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.
To know about CT
in details visit other post CT (CURRENT TRANSFORMERS) IN ELECTRICAL DISTRIBUTION SYSTEM
Sponsored:
Capacitor Voltage Transformer- CVT
On the other hand, VT stepdown the high voltage into low voltage and VT’s primary connected in parallel with monitored primary circuit.
Sponsored:
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.
Where,
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
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.
Voltage transformers are designed to present negligible load to the supply being measured and have an accurate voltage ratio and phase relationship to enable accurate secondary connected metering.
ReplyDeletePower transformers in India | Transformer Manufacturer in India