Unit protection scheme

A unit protection scheme protects a clearly defined zone (a generator, transformer, busbar or line section) by comparing electrical quantities at all the boundaries of the zone. It operates only for faults inside its zone and is inherently selective, so it can trip instantaneously without time grading. Differential protection (Merz-Price) is the main example.

Circuit diagram (circulating current differential)

           ┌──── Protected zone ────┐
  I₁ ──(CT1)───── equipment ─────(CT2)── I₂
         │                         │
      i₁ │      pilot wire         │ i₂
         ├─────────────────────────┤
         │           │             │
         │        [ Relay ]        │
         │     (i₁ − i₂ flows)     │
         ├─────────────────────────┤
              pilot wire

Operating principle

• Identical CTs are placed at both ends (or all terminals) of the zone, and their secondaries are connected through pilot wires so that their currents circulate.
• The relay is connected across the pilots at the equipotential point and carries the difference of the secondary currents (i₁ − i₂).
• Normal load or external (through) fault: current entering = current leaving (I₁ = I₂), so i₁ = i₂, no current flows in the relay and it does not operate, however large the through current.
• Internal fault: current entering ≠ current leaving (fault current is fed from one or both ends), so i₁ − i₂ flows in the relay. When it exceeds the setting the relay trips the breakers at all ends of the zone.
• To avoid false trips due to CT mismatch and saturation, percentage bias or high-impedance relays are used.

Types

• Circulating current (Merz-Price) differential.
• Balanced (opposed) voltage scheme (Translay, for feeders).
• Pilot wire and carrier (phase comparison) protection for lines.
• Restricted earth fault and frame leakage protection.

Applications

• Generators: stator phase and earth faults (biased differential).
• Transformers: differential protection with CTs in star/delta to correct phase shift.
• Busbars: high-impedance differential protection.
• Feeders and lines: pilot wire for short lines, carrier current/fibre-optic current differential for long lines.
• Large motors.

Merits and demerits

• Fast, sensitive and absolutely selective; no time grading needed.
• Does not give back-up for adjacent zones, so separate back-up (e.g. overcurrent) protection is required.
• Needs pilot or communication links and matched CTs.