Need of VT and CCVT construction

Why voltage transformers are required in protection

A voltage (potential) transformer reduces the system voltage to a standard value (110 V line or 110/√3 V phase) for relays and meters.

1. Many relays need voltage as an input: distance (impedance) relays, directional overcurrent and directional earth-fault relays, under/over-voltage relays, under/over-frequency relays, reverse power and loss-of-excitation relays, and synchronism-check relays.
2. Relays cannot be connected directly to 11 kV–400 kV lines; the VT gives isolation and safety.
3. Standard low-voltage relays can be used for any system voltage.
4. Open-delta VT connection provides the residual voltage (3V₀) for earth-fault detection and directional earth-fault relays.

Construction of CCVT (coupling capacitor voltage transformer)

 HV line
   │
  ═╪═ C1 ┐
   │     │ capacitor stack in
  ═╪═ C2 ┘ porcelain housing
   ├──── tap V₂ (≈10–20 kV)
   │       │
   │      [L] tuning reactor
   │       │
   │     ┌─┴──┐
   │     │IVT │→ 110/√3 V to relays
   │     └────┘   + ferro-resonance damper
  [Drain coil]──► PLC carrier equipment
   │
  ═══ earth

1. Capacitor divider: series-connected oil-impregnated capacitor elements (C1 high-voltage part, C2 low-voltage part) in a porcelain/polymer column. Tap voltage = V × C1/(C1 + C2).
2. Tuning reactor (L): in series with the IVT primary; resonates with C1 + C2 at 50 Hz (ωL = 1/ω(C1+C2)) so the ratio and phase do not change with burden.
3. Intermediate VT: small oil-immersed transformer stepping the tap voltage to the standard secondary.
4. Ferro-resonance suppression circuit across the secondary.
5. Drain coil and carrier terminal: low impedance at 50 Hz, high at carrier frequency; allows the capacitor stack to couple PLC signals to the line (hence "coupling capacitor").
6. Oil-filled base tank with terminal box and protective spark gap.

CCVTs are used at 66 kV and above because they are cheaper than electromagnetic VTs and also serve carrier communication.