The application for capacitor
voltage transformers, CVTs, is the same as for Inductive Voltage Transformers.
The main function of a Capacitive Voltage Transformer is as follows:

- To transform currents or voltages from a usually high value to a value easy to handle for relays and instruments.

- To insulate the metering circuit from the primary high voltage system.

- To provide possibilities of standardizing the instruments and relays to a few rated currents and voltages.

Thus we see that Capacitive
Voltage Transformer performs the same job that of a Potential Transformer or
Voltage Transformer. But there is some specific reason due to which Capacitive
Voltage Transformer is used instead of Potential transformer which I will
discuss in the next post. In this post we will discuss about Capacitive Voltage
Transformer.

Capacitive voltage
transformers (CVTs) are used on higher voltage levels, starting from 66 kV and
upwards. The type of the CVT is always a single-pole one, thus the connection
is between phase and earth.

**Construction of Capacitive Voltage Transformer:**

The CVT consists of two
parts, the capacitive voltage divider (CVD) with the two capacitances C

_{1}and C_{2}and the Electromagnetic Unit (EMU). The size of the capacitances C_{1}and C_{2}determines the voltage ratio of the CVD. The EMU contains an inductive voltage transformer, a tuning reactance and a protection against ferro-resonance.
The trimming windings are
used for fine tuning the output signal to correspond with the required accuracy
class requirements. The compensating reactor compensates the phase angle shift
caused by the capacitive voltage divider.

The capacitance C

_{1}in the voltage divider, in series with the inductance of the compensating reactor and the wound transformer, inside the electromagnetic unit EMU, constitutes a tuned resonance circuit. Unlike with the inductive type of voltage transformers, the CVTs usually have the**damping circuit inbuilt in the CVT itself.***Ferroresonance*
As shown in figure above,
secondary terminal (1a,1n), (2a,2n) etc. are used for the Protection Circuit or
Metering Circuit. Typical voltage ratio for a CVT is 400 kV / 110 V which means
that if a CVT is placed in a phase of 400 kV line then the secondary voltage
will be 110 V.

**Operating Principle:**

A Capacitive Voltage
transformer works on Capacitor Voltage Divider principle. For better
understanding, assume a simple circuit of CVT which is connected between a line
of 400 kV and Earth.

As the CVT is connected
between the line and earth, therefore phase voltage (400/1.732 = 230 kV) will
be applied.

Therefore,

Voltage across the Capacitor
C

_{1}= (230×C_{2})/(C_{1}+C_{2})
Voltage across the Capacitor
C

_{2}= (230×C_{1})/(C_{1}+C_{2})
Thus if an Electromagnetic
Unit is connected across the C2 then its voltage rating will reduce. The output
secondary voltage will depend on the impedance of EMU unit as below

Secondary Terminal Voltage =
(Zm× Voltage across the Capacitor C

_{2})/(Z_{1}+Z_{2}+Z_{m})
Where Zm = magnetizing
Reactance of EMU unit.

Z1 = Primary impedance of
EMU unit.

Z2 = Secondary impedance of
EMU unit.

This is the simple working
principle of a CVT.

**Thank you!**

## No comments:

Post a Comment