Capacitor voltage transformers (CVTs)

 This type of voltage transformer is normally meant for a high to an ultra-high voltage

system, say, 110 kV and above.

While a conventional wound-type (electromagnetic) voltage transformer will always be the

first choice, it may become costlier and highly uneconomical at such voltages. The size and

therefore the cost of a conventionally wound voltage transformer will be almost

proportional to the system voltage for which it is wound. As a cost consideration, therefore,

a more economical alternative is found in a capacitor voltage transformer (CVT)

A CVT consists of a capacitor divider unit in which a primary capacitor C1 and a secondary

capacitor C2 are connected in series between the line and the ground (Figure below). A

tapping point is provided at an intermediate voltageVe

, usually around 12 to 30 kV, which is

reasonably low compared to the high system voltage.

This helps restrict the phase error, on the one hand, and facilitates an economical

intermediate wound transformer Tr, the electromagnetic unit may be wound for two to

three secondary windings, one for metering and the others for protections.

The purpose of line capacitors is thus to step-down the high to very high system voltages to

an economically low value. The secondaries of the transformer are rated for the required

standard voltages, say, 110/ √3 (63.5V), to feed the auxiliary devices and components fitted

in the auxiliary circuit.

The capacitors connected in series act like potential dividers provided the current taken by

the burden is negligible compared with the current passing through the series-connected

capacitors. Generally, high voltage capacitors are enclosed in porcelain housing. A large

metal sheet box at the base encloses the tuning coil intermediate transformer.

Ve = VP.

C1

C1+C2

The reactor connected in series with the burden is adjusted to such a value that at supply

frequency it resonates with the sum of two capacitors. This eliminates the error.