03/01/2010  - Volume VI - Issue II

Stationary transformer drying system and moisture monitoring


The insulation materials on all power transformers will degrade with time in service. Insulation aging depends on several different factors including the original insulation material, oil temperature, moisture content, oxygen content, particle contamination and acids from oil and paper aging. Moisture content has a significant impact on transformer life expectancy, especially on the aging of the solid insulation.


Conventional methods of removing moisture, e.g. oil treatment plants, have focused on rapidly drying out transformer oil; however, 98% of the moisture is actually stored in the cellulose insulation. The rate of drying depends on the rate of water diffusion from the insulation into the insulating oil, which is a very slow process.


The solution


The SITRAM® DRY online oil dehydration system removes moisture from the insulating oil through disturbing the moisture equilibrium so that moisture diffuses from the wet insulating paper to the dried insulating oil, slowly removing the moisture from the solid insulation. The system has a modular construction and is well suited for transformers of varying ages, manufacturers, and ratings (from distribution to generator step-up transformers).


SITRAM® DRY utilizes an advanced molecular sieve absorbent material. As oil is circulated through SITRAM® DRY by a small pump motor, the sieve material contained in the cylinders absorbs the moisture. Moisture diffusion will slow down as the moisture content of the insulating paper converges to its equilibrium with the dried insulating oil.


The SITRAM® DRY system is capable of reducing transformer oil moisture levels from 50ppm to less than 10ppm. This process not only reduces the aging rate of the cellulose, but it also improves the dielectric strength of the insulation oil and can increase transformer reliability and life expectancy. Transformers can be run with a significantly reduced risk of failure and on higher load cycles.


Modular product concept


SITRAM® DRY has a modular product concept which is easily adaptable based on individual requirements. Two basic models of SITRAM® DRY are available: a frame version and a cabinet version. The frame version is fitted together with moisture and temperature sensors in an open steel frame. If the protection against environmental conditions of the system is important, the cabinet version would be the best option. Optional supervision modules for control are also available, e.g. for controlling the efficiency of the drying process.


Benefits for new transformers


SITRAM® DRY installed on a new or refurbished transformer can maintain the moisture content at or below the original 0.5% factory level. The system can significantly extend the expected life of a transformer at a fraction of its replacement costs. Typically, molecular sieve cylinders should not require replacement for several years.


Benefits for older or wet transformers (>2% ppm)


Legacy transformers generally have higher moisture content, so the target is to reduce the moisture content of the insulating paper to an acceptable level and then maintain that level. The amount of time required to dry out the unit will vary with the size, moisture content and operational temperature variation. Siemens can develop a moisture reduction plan which shall include a dry-out period and an estimated number of cylinder replacements. The time period is typically between 10 and 24 months.


Customer benefits include:

  • Continuous online removal of moisture from solid insulation

  • Based on molecular sieve technology

  • Easy to install on any energized transformer

  • Low maintenance system

  • Cartridge replacement and regeneration service

  • Temperature and moisture monitoring

  • Can be integrated with Siemens TMDS™ for remote monitoring and diagnostics.