winding resis doc

8/8/2019 Winding Resis Doc

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THE ART AND SCIENCE OF MEASURING

THE WINDING RESISTANCE OF

POWER TRANSFORMERS

Oleh W.Iw ansiw , P.EngTechnical Consultant - Eltel Industries

This a r t ic le descr ibes many of the

techniques and problems tha t may be

encoun tered wh en m easuring resistance in

the presence of inductance, especially as

app lied to measuring the wind ing resistance

of Power Transformers.

1. RESISTANCE MEASUREMENTS:

Resistance of a cond uctor can be determined

by d ividing the voltage across the condu ctor

by the current flow ing throu gh it. This

technique is employed wh en measuring

many resistance samples including the

windings of transformers to be discusses

here. It shou ld be pointed ou t that such

resistance measurement assumes steady

state cond itions where the current throughthe sample remains steady and so does the

voltage drop across it. This mu st be true

through out the measurement period. Most

professional knowledgeable in the field of

electrical measurements will recognize that

if inductance or capacitance is associated

wi th the res i s tance to be measured, a

t rans i en t wi l l be gene ra t ed on the

app lication of voltage/ current to the test

samp le. Accura t e measur ement s o f

resistance will not be possible in R/ L/ Ccircuits un til the transient is allowed to d ie

down and steady state conditions prevail.

Most electrical engineers w ill recognize that

the windings of most electrical power

equipm ent such as m otors, generators and

transformers have sizeable inductance

assoc ia ted wi th the res i s tance of the

wind ings. When measuring the wind ingsof such equipment, special techniques and

precautions need to be emp loyed if accurate

measurem ents are to result.

2. MEASURING RESISTANCE

ASSOCIATED WITH INDUCTANCE:

Electrical laws and principles tell us that

when induc t ance and re s i s t ance a re

associated in a circuit, a transient will occur

up on the app lication of power and that the

time constant of this transient will be equa lto L/ R seconds, where L is the indu ctance

in Henries and R is the Resistance in Ohm s.

To make measurements in such a circuit, a

t ime constant of several (5 to 9) t ime

constants w ill be required to settle the circuit

be fore meaningfu l measurement s o f

resistance can be undertaken. The number

of time constants required w ill depend on

the accuracy desired . 5 time constan t will

provide an accuracy of 1% and 9 t ime

constants w ill provide an accuracy of 0.1%,approximately.

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TWRM-10 ATWRM-25


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transformers, being relatively large, this

transient typ ically controls the time required

to complete a measurement on a del ta

connected transformer.

It shou ld be p ointed out that this transient

will occur regardless of which winding is

tested (primary or secondary) and regardless

of the measuring connection (phase-to-

ph ase-to-neutral). The time constan t of the

transient within the delta is pr opor tional to

the ratio of the inductance to resistance

within the delta circuit. This time constant

increases with the increase in the KVA

rating of the transformer as w ell as with the

efficiency of the transformer.

5. SAFETY CONSIDERATIONS:

Most p rofessional in the electric pow er field

will recognise that there is a danger when

testing inductive equipment using Direct

Current. The dan ger is due to the energy

that is stored in the ind uctance (E=1/ 2I.I.L).

This is the energy that mu st be app lied to

the specimen u pon initiation of the test. The

same energy must be dissipated in a safe

manner upon completion of the test andbefore opening of the current circuit. Shou ld

the current circuit be interrup ted d uring the

test, the store energy will be dissipated in a

High Voltage arc. Such an arc may dam age

the test specimen, the test equipment or

cause injury or death to personnel. The safe

d issipa tion of the energy cannot be stressed

enough. Safe ways of disposing of the stored

energy include, the shorting of the test

specimen (inductance) before d isconnection,

the use of sui table surge or t rans ientprotectors, or by u sing electronic techniques.

Shunt reactors due to their linearity store

more energy than t ransformers and are

therefore more d angerous.

6. DISCUSSION OF PROBLEMS:

6.1 INDUCTA NCE WITH RESISTA NCE

The ind uctance typ ically associated w ith the

measurement of winding resistance of

pow er equipment to be measured causes atransient up on initiation. This transient

The error in resistance measurements und er

un stable or transient cond ition is all due to

the ind uctance which will have a voltage

developed across it equal to V=L.di/ dt,

where L is the inductance in Henries and

di/ dt i t the rate of change of current inamp eres per second . In order to make an

error-free measurement, the inductance

must be eliminated from the circuit or the

current mu st remain p erfectly steady.

3. TRANSFORMERS:

It should be p ointed out that the indu ctive

effects are m ore serious w ith large p ieces of

equipm ent as large pieces of equipm ent will

have a larger L/ R time constant. The above

considerations assumed relatively linear

inductors, such as are associated with

moto rs , genera to rs or reac tors . An

addi t iona l compl ica t ion occurs when

measur ing the winding re s i s t ance of

transformers. The core of the transformer

comprises of a closed m agnetic circuit an d

therefore the winding exhibits a very large

ind u ctance. The core mu st be fully or

partially saturated in order to reduce the

high inductance and allow the current to

flow.

After the current is established one must still

wa it several time constan ts before read ings

can be taken. This time constant depends

on the ratio of the residual inductance of

the winding and the res i s tance of the

measuring circuit.

4. DELTA CONNECTED

TRANSFORMERS:

Del ta connected t ransformers present

spec i a l p rob lems when the i r winding

resistance is to be measu re. This problem is

due to the transient current that will be

established within the delta connection

du ring the initial transient (start-up) period.

As this current is coupled by transformer

ac t ion t o a l l t he windings on the

transformer, it mu st be allowed to d ie down

before accura t e measurement can be

accom pl ished . This t ran s ient i s anadditional complication when testing delta

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6.3 DELTA CONNECTIONS:

Transformers wi th de l ta connected

wind ings encoun ter an ad ditional transient

and causes a delay du e to the L/ R time

constant within the delta circuit. The testperson has li t t le control over this t ime

constant except to t ry and modi fy the

inductance by selecting a higher or lower

tes t curren t . IT MUST NO T be

autom atically assumed that a higher current

will reduce the ind uctance. In fact when

testing the low voltage wind ings of a large

transformer, the inductance of the winding

will increase with an increase in current, the

current in such situations m ust be r educed

or i nc reased pas t t he knee po in t t oeffect ively red u ce the ind u ctance. In

numerous t e s t l abora tor i e s , t he de l t a

connection is opened by m eans of a test link

to fac i l i t a te the winding res i s tance

measurement.

7. PRACTICAL SUGGESTIONS AND

SOLUTIONS:

7. 1 M E A S UR I N G M OT OR S A N D

GENERATORS:

The measurement of motor and generator

winding should present little difficulty to the

test person. Although the ind uctance of

such equipment can be substantial, it can

readily dampened by incorporating resistance

in the test circuit to reduce the time constant

of the start-up transient. Unless currents in

the order of 50 to 200 amperes are used the

energy stored in the circuit is modes and odes

not present a dangerous condition when

terminating the test.

7.2 USING DIGITAL MULTI-METERS OR

OHM-METERS:

A var iety of commercial digital multi-meters

or ohm -meters all using a constan t cur rent

source upto 1 amp will work well when

used on small power t ransformers and

motor/ generators. When the size of the test

spec imen increases and the L/ R ra t io

increases, the constant current sources

within these instruments become unstable

and the i ns t rument read ing becomesun stable. Instrum ents that use passive

may have a time constant of 1 to 100 secs.

A delay of 5 to 10 time constants may be

required before a stable reading is obtained.

This t ime constant may be effect ively

redu ced by suitable test equipment. This testequipm ent wou ld provide additional circuit

resistance, thereby reducing the L/ R ratio

i.e. Time Constant.

6.2 CORE SATURATION:

The measurement of transformer winding

resistance has an additional complication

that of saturating the core to allow the

curren t to flow in the wind ing. The typ ical

time requ ired to saturate the core depend s

on the voltage rating of the transformer

wind ing be ing t es t ed . Thus t he t e s t

equipment must apply vol t seconds

equiva lent to the ra ted vol tage of the

wind ing before any ap preciable cur rent w ill

be established. The volt-second s can be

calculated by integrating the area under on

half cycle of the voltage and dividing this

value by 2. Thus, a wind ing rated at 100KV

at 50 Hz will be rated at approx 450 volt-

second while a 100KV, 60 Hz w inding w ill

have a ra t ing of 375 vol t -seconds ,

app roximately. The 50 Hz wind ing will

need the ap plication of 6 volts for 75 second s

before the cur rent is establ ished . The

satura tion will occur m uch faster if a higher

voltage is app lied. Thus only 15 second s

will be required if a voltage of 30 volts is

applied.

I t must be pointed out tha t the above

calcula t ions assume no res idual f lux

(residual m agnetism) in the core of the test

t ransformer. As the residu al f lux in atransformer may be as high as 75% of the

saturation flux density, the actual time for

the current flow may be 0.25 to 1.7 times

the calculated value.

To reduce the time required to saturate the

core of a high voltage wind ing, a large test

voltage is ind icated. The use of a high test

voltage will autom atically provide a higher

resistance in the test circuit that red uces the

L/ R time constant. At the same time, the

use of a higher voltage increases the VArating and comp lexity of the pow er supp ly.

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circuitry (resistors) for controlling the test

current will work properly on all sizes of

equipment from smallest to the largest.

These passive cur rent sou rces waste a lot

of pow er in resistors wh ich convert vo ltage

to current to stabi les the measurementcircuit.

7.3 KELVIN A ND THOM PSON BRIDGES:

The traditional method of measuring low

resistance values has been the Kelvin or

Thomp son bridges. These bridges continue

to be used to d ate to measure the wind ing

resistance of pow er equipment. When u sed

to measure low resistance values, these

bridges would be equipped with a battery

and an adjustable resistor to provide thebridge with excitation curren t. The exact

value or stabi l i ty of the current is not

important when making measurements on

resistances using a brid ge circuit.

However, when measuring the value of

resistances associated w ith inductance, the

STABILITY of current is of great

importance. If the curren t is not stable, the

opera tor wi l l de termine a f ic t i t ious

resistance. This resistance will be stable overa considerably long period of t ime and

provide the operator w ith a false imp ression

that he has mad e an accurate measurement.

I t i s t he re fore ve ry impor t an t t o use

electronic current stabilizers for the test

current in addition to the usual current

setting resistor.

An e.g. of a typical fictitious resistance

measu remen t is as follows:

Assume a 12V battery source and a 1 ohmcurrent setting resistor that provides a test

curren t of 12 amp eres. The battery voltage

will fall continuously under the load of 12

amperes and for our example we wi l l

assum e a fall of 0.010 volt/ min (this is a

good battery !). This mean s that the test

curren t will also fall by 0.001 amperes/ min .

This type of reduction in test current of

battery voltage would not be noticeable, as

the battery would fall only to 11.400 volts

f rom 12.000 vol t s a f te r one hour of ope ra t i on . This cont inu ous cur ren t

reduction would allow the bridge to be

properly balanced and remain in balance

for a long time. The reading of the bridge,

however, would NOT BE CORRECT and

the error would d epend on the magnitude

of inductance associated with the sample(winding) being m easured.

Assuming that the 11kv winding of a 10

MVA t rans former i s be ing measured

(winding res i s tance 30 mi l l i ohms,

inductance about 5 Henries) the error in

resistance measurement under the above

conditions would be - 0.08 milli ohms or

0.25%. It shou ld be noted that the reading

will ALWAYS be lower under a falling

voltage battery cond ition an d that the errorbecomes larger for a low-loss transformer

(lower resistance and higher inductance)

Calculations:

R*I = 0.030 ohms * 12 amperes = 0.360 volts

L*di/ d t = 5 H enries * 0.0005 a/ s = 0.0025

volts

Error = 0.0025/ 0.36 = 0.0069 or 0.69%

7.4 VOLT-METER AM METER METHOD:

Many find it more convenient to u se a shun t

and two milli voltmeters to measure the

resistance of a wind ing. By using quality

digi ta l vol tmeters , one can make

measur ement s ove r a wid e range of

resistance provided that a suitable current

sour ce and shu nt are available. As this

method is very similar to the Kelvin bridge,

all the advantages, disadvan tages and errors

that w ere discussed in section 7.3 are equally

appl icable here inc luding the fa l se

reading examp les.

7.5 THERM A L EMFs:

As typical measurements discussed here

involve the m easurem ents of DC millivolts,

it is important to take precautions against

the introd uction of therm al EMFs. This

means that ALL connection to be made are

copper-to-copper. Quality instrumen ts for

this application will have copper binding

posts rather than the usual plated brass

term inals. Of extreme imp ortance is the

connection of the leads to the test specimen.

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As these are typically made using clips,

ON E MUST use SOLID COPPER clips. Very

large errors can be introduced by using

copper plated steel clips, especially du ring

heat run s i tua t ions where the bushing

terminals are hot.

7.6 SELECTION OF TEST CURRENT:

The statements by many in the industry is

that the more current you app ly, the faster

the reading will stabiles. Although th is

may be true of many cases, this is NOT true

in all cases. For best resu lts, the test curren t

should be selected according to requirements

of the test specimen. Thus, when m easuring

t ransformer winding res i s tance , i t i s

desirable to use a cur rent that is larger thanthe excitation curren t for that winding . This

will tend to saturate the core and reduce

the settling time for the measurement. The

selected current sh ould be less than 10% of

the ra ted current for tha t winding, as

excessive current will be heating up the

winding and caus ing i t s res i s tance to

increase. A curren t of 10% will cause only

1% of the normal heating of the winding.

Caution shou ld be exercised w hen selecting

the test current as some manufacturers claimthat excessive DC excitation of the core

prod uces un des i rable s t resses . These

stresses are similar to the stresses when a

transformer is energized, saturates, and

draws in-rush current.

Therefore it may be desirable to limit the test

current to 2-4 times the excitation current

for the w inding be ing tes ted. As the

excitation currents for larger transformers

are typically less than 1% of rated, one

should consider test currents in the rangeof 2-4% rated. This range would increase

for med ium and small pow er transformers

which draw a l a rge r pe rcen tage of

excitation current.

7.7 TIME REQUIRED FOR A

MEASUREMENT:

There are some individuals in the indu stry

wh o are asking for a time of only 30 seconds

to obtain a proper reading. Although this

is readily possible for small or even m edium

size transformers, this is very difficult to

obtain for large pow er transform ers. The

cost for equipment for accomplishing this

on large power transformer is rather large

and those who are asking for i t are not

willing to p ay the p rice.

Considering the measurement of winding

resistance after a heat ru n is one of the most

dem and ing app lication. The items that

requ ire time are as follows:

a) Shu tdow n of heat run .

b) Disconnection of heat run leads.

c) Connection of TWRM-10 leads.

d) Satur ation of transform er and

stabilisation of resistance reading .

AN SI C-57 specification allow 4 minu tes of

t ime be tween shut -down and the f i rs t

resistance read ing. This time is reasonable

for medium size transformers bu t becomes

un reasonable for the large transformers

being m anufactured tod ay. The test peop le

are pressed for time so much that they design

and procure high current short ing and

unshort ing equipment for high current

wind ings, as these typically take the longestto connect or d isconnect.

Anyway, allowing half of the allowable time

for tear ing down the connect ions and

connecting the Ohm Meter, the remaining

2 minutes i s typica l ly suff ic ient for

saturating even the largest transformers and

obtaining a reading with a TWRM-5.

The exception to the 2 minute time would

be delta connected transformers. As was

explained in section 6.3 there is a transient

circulating current set up within the delta

and this current m ust be allowed to decay

before accurate resistance readings can be

taken . The time constan t of this circu it

depend s on transformer characteristics and

is relatively ind ependen t of test conditions.

I t i s thi s prec ise reason tha t some

manufacturers of large pow er transformers

are prov iding a delta link wh ich can open

the del ta circui t to avoid the t ransient

condition within the delta for resistance

measurement.

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8. TWRM-10

The Eltel TWRM-10 is a second generation

ins t rument des igned espec ia l ly for

measur ing the winding re s i s t ance of

transform ers, inductors and other electrical

equipm ent. The TWRM-10 uses cur rentelectronic technology to p rovide a regulated

outpu t cur rent of up to 10A in a smaller size

and low er weight than the TWRM-5. The

TWRM-10 provides, many of the features

asked by the users of TWRM-5.

Some of its salient features are :

Test currents o f 10, 1, 0.1, and 0,01 Amperes

Resistance ranges of 1.9999 milli ohms to

1999.9 ohms

Resolution of 0.1 micro ohms

Two measuring inputs

Provides a safe shu t-dow n system

Provides protec t ion aga ins t induct ive

kickback

With the additional resolution, the TWRM-10

is ideal for measuring winding resistance during

heat run in factory tests. Without add itional

weight the TWRM-10 is as portable as its

predecessor the TWRM-5.

8.1 TAP CHANGER APPLICATION:

The des ign fea tures of the TWRM-10

ATWRM-25 mak es the ins t ru men t

especially useful in checking the proper

operation of on-load tap changers on p ower

transform ers. The protection circuit of the

TWRM-10/ ATWRM-25 autom at ica l ly

shuts down the instrument if the circuit is

interrupted even for a few microseconds.

This features is used for checking the proper,

make-before-break, operation of the tap

changer. The following procedu re is used :

a) Connect the wind ing equipp ed with

the on-load tap changer contacts to the

TWRM-10 for m easurem ent.

b) Energise the instrum ent and obtain a

reading.

c) Op erate the tap chan ger over its full

range, up and down, while watching the

indication of the instrument.

d) If the instrum ent continues to operate

after the up-d own cycle of the tap changer,

the tap changer op eration is prop er.

e ) The ins t rumen t wi l l shu t dow n

immediately when a contact of the tap

changer is operated incorrectly i.e. causes a

break-before-make operation.

9. ATWRM-25

The Eltel ATWRM-25 is a high current,

d ig i t a l d i r ec t r ead ing Au tomat ic

Transformer Winding Resistance Meter. It

is designed to m easure the DC resistance of

circuits associated with large Inductances.

It is specifically designed to measure the

resistances of highly inductive transformer

windings very quickly and accurately. The

operat ion of the ins t rument i s micro-

processor controlled and is fully au tomatic.

The ATWRM-25 test set can measure the

wind ing resistance of any transformer- small

or very large, distribution or power, single

or Three Phase , s tar , de l ta or z ig-zag

connected. There are four independent

measuring channels which can be used to

simultaneously measure the DC resistance

of upto four wind ings at the same time.

SCROLL 10-10-2004

OTHER PRODUCTS

G Manual & Automatic

Transformer Ratio Meters.

G Digital Micro Ohm Meters.

with built in 100Amp source.

G Manual & Automatic Transformer Winding

Resistance & On Load Tap Changer Test sets.

G Automatic CT/PT Test sets & Systems

G Automatic & Semi Automatic

HV Capacitance & Tan Delta Test sets,

10 kV Oil Test Cell.

G

Manual & Automatic Tan Delta & ResistivityTest sets for Transformer Oil, Solid Test Cell.

G Portable LV Manual & AutomaticCapacitance

and Tan Delta Test sets.

ELTEL INDUSTRIES311 EMBASSY CENTRE, CRESCENT ROAD,

BANGALORE-560 001, INDIA

TEL : 91-80-22255467, 22205686, 22284253, 22284298

FAX : 91-80-22252733

E-mail: [email protected]

Website: http//www.eltelindustries.com

Works : Plot No. 39, KIADB Industrial Area, Veerapura,

Doddaballapur, Bangalore 561 203, INDIA

TEL : 91-80-7630350, 7630366, 7630367, 7630368 FAX : 91-80-7630351

CHENNAI: 044-24312849/24339075 T KOLKATA: 033-24765536/ 24752394MUMBAI: 022-25383960/25398358 T NEW DELHI: 011-29810252/29815746

(SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE)

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