EETC Transformers 4 — Questions and Answers
Question 1: An instrument transformer used to measure high voltages safely by stepping them down to a standard 120V level is called a:
- Potential transformer (PT) (Correct answer)
- Current transformer (CT)
- Control transformer
- Isolation transformer
Correct answer: Potential transformer (PT)
A potential transformer (PT), also called a voltage transformer (VT), accurately steps down high voltage to a safe 120V level for metering and protection relays.
Question 2: The secondary circuit of a current transformer (CT) must never be open-circuited while the primary carries current because:
- It will develop a dangerously high voltage across the open secondary terminals (Correct answer)
- The primary current will drop to zero
- The CT core will immediately demagnetize
- Excessive current will flow through the primary winding
Correct answer: It will develop a dangerously high voltage across the open secondary terminals
With the secondary open, the primary MMF is unopposed, driving the core into saturation and inducing a very high dangerous voltage across the open secondary.
Question 3: In a three-phase transformer bank, what is the primary advantage of using a delta connection on the primary side?
- Third-harmonic currents circulate within the delta, preventing harmonic voltage distortion (Correct answer)
- It eliminates the need for grounding on the primary side
- It reduces the voltage stress on each winding by a factor of √3
- It allows single-phasing to occur without loss of three-phase output
Correct answer: Third-harmonic currents circulate within the delta, preventing harmonic voltage distortion
The closed delta path provides a circulating path for triplen (3rd, 9th, etc.) harmonic currents, keeping them out of the supply line.
Question 4: A transformer's short-circuit test (impedance test) is performed to determine:
- Full-load copper losses and series impedance parameters (Correct answer)
- Core losses and magnetizing reactance
- Maximum allowable secondary current
- Insulation resistance of the windings
Correct answer: Full-load copper losses and series impedance parameters
The short-circuit test is conducted at rated current with the secondary shorted to measure winding resistance, leakage reactance, and copper losses.
Question 5: What is the typical efficiency of a well-designed modern distribution transformer at full rated load?
- 97% to 99% (Correct answer)
- 85% to 90%
- 90% to 95%
- 80% to 85%
Correct answer: 97% to 99%
Modern distribution transformers achieve efficiencies of 97–99% because both core and copper losses are very small relative to the power transferred.
Question 6: Which condition represents maximum efficiency in a transformer?
- When core (iron) losses equal copper (I²R) losses (Correct answer)
- When copper losses are twice the core losses
- When the transformer operates at unity power factor regardless of loading
- When the transformer is operating at exactly half of rated load
Correct answer: When core (iron) losses equal copper (I²R) losses
Transformer efficiency is maximum when the variable copper losses equal the constant core losses, which typically occurs between 50–75% of full load.
Question 7: What is the function of a tap changer in a power transformer?
- To adjust the turns ratio and thereby regulate the output voltage (Correct answer)
- To disconnect one phase in an emergency
- To bypass the transformer during maintenance
- To limit inrush current during energization
Correct answer: To adjust the turns ratio and thereby regulate the output voltage
A tap changer varies the number of active turns on a winding to compensate for system voltage variations and maintain the desired output voltage.
EETC Transformers 4
An instrument transformer used to measure high voltages safely by stepping them down to a standard 120V level is called a: