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Current Transformrs(!.. 216. WlaJ t.V't tMin.ttnel1.t t r G t u
/ ~ 1 ? . Md #itir
a n U c a t l o t t ~ ? WJuJe"' tlt4: di.flerenee between A
1'(1tDtr f r u n s f ~ anda iMhmenl tmmJOf"'I'Ufr ?
Instrument Transformers. Instrument transformers includecurrent
transformers (CT) and voltage transformers (VT). VTs arealso called
potential. ttansformers.Current transformers are used for
reducing/stepping down.a.c. current from higher value to Jower
value for measurement/pro..tection/control. Voltage tran11formers
are used for stepping down vohage from"higher value to lower value
for measurcmentfpr?tection/control.Both crs and VTs' have low VA
rating and are, therefore, not
used for power transfer. The difference between instrument
transformers and power traos.former is mainly in their VA rating..
Power._transformers have high kV A rating (500 kVA, 1000 kVA
..upto1000 MVA). Instrument transformers have ra.ting ofonly a few
teo.s ..of VA (e.g."lO VA. 30 VA, 100 VA}.
Q.. 217 . W,_at are tha two main. g r t m f 1 8 / ~ u oJ
CUtf'Wftns/orm.trs aM w'Uage ltanljOY'fJ't,trB ?T h e r ~ are two
cJasses of instrument transformers :l . Mea.st.t!fi-ng current
trr:ntsformer' used for measurementpurposes.2. Prottdiw
eraliM/IJ'ff'l\t;J'S used for prottctive and controlrsystems.
aim5887
Q.. 218. WAat are 4M appliemimwl of eurrem l r a ' ! l . ! f ~
1~ t i v e tratuform.ers arc u:sed with overcu.rrentprteetion,
earth fault protection, dUferenHal protection and impe-
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BURDENONCTQ.. %%1. Ikjim e btwdcn em d 01.'.
l_mpedan.rs of secondary circuit exprl\l$sed in obms,
powerfactor 1JS called burtkJt. But it is usually expressed in
terms ofapparent power in kVA and rat! secondary euttent at
specifiedpower factor. This power factor is not the powtr factor of
$oecondarycumru orcr
The circuit connected to the secondary winding is. termed as~ b
u r d e n ' oftbe curre-nt transformer. If the term 'Joad' is used,
itrefers to the primary current magnitude. Burden is
expressedpreferably in terms of impedance of the cireuit connKted
to the$tcondary and its resistance and reactance. The British
metllod isspecity the burden on the cr in volt-ampere& at rated
secondary
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aim5887
JToltage Transfortne:rsQ.. 131. Whal are tlre anlit.t:UiMt-'
a:nd lf!Pu oJ t x : ~ l t o g c I r a n . ~ .
/Of'fl fDII 1Voltage transformers (potential transformers) are
used formeasurement and protection . Accordingly, tlley are either
measurin4 jtlpe or protective type voltage tntls.formers. They may
be eithersinsld phase or three phase. Voltage transformers are
necessaryfo,r voltage. d i r e c t i o n a ~ distance prcneetion.
The primary ot voltagetransformer is connected directly to power
.circuit between pbase andground depending upon rated voltage and
application. The volt-ampere rating of voltage trausformers is
lesser as compared with that1of power transfonners.
There are two types of constructions :-electromagn.etie
potential tran:srormer in which primary ands-econdary are wround on
magnetic corcJike io usual transfermer-capacitor potential t r a n
s f o r m e r ~ in which the primary \'oltil3\' is applied to a
series capacitor group. The voltage.a ~ r o s s one of the
capacitor is taken to auxiliary v o l t a g e ~ transformer. The
secondary of auxiliary voltage transformeris taken for measurement
or protection.
Spccif icatioa f'or Vohagc Traasfor . tU>enThe following
aspects should be determined wbil.e seJectins t h f : ~ voltage
transfotnter t. Rated primary voltage.2. Rated secondary voitlgc,3.
Rated burden.4. Supply freqllency.S. Number of phases.6. Clw of
accuracy.
1. I ~ s o n kvel. Power frequency .and impulse
voltagewitbltand.
Limits of dimensions, types of construction etc.
3
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aim5887
Q.. 235. E ~ r j J l < J i ~ the 'burden' on V!ra.Burdett
01'1 Voltage Tnafol"mer. Burdens are specified.in v o J t a m p ~ r
e s at a rated tecondary voltage at a particular powerfactor. Let
rated secondary voltage be v.. The ohmic impedance:nf burden be
.Zt. V o l t a m ~ r e burden =P,v.
Z&==- ohms." pLet burden power factor be l:os {I
Ba::::Za COS r/1X11e:rZo sin .pZo= 4R+X11.The total burde o on a
VT should be Jess than the rated burdeDt
of'VT.The capacitor voltage transfcrmer output is upto about
200VA. While that of wounded type is upto ab
