Development Of On-line Gas Chromatography Detection System

1.IntroductionThe power transformer is not only the key devices and most expensive inpower system. but also one of the most accident-prone equipments in electricsystem .Techniques for on-line monitoring of power transformer play an importantrole in keeping the reliability and stability of electricity supply. The traditionaloff-line oil meteorologic chromatogram method is mainly applied to judge itsrunning status. This preventive method is not able to detect the fault nowadays, dueto its shortcomings such as complexity of processing,time costing and so on. It isan urgent problem that how to find out the fault status in transformer, how tocontrol the system effectively and manage the system safely, using the oilmeteorologic chromatogram analysis method.Combined with the strongpoint of the oil chromatogram analysis and theon-line monitoring method, the oil chromatogram on-line monitoring system forpower transformer is able to monitor the the dissolved gas in the oil and analyze itwithout stopping the devices. Being the its key componet, the information systemis able to control the oil chromatogram on-line monitoring system, display thechromatogram result in real time and process the oil chromatogram chart data.In a certain extent, the component and content of dissolved gases intransformer indicate the insulation aging and malfunction degree of transformer. Inorder to ensure safety and credibility of the transformer, it is important to detect thecomponent, content and speed of gas producing in the oil of transformer, to findout the potential malfunction in the transformer. The portable gas chromatographydetection system can find out the potential malfunction in the transformer as soonas possible and forecast the developing trend of malfunction. It is an importantinstrumentality to monitor the running of electric transformer and ensure its safeand 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 on-line 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 dataprocessing, the signal is transmitted into computer to form chromatogram bywireless transmission and made a malfunction diagnosing. Under the control ofcomputer, the system will realize the flow control, the state control and detection,the data collection, chromatogram progressing, quantitative analysis, malfunctiondiagnosing etc.This paper realizes the detection system design of hydrogen concentration,the flame detecting circuit design, the signal processing, the circuit of keyboard anddisplay, the module of wireless transmission and the design of temperature controlin hardware and chromatogram showing in the working station, quantitativeanalysis of gas component and the malfunction diagnosing of transformer.(1)The Design of Detecting Unit. The detector of gas chromatography is anequipment to measure the gas component and concentration. Its function is totransform the different components in the carrier gas to electric signal in differentways. Flame ionization detector (FID) is adopted by this system. Because the ionstream collected by the detector is slender and variational direct current, it shouldbe amplified before sending to the working station. Because the ion stream is veryslender and the internal resistance is very high, choosing appropriate amplifier isthe key to design the signal detecting circuit. Because the system takes hydrogen ascarrier gas, the mixed gas should be detected before being injected chromatography.The detection system of hydrogen should be designed alone. The detector needshydrogen and air as carrier gas and combustion-supporting gas. The stability of gaspressure will influence directly the separating efficiency and sensitivity.(2) The Design of Temperature Control Unit. The temperature is an importantparameter of gas chromatography detecting system. It will influence the selectivityand separation efficiency of chromatography and the sensitivity and stability ofdetector. The general gas chromatographic instrument has three parts ofthermostatic control for invert furnace, chromatography and detector. To realize theaccurate thermostatic control, the system adopts PID control mode to adjusttemperature. The system can realize the accurate thermostatic control of ±0.1℃(3)This part realizes the A/D conversion of all kinds of the signal that acquisitionand the storage model of dual-port .,that is prepared to chromatogram showing inthe working station and the further data processing. We still use the HD7279 toexpand the keyboard to 3×3, and use the LCM3202401 LCD screen as the model ofshowing that can make the parameter of transformer running be showed at thescene real-time.(4) The design of the model of wireless transmission. This system use MAX232connecting the single chip and the MC35 GPRS DTU .to accomplish the fuction ofwireless transmission. The GPRS DTU have a smart design and low level of powerwaste and have the communication port with data terminal. Its ZIP port providesthe port of control and data , and the wire of audio and power. The GPRS DTUcommunicate with the single chip through serial-port. The communication rate canbe up to 115kb/s. The speed can achieve real-time monitoring requirements.(5) The Relation Research between Malfunction Type and the Dissolved Gas .Insulation material decomposes different gases for the different malfunction.Because of different degree, the amount of generated gas is different for the samemalfunction. According to the gas component and content dissolved in the oil, wecan judge the character and degree of malfunction. The paper introduces thecommon malfunction diagnosing methods such as character gas method, three-ratiomethod, non-coding ratio method and so on.(6)The Functional Design of Working Station for Data Processing. The designof working station have the function of data collection, chromatography peakdetection, retention time mensuration, peak area calculation, quantitative analysisetc. After analysis, the system will provide overflow alarm and malfunctionconclusion automatically.3. ConclusionThe paper realizes the collection of flame signal, Hydrogen ConcentrationDetecting, Pressure Detecting ,Multi-Channel temperature control, the signalprocessing, the circuit of keyboard and display and the module of wirelesstransmission in hardware and data collection, chromatography peak detection,retention time mensuration, peak area calculation, quantitative analysis in software.The system realizes the preconcerted aim basically.