Use case: Power Transformer in High Voltage Technology

Adsorber on Transformer

In operating equipment filled with insulating oil, such as power transformers in high-voltage engineering, moisture is one of the main causes of faults and progressive aging, along with oxygen and temperature. For this reason, dehumidifiers are used on the expansion tanks to prevent moisture ingress. GIEBEL offers adapted silica gels to protect transformers.

Function of the dehumidifier

In German-speaking countries, dehumidifiers are used in accordance with DIN 42562. These are mounted on the pipeline from the expansion vessel via a triangular flange. Depending on the oil volume of the operating medium, the dehumidifier consists of one or more chambers. Transformers with a load switch usually have another small dehumidifier mounted for its expansion tank.

When the oil level drops during operation, due to load reduction or because the air in the expansion vessel contracts due to the drop in ambient temperature, the transformer “breathes in” air via the expansion vessel and the connected dehumidifiers.

This air flows from the bottom to the top through the silica gel chambers where it is dehumidified. At the lower part of the dehumidifier the so-called oil pad is attached. This serves mainly as an air seal to prevent undesirable loading of the silica gel by the ambient air. The silica gel is usually equipped with a color indicator that changes color with increasing load and indicates a necessary change of the silica gel.

Function of the silica gel

Chemically, silica gel is silicon dioxide (SiO2). The silica gel used for drying is amorphous, has a large inner surface with many pores and is strongly hygroscopic. Pure silica gel is colorless. The loading state cannot be detected. To obtain an indication, a color indicator is added to the silica gel.

Dehumidification is based on the following reversible processes:

  • Polarity: Silica gel has a strong affinity for dipole molecules such as water (H2O). The water molecules are bound to the surface of the silica gel. Likewise, the loaded surface again has dipole character and can bind dipole molecules.
  • Capillary condensation: If the surface of the silica gel is increasingly loaded, the water molecules condense into the narrow pores of the silica gel.

Silica gel in dehumidifier

In the dehumidifiers described above with a longer flow path through the silica gel, the dehumidification can be divided into three zones:

– Zone 1 – Heavily or completely loaded silica gel.
– Zone 2 – Mass transfer zone (active zone)
– Zone 3 – Unloaded silica gel (Reserve)

Zone 1 is not present in new silica gel. With progressive loading, this zone increases in size until the upper edge of the dehumidifier is reached.

Zone 2 is largely responsible for dehumidification. The height of this zone depends on the flow velocity through the silica gel.

Zone 3 is the reserve. If the height of the reserve zone is low, then replacement of the silica gel is strongly recommended.

The binding velocity of the silica gel decreases with the loading condition. In the case of unloaded silica gel, the water molecules are bound at the beginning mainly by the dipole character of the surface of the silica gel. If the surface is visibly occupied, then mainly capillary condensation or bonding on a multimolecular level takes place. In practical terms, the probability that a water molecule will be bound decreases.

Range of application and regeneration

The silica gel used for dehumidification can absorb up to 40% of its own weight in water. At an absorption of approx. 30% of its own weight, the mono-molecular layer is completely occupied. Especially at higher temperatures, moisture can then no longer be bound.

When water binds to silica gel, energy (heat) is released. To dissolve the bond between silica gel and water again, energy (heat) must be supplied. Silica gel can be regenerated at approx. 120°C. The color indicator remains stable.

Color indicators

In the past, silica gel was used with the heavy metal cobalt chloride (CoCl) as an indicator, which turned from blue to pink. Since 2000, this silica gel has been classified as carcinogenic.
The successor silica gel (Chameleon, Sorbead Orange, …) has a color change from orange to white based on iron salt, which is harmless to health. However, this silica gel has the disadvantage that the color change is visible much earlier with a lower load and therefore the silica gel is replaced more often than necessary.

The latest silica gel Orange combines the properties of the two silica gels described above. The color change is produced by copper salt – it is therefore harmless to health. The color change occurs similarly late as with blue gel. The following figure shows dehumidifiers with 0.5 kg silica gel at 10%, 20% and 30% total load.

Conclusion

It can be clearly seen that the use of today’s standard Orange-Green or Orange-Colorless silica gels offers the best price/performance ratio for use in transformer dehumidifiers. Based on numerous tests and reference examples at public utilities, service companies and transformer manufacturers, the use of Blue Gel (not REACH compliant) and Sorbead Chameleon (short maintenance interval & high price) is not recommended.

Silica gel product portfolio

We recommend the use of silica gel orange-green because it has the following advantages:

  • High-contrast color change
  • Color change at approx. 15 % by weight

The use of silica gel orange colorless is also recommended because:

  • Color change at approx. 25 weight %.
  • Known color change when switching from BASF dry beads