Microprocessor relaying via generator field current

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

 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.

 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.