Download Basic Connection for three phase transformer File

Electrical Machine – I
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Aim: To Study the Basic connections of 3- Transformer.
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Specific Objective :
After having study the experiment, one should be able to :
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Identity the H.V and L.V winding.
Understand the basic fundamentals of 3- transformer
connections.
Machine Specification:
3- Transformer.
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Rationale :
Three phase operation can be either by means of a single three phase
transformer or three numbers of identical single phase transformers. So
altogether 3 sets of windings are used for this operation. Each of the three
phases will have one HV winding and one LV winding according to
construction or one primary winding and one secondary winding according to
the operation.
Let A1 A2 , B1 B2 and C1 C2 be terminals of the three primary winding
and a1a2 , b1 b2 and c1 c2 are the terminals of respective secondary windings.
The high –voltage and low-voltage winding terminals of a three-phase
transformer are connected either in star or in delta for connections to a three
–phase system. A bank of three single-phase transformers can also be
connected similarly. When the primary high-voltage windings terminals are
connected in, say, star and the secondary low-voltage winding terminals are
connected in, say, delta, it is said that the transformer windings are
connected in star-delta .Similarly, the following are some of the more
popular connection used.
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Star–Star (Y-Y),
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Delta –Delta (∆-∆)
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Star -Delta (Y-∆),
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Delta –Star (∆-Y)
There is a define time-phase relationship between the terminals
voltages of the high-voltage side and low-voltage side for three connections.
The time-phase relationship between the voltages of high-voltage and lowvoltage side will depend upon the manner in which the windings are
connected .
Star-Star Connection:
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Electrical Machine – I
This connection is symbolically denoted as Y-Y. Fig 1(a) shows the
connection diagram and the Vector Diagram for star-star connection. Here,
the finishing end of H.V windings A1, B1 and C1 are connected together to
form the primary neutral point N. The three phase input supply points R, Y,
and B are connected to the starting ends A1, B1 and C1 respectively.
In star-star connection line-to-line voltage ratio is same as the phase –tophase voltage ratio which is nothing but the secondary to primary turns
ratio K. Further, the secondary line-to-line voltage is in phase with the
primary line-to-line voltage i.e phase displacement is zero as shown in
vector diagram.
If the high-voltage side and low-voltage side windings are connected in
star-star as shown in fig (a), the phase displacement will be zero.
If ,however, the low-voltage windings connections are reversed, the timephase displacement in induced voltages between the high-voltage and lowvoltage winding will be 180 deg as shown in fig (b).
Advantages of The number of turns/phase and the amount of insulation is
minimum as Vphase = VL /√ 3 . Hence , Y-Y connection is the most economical
connection for small capacity, HV transformer.
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The possibility of utilizing both the star points N and n for a fourth
wire may be useful.
Disadvantages of
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Even if a sinusoidal voltage wave is impressed on the primary
side, the exciting current is not a pure sine wave, but contains
third harmonic component. Consequently , the three exciting
current do not add up to zero and neutral is needed to a carry the
resultant current. If the neutral is not present, the exciting
current in the three phases will be sine wave, leading to the flux
and hence the secondary phase voltage will have a third harmonic
component.
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If the loads are unbalanced, the secondary phase voltage becomes
unbalanced unless the secondary star point is earthed.
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For a satisfactory operation, another set of delta connected
secondary winding called tertiary winding may be required. This
delta connected winding suppresses the third harmonic
components as well as supply power to the local loads in the
generator station or sub-station.
Delta-Delta Connection
Symbolic representation for this connection is ∆-∆ Fig 1(c) Shows the
connection diagram and 4(b) shows the Vector Diagram for delta-delta
connection.
In ∆-∆ connection ,the line-to-line voltage ratio is same as the
transformation ratio K. similar to the condition seen in Y-Y Connection.
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Electrical Machine – I
Further , the output line-to-line voltages VRY , VYB and VBR are in phase with
input line-to-line voltages VRY , VYB and VBR respectively.
If the high-voltage side and low-voltage side windings are connected in
delta-delta as shown in fig1 (c), the phase displacement will be zero.
If ,however, the low-voltage windings connections are reversed, the timephase displacement in induced voltages between the high-voltage and lowvoltage winding will be 180 deg as shown in fig (d).
Advantages
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The cross-sectional area of conductor is reduced because the
current through the winding or phase current is 1/√ 3
times the
line current.
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Third harmonic component of magnetizing current can flow within
the delta connected primary windings without flowing in the line
wires.
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Large unbalance of load can be met without difficulty.
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If the operation is done with three units of single-phase
transformers , it is possible to operate the transformer on 58
percent of its normal rating in V-V connection, should one of the
transformer develop a fault.
Disadvantages
The absence of star point be the only disadvantage.
Star-Delta Connection
Symbolic representation for this connection is Y-∆ Fig 2(a) Shows the
connection diagram and the Vector Diagram for star-delta connection.
In Y-∆ connection , the line-to-line ratio becomes K/√ 3
and normally
used
for the step-down operation. Further the secondary line-to-line
voltage lags the input or primary line-to-line voltage by 300.
If the primary high-voltage windings are connected in star and the
secondary low-voltage windings are connected in delta, the phase
displacement will be -300as shown in fig 2(a) If the winding connections of
the low-voltage side are reversed the phase displacement will +300 as shown
in fig 2(b) .
Advantages
Delta connected secondary causes the third harmonics currents to get
circulated within the winding and not to appear on the lines.
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The connection can withstand the unbalanced operation in a stable
manner.
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Electrical Machine – I
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If the HV winding is in star-connected side, there is some saving in
the cost of insulation as the phase voltage is 1/√ 3
times line
voltage.
Delta-Star Connection
Symbolic representation for this connection is ∆-Y Fig 2(c) Shows the
connection diagram and the Vector Diagram for delta-star connection.
In ∆-Y connection, the line-to-line voltage ratio becomes √ 3
√3
K or
times the phase voltage ratio and hence used for step-up operation. Further
the output line-to-line voltage VRY , VYB and VBR leads the supply input line-toline voltages VRY , VYB and VBR be 300 respectively.
If the primary high-voltage windings are connected in delta and the
secondary low-voltage windings are connected in star, the phase
displacement will be +300 as shown of fig.2(c). If the winding connections of
the high-voltage side are reversed the phase displacement will -300 as shown
in fig 2(d).
Advantages of ∆-Y
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Delta connected primary causes the third harmonics currents to
get circulated within the winding and not to appear on the lines.
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The connection can withstand the unbalanced operation in a stable
manner.
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If the HV winding is in star-connected side, there is some saving in
the cost of insulation as the phase voltage is 1/√ 3
times line
voltage.
The phase difference between the high voltage and low-voltage
windings for different types of connections can be represented by comparing
it with the hour hand of a clock. When the hour hand of clock is at the 12
o’clock position it is considered zero displacement. When it is at the 11 clock
position the displacement is +300 (Anticlockwise is positive). When the hand
is at the 1 o’clock position the displacement is -300 and at the 6 o’clock
position it is 1800, as shown in fig.3
Thus the connections of fig (a), (b), (c) and (d) can respectively be
represented Yyo , Yy6 ,Dd0 and Dd6 . The connections of fig (a), (b), (c) and
(d) are often represented as Yd1 , Yd11 , Dy1 and Dy11 respectively.
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Fig-1
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Fig-2
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Electrical Machine – I
Y
Date:
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1
YZ 1
Sankalchand Patel College of Engineering
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Sankalchand Patel College of Engineering
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Date: