An oil-filled transformer "breathes": it expels air when the oil expands and draws in air when it contracts. The silica gel breather unit is the filter for this breathing process, and its job is to keep out the oil's most dangerous enemy: moisture.
Why Does the Transformer Need to Breathe?
As load and weather change, the oil expands and contracts within the conservator tank. During contraction, outside air enters to fill the gap, and this air carries moisture that lowers the oil's insulating ability and accelerates the aging of the paper insulation.
How Does the Breather Unit Work?
- Incoming air passes through a transparent cylinder filled with moisture-absorbing silica gel granules.
- An oil cup is usually located beneath the unit, acting as a primary barrier that traps dust and prevents continuous contact between the granules and the atmosphere.
- Dry granules are blue (or orange, depending on the type), and as they become saturated with moisture they turn pink/white (or dark green).
When Should We Intervene?
- A color change in most of the granules means saturation: the granules should be replaced or dried by heating (practical references mention a temperature of around 120°C), following the manufacturer's instructions.
- Check the oil level and cleanliness of the lower cup during every inspection round.
- Is the silica gel becoming saturated continuously and unusually fast? Look for a cause: excessive breathing, a leak, or a sharply fluctuating load cycle.
Sample answer: Silica gel absorbs moisture from the air entering the conservator tank as the oil contracts, protecting the oil and insulation from water contamination that lowers the breakdown voltage. A change in the granules' color (from blue to pink, for example, depending on the type) indicates saturation with moisture and the need to replace or re-dry them according to the manufacturer's instructions.
Neglecting the breather unit because it's "a small, cheap part." Saturated silica gel left for months means moisture is continuously seeping into the oil, eventually resulting in a failed BDV test and possibly a costly internal discharge.
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