Question

In which relay, the relay operation depends upon the ratio of voltage to current?

• A. directional
• B. differential
• C. distance
• D. reverse

Hint:

Remember that Distance relays are classified primarily into Impedance relays, Mho relays, and Reactance relays, all of which dynamically assess various mathematical characteristics of the fundamental ratio $\frac{V}{I}$.

Answer 1

The Correct Option is C

Concept: Protective relays operate by monitoring electrical characteristics and responding when specific thresholds are breached. A distance relay measures the ratio of voltage ($V$) to current ($I$) at the point where the relay is installed in the system. By Ohm's Law, this ratio directly equals the electrical impedance ($Z$): \[ Z = \frac{V}{I} \] Because the impedance of a transmission line is directly proportional to its physical length, measuring the electrical impedance provides an accurate estimation of the distance to a system fault.

Step 1: Define the operational behavior of a Distance Relay.
Distance relays (or impedance relays) are structured to compare the local voltage against the local current. Under sound operating network conditions, the ratio $\frac{V}{I}$ remains high because the load impedance is high. However, when a short-circuit fault occurs along the protected transmission line, the line voltage drops significantly, and the system current surges. Consequently, the measured ratio $\frac{V}{I}$ falls below a predetermined setting.

Step 2: Contrast with alternative options listed.
* Directional Relay: Operates based on the phase angle relationship between voltage and current to sense the direction of power flow. * Differential Relay: Operates based on the vector difference between the currents entering and leaving a protected zone. * Reverse Relay: Operates when power flows in a direction opposite to the intended configuration. Therefore, only the distance relay relies directly on the absolute ratio of voltage to current. This aligns with Option (C).