Differential relays are used on generators to guard against which of the following?

Differential relays are specifically designed to monitor the difference in electrical current between two or more circuits, making them particularly effective for detecting unbalanced phase currents. In the context of generators, if there is an unbalanced condition—such as an internal fault where one phase is carrying a significantly different current than the others—the differential relay will sense this discrepancy. By comparing the currents entering and exiting the generator, it can determine if an imbalance exists and subsequently initiate protective measures to prevent potential damage to the generator.

Unbalanced phase currents can occur due to various fault conditions, improper loading, or equipment failure. As such, the primary function of differential relays is to quickly detect these issues, ensuring that the generator operates safely and efficiently, and minimizing the risk of long-term damage or failure. The other options, while also important in protective schemes, do not align as closely with the specific function of differential relays as unbalanced phase currents do.