A voltage transformer or transformer is a step-down transformer. Voltage or voltage transformer is an instrument transformer. In principle, this type of instrument transformer is similar to a step-down power transformer. But its usage is different. The voltage transformer reduces the high system voltage to a measurable low level. According to the standard, the secondary voltage of almost all voltage transformers is 110V. Lower secondary voltage is used for system voltage indication, energy metering, protective relays, and synchronization.

Types of Voltage Transformers

There are two types of voltage transformers on the market. These are magnetic and capacitive types.

Electromagnetic voltage transformer

Like ordinary double winding transformers, magnetic voltage transformers operate based on mutual inductance between two magnetic coupled windings. Although the VA rating of this type of transformer is very small. Usually, this rated value is around several hundred VA.

Capacitive Voltage Transformer (CVT)

The main advantage of capacitive voltage transformers is that they can provide carrier frequency coupling while reducing system voltage. Due to the fact that this type of voltage transformer can be used as both a voltage transformer and a coupling capacitor, the use of capacitive voltage transformers has become economical. The transformer is composed of a series of capacitors. There is a tape attachment point in the capacitor column inside the CVT. This point is connected to the primary of the electromagnetic step-down transformer. The secondary of this electromagnetic type secondary transformer is the secondary of the CVT itself.

In fact, the entire phase voltage drops between a series of capacitors. Therefore, the tap point is located at a certain potential point on a series of capacitors. Obviously, this potential is the primary voltage of a two winding electromagnetic transformer. The standard primary voltage of this electromagnetic transformer can be 5KV, 10KV, 15KV, or 20KV, depending on the requirements of the load and accuracy.

Errors in AC voltage transformers

Ideally, the voltage ratio is equal to the turns ratio. However, in reality, due to magnetic current, iron core loss current, and voltage drop in the winding, voltage transformers cannot maintain a voltage ratio and turn ratio that are exactly the same. Therefore, there will be a ratio error. Due to the same reasons, there are phase angle errors in transformers. When PT is not connected to any load on the secondary coil, there will still be a voltage drop at the primary end due to excitation current. Similarly, the burden on transformers is constant. This depends on the rudeness of the connected relays, instruments, and wires. Therefore, by reducing the rudeness of PT windings alone, errors can be minimized. Therefore, reducing resistance and reactance is the main criterion for designing PT. For this purpose, we use several turns of larger cross-sectional wires in the winding. Similarly, PT requires a larger cross-sectional iron core.

In medium voltage transformers, the primary and secondary windings with cores are placed in stationary containers in oil fields. With the help of ceramic or polymer insulation sleeves, the primary terminals of the transformer come out of the container. However, for higher rated voltages, these active components of the transformer are contained within a ceramic casing.

A voltage transformer usually has one primary terminal. The other end of the primary winding is grounded.