EEEN 4372

Electrical Power Transmission and Distribution

Distribution Transformers

1

Three Phase Transformers

Applications

Construction

Transformer connections

Parallel operation

Three-phase connections

Special problems in transformers

Autotransformers

2

Applications

Power stations: To step-up generator output voltage, from (11kV~25kV)

to transmission voltage (132kV, 275kV, 400kV).

Transmission:

To interconnect lines operating at different voltages (for example 132kV

and 275kV lines).

To interface between transmission network and distribution network (

132kV to 33kV).

Distribution:

To step down system voltages (US 2.4-34.5kV UK 3.3-33kV) to

utilization voltages (US 120-600V, UK 230-415V), located at urban

substations,factories, rural areas (pole mounted), large buildings.

Input transformers for electrical equipment:

arc furnaces,

large power electronic drive systems (>5kW),

large dc power supply units (labs, body scanners, physics rigs..)

rectifier sets for dc traction.3

Construction

4

Three single-phase units One 3-phase unit

Three single phase units are more

flexible,

they can be rated individually to met

unbalanced loads,

in the case of a phase failure: only

one unit need be replaced

but,

heavier,

require more external wiring

A phase failure requires the

replacement of a complete unit,

lighter (15% less weight for the same

rating)

Cheaper

Requires less external wiring

Standard kVA and voltages

5

ANSI standard C57.12.20 gives standard transformer capacity and

voltage ratings for single-phase distribution transformers.

Designation of voltage ratings

6

Types of distribution transformers

(i) dry-type: air cooled and air insulated

(ii) liquid-filled type: oil-filled and inerteen-filled

In overhead distribution systems:

– Conventional transformers : no integral protection

devices provided as part of the transformer

– Completely self-protecting transformers (CSP): from

lightning/surges, overloads and short circuits

– Completely self-protecting for secondary banking

(CSPB): similar to CSPs, but provided with two sets

of circuit breakers. The second set is used to

sectionalize the secondary when it is needed.

7

8

In underground distribution systems:

– Subway transformers

– Low-cost residential transformers

– Network transformers

Transformer polarity

Transformer winding terminals are marked to

show polarity, to indicate the high-voltage

(HV) from the low-voltage (LV) side.

It is an indication of the direction of current

flow through the HV leads with respect to the

direction of current in the LV leads at any

given instant.

9

Transformer polarity

10

•With standard markings, the voltage from

H1 to H2 is always in phase with the

voltage from x1 to x2

•(a) Subtractive polarity : when H1 and x1

terminals are adjacent

•(b) additive polarity: when H1 and x1

terminals are diagonally opposite.

•To test the transformer polarity, two

adjacent terminals of the HV and LV

windings are connected together and a

moderate voltage is applied to the HV

winding, then the voltage between the HV

and LV terminals that are not connected

together is measured: The polarity is

subtractive if the voltage reading is less

than the applied voltage, and is additive if

the voltage reading is greater than the

supply voltage.

Single-phase transformer connections

Single-phase distribution transformers have one HV primary winding

and two LV secondary windings which are rated at 120V: these can be

connected in parallel to supply a two-wire 120V circuit, or in series to

supply a three-wire 120/240V single circuit.

11

12

Single-phase transformer connections (cont’d)

LV windings in series

Single-phase transformer connections (cont’d)

13

Single phase transformer connections for single and two-bushing

transformers to provide customers who require only 240-V single-

phase power. Used for small industrial applications.

Parallel operation of single-phase transformers

Conditions:

– All transformers have the same turns ratio

– All transformers are connected to the same primary

phase

– All transformers have identical frequency ratings

– All transformers have identical voltage ratings

– All transformers have identical tap settings

– The per unit impedance of one transformer is between

0.925 and 1.075 of the other in order to maximize

capability

14

Parallel operation of single-phase transformers

15

Additive

polarity

Additive

polarity

H1 H2

H1 H2 H1 H2

x2 x1x2 x1

x1 x2

Subtractive

polaritySubtractive

polarity

H1 H2

H1 H2 H1 H2

x1 x2x1 x2

x1 x2

Additive

polarity

Subtractive

polarity

H1 H2

H1 H2 H1 H2

x2 x1x1 x2

x1x2

Load sharing of single-phase transformers in parallel

16

The equivalent circuit of the low-voltage side is, with VH the voltage at the HV

side, „a‟ the transformer ratio and Zeq the equivalent transformer impedance:

Zeq1

LoadVH /a

Zeq2

I1

I2

VL

ILVoltage drop through each

transformer must be equal:

1

2

2

1

2211

eq

eq

eqeq

Z

Z

I

I

ZIZI

Using the percent impedances:

rating Voltage : and r,transforme

of rating : , 100)(%2

V

kVASSV

ZZ TT

TT

22

11

2

1

1

2

2

1

Ter transformof ratingkVA :

Ter transformof ratingkVA :

)(%

)(%

T

T

T

T

T

T

S

S

S

S

Z

Z

I

I

22

11

2

1

1

2

2

1

Trmer by transfo suppliedkVA :

Trmer by transfo suppliedkVA :

)(%

)(%

L

L

T

T

T

T

L

L

S

S

S

S

Z

Z

S

S

Example

A 250-kVA transformer with 2.4% impedance

is paralleled with a 500-kVA transformer with

3.1% impedance. Determine the maximum

load that can be carried without overloading

either transformer. Assume that the

maximum allowable transformer loading is

100% of the rating.

17

Three-phase connections

18

D-D connection formed by connecting three single phase transformers to

provide 240-V service at 0o angular displacement

D-D connection : 0o angular displacement

D-D connection: 180o angular displacement

19

D-D connection formed by connecting three single phase transformers to

provide 240-V service at 180o angular displacement

D-D connection

Often used to supply a small single-phase lighting load and three-phase power load simultaneously.

The mid-tap secondary winding of one of the transformers is grounded and connected to the secondary neutral conductor.

The single-phase loads are connected between the phase and neutral conductors

The transformer with the mid-tap carries two thirds of the 120/240-V single-phase load and one third of the 240-V three-phase load.

The other two units each carry one third of both the 120/240-V and 240-V loads.

20 D-D connection to provide 120/208/240 V 3-phase four-wire service

D-D connection (cont’d)

No problem from third-harmonic overvoltage

High-circulating currents will flow unless all three

single-phase transformers are connected on the same

regulating taps and have the same voltage ratios.

The transformer bank rating is decreased unless all

transformers have identical impedance values.

The secondary neutral bushing can be grounded on

only one of the three single-phase transformers.

Therefore to get balanced loading:

1. All transformer have identical voltage ratios.

2. All transformers have identical impedance values.

3. All transformers are connected on identical taps

21

Equivalent circuit of a D-Dconnected transformer bank

22Equivalent circuit referred to the LV side

Equivalent circuit of a D-D connected transformer bank

23

Voltage drop equation for the low-voltage winding:

0

cbacba

bccbcaacabbacbacba

VVV

where

ZIZIZIVVV

We have, for the D-connected secondary:

cbacc

bacbb

acbaa

III

III

III

-

-

-

0 bccbcaacabba ZIZIZIbut

hence:bccbcaacabba ZIZIZI --

cabcab

bcbcaaba

cababcba

ZZZ

ZIZII

ZIZI

-

: ngsubstituti and side, either to and adding

Equivalent circuit of a D-Dconnected transformer bank

24

similarly:

cabcab

cacabbcb

cabcab

ababccac

ZZZ

ZIZII

and

ZZZ

ZIZII

-

-

For equal percent impedance and equal ratios of

percent reactance to percent resistance (X/R ratio) :

caTbcTabT

bcT

b

caT

a

ba

SSS

S

I

S

I

I

,,,

,,

111

-

caTbcTabT

abT

a

bcT

c

ac

SSS

S

I

S

I

I

,,,

,,

111

-

caTbcTabT

caT

c

abT

b

cb

SSS

S

I

S

I

I

,,,

,,

111

-

ST,ab : kVA rating of the single-phase between phases a and b

ST,bc : kVA rating of the single-phase between phases b and c

ST,ca : kVA rating of the single-phase between phases c and a

Example:

25

Three single-phase transformers are connected D-D to provide power for a

three-phase Y-connected 200-kVA load with 0.8 lagging power factor and

80-kVA single-phase light load with a 0.9 lagging power factor.

Assume that the three single-

phase transformers have equal

percent impedance and equal

ratios of per-cent reactance to

percent resistance.

The primary side voltage of the

bank is 7620/13200V and the

secondary side voltage is

240V. Assume that the single-

phase transformer connected

between phases b and c is

rated at 100kVA and the other

two are rated at 75 kVA.

Determine:

1. The line current flowing in each secondary-phase wire

2. The current flowing in the secondary winding of each transformer

3. The load on each transformer in kVA

4. The current flowing in each primary winding of each transformer

5. The line current flowing in each primary-phase wire.

Solution:

26

oooan bc

oan bc

obcbc

bc

Lbc

oc

ob

oa

..-VI

VV,.VI

A.V

SI

A..I

A..I

A..I

811590825

90825

3333240

1080

8635198

721905765

9367481

3

--

-

-

-

by lags hence

,by voltage reference the lags butby lags

A...j.

....

SSS

S

I

S

I

I o

oo

ca,Tbc,Tab,T

ab,T

a

bc,T

c

ac 13311129555607116

75

1

100

1

75

175

9367481

100

8635198

111--

--

-

-

1.

2. Current in the secondary winding of the first transformer:

A...j.

SSS

S

I

S

I

I

:similarly

o

ca,Tbc,Tab,T

bc,T

b

ca,T

a

ba 3565301082834300111

-

A...j.

SSS

S

I

S

I

I

and

o

ca,Tbc,Tab,T

ca,T

c

ab,T

b

cb 72043270951126245111

--

-

27

kVA...IVS

kVA...IVS

kVA...IVS

acacca,L

cbcbbc,L

babaab,L

9930111292400

8864332702400

472653012400

A...

..

a

II

A...

..

a

II

A...

..

a

II

.a

oo

cbCB

oo

baBA

oo

acAC

72045187531

72043270

35597531

3565301

1330747531

13311129

7531240

7620

--

:ratio rtransforme

3. The kVA load on each transformer:

4. Currents in the primary windings :

5. Line Currents in the primary phase wires:

A...j.....III

A...j.....III

A...j.....III

oooACCBC

oooCBBAB

oooBAACA

8186221134114111330747204518

5147617444191772045183559

127086130563559133074

-----

--

-----

The Open D- Open D connection

28

•If one of the transformers becomes damaged or removed from service, the other

two can be operated in the open-delta or “V” connection

•The currents in the other two transformers increase by a ratio of 1.73

•The individual transformers now function at a power factor of 0.866

•One of the transformers delivers a leading load and the other a lagging load.

•To operate the openD-openD bank safely, the load has to be decreased by 57.7%.

%7.57577.03

1

:

10003

3

1000

3

or

D-D

-

-

D-D

S

S

hence

kVAVI

S

kVAVI

S

The Y-Y transformer connection

29

Wye-Wye connection to provide a 120/208-V grounded-

wye 3-phase four-wire multigrounded service

•This system provides a 208-V

three-phase power supply for

three-phase motors and 120-V

single-phase supply for small

single-phase loads.

• Advantage: when a system is

changed from D to a 4-wire Wye to

increase system capacity, existing

transformers can be used.

•Creates series disturbances in

communication circuits in the

vicinity.

•Primary neutral should be solidly

grounded and bonded to the

system neutral to avoid excessive

voltages on the secondary side.

• With an isolated neutral point,

third-harmonic voltages appear at

the transformer neutral.

•Possibility of resonance between

the line capacitance to ground and

the magnetizing impedance of the

transformer.

The Y-D transformer connection

30

Wye-delta connection to provide a 120/208/240-V 3-phase

four-wire secondary service

•Advantage: Transformers with

primary winding of only the voltage

to neutral can be used on a higher

distribution voltage (eg. 2.4-kV

primary single-phase transformers

can be connected in Y on the

primary to a 4.16-kV 3-phase Y-

circuit.

• The transformation ratio of the

bank is 1.73 times the

transformation ratio of the

individual transformers.

•When transformers of different

ratings are used, the maximum

safe bank rating is three times the

rating of the smallest transformers.

•Third-harmonic currents circulate

in the Delta winding, correcting

voltage deformations arising from

isolation of neutral on the primary

side.

The Y-D transformer connection

31

Wye-delta connection to provide a 240-V three-

phase three-wire secondary service at 210o angular

displacement

When the primary-side neutral of the

transformer bank is not isolated but

connected to the primary circuit neutral,

The Y-D transformer bank may burn-out

due to the following reasons:

• The transformer bank may act as a

grounding transformer bank for

unbalanced primary conditions and may

supply fault current to any fault on the

circuit to which it is connected, reducing

its own capacity for connected load.

•The transformer bank may be

overloaded if one of the protective fuses

opens on a line-to-ground fault, leaving

the bank with only the capacity of an

open-Y open-D bank.

•The transformer bank causes

circulating currents in the D in an

attempt to balance any unbalanced load

connected to the primary line.

The D-Y transformer connection

32

Delta-wye connection to provide a 120/208-V three-

phase four-wire grounde-Y service at 30o angular

displacement

The D-Y connection has many uses:

•It eliminates unbalanced primary currents occurring with other connections.

• Single-phase loads can be balanced on three-phases in each bank, and banks may be

paralleled.

•No operation if one of the transformers becomes damaged or is removed from service.

•The line voltages on the secondaries of Y-D and D-Y connections are 30o lagging behind

the line voltages on the primaries.

•The D-Y step-up and Y-D step down connections are suitable for high-voltage transmission

systems. They are economical and provide a stable neutral point for grounding.

Delta-wye connection to provide a 120/208-V three-

phase four-wire grounde-Y service at 210o angular

displacement

The auto-transformer

33

Construction:

•Use a single, tapped winding in each phase

rather than two separate windings

•A common flux passes through all the turns

1

2

1

21

1

21

1

444

444

n

n

n

nnk

V

V

IZIR

knf.

)n(nf.

E

E

x

H

tt

m

m

x

H

:is ratio rtransforme the drops, and neglecting

11

2 - kn

n

V

V

C

S

: is ratio voltage- windingThe

1--

kI

II

I

I

I

I

H

Hx

H

C

S

C

: ratio Current

IC: current in the common winding

IS: current in the series winding

xxHH IVIV rating VA sCircuit'

CCSS IVIV rating VA sWinding'

1-

-

-

k

k

V/)VV(

V/V

I)VV(

IV

IV

IV

xxH

xH

HxH

HH

SS

HH

rtransforme winding-two ascapacirty

ormerautotransf ascapacity

Designation of connections by vector group symbols (British Standard)

Y, D, Z for the star, delta and zigzag high-

voltage winding

y, d, z for the star, delta and zigzag low-

voltage and intermediate windings,

YN, ZN if a neutral connection is made on the

h.v winding,

yn, zn if a neutral connection is made on the

l.v winding.

34

Vector Group

In the British Standard, transformers are

designated according to the phase

displacement between the primary and

secondary winding voltages.

The clock-hour figure is used to express the

phase displacement.

35

Use of vector group symbols

• Use of the clock-hour

figure: The phase

displacement is expressed

as the hour number, shown

on a clock whose large hand

is pointing at 12 and

coincides with vector of the

voltage between the neutral

point and the HV line

terminal, and whose small

hand coincides with the

vector of the voltage

between the neutral and the

corresponding LV or

intermediate terminal.

•The vector relating to the

HV winding is taken as the

vector of origin.

HV transformer

37

38

39