HYPRO – TROJAN DryOut Systems for Power Transformers


Power transformers are the most important component in any power system. They are also the most costly and least easy to replace, with lead times for new transformers usually considerable.

In the course of normal operation, oil insulated power transformers will age. The two critical components of transformers' insulation and continued effective operation are the cellulose (paper) and oil. Together, the oil /paper insulation system provides:


  • A dielectric medium
  • A cooling medium
  • Protection for the core and coil assembly from chemical attack


The cellulose insulation is the most vulnerable component in a transformer. It is both a source and an accumulator of moisture. Over 98% of the moisture in a transformer is contained in the cellulose insulation.

The decline in its aging, as measured by the Depolymerisation Index, is not reversible, and as it deteriorates it produces other compounds that cause further deterioration. The quality of the paper and its water content are the primary indicators of the transformer condition. In essence, the life of the cellulose determines the life of the transformer.


The rate at which the paper ages is controlled by several factors:

  • Temperature
  • Moisture
  • Acidity
  • Oxygen


The oil is the main vehicle for heat extraction. Increasing moisture content in the oil leads to a steady decrease in the dielectric strength.


Moisture migration between the oil and paper in an oil-paper insulation system is governed by temperature.

The higher the temperature of the windings, the higher the rate of movement of water from the paper into the oil. However, when the transformer cools, the water usually returns at a far slower rate due to residual heat retained in the windings, this causes the relative saturation of the oil to increase significantly and the oil dielectric to decrease correspondingly. Under these circumstances, the transformer has an increased risk of failure. High levels of water in the cellulose (> 2%) can cause bubble evolution.


With a good understanding of the dynamics of the insulation system of power transformers, a suitable maintenance program can be put be in place to slow the aging process of both new and aged wet transformers.

An effective program is one that addresses the key issue - the removal of moisture from the cellulose insulation, not just the oil. This does not have to be an expensive process to be effective in reducing the aging process.


We have prepared a short introduction of the Trojan DryOut Sytems along with a technical document about Oil-Impregnated paper in Power transformers; both documents can be downloaded from this page.

Please don't hesitate to contact us for further explanations and/or for a proposal.

We will be more than happy to support you.