Distance Protection Questions and Answers

Answer: A

The two sources drive a current through the series path \(Z_A + Z_L + Z_B = 150\ \Omega\). With \(E_A\) and \(E_B\) in phase,

\[ I = \frac{430 - 330}{150} = 0.667\ \text{A} \]

The relay voltage is that of the bus after \(Z_A\):

\[ \begin{aligned} V = E_A - IZ_A = 430 - 0.667 \times 50 \\ &= 396.7\ \text{V} \end{aligned} \]

\[ \begin{aligned} Z_{relay} = \frac{V}{I} = \frac{396.7}{0.667} \\ &= 595\ \Omega \end{aligned} \]

Equivalently, \(Z = Z_L + Z_B + E_B/I = 100 + 495 = 595\ \Omega\). The answer is option A.

Answer: B

An impedance relay is an over current relay with a voltage restraint. In its operation the current produces the operating (positive) torque, tending to close the contacts, while the voltage produces the restraining torque, which opposes the closing. So the voltage produces a negative torque, and the relay operates when the ratio of voltage to current, the impedance, falls below the setting. The answer is option B.

Answer: D

A mho relay is a distance relay with a circular characteristic that passes through the origin, and it compares an operating quantity with a restraining quantity, with a polarising input that gives it direction. So its inputs are the operating input, the restraining input and the polarising input. Galvanic input is not one of the inputs of a mho relay (galvanic isolation is a property of circuits and not a relay input). The answer is option D.

Answer: D

The mho relay is usually employed for long transmission lines.

The mho (admittance) relay is a distance relay whose operating characteristic is a circle through the origin on the impedance diagram. It is inherently directional and covers the smallest area of the R-X plane for a given reach, so it is the least affected by power swings, which are a serious problem on long, heavily loaded lines.

The usual textbook matching is: reactance relay for short lines, where arc resistance would upset other relays; impedance relay for medium lines; mho relay for long lines. Transformers are protected mainly by differential relays, not distance relays.