Standards and Testing Cycles for Dissolved Gas Analysis in Transformer Oil Using Gas Chromatography

Gas chromatography for dissolved gas analysis (DGA) in transformer oil is an essential diagnostic tool for evaluating the health and reliability of transformers. This article provides a detailed guide to the testing cycle and standards for performing gas chromatography on transformer oil, aimed at helping new engineers and procurement professionals understand its importance and execution.

Importance of Dissolved Gas Analysis (DGA)

DGA measures the concentrations of gases dissolved in transformer oil, which are generated as a byproduct of thermal and electrical stress inside transformers. By analyzing these gases, engineers can:

• Detect early faults such as overheating, partial discharges, or arcing.
• Monitor operational conditions to ensure transformer reliability.
• Plan maintenance to prevent catastrophic failures.

Key gases monitored include hydrogen (H2), methane (CH4), acetylene (C2H2), ethylene (C2H4), and carbon monoxide (CO).

Testing Cycle for Dissolved Gas Analysis

The frequency of DGA testing depends on the voltage rating, capacity, and operational conditions of the transformer. Below are the recommended cycles for various transformer types:

1. 220 kV Transformers and Above 120 MVA Transformers:

   • Test every 3-6 months during normal operation.
   • Additional tests on the 4th, 10th, and 30th days after installation, overhaul, or winding replacement.

2. 110 kV Transformers:

   • Test once every 30 days and 180 days after installation, overhaul, or winding replacement.
   • Subsequently, test annually.

3. 35 kV Transformers:

   • Transformers rated 8 MVA and above: Test annually.
   • Transformers rated below 8 MVA: Test annually.

4. Ad-hoc Testing:

   • Perform tests when abnormal operational conditions arise, such as unexplained temperature rises, load imbalances, or oil discoloration.

Test Standards for Dissolved Gas Content

To ensure transformers operate safely and reliably, the following standards must be adhered to when analyzing dissolved gas content in transformer oil:

1. Newly Installed Transformers:

   • Total Hydrocarbon Gases (THG): ≤ 20 μL/L
   • Hydrogen (H2): ≤ 30 μL/L
   • Acetylene (C2H2): Not detectable.

2. Transformers After Major Maintenance:

   • Total Hydrocarbon Gases (THG): ≤ 50 μL/L
   • Hydrogen (H2): ≤ 50 μL/L
   • Acetylene (C2H2): Trace amounts allowed.

3. 110 kV and Above Transformers:

   • If C2H2 is detected, the testing frequency should be increased to track trends and monitor the equipment's condition closely.

4. Operating Transformers (Alert Thresholds):

   • Total Hydrocarbon Gases (THG): ≥ 150 μL/L
   • Hydrogen (H2): ≥ 150 μL/L
   • Acetylene (C2H2): ≥ 5.0 μL/L

5. Gas Production Rate:

   • If the relative gas production rate exceeds 10% per month, it is an indication of abnormal operation. This applies to both open-type and sealed transformers.

Key Principles of Gas Chromatography for DGA

Gas chromatography separates and quantifies individual gases dissolved in transformer oil. Here are the steps and principles involved:

1. Oil Sample Extraction:
   • Oil samples are collected from the transformer under controlled conditions to prevent contamination or gas loss.
2. Gas Extraction:
   • Dissolved gases are extracted from the oil using vacuum or gas stripping methods.
3. Chromatographic Analysis:
   • Gases are passed through a column where they separate based on molecular size and polarity.
   • A detector measures the concentration of each gas.
4. Data Interpretation:
   • Compare the measured gas concentrations to the standard thresholds to determine the condition of the transformer.

Applications and Benefits

1. Fault Diagnosis:
   • Identifies specific fault types (e.g., thermal faults, electrical discharges) by analyzing the gas composition.
2. Preventive Maintenance:
   • Allows early detection of issues, reducing the likelihood of unplanned outages or expensive repairs.
3. Regulatory Compliance:
   • Ensures transformers meet operational and safety standards.
4. Asset Management:
   • Extends transformer life by identifying and addressing faults before they escalate.

Conclusion

Gas chromatography-based dissolved gas analysis is an indispensable practice for maintaining the health and safety of transformers. By adhering to the recommended testing cycles and standards, engineers and procurement professionals can ensure early fault detection, minimize downtime, and enhance the reliability of power systems.

For new engineers and buyers, understanding these procedures provides a foundation for making informed decisions about transformer maintenance and diagnostics.