ELECTRICAL TEST DRY TYPE POWER TRANSFORMERS

VISUAL INSPECTION of DRY TYPE POWER TRANSFORMERS

Check complete assembly for damage and missing parts.
Compare equipment nameplate information with latest single line diagram and report discrepancies.
Ensure the enclosure is approved for the area classification.
Check conductor size and type.
Check for correct conduit connections.
Ensure that primary and secondary leads are connected for required voltage level.
Grounding installed.
Neutral wire correctly installed.
Check mounting and supports per drawings.
The spare conductors (pigtails) properly insulated.

MECHANICAL INSPECTION OF DRY TYPE POWER TRANSFORMERS

Verify proper auxiliary device operation such as fans, indicators.
Check tightness of accessible bolted electrical joints in accordance with manufacturers specifications or P-000 guidelines.
Perform specific inspections and mechanical tests as recommended by the manufacture.
Check operation of tap changer or tap connection block for correct number of shorting bars.

ELECTRICAL TESTS DRY TYPE POWER TRANSFORMER

Insulation resistance tests shall be performed winding to winding and winding to ground. Appropriate guard circuit shall be utilized under all bushings. Test voltage and minimum acceptable values in accordance with values in test section.
A dielectric absorption test shall be made winding to winding and winding to ground for ten (10) minutes. Compute the polarization index. The test voltages and minimum acceptable values are listed in the test section.
A turn’s ratio test shall be performed between windings for all tap positions. The final tap setting is to be determined by the engineer and set by the test engineer upon completion of the ratio testing. Acceptable values are in the test section.
Winding resistance tests shall be made for each winding on each tap position.
No load excitation current shall be measured for each phase in accordance with established procedure.
Perform special tests and adjustments as suggested by manufacturer for tap changer, fan and controls, and alarm functions.

TEST VALUES

Perform Insulation Resistance and Polarization Index test. Final values will be temperature corrected.
The absorption test polarization index should be above 2.0 unless an extremely high value is obtained at the end of one (1) minute that when doubled will not yield a meaningful value with the available test equipment.
Winding resistance test results should compare within one percent (1%) of adjacent windings.
Turns ratio test results should not deviate more than one-half of one percent ( +/- 0.5%) from the calculated ratio.

Data Sheet Name plate data

Insulation resistance test: Megger Voltage: 500/100/2500/5000.

Temperature ( deg. C)
Megger voltage

WINDING RATIO TEST

Energize transformer at no load, measure and record:

Primary voltage: ____________

Secondary voltage: ____________

Primary current: ____________