A voltage transformer is a type of instrument transformer.
So, what are the types of voltage transformers?
Let’s find out
What exactly is a voltage transformer?
A voltage transformer or potential transformer is an instrument transformer that is used for providing safe voltage levels to the relays or measuring instruments. The voltage transformer reduces the incoming high voltages of power systems to a safe standard lower voltage level.
Types of voltage transformers
There are 3 types of voltage transformers:
- Capacitor VT
- Optical VT
- Electromagnetic VT
Let’s discuss these types of voltage transformers in detail:
Capacitor voltage transformer
Capacitor voltage transformer consists of an auxiliary transformer with a reactor L and the capacitance-voltage divider.
The capacitance-voltage divider steps down the incoming voltage and the output voltage of the capacitance-voltage divider further steps down by the transformer.
Here, the reactor L helps in tuning the capacitor voltage transformer to reduce the ratio error and the phase angle errors with the variation of VA burden, frequency, etc.
Tuning of the reactor is done in such a manner that its value at system frequency resonates with the capacitors.
Another voltage transformer like electromagnetic VT becomes quite expensive for voltage above 132kV. Therefore, the capacitor VT is used for those voltages.
The capacitor voltage transformer tends to introduce the harmonics at the secondary voltage.
Optical voltage transformer
The optical voltage transformer operates based on the fact that the voltage difference between the conductor and ground manifests in an electric field between 2 electrodes.
In optical VT, a circularly polarized light beam flow through an optical fiber that contains three strategically placed cells. The light from each cell is sent back to optoelectronics at the ground.
So, measuring the change of polarization in 3 cells tells the voltage difference between the conductor and the ground.
Electromagnetic voltage transformer
Electromagnetic voltage transformer converts the primary voltage into secondary voltage by electromagnetic induction. This transformer operates for voltage levels up to 132kV.
For high voltages, oil-immersed voltage transformers are used and for low voltages, ( up to 33kV ) dry-type transformers with varnish impregnated and taped windings are used.
As the electromagnetic VT is preferably used for up to 132kV. When the electromagnetic VT is used for above 132kV, a few electromagnetic VT is used by connecting them in cascade.
In cascade connection, the primary windings are connected in series, and each primary is in a separated core.
Due to additional construction, the electromagnetic VT is not suitable for higher voltages. For higher voltages (above 132kV) capacitor-type voltage transformer is used.
These electromagnetic types of voltage transformers are used in all power system voltages, relay, etc.