**Explanation:**

To prevent the mal-operation of the relay in a transformer, the relay operating coil is biased with the 5th harmonic. This is done to ensure that the relay only operates when a fault occurs and not due to normal operating conditions.

**Harmonics in Transformer:**

When a transformer is subjected to non-linear loads or saturation, harmonics are generated in the transformer. Harmonics are multiples of the fundamental frequency (50 Hz or 60 Hz) and can result in distorted waveforms.

**Differential Protection Relay:**

A differential protection relay is used to protect transformers from internal faults. It compares the currents entering and leaving the transformer and operates when there is a significant difference between the two currents.

**Mal-operation of the Relay:**

Mal-operation of the relay can occur due to the presence of harmonics in the transformer. These harmonics can create imbalances in the currents entering and leaving the transformer, leading to false differential current and causing the relay to operate unnecessarily.

**Biasing the Relay Operating Coil:**

To prevent mal-operation of the relay, the relay operating coil is biased with the 5th harmonic. This means that a 5th harmonic voltage or current is injected into the relay operating coil in such a way that it cancels out the effect of the 5th harmonic component in the transformer currents.

**Effect of 5th Harmonic Biasing:**

By biasing the relay operating coil with the 5th harmonic, the relay becomes insensitive to the 5th harmonic component in the currents. This ensures that the relay only operates when there is a genuine fault and not due to the presence of harmonics.

**Other Harmonics:**

The 5th harmonic is chosen for biasing because it is the most commonly present harmonic in power systems. Other harmonics like the 3rd, 7th, or 2nd may also be present, but the 5th harmonic is considered the most effective for biasing purposes.

**Conclusion:**

In order to prevent the mal-operation of the relay in a transformer, the relay operating coil is biased with the 5th harmonic. This ensures that the relay only operates when there is a genuine fault and not due to the presence of harmonics.