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On-site low frequency heating (LFH) drying

Virtually all moisture in a transformer can be found in the fixed insulation material. The significance of water content is paramount: laboratory samples suggest that humidity of 3-4 percent may increase the degradation rate of paper by a factor of 10 or more compared to dry paper.

This underlines the importance of assessing both the moisture and temperature of the insulation system. Temperature and vacuum are the main factors for drying speed and drying quality.

For optimized drying, the transformer should be heated at the same time as the vacuum is applied (as is done during the vapor phase process). With the combination of the low-frequency heating (LFH) technique and hot oil spray or hot oil circulation, this can also be achieved on-site.

The low-frequency voltage is necessary in order to reduce the applied voltage on the transformer when it is under vacuum. The reason is that the breakdown voltage of the insulation system is much lower under vacuum than under atmospheric pressure. This is also known as Paschen's law.

In order to prevent hotspots during drying (due to reduced cooling), the applied current for heating power transformers should not exceed 50% of the nominal current. Consequently, the copper losses will be no more than Ľ of the nominal losses, and the risk of potential hot spots at points with higher resistance will be negligible.

Scope of work

Plant and process concept

In order to heat up both the low and high-voltage windings, a frequency of approximately 1 Hz is applied to the transformer.

The transformer can be heated uniformly with the combination of LFH drying and the conventional hot oil spray method. The LFH system heats the windings from the inside, and the hot oil spray supports the heating process by heating the outer parts of the insulation system.

Oil Regeneration or Reclamation

Hitachi Energy offers an oil reclamation or regeneration process whereby oil is processed to achieve a significant condition improvement, aimed at being as close to “as new” as possible by removing aging products.

Advanced Maintenance

Hitachi Energy provides advanced maintenance services for transformers to be performed on-site that aim to extend the life of your assets.

Parts & Maintain

Hitachi Energy offers genuine, high-quality spare parts along with basic and advanced maintenance services.


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Transformer service

Aging assets, rising energy demand, and the critical need to avoid unplanned outages are challenging worldwide utilities and industrials. The service portfolio of Hitachi Energy allows utilities and industrials to maximize the return on transformer assets by ensuring high reliability, reducing life cycle costs, and ensuring optimized performance while lowering environmental impact.
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Hitachi Energy offers distribution and power transformers, Dry-type and EconiQ™ Transformers as well as services, for industrial and commercial applications.