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Transformer FundamentalsTransformer
Fundamentals
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Transformers
• Transfer energy from one circuit to another by means of magnetic coupling
• Used to transform voltage levels- Minimize transmission losses
• S = VI; If V is high, I is low• Losses = I2Z, lower I = lower losses
• Used to act as sinks for harmonics- Delta windings absorb triplins (3rd, 9th, 15th, etc.)
• Applied in generation, transmission, distributionand utilization areas of the power system
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Transformers are used throughout the bulk electrical system:
Generation
Transmission
Distribution
Utilization
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Flux in Core Steel Core
PrimaryWinding
Secondary Winding
Basic Transformer
Transformer Fundamentals
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R = resistance; X = reactance (inductive); N = No of turns; E = voltage
Basic Equivalent Circuit
Winding Losses(≈1.5% at full load)
MagnetizingLosses (≈0.5%)
√
√
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• V1I1 = V2I2• N1V2 = N2V1
• N1I1 = N2I2
AA
I = 5 A I = 10 A
V = 100 V V = 50 V
N = 100 N = 50
Ideal Transformer – No Losses
Transformer Formulas
Transformer Fundamentals
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Transformer Formulas
PrimaryWinding
SecondaryWinding
TertiaryWinding
E1 = 1000N1 = 100E/N = 10
N2 = 50E2 = 50 X 10 = 500
N3 = 20E3 = 20 X 10 = 200
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• Found in generation, transmission, and distribution areas of the power system- Used to transfer large amounts of bulk power to different
voltage levels• Step Up, Step Down
- Used to regulate transmission and sub-transmission voltages• Autotransformer
• Typically iron core• Typically liquid insulation (wet vs. dry)• Two or Three Winding• With or without Taps• With or without Load Tap changers (LTC)
Power Transformers
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Bushing
Cooler
LTC
LTCControlCabinet
Cooler Main Tank
Power Transformers
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87T Ig• Two winding
transformer, with REF
Typical Applications
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87TIg
• Substation Differential Wrap, with REF
Typical Applications
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• Dual generator unit differential wrap
Typical Applications
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REF REF
87T 87THigh Speed Trip for Bus Faults
Main-Tie-Main Substation
Typical Applications
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From IEEE Press Book
• Small 500 to 10,000 kVA
• Medium 10,000 kVA to 100 MVA
• Large 100 MVA and above
• Less than 500 kVA not considered a powertransformer
Ratings and Classifications
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• Core Form- Single path for the magnetic circuit- Less $$$
• Shell Form- Multiple paths for the magnetic circuit- Better through-fault withstand
Windings
CoreCore
CoreCore
Core Types
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• Dry- Used where liquid spill cannot be tolerated- Small ratings, lower voltage distribution
• Wet- Offer smaller size, lower cost and greater overload
capacity- Liquids have greater coefficient of heat than dry
insulation- Vast majority of power transformers use wet (liquid)
insulation
Insulation Materials
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• Single Phase- Typical for lower voltage load-serving distribution- May be applied in higher capacities where a spare is
desired- 4 transformers on site, 3 connected for three phase duty,
1 as a spare
• Three Phase- Typical for T&D- Less expensive than 3 single phase transformers of the
same rating- Vast majority of power transformers
Single vs. Three Phase
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• No load taps - Taps are adjusted under no-load conditions to bring
secondary voltage to desired level- Cheaper than on-load tapchanger- Cannot dynamically adjust to voltage to load and line
drop conditions
• On-load tapchanger (LTC)- Taps are adjusted under load- Can respond dynamically to adjust voltage to load and
line drop conditions
Ratio Adjustment
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Autotransformer
Transformer Fundamentals
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• H1, H2, H3- Primary Bushings
• X1, X2, X3- Secondary Bushings
TransformerH1H2H3
X1X2X3
Wye-Wye H1 and X1 at zero degreesDelta-Delta H1 and X1 at zero degreesDelta-Wye H1 lead X1 by 30 degreesWye-Delta H1 lead X1 by 30 degrees
ANSI Standard
Bushing Nomenclature
Wye-Wye H1 and X1 at zero degreesDelta-Delta H1 and X1 at zero degreesDelta-Wye H1 lead X1 by 30 degrees or X1 Lags H1 by 30 degreesWye-Delta H1 lead X1 by 30 degrees or X1 Lags H1 by 30 degrees
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• Polarity – used to describe the phase relationship of single phase transformers- ANSI Standard
• Additive if voltage is 8660 or below and the kVA is 200 or less (voltage across any two bushings can be rated)
• Subtractive otherwise (voltage across any two bushings less than rated)
• Angular Displacement – used to describe the voltage phasing on three phase transformers- ANSI Standard
• Wye-wye and delta-delta; 0 degrees displacement• Wye-delta and delta-wye; X1 lags H1 by 30 degrees
or “High leads low by 30”
ANSI C57.12 & C57.105
Polarity & Angular Displacement
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• Wye-Wye– Cheaper than 2 winding if auto bank– Conducts zero-sequence between circuits– Provides ground source for secondary circuit
• Delta-Delta– Blocks zero-sequence between circuits– Does not provide a ground source
• Delta-Wye– Blocks zero-sequence between circuits– Provides ground source for secondary circuit
• Wye-Delta– Blocks zero-sequence between circuits– Does not provide a ground source for secondary
circuit
Winding Arrangements
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• ANSI Y-Y & Δ-Δ @ 0°• ANSI Y-Δ & Δ-Y @ H1 lead X1 by 30° or X1 lag H1 by 30°
Angular Displacement
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• ANSI Y-Y & Δ-Δ @ 0°
• ANSI Y-Δ & Δ-Y @ X1 lags H1 by 30°- ANSI makes our life easy
• Euro-designations use 30° CW increments from the H1 bushing to the X1 bushings- Dy1=X1 lags H1 by (1*30°) 30°
• or, H1 leads X1 by 30°- Think of a clock – each hour is 30
degrees
0
6
39
8
7
10
11 12
5
4
H1X1
• Dy1 = X1 lags H1 by 1*30 = 30, or H1 leads X1 by 30 (ANSI std.)• Dy1 equivalent to ANSI DabY
Transformer Fundamentals
Polarity & Angular Displacement
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*1
*1
*2
*2
*1 = ANSI std. @ 0°
*2 = ANSI std. @ X1 lag H1 by 30°, or “high lead low by 30°”
• IEC (Euro) practice does not have a standard like ANSI
• Most common GSU connection is Yd1 (High lead low by 30°)
• Obviously observation of angular displacement is extremely important when paralleling transformers!
Angular Displacement
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HV LV
H1
H2
H3
X1
X3
X2
A
B
C
a
b
c
a
b
c A
B
C
Assume 1:1 transformer
• H1 (A) leads X1 (a) by 30
• Currents on “H” bushings are delta quantities
Angular Displacement - Development
Transformer Fundamentals
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HV LV
H1
H2
H3
X1
X3
X2
a
b
c
A
B
C
IA-IC
IB-IA
IC-IB
A
B
C
ab
c
Assume 1:1 transformer
• H1 (A) leads X1 (a) by 30
• Currents on “X” bushings are delta quantities
Angular Displacement - Development
Transformer Fundamentals
©2008 Beckwith Electric Co., Inc.