Section 6—Maintenance

The frequency at which transformers should be inspected depends on operating conditions.

For clean, dry locations, annual inspections may be sufficient. However, for other locations, such as where the air is contaminated with dust or chemical fumes, an inspection at three- or six-month intervals may be required. Usually after the first few inspection periods, a definite schedule can be set up based on the transformer’s existing conditions.

1. Perform routine inspections, maintenance, and testing after any severe electrical short circuit, ground fault, or environmental event (e.g., flooding) to determine the operational status of the transformer.

2. Perform the inspections, maintenance and testing if the transformer has been out of service for an extended period of time. Place strip heaters to maintain the transformer temperature above ambient to prevent condensation from forming in the transformer during extended down time.

A clean contact surface area is necessary, and terminals must be clean. Where the transformers are installed indoors or outdoors or in a harsh environment, the connections must be sealed with an approved electrical joint compound.

Apply the compound to exposed connections and the surface area before making a bolted connection.

1. De-energize the transformer.

2. Follow the check list below to permit free circulation of air and to guard against insulation breakdowns and thermal performance issues:

   • Clean the transformer enclosure, windings and insulators by vacuuming or by blowing clean, dry, compressed air or nitrogen. Be sure to wear appropriate personal protective equipment (PPE) when cleaning.

     NOTE: Vacuuming is the preferred first step for cleaning. Compressed air should have a dew point of -50°F (-45.5°C) or less, and a pressure at or below 25 psi.

   • Clean lead supports, tap changers, terminal boards, bushings, and other major insulating surfaces with lint-free rags or soft-bristle brushes.

   • Do not use liquids, solvents, or detergents, as they may deteriorate insulating materials.

   • Carefully inspect and thoroughly clean the ventilation openings (ducts) and winding assemblies. Remove any blockages.

   • Remove any trash and/or combustible materials from the area around the unit.

1. Use a low resistance ohmmeter to measure contact resistance.

   1. Compare values with similar connections looking for unusually high contact resistance points.

   2. Investigate any connections that may deviate from common values by more than 50% of the lowest values.

2. An infrared scanning device may be used to look for abnormal temperatures at transformer contacts for energized units.

   NOTE: Take care to follow the manufacturer’s procedures, as small errors in scanner operation can cause significant errors in detected temperatures.

Insulation resistance checks may create a capacitive charge on a winding.

1. Ground each winding for at least one minute between tests to drain any static charge created during the test.

2. Perform insulation resistance tests with an appropriate device.

3. Test from winding-to-winding and winding-to-ground.

4. Record all test results for comparison for future readings.

   • Values are installation site dependent and not pre-set at the factory. All testing is compliant to NEMA ST-20.

   • Insulation resistance readings should be at least 1 megohm.

   • Lower readings may indicate the presence of moisture and require a drying procedure to correct.

On larger transformers (>500 kVA):

1. Measure the winding resistances and record for future reference.

2. Compare the results with the values obtained during the pre-commissioning checks. Readings should not vary by more than 5% with adjacent coils and the pre-commissioning checks.

3. Record the temperature at which these initial readings were taken.

   • Values are installation site dependent and not pre-set at the factory. All testing is compliant to NEMA ST-20.

4. Adjust the resistance values based on any differences in temperature between current readings and pre-commissioning values.

1. Check for proper values of resistance between the system ground lug (or point) and the transformer enclosure, core, frame and clamps.

2. Compare with pre-commissioning values, looking for trends that may indicate a compromise in the quality of the ground bond.

1. Perform a turns ratio test on all taps and on the full winding voltages for each phase. Significant changes may indicate a loss of winding continuity.

   1. Compare these values with those obtained during the pre-commissioning checks. Voltages between phases should not deviate by more than 0.5% from adjacent coils.

   2. Investigate abnormal readings found outside of this limit.

2. Verify input and output voltages are correct for the tap and output voltages expected. Investigate abnormal readings.

Inspect the sound isolation pads for signs of physical deterioration.

1. Inspect all coil cooling ducts for any accumulation of dust, dirt or other obstructions.

2. Follow standard cleaning procedures to ensure proper air flow.

Ensure terminals are in good working condition for the proper operation of transformers.

1. Inspect terminals for alignment, tightness (see torque requirements), pressure, burns or corrosion. Investigate and correct any signs of damage.

2. Repair minor pitting, as long as the plating on the terminal has not been compromised.