IR for power transformer, transformer testing, insulation resistance of power transformer, IR testing of transformer, IR...
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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