Transformer Oil Break Down Voltage (BDV)

1) When you take a series of reading for BDV, you should provide sufficient time gap (say 15-20 min) between successive readings so that the carbon formed due to breakdown of oil will settle at the bottom. A gap of 5 minutes can lead to inconsistency in the readings (ex. reading no. 3).

2) As per the Indian Standard IS 1866:2000, for unused transformer oil, the minimum BDV should be 40kV (for 1 minute at an electrode gap of 2.5mm). The miminum BDV for transformer oil in service is 30kV (for 1 minute at an electrode gap of 2.5mm).Your test results are well above the mimimum requirement. There is no cap on the maximum permissible BDV for transformer oil as per the standard.

Minimum Required BDV for used oil is 30KV/Min with a 2.5mm gap between the test electrodes. You need not bother about individual readings as long as each reading is above 30KV. Each depends on the impurity content in the oil between the electrodes. Hence as long as each BDV reading is above 30KV, the oil is suitable for use. Hence the there is no restriction on transformer charging from oil BDV point of view. You have to be craeful if the value is close to 30KV (above 30KV). You may have to check BDV more often than your schedule, i.e once in 6 months if your schedule is once a year.

As far as my knowledge is concerned, standard procedure for BDV testing is as follows.

1. first we will take filltered oil in test kit. 2. then maintained 2.5mm gap between electrode and applying rated 40 & 60 Kv for 1 minite . 3. the above test withstood for 1 minite its ok for energiseing the transformer.U may also go for DGA if ur trans. was out of service due to any fault.and my suggestion for u that dont charge ur transformer on the basis of ur BDV test data.

There is a reason behind every step of the test methods. You should follow a good one. ASTM has D1816; IEC has a corresponding one. The lab doing your testing should have these methods in their 'recipe book'. Read the method and compare it yourself with what is actually being done. The D877 one is bad for a couple of reasons; grit sinks and does not show up in the results. The agitator in D1816 clears floating micro-debris from the critical zone. Nothing short of abrasives will remove carbon on the metal electrodes themsselves, but such abrasives will, over time, deform the critical electrode shape. No need to 'clean them' for a year or so of testing. Test instruments automatically program the best delay and voltage ramp for the test. Any test instrument that cannot is not good enough to test this critical liquid.

Oil is plentiful. Do not keep testing the same aliquot. The sample process and container are subject to a lot of contamination error. If your numbers are not as high as you expect, get a new sample, and give up on one that may have dirt (which you cannot see) in it. Variability in Dielectric strength is expected. With D1816 and agitated oil, a few very low numbers can be due to floating fibers. A mix of high and low is typical. Look for a wide semi-symmetrical bell curve of voltage versus number of results.

i am suspecting something wrong with test kit ,pl go for dissolved gas analysis test and tandelta test cofirms 100% healthiness of transformer

Yes you can go head for charging the transformer the BDV is accaptable

In your case you can charge as the average value is more than 8o kV.