---
title: "Need of VT and CCVT construction"
subject: "Switchgear and Protection"
chapter: 2
key: "semester_6/switchgear_and_protection:vt-need-ccvt-construction"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/vt-need-ccvt-construction
site: BCT Engineering
---

# Need of VT and CCVT construction

Switchgear and Protection · Chapter 2: Current and Potential Transformers

- Asked in: 2077 Chaitra, 4 marks
- Video: [What is Capacitive Voltage Transformer (CVT) and CCVT? Difference between PT, CVT and CCVT](https://www.youtube.com/watch?v=1QFzO7qtA2A) — The Engineering Power
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/vt-need-ccvt-construction
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-2

## Question

Why voltage transformers are required in protection system? Also describe the construction of CCVT.

## Short answer

Voltage transformers provide isolated, standard-level voltage to protection relays like distance and directional types and supply residual voltage for earth-fault detection. A CCVT comprises a capacitor divider, a tuning reactor resonating at 50 Hz to compensate source impedance, an intermediate VT for 110/√3 V output, a ferro-resonance suppressor, and a drain coil for carrier signals.

## Answer

#### 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)
```text
 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which protection use of a VT is correctly stated?
   A. Distance and directional relays need a voltage input from the VT
   B. Overcurrent relays alone need VTs
   C. VTs increase fault current
   D. VTs are used only for energy meters
   Answer: A. Distance, directional, under/over-voltage, frequency and synchro-check relays all need voltage proportional to the line voltage. (Key point: VT need)

2. What does the open-delta VT connection provide?
   A. Phase-to-phase voltage doubling
   B. Residual voltage (3V0) for earth-fault detection and directional earth-fault relays
   C. Zero current
   D. Carrier coupling
   Answer: B. The broken-delta secondary sums phase voltages giving 3V0 when an earth fault occurs. (Key point: Open-delta)

3. What is the role of the drain coil in a CCVT?
   A. High impedance at 50 Hz, low at carrier frequency
   B. Suppresses ferro-resonance
   C. Low impedance at 50 Hz and high at carrier frequency, so carrier signals are coupled to the line via the capacitor stack
   D. Steps down the tap voltage
   Answer: C. The drain coil drains power-frequency current to earth while forcing carrier signals to the PLC equipment. (Key point: Drain coil)

4. How is the CCVT tap voltage and the tuning condition expressed?
   A. V x C2/(C1+C2); wL = 1/wC1
   B. V x (C1+C2); L = 1/C
   C. V/2; wL = wC
   D. V x C1/(C1+C2); wL = 1/[w(C1+C2)]
   Answer: D. The divider gives V x C1/(C1+C2) and L resonates with C1+C2 at 50 Hz so ratio and phase do not vary with burden. (Key point: CCVT tap voltage)

## Related in Switchgear and Protection

- [Need of instrument transformers; CT errors](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Ainstrument-transformers-need-ct-errors) — Switchgear and Protection, chapter 2
- [Need of capacitive voltage transformer](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aneed-of-cvt) — Switchgear and Protection, chapter 2
- [Construction of capacitor voltage transformer](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acapacitor-voltage-transformer-construction) — Switchgear and Protection, chapter 2
- [Need for protection and causes of faults](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aneed-protection-fault-causes) — Switchgear and Protection, chapter 1
- [Protection requirements; protective system vs scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aprotective-system-vs-scheme) — Switchgear and Protection, chapter 1
- [Lightning protection schemes](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Alightning-protection-schemes) — Switchgear and Protection, chapter 5

## Related in other subjects

- [T/F: protection ratio vs system voltage](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-protection-ratio-system-voltage) — Transmission and Distribution System Design, chapter 3
- [Need and conditions for parallel operation of transformers](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Atransformer-parallel-operation-conditions) — Electrical Machines I, chapter 2
- [T/F: single phase transformers for lower utilization voltage](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-single-phase-transformer-low-voltage) — Transmission and Distribution System Design, chapter 7
- [LV voltage of Δ–Y bank of 100 kVA transformers](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adelta-star-bank-100kva-lv-voltage) — Electrical Machines I, chapter 2