Question 1

When a transformer bank must supply both three-phase and single-phase loads, the utility company will often provide an open-delta connection with one transformer ____.

Accepted Answer

An open-delta connection with one transformer center-tapped is commonly used in situations where both three-phase and single-phase loads need to be supplied. The center tap on the transformer provides the neutral connection for the single-phase loads, while the open-delta connection provides the three-phase power. This setup is cost-effective, but it does have some limitations, such as reduced capacity and increased risk of power disruption if one of the transformers fails.

Question 2

High-leg voltage can be computed by increasing the single-phase center-tapped voltage value by ____.

Accepted Answer

High-leg voltage is the voltage between the high-leg and neutral in a three-phase four-wire system where one leg is center-tapped and grounded. The high-leg voltage is equal to the single-phase center-tapped voltage value multiplied by the square root of 3 (1.732). Therefore, to compute the high-leg voltage, we need to increase the single-phase center-tapped voltage value by 1.732.

Question 3

Connecting the center tap of a primary to the center tap of a secondary will solve an unbalance problem if a wye primary is used on a three-wire system.

Accepted Answer

Connecting the center tap of a primary to the center tap of a secondary in a transformer setup does not inherently solve an unbalance problem in a three-wire system. Unbalance issues are typically addressed by load management or phase balancing techniques, not by transformer center tap connections.

Question 4

The type of three-phase transformer connection to use is generally determined by the ____.

Accepted Answer

The type of three-phase transformer connection is generally determined by the amount of power needed to ensure the transformer can handle the required load efficiently.

Question 5

Typical voltages for a three-phase transformer connection with a delta-connected primary and wye-connected secondary are 208/120 and ____.

Accepted Answer

The typical voltages for a three-phase transformer connection with a delta-connected primary and wye-connected secondary are 208/120 volts on the secondary side and 480/277 volts on the primary side. Therefore, the correct answer is D, with the secondary voltage of 208/120 volts and the primary voltage of 480/277 volts.

Question 6

In a given three-phase transformer connection with a delta-connected primary and wye-connected secondary, the secondary has been center-tapped to form a neutral conductor. The neutral conductor will ____.

Accepted Answer

In a wye-connected system with a center-tapped neutral, the neutral conductor carries the vector sum of the unbalanced currents. If the system is perfectly balanced, there would be no current in the neutral. However, in the presence of unbalance, the neutral provides a return path for the unbalanced current, ensuring the system can operate properly.

Question 7

Three-phase transformer connections are often used to supply single-phase loads, which tend to unbalance the system.

Accepted Answer

Three-phase transformer connections can supply single-phase loads, but the loads can cause an imbalance in the system. Special precautions must be taken to ensure balanced voltage and current in the three-phase system.

Question 8

In a delta connection where one transformer is center-tapped to form a neutral conductor, the two lines are 1800 out of phase when compared with the center tap. In the wye connection, the lines will be ____ out of phase.

Accepted Answer

In a wye (or star) connection, each of the three phases is 120 degrees out of phase with the others, which is why the correct answer is 120 degrees.

Question 9

In a three-phase transformer connection with a delta-connected primary and wye-connected secondary, the neutral conductor will not carry the vector sum of the unbalanced current.

Accepted Answer

In a wye-connected secondary, the neutral conductor will carry the vector sum of the unbalanced current.

Question 10

If incoming power is a three-phase four-wire system, a wye-connected primary can be used without problems.

Accepted Answer

A wye-connected primary is compatible with a three-phase four-wire system.

Question 11

The amount of current flow in a neutral conductor will be the sum of the unbalanced load between lines L2 and L3, with the addition of the current flow in the high leg, L1. To determine the amount of neutral current, use the formula ____.

Accepted Answer

The formula for determining the amount of neutral current is AN = A1 + (A2 - A3). This takes into account the unbalanced load between lines L2 and L3, as well as the current flow in the high leg, L1. Option A is incorrect because it multiplies A2 and A3 instead of subtracting them. Option B is not the correct formula for determining neutral current. Option D only multiplies A1 by the sum of A2 and A3, which does not take into account the unbalanced load between L2 and L3.

Question 12

In a three-phase transformer connection with a delta-connected primary and wye-connected secondary, a wye-connected secondary with center tap can, under the right conditions, experience extreme unbalance problems.

Accepted Answer

This is because if the loads on the wye-connected secondary are not evenly balanced, then a large unbalance can occur due to the center tap. This can cause excessive currents to flow in the neutral conductor, potentially leading to overheating and damage to the system.

Question 13

In a given transformer bank, if the three loads have been connected directly across the three-phase lines, they form a delta-connected load.

Accepted Answer

When three loads are connected directly across the three-phase lines, they form a delta-connected load.

Question 14

In a given circuit, the three-phase load has a power requirement of 75 kVA, and the single-phase load requires an additional 50 kVA. Three 25-kVA transformers could be used to supply the three-phase power needed. The addition of the single-phase load, however, requires ____.

Accepted Answer

The answer of In a given circuit, the three-phase load...

Question 15

It is possible to use a high leg to supply power to a single-phase load.

Accepted Answer

It is possible to use a high leg (also known as a wild leg or stinger leg) in a three-phase system to supply voltage to a single-phase load. The high leg is typically 208V to ground, while the other two legs are 120V to ground, allowing for a 208V supply for single-phase loads that require it. However, it should be noted that proper precautions must be taken to ensure that the high leg is properly identified and isolated to prevent accidental connection to devices that cannot handle the higher voltage.

Question 16

Assume line L1 has a current flow of 100 amps, line L2 has a current flow of 75 amps, and line L3 has a current flow of 50 amps. The amount of current flow in the neutral conductor would be ____ amps.

Accepted Answer

The answer of Assume line L1 has a current flow...

Question 17

In many circuits, it is possible for the neutral conductor to carry more current than any of the three-phase lines.

Accepted Answer

This can happen in unbalanced three-phase loads, where one or two phases may draw significantly more current than the others, causing the neutral conductor to carry a higher current as a result of the unbalanced load.

Question 18

When true three-phase loads are connected to a three-phase transformer bank, there are problems in balancing the currents and voltages of the individual phases.

Accepted Answer

When three-phase loads are connected to a three-phase transformer bank, the currents and voltages of the individual phases are automatically balanced due to the nature of the three-phase system. Therefore, there are no problems in balancing the currents and voltages.

Question 19

In a given transformer bank, the three loads form a delta-connected load. If these three loads are intended to be used as single-phase loads, they will in all likelihood have ____.

Accepted Answer

The answer of In a given transformer bank, the three...

Question 20

When a line is known as a high leg, it is because the voltage between this line and the neutral conductor will be higher than the voltage between the neutral and either of the other two conductors.

Accepted Answer

This is correct. A high leg, also known as a "wild leg" or "stinger leg" in some systems, typically refers to a three-phase power system where the phase-to-neutral voltage on one of the phases is significantly higher (by about 208 volts in a 120/240V system) than the other two phases. This high voltage leg can create safety hazards if not properly identified and isolated.

Quiz 9: Single-Phase Loads for Three-Phase Transformers