Percentage differential protection of alternator

Percentage (biased) differential protection of an alternator compares the current entering and leaving each phase winding; it trips when the difference exceeds a fixed percentage of the through current. It protects the stator against phase-to-phase and phase-to-earth faults (Merz-Price scheme with bias).

Why "percentage" (biased) is needed

In a simple differential (circulating current) scheme, the two CTs of a phase never match perfectly. During heavy external faults, CT saturation and ratio errors make a spill current flow in the relay, which can cause a false trip. A restraining (bias) coil carrying the through current raises the operating current in proportion to the load, so the relay stays stable on external faults but is sensitive to internal faults.

Arrangement

 Neutral side                     Line side
   CT1  ┌──── stator winding ────┐  CT2
 ──(○)──┤                        ├──(○)──► to bus
   │ I₁                        I₂ │
   │                              │
   └──R/2──┬───────────┬──R/2────┘
           │ restraining coil R  │
           └────► O ◄─────────────┘
           operating coil (mid-point)
 (one such set per phase; star point of
  CTs connected to the relay neutral)

• Identical CTs are placed at the neutral end and the line end of each phase winding.
• CT secondaries are connected so that their currents circulate in the pilot wires.
• The restraining coil is connected in the pilot wires (carries (I₁ + I₂)/2).
• The operating coil is connected between the mid-point of the restraining coil and the CT star point (carries I₁ − I₂).

Operation

• Normal load or external fault: I₁ = I₂, so the operating coil current is nearly zero. Any small spill current due to CT mismatch is overcome by the large restraining torque.
• Internal fault: current in the faulty winding differs at the two ends (or one end gets reversed current when the generator is in parallel with others). A large differential current I₁ − I₂ flows in the operating coil, its torque exceeds the restraining torque and the relay trips the generator breaker and field breaker.

Operating condition:

|I₁ − I₂| > K × (|I₁| + |I₂|)/2  + I₀
K = bias slope (typically 5–20%), I₀ = minimum pick-up

Characteristic

 Operating  │        /  Trip zone
 current    │       /
 |I₁−I₂|    │      /  slope K
            │     /
            │____/   No-trip (restrain) zone
            └──────────────────────
              through current (I₁+I₂)/2

Features

• Protects about 85–90% of the winding against earth faults (when neutral is earthed through resistance, faults near the neutral give little current); a separate restricted earth fault or 100% stator earth fault relay covers the rest.
• Fast, selective; unaffected by load or external faults.
• Needs CTs of identical characteristics; pilot wires are short since both CTs are near the machine.