What is Voltage Transformer (VT)?

VT in abbreviation Voltage Transformer, on the other way PT or Potential Transformer is a transformer which convert voltage into standard measurable level. 

This converted voltage is then proportionally transform the measuring or controlling primary voltage. 

VT operates on the same principle as power transformer, different only purpose of uses. Unlike conventional transformer, voltage transformer have only one magnetic core attached with secondary winding. Physically VT may constructed in single pole or double pole.

Voltage transformers shall comply with the requirements of IEC 186 with amendments and supplements-

Instrument Transformers-CT&VT

Transformers which are used to instrumentation- like controlling, measuring etc. are known as instrument transformer. Instrument transformers are mainly used to coupling the main primary circuit and secondary controlling or measuring circuit.

Controlling or measuring devices are not capable to connect direct with high voltage or high level current primary power line. To reduce or stepdown the voltage or current level in standard ratio instrument transformers are used.

Instrument transformers are mainly two types- CT or Current Transformer and VT or Voltage Transformer. CT mainly step-down the high level current into low level and CT’s primary connected in series with the monitored primary circuit. 

On the other hand, VT stepdown the high voltage into low voltage and VT’s primary connected in parallel with monitored primary circuit.

Capacitor Voltage Transformer- CVT
Basic Components of Capacitor Voltage Transformer

Capacitor Voltage Transformer (CVT) is an instrument transformer that used in power system to step down the extra high voltage (EHV) signals and provide a low voltage signal what used for metering or operating a protective relay to control the circuit.

The basic components of capacitor voltage transformer are shown in the figure.

Voltage or Ratio Error and Phase or Phase Angle Error of VT or PT

The difference between the ideal value and actual value of the voltage is the voltage error or ratio error of the voltage transformer or potential transformer. The formula can expressed as,
% Voltage error= { (Vp-KT.Vs)/Vp}x100%

The angle between the primary system voltage Vp and the reversed secondary voltage vectors (KT.Vs ) is known as  phase error.

Vp - Primary terminal voltage,
Vs - Secondary terminal voltage,
KT - Turns ratio = Numbers of primary turns/number of secondary turns,

Voltage Transformer Using Classes

Class 1 accuracy for protection/indicating instruments
Class 0.5 accuracy for integrating meters
Class 0.2 accuracy will be for tariff metering or acceptance efficiency testing.
Class 0.1 accuracy for generator low forward power interlock relays or as specified.

Voltage Transformer Ratings

The VA output rating shall be 50% in excess of the design requirements except for tariff metering voltage transformers which shall be at least 10% in excess of the design requirements.

For tariff metering voltage transformers the Contractor shall check the total installed secondary burden and if necessary shall install dummy burdens to achieve the calibrated accuracy.

Voltage Transformer Primary & Secondary Connection

Voltage transformer secondary circuit shall be earthed at one point only and metal cases shall be separately earthed. The transformers core, where accessible, shall also be separately earthed.

All voltage transformers in the system at a given voltage level shall be earthed in the same manner.

Where it is required to earth the primary neutral of a metal clad three- phase voltage transformer, the neutral earthing connection shall be insulated and brought out separately from the tan earthing connection. Means shall be provided to maintain the tank earthing connection while the voltage transformer is being withdrawn.

Where three single-phase voltage transformers are supplied for protection purposes, star connected secondary windings shall have the star point formed by insulated connections and shall be earthed at a common point.

Where necessary for earth fault protection, voltage transformers shall be of five- limbed core construction.

Where possible primary windings shall be connect through fuses with current limiting features.

Secondary MCB’s shall be provided as close as possible to each voltage transformer and labeled to show their function and phase colour. The secondary circuits shall be monitored individually to detect and alarm individual fuse failure or MCB trip and to block protection operation if required.

Voltage transformers shall be designed that saturation of their cores does not occur when 1.732 times normal voltage is applied to each winding.

Magnetization curves shall be submitted for approval for each type of voltage transformer.

The standard secondary voltage between phases shall be 110 volts unless special circumstances dictate otherwise, and are approved by the Engineer.

Secondary circuits from different voltage transformers, or separate windings of the same transformer, shall not be connected in parallel.

Voltage transformers shall be connected on the non-busbar side of circuit breakers unless otherwise approved by the Engineer.

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