Transformer Diagnostics

With proper transformer diagnostics and control, the service life of transformers can be extended to up to 60 years. Accidents involving transformers in service pose serious risks to personnel and the environment, including explosions, fires, and oil leakage. Moreover, repairing or replacing damaged transformers is costly and can significantly increase expenses.

New transformers are designed with sufficient mechanical and electrical strength to withstand abnormal conditions such as atmospheric overvoltages and short circuits. However, as transformers age, their insulation system and overall stability may deteriorate, potentially to the extent where they can no longer withstand such conditions and may fail. Furthermore, certain types of faults may develop over time, accelerating wear and increasing the risk of accidents.

Common Transformer Failures

Common types of transformer failures include:

• Overheating of the active parts. Excessive thermal stress can cause localized heating of the windings and other components, leading to gradual degradation over time.
• Insulation breakdown. Damage to the insulation between components may result from poor-quality materials, manufacturing defects, or mechanical wear. Even minor insulation issues, if left unaddressed, can escalate into severe faults.
• Moisture ingress into insulation. Over time, insulation may absorb moisture, reducing its dielectric strength and increasing the risk of partial discharges or breakdown.
• Gas development in oil. Decomposition of oil under abnormal operating conditions can lead to gas formation, indicating internal faults such as overheating or arcing.
• Aging of insulation. Prolonged exposure to operational stresses causes insulation to degrade, diminishing its effectiveness and increasing the likelihood of failures.
• Deformation of transformer windings. Short circuits or mechanical stresses can deform transformer windings, disrupting their performance and potentially causing further short circuits.
• Wear of tap changer contacts. Repeated operation and thermal cycling can lead to wear or overheating of the tap changer contacts, compromising their functionality.
• Failure of the cooling system. Inefficient cooling due to blockages or component malfunctions can exacerbate overheating and damage internal parts.
• Partial discharges. Localized electrical discharges within the insulation may occur, often indicating a developing fault that requires attention.
• Factory defects or poor repairs. Issues such as faulty welding of leads or improper maintenance may compromise transformer reliability.

Monitoring and transformer diagnostics help identify these problems early. Evaluating the technical condition of a transformer involves analyzing various parameters and characteristics under specific environmental and operational conditions. Diagnostic methods often classify failures into categories such as auxiliary part malfunctions, winding and connection faults, or external influences like overloads and short circuits.

Minor insulation issues, if not promptly addressed, can escalate into major problems. These may be caused by mechanical wear, poor-quality core bolts, substandard winding insulation, or damage caused by operational stresses. Proper maintenance and early detection are critical for ensuring the long-term reliability of power transformers.

Diagnostic Methods for Transformer Failures

Transformers undergo a range of diagnostic tests to identify potential issues. These methods can be grouped into several categories:

1. Visual and thermal inspections: used to detect overheating or physical damage.
2. Electrical tests: used to assess insulation resistance, dielectric strength, and the integrity of transformer windings.
3. Chemical analysis of oil: used to determine the presence of contaminants or decomposition products.
4. Dissolved Gas Analysis (DGA): a specialized method that provides early warnings of developing faults.

Dissolved Gas Analysis (DGA) for transformer diagnostics

Dissolved Gas Analysis (DGA) is a diagnostic method used to assess the condition of power transformers. It identifies developing faults at an early stage without requiring a transformer shutdown, which makes it one of the most reliable and widely used diagnostic tools. DGA detects issues such as overheating, arcing, and partial discharges by analyzing the gases dissolved in the oil, which are byproducts of insulation or oil degradation.

How DGA Works

The process involves:

1. Sampling of oil using specialized equipment.
2. Correct transportation and storage of oil samples.
3. Extraction of dissolved gases.
4. Measurement of gas content using a gas chromatograph.
5. Diagnosis of faults based on gas composition and growth rates.

Each transformer has a unique "gas fingerprint," influenced by its design, cooling system, and operating conditions. Monitoring gas concentrations over time provides the basis for identifying abnormal patterns and determining the nature of developing faults.

Fault Identification Methods Commonly used transformer diagnostic techniques include:

• Rogers Ratio Method: evaluates ratios of key gas pairs (e.g., CH4/H2, C2H2/C2H4).
• Duval Triangle Method: plots CH4, C2H4, and C2H2 concentrations as a triangular diagram to identify specific faults.
• Doernenburg Ratio Method: combines gas ratios with threshold values to classify defects into thermal or electrical categories.

These methods are standardized under IEC guidelines, ensuring consistency and reliability in fault detection. The Duval Triangle, for instance, provides a visual representation that simplifies fault categorization and improves monitoring precision.

Importance of Gas Concentrations

Gas levels naturally increase throughout the transformer life cycle. However, sudden spikes or abnormal ratios indicate potential issues. The following points should be considered to ensure proper interpretation of DGA results:

• Baseline gas concentrations.
• Rate of gas increase.
• Ratios of gas pairs.

Ensuring Accuracy

Accurate results depend on proper sample collection, transportation, and analysis. Mishandling of samples may result in gas loss, particularly for low-solubility gases such as hydrogen, leading to incorrect diagnostic conclusions.

DGA is indispensable for maintaining transformer reliability, providing up to 70% of the information required for condition assessment. Its proactive approach minimizes unplanned outages and extends transformer service life.