Transformers and Power Distribution Flashcards

What is the purpose of a potential transformer (PT) in a power distribution system?

Answer: To provide a scaled-down voltage signal for metering and protection relays

Potential transformers step down high system voltages to a standard secondary voltage (typically 120 V) for safe connection to voltage meters, watt-hour meters, and protective relays.

What causes transformer humming noise during operation?

Answer: Magnetostriction — the expansion and contraction of the core laminations

Transformer hum is caused by magnetostriction — the physical expansion and contraction of the iron core laminations as the magnetic field alternates at 60 Hz, producing a 120 Hz mechanical vibration.

What is the primary purpose of laminating a transformer core?

Answer: To reduce eddy current losses

Laminating the core into thin, insulated sheets reduces eddy current losses by limiting the paths available for circulating currents induced in the core by the alternating magnetic flux.

In a three-phase transformer bank, what happens if one transformer fails in a delta-delta configuration?

Answer: It can operate as an open delta at reduced capacity (57.7%)

An open delta (V-V) connection uses only two transformers and can deliver approximately 57.7% (1/√3) of the original delta-delta bank capacity, providing emergency three-phase power.

What is the voltage regulation of a transformer?

Answer: The percentage change in secondary voltage from no-load to full-load

Voltage regulation = ((VNL - VFL) / VFL) × 100%, where VNL is the no-load secondary voltage and VFL is the full-load secondary voltage. Lower regulation percentage indicates better performance.

What type of cooling system is designated ONAN for a transformer?

Answer: Oil natural, air natural (self-cooled)

ONAN stands for Oil Natural Air Natural — the transformer is cooled by natural convection of oil internally and natural air circulation externally. This is the simplest and most common cooling method.