Voltage Transformer (VT)

Voltage transformer (VT) measures the voltage at the bus of relaying applications like distance relays, directional overcurrent relays. The principle of a voltage transformer is similar to the normally used transformer.

Hence, voltage transformer(VT) equivalent circuit can be represented as shown in figure

voltage transformer(VT) equivalent circuit

Typically, the secondary voltage of the Voltage Transformer(VT) is standardized to 110 V (ac). Hence, if the primary voltage increases, then turn ratio N1: N2 also increases and the transformer becomes large as the area increases.

To make the voltage transformer(VT) of small size and less costly, a capacitance potential divider is used as shown in the given below fig. Thus, a reduced voltage is fed to the primary of the transformer. This reduces the size of the voltage transformer(VT) that leads to the development of coupling capacitor voltage transformers (CCVT).

To cut down the voltage transformer(VT) size and cost, a capacitance potential divider is used. Thus, a reduced voltage is fed to the primary of the transformer. This reduces the size of the voltage transformer(VT). This leads to the development of coupling capacitor voltage transformers (CCVT).

Role Of Tuning Reactor In Voltage Transformer

Let’s take an ideal transformer, the Thevenin’s equivalent circuit of coupling capacitor voltage transformer (CCVT) is shown in fig

Assuming, the transformer to be ideal, the Thevenin's equivalent circuit of coupling capacitor voltage transformer (CCVT)

It is now clear that there is impedance Zth due to the capacitance divider, it has some effects on the voltage received by the relay. A tuning inductor is connected to compensate for this voltage drop to achieve a high level of accuracy. The value of tuning inductors is so chosen that it compensates for the ‘net C’ at power frequency (50Hz). The phasor diagram across the resistive load is as shown in the above-given figure.

It is now obvious that Zth due to the capacitance divider, it affects the voltage received by the relay. To achieve a high level of accuracy, it is, therefore, necessary to compensate for this voltage drop by connecting a tuning inductor. The tuning inductor‘s value is so chosen that it compensates for the ‘net C' at power frequency (50Hz). The phasor diagram across the resistive load is as shown.


From the respectively equivalent above circuit, it is clear that, if \omega L=\frac { 1 }{ \omega (C_1+C_2) } then the voltage drop across C is neutralized and the relay can easily measure the actual voltage.

CCVT In Power Line Communication

The capacitance potential divider also used for the dual purpose of providing a shunt path to the high-frequency signal used in power line carrier communication. Mostly, CCVT is used in HV/EHV systems where carrier line communication is used. High frequency i.e. Radio Frequency (RF) signals of range between 50 – 400 kHz can be coupled to the power line for communication.

The capacitance potential divider also serves the dual purpose of providing a shunt path to the high-frequency signal used in power line carrier communication. Normally, CCVT is used in HV/EHV systems where carrier line communication is used. High frequency i.e. Radio Frequency (RF) signals (50 - 400 kHz) can be coupled to the power line for communication.

At high frequency, the capacitive shunt impedance is very small because the frequency is inversely proportional to the impedance and hence these signals can be tapped by the potential divider. A small drainage reactor is connected in series with the capacitance divider to block the path to ground for the RF signal. There is a very small impedance at high frequencies. Thus, the role of capacitance potential divider at power frequency is not split into difference (compromised). On the other hand, at RF, the impedance of the drainage reactor is large and it blocks the RF signal.

RF signal is blocked by the inductive nature of compensating reactance and transformer leakage reactance. This signal is then exploited by a tuning pack which provides a low impedance to the RF signal.

Transient Response of CCVT

It is now obvious that Zth due to the capacitance divider, it affects the voltage received by the relay. To achieve a high level of accuracy, it is, therefore, necessary to compensate for this voltage drop by connecting a tuning inductor. The tuning inductor‘s value is so chosen that it compensates for the ‘net C' at power frequency (50Hz). The phasor diagram across the resistive load is as shown in the above-given figure.

CCVT equivalent circuit consisting of an R-L-C. If the transformer is considered to be ideal it can be described by the equation given below

v(t)=Ri+\frac { 1 }{ C_{eq} } \int _{ -\infty }^{ t }{ idt+L\frac { di }{ dt } }

The corresponding differential equation is given by

\frac { dv }{ dt } =R\frac { di }{ dt } +\frac { 1 }{ C_{eq} } i+L\frac { d^2i }{ d^2t }

For a solid three-phase fault near to the CCVT bus at t=t0. v(t) = 0 for t\ge t_0. Thus, during fault

\frac { R }{ L } \frac { di }{ dt } +\frac { 1 }{ LC_{eq} } i+\frac { d^2i }{ d^2t } =0

This equation is expressed in standard form as follows \frac { d^2i }{ d^2t } +2\zeta \omega_n +\omega^2_n i=0 where \omega_n is natural frequency in radians per second and \zeta is the damping constant.

By comparing both the equations thus \omega_n =\frac { 1 }{ LC_{eq} } and 2\zeta \omega_n =\frac { R }{ L } Because of the property of tuning reactor \omega =\frac { 1 }{ \sqrt { LC_{eq} } } =2\pi \times f_0

The response depends upon the damping <span class="katex-eq" data-katex-display="false">\zeta</span> and <span class="katex-eq" data-katex-display="false">\omega_n</span> and point on the voltage waveform where the fault strikes. Such transients are known as subsistence transients. The figure shows subsistence transients of CCVT. It can be seen that subsistence transients can reduce the accuracy of distance relays.

The response of the equation depends upon the damping \zeta and \omega_n and point on the voltage waveform where the fault strikes. Such transients are known as subsistence transients. The figure shows the subsistence transients of CCVT. Distance relays accuracy can also be affected by the substance transient as we can also see in the above figure.

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