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Overview
•
•
•
•
•
18-Specialty Transformers
Autotransformers
Center-tapped Transformers
Current Transformers
Potential Transformers
Regulating Transformers
ECEGR 450
Electromechanical Energy Conversion
2
Dr. Louie
Autotransformers
Autotransformer Benefits
• Transformers discussed so far have electrical
isolation from primary to secondary
• Possible to connect primary and secondary
electrically
• When compared to conventional two-winding
transformers, autotransformers offer:




 Series mutual inductive circuits
• Power is transferred to load via conduction and
induction
• Autotransformers are particularly useful when
voltage transformation is between 1:3 and 3:1
• Often grounded Y-connected if three phase
Dr. Louie
More inexpensive construction
Greater power density
Higher efficiency
Lower excitation current to establish the same flux
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Autotransformer Drawbacks
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Dr. Louie
Ideal Autotransformer Model
• When compared to conventional two-winding
transformers, autotransformers offer:
Primary and secondary windings connected
(“common winding”)
 No electrical isolation between primary and
secondary
 Higher short-circuit current
I1
+
V1
-
I2
+
e2
-
primary N1
+
e1
-
+
ZL
V2
-
secondary N2
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Dr. Louie
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10/21/2012
Ideal Autotransformer Model
Ideal Autotransformer Model
Transformer ratio:
By Ampere’s Law (sum of mmf = 0 due to ideal
assumption)
V1  e1  e 2
V2  e 2
V1 e1  e 2 N1  N2


 1  a  aT
V2
e2
N2
Recall: a 
(N1  N2 )I1  N2I2
N1
N2
I2 N1  N2

 1  a  aT
I1
N2
I1
I1
+
+
e1
-
V1
I2
+
e2
-
-
+
ZL
V2
e1
since aT >1, this
configuration
is a step-down
autotransformer
+
e2
ZL
-
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V2
-
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Dr. Louie
Ideal Autotransformer Model
Apparent power of ideal autotransformer:
• Other autotransformer configurations are possible
• Example: step-down autotransformer
V
S  V2I2  2 aT I2  V1I1
aT
V1  e 2
V2  e1  e 2
Real, imaginary and complex power is conserved
V1
e2
N2


 aT  1
V2 e1  e 2 N1  N2
I1
+
e1
-
I2
+
e2
-
+
-
Ideal Autotransformer Model
V1
I2
-
V1
-
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+
+
+
-
(How does this compare
with power from a twowinding xfmr? See text
page 247)
ZL
I1
+
+
V2
-
Non-Ideal Autotransformer Model
ZL
+
e2
-
-
V2
aT depends
on connection
-
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Dr. Louie
Center-Tapped Transformers
• Non-ideal autotransformer model has similar
elements as non-ideal two winding transformer
Consider the shown winding arrangement:
Let N2 = N3
 See ages 250-251 of text
 See Example 4.10 and 4.11 of text
I2
• Impedances can be transformed from secondary
2
to primary by: Z2aT
I1
-
11
+
N2
V2
N3
V3
+
V1
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+
e1
V1
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I2
+
N1
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-
-
+
I3
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Center-Tapped Transformers
Center-Tapped Transformers
Let N2 = N3
Center tapped transformers used to obtain
120/240V in residences
V1 = V2a
V1 = V3a
V23 = V2 + V3
I2
I1
N2
+
V3G
-
+
-
V23
N1
V1
V2G
I2
+
V2
N3
+
V3
-
I3
I1
120V
V3G
120V
V2G
V2
N3
V3
+
-
-
+
-
V23
N1
V1
+
N2
+
-
I3
-
240V
V2G, V3G are 180 deg.
apart wrt ground
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NEMA Outlet types
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Instrument Transformers
• Direct measurement of high voltage or high
current is not practical
 Relays, meters, etc require much lower voltage and
current
• Solution: use specialty xfmrs to step down
voltage or current
• Two general types:
 Current transformers (CTs)
 Voltage transformers (VTs, also called PTs)
Note: two
“hot” lines
for 240VAC
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• Polarity is critically important when CTs, PTs are
used as inputs to relays
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Instrument Transformers
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Voltage Transformer
• CTs, PTs have specified ratios
• Step-down transformer
 Example: a 100:1 PT will output 10V if the primary
voltage is 1,000V
 Example: a 120:5 CT will output 5A if the primary
current is 120 A
• Insulation rating, accuracy classes (ANSI C57.13)
and burden are also important specifications
 Primary in the kV range, secondary usually ~120V
 Large number of primary turns, small number of
secondary turns
 VA rating usually less than 500 VA
• Insulation of VTs often a challenge due to large
primary voltage
 one approach is to capacitively couple the high
voltage side
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Voltage Transformer
Voltage Transformer
High voltage
line
VT
V
Source: www.electro-meters.com/
Source: www.technovidyut.com
Secondary
grounded
for safety
Soruce://tucsontransformer.com
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Voltage Transformer
Question
• Capacitance-coupled voltage transformers are
often used to measure voltages in the high 100kV
range
• Utilizes voltage divider to measure lower voltage
To reduce the insulation requirements of the
transformer, should C1 be larger or smaller than C2?
CCVT
C1
High voltage
line
C2
V
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Dr. Louie
Question
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Current Transformers
To reduce the insulation requirements of the
transformer, should C1 be larger or smaller than C2?
C1 < C2
1
By voltage divider, |XC1| > |XC2|, and XC  C
so we want C1 < C2
• Designed to measure current
 Step down primary current (usually 5A secondary)
 Large number of secondary turns, small number of
secondary turns (often one turn)
 Very small VA rating (<100)
• Secondary terminal must have low impedance
connected (nearly shorted)
Example: C1: 2,284 pF, C2: 87,411 pF
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Current Transformers
Current Transformers
Line current
High voltage
line
VT
A
Secondary
grounded
for safety
Source: www.electro-meters.com/
Source: www.ABB.com/
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Dr. Louie
Current Transformers
Current Transformers
• Under normal operation, secondary is nearly shorted
• |e1| << |Vload|
• What happens if the secondary is opened?
I1
I0
•
•
I0 does not appreciably change
Excitation current dramatically increases
 Rc losses increase (CT may overheat)
 Im increases (may saturate CT and affect calibration)
•
I2
If
|e2| may increase to dangerous levels
I2 = 0
I1 = 0
I0
If = I0
+
Xm
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Dr. Louie
Rc
e1
-
+
e2
A
Internal resistance of
ammeter is nearly zero
+
Xm
Rc
e1
-
-
a: very small
e2
-
a: very small
+
Vload
+
+
Vload
load
-
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load
-
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Current Transformers
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Regulating Transformers
• It is very important to never open-circuit a CT
• Short terminals before removing ammeter, relay,
etc
• Tap Changing Xmfr: transformer with adjustable
turns ratio. Used to control (or maintain) the
voltage magnitude on one side of the
transformer.
• Phase-shifting Xfmr: transformer capable of
controlling the voltage phase shift from primary
to secondary. Used to control flow of real power
in transmission systems
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Summary
• Autotransformers are commonly used as they are
cheaper and have greater power density than
two-winding transformers
• CTs and VTs (PTs) are instrument transformers
used to step down high current and voltage to
practical levels for meters and relays
• CTs should never be open-circuited
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