Carrier current and bus-bar protection

i) Carrier current protection

Carrier current protection is a unit protection for long transmission lines in which information about the currents at both ends is sent over the power line itself using a high-frequency carrier signal (50–500 kHz). It allows high-speed, simultaneous tripping at both ends for faults anywhere on the line, where pilot wires would be too long.

Equipment at each end

• Transmitter and receiver (carrier set) producing and detecting the HF signal.
• Coupling capacitor: connects the carrier set to the HV line and blocks power-frequency voltage.
• Line trap (wave trap): a parallel LC circuit tuned to the carrier frequency, in series with the line; it confines the carrier to the protected section and passes 50 Hz.
• Relays: directional or distance relays and a fault detector.

 Bus A                                        Bus B
  │  CB  Line trap ═══════ Line ═══════ Line trap  CB  │
  ├──□───[LC]──┬────────────────────────┬──[LC]───□──┤
               │                        │
          Coupling cap.            Coupling cap.
               │                        │
       Transmitter/Receiver      Transmitter/Receiver
               │                        │
            Relays ───── trip ───── Relays

Directional comparison (blocking) scheme

1. Directional relays at both ends see the fault direction.
2. For an internal fault, power flows into the line at both ends; both relays see "forward" and no blocking carrier is sent; both breakers trip.
3. For an external fault, the relay at the end nearest the fault sees power flowing out of the line; it starts its transmitter and sends a blocking signal to the remote end, which prevents tripping.

Phase comparison scheme: the phase of currents at the two ends is compared via the carrier; in-phase (internal fault) → trip; 180° out of phase (through fault) → block.

Advantages: fast (1–3 cycles), simultaneous tripping at both ends, permits high-speed auto reclosing, no separate communication channel needed; the carrier can also be used for telephony and telemetry.

ii) Bus-bar protection

Bus-bar protection detects faults on the busbars of a substation and trips all breakers connected to the faulty bus section. Busbar faults are rare but severe, so the scheme must be fast and very stable for external faults.

Methods

1. Back-up protection by remote relays: the bus is cleared by the time-delayed relays of the connected circuits; simple but slow.
2. Fault bus (frame leakage) protection: used for metal-clad switchgear; the frame is insulated from earth and earthed through a single CT and relay; an earth fault to the frame trips all breakers.
3. Differential protection (most common): based on Kirchhoff's current law: the sum of currents entering a healthy bus is zero.

   Feeder1     Feeder2     Feeder3    Source
     │CT          │CT        │CT       │CT
 ════╪════════════╪══════════╪═════════╪════ Bus
     │            │          │         │
     └─────┬──────┴────┬─────┴────┬────┘
           │  CT secondaries paralleled
           └────────[ R ]─────────┘
                differential relay

• CTs of the same ratio are placed in every circuit connected to the bus, and their secondaries are connected in parallel with the relay.
• Normal load or external fault: ΣI = 0, so no current flows in the relay.
• Bus fault: all circuits feed the fault; ΣI = total fault current, and the relay trips all breakers on that bus.
• High impedance differential scheme is widely used: a high-resistance relay with a stabilising resistor prevents false tripping when a CT saturates during a heavy external fault. Biased (percentage) differential relays are also used.
• Large stations divide the bus into zones with separate protection, plus a check zone to avoid false trips.