What are the common faults of oil-immersed transformers, and what are the best solutions?

GNEE Electric, a premier manufacturer and service provider with over 18 years of field-tested expertise, confronts this critical question daily as we support utilities and industries worldwide. A clear understanding of the common faults of oil-immersed transformers and the implementation of scientifically backed solutions are essential to prevent costly outages and extend asset life. Transformer faults account for a significant percentage of power system equipment failures, but with early identification and correct intervention, most units can be restored to full reliability.

 

This article distills our engineering team's deep diagnostic knowledge into a practical guide for maintenance managers and technical decision-makers.

Learn more about GNEE Oil-Immersed Transformer

 

 

Winding faults represent the most frequent category among the common faults of Oil-Immersed Transformers, accounting for roughly 30-40% of all internal failures. These faults directly compromise the core electromagnetic function of the unit.

 

Types and Root Causes:

• Turn-to-Turn Short Circuit: The most typical winding failure, starting from localized insulation paper degradation. A small short-circuit creates a heating point that rapidly degrades adjacent turns, potentially leading to winding burnout.
• Winding Grounding: Insulation between the conductor and the grounded core or tank fails, creating an unintended path to earth and tripping protective relays.
• Conductor Breakage: Mechanical stress from severe short-circuit forces or poor joint welding can cause conductor separation, resulting in abnormal voltage outputs.

 

Best Solutions:
For minor turn-to-turn damage where the hotspot is accessible, the transformer tank can be opened, the fault point identified, and the shorted turns carefully separated with additional insulation barriers, followed by localized vacuum drying. For severe, widespread winding damage, the definitive solution is a complete winding replacement.

 

GNEE Electric performs this specialized work at our ISO-certified facility, using original-grade copper conductors and state-of-the-art winding machines. After any winding repair, comprehensive DC resistance tests, turn ratio verification, and partial discharge measurements must validate the fix. Preventing such faults starts with avoiding sustained overload operation and conducting regular dissolved gas analysis to catch early signs of overheating.

 

 

The iron core operates with only a single intended ground point. When this design principle is violated, it becomes a direct trigger for common faults of oil-immersed transformers that manifest as chronic local overheating.

 

 

Recognizing the Fault Mechanism:
A healthy transformer core has precisely one grounding connection to prevent floating potential. A multi-point grounding fault occurs when a second, unintended contact forms-caused by metallic debris, rust accumulation, or a damaged insulation spacer. This creates a closed loop, and the induced circulating current causes intense localized heating that degrades oil and insulation near the core. You will observe rising methane and ethylene levels in the DGA results without an obvious electrical reason.

 

Systematic Solution Approach:
The first line of action is diagnostic: measure the iron core insulation resistance to confirm the loss of single-point grounding. In minor cases, applying a limited current impulse technique can vaporize microscopic metal bridges without opening the tank. For persistent faults, a core lifting inspection becomes necessary. Our service engineers lift the active part, visually scan for foreign objects or rust bridges, and meticulously restore the insulation spacer. We often install a permanent core grounding current monitor post-repair so that the operator can continuously verify that the circulating current remains below the safe threshold of 100 milliamperes.

 

 

Aging and breakdown of the solid and liquid insulation represent the most dangerous common faults of oil-immersed transformers, as they can cascade from minor aging to catastrophic dielectric failure.

 

Critical Symptoms:

Significant decrease in insulation resistance and polarization index during routine megger testing.

A noticeable rise in carbon monoxide and carbon dioxide in the DGA chromatogram, signaling paper decomposition.

Increasing partial discharge levels, especially at operating voltage.

 

Structured Intervention Strategy:
For bushings showing surface flashover and minor cracks, cleaning and applying a room-temperature-vulcanizing silicone rubber coating often restores adequate creepage distance. However, for main insulation breakdown between the winding and tank, a major overhaul is non-negotiable.

 

GNEE Electric's workshop removes the aged insulation, re-wraps conductors with high-grade crepe paper, and subjects the active part to multiple cycles of vapor-phase drying to restore its water content to below 0.5%. The most effective prevention strategy combines conservative thermal management-ensuring top oil temperature never sustains above 85°C-and installing online partial discharge monitors that provide real-time alerts of developing insulation defects.

 

 

Transformer mineral oil functions as both a dielectric and a coolant, so its condition directly reflects the asset's overall health. Oil quality problems are common faults of oil-immersed transformers that accelerate all other failure modes if left uncorrected.

 

Key Diagnostic Indicators:
The best solutions begin with regular laboratory screening of oil samples for breakdown voltage, acid number, moisture content, and dissolved gas profile. Oil deterioration typically appears as a reduced breakdown voltage, increased acidity, and darker color. Excessive dissolved gas content reduces dielectric strength, while high moisture actively accelerates cellulose paper aging.

 

Precision Solutions:
For oil with elevated moisture or gasses but stable chemical properties, on-line vacuum degassing and dehydration units running continuously can restore parameters while the transformer stays energized. When the acid value rises unacceptably, Bentonite clay treatment or full regeneration using mobile oil reclamation plants strips out acidic compounds and colloidal contaminants. In cases of extreme oxidation, a complete oil change is the best solution.

 

GNEE Electric supplies premium Class I mineral oil pre-tested to IEC 60296, ensuring your transformer returns to service with factory-fresh dielectric integrity.

 

 

Accessories such as tap changers, cooling pumps, and protective relays collectively account for 20-30% of common faults of oil-immersed transformers failures. While they don't directly involve the core or coils, their failure can rapidly induce thermal or mechanical damage to the main body.

Focus Areas and Corrective Actions:

• On-Load Tap Changer (OLTC) Issues: Poor contact wear generates acetylene in the diverter tank oil. The best solution involves a scheduled OLTC maintenance dive-cleaning or replacing arcing contacts and filtering the separate OLTC oil compartment.
• Cooling System Blockage: Radiators fouled by external debris or internal sludge cause high oil temperature alarms. High-pressure nitrogen or hot-oil flushing cleans internal pathways effectively. Faulty fan motors and oil pump seals demand immediate replacement from stock.
• Protection Device Malfunction: Buchholz relays or pressure relief valves that fail to operate or generate false trips require calibration on a test bench. GNEE recommends annual functional testing of all mechanical protection devices as a fixed element of the maintenance calendar.

 

 

Preventing the recurrence of these common faults of oil-immersed transformers requires transitioning from reactive repair to a condition-based maintenance philosophy.

 

Our engineering support team recommends a layered strategy:

• Continuous Monitoring Foundation: Install on-line DGA monitors and partial discharge sensors on critical transformer assets. Real-time acetylene and hydrogen trending gives you the earliest possible notification of developing winding or core faults.
• Scheduled Electrical Testing: Perform DC winding resistance, short-circuit impedance, and insulation tests at least every two years. Comparative trending against factory baseline data flags slow-developing deformation or contact wear.
• Thermal and Physical Survey: Use infrared thermography annually to identify loose bushing connections or uneven radiator flow without any service interruption.
• Full Lifecycle Analysis: For transformers exceeding 15 years of service, conduct furan compound analysis of the paper insulation during oil tests. This non-invasive method tells you the actual remaining mechanical strength of the solid insulation, guiding rational replacement decisions.

 

 

Recognizing common faults of oil-immersed transformers is the first step; executing the best solutions with technical precision is where GNEE Electric consistently delivers. From winding repairs and oil regeneration to complete on-site fault investigations, our factory-trained teams bring the process discipline and testing rigor necessary for long-term grid reliability.

 

Do not wait for a minor anomaly to become a catastrophic failure. Contact our technical support division today with your transformer's recent test data. We will return a detailed fault analysis and a competitive quotation for corrective service within 24 hours. Let's restore your asset's health and secure your power supply together.

 

Request A Quote