Transcript Electro Mechanical System

Delta-Delta Connection
 Three single-phase transformers connected delta-delta
 H1 terminal of each transformer is connected to the H2 of the
next transformer. X1 terminal of each transformer is connected
to the X2 of the next transformer
 Primary and secondary voltages and currents are inphase.
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Delta-Wye Connection
 Three single-phase transformers connected delta-wye.
 H1 terminal of each transformer is connected to the H2 of the
next transformer the X2 terminals of all transformers are
connected together to form a neutral terminal.
 Delta-wye connection produces a 30° phase shift between the
primary and secondary voltages and currents.
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Wye-Delta Connection
 Connection are identical to those in delta-wye
 The primary and secondary connections are interchanged.
 The H2 terminals of all transformers are connected together to
form a neutral terminal.
 The X1 terminal of each transformer is connected to the X2
terminal of the next transformer.
 Connection results in a 30° phase shift between the
primary and secondary voltages and currents.
Wye-wye Connection
 In wye-wye, special precautions have to he taken to
prevent severe distortion of the line-to-neutral
voltages, when load becomes unbalance.
 One way to prevent is to connect the neutral of the
primary to the neutral of the source, usually by ground.
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Wye-wye Connection
 Another way is to provide each transformer with a third winding,
called tertiary winding.
 The tertiary windings of the three transformers are connected
in delta.
 They often provide the substation service voltage where the
transformers are installed.
 There is no phase shift between the line voltages of a wye-wye
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Open-Delta Connection
 It is possible to transform the voltage of a 3-phase
system using only 2 transformers.
 Identical to a delta-delta connection, less one transformer.
 Limited use due to a 86.6% reduction in the installed power
capability.
 Two 50 kVA transformers could carry 100 kVA, but
limited to 86.6 kVA in an open-delta connection.
 Used in emergency conditions.
 Used in metering of power system voltages.
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Three-phase Transformers
 One 3-phase transformer may replace a
transformer bank of three single-phase units.
 The magnetic core has three flux-carrying legs.
 Two yokes, bottom and top, couple the flux of
the three legs.
 The sum of the three fluxes equals zero.
 Each leg contains the primary and secondary
windings of one of the phases.
 The windings are connected delta or wye,
internal to the transformer tank.
 Only six terminals are brought out of the tank.
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Step-up & step-down
autotransformers
 When a small change in voltage is required in
3-phase, it is economical to use 3 single phase
transformers to create wye-connected
autotransformer
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Home Work & Assignment
Home Work
Page 245, Example 12.1
Page 247, Example 12.2
Page 249, Example 12.3
Page 251, Example 12.4
Page 259, Example 12.7
Assignment 4
Page 259, Problems: 12-2, 12-4, 12-6, 12-8,
12-10
Due Date: Tuesday 1st March, 2011
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