Need of capacitive voltage transformer

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)

 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.