Our final discussion on moisture will cover the three most commons ways to reduce the moisture levels in a wet transformer.
Method 1 – Field Vacuum Dry-Out
Field vacuum dry-out is where heat and direct vacuum are applied to the transformer tank after the unit is completely drained of fluid. The transformer must be rated for full vacuum and be leak free. This usually results in an extended outage due to processing times, as well as set and idle times. Often multiple cycles of heat and vacuum are applied. Cold traps are used to collect and measure the drying process; temperature will affect the efficiency of this process. Electrical testing can also be a useful tool to determine the effectiveness of the procedure. Insulation power factors of less than 1% are desirable for most service aged transformers.
Method 2 – Factory Dry-Out
A transformer can be effectively factory dried in a transformer repair facility, but the transformer must be removed from service, drained normally, and then transported. Once in the factory, there are several methods for drying including the use of ovens, heat and vacuum equipment, and vapor phase drying. This method will normally require a spare transformer to be installed and is the most expensive option.
Method 3 – Online Dryer
A third effective and recommended field procedure to dry a transformer is the application of an online dryer. Online dryers have recently gained popularity and S. D. Myers Inc, has pioneered a segment of the market with their own model called DryMax. This process has advantages in that it does not require de-energizing, and it can be applied to transformers that are not designed for full vacuum.
When the moisture levels in the paper insulation exceed 1.5% moisture by dry weight, the moisture has migrated and settled into the inner layers of solid insulation. This moisture cannot be removed by simply dehydrating the oil with 10 or even 20 passes.
This water can be removed by an online dryer that will subject the core and coil to dry oil for an extended period of time (measured in months). The moisture will migrate from the paper insulation and eventually become dry. Investigation and correction procedures should be performed wherever the existence of moisture is at a level of concern (ie. leaks must be repaired).
The online dryer connects to the bottom valve by a hose (or hard-piped) and returns to a suitable connection at the top. The online dryer slowly pumps oil and continually removes the water from the oil and over time, this continuous removal of the moisture will dry the transformer. Insulation power factor tests have shown dramatic improvement in oil quality. The time required to dry the transformer will depend on the initial oil quality and moisture content.