Which protective relay is most likely to operate while adjusting the tap changer on a power transformer?

The differential relay is specifically designed to detect differences in electrical current between two or more points within a system, such as the primary and secondary winding of a transformer. When adjusting the tap changer on a power transformer, there are changes in the transformer's output voltage and current due to the alteration in the turns ratio. This operation can potentially cause internal faults or conditions that need to be monitored closely.

If there’s an imbalance in current through the transformer windings while the tap changer is being adjusted, the differential relay will recognize this as a potential fault condition. It compares the current entering and leaving the transformer and will operate if these currents differ beyond a pre-set threshold, indicating that something is wrong. Therefore, the differential relay is well-suited to respond during the tap-changing process, ensuring any discrepancies that may arise, including potential faults, are promptly detected and addressed.

Other relays, such as overcurrent, voltage, and distance relays, serve different functions and may not be as directly involved in the specific operational monitoring needed during tap changer adjustments. For instance, an overcurrent relay primarily responds to excessive current flowing through a circuit, while a voltage relay monitors voltage levels, and a distance relay is applicable for detecting faults over transmission lines based on