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Transformer is overheating

Transformer insulation is generally rated for 220°C but may be lower for some designs including control or encapsulated. Standards permit the temperature of the transformer enclosure cover to be 65°C over ambient. When temperatures exceed the rating for the insulation system or enclosure, overheating occurs.

Burned, darkened or damaged insulation may be apparent along with a burnt smell. The hottest part of a transformer is the coil near the top of the core. Energized transformers should not be touched. If the insulation is damaged or smoke is visible, the unit may need to be returned for testing and replaced or repaired.

Check: Solution:
Verify total load doesn’t exceed transformer kVA rating. Reduce size or load or replace with larger transformer. In some cases fans can be added to increase cooling and
maximum load.
Verify ambient temperature does not exceed transformer ratings. Relocate to area with lower ambient temperature, reduce load, reduce ambient temperature at primary location or
replace with a low temperature rise transformer. Transformers installed in small rooms will need proper room
ventilation.
Verify tap connections are set up identically on all coils.
Verify transformer is correctly rated for harmonic load, check for high neutral currents. Reduce or remove harmonic loads or replace transformer with a larger unit or unit with the proper k-rating.
Verify that the transformer’s ventilation openings are not blocked. Transformers purchased as core and coil
units and placed in enclosures not supplied by HPS require that the integrator properly size the enclosure and cooling
requirements.
Relocate the transformer to an area of better ventilation. Move the transformer away from walls, equipment or
overhead projections that may impede airflow. Do not install fans to cool a transformer. Improperly installed fans may
actually impede airflow and could result in transformer damage.
Improper Input Voltages Verify taps are correctly set for the input voltage. Depending on the load and transformer type, continuous
overvoltages or undervoltages as low as 5-10% may cause overheating.
Check no load current. If no load current is high (varies with transformer efficiency but no load current is typically less than 2-3% of
total kVA), inspect the core and coils for damage. In most cases you will not be able to inspect the insulation between
the core and coil without returning to the factory for testing and disassembly. If there is a short between the core
and coil, the unit will have to be replaced or repaired.
Excessive and sustained airflow caused by exterior winds or fans generally moving horizontally to the ground can
disrupt convection cooling and cause overheating at high loads.
Relocate the transformer to an area with less wind or block the wind.
Fan cooled transformers have broken or misaligned fans. Fans need to be replaced or realigned.
Low Power Factor Low power factor can cause excessive current and higher overall loads. Current meters need to be able to register
total current. Some digital meters may not be accurate.
Unbalanced loads may cause excessive heating. Loads should be balanced to within 20 % of maximum kVA. No individual load should exceed the load specific load for
each phase (1/3 of total kVA for three phase units).
Transformer is installed above a heat source such as another transformer. Move either the transformer or the heat source. Redirect the hot airflow from the lower object away from the
cooling entrances and surfaces of the higher object. Replace the top unit with a low temperature rise transformer.
Check if output voltage is distorted. A highly distorted output voltage may be a sign that there is a turns to turns fault and the transformer is in
danged of immediate failure. The transformer needs to be denergized and meggered. The damaged coil may need to be
replaced or the transformer scrapped.
Check the output circuits to make sure each leg of the transformer is functioning and overcurrent protection is
ok.
If a fuse on one or more of the legs has opened, determine and clear the fault and replace the fuse. This is more
commong on delta transformer outputs, especially if three single phase units are used in a Delta bank.
If a Drive Isolation Tranformer (DIT) is being used, verify the DIT kVA has been derated per the HP sizing charts in the catalog. If motor HP is unknown, use .746 kW/HP to determine the equivalent HP of the load. DIT’s are not current rated
devices, the HP selection charts must be used to properly size a DIT. Extrusion applications tend to be the worst.
Check if two or more transformers are operating in parallel to power one load. Transformers operating in parallel are rare. Large circulating currents and uneven load can result from
transformers wired in parallel. The transformer s may have to replaced with one unit capable of power the entire
load.
Cable connections are discolored by heating. Cables should be periodically tightened. The surface should be cleaned of any insulation applied during the vacuum
pressure impregnation process. Rough edges must be smoothed.
Sparks or smoke is visible from the base of the transformer but the transformer has not failed and there isn’t any sound of arcing. During the VPI process, icicles of insulation can form under the tarnsformer and occassionally act as a ground. If
discovered early enough the icicle can be removed and the transformer will not be damaged.
Excessive dust could block air vents Dust needs to be blown out while transformer is denergized.
Discolored Insulation The transformer’s insulation may have been damaged and may need to be repaired or replaced.
Visible Flames or Smoke The transformer’s insulation may have been damaged and may need to be repaired or replaced.

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