NETA Transformer Testing Procedures 2

Question 1

The turns ratio test on a transformer is performed to:

Accepted Answer

Verify that the actual voltage transformation ratio matches the nameplate ratio

Answer

Measure the insulation resistance between windings

Answer

Determine the leakage reactance of the windings

Answer

Test the transformer's short-circuit impedance

Question 2

During a transformer oil dielectric strength test, the oil sample fails if:

Accepted Answer

The breakdown voltage is below the minimum specified value (typically 30 kV for ASTM D877)

Answer

The oil is darker than normal in color

Answer

The oil temperature is above 25 degrees C during testing

Answer

The oil sample contains any visible moisture

Question 3

The excitation current test on a power transformer measures current drawn with:

Accepted Answer

Secondary open-circuited and rated voltage applied to the primary

Answer

Secondary short-circuited and rated voltage applied to the primary

Answer

Both windings energized simultaneously

Answer

Reduced voltage applied to verify linearity

Question 4

Dissolved Gas Analysis (DGA) of transformer oil is used to:

Accepted Answer

Detect and identify internal faults by analyzing gases dissolved in the oil

Answer

Measure the breakdown voltage of the insulating oil

Answer

Determine the acid content of aged oil

Answer

Measure the moisture content of the oil in parts per million

Question 5

A transformer winding resistance test is typically performed using:

Accepted Answer

A micro-ohmmeter or DLRO injecting DC current and measuring voltage drop

Answer

A megohmmeter (Megger) applying 500 to 5000V DC

Answer

An AC bridge at power frequency

Answer

A clamp-on ammeter with the transformer energized

Question 6

Transformer power factor (dissipation factor) testing measures:

Accepted Answer

The insulation quality, with higher values indicating more insulation degradation

Answer

The ratio of real power to apparent power in the primary winding load

Answer

The efficiency of power transfer from primary to secondary

Answer

The magnetic core losses under rated flux density