On a delta-wound transformer with a 120 VAC line-to-line secondary, what is the voltmeter reading from line to ground?

On a delta-wound transformer with a 120 VAC line-to-line secondary, what is the voltmeter reading from line to ground?

• A. Approximately 67 volts for a normal balanced system with no faults
• B. 0 VAC for a system with that phase faulted to ground
• C. 120 VAC when another phase is faulted to ground
• D. Any of the above

Answer: (D)

In a delta-wound transformer system, the secondary winding is configured such that the three phases are interconnected in a loop. When measuring the voltage from line to ground, the reading can vary depending on the system's balance and whether there are any faults present. When the system is normal and balanced, the voltage from line to ground for each phase is approximately 67 volts. This is derived from the line-to-line voltage divided by the square root of three \( (120 \text{ VAC} / \sqrt{3} \approx 67 \text{ VAC}) \). Thus, a normal condition results in a reading of about 67 volts from line to ground. If a phase becomes faulted to ground, the voltage to ground would effectively be 0 volts for that specific phase because the ground reference has taken over. This is crucial in understanding how fault conditions can impact voltage readings. On the other hand, if a different phase is faulted to ground while another remains normal, the voltmeter could read up to the full line voltage, which in this scenario is 120 VAC, because the ungrounded phase would maintain its voltage relative to the ground. Therefore, the range of possible voltmeter readings reflects various operating conditions: approximately 67

In a delta-wound transformer system, the secondary winding is configured such that the three phases are interconnected in a loop. When measuring the voltage from line to ground, the reading can vary depending on the system's balance and whether there are any faults present.

When the system is normal and balanced, the voltage from line to ground for each phase is approximately 67 volts. This is derived from the line-to-line voltage divided by the square root of three ( (120 \text{ VAC} / \sqrt{3} \approx 67 \text{ VAC}) ). Thus, a normal condition results in a reading of about 67 volts from line to ground.

If a phase becomes faulted to ground, the voltage to ground would effectively be 0 volts for that specific phase because the ground reference has taken over. This is crucial in understanding how fault conditions can impact voltage readings.

On the other hand, if a different phase is faulted to ground while another remains normal, the voltmeter could read up to the full line voltage, which in this scenario is 120 VAC, because the ungrounded phase would maintain its voltage relative to the ground.

Therefore, the range of possible voltmeter readings reflects various operating conditions: approximately 67