The fundamental goal of transformer protection is to identify faults internally in the transformer very precisely and de-energize it as a result, while simultaneously protecting it from external faults. The damage caused by the malfunction and the repairs that are required can be minimized with sensitive detection and de-energization. But it needs to have backup protection in case the system has through faults, which could cause the insulation on the transformer windings to fail from overheating and the high impact forces generated by the fault currents, which accelerate aging and deterioration.

A transformer can fail internally or experience premature deterioration due to abnormal system circumstances including overexcitation, overvoltage or loss of cooling, in addition to internal problems. Therefore, a complete transformer protection plan should take these failures into account. In general, transformers are protected electrically by sensing the frequency, current and voltage that flow through them, and mechanically by sensing operational parameters such as winding temperature, gas evolved, oil pressure/level and oil temperature. Based on this, solutions can be broadly classified as:

Electrical protection

The following, each of which is described in greater detail, make up the electrical protection of a transformer:

Fused protection

• Principle: Fuses are overcurrent protection devices that melt and open the circuit when the current exceeds a predetermined value.
• Application: Fused protection is often used for low-voltage transformers or as a backup protection for other schemes.
• Advantages: Simple and inexpensive.
• Disadvantages: Can be slow to operate, leading to excessive damage.

Differential current protection

• Principle: Compares the currents flowing into and out of the transformer. If there's a significant difference, it indicates a fault within the transformer, triggering protective action.
• Application: Widely used for all transformer sizes.
• Advantages: Fast and sensitive to internal faults.
• Disadvantages: Can be susceptible to external faults or transformer saturation.

Overcurrent protection

• Principle: Detects excessive current flow in the transformer's primary or secondary winding.
• Application: Used as backup protection or for phase or ground fault conditions.
• Advantages: Simple and inexpensive.
• Disadvantages: Can be slow to operate, leading to excessive damage.

Over excitation protection

• Principle: Detects excessive magnetization of the transformer core, which can lead to overheating and insulation failure.
• Application: Used for transformers operating in weak grid conditions or with variable loads.
• Advantages: Helps prevent transformer damage.
• Disadvantages: Can be sensitive to normal operating conditions.

Over voltage protection

• Principle: Detects excessive voltage levels in the transformer's windings.
• Application: Used to protect against lightning strikes, switching surges or other voltage transients.
• Advantages: Helps prevent insulation failure.
• Disadvantages: Can be sensitive to normal operating conditions.

Mechanical protection

When any of the above-mentioned mechanical parameters exceed predetermined limits, the protection system should initiate actions such as a warning signal may be generated to alert operators; the transformer's load may be reduced to prevent further overheating; and the transformer can be isolated from the power system to prevent damage. The following are the details of mechanical protection methods:

Buchholz relay

• Purpose: To detect internal faults or abnormal operating conditions in transformers that produce gases. These gases can be indicative of faults such as short circuits, overheating or insulation breakdown.
• Principle: The Buchholz relay is a mechanical device installed in the oil-filled conservator tank of a transformer. It works by monitoring the flow and pressure of the oil. When gas is produced within the transformer, it rises to the top of the oil and displaces the oil in the conservator tank. This change in oil level triggers the Buchholz relay.

Winding temperature monitoring

• Purpose: To prevent overheating of the transformer windings, which can lead to insulation failure.
• Methods:
  • Hot-spot detector: Measures the temperature of the hottest point in the winding.
  • Winding temperature sensor: Measures the average temperature of the winding.
  • Oil temperature sensor: Assumes that the winding temperature is proportional to the oil temperature.

Gas evolved monitoring

• Purpose: To detect internal faults or abnormal operating conditions that may produce gases.
• Methods:
  • Buchholz relay: Detects gas production and initiates protective actions.
  • Dissolved gas analysis: Analyzes the gases dissolved in the transformer oil to identify the type of fault.

Oil pressure/level monitoring

• Purpose: To ensure adequate oil circulation and cooling of the transformer.
• Methods:
  • Oil pressure sensor: Monitors the pressure of the transformer oil.
  • Oil level indicator: Monitors the level of the transformer oil.

Oil temperature monitoring

• Purpose: To prevent overheating of the transformer oil, which can lead to insulation failure.
• Methods:
  • Oil temperature sensor: Measures the temperature of the transformer oil.

Conclusion

The economics of the protection scheme, taking into account the likelihood of a specific failure type, the cost of replacing and repairing the transformer, and the possibility of the failure causing damage to nearby equipment or infrastructure, determine the extent to which protective devices are applied to a particular transformer.