Restricted earth fault for generator

Restricted earth fault (REF) protection is a unit protection that responds only to earth faults inside the generator stator winding (the zone between the neutral CT and the line CTs). It is not affected by external earth faults.

Connection

• Three line CTs (one per phase at the generator terminals) have their secondaries connected in parallel (residual connection); their sum is the residual current 3I₀ on the line side.
• One CT of the same ratio is placed in the neutral-to-earth connection of the solidly grounded star point.
• The residual circuit of the line CTs and the neutral CT secondary are connected so that their currents circulate; an earth fault relay (a high-impedance relay with stabilising resistor R_st and a non-linear resistor/metrosil) is connected across them.

                Generator stator          Line CTs
          ┌──── R winding ──────────────(CT)──── R
   star ──┼──── Y winding ──────────────(CT)──── Y
   point  └──── B winding ──────────────(CT)──── B
     │                                   │ │ │
   (CT) neutral CT                       └─┼─┘ secondaries
     │                                     │   in parallel
    ═╧═ earth (solid)                      │
                                           │
   neutral CT sec ──────────┬──────────────┘
                            │
                         [R_st]
                            │
                         [ 64 ] REF relay
                            │
   common return ───────────┴── other ends of all CT secondaries

Operation

• Normal or external earth fault: the residual current of the line CTs equals the current in the neutral CT; the two secondary currents circulate and no current flows in the relay.
• Internal earth fault: fault current flows through the neutral CT but not through the line CTs (or flows in the opposite sense if other sources feed it); the unbalance current passes through the relay, which trips the generator breaker and field breaker.
• Since the neutral is solidly grounded, the earth fault current is large and almost the whole winding (about 90–95%) is protected.