309A Transformers and Power Distribution 3 — Questions and Answers
Question 1: What is the purpose of a potential transformer (PT) in a power distribution system?
- To step up voltage for transmission
- To provide a scaled-down voltage signal for metering and protection relays (Correct answer)
- To correct power factor
- To provide backup power
Correct 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.
Question 2: What causes transformer humming noise during operation?
- Loose electrical connections
- Magnetostriction — the expansion and contraction of the core laminations (Correct answer)
- Overloaded secondary windings
- Defective insulation
Correct 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.
Question 3: What is the primary purpose of laminating a transformer core?
- To increase mechanical strength
- To reduce eddy current losses (Correct answer)
- To improve heat dissipation
- To reduce the weight of the core
Correct 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.
Question 4: In a three-phase transformer bank, what happens if one transformer fails in a delta-delta configuration?
- The entire system shuts down
- It can operate as an open delta at reduced capacity (57.7%) (Correct answer)
- It continues at full capacity
- It automatically switches to wye configuration
Correct 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.
Question 5: What is the voltage regulation of a transformer?
- The ratio of primary to secondary voltage
- The percentage change in secondary voltage from no-load to full-load (Correct answer)
- The efficiency of the transformer
- The maximum voltage the transformer can handle
Correct 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.
Question 6: What type of cooling system is designated ONAN for a transformer?
- Oil natural, air natural (self-cooled) (Correct answer)
- Oil natural, air forced (fan-cooled)
- Oil forced, air natural
- Oil forced, air forced
Correct 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.
309A Transformers and Power Distribution 3
What is the purpose of a potential transformer (PT) in a power distribution system?