The BDV (Breakdown Voltage) test is the most well-known test performed on a transformer oil sample: voltage is raised between two electrodes submerged in the oil until a breakdown spark occurs, and the voltage value at that point tells us about the oil's quality as an insulator.
The Concept of the Test and Its Steps
- An oil sample is taken from the dedicated drain valve at the bottom of the transformer — not all the oil is drawn.
- The sample is placed in the standard test cell between two electrodes at a specified gap (2.5 mm in common specifications).
- The voltage is raised gradually at a steady rate until breakdown occurs (a spark between the electrodes).
- The breakdown voltage is recorded, the measurement is repeated several times, and the average is taken.
What Does the Result Mean?
The higher the breakdown voltage, the cleaner and drier the oil. If the oil is designed to withstand, say, up to 50 kV but breaks down at 45 kV, this is a warning sign that warrants investigation: moisture? impurities? aging? Acceptable values vary depending on the transformer's voltage and the applicable specification, so always refer to the manufacturer's specification and local code.
Moisture: The Number One Enemy
A few parts per million of water are enough to noticeably lower the breakdown voltage. This is why BDV results are directly linked to the condition of the breather and silica gel unit; saturated silica gel means damp air is entering the tank and gradually degrading the oil.
Cleanliness of the sampling equipment is critical. A contaminated sample from an unclean container gives a falsely low reading and may lead to unnecessary maintenance decisions.
Sample answer: BDV is a breakdown voltage test on a sample of transformer oil: voltage is raised between two standard electrodes submerged in the oil until breakdown occurs, and the recorded value is a measure of the oil's insulating quality. The value decreases mainly due to moisture, as well as solid impurities, oxidation byproducts, and aging, and a drop below the specification limits calls for filtering, treating, or replacing the oil.
Judging the oil based on a single measurement. The correct approach is to repeat the measurement several times and take the average, then compare the result to the record of previous measurements and to the manufacturer's specification, not to a single absolute number.
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