Transformers: Chapter 4 (Nameplates) & 5 (Winding Designations) - Possible Test Questions - WEEK 12

Question 1

The nameplate provides:

Answer 1

• technical information used to safely store, transport, install, maintain, and inventory the unit. It provides information that the utility can track for the life of the transformer.

Question 2

When To Use The Nameplate:

Answer 2

• Prior to installation, linemen always refer to the nameplate to:
• Ensure that the desired primary voltage is chosen
• Ensure that the desired secondary voltage is chosen
• Ensure that the impedances are within +/- 10% if banking or paralleling
• Determine the weight of the transformer for rigging purposes

Question 3

Nameplate Location & Information:

Answer 3

• The nameplate can usually be found on or near the lower mounting bracket on overhead transformers and inside the door or on the face of underground transformers.

Question 4

Voltage Ratings:

Answer 4

• The input (primary) and output (secondary) voltage ratings are clearly printed. The primary coil rating is always the smaller of the two primary voltages listed.

Question 5

Kilovolt-Amperes (kVA):

Answer 5

• This is the wattage rating. The average home uses 6000 watts. Therefore, a 10 kVA (10,000 watt) transformer would be used.

Question 6

Serial Number:

Answer 6

• This number is unique to every transformer in a company’s system and is used for inventory and tracking purposes.

Question 7

Polarity:

Answer 7

• Polarity is the direction of current flow. Additive transformers will have the x1 on the right (facing the transformer) and subtractive transformers will have the x1 on the left.

Question 8

Impedance:

Answer 8

• Total opposition to current flow caused by resistance and inductive reactance. If a transformer is 98% efficient, the impedance is 2%.

Question 9

Schematic:

Answer 9

• Shows the primary and secondary coils and their relationship.

Question 10

Engineers can choose from three different primary voltage designations when purchasing transformers. The selection is based on the particular application of the transformer on that utility company’s system. The engineer can purchase a transformer that is designed for:

Answer 10

1. Phase-to-phase connections only
2. Phase-to-phase or phase-to-ground connections
3. Phase-to-ground connections only

Question 11

If a transformer has only one number for a primary voltage rating, it should be:

Answer 11

• connected phase-to-phase.

Question 12

Phase-to-Phase Connections Only:

Answer 12

• These transformers will have a primary voltage rating that depicts only one number. This number serves as both the primary voltage rating and the system voltage. Transformers with this designation have two primary bushings and should be connected between two primary phases that will provide the rated voltage. Although they can be connected between a phase and ground, they are not designed to have the rated voltage applied to one bushing and zero volts applied to the other. Doing this will create radio interference and corona problems.

Question 13

Phase-to Phase or Phase-to-Ground Connections:

Answer 13

These transformers will have a primary voltage rating that depicts two numbers separated by a slash (/). The first number is the primary coil rating (the smaller number), and the second number is the system voltage. Transformers with this designation have two primary bushings and can be connected between two phases or between a phase and ground that will provide the rated voltage. These are clearly the most versatile transformers because they can be used for both types of connections.

Question 14

Phase-to-Ground Connections Only:

Answer 14

• These transformers also have a primary voltage rating that depicts two numbers separated by a slash (). The first number is the system voltage, and the second number is the primary coil rating (the smaller number). Transformers with this designation have one primary bushing and can only be connected between a phase and ground that will provide the rated voltage. The insulation of the primary coil of these transformers is reduced at the neutral end.

Question 15

When the primary coil rating is printed after the system voltage, the transformer will have:

Answer 15

• one bushing and it must be connected phase-to-ground.

Question 16

The dual-voltage switch should only be operated when:

Answer 16

• the transformer is de-energized

Question 17

Remember that the taps on distribution transformers must:

Answer 17

• be adjusted with the transformer de-energized.

Question 18

Depending on the manufacturer, each tap raises or lowers the:

Answer 18

• secondary voltage by 2.5% or 4.5%

Question 19

The tap changer is connected to:

Answer 19

• the primary coil. If a tap is raised, the number of turns of the primary coil is reduced, which maintains the output voltage as the primary voltage drops.

Question 20

Continuity Testing:

Answer 20

• Continuity testing is easily achieved with a multi-tester. The multi-tester will give off an audible sound when the two probes contact one another, either directly or through a conductive object. This feature is especially useful when proving the integrity of transformer coils. Knowing that the primary coil should be connected between the H1 and the H2, an audible sound will be heard when the probes touch the bushings and the multi-meter is set to continuity. An audible sound will also be heard between all combinations of the secondary bushings. An audible sound should not be heard between the primary and secondary windings of a two-primary-bushing transformer.

Question 21

Hastings manufactures a transformer tester called the “TLC Meter.”:

Answer 21

• TLC stands for transformer last check. The instrument can be used to verify that distribution transformers do not have shorted or open windings prior to energizing. The hand-held unit is easy to use. It has two leads that can be connected in the desired combination between the bushings. A button is then pressed and one of three lights signal the condition of the transformer, which can be either pass, open, or shorted.

Question 22

Insulation Testing:

Answer 22

• The insulation tester is sometimes referred to as a “Megger” and is used in troubleshooting and preventative maintenance programs to test the insulation in electrical apparatus, cables, wire, or networks. This device can be used to check for transformer coil insulation damage (and coil continuity). All connections, including the ground on the X2 terminal, must be removed for this test. When testing the insulation, the readings between any primary and secondary bushings should read infinity, and the reading between any bushing and the transformer case should read infinity. This would indicate that the insulation is not damaged.

Question 23

Turns-Ratio Testing:

Answer 23

• Transformer turns-ratio testers measure exactly what their name indicates- the turns ratio of transformers. A standard turns-ratio test will also give a good indication that the transformer is operational and that there are not shorts between the turns in the windings. A turns-ratio test will not show a weakness in the insulation.
• The test is carried out by energizing the high-voltage coil with a low voltage, such as 120 volts. The device can then compare the voltage output voltage with the known input voltage. The ratio on the nameplate and the measured ratio should be equal. The turns ratio is calculated by dividing the input voltage by the output voltage.
  When doing this test, remember that transformers work both ways. For this test, the low-input voltage must be connected to the high-voltage winding. If the input voltage is applied to the low-voltage winding, a high voltage will be present on the primary bushings.

Question 24

For Turns-Ratio test, the low-input voltage must be:

Answer 24

• connected to the high voltage winding.