Three-Phase Transformers One 3-phase or 3 single-phase?

Unit 8
3-Phase Transformers
Objectives
• After studying this unit, you should be able to:
– Discuss the operation of three-phase transformers
– Connect three single-phase transformers to form a
three-phase bank
– Calculate voltage and current values for a threephase
transformer connection
Three-Phase Transformers
• Constructed by winding three single-phase
transformers on a single core
– Mounted in an enclosure full of dielectric oil
• Oil provides insulation and cooling and prevents
formation of moisture
Figure 8-1 Basic
construction of a
three-phase
transformer.
Unit 8 Homework
• Review Questions and Problems, Pages 189
and 190.
• We will Discuss the Open Delta with Unit 9, so
questions 1 and 2 will be covered then.
Objectives (cont'd.)
– Connect two single-phase transformers to form a
three-phase open-delta connection
– Discuss the characteristics of an open-delta
connection
One 3-phase or 3 single-phase?
• A single 3-Phase transformer is less expensive
than 3-Single-Phase Transformers, but if on of
the coils goes out you need to replace the
while thing.
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Connecting Single-Phase Transformers in
Three-Phase Bank
• Using three single-phase transformers to make a
three-phase bank:
– Connect primary and secondary windings in wye or
delta
– Label ends of each primary winding H 1 and H 2
– Label ends of each secondary winding X 1 and X 2
Figure 8-5 Identifying
the windings.
Figure 8-7 Connecting
three single-phase
transformers to form a
wye-delta three-phase
bank.
Three-Phase Transformer Calculations
• Follow formulas for transformer and threephase
calculations when computing voltage and
current for three-phase transformers
– Focus on the voltages and currents on the coils,
which will be the “phase values,” then look at the
voltages on the rest of the circuit.
Three-Phase Transformer Connections
• Wye-delta transformer has its primary winding
connected in a wye and secondary connected in
a delta
• Delta-wye transformer has its primary winding
connected in a delta and secondary connected
in a wye
One-line diagrams (Shorthand)
• One-line diagram: alternative method for
illustrating three-phase transformer connections
– Used to show main power distribution system of a
large industrial plant
Figure 8-8 One-line diagram symbol used to represent a deltawye
three-phase transformer connection.
Simple 3- Phase Connections
Δ→Y (Delta Primary, Wye Secondary)
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Δ→Y Phasor Diagram Y→Y Connection
Y→Y Phasor Diagram Special Note on Y-Y connections
Stable Y→Y
Y→Y stabilized by tying neutral together to the source-neutral
• This is not a popular connection because if the
currents in a Y-Y system are not balanced the
phase voltages will not be stable. (We will see
this in lab).
• The only way to totally fix this is to tie the
center of the Primary to the source Neutral
and tie the secondary Neutral to the same
point. This eliminates “Isolation.”
Y→Δ Connections
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Y→Δ Phasor Diagram Special note about Power Transformers
• Power Transformers frequently have dualsecondaries.
– This allows more options when connecting them.
EET150 Chapter 4- Transformer Connection 20
Pop Example: Connect a Δ→Y Δ →Δ Connection
Δ →Δ Phase Diagrams Winding Markings
After Manufacturing, the High Voltage Leads are
labeled with “H” and the low Voltage with “X”
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Delta Connection, maintaining
polarity
Closing a Delta
• Always check for proper polarity before making
final connection and applying power
– Test for proper phasing with a voltmeter
• If one phase winding is reversed, voltmeter will read
double the applied voltage rather than 0 volts when
applying power before connection is closed
• Test does not reveal whether reversed winding is in
primary or secondary
• Voltmeter may indicate voltage even if connection is
correct (usually close to normal output voltage)
T-Connected Transformers
• T-connection uses two transformers: main and
teaser
– Main transformer contains center or 50% tap for
both primary and secondary windings
– Teaser contains 86.6% voltage tap (preferred) for
both primary and secondary
• Connection can also be made with teaser with same
voltage rating as main transformer, but teaser operates at
reduced flux
Delta Polarity notes
• If you accidentally reverse the polarity of one
of the windings in a Delta configuration you
will severely unbalance the Secondary
Voltages so always check the polarity before
closing the delta.
(Always Check the fine print before you seal the
deal!)
Closing the Delta
When Closing the Delta a Δ primary to Δ secondary
will usually give you 0 Volts out. A Y to Δ will read
close to the output voltage, but if a coil is reversed it
will read double the input voltage.
T-Connected Transformers (cont'd.)
– Main transformer connects directly across power
line
• One primary lead of teaser connects to center tap of
main, 86.6% tap connects to power line, and same basic
connection for secondary leads
– Advantage: T-connection maintains better phase
balance than open-delta
– Disadvantage: one transformer must have a centertap
on both the primary and secondary windings
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T Connection
Figure 8-16 T-connected transformers.
Scott Connection
• Scott connection
– Converts three-phase power into two-phase using
two single-phase transformers
• Main has 50% tap and teaser has 86.6% tap on primary,
like T-connection, but secondary of each transformer
provide phases of two-phase system
Scott Connection Application of Scott Connection
Figure 8-19 Scott connection.
Moral of the Story:
• You can use a Scott T to power larger 2-Phase
Loads without pulling your 3-Phase out of
Balance
Single-Phase Zone
Heating/Blower
units in a 3Φ
system can be
supplied using a
Scott T.
If the 2 units were
run on 2 of the 3
phases, the 3Φ
system would be
unbalanced.
Zig-Zag Connection
• Zig-zag or interconnected-wye connection:
– Three-phase autotransformer with windings
divided into six equal parts
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Figure 8-21
Schematic diagram of
a zig-zag connection.
Figure 8-20 Zig-zag
connection.
Three-Phase to Six-Phase Connections
• Power system with more than three phases:
desirable for converting AC to DC with minimal
ripple.
– Common example: diametrical connection
• Requires only one low-voltage winding on each
transformer
• If center tapped, a neutral conductor for the six-phase
output permits half-wave rectification
• High-voltage windings can be connected in wye or delta
(preferred)
Double-Delta Connection and Double-
Wye Connection
• Double-delta connection:
– Transformers have two secondary windings that
provide equal voltage, form two delta connections
• Delta windings are reverse connected 180° out of
phase; primary may be wye or delta
Using a Zig-Zag Connection to get 3Φ
with Neutral From a Grounded-Delta
In this case I have a 480V
Grounded-Delta system
(one phase is grounded) and
I want 277/480 for lighting
with a separate Neutral that
is grounded. A 480/277V
transformer could be used,
but that would be a special
size. This solution uses a
common 480/240V
Transformer connected Z-Z.
A Zig-Zag Configuration can
do that, while adding in
Harmonic Suppression as a
bonus.
Figure 8-24 Three single-phase
center-tapped transformers are
used to convert three-phase into
six-phase.
Figure 8-22
Diametrical
connection.
Double-Delta Connection and Double-
Wye Connection
• Double-wye connection:
– Like double-delta but secondary windings form
two wyes
• Windings are reverse connected 180°
• Resembles diametrical connection with no center tap
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Figure 8-26 Schematic diagram
of a double-delta connection with
a wye-connected primary.
Figure 8-25
Double-delta
connection.
Problems 1 and 2
Calculate the Primary Current (and a bunch of other
stuff along the way)
Solution sequence
1) Use the Turns Ratio to figure out the secondary
voltage (VP →N → VS)
2) Use the Secondary Voltage to figure out the load
current (VS →Z →ILoad)
3) Use the Load Current to figure out the Secondary
Current (ILoad →Configuration →ISecondary)
4) Use the Secondary Current to figure out the Primary
Current (ISecondary →N →IPrimary)
Figure 8-28 Schematic
diagram of a double-wye
connection.
Figure 8-27 Doublewye
connection.
Primary Current
Bear in mind that with a transformer, the primary
current is determined by the current drawn by the load.
Example
Let’s say that the Primary Voltage is 1380 V and the
transformer N=6.63:1. What is the Primary Current and
Transformer Power rating?
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V
I
I
Phase
Phase
Example Detail 1
First Detail: If the Transformer Primary winding is rated
for 1380V, and is connected in a Wye configuration,
what are the Phase and Line Voltages?
•Recall that the Phase voltage is the voltage across an element (like a
Primary Coil) and the Line Voltage is the Voltage from “Line to Line.”
E
E
E
Phase
Line
Line
1380 V
E
Phase
2390 V
3
1380 V
Example Solution Step 2
Determine Load Current
Phase
I
I
V
208V
3 3
VPhase
120V
Z 2.
77
I 43.
3A
Line
Line
120V
43.
3A
Example Solution Step 4
Determine Primary Currents
Pr i Phase
Line
I
Phase
I
Sec Phase
N
3.
77 A
25A
6.
63
3.
77 A
3
2390 V
Example Solution Step 1
Determine Secondary Voltage
V
In a
V
Sec
Phase
VPr
i
N
1380V
6.
63
208V
configuration,
V
V
Line
208V
Phase
V
Line
Example Solution Step 3
Determine Secondary Current
I
Phase
I
Line
3
43.
3A
3
25AV
Example Solution Step 5
Determine Transformer Power
Power ( VA)
VPhaseIPhase
1380
V 3.
77 A 5.
2kVA
Each
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Example Problem Summary
• Given this circuit, the loads will receive 120V
each IF EACH OF the transformers have a
power rating of AT LEAST 5.2 kVA each.
Summary (cont'd.)
• Use phase values for voltage and current
when computing transformer values
• Open-delta connects only two transformers
• Two T-connected transformers can supply
three-phase power
• T-connection: main contains center tap,
teaser ideally contains 86.6% voltage tap
Summary
• Three-phase transformers are constructed
by winding three separate transformers on
same core material
• Single-phase transformers can be used as
three-phase bank by connecting primary and
secondary windings wye or delta
• Follow three-phase circuit/transformer rules
to compute three-phase transformer values
Summary (cont'd.)
• Scott connection converts three- to twophase
power
• Three-phase transformers can connect to
produce six-phase power
• Double-delta and -wye require two
secondary windings on each transformer
without a center-tapped winding
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