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

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.

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

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.

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

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.

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

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.

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

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.

(NETA) InterNational Electrical Testing Association Certified Technicians Practice Test

(NETA) InterNational Electrical Testing Association Certified Technicians Practice Test

NETA Transformer Testing Procedures 2