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A Presentation on Insulation Testing of Transformer Dhritiman Nath, ET

Contents Transformer Insulation Insulation Tester Insulator Model Significance of Records

Transformer Transformer is an electromagnetic device that is used to change the electrical parameters keeping the frequency constant It is the most efficient equipment in the field of electrical engineering having efficiency upto 99 % Availability of transformer makes the use of alternating current more advantageous than direct current We have total 104 nos of transformer in 103 nos of sub-station

Insulation Insulator prevents the flow of current in a unwanted direction Transformers have transformer oil and paper as it’s insulation Causes of insulation failure are – Thermal Breakdown Chemical deterioration Effect of oxygen and humidity Electro-chemical deterioration

insulation tester We use insulation tester of megger D.C. voltage is applied to the insulator under measurement The voltage levels that are available are – 250V, 500 V, 1000V, 2500V, 5000V Measurement is taken between H.V. - Earth, L.V.- Earth and H.V.-L.V.

Procedure Shutdown the transformer and isolate it Discharge the winding capacitance Clean the bushings Short Circuit the windings Guard the terminals to eliminate surface leakage over terminal bushings Connect “LINE”, “EARTH” leads Transformer neutral bushing is to be disconnected from earth during test Apply test voltage and note the readings

Continued….. Records are taken at an interval of – After 30 secs After 1 minute After 10 minutes The total current is the sum of following – Capacitive current (IC ) Conduction current ( IR ) Surface leakage current (IL ) Absorptive current ( IA )

+ - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - Dipoles + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + -

INSULATOR MODEL Total Current = Conduction current C R0 R1 = Leakage current = Conduction current = Capacitive current & absorptive current Total Current

Time of Voltage Application(minutes) Graph = Total Current = Absorptive Current = Capacitive current = Leakage Current = Conduction Current Relative Current Time of Voltage Application(minutes)

Continued…. Reading of 10 Mins Reading of 1 Min Reading of 1 Min Insulation resistance reading : After 30 secs the reading is of, IT = IC+ IA + IL + IR After 1 minute, the reading is of, IT = IA + IL + IR After 10 minutes, the reading is of, IT = IL + IR Polarization Index = Dielectric Absorption Ratio = Reading of 10 Mins Reading of 1 Min Reading of 1 Min Reading of 30 sec

Significance IL + IC + IA IL + IC Polarisation Index : One minute measurement is not reliable and scientifically acceptable as IR is temperature dependent P.I. gives result which is temperature independent and this gives more accurate result. P.I. gives better idea of conduction and leakage current If IL and IC are low compared to IA, then PI will be more than 2 If IL and IC are high compared to IA, then PI will be around 1 IL + IC + IA IL + IC

Continued….. AS per IEEE, Insulation Resistance Polarisation Index Dangerous Less than 1 poor 1.0-1.1 Questionable 1.1-1.25 Fair 1.25-2.0 Good Above 2 Source : grouper.ieee.org

Source: www.openelectrical.org Continued….. Dielectric Absorption Ratio : The DAR test is performed only when P.I. is found to be less than 2 even for new materials.Under such cases a minimum DAR value of 1.25 is required. IC +IA +IL + IR IA + IL + IR Insulation Resistance DAR value Questionable Less than 1.25 Adequate Less than equal to1.6 Good 1.6 and above Source: www.openelectrical.org

Recall Transformers Procedure Significance of data

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