The correct answer is: (d) Reverse current relay

A reverse current relay is specifically designed to prevent a DC generator from becoming motorized. If the generator loses its prime mover (e.g., due to fuel loss), its voltage drops. When operating in parallel, the other generator(s) in the system will cause current to flow back into the faulty generator, leading to reverse current. The reverse current relay detects this and trips the circuit breaker, protecting the generator from damage.

Reasons for not considering other options:

Overcurrent Relay (a):

• This relay is designed to trip the generator during high overcurrent events, such as those caused by a short circuit. Motoring of a DC generator does not result in an overcurrent condition, so an overcurrent relay cannot detect or prevent motorization.

Motoring Trip (b):

• Motoring trips are primarily used in motors where reverse operation occurs due to reverse flow in connected machinery, such as pumps. While a motoring trip could detect motoring, it typically responds after the motoring has occurred, which may already cause damage. Therefore, it is unsuitable for preventing DC generator motorization.

Reverse Power Relay (c):

• Reverse power relays work based on power flow direction and are designed for AC systems. In a DC generator, the current is unidirectional, so a reverse power relay cannot be used effectively in this context.