Directional static overcurrent relay

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

     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

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.