minimum ir result

8/12/2019 Minimum Ir Result

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8.5.1 - Testing insulation resistance

A low resistance between phase and neutral conductors, or from live conductors to earth, will result in a leakage current. This current couldcause deterioration of the insulation, as well as involving a waste of energy which would increase the running costs of the installation. Thus,

the resistance between poles or to earth must never be less than half of one meg ohm (0.5 M hms! for the usual supply voltages. "naddition to the leakage current due to insulation resistance, there is a further current leakage in the reactance of the insulation, because itacts as the dielectric of a capacitor. This current dissipates no energy and is not harmful, but we wish to measure the resistance of theinsulation, so a direct voltage is used to prevent reactance from being included in the measurement. "nsulation will sometimes have highresistance when low potential differences apply across it, but will break down and offer low resistance when a higher voltage is applied. #or

this reason, the high levels of test voltage shown in $Table %.%& are necessary. $%.'.&gives test instrument re)uirements.

Before commencing the test it is important that:

1.* electronic e)uipment which could be damaged by the application of the high test voltage should be disconnected. "ncluded in thiscategory are electronic fluorescent starter switches, touch switches, dimmer switches, power controllers, delay timers, switches associatedwith passive infrared detectors (+"s!, -s with electronic operation etc. An alternative to disconnection is to ensure that phase and neutralare connected together before an insulation test is made between them and earth.

2.* capacitors and indicator or pilot lamps must be disconnected or an inaccurate test reading will result.

Table 8.8 - Required test voltages and minimum resistance

Nominal circuit voltageTest voltage

(V)

Minimum insulationresistance(M Ohms)

/tra*low voltage circuits supplied from asafety transformer

150 0.15

2p to 500 3 ecept for above 500 0.5

Above 500 3 up to 000 3 000 .0

The insulation resistance tester must be capable of maintaining the re)uired voltage

when providing a steady state of current of mA.

4here any e)uipment is disconnected for testing purposes, it must be subected to its own insulation test, using a voltage which is not likelyto result in damage. The result must conform with that specified in the 6ritish 7tandard concerned, or be at least 0.5 M hms if there is no7tandard.

The test to earth $#ig %.0& must be carried out on the complete installation with the main switch off, with phase and neutral connectedtogether, with lamps and other e)uipment disconnected, but with fuses in, circuit breakers closed and all circuit switches closed. 4here two*way switching is wired, only one of the two strapper wires will be tested. To test the other, both two*way switches should be
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ig 8.1! - "nsulation test to earth

ig 8.11 - "nsulation tests bet#een poles

operated and the system retested. "f desired, the installation can be tested as a whole, when a value of at least 0.5 M hms should beachieved, see $#ig %.0&. "n the case of a very large installation where there are many earth paths in parallel, the reading would be epectedto be lower. "f this happens, the installation should be subdivided and retested, when each part must meet the minimum re)uirement.

The tests to earth $#ig %.0& and between poles $#ig %.& must be carried ut as indicated, with a minimum acceptable value for each test of

0.5 M hms. 8owever, where a reading of less than 1 M hms is recorded for an individual circuit, (the minimum value re)uired by the8ealth and 7afety /ecutive!, there is the possibility of defective insulation, and remedial work may be necessary. A test result of 1 M hmsmay sometimes be unsatisfactory. "f such a reading is the result of a re*test, it is necessary to consult the data from previous tests to identifydeterioration. A visual inspection of cables to determine their condition is necessary during periodic tests9 perished insulation may not alwaysgive low insulation readings

As indicated above, tests on 7/:3 and +/:3 circuits are carried out at 150 3. 8owever tests between these circuits and the live conductorsof other circuits must be made at 500 3. Tests to earth for +/:3 circuits are at 150 3, whilst #/:3 circuits are tested as :3 circuits at 5003. eadings of less than 5 M will re)uire further investigation.

Source : http://www.tlc-direct.co.uk/Book/8.5.1.htm
http://www.tlc-direct.co.uk/Book/8.5.1.htmhttp://www.tlc-direct.co.uk/Book/8.5.1.htm


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$easurement of insulation resistance%"R& ' (art 2

Posted APR 29 2012 ! "# $% PAR &A R i' (#$( )*+,A$ &A#,A &*,*R ,R A S *R & RS ,R A S& #S S# * A

#S,R#B%,#* with 13 *&&,S
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Fluke insulation resistance tester up to 10kV - Allows testing of high voltage systems such as control gears, engines, generators and cables. It can be adjusted

to all testing voltages that are specified in IEEE 43-2000. Ideal for Electricity Board and industrial companies for predictive and preventive maintenance.

Continued from first part:Measurement of insulation resistance (IR) Part 1

"nsulation esistance ("! 3alues ; "nde
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1.IR Values For Electrical Apparatus & Systems

2.IR Value for Transformer

3.IR Value for Tap Changer

4.IR Value for Electric motor

5.IR Value for Electrical cable and wiring

6.IR Value for Transmission / Distribution Line

7.IR Value for Panel Bus

8.IR Value for Substation Equipment

9.IR Value for Domestic /Industrial Wiring

0.Required Precautions

Index

. " 3alues #or /lectrical Apparatus < 7ystems

(PEARL Standard / NETA MTS-1997 Table 10.1)

Max.Voltage

Rating Of

Equipment

Megger Size Min.IR

Value

250 Volts 500 Volts 25 M

600 Volts 1,000 Volts 100 M

5 KV 2,500 Volts 1,000 M

8 KV 2,500 Volts 2,000 M

15 KV 2,500 Volts 5,000 M

25 KV 5,000 Volts20,000 M

35 KV 15,000 Volts 100,000M

46 KV 15,000 Volts 100,000

M

69 KV 15,000 Volts 100,000

M

ne Meg ohm ule for " 3alue for /)uipment

Based upon equipment rating:

< 1K V = 1 M minimum

>1KV = 1 M /1KV

As per IE Rules-1956

At a pressure of 1000 V applied between each live conductor and earth for a period of one minute theinsulation resistanceof

HV installations shall be at least 1 Mega ohm or as specified by the Bureau of Indian Standards.
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Medium and Low Voltage Installations- At a pressure of 500 V applied between each live conductor and earth for a period of one

minute, the insulation resistance of medium and low voltage installations shall be at least 1 Mega ohm or as specified by the

Bureau of Indian Standards] from time to time.

As per CBIP specifications the acceptable values are 2 Mega ohms per KV

Index1. " 3alue for Transformer

Insulation resistance tests are made to determine insulation resistance from individual windings to ground or between individual

windings. Insulation resistance tests are commonly measured directly in megohms or may be calculated from measurements of

applied voltage and leakage current.

The recommended practice in measuring insulation resistance is to always ground the tank (and the core). Short circuit each

winding of the transformer at the bushing terminals. Resistance measurements are then made between each winding and all other

windings grounded.

Insulation resistance testing: HV - Earth and HV - LV

Transformer windingsare never left floating for insulation resistance measurements. Solidly grounded winding must have the

ground removed in order to measure the insulation resistance of the winding grounded. If the ground cannot be removed, as in the

case of some windings with solidly grounded neutrals, the insulation resistance of the winding cannot be measured. Treat it as part

of the grounded section of the circuit.

We need to test winding to winding and winding to ground ( E ).For three phase transformers, We need to test winding ( L1,L2,L3 )

with substitute Earthing for Delta transformer or winding ( L1,L2,L3 ) with earthing ( E ) and neutral ( N ) for wye transformers.

IR Value for Transformer

(Ref: A Guide to Transformer Maintenance by. JJ.

Kelly. S.D Myer)

Transformer Formula
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1 Phase Transformer IR Value (M) = C X E /

(KVA)

3 Phase Transformer

(Star)

IR Value (M) = C X E (P-n) /

(KVA)

3 Phase Transformer

(Delta)

IR Value (M) = C X E (P-P) /

(KVA)

Where C= 1.5 for Oil filled T/C with Oil Tank, 30 for

Oil filled T/C without Oil Tank or Dry Type T/C.

Temperature correction #actor (6ase 10=-!>

Temperature correction Factor

OC OF Correction Factor

0 32 0.25

5 41 0.36

10 50 0.50

15 59 0.720

20 68 1.00

30 86 1.9840 104 3.95

50 122 7.85Example:For 1600KVA, 20KV/400V,Three Phase Transformer

IR Value at HV Side= (1.5 x 20000) / 1600 =16000 / 40 = 750 M at 200C

IR Value at LV Side = (1.5 x 400 ) / 1600= 320 / 40 = 15 M at 200C

IR Value at 300C =15X1.98= 29.7 M

"nsulation esistance of Transformer -oil

Transformer Coil Voltage Megger Size Min.IR Value Liquid Filled T/C Min.IR Value Dry Type

T/C

0 600 V 1KV 100 M 500 M

600 V To 5KV 2.5KV 1,000 M 5,000 M

5KV To 15KV 5KV 5,000 M 25,000 M


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15KV To

69KV

5KV 10,000 M 50,000 M

" 3alue of Transformers

Voltage Test Voltage (DC) LV

side

Test Voltage (DC) HV

side

Min IR Value

415V 500V 2.5KV 100M

Up to 6.6KV 500V 2.5KV 200M

6.6KV to 11KV 500V 2.5KV 400M

11KV to 33KV 1000V 5KV 500M

33KV to 66KV 1000V 5KV 600M

66KV to

132KV

1000V 5KV 600M

132KV to

220KV

1000V 5KV 650M

7teps for measuring the " of Transformer>

Shut down the transformer and disconnect the jumpers andlightning arrestors.

Discharge the winding capacitance.

Thoroughly clean all bushings

Short circuit the windings.

Guard the terminals to eliminate surface leakage over terminal bushings.

Record the temperature.

Connect the test leads (avoid joints).

Apply the test voltage and note the reading. The IR. Value at 60 seconds after application of the test voltage is referred

to as the Insulation Resistance of the transformer at the test temperature.

The transformer Neutral bushing is to be disconnected from earth during the test.

All LV surge diverter earth connections are to be disconnected during the test.

Due to the inductive characteristics of transformers, the insulation resistance reading shall not be taken until the test

current stabilizes.

Avoid meggering when the transformer is under vacuum.

Test -onnections of Transformer for " Test (?ot :ess than 100 M@!

Two winding transformer

1. (HV + LV) GND

2. HV (LV + GND)

3. LV (HV + GND)

Three winding transformer

1. HV (LV + TV + GND)

2. LV (HV + TV + GND)
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3. (HV + LV + TV) GND

4. TV (HV + LV + GND)

Auto transformer (two windings)

1. (HV + LV) GND

Auto Transformer (three winding)

1. (HV + LV) (TV + GND)

2. (HV + LV + TV) GND

3. TV (HV + LV + GND)

For any installation, the insulation resistance measured shall not be less than:

HV Earth 200 M

LV Earth 100 M

HV LV 200 M

#actors affecting on " value of Transformer

The IR value of transformers are influenced by

Surface condition of the terminal bushing

Quality of oil

Quality of winding insulation Temperature of oil

Duration of application and value of test voltage

Index

. " 3alue for Tap -hanger

IR between HV and LV as well as windings to earth.

Minimum IR value for Tap changer is1000 ohm per volt service voltage

Index

B. " 3alue for /lectric motor

Forelectric motor, we used a insulation tester to measure the resistance of motor winding with earthing (E).

For rated voltage below 1KV, measured with a 500VDC Megger.

For rated voltage above 1KV, measured with a 1000VDC Megger.

In accordance with IEEE 43, clause 9.3, the following formula should be applied.

Min IR Value (For Rotating Machine) =(Rated voltage (v) /1000) + 1

Insulation resistance (IR) value for electric motor

As per IEEE 43 Standard 1974, 2000
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IR Value in M

IR (Min) = kV+1 For most windings made before

about 1970, all field windings, and

others not described below

IR (Min) = 100

M

For most dc armature and ac

windings built after about 1970

(form wound coils)

IR (Min) = 5 M For most machines with random

-wound stator coils and form-

wound coils rated below 1kVExample-1:For 11KV, Three Phase Motor.

IR Value =11+1=12 M but as per IEEE43 It should be 100 M

Example-2: For 415V,Three Phase Motor

IR Value =0.415+1=1.41 M but as per IEEE43 It should be 5 M.

As per IS 732 Min IR Value of Motor=(20XVoltage(p-p/(1000+2XKW)

" 3alue of Motor as per ?/TA AT7 100'. 7ection '.5.

Motor Name Plate

(V)

Test

Voltage

Min IR Value

250V 500V DC 25 M

600V 1000V DC 100M1000V 1000V DC 100M

2500V 1000V DC 500M

5000V 2500V DC 1000M

8000V 2500V DC 2000M

15000V 2500V DC 5000M

25000V 5000V DC 20000M

34500V 15000V DC 100000M

" 3alue of 7ubmersible Motor>

IR Value of Submersible Motor

Motor Out off Well (Without Cable) IR Value


11/22

New Motor 20 M

A used motor which can be reinstalled 10 M

Motor Installed in Well (With Cable)

New Motor 2 M

A used motor which can be reinstalled 0.5 M

Index

5. " 3alue for /lectrical cable and wiring

For insulation testing, we need to disconnect from panel or equipment and keep them isolated from power supply. Thewiring and

cablesneed to test for each other ( phase to phase ) with a ground ( E ) cable. The Insulated Power Cable Engineers Association

(IPCEA) provides the formula to determine minimum insulation resistance values.

R = K x Log 10 (D/d)

R= IR Value in Ms per 1000 feet (305 meters) of cable.

K= Insulation material constant.( Varnished Cambric=2460, Thermoplastic Polyethlene=50000,Composite Polyethylene=30000)

D= Outside diameter of conductor insulation for single conductor wire and cable ( D = d + 2c + 2b diameter of single conductor

cable )

d Diameter of conductor

c Thickness of conductor insulation

b Thickness of jacket insulation

83 test on new C:+/ cable (As per /T7A 7tandard!

Application Test VoltageMin IR Value

New cables Sheath 1KV DC 100 M

New cables

Insulation

10KV DC 1000 M

After repairs Sheath 1KV DC 10 M

After repairs

Insulation

5KV DC 1000M

k3 and k3 -ables between -ores and /arth (As per /T7A 7tandard!

Application Test VoltageMin IR Value

11KV New cables

Sheath

5KV DC 1000 M

11KV After repairs 5KV DC 100 M
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Sheath

33KV no TFs connected 5KV DC 1000 M

33KV with TFs

connected.

5KV DC 15M

11kV and 33kV Cables between Cores and Earth

" 3alue Measurement (-onductors to conductor (-ross "nsulation!!

The first conductor for which cross insulation is being measured shall be connected to Line terminal of the megger. The

remaining conductors looped together (with the help of crocodile clips) i. e. Conductor 2 and onwards, are connected to

Earth terminal of megger. Conductors at the other end are left free.

Now rotate the handle of megger or press push button of megger. The reading of meter will show the cross Insulation

between conductor 1 and rest of the conductors. Insulation reading shall be recorded.

Now connect next conductor to Line terminal of the megger & connect the remaining conductors to earth terminal of themegger and take measurements.

" 3alue Measurement (-onductor to /arth "nsulation!

Connect conductor under test to the Line terminal of the megger.

Connect earth terminal of the megger to the earth.

Rotate the handle of megger or press push button of megger. The reading of meter will show the insulation resistance of

the conductors. Insulation reading shall be recorded after applying the test voltage for about a minute till a steady reading is

obtained.

" 3alue Measurements>

If during periodical testing, insulation resistance of cable is found between5 and 1 M/kmat buried temperature, thesubject cable should be programmed for replacement.

If insulation resistance of the cable is found between1000 and 100 K /km, at buried temperature, the subject cable

should be replaced urgently within a year.

If the insulation resistance of the cable is found less than 100 kilo ohm/km., the subject cable must be replaced

immediately on emergency basis.

Index

D. " 3alue for Transmission E istribution :ine
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Equipment Megger SizeMin IR Value

S/S .Equipments 5 KV 5000M

EHVLines. 5 KV 10M

H.T. Lines. 1 KV 5M

LT / Service

Lines.

0.5 KV 5M

Index

'. " 3alue for +anel 6us

IR Value for Panel = 2 x KV rating of the panel.

Example, for a 5 KV panel, the minimum insulation is 2 x 5 = 10 M.

Index

%. " 3alue for 7ubstation /)uipmentGenerally meggering Values of Substation Equipments are.

Typical IR Value of S/S Equipments

Equipment Megger

Size

IR Value(Min)

Circuit

Breaker

(Phase-Earth)5KV,10 KV 1000 M

(Phase-

Phase)

5KV,10 KV 1000 M

Control Circuit0.5KV 50 M

CT/PT (Pri-Earth) 5KV,10 KV 1000 M

(Sec-Phase) 5KV,10 KV 50 M

Control Circuit 0.5KV 50 M

Isolator (Phase-Earth)5KV,10 KV 1000 M(Phase-

Phase)

5KV,10 KV 1000 M

Control Circuit0.5KV 50 M

L.A (Phase-Earth)5KV,10 KV 1000 M
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Electrical

Motor

(Phase-Earth)0.5KV 50 M

LT Switchgear(Phase-Earth)0.5KV 100 M

LTTransformer

(Phase-Earth)0.5KV 100 M

IR Value of S/S Equipments As per DEP Standard

Equipment Meggering IR Value at

Commissioning

Time (M)

IR Value at

Maintenance

Time

Switchgear HV Bus 200 M 100 M

LV Bus 20 M 10 M

LV wiring 5 M 0.5 M

Cable(min 100

Meter)

HV & LV (10XKV) / KM (KV) / KM

Motor & Generator Phase-Earth 10(KV+1) 2(KV+1)

Transformer Oil

immersed

HV & LV 75 M 30 M

Transformer DryType

HV 100 M 25 MLV 10 M 2 M

Fixed

Equipments/Tools

Phase-Earth 5K / Volt 1K / Volt

Movable

Equipments

Phase-Earth 5 M 1M

Distribution

Equipments

Phase-Earth 5 M 1M

Circuit Breaker Main Circuit 2 M / KV -

Control Circuit 5M -

Relay D.C Circuit-

Earth

40M -


15/22

LT Circuit-

Earth

50M -

LT-D.C Circuit 40M -

LT-LT 70M -Index

F. " 3alue for omestic E"ndustrial 4iring

A low resistance between phase and neutral conductors, or from live conductors to earth, will result in a leakage current. This

cause deterioration of the insulation, as well as involving a waste of energy which would increase the running costs of the

installation.

The resistance between Phase-Phase-Neutral-Earth mustnever be less than 0.5 M Ohmsfor the usual supply voltages.

In addition to the leakage current due to insulation resistance, there is a further current leakage in the reactance of the insulation,

because it acts as the dielectric of a capacitor. This current dissipates no energy and is not harmful, but we wish to measure the

resistance of the insulation,so DC Voltage is used to prevent reactance from being included in the measurement.

1 Phase Wiring>The IR test between Phase-Natural to earth must be carried out on the complete installation with the main switch off, with phase

and neutral connected together, with lamps and other equipment disconnected, but with fuses in, circuit breakers closed and all

circuit switches closed.

Where two-way switching is wired, only one of the two stripper wires will be tested. To test the other, both two-way switches should

be operated and the system retested. If desired, the installation can be tested as a whole, when a value of at least 0.5 M Ohms

should be achieved.

1 Phase Wiring

3 Phase WiringIn the case of a very large installation where there are many earth paths in parallel, the reading would be expected to be lower. If

this happens, the installation should be subdivided and retested, when each part must meet the minimum requirement.
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3 Phase Wiring

The IR tests must be carried out between Phase-Phase-Neutral-Earth with a minimum acceptable value for each test of 0.5 M

Ohms.

IR Testing for Low voltage

Circuit voltage Test voltage IR

Value(Min)

Extra Low Voltage 250V DC 0.25M

Up to 500 V except for

above

500 V DC 0.5M

500 V To 1KV 1000 V DC 1.0MMin IR Value = 50M / No of Electrical outlet. (All Electrical Points with fitting & Plugs)

Min IR Value = 100M / No of Electrical outlet. (All Electrical Points without fitting & Plugs).

Index

e)uired +recautions

Electronic equipment like electronic fluorescent starter switches, touch switches, dimmer switches, power controllers, delay timers

could be damaged by the application of the high test voltage should be disconnected.

Capacitors and indicator or pilot lamps must be disconnected or an inaccurate test reading will result.

Where any equipment is disconnected for testing purposes, it must be subjected to its own insulation test, using a voltage which is

not likely to result in damage. The result must conform with that specified in the British Standard concerned, or be at least 0.5 M

Ohms if there is no Standard.

Source: http://electric4l-e'i'eeri'-port4l.com/me4sureme't-o6-i'sul4tio'-

resist4'ce-272.
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17/22

Transformer Winding ResistanceMeasurement.........Previous

Transformer winding resistance measurement

Transformer Ratio Test

Vector Group Test of Transformer

Dielectric Test of Transformer

Temperature Rise Test of Transformer

Transformer Impulse Test

Sweep Frequency Response Analysis

Tan Delta or Dissipation Factor Test

Over Fluxing in Transformer

Earthing or Grounding Transformer

External & Internal Faults in Transformer

Backup Protection of Transformer

Differential Protection of Transformer

Restricted Earth Fault Protection

Maintenance of Transformer

Distribution Transformer

Air Core Transformer

High Voltage Transformer

Procedure of Transformer Winding Resistance Measurement

Current Voltage Method of Measurement of Winding ResistanceProcedure of Current Voltage Method of Measurement of Winding Resistance

Bridge Method of Measurement of Winding Resistance

Transformer winding resistance measurementis carried out as a type test, routine test and also as a field test.

In the factory, it helps in determining the following :

1. Calculation of the I2Rlosses in transformer.

2. Calculation of winding temperature at the end of temperature rise test of transformer.

3. As a benchmark for assessing possible damages in the field.

It is done at site in order to check for abnormalities due to loose connections, broken strands of conductor, high contactresistanceintap changers, high voltage leads and bushings.

Procedure of Transformer Winding Resistance Measurement

For star connected winding, theresistanceshall be measured between the line and neutral terminal.
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18/22

For star connected auto-transformers the resistance of the HV side is measured between HV terminal and IV terminal, then between

IV terminal and the neutral.

For delta connected windings,measurement of winding resistanceshall be done between pairs of line terminals. As in delta

connection the resistance of individual winding can not be measured separately, the resistance per winding shall be calculated as

per the following formula:

Resistance per winding = 1.5 x Measured value

Theresistanceis measured at ambient temperature and then converted toresistanceat 75C for all practical purposes of

comparison with specified design values, previous results and diagnostics.

WindingResistanceat standard temperature of 75 C

Rt= Windingresistanceat temperature t.

t = Winding temperature.

Generally transformer windings are immersed in insulation liquid and covered with paper insulation, hence it is impossible to

measure the actual winding temperature in a de-energizing transformer at time oftransformer windingresistancemeasurement.

An approximation is developed to calculate temperature of winding at that condition, as follows

Temperature of winding = Average temperature of insulating oil.

(Average temperature of insulating oil should be taken 3 to 8 hours after de-energizing transformer and when the difference between

top & bottom oil temperatures becomes less than 5 C.)

Theresistancecan be measured by simple voltmeter ammeter method, Kelvin Bridge meter or automaticwindingresistancemeasurement kit. (ohm meter, preferably 25 Amps kit)

Caution for voltmeter ammeter method: Current shall not exceed 15% of the rated current of the winding. Large values may cause

inaccuracy by heating the winding and thereby changing its temperature andresistance.

NB: - Measurement of windingresistanceof transformer shall be carried out at each tap.

Current Voltage Method of Measurement of Winding

Resistance

The transformer windingresistances can be measured by current voltage method. In this method of measurement ofwindingresistance, the test current is injected to the winding and corresponding voltage drop across the winding is measured.

By applying simpleOhm's lawi.e. Rx= V I, one can easily determine the value ofresistance.

Procedure of Current Voltage Method of Measurement of Winding

Resistance
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19/22

1. Before measurement the transformer should be kept in OFF condition without excitation at least for 3 to 4 hours. During

this time the winding temperature will become equal to its oil temperature.

2. Measurement is done with D.C.

3. To minimize observation errors, polarity of the core magnetization shall be kept constant during allresistancereadings.

4. Voltmeter leads shall be independent of the current leads to protect it from high voltages which may occur during switching

on and off the current circuit.

5. The readings shall be taken after the electric current and voltage have reached steady state values. In some cases this

may take several minutes depending upon the winding impedance.

6. The test current shall not exceed 15% of the rated current of the winding. Large values may cause inaccuracy by heating

the winding and thereby changing itsresistance.

7. For expressingresistance,the corresponding temperature of the winding at the time of measurement must be mentioned

along withresistancevalue. As we said earlier that after remaining in switch off condition for 3 to 4 hours, the winding

temperature would become equal to oil temperature. The oil temperature at the time of testing is taken as the average of

top oil and bottom oil temperatures of transformer.

8. For star connected three phase winding, theresistanceper phase would be half of measuredresistancebetween two line

terminals of the transformer.

9. For delta connected three phase winding, theresistanceper phase would be 0.67 times of measured resistance between

two line terminals of the transformer.

10.This current voltage method ofmeasurement of winding resistanceof transformer should be repeated for each pair of

line terminals of winding at every tap position.
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20/22

Bridge Method of Measurement of Winding Resistance

The main principle of bridge method is based on comparing an unknownresistancewith a knownresistance. When electric currents

flowing through the arms of bridge circuit become balanced, the reading of galvanometer shows zero deflection that means at

balanced condition no electric current will flow through the galvanometer. Very small value ofresistance( in milli-ohms range) can be

accurately measured by Kelvin bridge method whereas for higher value Wheatstone bridge method ofresistancemeasurement is

applied. In bridge method of measurement of windingresistance, the errors is minimized.
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21/22

Theresistancemeasured by Kelvin bridge,

All other steps to be taken during transformer windingresistancemeasurement in these methods are similar to that of current voltage

method ofmeasurement of windingresistanceof transformer, except the measuring technique of resistance.

Theresistancemeasured by Wheatstone bridge,

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22/22

Source : http://www.electric4lu.com/tr4's6ormer-wi'di'-resist4'ce-me4sureme't/

Minimum insulation resistance (M Ohms) - 0.15
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minimum ir result

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