GATE EE 2023 Set 1 — Question 20
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Power Systems → Power System Protection → Differential & Directional
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Question
For the three-bus power system shown in the figure, the trip signals to the circuit breakers to are provided by overcurrent relays to , 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
Correct answer
(A) R₃ and R₄ are directional overcurrent relays blocking faults towards bus 2
Solution
In a power system with parallel feeders (Line 1 and Line 2), directional overcurrent relays are required at the receiving end (load end) to ensure selectivity.
1.Parallel Feeders: For a fault on Line 1, fault current flows from the source at Bus 1 through . Additionally, current flows through the healthy Line 2 to Bus 2 and then back through to the fault.
2.Selectivity: To isolate only the faulted Line 1, relay must trip, while must remain stable. If and were non-directional, both might trip for a fault on either line.
3.Directional Placement: By making and directional such that they trip only for faults in the direction of the lines (away from Bus 2) and block for faults towards Bus 2, we ensure that for a fault on Line 1, sees a forward fault and trips, while sees a reverse fault (towards the bus) and blocks.
4.Radial Feeders: Line 3 is a radial feeder. Non-directional relays at and are sufficient for its protection when coordinated with time-grading.
Therefore, the minimum requirement for directional relays is that and are directional, blocking faults towards Bus 2.Continue learning with Success Tracker
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