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Introduction v

TABLE OF CONTENTS

Chapter 1 Transformer Concepts

Introduction to Distribution Transformers . . . . . . . . . . . . . . . 1The Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Formulas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Power Triangle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Transformer Ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Transformer Taps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Transformer Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Transformer Terminal Designations . . . . . . . . . . . . . . . . . . . . . 9Polarity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Wye, Delta Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Transformer Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Completely Self-Protected Transformers . . . . . . . . . . . . . . . . . . 17Ferroresonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Vectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Why , or 1.73? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Angular Displacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Paralleling Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Chapter 2 Transformer Connections

Index to the Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Notes to the Connection Diagrams . . . . . . . . . . . . . . . . . . . . . . 28Single-Phase Installations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Transformer BanksWye - Wye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32Wye - Delta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35Open Wye - Open Delta . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Delta - Wye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39Delta - Delta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Three-Phase Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3

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vi Distribution Transformer Handbook

Chapter 3 Installing Transformers

Lifting and Handling Transformers . . . . . . . . . . . . . . . . . . . . . . 45Nameplates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Safety Tips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48Installation Procedure for Overhead Transformers . . . . . . . . . 50Backfeed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51Single-Phase Transformer Loads . . . . . . . . . . . . . . . . . . . . . . . . 52Load Checks on Single-Phase Transformers . . . . . . . . . . . . . . . 53Three-Phase Bank Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54Load Checks on Delta, Wye Banks . . . . . . . . . . . . . . . . . . . . . . . 55Make or Break Parallel Circuits at Transformer Banks . . . . . . 57

Phasing and Paralleling Three-Phase Installations . . . . . . . . . . 58Minimum Pole Class Guidelines . . . . . . . . . . . . . . . . . . . . . . . . 62Strength of Wood Poles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63Grounding Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64Fusing Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71Padmounted Transformer Installations . . . . . . . . . . . . . . . . . . . 73Safety Clearances Around Padmount Transformers . . . . . . . . 77Work Clearances Around Padmount Transformers . . . . . . . . . . 78Guard Posts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

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16 Distribution Transformer Handbook

Arresters create a safe, low-resistance path for lightning to get toground, that bypasses the transformer. Lightning strikes can exceedone million volts, so the connections at the arrester must be tight,

and the ground wire properly sized for surge currents that accompanythese high voltages.

Transformer protective devices.

CutoutArrester

Shunts lightning andother high voltagespikes to ground

Discourages wildlife fromresting on a transformer

primary wire and shorting a high-side

line to ground

Fuse opens theprimary line when

current is excessive

Allows excessive tankpressure to escape to

the atmosphere

Wildlife protector Pressure relief valve

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C

N

n

B

A

c

b

a

A

B

CN

a

x3 x1x2

b

c

n

A

B

C

N

n

a

b

c

Three-phase, four-wirewye primary

Three-phase, four-wirewye secondary

0

angular displacement

Wye-Wye

Three-phase, four-wire wye primaryThree-phase, four-wire wye secondary

Secondary services120 volts, phase-to-neutral208 volts, phase-to-phase

H1

H2 H2

x3 x1x2

H1

x3 x1x2

H1

H2

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Transformer Connections 35

C

B

A

b

n

c

a

A

N

B

C

-a

H1

x2x1x3

-b

-c208 V

n

AB

C

-a

-b

-c

n

Three-phase, three-wirewye primary

Three-phase, four-wiredelta secondary

210 angular displacement

H2

Do notpermanently ground

H1

x2x1x3

H2H1

x2x1x3

H2

Wye-Delta

Three-phase, three-wire wye primaryThree-phase, four-wire delta secondary

Secondary services240 volts, phase-to-phase120 volts, b-to-neutral, c-to-neutral208 volts, a-to-neutral

Float the primary neutral

When concerned about ferroresonance, install a

temporary grounding jumper between the floatingprimary neutral and ground, when switching thetransformer bank in or out of service.

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A

BC

a

H1

x2x1x3

b

c

n

A

B

C

a

b

c

n

H2H1

x2x1x3

H2H1

x2x1x3

H2

Delta-DeltaThree-phase, three-wire delta primaryThree-phase, four-wire delta secondary

Secondary services240 volts, phase-to-phase120 volts, a-to-neutral, b-to-neutral208 volts, c-to-neutral

Three-phase,four-wire delta secondary

0 angular displacement

a

n

c

b

Three-phase,three-wire delta primary

A C

B

208 V

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45

CHAPTER

INSTALLING TRANSFORMERS

LIFTING AND HANDLING TRANSFORMERS

Lift overhead transformers by their lifting lugs only, using a nylon

web sling or a rope sling. Sling angles of greater than 45 arepreferred. Avoid sling angles of less than 30 because of the hightension in the sling. If the vertical clearance above the transformer islimited, use a spreader bar in place of a sling, and install cover-up onany energized conductors nearby.

Do not lift a transformer from beneath a bushing, pressure reliefvalve, drain plug, or any other attachment not specifically designedfor lifting.

Do not move or shift a transformer by grasping the bushings, fins,or plugs. Porcelain bushings can be damaged during handling in waysnot visually obvious, then fail when the unit is put into service.

The windings can be damaged if the transformer is dropped or

severely jolted.When handling a transformer, take care to not damage the tankfinish. Paint scratches can lead to rust.

Slingangle

3

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NAMEPLATES

Each transformer has a manufacturers nameplate with importanttechnical information.

The nameplate shown here is for an overhead single-phasetransformer.

1 HV High-voltage (primary) winding. The low number (7200)is the phase-to-neutral (coil) voltage. The high number (12470)is the phase-to-phase voltage. This transformer is for installationin a wye system. Note: On nameplates for padmount transform-ers and single-bushing overhead transformers, the low numberappears last instead of first.

2 LV Low-voltage (secondary) winding delivers 120 and 240volts.

HV TAP POSIT ION

100%

97.5%

95%

92.5%

90%

1 OR A

2 OR B

3 OR C

4 OR D

5 OR E

NON-PCB MINERAL OIL

WHEN MANUFACTURED CONTAINED

LESS THAN 1PPM PCB

GAL 11%IZ WT 202 LBHV AL 95 BIL

HV

CUSTI.D.

LV

10 60 HZ

CLASS OA

READ INSTALLATION AND OPERATION INSTRUCTIONS S201-10-1

LV AL 50 BIL 65 C RISE

A D D P O L . 1.8

E 211072 05W5

7200 / 12470Y

120/240 CP123456790

000000556MFG DATE AUG 98MS 85C

NOTES:

CAT

SER

MFG

KVA

TK

5

CP

C B C B

DA D IA

x1x2x3 x1

H1 H2

x2x3

SW

1

4

5

6

7

2 9

14

3

10

11

12

13

8

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Installing Transformers 53

LOAD CHECKS ON SINGLE-PHASE TRANSFORMERS

Quick Check

To determine the approximate kVAload on a single-phase transformer:

1. Measure the current in the twoline conductors

2. Add the amps together3. Multiply by 1204. Move the decimal point three

places to the leftExample: If the readings are55A and 60A:

55A+60A

115A

115 120 = 13800

The load is 13.8 kVA

Complete Calculation

Calculate the load on each half of the transformer separately, thenadd them together to determine the full load.

Example: The readings on a 25 kVA transformer are 30A and 160A:

The total load is within the transformer rating, but one secondarywinding exceeds 12.5 kVA and is severely overloaded.

30 120

1,000+

160 120

1,000

= 3.6 + 19.2

=22.8 kVA

Total load in kVA =

Total load in kVA =current a voltage a-to-n

1,000

current b voltage b-to-n1,000

+

120V

240V

120V

an

b

Note: Dont use this quick check if there isconsiderable imbalance between the twocurrent readings. Instead, use the CompleteCalculation, and verify that each load is less

than half the kVA rating of the transformer.

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Grounding Single-Phase Two-Bushing Transformers,Neutral on the Crossarm

Recommendation: Keep the primary neutral separate and distinct fromthe many other connections at the secondary rack. Failure to keep the

primary neutral separated introduces the high risk of inadvertentlycutting the primary neutral while the transformer is energized. This will

result in a primary voltage across the cut!


Tankgrounding

lug

Poleground

Secondary neutral

Connect

serviceneutralto tail

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Installing Transformers 73

PADMOUNT TRANSFORMER INSTALLATIONS

The transformers illustrated here are typical, but many differentconfigurations exist.

All padmount transformers have primary connections on the leftside of the panel, secondary connections on the right.

Single-phase minipad transformer labeling.

Single-phase minipad transformer layout.

75kVA rating

Company ID

Landscaping

suggestions

Grid number

Warning High Voltage

Secondaryvoltage

Used onreducedflammablity

transformersonly

Lock

Removable sill

Flip-top hoodopens 180

NEMA safety labelsLock mechanism

E e for adlock

X1

X3

X2

H1B

H1A

Removable ground strap

Low voltage threadedstud bushings

Fill plug

Pressure relief valve

k ground provision

Drain plug

High voltagebushing wells

Parking stand forelbow connectors

Externally accessibleexpulsion fusewith drip cup

X3

X1

X2

H1B

H1A

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