--- title: "Need of capacitive voltage transformer" subject: "Switchgear and Protection" chapter: 2 key: "semester_6/switchgear_and_protection:need-of-cvt" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/need-of-cvt site: BCT Engineering --- # Need of capacitive voltage transformer Switchgear and Protection · Chapter 2: Current and Potential Transformers - Asked in: 2076 Bhadra, 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/need-of-cvt - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-2 ## Question Explain the need of capacitive voltage transformer in power system. ## Short answer CVTs are needed at EHV levels (≥66 kV) because they are cheaper and lighter than electromagnetic VTs, and the same capacitor stack serves as a coupling capacitor for power line carrier communication. They step down voltage accurately via a tuned reactor, though transient response is poorer. ## Answer A **capacitive (capacitor) voltage transformer (CVT)** steps down extra-high voltage using a capacitor divider followed by a small intermediate electromagnetic VT and tuning reactor. #### Need of CVT in power system 1. **Economy at high voltage:** an electromagnetic VT for 132 kV and above needs very heavy insulation, so it becomes large and costly. A capacitor stack handles the high voltage cheaply; only the small IVT works at 10–20 kV. The higher the voltage, the larger the saving. 2. **Voltage signal for protection and metering:** distance relays, directional relays, synchronising, voltmeters and energy meters require a voltage proportional to line voltage (110/√3 V). 3. **Power line carrier communication (PLCC):** the same capacitor stack acts as the **coupling capacitor** for carrier signals (30–500 kHz) used for teleprotection, telephone and SCADA. One equipment serves two purposes. 4. **Smaller size and weight:** easier transport and installation in EHV switchyards. 5. **Reduced risk of ferro-resonance with breaker grading capacitors** compared with some inductive VTs (with suppression circuit). 6. **Surge protection:** the capacitance helps to slope steep wave fronts. #### Working principle (brief) ```text V_tap = V_line × C1/(C1 + C2) Tuning: ωL = 1 / [ω(C1 + C2)] ``` The tuning reactor compensates the capacitive source impedance so that the output is nearly independent of burden. #### Limitations - Poorer transient response: after a fault, the energy stored in capacitors and reactor causes transients that may affect high-speed distance relays. - Accuracy is affected by frequency and temperature. Therefore CVTs are standard for line voltage measurement at 66 kV and above. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a CVT preferred to an electromagnetic VT at 132 kV and above? A. A capacitor stack handles the high voltage cheaply, so only a small IVT is needed; the saving grows with voltage B. Electromagnetic VTs are more accurate in transients C. CVTs need no insulation D. CVTs have better transient response Answer: A. Electromagnetic VTs need very heavy insulation at EHV and become costly; the CVT only subjects a small IVT to 10-20 kV. (Key point: Economy at EHV) 2. What second function does the capacitor stack of a CVT serve? A. Acts as a surge arrester B. Acts as coupling capacitor for power-line carrier signals (30-500 kHz) C. Acts as a current limiter D. Supplies reactive power Answer: B. The same stack couples carrier signals for teleprotection, telephony and SCADA, so one equipment serves two purposes. (Key point: PLCC role) 3. What is a main limitation of CVTs? A. Cannot be used above 66 kV B. Very large size C. Poorer transient response; stored energy causes transients that may affect high-speed distance relays D. They cannot feed energy meters Answer: C. After a fault the energy stored in capacitors and reactor causes transients; accuracy also depends on frequency and temperature. (Key point: Limitation) 4. What relation is used for tuning the CVT? A. wL = C1/C2 B. L = C1 + C2 C. wL = w(C1+C2) D. wL = 1/[w(C1+C2)] Answer: D. The reactor resonates with C1+C2 at system frequency, compensating the capacitive source impedance. 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