NETA Transformer Testing Procedures 2 — Questions and Answers

Question 1: The turns ratio test on a transformer is performed to:

Measure the insulation resistance between windings
Verify that the actual voltage transformation ratio matches the nameplate ratio (Correct answer)
Determine the leakage reactance of the windings
Test the transformer's short-circuit impedance

Correct answer: Verify that the actual voltage transformation ratio matches the nameplate ratio

The turns ratio test (TTR) verifies that the ratio of primary to secondary turns (and thus voltage) matches the nameplate specifications, detecting shorted turns, wrong tap settings, or internal connection errors.

The transformer turns ratio (TTR) test applies a low voltage AC signal to one winding and measures the output on the other winding. The ratio should match the nameplate voltage ratio to within plus or minus 0.5% per NETA acceptance criteria. Deviations indicate shorted turns within a winding, incorrect tap changer position, wrong internal connection, or winding damage. Testing is performed on all tap positions and all winding combinations. This test is essential at initial commissioning and after any repair or suspected fault event.

Question 2: During a transformer oil dielectric strength test, the oil sample fails if:

The oil is darker than normal in color
The breakdown voltage is below the minimum specified value (typically 30 kV for ASTM D877) (Correct answer)
The oil temperature is above 25 degrees C during testing
The oil sample contains any visible moisture

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

Oil dielectric strength is measured by the breakdown voltage in kV. For ASTM D877 (disc electrodes), minimum acceptable values are typically 30 kV. Below this indicates contamination with moisture or particles.

Transformer oil dielectric strength tests per ASTM D877 or D1816 measure the voltage at which oil breaks down electrically. Minimum acceptable values depend on voltage class but are typically 30 kV for distribution transformers and higher for transmission. Low dielectric strength indicates water contamination (even small amounts dramatically reduce breakdown voltage), particulate contamination, or aging products. Additional oil tests include dissolved gas analysis (DGA), acid number, interfacial tension, power factor, and color/appearance.

Question 3: The excitation current test on a power transformer measures current drawn with:

Secondary short-circuited and rated voltage applied to the primary
Secondary open-circuited and rated voltage applied to the primary (Correct answer)
Both windings energized simultaneously
Reduced voltage applied to verify linearity

Correct answer: Secondary open-circuited and rated voltage applied to the primary

Excitation (no-load) current is measured with the secondary open-circuited and rated voltage applied to the primary. It indicates core condition, detecting shorted laminations or other core defects.

The excitation current (no-load current) test applies rated voltage to the primary winding with the secondary open-circuited. The measured current flows to magnetize the core and supply core losses. Normal excitation current is typically 1 to 5 percent of rated current. Elevated excitation current indicates shorted laminations in the core, which increase eddy current losses. Comparing excitation current on all three phases and comparing to baseline measurements helps identify developing problems. NETA acceptance criteria typically require excitation current values within 50% of each other between phases.

Question 4: Dissolved Gas Analysis (DGA) of transformer oil is used to:

Measure the breakdown voltage of the insulating oil
Detect and identify internal faults by analyzing gases dissolved in the oil (Correct answer)
Determine the acid content of aged oil
Measure the moisture content of the oil in parts per million

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

DGA analyzes gases dissolved in transformer oil to detect thermal and electrical faults. Different fault types produce characteristic gas patterns interpretable using methods like Rogers Ratio or Duval Triangle.

Dissolved Gas Analysis (DGA) is one of the most powerful transformer diagnostic tools. Electrical and thermal faults within a transformer decompose oil and insulating paper, producing gases that dissolve in the oil. Key gases include hydrogen (H2), methane (CH4), ethylene (C2H4), acetylene (C2H2), carbon monoxide (CO), and carbon dioxide (CO2). Acetylene indicates high-energy arcing; excessive CO and CO2 indicates paper insulation degradation; ethylene indicates overheating. Interpretation methods include IEC 60599 ratios, Rogers Ratios, Duval Triangle, and IEEE C57.104 guidelines. Regular DGA trending is essential for condition-based maintenance.

Question 5: A transformer winding resistance test is typically performed using:

A megohmmeter (Megger) applying 500 to 5000V DC
A micro-ohmmeter or DLRO injecting DC current and measuring voltage drop (Correct answer)
An AC bridge at power frequency
A clamp-on ammeter with the transformer energized

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

Winding resistance is measured using DC current (micro-ohmmeter or DLRO) to determine actual copper resistance. This detects loose connections, shorted turns, broken strands, and contact problems in tap changers.

Winding resistance measurement uses a precision micro-ohmmeter or DLRO that injects a stable DC current and measures the resulting voltage drop. DC is used because inductance does not impede steady DC current flow. Results are corrected to a reference temperature (usually 75 degrees C) using the conductor temperature coefficient. Comparing measurements to factory values and between phases identifies high-resistance connections, broken conductor strands, or shorted turns. Tap changer resistance uniformity across all positions verifies contact integrity.

Question 6: Transformer power factor (dissipation factor) testing measures:

The ratio of real power to apparent power in the primary winding load
The insulation quality, with higher values indicating more insulation degradation (Correct answer)
The efficiency of power transfer from primary to secondary
The magnetic core losses under rated flux density

Correct answer: The insulation quality, with higher values indicating more insulation degradation

Transformer power factor (or dissipation factor/tan delta) testing measures insulation quality. Higher power factor values indicate moisture, contamination, or deterioration in the oil-paper insulation system.

Transformer insulation power factor (also called dissipation factor or tan delta) testing measures the dielectric losses in the insulation system. An ideal capacitor (insulator) has zero losses and zero power factor; real insulators have some losses due to polarization and conduction. Power factor test voltage is applied at power frequency using a Doble or similar test set. New, dry insulation has very low power factor (below 0.5%). Values above 1% indicate moisture ingress, oil contamination, or insulation aging. The test is performed on winding-to-winding and winding-to-ground insulation, with comparisons to initial acceptance values for assessment.

NETA Transformer Testing Procedures 2

The turns ratio test on a transformer is performed to: