Transformers are stationary devices that operate continuously and are highly reliable, with minimal chances of failure. However, as most transformers are installed outdoors and affected by operational load and short-circuit faults in the power system, various faults and anomalies are unavoidable during operation.

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1. Common Transformer Faults and Anomalies

(1) Internal Faults

Internal faults occur within the transformer's casing, such as:

• Phase-to-phase short circuit in windings.
• Inter-turn short circuit within a single winding.
• Short circuit between winding and core.
• Winding disconnection.

 

(2) External Faults

External faults include:

• Phase-to-phase short circuits on transformer output lines.
• Single-phase grounding faults caused by bushing flashover through the casing.

Transformers can suffer significant damage from faults, especially internal faults. High-temperature arcs generated by short-circuit currents can damage insulation, overheat the core, and cause transformer oil to decompose, releasing gases that may deform or even explode the casing. Therefore, transformers must be disconnected immediately in case of faults.

 

(3) Anomalies

Common anomalies during transformer operation include:

• Overload conditions.
• Lowering of oil level.
• Overcurrent caused by external short circuits.
• Excessive oil or winding temperature.
• High transformer pressure.
• Cooling system failures.

For such anomalies, warning signals should be generated.

 

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2. Transformer Protection Configuration

(1) Main Protection for Short-circuit Faults

• Differential protection
• Heavy gas relay protection (Buchholz relay)

 

(2) Backup Protection for Short-circuit Faults

• Overcurrent protection with composite voltage blocking.
• Zero-sequence (directional) overcurrent protection.
• Low-impedance protection.

 

(3) Anomaly Protection

• Overload protection.
• Overexcitation protection.
• Light gas relay protection.
• Neutral point gap protection.
• Temperature, oil level, and cooling system fault protection.

 

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3. Non-electrical Quantity Protection

Protection mechanisms that utilize transformer oil, gas, temperature, and other non-electrical indicators are known as non-electrical quantity protection. These include:

(1) Gas Relay Protection

When internal faults occur, short-circuit currents and arcs generate large amounts of gas, accelerating oil flow. Gas relay protection uses gas and oil flow as indicators.

• Light gas protection: Detects minor faults or abnormalities that produce gas bubbles, triggering alarms.
• Heavy gas protection: Detects severe faults generating significant gas and oil flow, triggering tripping signals to isolate the transformer.

 

(2) Pressure Protection

Monitors transformer oil pressure through pressure release or sudden pressure change detection.

 

(3) Temperature and Oil Level Protection

• Temperature protection: Issues alerts for high temperatures and activates backup coolers.
• Oil level protection: Triggers alerts for oil leakage or reduced oil levels.

 

(4) Cooling System Protection

If the transformer cooler fails completely, temperatures may rise, leading to insulation damage. This protection triggers an alert and, after a delay, disconnects the transformer.

 

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4. Differential Protection

Differential protection serves as the primary protection for electrical quantities in transformers. It covers faults like phase-to-phase or inter-turn short circuits within the range enclosed by current transformers (CTs).

 

(1) Transformer Inrush Current

When a transformer is energized, magnetizing inrush currents arise, influenced by factors such as transformer design, energizing angle, capacity, and residual magnetism. These currents are often 2–6 times the rated current and may reach up to 8 times in extreme cases. Inrush currents can cause false tripping of differential protection due to high differential currents in the circuit.

Characteristics of inrush current:

• High amplitude with distinct aperiodic components.
• Sharp, discontinuous waveforms.
• Significant harmonics, particularly the second harmonic.
• Decaying waveforms.

To prevent false tripping, differential protection uses:

• Second harmonic restraint: Differentiates fault currents from inrush currents by analyzing harmonic components.
• Waveform asymmetry and interruption angle analysis.

 

(2) Second Harmonic Restraint Principle

This method identifies inrush currents by measuring the second harmonic component in the differential current. If the second harmonic exceeds a threshold (typically 20%), differential protection is restrained.

 

(3) High-speed Differential Tripping

For severe internal faults with large fault currents that saturate CTs, differential protection might be delayed. High-speed differential tripping, with higher sensitivity thresholds, ensures prompt isolation without relying on harmonic restraint.

 

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5. Backup Protection Overview

(1) Overcurrent Protection with Voltage Blocking

This serves as backup protection for phase-to-phase faults in medium and large transformers. Composite voltage, derived from negative-sequence and low-voltage signals, enhances sensitivity and reduces overcurrent settings.

 

(2) Grounding Protection

Backup protection for grounding faults includes zero-sequence overcurrent, zero-sequence overvoltage, and gap protection, tailored to different neutral grounding configurations:

• Direct grounding: Zero-sequence current protection with directional settings.
• Ungrounded: Zero-sequence voltage protection to address arcing ground faults.
• Grounding through gaps: Gap protection ensures the safety of neutral point insulation.

This comprehensive configuration ensures transformers operate reliably under varying conditions while minimizing the risk of faults and anomalies.