Introduction to Checking out Dielectric Strength for Oil

Dielectric failure strength of transformer oil is one of the very much reliable tests for proving the condition of the oil, and so good care is essential in conducting the test, because the slightest trace of contamination or existence of moisture brings down the breakdown value very sharply. For instance, if finger tips are immersed for a moment or two into an oil cup consisting of transformer oil that has been checked for a breakdown value of say 45 kV and test again, it is surprising to note that it might now breakdown at 20 kV or even less. Thus if low values are recorded in a test, a possible cause might be careless handling. Unless staffs are particularly trained and they completely appreciate the significance of perfect cleanliness at each stage of the test, the results will be totally misleading.

A number of the points to be considered are detailed below:

i) The breakdown values (BDV) given consider to the RMS (root mean square) voltage while tested according to the  IS 335:1993, using a standard test cell with two 13 mm dia. polished spheres, and a test gap of 2.5 mm. The shape and spacing of the electrodes comprise a great effect on the test values

ii) The test should be conducted while the oil is cold and not while hot. The dielectric strength changes with temperature as displayed below:

Temperature ⁰C                       30       40        50        60        70        80

BDV, kV                                     33      35        36        37        38        39

iii) Rubber is influenced by oil. Hence use plastic tubes for drawing out the sample. Sampling bottles should comprise glass stoppers; cork absorbs moisture and might contaminate the oil.

iv) The oil's sample should preferably be drawn from the bottom of the transformer tank.  Like water is heavier than oil, it settles down at the base. The first sample or two might be thrown away if it consists of sludge or droplets of water; the next sample drawn will certainly not fail to reveal high moisture content if the oil has been showing to drops of water. The major complexity in drawing samples from the bottom of the tank is while it is not fitted with a drain cock but simply with a drain plug. The sample may after that drawn through siphoning off the conservator tank. In circuit breakers of good make special oil sampling cocks are frequently fitted to facilitate drawing out a little quantity of oil for test purposes without any spilling.

v) The glass bottle in which oil is drawn should be completely clean, clear, transparent and dry. It should then be carefully rinsed along with oil known to be good. Collecting the oil straight into the oil test cup may emerge more convenient but is not suggested, to avoid the probability of damaging the cup through handling it not necessarily; it is as well impracticable if a number of oil samples must be tested. The sampling bottles should have enough capacity to permit at least two tests. After collecting the oil, test the oil level in the transformer and make good if deficient. Each bottle should be noticeably labeled and dated.

vi) The oil testing that set is best kept at the central maintenance depot and run through trained and intelligent staff. The testing equipment having:

a)  A standard test cup that can be readily removed for cleaning fitted along with two spheres each 13 mm dia. with some sources of adjusting the gap. This should be set at 4 mm by using the calibrated gauge, generally supplied with the equipment. The gap should be tested every time the test set is employed.

b)  A set ratio transformer is employed to step up the voltage. A small Variac connected to the AC (alternate current) 230 volt supply allows application of variable voltage to the primary that is why the HT voltage applied to the test gaps may be increased gradually from 0 to 40 kV or more. A voltmeter is given on the primary side but calibrated to depict the secondary voltages directly.

Diagram - Transformer oil testing kit

c) A circuit breaker to trip off the supply to the transformer, instantaneously if a spark over takes place.

(vii) The actual testing is completed as follows:

a) The gap is first tested with gauge and the test cup and the electrode gap carefully cleaned and washed along with oil known to be good. The cup after that is filled with the sample oil to be tested equal to about one cm higher than the electrodes.  The cup top after that should be covered with a clean glass plate and permitted to rest for at least 5 minutes thus, that all air bubbles may vanish. Any bubbles still standing on the surface might be eliminated with a clean glass rod. Make Use of thin rubber gloves if possible so that the sweat on your fingers may not cause any contamination of the oil.

b) After making certain that the test area is apparent of all men and the voltage regulator is in the 0 position, switch on the supply. Increase the voltage slowly from zero so that the full voltage is arrives at in about 20 to 30 seconds. It is fairly feasible that there might be one or two spark over beyond the electrodes in the extremely early stage itself though while the voltage is 20 kV or less. These should be ignored like they are generally because of some extraneous matter such as microscopic strands of cotton, dust, etc. that has a tendency to obtain aligned along the strong electrostatic field in the spark gap. They get burnt out and do not influence the test. Hence, the test should be continued and the voltage increased until there is positive and final breakdown of the oil accompanied through blackening of the oil near the gap. The circuit breaker will as well get tripped out. If it is closed again it will instantaneously trip. This is completely dissimilar from the spurious temporary breakdown earlier, while re-closure will not cause tripping.

c) Later than emptying the oil cup the electrodes and the cup are again carefully cleaned and rinsed with good oil, after which they are prepared for conducting other test. There is no opposition at all against carrying out the tests on two or more samples; the highest most value recorded should be taken as correct.

There are some environmental variables, like temperature, precipitation, etc., to refer before collecting a sample. The perfect condition for collecting a sample from an electrical apparatus is 35°C (95°F) or higher, 0% humidity and no wind. Cold conditions, or conditions while relative humidity is in excess of 70%, should be avoided, like this will raise moisture in the sample. Collecting a sample throughout windy conditions is also not suggested because dust and debris enter the clean sample simply and disrupt correct particle counts. If sampling the oils is not avoidable while the outside temperatures are at or below 0°C (32°F), it should not be checked for water content or any properties which are influenced by water like dielectric breakdown voltage. Fluids with particular gravity that is greater than 1.0, like askarels, should be sampled from the top since free water will float. For fluids with a particular gravity that is less than 1.0, like mineral-based synthetic fluids, transformer oils, and silicone oils, the sample should be taken from the bottom because water will be apt to drop to the bottom in these fluids.

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