Microprocessor-based transmission line protection

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

 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.