Insulation-resistance-of-power-transformer

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 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 se c k = R v a lu e a t 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 = 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 0 se c k = R v a lu e a t 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 to 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 melvinkoshy@gmail.com