EETC Power Distribution 2 — Questions and Answers
Question 1: What is the primary purpose of a distribution transformer's tap changer?
- To adjust the output voltage to compensate for load variations (Correct answer)
- To disconnect the transformer during fault conditions
- To regulate the transformer's cooling system
- To balance the three-phase loads automatically
Correct answer: To adjust the output voltage to compensate for load variations
Tap changers adjust the transformer turns ratio to maintain proper secondary voltage as load and primary voltage fluctuate.
Question 2: In a radial distribution system, what is the main disadvantage compared to a loop or network system?
- Higher installation cost
- Loss of service to all customers downstream of a fault (Correct answer)
- Inability to handle three-phase loads
- Reduced voltage regulation capability
Correct answer: Loss of service to all customers downstream of a fault
In a radial system, a single fault interrupts power to all customers downstream of the fault point since there is only one supply path.
Question 3: Which conductor arrangement is used in most North American overhead distribution systems serving residential areas?
- Delta three-wire
- Wye four-wire multigrounded neutral (Correct answer)
- Delta four-wire
- Wye three-wire ungrounded
Correct answer: Wye four-wire multigrounded neutral
The multigrounded wye four-wire system is standard in North America because it provides both single-phase 120/240V service and neutral grounding at multiple points.
Question 4: A capacitor bank installed on a distribution feeder primarily improves which system parameter?
- Short-circuit current capacity
- Power factor and reduces reactive power demand (Correct answer)
- Transformer tap voltage
- Conductor ampacity rating
Correct answer: Power factor and reduces reactive power demand
Shunt capacitor banks supply reactive power locally, improving power factor, reducing current draw, and decreasing line losses.
Question 5: What does the term 'feeder' refer to in power distribution?
- The high-voltage transmission line entering a substation
- The medium-voltage circuit leaving a substation to serve distribution loads (Correct answer)
- The service entrance cable at a customer's meter
- The busbar connecting multiple transformers in a substation
Correct answer: The medium-voltage circuit leaving a substation to serve distribution loads
A distribution feeder is the medium-voltage (typically 4–35 kV) circuit that originates at a substation and carries power to distribution transformers serving customers.
Question 6: Voltage unbalance in a three-phase distribution system primarily causes which problem in three-phase motors?
- Reduced insulation resistance in transformers
- Overheating and reduced motor efficiency (Correct answer)
- Increased capacitor bank losses
- Lower fault current magnitude
Correct answer: Overheating and reduced motor efficiency
Voltage unbalance creates negative-sequence currents in motors that produce opposing torque and significant heat, reducing efficiency and shortening motor life.
Question 7: What is the function of a sectionalizer in a distribution system?
- To limit fault current magnitude
- To count fault current operations and open after the recloser locks out (Correct answer)
- To regulate voltage along the feeder
- To provide overcurrent protection independent of upstream devices
Correct answer: To count fault current operations and open after the recloser locks out
A sectionalizer counts the number of fault interruptions by an upstream recloser and opens during a dead interval after a preset count, isolating the faulted section.
EETC Power Distribution 2
What is the primary purpose of a distribution transformer's tap changer?