Universal Meter,Multi-Function Multimeter,Electronic Multimeter,Electronic Volt-Ohm-Ma Meter YINTE TOOLS (NINGBO) CO., LTD , https://www.yinte-tools.com
**I. Overview**
The chromatographic analysis method for dissolved gases in insulating oil is widely used both domestically and internationally, and it has proven to be highly effective in predicting latent failures in oil-filled electrical equipment. The "pre-regulation" provides clear guidelines for various types of oil-filled electrical equipment.
Gas generation in oil typically results from local overheating, corona discharge, or arcing. The primary gases produced include methane (CH₄), ethane (C₂H₆), ethylene (C₂H₄), acetylene (C₂H₂), propane (C₃H₈), propylene (C₃H₆), carbon monoxide (CO), and carbon dioxide (CO₂).
Table 2-3 outlines the gas production characteristics associated with different fault types. To aid in further fault identification, Table 2-4 lists the attention values for dissolved gases in the oil as per the pre-regulation.
Note:
1. Total hydrocarbons refer to the sum of methane, ethane, ethylene, and acetylene.
2. The attention value should not be used as the sole criterion for determining faults; it should be followed up and analyzed to identify the root cause.
3. Several factors can affect the nitrogen content in transformer and capacitor oil. If hydrogen levels are below the median but increasing rapidly, this should be noted. If only hydrogen is slightly above the median without a clear trend, the condition may still be considered normal.
4. Due to differences in structure and oil type among imported equipment, some domestic standards may not apply directly, and foreign standards may vary. Therefore, domestic standards should be used as a reference.
5. This guideline does not apply to gas samples taken from the gas relay bleeder.
6. The "Guidelines" mentioned in the table refer to DL/T 722-2000, titled “Guidelines for Analysis and Judgment of Dissolved Gases in Transformer Oil.â€
The pre-regulation also sets limits on gas production rates. For transformers, if the total hydrocarbon gas production rate exceeds 0.25 mL/h (open) or 0.25 mL/h (sealed), or if the relative gas production rate exceeds 10% per month, the equipment is considered abnormal. The gas production rate is calculated using the following formulas:
(1) Absolute gas production rate of total hydrocarbons:
$$ r_s = \frac{(C_{i2} - C_{i1})}{\Delta t} \times \frac{G}{P} $$
Where:
- $ r_s $: absolute gas production rate, mL/h
- $ C_{i2} $: concentration of a gas component in the second sample, *10â»â¶
- $ C_{i1} $: concentration of a gas component in the first sample, *10â»â¶
- $ \Delta t $: actual running time between two sampling intervals, h
- $ G $: total oil volume of the equipment, t
- $ P $: oil density, t/m³
(2) Relative gas production rate:
$$ r_s = \frac{(C_{i2} - C_{i1})}{C_{i1}} \times \frac{1}{\Delta t} \times 100\% $$
Where:
- $ \Delta t $: actual running time between two sampling intervals, months
**II. Fault Diagnosis Methods**
There are several methods for fault diagnosis. This section introduces the characteristic gas method and the three-ratio method.
**Characteristic Gas Method**
When one or more dissolved gas concentrations exceed the attention values listed in Table 2-4, Table 2-5 can be used to determine the nature of the fault.
**Three-Ratio Method**
As recommended in DL/T 722-2000, the three-ratio method uses three ratios of five characteristic gases to diagnose faults. This method is based on the relationship between gas component concentrations and temperature during oil and paper insulation cracking. It provides a more detailed classification than the characteristic gas method.
Table 2-6 presents the coding rules for this method, while Table 2-7 shows typical code combinations and their corresponding fault classifications. For combinations outside the table, such as "2.0.2," "1.2.1," and "1.2.2," additional testing is required. The "0.1.0" combination may indicate multiple causes, and further analysis is necessary.
When applying the three-ratio method, consider the following:
1. It should only be used when a fault is already suspected based on gas content and production rate.
2. Multiple faults may not correspond to known combinations, requiring further analysis.
3. For open-type transformers, hydrogen and methane may escape, so corrections are needed when calculating CHâ‚„/Hâ‚‚.
4. Gas analysis should be combined with other test results for accurate judgment.
5. Some ratio combinations may not be included in the table, as certain judgments are still under study.
**III. Comprehensive Judgment Method**
To analyze test results comprehensively, consider the following aspects:
1. First, eliminate external influences. Check if oil from the on-load tap changer has leaked into the main tank, or if the tank is properly sealed.
2. Use fault judgment methods (such as the characteristic gas method or three-ratio method) to determine the nature of the fault. Track and analyze multiple times for final confirmation.
3. Combine the identified fault type with other tests, such as DC resistance measurement, no-load testing, and partial discharge testing, to further confirm the location and nature of the fault.
4. If the fault is minor and the location is unclear, continue monitoring and adjust operations accordingly. If power outage is difficult, limit the load and plan for maintenance. If the fault is severe, stop the equipment immediately for repairs.
**IV. Brief Description of Gas Chromatography**
Gas chromatography is performed using a gas chromatograph. In addition to the column and detector, the system includes pneumatic, electrical, regulating, and temperature control components. Table 2-8 illustrates a typical chromatographic process, while Figure 2-20 shows the flow chart of the SP-5A gas chromatograph.
The column is a narrow stainless steel or glass tube filled with an adsorbent. The gas sample enters the column at one end and separates due to differences in adsorption properties. The separated gases exit the column and are detected by the evaluator.
Detectors include thermal conductivity cells and hydrogen flame ionization detectors. Thermal conductivity cells are simple and stable but less sensitive, while hydrogen flame detectors offer high sensitivity but do not detect inorganic gases like Nâ‚‚, Oâ‚‚, CO, and COâ‚‚. Both can be used together to meet various analytical needs.
Figure 2-20 shows a dual-gas system, with nitrogen as the carrier gas for hydrogen analysis and hydrogen for other gases. After purification, the carrier gas passes through the column and reaches the thermal conductivity cell. The difference in heat conduction between the reference and measuring cells allows for the detection of gas concentration, which is recorded as a chromatogram.
The height or area of a chromatographic peak indicates the concentration of a particular gas. Calibration with standard samples ensures accuracy. Retention time helps identify the type of gas present.
**V. Sampling and Injection**
1. Gas chromatographic analysis must be conducted using a 100 mL medical syringe with good air tightness, cleanliness, and dryness. Ensure no air bubbles remain after sampling.
2. Before sampling, purge any accumulated oil from dead spaces. Typically, 2–3 liters should be discharged. If the pipe is long, discharge at least twice its volume.
3. Use dedicated connecting pipes for sampling. Do not use rubber tubes welded with acetylene.
4. Keep the syringe core clean after sampling to avoid jamming.
5. Protect samples from light and deliver them promptly to ensure analysis within four days.
6. Transport oil samples carefully to avoid vibration, preventing low-solubility gases from diffusing.
July 03, 2025