Drying out a Transformer after a Flood: Part 4

6 May 2019


Your area just went through a massive storm.  Your facility has become flooded and your transformer has taken on water in the flood.  Is there a way to dry it out and have it work like new again?  The answer is, sure!  But there are several steps you must follow in order to dry out this precious piece of equipment.

Remove covers or inspection cover plates

Your first step in restoring your transformer is to remove covers or inspection cover plates. Upon doing this, make note of the condition of the oil and windings and check for signs of failure.  Many transformers are oil insulated and flooding has been known to introduce contaminants that will reduce dielectric strength and deteriorate performance.

In order to test dielectric strength use an oil test set.  Inspect for loose connections or heating. Draw oil samples from top to bottom.  Breakdown strength should be at least 22 kV.  Check the insulation resistance.  That value should be at least 2 megohms. 

It is best to use a tester that can measure values all the way to the values of pre-flood conditions.  The best type of tester would be one with extended range (into tera-ohms).  For equipment rated at higher voltages, a 5- to 10- kV tester should be employed to apply the higher voltage stresses that such equipment will experience in operation.

Be sure to examine the condition of bushings, external connections, operating switches and protective devices.  Also be sure to clean the transformer externally and paint the tank if necessary.

What to do if water has entered the tank

If water has entered the tank, don’t panic.  It should be salvageable.  Remove the tank cover.  Flush the windings and core with clean, moisture-free insulating oil.  If the transformer is small, remove the coil and core and dry in an oven at a temperature that does not exceed 194°F. 

Dip and bake the windings if it appears those steps need to be done.  If you are working with a larger unit, they can be dried in the tank by forcing hot, dry air that does not exceed 194°F in and around the windings after the oil has been removed from the tank, by short-circuiting one winding and energizing the other with low ac voltages, or by a combination of these two methods.


Insulation resistance checks will determine the progress of drying.  A curve should be plotted showing resistance against time.  It is best to start at the 5-volt level of the kilohm range, and then test periodically throughout the drying process.

You will know if the drying process is working by the steady rise in insulation values. If there is mechanical damage, like pinholes or burn tracks, the insulation will not spontaneously regenerate and the drying process can be discontinued without costly expenditure of time in favor of rewinding or similar repair actions.

If all goes well

If the drying process is successful, it can be monitored into the megohm range and higher voltage tests can be applied without damage until the final desired value is reached.  Once the drying has restored insulation resistance to acceptable values, full performance can be further verified by performing a turns-ratio test. 

Damage to insulation can produce turn-to-turn shorts that effectively alter the turns ration between primary and secondary windings.  A turns-ratio tester is the most precise and sensitive means of determining the existence and extent of such problems.

It is important to remember to filter the oil

You must remember that oil should be filtered to standard dielectric test value of not less than 22 kV, or a new supply of oil of high dielectric strength should be used.

In our final installment of salvaging flood-damaged electrical equipment, we will discuss the drying out of cable and wiring as well as the reconditioning of electrical tools. Review Part 1, Part 2 and Part 3, or view the entire series of tips on page 28.