Transformer overexcitation occurs when the magnetic flux in the core exceeds its saturation level, typically caused by overvoltage, undervoltage, or frequency deviations in the power system. Overexcitation protection is designed to detect this condition and prevent damage to the transformer, as saturated cores lead to increased eddy current losses, excessive heating, and potential insulation failure.

The core principle relies on monitoring the volts-per-hertz (V/Hz) ratio. A transformer is designed to operate at a specific V/Hz value (e.g., 50Hz or 60Hz). When the voltage increases or the frequency decreases, this ratio rises, causing the core to saturate. Overexcitation protection relays continuously measure the input voltage and frequency, calculating the V/Hz ratio in real-time.

Once the ratio exceeds a predefined threshold—usually 10-15% above the rated value—the protection system activates. The response typically involves issuing an alarm to alert operators and, if the condition persists, tripping the transformer’s circuit breaker to disconnect it from the power source. This prevents prolonged saturation, which would otherwise lead to overheating of the core and windings, degradation of insulation materials, and eventual transformer failure.

Modern overexcitation protection relays may also incorporate time-delay settings to avoid unnecessary tripping during transient conditions, such as momentary voltage spikes. They may use additional sensors to monitor core temperature, providing a secondary layer of protection. By continuously safeguarding the V/Hz ratio, overexcitation protection ensures transformers operate within safe limits, extending their service life and maintaining grid stability.

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