The correct answer is: A. power
Directional relays are based on the flow of power. They are used to protect power systems from faults that occur in reverse power flow.
Power is the rate at which energy is transferred. It is calculated by multiplying the voltage and current. The voltage is the difference in electrical potential between two points. The current is the flow of electrical charge.
The direction of power flow is from the point with higher voltage to the point with lower voltage. When a fault occurs, the direction of power flow can reverse. This can damage equipment and cause outages.
Directional relays are used to detect reverse power flow and to trip the circuit breaker to protect the system.
Option B, current, is not correct because directional relays are not based on the flow of current. Current is the flow of electrical charge. It is measured in amperes.
Option C, voltage wave, is not correct because directional relays are not based on the shape of the voltage wave. The voltage wave is the graph of the voltage as a function of time.
Option D, all of the above, is not correct because directional relays are not based on the flow of power, current, or voltage wave. They are based on the direction of power flow.