Types of Transformer Faults and Effective Ways to Prevent Them

 

1. Types of Transformer Faults

Transformer faults are broadly classified into internal and external faults.

A. Internal Faults

Internal faults occur inside the transformer and are often more severe.

1. Winding Faults
• Turn-to-turn fault: Short circuit between turns of the same winding.
• Occurrence: ~10–15% of internal faults.
• Effect: Localized overheating, partial discharge, insulation failure.
• Phase-to-phase fault: Short between different phases.
• Occurrence: ~5–8%.
• Effect: Severe current surge, can damage the transformer permanently.
• Ground fault: Short between winding and earth.
• Occurrence: ~10%.
• Effect: Leakage currents, oil deterioration, and insulation damage.
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2. Core Faults
• Magnetization faults: Due to core lamination faults.
• Occurrence: ~2–5%.
• Effect: Increased no-load current, heating, vibrations.
3. Tap Changer Faults
• On-load tap changer (OLTC) or off-load tap changer faults.
• Occurrence: ~5–10%.
• Effect: Arcing, contact wear, overvoltage.
4. Insulation Failure
• Between windings, turns, or winding-to-core.
• Occurrence: ~20–25%.
• Effect: Leads to internal short-circuits or winding damage.
5. Oil Insulation & Paper Degradation
• Faults due to aging or moisture.
• Occurrence: ~15–20%.
• Effect: Reduced dielectric strength, overheating, partial discharge.

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B. External Faults

External faults occur outside the transformer but can affect it.

1. Overvoltage
• Lightning or switching surge.
• Occurrence: ~10–15% of faults affecting transformers.
• Effect: Insulation breakdown, bushing failure.
2. Overcurrent / Short Circuit in Network
• Faults in connected lines or equipment.
• Occurrence: ~20–30%.
• Effect: Excessive heating, mechanical stress, winding deformation.
3. Environmental Factors
• Moisture, dust, or chemical contamination.
• Occurrence: ~5–10%.
• Effect: Accelerates insulation degradation, partial discharges.
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2. Methods to Minimize Transformer Faults

Minimizing faults is critical for reliability and transformer longevity.

A. Design and Manufacturing Measures

1. High-quality insulation
• Use modern cellulose paper, epoxy, and oil with high dielectric strength.
2. Proper winding design
• Interleaving and proper spacing reduce turn-to-turn faults.
3. Core selection
• Use laminated, low-loss cores to reduce heating.

B. Installation & Operation

1. Correct grounding
• Prevents ground faults and stabilizes voltage.
2. Proper load management
• Avoid overloading; maintain rated current and voltage.
3. Tap changer operation
• Periodic maintenance of OLTC to avoid arcing faults.

C. Monitoring and Maintenance

1. Dissolved Gas Analysis (DGA)
• Detects early signs of internal faults in oil-immersed transformers.
2. Partial Discharge (PD) Monitoring
• Detects insulation deterioration before catastrophic failure.
3. Thermal Imaging & Load Monitoring
• Identifies hot spots due to overloading or winding issues.
4. Periodic Insulation Testing
• Power factor, insulation resistance, and oil tests.

D. External Protection

1. Surge Arresters
• Protect from lightning and switching surges.
2. Circuit Breakers / Relays
• Isolate short circuits and overcurrent events promptly.
3. Proper Earthing & Shielding
• Reduces environmental impact and improves safety.

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3. Summary Table of Faults & Occurrence

Fault Type	Occurrence (%)	Effect	Mitigation Method
Turn-to-turn winding	10–15	Local overheating, partial discharge	High-quality insulation, PD monitoring
Phase-to-phase winding	5–8	Severe current surge	Protective relays, load management
Ground fault	10	Leakage currents, insulation damage	Proper grounding, DGA
Core fault	2–5	No-load current rise, vibrations	Quality core laminations
Tap changer fault	5–10	Arcing, contact wear	Regular maintenance, proper operation
Insulation failure	20–25	Internal shorts, winding damage	Insulation testing, oil monitoring
Oil/paper degradation	15–20	Dielectric strength drop	Oil filtration, DGA
Overvoltage	10–15	Insulation breakdown	Surge arresters, shielding
External overcurrent/short circuit	20–30	Heating, mechanical stress	Circuit breakers, protective relays
Environmental factors	5–10	Accelerated insulation aging	Proper housing, clean environment

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✅ Key Takeaways

• Insulation failures are the most common internal faults.
• External network issues and overloading cause the highest occurrence of transformer-impacting faults.
• Preventive measures like DGA, PD monitoring, and surge protection are vital for minimizing faults.

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