Combined phase and earth fault protection of generator

In the combined phase and earth fault protection scheme, a single set of CTs on each side of the generator winding feeds a differential (Merz-Price) scheme for phase faults and, through the residual (neutral) connection of the same pilots, an earth fault relay. This saves CTs and covers both types of fault.

Connection diagram

 Neutral-side CTs      Stator         Line-side CTs
  (CT)──────────────── R ───────────────(CT)
  (CT)──────────────── Y ───────────────(CT)
  (CT)──────────────── B ───────────────(CT)
   │││   star pt                         │││
   │││     │                             │││
   │││   R_n (earthing)                  │││
   │││     ⏚                             │││
   ││└───── pilot B ──────┬───────────────┘││
   │└────── pilot Y ──────┼──┬─────────────┘│
   └─────── pilot R ──────┼──┼──┬───────────┘
                         [P][P][P]  phase relays
                          │  │  │  (87)
                          └──┴──┴───[E]── neutral
                                  earth fault
                                  relay (64)

Arrangement

• Identical CTs at the neutral end and the line end of each phase; the two CT groups are each connected in star.
• Pilot wires join corresponding phases, forming a circulating-current loop.
• Two (or three) phase-fault relays (P) are connected between the pilots and a common star point.
• An earth-fault relay (E) is connected between this common point and the star point of the CTs (neutral pilot), so it carries the residual current.

Operation

• Normal and external faults: currents at both ends are equal, the secondary currents circulate and no current flows in any relay.
• Phase-to-phase fault inside the winding: the out-of-balance current returns through the phase relays of the faulted phases, which trip.
• Earth fault inside the winding: the unbalance current returns through the earth-fault relay (with a lower setting), giving sensitive earth fault protection.
• The relays trip the generator breaker, field breaker and prime mover.