Transformer Protections: Types, Working, and Applications

 

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Transformer Protections: Types, Working, and Applications

Transformers are critical equipment in power systems, and any fault can lead to severe damage, downtime, or even hazards. Hence, multiple protection schemes are applied depending on transformer size, voltage level, and operational criticality.

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1. Overcurrent Protection (OC)

Purpose: Protects the transformer from currents exceeding its rated capacity, which can occur due to short circuits or overloads.

Types:

• Instantaneous Overcurrent (IOC): Trips immediately when current exceeds a threshold.
• Time-Delayed Overcurrent (TOC):
  
  Trips after a set time to allow transient conditions like motor starting currents.

Working:

• Current transformers (CTs) measure primary and secondary currents.
• If the current exceeds the set value, the relay sends a trip signal to the circuit breaker.

Applications:

• Backup protection for line-to-line or line-to-ground faults.
• Often used in conjunction with other protections for complete coverage.

Figure:
Imagine a simplified diagram:

         ---- CT ----> Overcurrent Relay ----> Trip CB

Primary Bus ----------------------------- Transformer

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2. Differential Protection (87T)

Purpose: Detects internal transformer faults (winding-to-winding or winding-to-earth faults) while remaining stable for external faults.

Working:

• Measures current at both ends of the transformer winding (primary and secondary).
• If the difference exceeds a set percentage (typically 20–30%), the relay trips the breaker.
• Operates on the principle: Internal fault → Current in ≠ Current out

Key Feature: High-speed protection (milliseconds) to prevent winding damage.

Figure:

Primary Current --->|    Differential Relay    |--- Trip CB

Secondary Current -->|                          |

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3. Buchholz Relay (Gas & Oil Protection)

Purpose: Protects oil-immersed transformers from internal faults like insulation failure or overheating.

Working:

• A Buchholz relay is installed on the pipe between transformer tank and conservator.
• Gas generated due to insulation failure or minor faults accumulates in the relay float chamber → actuates alarm or trips the transformer.
• Severe oil surge → Trip instantly.

Applications:

• Essential for large oil-filled transformers.
• Provides early warning before catastrophic failure.

Figure Concept:

 Transformer Tank ----> Buchholz Relay ----> Conservator

 Gas Accumulation triggers Alarm/Trip

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4. Overvoltage & Undervoltage Protection

Purpose: Protects transformers from abnormal voltages.

Working:

• Overvoltage: If voltage exceeds rated level → relay triggers load shedding or disconnects transformer.
• Undervoltage: Prevents transformer operation under low voltage, which can cause overheating or core saturation.

Applications:

• Critical in distribution networks with fluctuating voltage.

Figure Concept:

Voltage Transformer --> OV/UV Relay --> Trip Signal --> CB

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5. Earth Fault Protection

Purpose: Detects insulation breakdown to ground.

Working:

• CTs detect zero-sequence current (sum of three phase currents).
• Non-zero sequence → fault to ground → relay trips the breaker.

Applications:

• High importance in grounded neutral systems.
• Can be phase-wise or system-wide protection.

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6. Thermal / Overheating Protection

Purpose: Protects transformer from thermal stress due to overload.

Working:

• Thermocouples or winding temperature sensors measure real-time winding/oil temperature.
• If temperature exceeds safe limit → alarms or trips transformer.
• Common standard: IEEE C57.91 for transformer loading and temperature rise.

Applications:

• Large oil-filled transformers with conservators.
• Can be integrated with auto-cooling control systems.

Figure Concept:

 Winding Temperature Sensor --> Temperature Relay --> CB Trip

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7. Pressure Relief & Explosion Venting

Purpose: Prevents catastrophic failure in case of internal fault or oil overheating.

Working:

• Pressure relief valves or disc vent in transformer tank.
• Relieves excess pressure due to oil expansion, preventing tank rupture.

Applications:

• All large oil-immersed transformers.

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8. Surge Protection

Purpose: Protects transformer from lightning or switching surges.

Working:

• Lightning Arresters connected at high voltage terminals.
• Diverts transient high-voltage surge to ground.

Applications:

• Outdoor transmission transformers.
• Often coordinated with system-level protection for insulation safety.

Figure Concept:

 HV Bus ---LA--- Transformer HV Winding

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Summary Table: Transformer Protections

Protection Type	Function	Trigger Parameter	Typical Use
Overcurrent (OC)	Overload/Short circuit	Current	Backup & Line protection
Differential (87T)	Internal faults	Current difference	Fast primary protection
Buchholz Relay	Internal oil fault	Gas/Oil surge	Oil transformers
Over/Undervoltage	Voltage anomalies	Voltage	System voltage monitoring
Earth Fault	Ground insulation failure	Zero-sequence current	Grounded transformers
Thermal/Overheating	Overload & insulation heating	Temperature	Auto-cooling & alarms
Pressure Relief	Prevent tank rupture	Pressure	Oil-filled transformers
Surge Protection	Lightning & switching surges	Voltage surge	Outdoor transformers

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