Transformer FundamentalsTransformer

Fundamentals

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

Transformer Fundamentals

Transformers are used throughout the bulk electrical system:

Generation

Transmission

Distribution

Utilization

Transformer Fundamentals

Flux in Core Steel Core

PrimaryWinding

Secondary Winding

Basic Transformer

Transformer Fundamentals

R = resistance; X = reactance (inductive); N = No of turns; E = voltage

Basic Equivalent Circuit

Winding Losses(≈1.5% at full load)

MagnetizingLosses (≈0.5%)

√

√

Transformer Fundamentals

• 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

Transformer Formulas

PrimaryWinding

SecondaryWinding

TertiaryWinding

E1 = 1000N1 = 100E/N = 10

N2 = 50E2 = 50 X 10 = 500

N3 = 20E3 = 20 X 10 = 200

Transformer Fundamentals

• 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

Transformer Fundamentals

Bushing

Cooler

LTC

LTCControlCabinet

Cooler Main Tank

Power Transformers

Transformer Fundamentals

87T Ig• Two winding

transformer, with REF

Typical Applications

Transformer Fundamentals

87TIg

• Substation Differential Wrap, with REF

Typical Applications

Transformer Fundamentals

• Dual generator unit differential wrap

Typical Applications

Transformer Fundamentals

REF REF

87T 87THigh Speed Trip for Bus Faults

Main-Tie-Main Substation

Typical Applications

Transformer Fundamentals

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

Transformer Fundamentals

• 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

Transformer Fundamentals

• 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

Transformer Fundamentals

• 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

Transformer Fundamentals

• 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

Transformer Fundamentals

Autotransformer

Transformer Fundamentals

• 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

Transformer Fundamentals

• 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

Transformer Fundamentals

• 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

Transformer Fundamentals

• ANSI Y-Y & Δ-Δ @ 0°• ANSI Y-Δ & Δ-Y @ H1 lead X1 by 30° or X1 lag H1 by 30°

Angular Displacement

Transformer Fundamentals

• 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

*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

Transformer Fundamentals

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

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.