Insulation Resistance of power transformer

August 9, 2017 | Author: MELVIN | Category: Capacitor, Transformer, Electric Current, Quantity, Electric Power
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IR for power transformer, transformer testing, insulation resistance of power transformer, IR testing of transformer, IR...

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INSULATION RESISTANCE OF TRANSFORMERS

Melvin Koshy

Introduction 





IR test is a non-destructive test to analyse quality of transformer insulation system Absorption ratio, Polarisation Index, Step Voltage etc. correlate IR results Components of Insulation current (total insulation current)  Capacitance current  Absorption current  Leakage current

Capacitance current 





Dielectric system encountered can be considered as a capacitor Like capacitors, charging current decreases from a high value to a low value in a short period as insulation is getting charge to full voltage Charging current can be expressed as

E Ic = e R

 t     RC 

E C R t

– Open circuit Voltage – Capacitance – Resistance – Time constant

Absorption current 



Due to imperfect dielectric caused by polarization charges Absorption current initially starts from a high value but decreases at a slower rate than capacitor charging current

Leakage current 

Resistive loss Comprises leakage current either through the insulation or over leakage surfaces



Total insulation current



IT = I c + I a + I l

How to measure IR?   





One minute test Gives IR value corresponding to one minute IR values depend on applied voltage temperature etc. hence any measurements should be taken under identical conditions such as same applied voltage, instrument etc. IR values taken at different temp are brought to base temp of 20°C Temperature correction factors are applied

Contd… 





IR values may be checked periodically for a transformer Large reduction in values warraties investigation for the cause For every 10°C rise in temp, IR value is reduced to half

Temperature correction factor Temp °C

Correction factor

Temp °C

Correction factor

5

0.25

40

3.95

10

0.36

45

5.6

15

0.5

50

7.85

20

0.72

55

11.2

25

1.0

30

1.4

60

15.85

65

22.4

70

31.75

35

1.98

40

2.8

45

3.95

Interpretation of IR results 







Absorption current varies with time, hence one minute withstand alone will not give any reliable indication of quality of insulation structure For a good dielectric system, IR value increases with time as current decreases Hence Absorption Ratio and Polarisation Index are factors can be used to interpret IR result Polarisation Index and Absorption Ratio are independent of temperature

Absorption ratio 

Absorption ratio (k)

IR v a lu e a t 6 0 se c . k = R v a lu e a t 1 5 se c . 



a fte r th e a p p lic a tio n o f D C v o lta g e

In the initial measurement, insulation system can be considered to comprise paper & oil Hence Insulation Current is given by

It = I l + I a e

(t / τ )

Contd… 

 

When the insulation is wet, leakage current is very large compared to absorption current Hence Absorption Ratio is nearly = 1 When the insulation is dry, absorption time constant becomes high and leakage current is negligible

Polarisation Index 





If absorption time of a transformer is very high, there will be a large difference between IR values after 60 sec. Therefore 10 minute IR value gives more reliable data on properties of insulation Polarisation Index is given by

IR v a lu e a t 6 0 0 se c . k = R v a lu e a t 6 0 se c . 

a fte r th e a p p lic a tio n o f D C v o lta g e

PI value greater than 1.5 is acceptable

Step Voltage Test  



Used to detect weak and moist insulation One minute IR value is measured by applying voltages are in the order of 1 to 5 times i.e. 500V and 2500V DC Decrease in IR value of 25% or greater at higher test voltage is an indication of excess moisture

IR values 

IR value of winding to ground with other winding grounded or between windings in Megaohms at 20°C for oil filled transformer is given by C×E IR = P

E – per phase voltage rating of winding C – 0.8 for unguarded terminal – 1.6 for guarded terminal P – kVA rating of transformer

Recommended values of IR CBIP Recommendations No. 198: 1987 Rated voltage Minimum safe IR value in Megaohm at the of winding temperature 30°C

40°C

50°C

60°C

Upto 6.6 kV

200

100

50

25

6.6 to 11 kV

400

200

100

50

11 to 33 kV

500

250

125

65

> 66 kV

600

300

150

75

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