Transformers: Chapter 6 - Possible Test Questions - WEEK 13

Question 1

WYE - WYE BANK:

Answer 1

• With a Wye - Wye bank, each transformer is connected to the system between a phase and grounded neutral.
• In addition, each of the three transformers provide one hot leg on the secondary side to deliver three- phase power. For this reason, three-phase power can only be supplied with three transformers.

Question 2

An open bank would not provide three-phase power if the secondary were connected:

Answer 2

• Open WYE

Question 3

WYE - WYE BANK (SECONDARY)

Answer 3

• The secondary coils are paralleled, leaving the x3 lug idle. It is strongly recommended that paralleling be done between the xl and x2 rather than the x1 and x3. The x2 bushing is considered the neutral position and should be treated as such.

Question 4

Wye connected banks have two main advantages:

Answer 4

• 1. All three transformers can share the single-phase load equally.
• 2. They can be built using single or dual primary bushing transformers.

Question 5

The disadvantage for a WYE-WYE Bank:

Answer 5

• is that if one transformer fails, the customer is down to one phase until the defective transformer is fixed or replaced.

Question 6

WYE - DELTA BANK:

Answer 6

• With a Wye - Delta bank, each transformer is connected to the system and the Wye point is floated (not grounded). Because the Wye point is floated, transformers with two primary bushings must be used.

Question 7

WYE - DELTA BANK

- With one unit out of service, a bank of single-phase units can be reconnected as an:

Answer 7

• open Wye - open Delta, which may temporarily supply power to the customer until permanent repairs are made.

Question 8

WYE - DELTA BANK

- Grounding the Wye point would create a:

Answer 8

• ground source for the primary system. This could subject the transformer to severe overloading during a primary system disturbance or load imbalance. Because the Wye point is floated, this connection can be used on both three- and four-wire systems.

Question 9

WYE - DELTA BANK

- With one unit out of service, a bank of single-phase units can:

Answer 9

• be reconnected as an open Wye - open Delta, which may temporarily supply power to the customer until permanent repairs are made.

Question 10

WYE - DELTA BANK

- How are he secondary coils are connected:

Answer 10

• in series.

Question 11

WYE - DELTA BANK

- The voltage between the neutral and the power leg (designated as ‘p’ above) is:

Answer 11

• 1.73 times greater than the neutral to hot leg voltage.

Question 12

WYE - DELTA BANK

- The power leg has no use electrically and is simply a by-product of a:

Answer 12

• Delta secondary. Three-phase motors only see the h - h voltage and do not “notice” the power leg. It is very important to avoid confusing the power leg with hot legs when connecting to 120-volt lighting loads.

Question 13

WYE - DELTA BANK

- The floating Wye point can be a:

Answer 13

• hazard to lineworkers because it is not grounded.

Question 14

WYE - DELTA BANK

A power leg only exists on:

Answer 14

Delta and open Delta secondaries when a ground is attached to the winding midpoint of one of the transformers.

Question 15

WYE - DELTA BANK

Delta secondaries do not allow all three transformers to:

Answer 15

share the single- phase load equally; therefore, one transformer of the bank must provide the majority of the lighting load. It will always be the transformer with the x2 grounded serving as a neutral. Because this transformer must provide the majority of the single-phase load, it must be sized larger than the other two transformers in the bank, and is referred to as the lighter transformer because it handles the “lighting” load. The other transformers share somewhat equal portions of single-phase and three-phase load and are referred to as power transformers.

Question 16

DELTA - WYE BANKS

-

Answer 16

• With a Delta - Wye bank, the H2s are connected to the H1s, creating a symmetrical pattern, and then connected to each phase of the system. The primary Delta connection is suitable for both three- and four-wire systems.

Question 17

DELTA - WYE BANKS

- The secondary Wye connection of a Delta - Wye is the same as:

Answer 17

with the Wye - Wye bank and as shown previously.

Question 18

DELTA - DELTA BANKS

-

Answer 18

With a Delta - Delta bank, the primary connection is suitable for either a three- or four-wire system. With one unit out of service, a bank of single-phase units can be reconnected as an open Delta, open Delta bank. With one of three identical units out of service, the rating of the bank when supplying only three-phase load is about 57.7% of the bank rating when all three units are in service.

Question 19

DELTA - DELTA BANKS

- As mentioned before, Delta secondaries do not allow:

Answer 19

all three transformers to share the single-phase load equally; therefore, one transformer of the bank must provide the lighting load. It will always be the transformer with the x2 grounded serving as a neutral. Because this transformer must provide the majority of the single-phase load, it must be sized larger than the other two transformers in the bank and is referred to as the lighter transformer.

Question 20

When supplying three-phase power from underground systems, the transformers used are:

Answer 20

• three-phase transformers. The coils are banked internally and the leads are brought up to the primary and secondary bushings.

Question 21

When pad-mounted or subsurface transformers are used,:

Answer 21

• the system voltage and the service voltage needed can be found on the nameplate of the unit.

Question 22

THREE-PHASE UNDERGROUND TRANSFORMERS

- The lineworker must rely entirely on

Answer 22

the nameplate to determine which primary and secondary connections were made at the factory.

Question 23

Although banking transformers in an underground system is rare, some utilities install

Answer 23

open Wye - open Delta banks using two single-phasepad-mounted transformers. The banking is performed with the secondary cables through short ducts.

Question 24

NON-STANDARD CONNECTIONS

- Three-Phase 120/208 from V-Phase Primary

Answer 24

A three-phase 120/208-volt service can be supplied even if only two phases of a Wye primary (V-phase) are available. One transformer in the bank should not be less than twice the size of either one of the other two transformers, and its secondary coils must be connected in series.

Question 25

NON-STANDARD CONNECTIONS

- 120/240 Single-Phase and 120/208 Three-Phase:

Answer 25

The primary connection can be either Wye or Delta depending upon the system voltage and the primary coil voltage ratings. One transformer in the bank should not be less than twice the size of either one of the other two transformers, and its secondary coils must be connected in series.

Question 26

NON-STANDARD CONNECTIONS

-Corner Ground:

Answer 26

Some customers need only three-phase three-wire services. Examples of this may be a customer with an irrigation pump remotely located. There may be no need for lights at the pump location. However, the NESC still requires a ground reference between the pump and the transformer bank. To satisfy this requirement without installing a neutral conductor, some utilities install corner grounded banks with the most common being the Delta corner ground.

Question 27

NON-STANDARD CONNECTION:

- Corner Ground II:

Answer 27

Nearly every lineworker who is first exposed to this connection is surprised that it can work. The best explanation is that a Delta or Wye can be grounded one time at any coil terminal and still operate properly. Grounded Wye connections are grounded one time at the H2 or x2 terminals. Delta secondaries are normally grounded at the midpoint terminal of one transformer. With a corner ground, the ground is installed on one of the corners.