Question

Which relay is most affected by power swings?

• A. Mho Relay
• B. Differential Relay
• C. Over-current Relay
• D. Buchholz Relay

Hint:

Among the three main distance relays, the susceptibility to power swings is: \[ \text{Impedance Relay} > \text{Mho Relay} > \text{Reactance Relay} \] To prevent incorrect tripping during power swings, distance relays are equipped with "Power Swing Blocking" (PSB) logic.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks to identify which of the given protection relays is most susceptible to maloperation (tripping incorrectly) during power swings in a power system.

Step 2: Detailed Explanation:

• A power swing refers to transient oscillations in rotor angles and active power flow across a transmission grid following a major disturbance (such as a fault clearance or line switching).

• During a power swing, the voltage magnitudes and phase angles at various buses fluctuate, causing the apparent impedance (\(Z_{\text{apparent}} = \frac{V}{I}\)) measured by distance relays at transmission line terminals to swing.

• If the trajectory of this apparent impedance enters the operating characteristic zone of a distance relay on the R-X plane, the relay may misinterpret the swing as a line fault and trip the line.

• Distance relays (like Mho, Impedance, and Reactance relays) are directly affected by these impedance variations. Among the options listed, the Mho relay is a distance relay and is therefore highly affected by power swings.

• Other relays like Differential relays, Over-current relays, and Buchholz relays (used for internal transformer faults) do not measure terminal impedance and are generally immune to power swings.

Step 3: Final Answer:
The Mho relay is most affected by power swings.