USE FULL INFORMATION OF TRANSFORMERS Temperature correction Factor (Base 20°C): 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 1.98 40 104 3.95 50 122 7.85 Example: For 1600KVA, 20KV/400V,Three Phase Transformer • IR Value at HV Side= (1.5 x 20000) / √ 1600 =16000 / 40 = 750 MΩ at 200C • IR Value at LV Side = (1.5 x 400 ) / √ 1600= 320 / 40 = 15 MΩ at 200C • IR Value at 300C =15X1.98= 29.7 MΩ Insulation Resistance of Transformer Coil Transformer Coil Voltage Megger Size Min.IR Value Liquid Filled T/C Min.IR Value Dry Type T/C 0 – 600 V 1KV 100 MΩ 500 MΩ 600 V To 5KV 2.5KV 1,000 MΩ 5,000 MΩ 5KV To 15KV 5KV 5,000 MΩ 25,000 MΩ 15KV To 69KV 5KV 10,000 MΩ 50,000 MΩ IR Value of Transformers Voltage Test Voltage (DC) LV side Test Voltage (DC) HV side Min IR Value 415V 500V 2.5KV 100MΩ Up to 6.6KV 500V 2.5KV 200MΩ 6.6KV to 11KV 500V 2.5KV 400MΩ 11KV to 33KV 1000V 5KV 500MΩ 33KV to 66KV 1000V 5KV 600MΩ 66KV to 132KV 1000V 5KV 600MΩ 132KV to 220KV 1000V 5KV 650MΩ Steps for measuring the IR of Transformer: • Shut down the transformer and disconnect the jumpers and lightning 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 Connections of Transformer for IR Test (Not Less than 200 MΩ) 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) 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 Ω • LV – Earth 100 M Ω • HV – LV 200 M Ω Factors affecting on IR 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↑ 3. IR Value for Tap Changer • IR between HV and LV as well as windings to earth. • Minimum IR value for Tap changer is 1000 ohm per volt service voltage
Posted on: Mon, 19 Aug 2013 09:30:24 +0000
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