gas to inpour in the chromatographyand Flame Ionization Detector (FID). The system transforms each of theconcentration into analog signal pro direct ratio to concentration. After ADconversion, the signal is transmitted into computer to form chromatogram andmade a malfunction diagnosing. Under the control of computer, the system willrealize the flow control, the state control and detection, the data collection,chromatogram progressing, quantitative analysis, malfunction diagnosing etc. Thispaper realizes the detection system design of hydrogen concentration, the flamedetecting circuit design and the design of temperature control in hardware andchromatogram showing in the working station, quantitative analysis of gascomponent and the malfunction diagnosing of transformer.(1) The Design of Flame Signal Detecting UnitThe detector of gas chromatography is an equipment to measure the gascomponent and concentration. Its function is to transform the different componentsin the carrier gas to electric signal in different ways. According to the advantagesand disadvantages of all kinds of detectors and the using condition of the system,flame ionization detector (FID) is adopted. Because the ion stream collected by thedetector is slender and variational direct current, it should be amplified beforesending to the working station. Because the ion stream is very slender and theinternal resistance is very high, choosing appropriate amplifier is the key to designthe signal detecting circuit. The amplifier should have high sensitivity and highinput impedance. At present, the multilevel current amplifier with deep negativefeedback is adopted. The system adopts the ultralow input bias current operationalamplifier-AD549. Consideed the slender signal and biggish interference, thesystem takes some improvement measures in anti-interference.(2) The Design of Temperature Control UnitThe temperature is an important parameter of gas chromatography detectingsystem. It will influence the selectivity and separation efficiency ofchromatography and the sensitivity and stability of detector. The general gaschromatographic instrument has three parts of thermostatic control for invertfurnace, chromatography and detector. To realize the accurate thermostatic control,the system adopts platinum Pt-100 as the sensor and adopts PID control mode toadjust temperature. The system adopts temperature control unit based on MCS-5lsingle chip microcomputer. The A/D circuit detects the temperature signal andcalculates the error and control variable. The D/A circuit transforms the signal andis exported to executive committee to stabilitate the temperature on theinitialization. The system can realize the accurate thermostatic control of ±0.1℃.(3) The Design of Hydrogen Concentration Detecting UnitBecause the system takes hydrogen as carrier gas, the mixed gas should bedetected before being injected chromatography. The detection system of hydrogenshould be designed alone.By request of DL/T722-2000, < Guide to the analysis and the diagnosis ofgases dissolved in transformer oil >, the request for the detecting sensitivity ofhydrogen should be better than 10ppm(ml/L). Considering the structure of gas-pathsystem, the sensor of hydrogen concentration should be satisfied with the request ofhigh precision, small volume. H2/M-1000 produced by SwitzerlandMEMBRAPOR, the sensor of hydrogen concentration, is chosen. The sensor hasthe following character: the measuring range is 0～1000ppm, the resolution is1ppm. The sensor has small volume for the measurement of minim gas.(4) The Design of Pressure Detecting UnitThe detector needs hydrogen and air as carrier gas and combustion-supportinggas. The stability of gas pressure will influence directly the separating efficiencyand sensitivity. Considered the reducing of host machine volume and theimprovement of system configuration, the system adopts the vision mode in theworking station instead of traditional press gauge vision. The system adoptsMCY-W subminiature pressure sensor produced by Beijing Zhengkai Co. toacquire the detecting signal of gas pressure. (5) The Research of Quantitative Analysis Method The working station processes the collected data and forms chromatographyand realizes the quantitative analysis of component through normalization method,internal standard method, external standard method and standard addition methodetc. It provides theory foundation for the following malfunction diagnosing. (6) The Relation Research between Malfunction Type and the Dissolved Gas Generally speaking, insulation material decomposes different gases for thedifferent malfunction. Because of different degree, the amount of generated gas isdifferent for the same malfunction. According to the gas component and contentdissolved in the oil, we can judge the character and degree of malfunction. Thepaper introduces the common malfunction diagnosing methods such as charactergas method, three-ratio method, non-coding ratio method and so on. (7) The Design of Working Station for Data Processing The hardware of working station for data processing is constituted ofcomputer and its interface. The software of working station is a program with thefunction of data collection, chromatography peak detection, retention timemensuration, peak area calculation, quantitative analysis etc. the advantage iscreate a bulky data base for the user. The analysis chromatography, data, Gasproduction rate for every equipment is saved disciplinarily in data base. The usercan transfer the data to analyze and calculate at any moment. After analysis, thesystem will provide overflow alarm and malfunction conclusion automatically.3. Conclusion The paper realizes the collection of flame signal, Hydrogen ConcentrationDetecting, Pressure Detecting and the Multi-Channel temperature control in...