Dissolved Gas Analysis (DGA) Abnormal? How to Remove Combustible Gases from Insulation Oil?

When transformers exhibit unusual DGA results, and the transformer oil has high concentrations of combustible gases such as acetylene or ethylene, then using mechanical filters to restore stability in the equipment is inadequate. This calls for a high-vacuum degassing process rather than a standard filter that removes solid contaminants. While the mechanical filter takes out the solid materials (carbon dust and metal), the high-vacuum degassing tackles the dissolved gas contaminants in the oil molecules.

In the event of gas peaks that are active, the multi-pass thermal vacuum cycle needs to be carried out. The transformer oil purifier system should be programmed to manage pressure and temperature together. This ensures that the gases that have been dissolved in the liquid medium are brought back to their gas form, allowing them to be sucked up by the vacuum system. In the event the oil is simply filtered physically, without the application of a deep vacuum, the combustible gases would remain locked within the hydrocarbons, compromising the dielectric properties of the oil.

Decoding DGA: Key Gases and What They Mean

The Dissolved Gas Analysis (DGA) is a critical diagnostic technique used in checking the condition of transformers. The mineral oil and cellulose paper used in transformers are decomposed under conditions of thermal and electrical stress to form certain gases. The detection of such gases helps engineers identify the type of internal fault in transformers.

Some of the key gases identified through diagnostic tests include:

• Acetylene (C2H2): The most critical signature, which shows high-energy arc or spark discharge.
• Hydrogen (H2): Usually formed as a result of partial discharge (corona) or electrolysis due to high moisture content.
• Ethylene (C2H4): Shows high temperatures above 700 degrees C, resulting from excessive heating of oils or loose connections.
• Methane (CH4) & Ethane (C2H6): Signs that show medium or low temperatures inside the core and windings.
• Carbon Monoxide (CO) & Carbon Dioxide (CO2): High amounts of these components signify that thermal breakdown of cellulose paper insulation has occurred.

Based on the amount of these components using the guidelines provided in IEC 60599 or IEEE C57.104 ratio, one can determine if there is any active faulting of the equipment due to electrical faulting or overloading, dictating immediate oil processing intervention.

Why Standard Filtration Isn’t Enough for DGA Faults

One of the misconceptions concerning maintenance procurement is that standard mechanical filtration will take care of an unacceptably high DGA reading. Standard filtration equipment like particulate filters, centrifuges, and regular filter presses works on the principle of using physical filters to remove solid contaminants. These filtration units work effectively for carbon soot, metals, and sludge.

But then again, the gaseous components are not actually particles that are suspended in air; rather, they are molecules that have dissolved into the hydrocarbon base of the insulation oil. No matter how many times you run the oil containing these gases through a standard filtration system, you will never be able to affect the molecular bond between the gas and the insulation oil. You cannot separate a gas from its liquid medium unless you change the physical properties of the fluid through the manipulation of pressure and temperature.

How a Vacuum Degassing System Removes Combustible Gases

To separate dissolved gases, a transformer oil purifier employs the physical phenomena of Henry’s Law and the vacuum distillation process. Henry’s Law suggests that the volume of gases dissolved in liquids is directly proportional to their partial pressures on the liquid. This means that when atmospheric pressure is decreased in a special chamber, the gases will become insoluble in the liquid.

The vacuum distillation system achieves the separation in a carefully devised three-step thermodynamic process:

• Reducing Oil Viscosity via Heating: The crude insulation oil goes through a heating process using low-watt density indirect heating, which increases its temperature up to 50°C to 65°C, reducing its viscosity and weakening the chemical bonds between the gases.
• Surface Area Enhancement: After heating, the oil is introduced into the main vacuum chamber using custom spray nozzles or specialized three-dimensional flash evaporation units. It forms films or droplets, increasing the surface area where the oil meets the vacuum.
• Ultimate Vacuum Extraction: The gases (H2, C2H2, C2H4), maintained at an ultimate vacuum below 10 Pa, immediately vaporize and detach from the oil films. They are extracted using a high-efficiency Roots pump.

Steps to Resolving DGA Anomalies in the Field

When handling an active fault in a DGA analysis at a substation, the maintenance personnel are required to take the following steps:

Step 1: Establishing a baseline

Take a new oil sample from the transformer at the bottom valve using a closed syringe. Conduct a full lab analysis to identify precise baseline ppm values for all major gases.

Step 2: System Isolation

If there is a high gas content in the oil, specifically a high level of acetylene, then the transformer should be isolated from the system.

Safety Precaution: In case of conducting filtration online (on an energized transformer), make sure that the purifier is fitted with automatic oil-level synchronizing and shutdown valves to avoid any air/oil hazards.

Step 3: Installation of the Purifier

Attach the oil purifier input to the bottom valve of the transformer and output to the top valve or conservator tank, creating a continuous circulation circuit.

Step 4: Setting Parameters

Perform initialization of the system. Define the vacuum chamber operating pressure (e.g., not exceeding 10 Pa) and set the parameters of the heating system to maintain the temperature of the oil at about 60 degrees C.

Step 5: Continuous Circulation in the Closed Loop

Purify oil in the purifier for several cycles. In order to ensure the absence of gas dissolution and reduce moisture content to less than 10 ppm in a safe way, usually, it is necessary to circulate all the transformer oil through the degassing unit 3 to 5 times.

Step 6: Post-Sample Testing

Once fluid circulation is completed, take an oil sample from the same spot. Check that the amount of gas in the oil is below the limit value before restoring the machine’s operation.

Right Transformer Oil Purifier for DGA Correction

The selection of the proper processing equipment relies mainly on the electrical power rating, total volume of the oil to be processed, and the actual composition of the gases present in the oil. The following table outlines the major distinctions between the two types of vacuum purifiers available today.

Technical Features	Single-Stage Vacuum Purifier	Double-Stage Vacuum Purifier (with Roots Pump)
Optimal Application	Distribution Transformers (under 35 kV)	Transmission and Grid Transformers (110 kV and above)
Typical Vacuum Level	133 Pa (1 Torr)	Less than or equal to 10 Pa (0.075 Torr)
Gas Extraction Velocity	Moderate capability (Best for light H2 spikes)	Ultra-high velocity (Essential for deep C2H2 extraction)
Moisture Capability	Reduces water content to 15 ppm or less	Reduces water content to 5 ppm or less
Dielectric Breakdown Voltage	Restores BDV up to 50 kV or more	Restores BDV up to 70 kV or more

In the case where substation maintenance involves a large volume of work or when handling situations where critical concentrations of acetylene appear in transformers, a Double-Stage Vacuum Transformer Oil Purifier is recommended. With the added Roots blower pump, the efficiency of pumping out air becomes much higher in a deep vacuum setting.

Effective measures must be taken in the event of abnormal DGA results.

The presence of explosive gases such as acetylene or hydrogen dissolved in the insulating oil due to internal defects makes filtration using conventional physical methods utterly ineffective.

Use of a highly efficient Double-Stage Vacuum Transformer Oil Purifier uses thermal processing and vacuum flash distillation to safely remove the volatile compounds from the oil. This process greatly improves the dielectric properties of the oil while prolonging the life of the transformer.