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**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 potential failures in oil-filled electrical equipment. The "pre-regulation" provides clear guidelines for various types of oil-filled electrical devices.
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 further fault identification, Table 2-4 lists the attention values for dissolved gases in the oil as specified in the pre-regulation.
Note:
1. Total hydrocarbons refer to the sum of methane, ethane, ethylene, and acetylene.
2. The attention value is not the sole criterion for determining faults; continuous monitoring and analysis are essential to identify the root cause.
3. Several factors can influence hydrogen content in current transformers and capacitor bushings. If the increase is rapid, even if below the median, it should be closely monitored. Some cases may only show elevated hydrogen levels without a significant trend, which can still be considered normal.
4. Due to differences in equipment structure and oil type, domestic standards may not always align with foreign ones, so they should be used as a reference.
5. This method does not apply to gas samples taken from the gas relay.
6. The "Guidelines" mentioned refer to DL/T 722-2000, "Guidelines for Analysis and Judgment of Dissolved Gases in Transformer Oil."
The pre-regulation also sets limits on gas production rates. For transformers, if the total hydrocarbon gas production rate exceeds 0.25 mL/h (for open systems) or 0.25 mL/h (for sealed systems), or if the relative gas production rate exceeds 10% per month, the device 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} $: gas component concentration in the second sample, *10â»â¶
- $ C_{i1} $: gas component concentration in the first sample, *10â»â¶
- $ \Delta t $: actual operating time between two sampling intervals, hours
- $ G $: total oil volume of the equipment, tons
- $ P $: oil density, tons/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 operating time between two sampling intervals, months
**II. Fault Diagnosis Methods**
There are several methods for diagnosing faults. 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 IEC three-ratio method uses three ratios of five characteristic gases to judge faults. This method is based on the correlation between the relative concentration and temperature of gases generated by the cracking of oil and paper insulation. It improves upon the characteristic gas method by considering the ratio of similar gas groups.
Table 2-6 outlines the coding rules for this method, while Table 2-7 shows typical code combinations and their corresponding fault judgments. For codes outside the table, such as "2.0.2," "1.2.1," or "1.2.2," additional testing and analysis are required. For the "0.1.0" combination, there are multiple possible causes, and further methods should be used for accurate judgment.
Key considerations when applying the three-ratio method:
1. This method is applicable only when a fault is suspected based on gas content and production rate.
2. Multiple faults may not correspond to any specific code in the table, requiring detailed analysis.
3. For open-type transformers, hydrogen and methane may escape, so CHâ‚„/Hâ‚‚ ratios should be corrected.
4. Gas analysis results should be combined with other tests for accuracy.
5. Some ratio combinations may not be included in the table, as some classifications are still under study.
**III. Comprehensive Judgment Method**
For test results, a comprehensive analysis should consider the following aspects:
1. First, eliminate external influences. Check if oil from the on-load tap changer might have leaked into the main tank, and whether the tank is properly sealed.
2. Use fault diagnosis methods (e.g., characteristic gas or three-ratio method) to determine the fault type. Track and analyze over time to reach a final conclusion.
3. Combine the fault type with other tests like DC resistance, no-load tests, and partial discharge tests to confirm the location and severity.
4. If the fault is minor and the location is unclear, continue monitoring and adjust operations accordingly. If serious, stop immediately for repairs.
**IV. Brief Description of Gas Chromatography**
Gas chromatography is performed using a gas chromatograph. The instrument includes key components such as the column, detector, pneumatic system, electrical system, and temperature control system. Table 2-8 illustrates a typical chromatographic process, and Figure 2-20 shows the flow chart of the SP-5A gas chromatograph.
The column is a stainless steel or glass tube filled with an adsorbent. The sample gas enters the column and separates due to differences in adsorption. Each gas exits the column at a different time, allowing for separation.
The detector identifies the separated gases. A thermal conductivity cell detects changes in thermal conductivity, while a flame ionization detector offers higher sensitivity but does not detect inorganic gases. Both detectors can be used together for broader applications.
Figure 2-20 shows a dual-gas system, with nitrogen and hydrogen as carrier gases for different analyses. The carrier gas is purified before entering the column and reaches the thermal conductivity cell. The cell compares heat conduction between the reference and measurement chambers, converting gas concentration into an electrical signal that is recorded as a chromatogram.
The height or area of a chromatographic peak indicates the concentration of a particular gas. This is determined using an external standard method, where a known concentration sample is injected, and the relationship between peak height/area and concentration is established.
Retention time is the time it takes for a gas to pass through the column and reach the detector. Different gases have distinct retention times, enabling qualitative and quantitative analysis.
**V. Sampling and Injection**
1. Sampling must be done using a 100 mL medical syringe with good air tightness and cleanliness, ensuring no air bubbles are present after sampling.
2. Before sampling, purge the oil from dead zones, usually 2–3 liters. For longer pipes, purge at least twice the volume.
3. Use dedicated sampling tubing; avoid rubber tubes welded with acetylene.
4. Keep the syringe plunger clean to prevent jamming.
5. Protect samples from light and deliver them promptly to ensure testing within four days.
6. Transport samples carefully to avoid vibration, which could cause low-solubility gases to diffuse.
July 03, 2025