Resistance testing verifies moisture presence; simple dryout methods eliminate the problem.

Failure of dry type transformers can occur during operation when moisture is present in the windings. Experience has shown this to be a particular problem in transformers ranging in size from 500kVA and larger; and with a primary voltage greater than 600V. As such, you should test these transformers before energizing to verify that windings are dry. This is especially important for units that have been stored and/or deenergized in locations with high humidity, dampness, or wide temperature fluctuations. If this testing is not done, catastrophic failure may occur.

Testing for moisture presence

The test method used in determining whether or not windings have taken on moisture is relatively simple. All you have to do is test for resistance between individual windings and between each winding and ground. (On existing transformers that are connected, make sure to disconnect primary and secondary leads from everything, including feeders, secondary bus, lightning arresters, etc. before beginning this test.) This should be done for both the primary and secondary windings.

The test voltage should be a maximum of 1kV above the rated winding voltage, unless other voltage limits are recommended by the transformer manufacturer. The minimum resistance readings taken should be as recommended by the manufacturer before energizing the transformer. If manufacturer recommendations are unavailable, the minimum resistance readings at 68 [degrees] F should at least be 20,000 ohms per volt for the rated voltage of the transformer coils being tested. If any of the tests result in a reading less than these recommendations, the transformer should be dried out and retested before being energized and put into service.

For example, on the primary side of a 13.8kV/480V transformer, the test voltage should be a maximum of 14,800V and the minimum acceptable resistance readings should be at least 13,800V x 20,000 ohms, or 276 megohms. The recommended resistance readings are very dependent on the ambient temperature during testing. Correction factor tables are available for other ambient temperatures. One source for these tables is the InterNational Electrical Testing Association's (NETA's) MTS-1993 standard, Maintenance Testing Specification for Electrical Power Distribution Equipment and Systems. (Call 1-303-697-8441.)

A resistance reading less than 20,000 ohms per volt would suggest moisture in the windings, unless the reading is extremely low (less than 100 ohms per volt), which could suggest a short circuit. If a short circuit is suspected, further testing should be done.

The above testing can be done with a megohmmeter or a high potential (hi-pot) tester. A hi-pot, with a knowledgeable individual running the tests, is the safest due to the controlled rate of increase in voltage.

For reference, the voltage (applied volts) divided by the microamps (leakage current read from hi-pot tester) is approximately equal to megohms.

Leakage current should remain constant over time at any constant voltage or the test should be immediately discontinued to avoid a failure.

If test results are OK, you should energize the unit as soon as possible.

If the results of the tests are marginally less than these readings, it may be possible to energize without drying out the transformer, but an electrical engineer or trained testing technician should be consulted to make such a decision.

Drying out a transformer

Drying out a transformer should be done as per the manufacturer's recommendations. If no recommendations are available, you can use either of the following methods.

Method 1. Place a 60W to 100W incandescent lamp under the front and back of each coil and leave them on for a minimum of two weeks if possible. Then, retest as per above and decide whether or not more drying out is required.

Method 2. Disconnect the primary and secondary leads from everything, including feeders, bussing, lighting arresters, etc. Then, short all the load ends of the secondary windings together but not to ground. If you have to use a shorting jumper, the calculation below will help you determine its size. If you can, just bolt all the secondary leads together.

Connect the primary leads to a voltage source as determined by the calculations below. If possible, the drying out period should be one week at a minimum. The transformer should then be retested as noted above. The resultant resistance readings will determine whether or not more drying out is required.

This method is probably more reliable than Method I in uniformly drying out the entire transformer.

Voltage source calculation

Step 1. Determine maximum primary dryout voltage with secondary shorted.

The maximum primary dryout voltage with secondary shorted ([V.sub.MPDV]) is found by using the following equation.

[V.sub.MPDV] = [V.sub.p] X Z (eq. 1)

where [V.sub.p] = primary voltage (volts).

Z = transformer impedance/100

Step 2. Determine connected primary dryout voltage.

The connected primary dryout voltage ([V.sub.CPDV]) must be equal to or less than [V.sub.MPDV]. Therefore, depending on the available voltages at the respective site, a choice is made.

Step 3. Determine primary amps with connected primary dryout voltage applied.

First, calculate the primary amps with the normal application voltage applied ([I.sub.P]) using the following equation.

[I.sub.P] = VA / ([V.sub.P] 1.732) (Eq. 2)

Next, calculate the transformer's primary with [V.sub.CPDV] applied ([I.sub.CPDA]) using the following equation.

[I.sub.CPDA] = ([V.sub.CPDV] X [I.sub.P]) / [V.sub.MPDV] (Eq. 3)

This value is then used to determine appropriate circuit overcurrent protection and feeder sizing.

Step 4. Determine the magnitude of current flowing in the secondary jumper.

The current in the secondary jumper ([I.sub.SJ]) must be equal to or greater than the secondary amps with [V.sub.CPDV] applied. To determine [I.sub.SJ], we use the following equation.

[I.sub.SJ] = ([V.sub.P] X [I.sub.CPDA]) / [V.sub.S] (Eq. 4)

where [V.sub.S] = secondary voltage.

This looks complicated but really isn't. Let's do a sample calculation to see how easy it is.

Sample calculation

Suppose we have a 1500kVA, 13.8kV-480/277V transformer with an impedance of 8% that needs drying out. What is the maximum primary dryout voltage (with secondary shorted) needed? What is the magnitude of current the secondary jumper will have to conduct?

Step 1. Determine maximum primary dryout voltage with secondary shorted.

Using Equation 1, we have:

[V.sub.MPDV] = [V.sub.P] X Z

= 13,800 x 0.08 = 1104V

Step 2. Determine connected primary dryout voltage.

Since [V.sub.MPDV] is 1104V, the next lower readily available voltage is 480V. Therefore, [V.sub.CPDV] is 480V.

Step 3. Determine primary amps with connected primary dryout voltage applied.

First, we have to find the transformer's primary current with normal application voltage applied ([I.sub.P]) by using Equation 2.

[I.sub.P] = VA / ([V.sub.P] x 1.732)

= 1,500,000 / (13,800 x 1.732)

= 62.8A

We then insert this value into Equation 3 and solve for [I.sub.CPDA].

[I.sub.CPDA] = ([V.sub.CPDV] X [I.sub.P]) / [V.sub.MPDV]

= (480 x 62.8) / 1104 = 27.3A

Therefore, we should connect the primary windings to a 480V, 35A, 3P breaker (or fuse) for dry out.

Step 4. Determine the magnitude of current flowing in the secondary jumper.

We insert [I.sub.CPDA] as determined from Step 3 into Equation 4 and calculate the amount of current that will flow in the shorting jumper ([I.sub.SJ]) as follows.

[I.sub.SJ] ([V.sub.P] X [I.sub.CPDA]) / [V.sub.S]

= (13,800 X 27.3) + 480 = 785A

Therefore, we need a jumper of 785A minimum capacity to short the secondary windings of the transformer to each other (but not to ground).

We strongly recommended that you check with the transformer manufacturer for methods, voltage levels, resistance readings, dryout procedures; etc.