Transformer dga test

Date: November 2, 2025 18:50:27

Transformer DGA detection (dissolved gas analysis), the core is through the analysis of transformer oil dissolved in the fault characteristics of the gas, to determine whether there is overheating, discharge and other latent faults within the equipment, is the transformer condition monitoring "core diagnostic means".

core principle

Transformer internal insulating oil and solid insulating materials, in overheating, discharge and other faults will decompose, producing hydrogen, methane, ethane, ethylene, acetylene and other gases. These gases are dissolved in the insulating oil, through the detection of gas components, content and change trend, it can be reversed to infer the type and severity of the fault.

Core Detected Gases and Corresponding Faults

  1. Hydrogen (H₂) + methane (CH₄): mainly corresponds to low-energy discharges or localized overheating (temperature <300°C).
  2. Ethane (C₂H₆) + Ethylene (C₂H₄): mainly corresponds to high superheat temperatures (300°C-700°C), the higher the ethylene content, the higher the temperature may be.
  3. Acetylene (C₂H₂): the core high-energy discharge characteristic gas, as long as a significant level is detected, there is a high probability of serious faults such as arc discharge.
  4. Total Hydrocarbons (sum of the above hydrocarbon gases): An abnormally high total indicates a developing malfunction and requires focused attention.

central role

  1. Fault early warning: When there is no abnormality in the appearance of the transformer and the electrical parameters are normal, internal latent faults are detected in advance to avoid sudden shutdown.
  2. Fault location: Determine whether the fault is overheating, discharge or insulation aging through the gas components, providing direction for maintenance.
  3. Condition assessment: tracking trends in gas levels, assessing the rate of fault development, and formulating a reasonable maintenance plan (e.g., emergency outage or planned maintenance).

Common Detection Methods

  1. Offline testing: regular sampling of transformer oil, sent to the laboratory with chromatograph analysis, low cost, high precision, but the detection cycle is long, suitable for routine inspection.
  2. Online detection: real-time collection of oil samples and analysis of gases through online chromatographic monitoring device, which can be remotely monitored in real time and is suitable for unattended substations or important transformers.

 

Dissolved Gas Analysis (DGA) in oil has a list of faulty gases corresponding to fault types. Combined with the international standard IEC 60599 and the domestic standard GB/T 7252, it systematically sorts out from gas characteristics, fault mechanisms to treatment suggestions:

I. Correspondence table of core fault types and characteristic gases

Fault type Main characteristic gases Secondary characteristic gases temperature range Typical Failure Scenarios
Partial discharge (low energy discharge) H₂ (hydrogen), CH₄ (methane) C₂H₆ (ethane), CO (carbon monoxide) ordinary temperatures Partial discharges triggered by air bubbles, impurities or burrs within the insulation are commonly caused by defective winding insulation or poor casing sealing.
Low temperature overheating (150-300°C) CH₄ (methane), C₂H₄ (ethylene) C₂H₆ (ethane) 150-300°C Minor winding overload, poor tap changer contact, localized eddy current losses in the core.
Medium temperature overheating (300-700°C) C₂H₄ (ethylene), CH₄ (methane) C₂H₆ (ethane) 300-700°C Overheating due to severe winding overload, cooling system failure, and core multi-point grounding.
High temperature overheating (>700°C) C₂H₄ (ethylene), CH₄ (methane) C₂H₂ (acetylene) (trace) ＞700℃ Overheating of metal parts caused by winding turn-to-turn short circuits, poorly soldered leads, and leakage of magnetic flux.
Arc discharge (high energy discharge) C₂H₂ (acetylene), H₂ (hydrogen) CH₄ (methane), C₂H₄ (ethylene) ＞1000℃ Violent discharges such as winding insulation breakdown, tap changer arcing, casing flashover, etc., may result in equipment explosion.
Insulation aging (solid insulation breakdown) CO (carbon monoxide), CO₂ (carbon dioxide) H₂ (hydrogen), CH₄ (methane) Long-term operating temperature rise Deteriorated, damp or oxidized insulating paper/board, commonly found in older transformers or chronically overloaded equipment.

II. Diagnostic value of key gases

  1. Acetylene (C₂H₂)
    • Core indicators: The only gas that clearly reflects a high energy discharge, and levels >5 μL/L require urgent attention.
    • typical scenario: Arc discharge, insulation breakdown, e.g. short-circuiting between turns of the winding or ablation of the on-load tap-changer contacts.
    • Recommendations for handling: Immediately shut down power for maintenance to avoid equipment damage.
  2. Hydrogen (H₂)
    • early warning: Characteristic gas of partial discharge or moisture, levels >100 μL/L need to be tracked for trends.
    • disruptive factor: It may rise briefly after oiling a new transformer (<1000 μL/L is normal), and should be judged in conjunction with historical data.
  3. Ethylene (C₂H₄)
    • temperature indication: The higher the content, the higher the superheating temperature. For example, if C₂H₄ is >70% of total hydrocarbons, the temperature may be >500 °C.
    • typical scenario: Winding overheating, core eddy current losses.
  4. Carbon monoxide (CO) and carbon dioxide (CO₂)
    • Insulation aging: Elevated CO/CO₂ ratio reflects solid insulation breakdown. For example, if CO > 1,000 μL/L and CO₂ > 10,000 μL/L, insulation paper deterioration may be present.
    • disruptive factor: The CO/CO₂ ratio may slowly increase due to oil oxidation during normal operation, and needs to be combined with a furfural content test to make a comprehensive judgment.

Three, three ratio method of fault diagnosis logic

The trigonometric method calculates C₂H₂/C₂H₄,CH₄/H₂,C₂H₄/C₂H₆ The ratio of the result is converted to a code combination, corresponding to the fault type as follows:
code combination Fault type characteristic
0 0 0 trouble-free All gas levels were within the noted values (e.g., C₂H₂ < 5 μL/L, total hydrocarbons < 150 μL/L).
0 0 1 Low temperature overheating (150-300°C) CH₄ and C₂H₄ were predominant and C₂H₂ was not detected.
0 2 2 High temperature overheating (>700°C) C₂H₄ accounted for >70% of total hydrocarbons, with trace amounts of C₂H₂.
1 0 1 partial discharge H₂ and CH₄ are dominant and CO may be elevated.
2 0 2 electric arc discharge C₂H₂ was significantly elevated (>5 μL/L) with concomitant increases in H₂ and C₂H₄.
2 1 2 Arc discharge + overheating C₂H₂, H₂, and C₂H₄ are all high, probably due to arc overheating triggered by insulation breakdown.

IV. Gas Content Noted Values and Treatment Recommendations

Gas type Attention value (μL/L) Recommendations for handling
Acetylene (C₂H₂) ＞5 Immediately shut down the power for maintenance and troubleshoot the arc discharge source.
Ethylene (C₂H₄) ＞100 Track the trend and analyze the cause of overheating (e.g., load, cooling system) if it continues to grow.
Hydrogen (H₂) ＞1000 Combine with partial discharge monitoring to troubleshoot moisture or insulation defects.
total hydrocarbon ＞150 Analyze the gas production rate, if the absolute gas production rate is >10 mL/day increased monitoring or power outage is required.
CO/CO₂ ratio >0.3 (insulation aging) Test for furfural content to assess solid insulation life and replace insulation paper if necessary.

V. Comprehensive diagnostic process

  1. make a preliminary judgment: Compare the gas content with the noted values and initiate further analysis if a gas exceeds the standard.
  2. Trend analysis: Calculate the rate of gas production (absolute/relative) to determine the rate of development of the fault. For example, an absolute gas production rate > 10 mL/day indicates an active fault.
  3. trinomial method (math.): Target fault types based on code combinations and eliminate interfering factors (e.g., equipment model, operating history).
  4. means of verification: Confirm the diagnosis by combining partial discharge monitoring, winding deformation testing, insulation resistance testing, etc.
  5. Recommendations for decision-making::
    • emergency treatment: If C₂H₂＞5 μL/L or the gas production rate surges abnormally, immediately shut down the power supply for maintenance.
    • scheduled maintenance: Arrange for preventive testing or hood inspection when total hydrocarbons continue to rise but do not reach the emergency threshold.
    • Condition monitoring: Periodic sampling and tracking to establish a DGA trend curve to predict equipment life.

VI. Statement of special circumstances

  1. Newly commissioned equipment::
    • Transient elevation of H₂ and CH₄ may occur after oil injection (<1000 μL/L is normal), and data stability should be observed after 3-6 months.
  2. on-load tap-changer::
    • The DGA data of the diverter switch oil compartment should be analyzed separately. C₂H₂ > 2 μL/L may indicate contact burnout.
  3. Non-mineral oil insulation::
    • The criteria for determining DGA for ester oils or silicone oils are different from those for mineral oils, and reference should be made to specialized guidelines (e.g. IEC 62770).

VII. Reference to standards and tools

  1. international standard: IEC 60599 Interpretative Guidelines for the Analysis of Dissolved Gases in Mineral Oil-Filled Electrical Equipment in Service.
  2. domestic standard: GB/T 7252 Guidelines for Analysis and Judgment of Dissolved Gases in Transformer Oil.
  3. analysis tool::
    • offline testing: Gas Chromatograph
    • Online monitoring: Photoacoustic Spectrometer
With the above list, fault types can be quickly located based on DGA data, and targeted maintenance strategies can be formulated in conjunction with gas production rate and trend analysis to minimize the risk of sudden transformer failure.