Transformer Faults causes

Major Causes of Transformer Faults and Protection Mechanisms

Transformers are critical assets in power systems, but they are also vulnerable to faults due to electrical, mechanical, and thermal stresses. A fault in a transformer not only affects reliability but may also lead to system instability and fire hazards. Below are the major causes of transformer faults and the protective schemes used to mitigate them.

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Major Causes of Faults in Transformers

1. Insulation Failure

   • The most common cause of transformer faults is insulation breakdown in windings. Ageing, overheating, moisture ingress, and over-stressing weaken insulation, leading to turn-to-turn short circuits or winding-to-core faults.

2. Tap Changer Failures

   • On-load and off-load tap changers are prone to mechanical wear, carbonization, and contact failures, which may lead to arcing and insulation damage.

3. Oil Leakage

   • Large transformers are typically oil-immersed. Leakage not only reduces insulation strength but also creates a fire hazard.

4. Inrush Currents

   • Switching an unloaded transformer generates high inrush currents rich in harmonics. These may cause misoperation of protective relays and local heating.

5. Inter-Turn Faults

   • Short circuits between turns in the same winding create localized hot spots, which accelerate insulation ageing and may escalate into severe internal faults.

6. Over-Fluxing

   • Operation at rated voltage with reduced frequency (under-frequency) or application of over-voltage at rated frequency causes magnetic core saturation, leading to excessive core losses and heating.

7. Sustained Overloads

• Prolonged overloading increases winding temperature beyond design limits, accelerating insulation deterioration and potentially causing catastrophic failure.

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Protection Schemes for Power Transformers

(A) Differential Protection

• Purpose: Detects internal short circuits.

• Principle: Compares input and output currents of the transformer. A difference indicates an internal fault.

• Challenges:

  • Magnetizing inrush current: Can be 8–30 times rated current during energization, rich in harmonics.

  • Over-excitation: Core saturation due to over-voltage or under-frequency operation increases excitation current.

  • CT Saturation: External faults may cause CT distortion, leading to false differential current.

Solution:

• Percentage differential relays (1–5% slope) with harmonic restraint are used. Harmonic restraint blocks operation during inrush by detecting high harmonic content.

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(B) Restricted Earth Fault (REF) Protection

• Protects against low-magnitude earth faults within the transformer winding.

• Especially useful for high-resistance winding-to-core faults that may not trigger differential protection.

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(C) Overcurrent Protection

• Provides backup protection for large transformers (≥5 MVA).

• Typically includes two-phase fault relays and one earth-fault relay in star-delta configurations.

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(D) Overfluxing Protection

• Principle: Flux (Φ) ∝ Voltage / Frequency (E/f).

• If E/f exceeds a set threshold, flux density rises beyond safe limits.

• Protection: Relays monitor E/f ratio and initiate tripping after a time delay (to prevent tripping for transient over-voltages).

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(E) Overheating Protection

• Thermal Image Protection: Uses temperature detectors and thermal models to monitor winding heating.

• Implementation:

  • Resistance Temperature Detectors (RTDs) or thermistors in oil.

  • CT-fed heaters simulate winding heating effect.

  • Trips when winding exceeds ~95 °C.

• Cooling fans and pumps can be automatically controlled.

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(F) Protection Against Incipient Faults

1. Buchholz Relay (Gas-Actuated Relay):

   • Installed between the conservator and transformer tank.

   • Detects slow-developing faults (incipient faults) that release gases from oil decomposition.

   • Provides alarm for small gas accumulation and tripping for severe oil surges.

2. Gas Analysis:

   • Dissolved Gas Analysis (DGA) identifies specific gases (H₂, CH₄, C₂H₂, etc.) to indicate the type of fault.

3. Pressure Relief Device:

   • Operates when internal oil pressure rises above ~10 psi.

   • Prevents tank rupture by releasing oil.

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(G) Fire and Lightning Protection

1. Fire Protection:

   • Causes: insulation failure, arcing, overheating, or oil leakage.

   • Mitigation: regular maintenance, lightning arresters, grounding, suppression systems, and oil fire protection systems.

2. Lightning Protection:

   • Lightning overvoltage surges can reach several times the rated current within microseconds.

   • Solution: Surge arresters placed close to transformer bushings.

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Conclusion

Transformer faults are inevitable due to ageing, operating stresses, and external disturbances. However, with comprehensive protection schemes—such as differential relays, REF protection, overcurrent, overfluxing relays, Buchholz relays, and fire/lightning protection systems—transformer reliability and safety can be significantly enhanced.

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