--- title: "Directional static overcurrent relay" subject: "Switchgear and Protection" chapter: 8 key: "semester_6/switchgear_and_protection:directional-static-overcurrent" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/directional-static-overcurrent site: BCT Engineering --- # Directional static overcurrent relay Switchgear and Protection · Chapter 8: Static and Digital Relays - Asked in: 2075 Baisakh, 8 marks; 2079 Chaitra, 8 marks (asked 2 times) - Video: [Directional Static Over Current Relay](https://www.youtube.com/watch?v=JhTY9TUo6k8) — Engineering Science Videos - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/directional-static-overcurrent - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-8 ## Question With the help of neat block diagram, explain the function of directional static overcurrent relay. ## Short answer A directional static overcurrent relay trips only when current exceeds the set value and fault power flows in the tripping direction. It combines a static overcurrent unit with a phase comparator that checks the angle between current and polarizing voltage. The block diagram includes CT/PT inputs, phase-shift network, squaring circuits, coincidence-type phase comparator (operate if coincidence >5 ms), overcurrent level detector with timer, AND logic, and output amplifier driving the CB trip coil. ## Answer A **directional static overcurrent relay** trips only when (a) the current exceeds the set value and (b) the power (fault current) flows in the tripping direction. It combines a static overcurrent unit with a static **phase comparator** that checks the angle between the current and a reference (polarising) voltage. #### Block diagram ```text CT PT │ │ [Aux CT] [Aux PT] │ │ ├──────────┐ [Phase shift τ] │ │ │ [Rectifier] [Squaring] [Squaring] │ └────┬───────┘ [Level det.] [Phase comparator] │ (coincidence > 5 ms) [Timer] │ └──────┬────────┘ [AND] │ [Amplifier] │ [Output relay] ──► CB trip coil ``` #### Function of the blocks 1. **Auxiliary CT and PT:** reduce current and voltage to electronic levels and isolate the circuit. 2. **Phase-shift network:** shifts the polarising voltage by the relay characteristic angle (e.g. 30° or 45°) so that maximum sensitivity occurs at the typical fault angle. 3. **Squaring (zero-crossing) circuits:** convert current and voltage to square waves. 4. **Phase comparator (coincidence circuit):** an AND gate measures the time for which both square waves are positive. If coincidence > 5 ms (in a 50 Hz system, i.e. angle within ±90°), the fault is in the forward direction and a directional output is given. 5. **Overcurrent unit:** the current is rectified and compared by a level detector with the plug setting; an RC/digital timer gives definite or inverse time delay. 6. **AND logic:** a trip signal is given only when both the directional and the overcurrent conditions are true. 7. **Amplifier and output:** drive the auxiliary relay/thyristor which energises the CB trip coil. #### Principle ```text Forward fault: −90° < (θ − τ) < +90° → operate Reverse fault: angle outside ±90° → block ``` #### Applications Ring mains, parallel feeders, and lines fed from both ends; generator and transformer reverse-power or back-feed protection; directional earth-fault protection. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. When does a directional static overcurrent relay trip? A. Current exceeds setting and flows in the tripping direction B. Whenever current flows in any direction C. Only when voltage is zero D. Only when current is below setting Answer: A. Both the overcurrent and directional conditions must be satisfied, combined in an AND gate. (Key point: Trip conditions) 2. How does the coincidence phase comparator detect a forward fault? A. Squared current and voltage waves overlap (both positive) for more than 5 ms B. Their magnitudes are equal C. Overlap is less than 1 ms D. The voltage is zero Answer: A. A coincidence of more than 5 ms at 50 Hz means the angle is within +/-90 degrees. (Key point: Phase comparator) 3. Why is a phase-shift network used on the polarising voltage? A. To set the relay characteristic angle so sensitivity is maximum at the typical fault angle B. To reduce the voltage to zero C. To rectify the voltage D. To delay tripping by 1 s Answer: A. Shifting by tau (e.g. 30 or 45 degrees) centres the operating region on the usual fault angle. (Key point: Phase-shift network) 4. What is the purpose of the AND gate before the amplifier? A. Trip only when both directional and overcurrent units operate B. Trip when either unit operates C. Generate square waves D. Supply DC power Answer: A. A reverse-direction fault or low current leaves one input false, so no trip. 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