Voltage transformers or potential transformers are step-down transformers. A voltage or potential transformer is a kind of instrument transformers. Principally this type of instrument transformer is similar to a step-down power transformer. But its use is different. A potential transformer steps down the high system voltage to a measurable low level. As per standard, the secondary voltage of almost all the potential transformers is 110V. This low secondary voltage serves for system voltage indication, energy metering, protection relaying and synchronizing.
Types of Voltage Transformer
There are two types of voltage transformer available in the market. These are magnetic type and capacitive type.
Magnetic Type Voltage Transformer
Like a normal two-winding transformer, the magnetic type potential transformer works on the basis of the mutual induction between two magnetically coupled windings. Although the VA rating of such transformers is quite small. Normally this rating is about a few hundreds VA.
Capacitive Voltage Transformer (CVT)
The main advantage of a capacitive voltage transformer is that it can provide carrier frequency coupling simultaneously with stepping down the system voltage. Since this type of potential transformer serves as a potential transformer as well as a coupling capacitor, it becomes economical to use a capacitive voltage transformer.
This transformer consists of a series of capacitors. There is a taping point in the column of the capacitors inside the CVT. This point connects the primary of an electromagnetic type step down transformer. The secondary of this electromagnetic type secondary transformer is the secondary of the CVT itself.
Actually, the entire phase voltage drops across the series of the capacitors. Hence the tapping point is at a certain potential point on the series of the capacitors. Obviously, this potential is the primary voltage of the two-winding electromagnetic transformer. The standard primary voltage of that electromagnetic transformer may be 5KV, 10KV, 15KV or 20KV depending on the requirement of burden and accuracy.
Errors in Alternating Type Voltage Transformer
Ideally, the voltage ratio equals to the turns ratio. But practically due to magnetic currents, core loss currents and voltage drops in the winding, a potential transformer can not maintain the voltage ratio exactly the same as the turns ratio. So there will be a ratio error. Also due to the same reasons, there is a phase angle error in the transformer.
When the PT does not connect any burden across the secondary, still there will be a voltage drop in the primary side due to the magnetizing current. Again the burden of a voltage transformer is constant. It depends on the impudence of the connected relays, meters, and leads, etc. Hence, by only reducing impudence of the winding of PT we can reduce the errors. So, the reducing of the resistances and reactance is the main criteria for designing a PT. We do this by using a few turns of a larger cross-sectional wire in the winding. For the same, a PT needs to have a larger cross-sectional iron core.
In the medium voltage potential transformers, the primary and secondary winding with core are placed in an oil field still container. The primary terminal of the potential transformer comes out from this container with the help of porcelain or polymer insulated bushing. But for higher voltage ratings, these active parts of the transformer are contained in porcelain housing.
A potential transformer normally has one primary terminal. The other end of the primary winding is connected with earth.