scott-t connection of transformer power point- edited (2)
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8/9/2019 Scott-T Connection of Transformer Power Point- Edited (2)
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Scott-T Connectionof Transformer
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This is a connection by which 3-phase to 3-phase or 3-phase to 2-phase transformation is accomplished
with the help of two transformers .Since it was proposed by Charles F.Scott, it is frequently referred to asScott Connection. This conuration of
transformer can be used when one ofthe transformer of a transformer ban!is faulty or damaed.
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VA1
VB1
VC1
."## tap
Centertap
Teaser Transformer
$ain Transformer
%a2
%b2
%c2
Scott Connection Conuration &3-phase to 3-phase'
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Characteristics of T-T Connection (3phase – 3phase)
(. The main transformer is center tap both on primary
and secondary.2. The other transformer, !nown as the teaser
transformer, is ."## tap,. )ne end of both primaryand secondary is *oined to center taps of the main
transformer. imae
3. +oth transformer uses only "#.# of their !% ratinwhich is the same as %-% connection if two identicalunits are used.
. /t will be more economical than %-% connectionoperatin at 0.12" of its !% ratin if teasertransformer will be desined only to use "#.# ofthe mains windins.
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%(
%+( %C(
%a2
%b2
."## tap
Centertap
Teaser Transformer
Scott Connection Conuration &3-phase to 2-phase, -wire'
$ain Transformer
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Characteristics of Scott Connected Transformer 3-phaseto 2-phase(. Teaser transformer primary has 0."## times the
turns of main primary. +ut 4olt5turn is the same. Their secondaries ha4e the same number whichresults in equal secondary 4oltaes. imae
2. /f main primary has turns and main secondary has
turns, then the main transformation ratio is 5 .6owe4er, the teaser transformation ratio is
57 0.5 7 0."##a3. /f the load is balanced on one side, it is balanced on
the other side as well.. 8nder balanced load conditions, main transformerratin is 9(: reater than that of the teaser.
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There are two main reasons forthe need to transform from threephases to two phase
• To i4e a supply to an e<istin two
phase system from a three phasesupply.
• To supply two phase furnace
transformers from a three phasesource.
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• Two-phase systems can ha4e 3-wire, -wire, or :-wirecircuits. /t is needed to be considerin that a two-phase system is not 253 of a three-phase system. The
phase-to-neutral 4oltaes are 10= out of phase witheach other.
• Two phase -wire circuits are essentially *ust twounrounded sinle-phase circuits that are electrically10= out of phase with each other. Two phase :-wirecircuits ha4e four wires plus a neutral> the two phase4oltaes are 10= out of phase with each other.
• The easiest way to transform three-phase 4oltaesinto two-phase 4oltaes is with two con4entional
sinle-phase transformers. The rst transformer isconnected phase-to-neutral on the primary &three-phase' side and the second transformer is connectedbetween the other two phases on the primary side.
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• Scott T Transformer is built with two sinle phasetransformers of equal power ratin. The $/?and Teaser sections can be enclosed in a @oormount enclosure with $/? on the bottom and
Teaser on top with a connectin *umper cable. They can also be placed side by side in separateenclosures.
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Key oint!
• /f the main transformer has a turns ratio of (
(, then the teaser transformer requires a turnsratio of 0."## ( for balanced operation. Theprinciple of operation of the Scott connectioncan be most easily seen by rst applyin acurrent to the teaser secondary windins, and
then applyin a current to the main secondarywindin, calculatin the primary currentsseparately and superimposin the results.
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Ad"anta#es of the Scott TConnection!
•
/f desired, a three phase, two phase, or sinlephase load may be supplied simultaneously
• The neutral points can be a4ailable forroundin or loadin purposes
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$isad"anta#es when %sed for 3 hase &oadin#
• This type of asymmetrical connection &3phases, 2 coils', reconstructs three phasesfrom 2 windins. This can cause unequal
4oltae drops in the windins, resultin inpotentially unbalanced 4oltaes to be appliedto the load.
• The transformation ratio of the coils and the
4oltae obtained may be slihtly unbalanceddue to manufacturin 4ariances of theinterconnected coils.
• This desins neutral has to be solidly
rounded. /f it is not rounded solidly, thesecondary 4oltaes could become unstable.
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App'ication
• For /ndustrial Furnace Transformer.
• or Traction %rpose! The power is obtainedfrom the 220 !% or (32 !% or ((0 !% or ## !%,three-phase, eEecti4ely earthed transmission
networ! of the State lectricity +oard, throuhsinle-phase transformers or Scott connectedtransformer installed at the Traction Substation. The primary windin of the sinle-phase
transformer is connected to two phases of thetransmission networ! or here Scott-connectedtransformer is used, the primary windins areconnected to the three phases of thetransmission networ!.
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• The sinle-phase transformers at a TractionSubstation are connected to the same twophases of the transmission networ! &referred
as sinle-phase connection', or alternati4ely todiEerent pairs of phases- the three sinlephase transformers formin a delta-connectionon the primary side. )ut of three sinle-phase
transformers, one transformer feeds the
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(. Two T-T connected transformers are used tosupply a 0%, 33!% balanced 3-phase supply of
3300%. Calculate &a' 4oltae and current ratin ofeach coil. &b' !% ratin of the main and teasertransformer.
3300
%
3300
%
0%
0%
3 3 !
% - l o
a d
ro'ems
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Solution
a.
Drimary %oltae ratin
Drimary Current
Secondary %oltae
Secondary Current
b. !% ratin of the each transformer
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2. Two transformers are required for a Scott connectionoperatin at 0% 3-phase supply for supplyin two sinle-phase furnace at 200% on the two-phase side. /f the totaloutput is (:0!%, calculate the secondary to primary turnratio and the windin currents of each transformer.
0%
+
%a7200%
%b7200%
C
0%
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2-phase, G.: ! &9(0 hp', 20 %, #0 6B motor has aneHciency of 0."3 and a power factor of 0."0. /t is to be fedfrom a #00 %, 3-phase line usin a Scott-connected
transformer ban! &Fiure shown '.
Calculate
&a' The apparent power drawn by the motor&b' The current in each 2-phase line&c' The current in each 3-phase line
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(0 hpI 7 "3pf 70."0
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Solution
The of the motor is the of the transformer.
a. I 7 &motor' 7 7 7
S 7 D5cosJ 7 103#.((#50."
S 7 ((21:.("0G %
The apparent power per phase is
S 7 ((21:.("0G52 7
•
103#.(#
:#G.:10%
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b. Thee current in the two phase line
/ 7 S5% 7 :#G.:10520
c. The three-phase line current is
/ 7 S5&%K3' 7 ((21:.("0G5�<K3'
/ 7 23.:3(#
/ 7 (0."#""
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T6 ?A
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%
%+ %C
%a
%b
."## tap
Center
tap
Scott Connection Conuration &2-phase
wire'
TeaserDrimary
TeaserSecondary
$ain Drimary
$ainSecondary
bac!
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VA1
VB1
VC1
."## tap
Centertap
Teaser Transformer
$ain Transformer
%a2
%b2
%c2
Scott Connection Conuration &3-phase to 3-
phase' bac!