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GATE 2023 EE Q.20: solution and answer

GATE 2023 · EEQ.20 · 1 mark · MCQ

For the three-bus power system shown in the figure, the trip signals to the circuit breakers B1 to B9 are provided by overcurrent relays R1 to R9, respectively, some of which have directional properties also. The necessary condition for the system to be protected for short circuit fault at any part of the system between bus 1 and the R-L loads with isolation of minimum portion of the network using minimum number of directional relays is

GATE 2023 EE question 20: Single-line diagram.Tap to open full size
Figure from the GATE 2023 EE paper. It is part of the question.
Figure description

Single-line diagram. A generator at the left connects through breaker B9 to bus 1. Two parallel lines run from bus 1 to bus 2: Line 1 (top) has B1 at the bus 1 end and B3 at the bus 2 end; Line 2 (bottom) has B2 at the bus 1 end and B4 at the bus 2 end. At bus 2, breaker B5 leads to Line 3, which runs to bus 3 with breaker B7 at the bus 3 end. Breaker B6 at bus 2 feeds an R-L load. At bus 3, breaker B8 feeds another R-L load. The only source is the generator at bus 1.

  1. (A)

    R3 and R4 are directional overcurrent relays blocking faults towards bus 2

  2. (B)

    R3 and R4 are directional overcurrent relays blocking faults towards bus 2 and R7 is directional overcurrent relay blocking faults towards bus 3

  3. (C)

    R3 and R4 are directional overcurrent relays blocking faults towards Line 1 and Line 2, respectively, R7 is directional overcurrent relay blocking faults towards Line 3 and R5 is directional overcurrent relay blocking faults towards bus 2

  4. (D)

    R3 and R4 are directional overcurrent relays blocking faults towards Line 1 and Line 2, respectively.

Answer and solution

ANSWER

Option A: only R3 and R4 need to be directional, and they block faults towards bus 2.

Official answer: ASource: Official answer key, GATE 2023 EE (IIT Kanpur)

  1. The only source is the generator at bus 1, so in normal operation power flows from bus 1 to bus 2 and then to bus 3 and the two R-L loads.

  2. Line 3 and bus 3 are radial. A fault there is fed from one side only, so the fault current always passes R5 and R7 in the same direction. Ordinary time-graded overcurrent relays are enough, so R5 and R7 need no direction.

  3. The two parallel lines are different. For a fault on Line 1, current reaches the fault from bus 1 through B1. It also reaches the fault from bus 1 through Line 2, bus 2 and then back into Line 1 through B3.

  4. So R3 sees fault current flowing from bus 2 into Line 1, which is against the normal flow. R4 sees the same reversed flow for a fault on Line 2.

  5. Without direction, R4 would also trip for a Line 1 fault, and R3 for a Line 2 fault. That would isolate both lines and cut the supply to bus 2. To isolate only the faulty line, R3 and R4 must operate only for current flowing from bus 2 into their own line.

  6. A fault at bus 2 or beyond (Line 3, bus 3, the loads) drives current from the line toward bus 2 through R3 and R4. That is the opposite direction, and R3 and R4 must stay stable for it. This means R3 and R4 are directional relays that block faults on the bus 2 side, which is the side behind each relay.

  7. Option D reverses the blocking direction. Blocking faults towards Line 1 and Line 2 would stop R3 and R4 from tripping for faults on their own lines. Options B and C add directional relays R7 and R5, which step 2 showed are not needed, so they do not use the minimum number of directional relays. Option A is the answer.

KEY IDEA

Directional relays are needed only where fault current can reach the relay from both sides. Here that is the bus 2 end of each parallel line, where the other line can feed a fault through bus 2. The radial part (Line 3 and bus 3) needs none.

WATCH OUT FOR

Adding directional relays on the radial Line 3 (R5, R7) because they feel safer. With a single source they are not needed, and they break the "minimum number of directional relays" condition. Another slip is reading "blocking towards bus 2" as the upstream direction.

From the GATE 2023 EE paper. Every question of this paper · Download the full paper · All 5 Protection questions, with notes

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