Distribution Transformer Testing

Testing of Distribution Transformer

Distribution Transformer

Construction of Transformer

Transformer Name plate details

Routine Test

Factory test

Transformer failure and causes

Type Test

Principle of Transformer

2

Principle of transformer action

3

A current flowing through a

coil produces a magnetic field

around the coil. The magnetic

field strength H,

required to produce a

magnetic field of flux density

B, is proportional to the

current flowing in the coil.

Principle of transformer action

4

A transformer is a static piece of

apparatus used for transferring

power from one circuit to

another at a different voltage,

but without change in frequency.

It can raise or lower the voltage

with a corresponding

decrease or increase of current.




Routine Test

Factory test


Type Test

Principle of transformer actions

5

CONSERVATOR TANK

TAP SWITCH

HV / LV WINDING

OIL

EXPLOSION VENT

BREATHER

RADIATOR

L.T BUSHING

BOTTOM VALVE

H.T BUSHING


6


DELTA / STAR

7

.

R

Y

B

y

r

b

n

HV LV

ETH

6/25/2015 8 Distribution Transformer

6/25/2015 9 Core and Coil Assembly




Routine Test

Factory test


Type Test


10

Transformer Name Plate Details

- Before any testing study of name plate details is must, The HV/LV voltage

,HV/LV current, Size of transformer, Vector group & Impedance volts play

an important roll for calculating test values.

MAKE : ( XYZ )

KVA : (315)

NO LOAD VOLTS HV / LV ( 11000 / 415 ) Volts

FULL LOAD AMPS. HV / LV ( 16.5 / 438.2 ) Amps

FREQUENCY : 50 HZ

TAPPING : No. of Taps :- 7

VECTOR GROUP : Dyn11

TYPE OF COOLING : ONAN

IMPEDANCE VOLTS : 5 %

11

Name plate of Distribution transformer-Sample

12




Routine Test

Factory test


Type Test


13

Routine Test

Generally routine test is carried out at stores where we received New /

Recovered transformers from manufacturer and site.

Following Test are perform as routine test.

Insulation Resistance Test

Continuity test

Ratio test (Voltage / Turns ratio)

Short circuit test (Short circuit current)

Open circuit test (Magnetizing current )

Magnetic balance test

Vector Group test

Oil test

14

Insulation Resistance Test

(IR VALUE)

Insulation resistance test to be carried out by 1 / 2.5 / 5 KV manual or motor driven megger, It gives us the status of insulation of HV

winding and LV winding with respect to earth and between the

winding.

HV WDG. TO EARTH = _____ M

LV WDG. TO EARTH = _____ M

HV WDG.TO LV WDG. = _____ M

15

0

M

MEGGER

HV

R

Y

B

TR BODY TR BODY

0

M

MEGGER

LV

R

Y

B

N

Insulation resistance measurement

between LV winding and earth

Insulation resistance measurement

between HV winding and earth

16

TR BODY

0

M

MEGGER

HV

R

Y

B

LV

R

Y

B

N

Insulation resistance measurement between LV winding and HV

winding

17

Continuity Test

CONTINUITY TEST CARRIED OUT BY MEGGER OR AVO METER TO CHECK ANY OPEN CIRCUIT FAULT IN THE HV & LV WINDING OF THE

TRANSFORMER.

HV SIDE LV SIDE

RY = _____ rn = _____

YB = _____ yn = _____

BR = _____ bn = _____

VALUE = ZERO ( CONTINUITY O.K )

VALUE = INFINITY ( CONTINUITY NOT O.K)

18

R

Y

B

r

y

n

b

TR BODY

0

M

MEGGER

LV

R

Y

B

N

TR BODY

HV

R

Y

B

0

M

MEGGER

Continuity test for HV

winding

Continuity test for LV

winding

19

Voltage ratio test

Objective:

To check that the ratio of voltages ( in 3-phase transformers, line to line voltages) is as per designed

value in all tap position.

The % Error of the voltage ratio is not more that +/- 0.5 % as per the IS 2026 and IEC 60076

Apparatus:

3 phase supply -415V (Applied to HV side)

Clip on meter for measuring voltage at LV side.

20

Voltage ratio test

Procedure:

Apply 3 phase 415V supply on HV side and LV side voltage is to be measured.

Output voltage will be measure between phase & neutral (VRN, VYN, VBN). Ratio will be changed in respect to tap positions.

21

Tap position Vrn Vyn Vbn Vry Vyb Vbr

1

2

3

4

5

6

7

400v AC

supply

ICTP

TR BODY

LV R

Y

B

N

HV

R

Y

B

VOLT-METER

Voltage ratio test

22

Voltage ratio test

23

HV side 415V

applied

Tap switch

position

LV side voltage

measurement

Short circuit test

Objective:

To determine the Short Circuit impedance voltage.

To determine the Load losses of the transformer

Short circuit loss = I2R Loss + stray loss

Apparatus:

Voltage injection set (Applied on HV side)

Clip on meter (Measuring current at LV side).

Shorting Link (For passing full load current).

24

Short circuit test

Procedure: LV side will be shorted through

shorting link.

Applied the 3 phase voltage on

HV side and increased the voltage

till the full load current passed

through the transformer.

Unbalance current and phase to phase current measure with the help of Clip on meter.

Unbalance current should not be more than 2% of full load current.

Load loss and impedance voltage measured on power analyser. This Load loss measured at Room temperature Converted this loss into 75

Degree C and it should be within the specified limit.

25

400v AC

supply

ICTP

TR BODY

LV R

Y

B

N HV

R

Y

B

Short circuit test

Load loss at 75 Degree C = Load loss at RT (A+75) / (A+RT)

Here RT= Room Temperature

A= 235 for CU wound

A= 225 for Al wound

26

Open circuit test

Objective:

To know the Excitation losses and currents of Transformer when excited at rated kV at rated Hz

To determine the No load losses(Core loss) of the transformer

To check healthiness of winding and stampings.

No load loss = hysteresis loss + eddy current loss

Apparatus:

Voltage injection set (Applied on LV side)

Clip on meter for measuring no load current at LV side.

27

Hysteresis Loss

28

when a magnetic field is applied all the

grains of the magnetic material will

orient in the direction of magnetizing

force. In another cycle this grains will

orient in opposite direction in the

direction of magnetizing force. The

energy required to change the

orientation of the magnetic grains in the

direction of the magnetic field is lost in

the form of heat. This loss is called

hysteresis loss.

Eddy current Loss

29

Eddy Current Loss:

When an alternating magnetic flux is applied to the iron

core small emf will be induced due to change in flux

linkage. This induced emf will cause small circulating

current s called eddy currents. Eddy current flowing

through the material causes I2R losses in the material.

In order to reduce the Eddy Current loss:

Use of Material having high electrical resistivity:

By using superior grain orientation CRGO steel will have

higher electrical resistivity. Thus eddy current loss can

be reduced.

Lamination Cores are used:

By using thin laminations the core thickness is reduced

so eddy current losses are reduced. So CRGO steel

laminations are used as a core material.

Insulation between laminations:

Oxide coating is provided for CRGO laminations to

reduce eddy current losses.

Open circuit test

Procedure:

Rated voltage applied on LT side

of distribution transformer for

energizing transformer.

No load current measured by

clip on meter and no load loss

measured by power analyzer.

No load current should not be

more than 3% of full load current.

Note: Nobody can go closer to HV side

because it is subject to 11KV.

30

TR BODY

LV R

Y

B

N

HV

R

Y

B

400v AC

supply

ICTP Open circuit test

31

32

Rating 1 star 2 star 3 star 4 star 5 star

KVA Max Losses

at 50% (Watts)

Max Losses at 100% (Watts)

Max Losses at

50% (Watts)

Max Losses at

100% (Watts)

Max Losses at

50% (Watts)

Max Losses at

100% (Watts)

Max Losses at

50% (Watts)

Max Losses at

100% (Watts)

Max Losses at

50% (Watts)

Max Losses at

100% (Watts)

16 200 555 165 520 150 480 135 440 120 400

25 290 785 235 740 210 695 190 635 175 595

63 490 1415 430 1335 380 1250 340 1140 300 1050

100 700 2020 610 1910 520 1800 475 1650 435 1500

160 1000 2800 880 2550 770 2200 670 1950 570 1700

200 1130 3300 1010 3000 890 2700 780 2300 670 2100

Star Rating of Transformer(BEE)

BEE Star Labeling

33

Full load current and loss of different rating

transformers

KVA

HV

(Amp)

LV

(Amp)

NO LOADLOSS

( Watts)

LOAD LOSS

(Watts)

160 8.4 222.6 300 1600

315 16.5 438.2 515 2750

630 33.1 876.5 800 4700

990 52 1377 1250 6200

34

Magnetic Balance Test:

rn yn bn

rn= 240 240 156 84

yn= 240 120 240 120

bn =240 84 156 240

Applied 240V on LV side and voltage measured on respective LV terminals

as under.

Division should 65:35 between central and the other extreme limb if one

of the extreme limbs is excited and is 50:50 for central limb excitation

35

Break down voltage test of Oil

Transformer oil is used insulation as well as cooling

purpose.

BDV tester ranging from 0-100KV used for test.

Gap between two electrodes must be maintain 2.5mm.

BDV of new insulating oil

Should be more than 60KV.

37

Acidity test of Oil

In this test we will measure the value of free carbonic and non-carbonic acid in oil and this will be measured

with the help of acidity testing kit.(mgKOH/gm)

Acidity must be less than 0.03 for new insulating oil.

38




Routine Test

Factory test


Type Test


39

Factory Test

In addition to routine test some more test are carryout at factory, This

type of test required some specific testing instruments and testing

facilities, even through some of the test carryout at site also.

Winding resistance test

Vector group test

Separate source voltage test

Temperature rise test

Induced overvoltage test

40




Routine Test

Factory test


Type Test


41

Over and above routine test some important test are carried out at

factory or laboratory like ERDA & CPRI, this type of test required

some special testing instrument and testing facility.

Impulse Voltage Test - 95KVp

Short circuit withstand test - 18.4 KA for 3 sec

Partial discharge test

Type Test

42




Routine Test

Factory test


Type Test


43

Failure & Causes

Insufficient Oil level.

Seepage of water in oil.

Prolonged Over loading.

Single Phase loading.

Unbalanced loading.

Faulty Termination (Improper sized lugs etc)

Power Theft.

Prolonged Short Circuit.

Faulty operation of tap changer switch.

Lack of installation checks.

Faulty design

Poor Workmanship Improper formation of core.

Improper core bolt insulation.

Burr to the lamination blades

Improper brazing of joints.

Burr /sharp edges to the winding conductor.

Incomplete drying.

Bad insulation covering.

Insufficient cooling ducts in the winding.

44

Failure & Causes

Faulty design

Poor Workmanship

Improper formation of core. Improper core bolt insulation. Burr to the lamination blades Improper brazing of joints. Burr /sharp edges to the winding conductor. Incomplete drying. Bad insulation covering. Insufficient cooling ducts in the winding.

Bad Quality of raw material.

Transit damaged transformers. After failure , transformer is removed and replaced with

new/repaired one without removing the cause of failure which results in immediate or short time failure.

45

6/25/2015 47

6/25/2015 48

Thank You

Documents

Distribution Transformer Testing