Transformer Testing Procedures Flashcards
6 cards from real NETA practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Transformer Testing Procedures flashcards as text
What does polarity testing of a transformer verify?
Answer: The instantaneous direction of induced voltage in the secondary relative to the primary
Polarity testing verifies whether the secondary voltage is in phase (additive polarity) or out of phase (subtractive polarity) with the primary voltage. Correct polarity is essential for parallel operation and proper protection relay connections.
The percent impedance of a transformer (measured by the short-circuit test) directly affects:
Answer: The maximum available fault current on the secondary side
The percent impedance limits the maximum fault current available at the transformer secondary. Lower impedance equals higher available fault current. This is critical for calculating available fault current and selecting protective devices.
During transformer core ground testing, a technician finds two core ground connections. This indicates:
Answer: A potentially serious fault allowing circulating currents in the core
The transformer core must have exactly one intentional ground. Two ground points create a closed loop that allows stray flux to induce circulating currents in the core, causing excessive heating.
What is the primary purpose of performing a frequency response analysis (FRA) on a power transformer?
Answer: To detect mechanical displacement of windings and core due to through-fault forces
Frequency Response Analysis (FRA) measures the transformer's frequency response signature. Changes from baseline indicate winding deformation, loosening, or core movement caused by through-fault forces.
A transformer's nameplate shows OA/FA/FOA cooling designation. What does FOA indicate?
Answer: Forced Oil and Forced Air cooling — oil is pumped and external fans blow air over radiators
In ANSI cooling designations: F = Forced, O = Oil, A = Air. FOA = Forced Oil, Forced Air — oil is circulated by pumps and radiators are cooled by forced-air fans, providing the highest cooling efficiency.
When performing a transformer insulation power factor test, the UST (Ungrounded Specimen Test) configuration is used to:
Answer: Eliminate the effect of surface leakage currents from the measurement
The UST (Ungrounded Specimen Test) or guarded measurement configuration eliminates stray surface leakage currents by routing them to the guard terminal rather than the measurement circuit, providing accurate bulk insulation power factor.