Electrical Engineering Association

Capacitive Voltage Transformer (CVT)

Definition:

The capacitive voltage transformer step-down the high voltage input signals and provide the low voltage signals which can easily measure through the measuring instrument. The Capacitive voltage transformer (CVT) is also called capacitive potential transformer

Construction:

In its most basic form, the device consists of three parts: two capacitors across which the transmission line signal is split, an inductive element to tune the device to the line frequency, and a voltage transformer to isolate and further step down the voltage for metering devices or protective relay.

The tuning of the divider to the line frequency makes the overall division ratio less sensitive to changes in the burden of the connected metering or protection devices. The device has at least four terminals: a terminal for connection to the high voltage signal, a ground terminal, and two secondary terminals which connect to the instrumentation or protective relay.

Capacitor C1 is often constructed as a stack of smaller capacitors connected in series. This provides a large voltage drop across C1 and a relatively small voltage drop across C2. As the majority of the voltage drop is on C1, this reduces the required insulation level of the voltage transformer. This makes CVTs more economical than the wound voltage transformers under high voltage (over 100 kV), as the latter one requires more winding and materials.

Capacitive Voltage Transformer Working:

The capacitive potential divider is used in combination with the auxiliary transformer and the inductive element. The capacitive potential divider step-down the extra high voltage signals into a low voltage signal. The output voltage of the capacitive potential transformer is further step-down by the help of the auxiliary transformer.

Consider the circuit diagram of the capacitive potential transformer in figure 2.

The capacitor or potential divider is placed across the line whose voltage is used to be measured or controlled. Let the C1 and C2 be the capacitor placed across the transmission lines. The output of the potential divider acts as an input to the auxiliary transformer.

The capacitor places near to the ground have high capacitances as compared to that placed near the transmission line. The high value of capacitances means the impedance of that part of the potential divider becomes low. Thus, low voltages pass to the auxiliary transformer. The auxiliary transformer further step-down the voltages.

The N1 and the N2 are the numbers of turns on the primary and the secondary winding of the transformer. The meter used for measuring the low value of voltage is resistive, and the potential divider is capacitive. Thereby, the phase shift occurs, and the output will be affected. To overcome this problem, the inductance is placed in series with the auxiliary transformer.

This inductance L consists the leakage flux of the auxiliary winding of the auxiliary transformer. The value of inductances is given asThe value of inductances is adjustable. The inductance compensates the voltage drops occurs in the transformer because of the reduction of the current from the potential divider. But, in actual practice, the compensation is not possible because of the inductance losses.

L= 1/(ω^2 (C1+C2))

The voltage turn ratio of the transformer is expressed as As the value of C1 is greater than the C2. Thus the value C1/(C1+C2) is small. The low value of voltage is obtained.

V0/V1 = (C1/(C1+C2)) x (N1/N2)

The voltage transformation ratio of the capacitive potential transformer is free from the burden. The burden is the load on the secondary winding of the transformer.

Other Applications:

The CVT is also useful in communication systems. CVTs in combination with wave traps are used for filtering high-frequency communication signals from power frequency. This forms a carrier communication network throughout the transmission network, to communicate between substations. The CVT is installed at a point after Lightning Arrester and before Wave trap.