Chromatographic analysis of dissolved gases in insulating oil

        Note 1. The total hydrocarbons are the sum of methane, ethylene, ethylene, and acetylene.

            2. The attention value is not the only criterion for dividing the equipment for faults. Instead, it should be followed up and analyzed to find out the cause.

            3. There are many factors affecting the nitrogen content in the current transformer and the capacitor casing oil. Some hydrogen content is lower than the middle value of the table. If the increase is faster, it should also attract attention; some only have the hydrogen containing cloud exceeding the table median value. If there is no obvious increase trend, it can be judged as normal.

            4. Due to the different structure and oil type of imported equipment, the ministerial standards are often not suitable, and the foreign standards are not the same. Therefore, the domestic standards can only be used for reference.

            5. This watch does not apply to gas samples taken from the gas relay bleeder.

            6. The “Guidelines” in the table refer to DL/T 722-2000 “Guidelines for Analysis and Judgment of Dissolved Gases in Transformer Oil”.

        (1) The absolute gas production rate of total hydrocarbons is

r _s =(C _i2 -C _i1 )/Δt(G/p)

        Where rs - absolute gas production rate; mL / h;

        C _i2 — the content of a gas component measured in the second sampling, *10 ^-6 ;

        C _i1 , the content of a gas component measured by the first sampling, *10 ^-6 ;

        Δt—the actual running time in the two sampling interval, h;

        G-one equipment total oil quantity, t;

        P—oil density, t/m3.

        (2) Relative gas production rate is

r _s =(Ci2 - Ci1) / cil(1/Δt) * 100%

        Δt——the actual running time in two sampling intervals, month.

        When one or more dissolved gas contents exceed the attention values ​​of Table 2-4, Table 2-5 can be used to judge the nature of the fault.

        2. Three ratio method

        In DL/T 722-2000 "Guidelines for Analysis and Judgment of Dissolved Gases in Transformer Oils", it is recommended to use three contrast values ​​of five characteristic gases as the main method for judging faults, called IEC three-ratio method.

        Table 2-6 is the coding rule of this method. It is based on the correlation between the relative concentration and temperature of the gas components generated by the cracking of the oil and paper insulation in the electrical equipment. The two gas groups with similar solubility and diffusion coefficient are similar. The ratio of points is used as the basis for judging the nature of the fault. It goes a step further than the characteristic gas method.

        Table 2-7 shows the typical coding combination and the three-ratio method for judging the nature of the fault. For the coding combinations outside the scope of this table, such as "2.0.2", "1.2.1" and "1. 2. 2", A comprehensive analysis should be performed in conjunction with the necessary electrical tests. It is generally believed that both superheat and discharge are present. For the "0.1.0" combination, there are many reasons for causing H: high. If it is difficult to judge, it should be judged in combination with other methods.

        When applying the three-ratio method, you should pay attention to:

        (1) This method is applied only when it is judged that there is a fault according to the content and gas production rate attention value. This method has no meaning for normal equipment.

        (2) The situation of joint action on multiple faults may not find the corresponding combination from the table. At this time, specific analysis should be made.

        (3) For open-type transformers with free breathing, hydrogen and methane escape from the oil surface, and correction should be made when calculating CH4/H2.

        (4) The gas analysis results should be combined with other test results to judge the accuracy of the judgment.

        (5) Actually, there may be a combination of ratios not included in Table 2-7, as some judgments are still under study.

Third, comprehensive judgment method

        For the test results, comprehensive analysis and judgment can be made from the following aspects:

        (1) The external influence should be excluded first. If the oil of the on-load tap changer may leak into the body tank; whether the fuel tank is welded or not.

        (2) Determine the nature of the fault by using a fault judgment method (such as a characteristic gas method, a three-ratio method, etc.). As far as possible, based on the method of tracking and analyzing multiple times, it is finally determined.

        (3) With the nature of the fault, it is also necessary to combine other inspection tests, such as measuring DC resistance, no-load test, partial discharge test, etc., to further determine the nature and location of the fault.

        (4) If the nature is not serious and the location cannot be determined, the follow-up analysis can be continued to strengthen supervision. If the power outage is difficult, you can first limit the load to arrange for recent processing. If the fault is serious, it should be stopped immediately for processing.

Fourth, a brief description of gas chromatography

        Gas chromatography analysis was carried out using a gas chromatograph. In the gas chromatograph, in addition to the two key components of the column and the evaluator, there are pneumatic systems, electrical systems and regulating measurement systems and temperature control systems. Table 2-8 shows a typical chromatographic process and Figure 2-20 shows a flow chart of the SP-5A gas chromatograph.

        Figure 2-20 shows a dual gas system, one with nitrogen as the carrier gas (for hydrogen analysis) and the other with hydrogen as the carrier gas (for the analysis of several gases other than hydrogen). The carrier gas is discharged from the high-pressure cylinder, and the water and solid liquid impurities are removed through a pressure reducing valve, a drying tube, and a purification tube, and then passed through the column to reach the thermal conductivity cell. The thermal conductivity cell has two cell cavities, one is the reference cell, and only the carrier gas does not pass through the sample gas; the other is the measuring cell, which passes through various gases to be measured. After the two cells are compared for heat conduction, the concentration of the gas to be measured is converted into an electrical signal, which is recorded by the recorder and arranged in a sequence of chromatograms (see Figure 2-21). The gas to be measured can be qualitatively analyzed from the chromatogram, and quantitative analysis can also be performed.

        The height (h) or area of ​​the chromatographic peak [S = 1.065h (b/2), b/2 is the full width at half maximum] indicates the concentration of a certain gas. This concentration can be checked by an external standard method. The chromatograph is injected with a standard gas sample of known concentration, and the relationship between the concentration and the peak height (or area) is obtained from the height (or area) of the peak, and the concentration of the gas to be measured is obtained by comparison. The tr in Figure 2-21 is called the retention time. The retention times of various gases are different. As long as there are the order of the peaks and the peak time, it can be determined which gas is the peak.