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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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For protection of parallel feeders fed from one end the relays required at load end are
Non directional
Directional
Both A and B
None of above
Non directional
Quick Summary: In parallel feeder systems fed from a single source, the load end requires directional relays to ensure selectivity. Directional relays are sensitive to the direction of power flow, allowing the protection system to isolate only the faulty feeder while keeping the healthy feeder in operation.
In parallel feeder systems fed from a single source, the load end requires directional relays to ensure selectivity. Directional relays are sensitive to the direction of power flow, allowing the protection system to isolate only the faulty feeder while keeping the healthy feeder in operation.
P=VIcosϕ — The power flow principle used by directional relays to detect fault direction
When a fault occurs on one of the parallel feeders, current flows from the source into the fault from both directions at the load end. Without directional elements, a non-directional relay at the load end might trip for a fault on the parallel line, causing unnecessary outage of healthy circuits. The directional relay utilizes the phase relationship between voltage and current to sense power flow direction, ensuring the relay operates only when current flows toward the load bus.
Directional relays respond to the direction of power flow.
Parallel feeders are used to increase reliability and reduce line impedance.
Load end relays must discriminate between faults on their respective feeders.
Non-directional relays are sufficient at the source end for radial configurations.
Improved selectivity in complex network topologies.
Minimizes unnecessary tripping of healthy feeders during fault conditions.
Higher complexity compared to non-directional relays.
Requires both voltage and current references, increasing sensing hardware costs.
Ring main distribution systems.
Parallel transmission lines.
Interconnected power grids.
At the source end, overcurrent relays are typically non-directional because faults on either parallel line appear in the same direction from the source.
Option A (Non-directional) is incorrect for the load end as it cannot distinguish which of the two parallel lines is faulted, leading to loss of supply for the entire load.
B is correct — Directional relays are required at the load end to ensure that only the faulty feeder is disconnected, maintaining service continuity on the healthy parallel feeder.
Always remember the 'Directional' rule: use them when current can flow in either direction through the protected element under fault conditions.