Restricted earth fault protection

Restricted earth fault (REF) protection is a sensitive unit protection that detects earth faults inside a defined zone only, usually a star-connected transformer or generator winding. It compares the sum of the three phase currents with the neutral current.

Principle

• CTs are placed in each phase and in the neutral-to-earth connection, all of the same ratio.
• The three phase CTs are connected in parallel (residual connection) and then in parallel with the neutral CT, with a relay across them.
• External earth fault (outside the zone): the residual current from the phase CTs equals the neutral CT current, so they circulate between CTs and no current flows through the relay.
• Internal earth fault (inside the winding): the neutral current has no matching residual current, so the difference flows through the relay and it trips.

        A ---[CT]-----+---- to system
        B ---[CT]-----+
        C ---[CT]-----+
   Star |   |   |     |
  winding \ | /       |  residual
         \|/          |
     N ---+--[CT_N]---+---[ R ]--+
          |               relay  |
         earth   (high-impedance, with
                 stabilising resistor
                 and Metrosil)

Features

• Usually a high-impedance relay with a stabilising resistor so CT saturation on heavy external faults does not cause false trips. A non-linear resistor (Metrosil) limits voltage during internal faults.
• Very sensitive: it covers faults close to the neutral end, where the ordinary transformer differential (87T) is not sensitive enough. On a resistance-earthed winding a fault near the neutral gives very little phase current.
• Fast and stable.

Use

• HV star winding of the generator step-up transformer (solidly earthed side).
• Star winding of station/auxiliary transformers.
• Small generators with resistance earthing.

For a delta winding, REF is applied using an earthing transformer or the three phase CTs alone (balanced earth fault protection).