After power transformer (10MVA) oil filtering, BDV value reached about 86kV but in megger process HT-LT,HT-G,LT-G, are in the range of 100M ohm.why?

Question

مستخدم محذوف‎ · Answer

due to winding insulation breakdown

Himadri Roy · Answer

After filtration BDV Value definitely increase.As IR value is100 Mega-Ohm, then there is no problem, the Transformer is not a new one.You can charge it.

Mahmood Shaikh · Answer

Dear Abin Varghese, The dielectric strength of new insulating oil shall be measured in accordance with the following ASTM testing methods: 1. ASTM D877, "Test Method for Dielectric Breakdown Voltage of Insulating Liquids using Disk Electrodes (30 KV minimum)" 2. ASTM D1816, "Test Method for Dielectric Breakdown Voltage of Insulating Oils from Petroleum Origin Using VDE Electrodes ~0.04 inch, (28 kV minimum)" The Breakdown Voltage (BDV) Value of your Transformer Oil is86KV which is pretty much in acceptable limits but the Winding resistance is100 Mega Ohms which is absolutely worrying situation assuming that all power connections are Isolated properly.

If connections are isolated properly then in this case the10MVA Transformer cannot be energized or in other words it is dysfunctional so need to check the winding inside the shell or replace the transformer or Reinsulating of HV/LV windings along with all accessories can be done.

deepak n d · Answer

Dear Sir,

The low IR value (megger value) is due to heigh tan-delta value of your transformer oil.

In the IR results you have obtained, you will find the HT-G showing the least value.

From doing filteration the BDV and ppm values of oil will improve but tan-delta remains the same.

Please conduct a tan-delta test of the transformer oil and verify. Also please inform me rgarding the same

Rajendra Deshpande · Answer

This may be due to sludge deposition on shell winding. Transformer needs oven baking. PPM may be checked also check PI ( Polarisation index of winding)

kukuh nugrahadi Nugrahadi · Answer

because insulation test is testing resistance between two points, so, current that megger applied is flow through low resistance. and sample of oil is taken on the bottom transformer. and also winding clearance distance too short.

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After power transformer (10MVA) oil filtering, BDV value reached about 86kV but in megger process HT-LT,HT-G,LT-G, are in the range of 100M ohm.why?

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