What Are the Methods for Drying a Wet Transformer?

This article is Part 3 of a three-part series on moisture in transformers. Part 1 addressed why paper insulation is the critical moisture target, and Part 2 covered the forms moisture takes and the pathways through which it enters or is generated. Here we will address the four primary drying methods available — online drying, field dehydration, field vacuum dry-out, and factory dry-out — and the technical and operational factors that determine which is appropriate for a given situation.

Important note: Leak repairs should be addressed prior to proceeding with the chosen method of dehydration.

Method 1: Online Drying

How online dryers work

Online drying operates while the transformer remains energized and in service. The dryer connects to the bottom valve — either via hose or hard-piped — and returns processed oil through a suitable connection at the top of the tank. Oil is circulated slowly and continuously through the dryer, which removes moisture from the oil on each pass. As the oil moisture level is repeatedly reduced, a sustained concentration gradient is established between the wet paper insulation and the dry circulating oil, driving moisture diffusion out of the solid insulation and into the oil over time.

The drying timeline is measured in months rather than days or weeks, reflecting the rate at which moisture can diffuse through multiple layers of paper, pressboard, and structural insulation to reach the oil interface. Time to completion depends on initial oil quality, starting moisture content in the paper, transformer geometry, and operating temperature.

SDMyers’ DryMax® online dryers allow you to remove the moisture from your transformer’s paper insulation while staying energized.

When online drying is the appropriate method

Online drying is the only method that dries the paper insulation while the transformer remains in service. It is particularly well-suited to situations where the transformer cannot be de-energized for an extended period, or where the unit is not rated for full vacuum. It is also the indicated method when paper moisture exceeds approximately 1.5% by dry weight — the threshold at which moisture has migrated into the inner insulation layers. At that level, recirculating oil through the transformer without a dedicated drying unit, even at high volumes, will not extract moisture from depth. Dehydrating the oil alone — regardless of the number of passes — does not replicate the sustained concentration gradient that an online dryer maintains continuously over an extended treatment period.

Insulation power factor testing has shown measurable improvement in oil quality over the course of online drying treatment and serves as a useful monitoring tool throughout the process. Because online dryers do not require de-energization, a single unit can be installed and reinstalled across an entire transformer fleet, making this method practical for portfolio-level moisture management programs.

Method 2: Field Dehydration

Field dehydration circulates transformer oil through a mobile processing unit brought onsite. The unit applies heat and filtration to remove moisture from the oil as it cycles through, typically completing a treatment pass in one to two days. This method is effective at reducing dissolved moisture in the oil and is a common first-line response when oil moisture readings are elevated.

However, field dehydration has a fundamental limitation that mirrors the problem described in Part 1: it addresses moisture in the oil, not in the paper insulation. When paper moisture is the primary concern — particularly when moisture-by-dry-weight exceeds 1.5% — field dehydration will produce a temporary reduction in oil ppm readings, but moisture will re-migrate from the paper back into the oil within weeks. Field dehydration is most appropriately used when moisture is confirmed or strongly suspected to be confined to the oil, or as a preparatory step before a more comprehensive drying procedure.

Method 3: Field Vacuum Dry-Out

Field vacuum dry-out involves completely draining the transformer of oil, then applying heat and direct vacuum to the tank to drive moisture out of the solid insulation. Because this procedure applies vacuum directly to the transformer tank, the unit must be rated for full vacuum and confirmed leak-free before processing begins — a vacuum applied to a compromised tank presents structural risk.

Processing typically requires multiple heat-and-vacuum cycles to achieve adequate moisture reduction in the paper insulation, and the combination of setup, active processing, and idle time between cycles results in an extended outage. Cold traps are used to capture and quantify the moisture extracted during each cycle, providing a measurable record of drying progress.

Electrical testing is a valuable complement to cold trap data for evaluating procedure effectiveness. For most service-aged transformers, an insulation power factor below 1% is considered a desirable endpoint. Ambient and insulation temperature during processing directly affects drying efficiency and should be monitored and controlled where possible.

Method 4: Factory Dry-Out

Factory dry-out relocates the transformer to a repair facility where a broader range of drying equipment is available, including industrial ovens, heat-and-vacuum systems, and vapor phase drying — a method that uses solvent vapor at elevated temperature to penetrate and displace moisture from deep within the insulation structure.

This method is generally the most thorough available, but it carries the highest logistical burden. The transformer must be taken out of service, drained, and transported — requiring route assessment for large power transformers. A spare unit must typically be installed to maintain service continuity during the period the transformer is out of operation. For these reasons, factory dry-out is usually reserved for units where field methods are not sufficient or not feasible, or where the transformer is already at the repair facility for other corrective work.

Correcting Ingress Sources

Regardless of drying method selected, any identified moisture ingress pathway must be corrected before or concurrent with treatment. Applying a drying procedure to a transformer with an active leak, exhausted breather desiccant, or off-specification nitrogen supply will reduce the effectiveness of treatment and increase the likelihood of recurrence. Moisture removal and source correction are not sequential steps — they are parallel requirements.

Early and consistent moisture remediation, with ingress sources addressed, is considerably less costly than the alternative: unplanned downtime, accelerated insulation aging, and premature transformer replacement. As established in Part 1, the damage moisture inflicts on paper insulation is irreversible — which makes early intervention the only strategy that preserves full service life.

Summary:

• Four drying methods are available — online drying, field dehydration, field vacuum dry-out, and factory dry-out. Method selection depends on moisture severity, transformer design constraints, outage availability, and cost tolerance.
• Online drying is the only method that dries paper insulation in service — it does not require de-energization, can be applied to transformers not rated for full vacuum, and is practical for fleet-wide deployment. It is the recommended method when paper moisture exceeds 1.5% by dry weight and outage is not available.
• Field dehydration addresses oil moisture only — it is effective as a short-term measure or preparatory step, but will not extract moisture from the paper insulation and should not be used as a standalone treatment when solid insulation moisture is the primary concern.
• Field vacuum dry-out requires full vacuum rating — the transformer must be completely drained, rated for full vacuum, and confirmed leak-free. Multiple heat-and-vacuum cycles are typically required, and insulation power factor below 1% is the target endpoint for most service-aged units.
• Factory dry-out is the most thorough but most disruptive option — it requires removal from service, transport to a repair facility, and a spare unit for continued operation. It provides access to the widest range of drying methods, including vapor phase drying.
• When paper moisture exceeds 1.5% by dry weight, oil dehydration alone is insufficient — at that level, moisture has migrated into the inner insulation layers and cannot be extracted by oil passes alone, regardless of volume.
• Ingress source correction is a parallel requirement, not a sequential step — drying without addressing the entry mechanism will produce diminishing returns over successive treatment cycles.

In This Series:

• Part 1: Why Is Drying Transformer Paper Insulation So Difficult?
• Part 2: Where Does Transformer Moisture Come From?
• Part 3: What Are the Methods for Drying a Wet Transformer?