Dissolved Gas Analysis

Dissolved Gas Analysis of Transformer Oil

ASTM D3612: Standard Test Method for Analysis of Gases Dissolved in Electrical Insulating Oil by Gas Chromatography

Dissolved Gas Analysis (DGA) is a critical diagnostic tool used to assess the health and integrity of oil-filled transformers. This method involves sampling the transformer oil and analyzing the gases dissolved within it. The presence and concentration of specific gases can indicate various types of faults and deterioration within the transformer.

Transformers are vital components in electrical power systems, and their failure can lead to significant disruptions and costly repairs. DGA helps in early detection of potential issues, allowing for timely maintenance and preventing catastrophic failures. By monitoring the types and levels of gases dissolved in the oil, operators can gain insights into the internal conditions of the transformer.

The DGA process includes:

Oil Sampling: A sample of the transformer oil is drawn from the unit. This sample must be handled carefully to avoid contamination.
Gas Extraction: The gases dissolved in the oil are extracted using specialized equipment. This step is crucial as it ensures that the gas analysis is accurate.
Gas Chromatography: The extracted gases are analyzed using gas chromatography (GC). This technique separates the gases and measures their concentrations. The key gases typically analyzed include hydrogen (H₂), methane (CH₄), ethane (C₂H₆), ethylene (C₂H₄), acetylene (C₂H₂), carbon monoxide (CO), and carbon dioxide (CO₂).

The DGA helps on identifying the potential issues before they become severe, preventing expensive repairs and downtime by enabling proactive maintenance, extending transformer life by Regular monitoring and maintenance of the transformers, and reducing the risk of transformer failures, which can be hazardous and lead to power outages

The frequency of performing Dissolved Gas Analysis (DGA) on transformer oil can vary based on several factors, including the transformer’s age, load conditions, and operating environment. However, here are some general guidelines:

New Transformers: For new transformers, it’s recommended to perform DGA annually during the initial years of operation to establish a baseline for future comparisons.
Older Transformers: For transformers that are older or have shown signs of deterioration, DGA should be performed more frequently, typically every 6 months.
Critical Transformers: For transformers that are critical to the operation of the power system, such as those in key substations, DGA might be performed quarterly or even monthly.
After Faults or Repairs: If a transformer has experienced a fault or undergone significant repairs, a DGA should be conducted immediately after the event and then monitored closely for a period of time.
Routine Maintenance: As part of routine maintenance, DGA is often included in the comprehensive testing schedule, which might be every 1 to 3 years depending on the utility’s maintenance practices.

Regular DGA helps in early detection of potential issues, ensuring timely maintenance and extending the life of the transformer.

The most common faults that can be identified through DGA is listed below:

1. Partial Discharge

Description: Partial discharges are small electrical sparks that occur within the transformer, often due to insulation defects or moisture.

Key Gases: Hydrogen (H₂), Methane (CH₄).

Significance: Early detection of partial discharges can prevent more severe electrical faults and insulation breakdown.

2. Thermal Faults

Description: These faults occur when parts of the transformer overheat, leading to the decomposition of oil and insulation materials.

Key Gases: Methane (CH₄), Ethane (C₂H₆), Ethylene (C₂H₄), Carbon Monoxide (CO), Carbon Dioxide (CO₂).

Significance: Thermal faults can range from moderate overheating (below 300°C) to severe overheating (above 300°C), potentially causing significant damage if not addressed.

3. Arcing Faults

Description: Arcing faults are high-energy electrical discharges that can cause severe damage to the transformer.

Key Gases: Acetylene (C₂H₂), Ethylene (C₂H₄), Hydrogen (H₂).

Significance: The presence of acetylene is a strong indicator of arcing, which requires immediate attention to prevent catastrophic failure.

4. Corona Discharge

Description: Corona discharge is a type of partial discharge that occurs in the presence of high voltage and can lead to the formation of ozone and other reactive gases.

Key Gases: Hydrogen (H₂), Methane (CH₄).

Significance: While less severe than arcing, corona discharge can still degrade insulation over time and should be monitored.

5. Cellulose Insulation Degradation

Description: The degradation of cellulose insulation (paper) within the transformer can lead to reduced dielectric strength and mechanical integrity.

Key Gases: Carbon Monoxide (CO), Carbon Dioxide (CO₂).

Significance: Monitoring these gases helps in assessing the aging and condition of the insulation, allowing for timely maintenance or replacement.

6. Low-Energy Arcing

Description: Low-energy arcing involves smaller electrical discharges that can still cause localized heating and damage.

Key Gases: Methane (CH₄), Ethane (C₂H₆).

Significance: Detecting low-energy arcing early can prevent it from escalating into more severe faults.

7. High-Energy Arcing

Description: High-energy arcing is a severe fault involving intense electrical discharges that can cause significant damage to the transformer.

Key Gases: Acetylene (C₂H₂), Ethylene (C₂H₄), Hydrogen (H₂).

Significance: Immediate action is required to address high-energy arcing to avoid catastrophic transformer failure.

By identifying these common faults through DGA, operators can take proactive measures to maintain transformer health, prevent failures, and ensure reliable operation of the power system. Regular monitoring and analysis of dissolved gases provide valuable insights into the internal conditions of transformers, enabling timely maintenance and extending their operational life.

The interpretation of DGA results involves understanding the significance of different gases and their concentrations, as well as the ratios between them. Monitoring the trend of gas concentrations over time is crucial for effective DGA interpretation. Sudden increases in gas levels can indicate the onset of a fault, while stable or decreasing levels suggest that the transformer is operating normally. Various standards and guidelines provide reference values for interpreting DGA results. For example, the IEEE Standard C57.104-2019 offers detailed procedures and reference values for assessing transformer conditions based on DGA results.

I. Key Gas Method

The Key Gas Method is a straightforward and effective approach used in Dissolved Gas Analysis (DGA) to diagnose faults in transformers. This method focuses on identifying specific “key gases” that are indicative of particular types of faults. Here’s a detailed explanation:

Principle of the Key Gas Method

The Key Gas Method is based on the principle that different types of faults generate specific gases in significant quantities. By identifying and measuring these key gases, operators can diagnose the type of fault occurring within the transformer.

Key Gases and Their Associated Faults

Hydrogen (H₂)

Fault Type: Partial Discharge
Significance: Hydrogen is often produced in significant quantities during partial discharges, which are small electrical discharges within the transformer.

Methane (CH₄)

Fault Type: Low-Energy Arcing, Thermal Faults
Significance: Methane is generated during low-energy arcing and thermal faults at moderate temperatures.

Ethane (C₂H₆)

Fault Type: Thermal Faults
Significance: Ethane is produced during the decomposition of oil at moderate temperatures, indicating thermal faults.

Ethylene (C₂H₄)

Fault Type: High-Temperature Thermal Faults
Significance: Ethylene is a key gas for high-temperature thermal faults, indicating severe overheating of the oil.

Acetylene (C₂H₂)

Fault Type: High-Energy Arcing
Significance: Acetylene is produced during high-energy arcing, which is a severe electrical fault.

Carbon Monoxide (CO)

Fault Type: Cellulose Insulation Degradation
Significance: Carbon monoxide is generated by the decomposition of cellulose insulation, indicating insulation degradation.

Carbon Dioxide (CO₂)

Fault Type: Cellulose Insulation Degradation
Significance: Like carbon monoxide, carbon dioxide is produced by the breakdown of cellulose insulation.

Advantages of the Key Gas Method

Simplicity: The method is straightforward and easy to apply.
Quick Diagnosis: Provides a quick and effective way to identify faults.
Cost-Effective: Requires minimal equipment and resources compared to more complex methods.

Limitations

Sensitivity: May not detect very early stages of faults.
Complex Faults: Less effective for diagnosing complex faults involving multiple issues.

II. Rogers Ratio Method

The Rogers Ratio Method is a widely used technique in Dissolved Gas Analysis (DGA) for diagnosing transformer faults. This method relies on the ratios of specific gases dissolved in the transformer oil to identify different types of faults. Here’s a detailed look at how the Rogers Ratio Method works:

Key Gas Ratios

The Rogers Ratio Method uses the following gas ratios to diagnose faults:

Methane to Hydrogen (CH₄/H₂)
Ethylene to Ethane (C₂H₄/C₂H₆)
Acetylene to Ethylene (C₂H₂/C₂H₄)

Diagnostic Ratios and Fault Types

Each ratio provides insights into specific fault conditions within the transformer:

CH₄/H₂ (Methane to Hydrogen)

Low Ratio (<0.1): Indicates partial discharge.
Medium Ratio (0.1 - 1): Suggests low-energy arcing.
High Ratio (>1): Points to thermal faults.

C₂H₄/C₂H₆ (Ethylene to Ethane)

Low Ratio (<1): Indicates low-temperature thermal faults.
Medium Ratio (1 - 3): Suggests medium-temperature thermal faults.
High Ratio (>3): Points to high-temperature thermal faults.

C₂H₂/C₂H₄ (Acetylene to Ethylene)

Low Ratio (<0.1): Indicates thermal faults.
Medium Ratio (0.1 - 3): Suggests low-energy arcing.
High Ratio (>3): Points to high-energy arcing.

Interpretation Table

The Rogers Ratio Method uses a table to interpret the ratios and diagnose faults. Here’s a simplified version of the interpretation table:

Fault Type	CH₄/H₂	C₂H₄/C₂H₆	C₂H₂/C₂H₄
Partial Discharge	<0.1	-	-
Low-Energy Arcing	0.1 - 1	-	0.1 - 3
High-Energy Arcing	>1	-	>3
Low-Temperature Thermal	-	<1	-
Medium-Temperature Thermal	-	1 - 3	-
High-Temperature Thermal	-	>3	-

Advantages of the Rogers Ratio Method

Simplicity: The method is straightforward and easy to apply.
Effectiveness: It effectively distinguishes between different types of faults.
Early Detection: Helps in early detection of faults, allowing for timely maintenance.

Limitations

Sensitivity: The method may not be sensitive enough to detect very early stages of faults.
Complex Faults: It may not accurately diagnose complex faults involving multiple types of issues.

Practical Application

To apply the Rogers Ratio Method, follow these steps:

Sample Collection: Collect an oil sample from the transformer.
Gas Extraction: Extract the dissolved gases from the oil.
Gas Chromatography: Analyze the gases using gas chromatography to determine their concentrations.
Calculate Ratios: Calculate the key gas ratios (CH₄/H₂, C₂H₄/C₂H₆, C₂H₂/C₂H₄).
Interpret Results: Use the interpretation table to diagnose the fault type based on the calculated ratios.

III. Duval Triangle Method

The Duval Triangle Method is a graphical approach used in Dissolved Gas Analysis (DGA) to diagnose faults in oil-filled transformers. This method was introduced by Michel Duval and has become one of the most widely used techniques for interpreting DGA results. Here’s a detailed explanation:

How the Duval Triangle Method Works

1. Key Gases Used The Duval Triangle Method focuses on three key gases:

Methane (CH₄)
Ethylene (C₂H₄)
Acetylene (C₂H₂)

These gases are chosen because they are indicative of different types of faults within the transformer.

2. Plotting the Gases The concentrations of these gases are plotted on an equilateral triangle. Each corner of the triangle represents 100% concentration of one of the gases:

Top Corner: 100% Acetylene (C₂H₂)
Bottom Left Corner: 100% Methane (CH₄)
Bottom Right Corner: 100% Ethylene (C₂H₄)

The relative proportions of the three gases are used to locate a point within the triangle.

3. Fault Zones The interior of the triangle is divided into several fault zones, each corresponding to a different type of fault. The position of the plotted point within these zones helps in diagnosing the specific fault type. The main fault zones are:

PD (Partial Discharge)
D1 (Low Energy Discharge)
D2 (High Energy Discharge)
T1 (Thermal Fault <300°C)
T2 (Thermal Fault 300-700°C)
T3 (Thermal Fault >700°C)

Interpretation of Faults

1. Partial Discharge (PD)

Location: Near the bottom left corner (high CH₄, low C₂H₂ and C₂H₄).
Significance: Indicates partial discharges, which are small electrical discharges within the transformer.

2. Low Energy Discharge (D1)

Location: Between the PD and D2 zones.
Significance: Indicates low-energy arcing or sparking.

3. High Energy Discharge (D2)

Location: Near the top corner (high C₂H₂).
Significance: Indicates high-energy arcing, which is a severe fault.

4. Thermal Faults (T1, T2, T3)

Location: Along the bottom right side (high C₂H₄).
Significance: Indicates thermal faults at various temperature ranges:

T1: Thermal faults below 300°C.
T2: Thermal faults between 300°C and 700°C.
T3: Thermal faults above 700°C.

Advantages of the Duval Triangle Method

Visual Representation: Provides a clear and intuitive visual representation of the fault type.
Accuracy: Effective in distinguishing between different types of faults.
Ease of Use: Simple to apply and interpret.

Limitations

Complex Faults: May not accurately diagnose complex faults involving multiple issues.
Gas Concentrations: Requires accurate measurement of gas concentrations for reliable results.

Practical Application

To use the Duval Triangle Method:

Sample Collection: Collect an oil sample from the transformer.
Gas Extraction: Extract the dissolved gases from the oil.
Gas Chromatography: Analyze the gases using gas chromatography to determine their concentrations.
Plotting: Calculate the relative proportions of CH₄, C₂H₄, and C₂H₂ and plot the point on the Duval Triangle.
Interpretation: Determine the fault type based on the location of the point within the triangle.