--- title: "Microprocessor relaying via generator field current" subject: "Switchgear and Protection" chapter: 8 key: "semester_6/switchgear_and_protection:microprocessor-generator-field" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/microprocessor-generator-field site: BCT Engineering --- # Microprocessor relaying via generator field current Switchgear and Protection · Chapter 8: Static and Digital Relays - Asked in: 2072 Magh, 8 marks - 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-generator-field - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-8 ## Question Describe with neat block diagram, the microprocessor based relaying scheme for the protection of generator by monitoring the field current of the alternator. ## Short answer A microprocessor-based loss-of-field protection monitors the generator field current via a shunt or Hall sensor. The signal is isolated, filtered, scaled, sampled, and digitized. The microprocessor compares the field current with a set minimum (20–40% of no-load value); if below for a set delay (0.5–2 s), it issues an alarm and trips the generator and field circuit breakers. The scheme includes self-checking, data logging, and can add undervoltage or reactive power confirmation. ## Answer **Loss-of-field (field failure) protection** of an alternator can be done by a microprocessor that continuously monitors the field (excitation) current. If the field current falls below a set minimum (field open, short-circuited, or AVR/exciter failure), the generator loses synchronism and draws large reactive power from the system; the scheme gives an alarm and trips the generator after a short delay. #### Block diagram ```text Exciter ──► Field winding of alternator │ [Shunt / Hall-effect current sensor in field circuit] │ V ∝ I_f [Isolation amplifier / filter] │ [Signal conditioner (0–5 V)] │ [Sample & hold] ─► [A/D converter] │ ┌─────────────────────────────▼─────┐ │ MICROPROCESSOR │ │ ROM: program RAM: data │ │ Set value I_f(min), time delay │ └──────┬──────────┬───────────┬─────┘ │ │ │ [Output port] [Display] [Clock/timer] │ [Driver + aux relay]──► Alarm └──────────────► Trip gen. CB & field CB ``` #### Working 1. **Sensing:** the DC field current I_f is measured by a shunt (or Hall sensor) giving a small voltage proportional to I_f. An isolation amplifier separates the field circuit from the electronics and filters ripple. 2. **Conversion:** the signal is scaled to the ADC range (e.g. 0–5 V), sampled and converted to a digital number. 3. **Program logic:** - Read the ADC value at regular intervals and convert to field current. - Compare with the stored minimum value I_f(min) (e.g. 20–40% of no-load field current; set from the generator's capability curve). - If I_f ≥ I_f(min): healthy, repeat. - If I_f < I_f(min): start a timer (about 0.5–2 s) to avoid tripping on transient dips or swings. - If the low value persists for the set time: give alarm, then send trip signal to the generator CB and field CB. 4. **Output:** the output port drives a transistor/auxiliary relay that trips the breakers; the display shows field current and status. ```text Start → Read I_f → I_f < I_min ? ─No─► loop │Yes Start/continue timer │ t > t_set ? ─No─► loop │Yes Alarm + Trip CBs ``` **Advantages:** simple, accurate settings, self-checking, and the same microprocessor can also log data and add an under-voltage or reactive-power check to confirm loss of field. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the microprocessor scheme for loss-of-field protection, which quantity is continuously monitored? A. Stator terminal voltage B. Field (excitation) current of the alternator C. Rotor speed D. Stator neutral current Answer: B. The scheme senses the DC field current I_f with a shunt or Hall sensor and trips if it falls below a set minimum. (Key point: What is monitored) 2. Why must the generator be tripped if its field current falls below the minimum value? A. It over-speeds and produces excess active power B. It loses synchronism and draws large reactive power from the system C. Its stator voltage rises dangerously D. Its field winding overheats from excess current Answer: B. Loss of field makes the machine lose synchronism and absorb large reactive power from the system, so it is alarmed and tripped. (Key point: Effect of field loss) 3. What is the correct order of the signal path from the field circuit to the microprocessor? A. Sensor, A/D converter, isolation amplifier, sample and hold B. Isolation amplifier, sensor, A/D converter, signal conditioner C. Sensor, isolation amplifier and filter, signal conditioner, sample and hold, A/D converter D. Signal conditioner, sensor, A/D converter, sample and hold Answer: C. The shunt/Hall sensor voltage is isolated and filtered, scaled to 0-5 V, sampled and held, then digitised before the microprocessor reads it. (Key point: Signal chain) 4. Why does the program start a timer (about 0.5-2 s) when I_f < I_f(min) instead of tripping immediately? A. To let the ADC finish converting B. To avoid tripping on transient dips or swings; trip only if the low value persists C. To allow the field breaker to close D. To charge the output relay Answer: B. A short delay prevents false trips from momentary dips; the generator and field CBs are tripped only if the low value persists for the set time. 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