# Voltage Transformer or Potential Transformer Theory

## Potential Transformer Definition

**Potential transformer**or

**voltage transformer**gets used in electrical power system for stepping down the system voltage to a safe value which can be fed to low ratings meters and relays. Commercially available relays and meters used for protection and metering, are designed for low voltage. This is a simplest form of

**potential transformer definition**.

## Voltage Transformer or Potential Transformer Theory

A**voltage transformer theory**or

**potential transformer theory**is just like a theory of general purpose step down transformer. Primary of this transformer is connected across the phase and ground. Just like the transformer used for stepping down purpose, potential transformer i.e. PT has lower turns winding at its secondary.

The system voltage is applied across the terminals of primary winding of that transformer, and then proportionate secondary voltage appears across the secondary terminals of the PT.
The secondary voltage of the PT is generally 110 V. In an ideal **potential transformer** or **voltage transformer**, when rated burden gets connected across the secondary; the ratio of primary and secondary voltages of transformer is equal to the turns ratio and furthermore, the two terminal voltages are in precise phase opposite to each other. But in actual transformer, there must be an error in the voltage ratio as well as in the phase angle between primary and secondary voltages.
The errors in potential transformer or voltage transformer can be best explained by phasor diagram, and this is the main part of **potential transformer theory**.

## Error in PT or Potential Transformer or VT or Voltage Transformer

I_{s}- Secondary current. E

_{s}- Secondary induced emf. V

_{s}- Secondary terminal voltage. R

_{s}- Secondary winding resistance. X

_{s}- Secondary winding reactance. I

_{p}- Primary current. E

_{p}- Primary induced emf. V

_{p}- Primary terminal voltage. R

_{p}- Primary winding resistance. X

_{p}- Primary winding reactance. K

_{T}- Turns ratio = Numbers of primary turns/number of secondary turns. I

_{0}- Excitation current. I

_{m}- Magnetizing component of I

_{0}. I

_{w}- Core loss component of I

_{0}. Φ

_{m}- Main flux. β - Phase angle error.

As in the case of current transformer and other purpose electrical power transformer, total primary current I_{p} is the vector sum of excitation current and the current equal to reversal of secondary current multiplied by the ratio 1/K_{T}.
Hence,I_{p} = I_{0} + I_{s}/K_{T}.
If V_{p} is the system voltage applied to the primary of the PT, then voltage drops due to resistance and reactance of primary winding due to primary current I_{p} will come into picture. After subtracting this voltage drop from V_{p}, E_{p} will appear across the primary terminals. This E_{p} is equal to primary induced emf. This primary emf will transform to the secondary winding by mutual induction and transformed emf is E_{s}. Again this E_{s} will be dropped by secondary winding resistance and reactance, and resultant will actually appear across the burden terminals and it is denoted as V_{s}.

So, if system voltage is V_{p}, ideally V_{p}/K_{T} should be the secondary voltage of PT, but in reality; actual secondary voltage of PT is V_{s}.

### Voltage Error or Ratio Error in Potential Transformer (PT) or Voltage Transformer (VT)

The difference between the ideal value V_{p}/K

_{T}and actual value V

_{s}is the voltage error or ratio error in a potential transformer, it can be expressed as,

### Phase Error or Phase Angle Error in Potential or Voltage Transformer

The angle ′β′ between the primary system voltage V_{p}and the reversed secondary voltage vectors K

_{T}.V

_{s}is the phase error.

### Cause of Error in Potential Transformer

The voltage applied to the primary of the potential transformer first drops due to the internal impedance of the primary. Then it appears across the primary winding and then transformed proportionally to its turns ratio, to the secondary winding. This transformed voltage across the secondary winding will again drop due to the internal impedance of the secondary, before appearing across burden terminals. This is the reason of errors in potential transformer.Closely Related Articles Instrument TransformersCurrent Transformer CT class Ratio Error Phase Angle Error in Current TransformerKnee Point Voltage of Current Transformer PS ClassAccuracy Limit Factor and Instrument Security Factor of Current TransformerIsolation TransformerMore Related Articles What is transformer? Definition and Working Principle of TransformerEMF Equation of Transformer | Turns Voltage Transformation Ratio of TransformerIdeal TransformerTheory of Transformer on Load and No Load OperationResistance and Leakage Reactance or Impedance of TransformerEquivalent Circuit of Transformer referred to Primary and SecondaryHysteresis Eddy Current Iron or Core Losses and Copper Loss in TransformerVoltage Regulation of TransformerSingle Three Phase Transformer vs bank of three Single Phase TransformersParallel operation of TransformersMagnetizing Inrush Current in Power TransformerTransformer Insulating Oil and Types of Transformer OilDGA or Dissolved Gas Analysis of Transformer Oil | Furfural or Furfuraldehyde AnalysisTransformer Accessories | Breather and Conservator Tank | RadiatorSilica Gel Breather of TransformerConservator Tank of TransformerRadiator of Transformer | Function of RadiatorMagnetic Oil Gauge or MOG | Magnetic Oil Level Indicator of TransformerOil Winding and Remote Temperature Indicator of TransformerTransformer Cooling System and MethodsOn Load and No Load Tap Changer of Transformer | OLTC and NLTCTertiary Winding of Transformer | Three Winding TransformerCore of Transformer and Design of Transformer CoreRestricted Earth Fault Protection of Transformer | REF ProtectionBuchholz Relay in transformer | Buchholz Relay operation and principleWhat is Earthing Transformer or Grounding TransformerDifferential Protection of Transformer | Differential RelaysOver Fluxing in TransformerTransformer Testing | Type Test and Routine Test of TransformerTransformer Winding Resistance MeasurementVoltage and Turn Ratio Test of TransformerVector Group Test of Power TransformerOpen and Short Circuit Test on TransformerInsulation Dielectric Test of TransformerTransformer Oil and Winding Temperature Rise TestImpulse Test of TransformerMaintenance of TransformerSweep Frequency Response Analysis Test | SFRA TestInstallation of Power TransformerCommissioning of Power TransformerElectrical Power Transformer | Definition and Types of TransformerStep Up TransformerStep Down TransformerWhat is Auto Transformer ?High Voltage TransformerDistribution Transformer | All Day Efficiency of Distribution TransformerDry Type TransformerAir Core TransformerDesign of Inductor in Switched Mode Power Supply SystemsDesign of High Frequency Pulse TransformerSingle Phase TransformerToroidal TransformerNew Articles Voltage in ParallelVoltage in SeriesVoltage Regulator 7805How to Use a Digital Multimeter?Vector Impedance MeterVoltage Follower