What is Capacitive Voltage Transformer & Its Working

The device named as transformer ought to have the best credits of crucial and essential development in the industrial and electrical industry. The electrical transformer delivers many advantages, and they hold multiple applications across various domains. And the one kind type which got evolved from the transformer is “Capacitive Voltage Transformer”. This kind of transformer has more than 3 decades of development history. Even the device offers many benefits, there exist few regulations in the implementation of harmonic calculations. So, let us know in detail why this happens and gain knowledge on CVT working principle, testing approach, applications, and advantages.

What is Capacitive Voltage Transformer?

Similar to the potential transformer, this is also a step-down capacitive voltage transformer where it holds the ability to convert high-level voltages to a low level. These transformers also transform the transmission level of voltage to normalized minimum levels and to simply quantifiable values where these are implemented for safety, metering, and the regulation of the high level of voltage systems.

In general, in the case of high-level voltage systems, either the line current or voltage values cannot be calculated. So, this requires an instrument type of transformers like the potential or current transformers for the implementation. Whereas in the case of increased high voltage lines, the utilized potential transformer cost will be more due to the installation.

So as to decrease the installation cost, the CVT type of transformers is used in the place of a normal voltage transformer. Starting from the range of 73 kV and more, these capacitive voltage transformers can be used in the required applications.

What is the Need of CVT?

Above the range of 100 kV and increased voltage levels, there will be the requirement of a high-end insulated transformer. But the price of insulated transformers are extremely high and may not be chosen for every application. In order to lessen the price, potential transformers are used in the place of insulated transformers. The cost of CVTs is less but the performance is low when compared to insulated transformers.

Working of Capacitive Voltage Transformer

The device mainly consists of three sections and those are:

• Inductive element
• Auxiliary transformer
• Potential divider

The below circuit diagram clearly explains the capacitive voltage transformer working principle.

The potential divider is operated along with the other two sections which are the inductive element and the auxiliary transformer. The potential divider functions to minimize increased voltage signals to that of low voltage signals. The voltage level that is received at the output of the CVT is more lessened by the support of an auxiliary transformer.

The potential divider is located between the line where the voltage level is to be either regulated or calculated. Consider C1 and C2 are the capacitors that are placed in between the transmission lines. The output from the potential divider is fed as input to the auxiliary transformer.

The capacitance values of the capacitor that are placed near the ground level are more when compared with the capacitance values of the capacitors that are close to the transmission lines. The high value of capacitances indicates the electrical resistance of the potential divider as less. So, minimal voltage value signals move towards the auxiliary transformer. Then the AT again steps down the voltage value.

And N1 and N2 are the primary and secondary winding turns of the transformer. The meter that is utilized for the low voltage value calculation is resistive and so the potential divider holds capacitive behavior. So, because of this phase shift takes place and this shows an impact on the output. In order to eliminate this issue, both the auxiliary transformer and the inductance has to be in series connection. The inductance is included with the leakage flux that is present in the auxiliary of the AT and the inductance ‘L’ is represented as

L = [1/(ω²(C1 + C2))]

This inductance value can be adjusted and it compensates for the voltage drop that takes place in the transformer due to the decline of the current value from the divider section. Whereas in real situations, this compensation is not likely to take place due to the induction losses. The ratio of voltage turn of the transformer is shown as

V0/V1 = [C2/C2 + C1] × N2/N1

As C1 > C2, then the value is C1/(C1 + C2) will be reduced. This shows that the value of the voltage will get decreased.

This is the capacitive voltage transformer working.

CVT Phasor Diagram

To know about the phasor diagram of the capacitive voltage transformer, the equivalent circuit of the device has to be shown. With the above circuit diagram, its equivalent circuit can be drawn as below:

In between the meter and C2, a matching transformer is placed. The transformer proportion

n is selected depending on the economic bases. The high voltage rating value might be across 10 – 30 kV whereas the low voltage winding rating is across 100 – 500 V. The level of the tuning choke ‘L’ is selected in the way that the capacitive voltage transformer’s equivalent circuit is completely resistive or chosen to operate in a complete resonance state. The circuit is moved into resonance condition only when

ω(L + Lt) = [1/(C1 + C2)]

Here ‘L’ represents the choke inductance value and ‘Lt’ corresponds to the transformer’s equivalent inductance mentioned in the high voltage section.

The phasor diagram of the capacitive voltage transformer, when operated in a resonance condition, is shown below.

Here, the ‘Xm’ reactance value of the meter can be ignored and considered as resistance load ‘Rm’ when the load has a connection with the voltage divider. The voltage value at the potential transformer is given by

V₂ = Im.Rm

Whereas the voltage across a capacitor is given by

V_c2 = V₂ + Im (Re + j. Xe)

By considering V1 as the phasor reference, the phasor diagram is drawn. From the phasor diagram, it can be observed that both the reactance and resistance are not individually represented and these are represented along with the reactance ‘Xi’ and resistance ‘Ri’ of the tuning indicator ‘L’.

Then the voltage ratio is

A = V1/V2 = (V_c1 + V_Ri + V₂)/V₂

By ignoring the reactance drop ImXe, then the voltage drop at the tuning indicator and transformer resistance is given by V_Ri. The meter voltage and the input voltage will be in phase with each other.

CVT V/S PT

This section describes the difference between the capacitive voltage transformer and a potential transformer.

Advantages of Capacitive Voltage Transformer

A few of the benefits of CVT are:

• These devices can be utilized as enhanced frequency coupling units
• CVT devices are less expensive than that potential transformers.
• They utilize minimal space
• Simple to construct
• The voltage level is based on the type of capacitive element that is used

CVT Applications

A few of the applications of capacitive voltage transformer are:

• CVT devices have extensive applications in transmission power systems where the voltage value ranges from high to ultra-high
• Employed in voltage calculations
• Automatic management devices
• Protection relay devices

So, this is all about the concept of a capacitive voltage transformer. This article has provided a detailed concept of CVT working, applications, phasor diagrams, and benefits. In addition to these, know about capacitive voltage transformer testing and choose the one that suits for the specific application.