--- title: "Microprocessor-based transmission line protection" subject: "Switchgear and Protection" chapter: 8 key: "semester_6/switchgear_and_protection:microprocessor-line-protection" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/microprocessor-line-protection site: BCT Engineering --- # Microprocessor-based transmission line protection Switchgear and Protection · Chapter 8: Static and Digital Relays - Asked in: 2071 Magh, 8 marks; 2075 Baisakh, 8 marks (asked 2 times) - Video: [SGP Lec 41: Microprocessor Based Distance Relay](https://www.youtube.com/watch?v=qsyGT6hPxrg) — Polytech Plus - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/microprocessor-line-protection - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-8 ## Question Draw the block diagram of microprocessor based protective scheme for protection of transmission line. ## Short answer A microprocessor-based line protection scheme samples line voltages and currents via CTs/PTs, scales and filters them, then converts to digital via multiplexed ADC. The microprocessor computes phasors using DFT, calculates apparent impedance for each zone, and compares with mho/quadrilateral characteristics. Zone 1 trips instantly; zones 2 and 3 trip after set delays. Output drivers operate auxiliary relays to trip the breaker, with event recording and SCADA communication. ## Answer In a **microprocessor-based line protection scheme**, the voltages and currents of the transmission line are sampled, converted to digital form and processed by a microprocessor that computes the apparent impedance (or other quantities) and issues a trip when a fault lies inside a protected zone. Usually it works as a **distance (impedance/mho/quadrilateral) relay** with three zones. #### Block diagram ```text Transmission line ══CB══════════════════ │ CT (Ia,Ib,Ic) │ PT (Va,Vb,Vc) ▼ ▼ ┌──────────────────────────────────┐ │ Aux CTs/PTs, surge suppressors │ │ (isolation, scaling to ±10 V) │ └───────────────┬──────────────────┘ [I→V converters] │ [Anti-aliasing LP filters] │ [Sample & hold circuits] │ [Analog multiplexer] ◄─ control │ [A/D converter] │ ┌───────────────▼──────────────────┐ │ MICROPROCESSOR / DSP │ │ ROM: program RAM: samples │ │ EEPROM: zone settings │ └──┬────────┬────────┬────────┬────┘ │ │ │ │ [Timer] [Output [Display/ [Comm. interface] keypad] port] │ [Driver + aux relay]──► CB trip coil [Digital inputs: CB status, carrier] [DC-DC power supply from battery] ``` #### Working 1. **Input stage:** CT and PT outputs pass through auxiliary transformers which isolate and scale them; current signals are converted to voltages across precision resistors. 2. **Anti-aliasing filter:** removes components above half the sampling frequency. 3. **Sample-and-hold + multiplexer:** all six signals are sampled at the same instant (e.g. 12–20 samples per cycle) and passed one by one to the ADC. 4. **ADC:** gives 12–16 bit binary numbers to the microprocessor. 5. **Microprocessor program:** - Computes phasors of V and I by Fourier (DFT) or least-squares algorithms. - Computes apparent impedance Z = V/I (R and X) for each phase and phase pair, with residual compensation for earth faults. - Compares Z with zone 1, zone 2 and zone 3 characteristics (mho or quadrilateral). - Zone 1: instantaneous trip; zone 2 and zone 3: trip after timer delays (e.g. 0.3 s and 1 s). - Checks power swing, VT fuse failure, and carrier signals. 6. **Output interface:** opto-isolated drivers operate auxiliary relays to trip the breaker and give alarms. 7. Records events and disturbance data; communicates with SCADA. **Advantages:** one hardware for many characteristics, self-checking, fault location, low burden and easy setting changes. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does the microprocessor compute to protect a transmission line? A. Apparent impedance Z = V/I, compared with zone characteristics B. Only the frequency C. The line temperature D. Only the CT ratio Answer: A. It works as a distance relay with zone 1, 2 and 3 comparisons. (Key point: Scheme function) 2. What is the correct order of the front-end? A. Aux CT/PT, anti-aliasing filter, sample-and-hold, multiplexer, ADC B. ADC, filter, aux CT/PT, multiplexer C. Multiplexer, sample-and-hold, ADC, filter D. Sample-and-hold, aux CT, ADC, filter Answer: A. Signals are scaled, band-limited, sampled simultaneously, multiplexed and digitised. (Key point: Signal chain order) 3. Which algorithms estimate the phasors of V and I? A. Fourier (DFT) or least-squares B. Binary search C. Bubble sort D. Huffman coding Answer: A. Fourier or least-squares algorithms extract the fundamental phasors used to compute R and X. (Key point: Phasor algorithms) 4. How are the zones treated? A. Zone 1 instantaneous; zones 2 and 3 after timer delays such as 0.3 s and 1 s B. All zones instantaneous C. Zone 1 delayed 1 s D. Only zone 3 is used Answer: A. Delayed outer zones coordinate with adjacent line protection. (Key point: Zone timing) ## Related in Switchgear and Protection - [Protection requirements; protective system vs scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aprotective-system-vs-scheme) — Switchgear and Protection, chapter 1 - [Unit protection scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aunit-protection-scheme) — Switchgear and Protection, chapter 7 - [Microprocessor relaying via generator field current](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Amicroprocessor-generator-field) — Switchgear and Protection, chapter 8 - [Basic requirements of a protection scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Abasic-requirements-protection-scheme) — Switchgear and Protection, chapter 1, asked 3× - [Converting IDMT scheme to differential protection](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-to-differential-transformer) — Switchgear and Protection, chapter 7 - [Knee point voltage; measuring vs protective CT](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aknee-point-measuring-protective-ct) — Switchgear and Protection, chapter 2 ## Related in other subjects - [Microprocessor based system and bus organization](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Amp-based-system-bus-organization) — Microprocessor, chapter 1, asked 5× - [Features of microprocessor based instrumentation](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ambi-features) — Instrumentation II, chapter 1, asked 4× - [Types of microprocessor based instrumentation systems](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ambi-types) — Instrumentation II, chapter 1 - [MBI block diagram and basic features](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ambi-block-features) — Instrumentation II, chapter 1, asked 4×