Chapter 6 Week 4

Question 1

What are the four different combinations when hooking up a three pot bank?

Answer 1

1. ) WYE-WYE
2. ) WYE-DELTA
3. ) DELTA-DELTA
4. ) DELTA-WYE

Question 2

How is the primary connection determined?

Answer 2

By comparing the primary coil voltage rating of the transformers, as printed on the nameplate, with the system voltage

Question 3

How is the secondary connection determined?

Answer 3

By the service voltage of the customer

Question 4

What are the WYE secondary voltages?

Answer 4

• 120/208
• 277/480
• 240/416
• 347/600

Question 5

If the two secondary coils are connected together in series inside the tank, what will the secondary voltages be?

Answer 5

120/240

Question 6

If the two secondary coils are connected together in parallel inside the tank, what will the secondary voltages be?

Answer 6

120

Question 7

How do you check the configuration of the secondary coils?

Answer 7

Either a turns-ratio test or a physical check by taking the lid off the transformer

Question 8

What happens if the secondary coils are not paralleled on WYE secondary connections?

Answer 8

The proper voltage would still be delivered, but the kVA rating of the bank would be cut in half

Question 9

How do you connect the secondary connections on a WYE Hook-up?

Answer 9

Alley Cat Bad Dog

Question 10

What is industry standard about the X2 bushing?

Answer 10

It serves as the neutral

Question 11

How is a 3 pot bank connected on a WYE-WYE bank?

Answer 11

Each transformer is connected to the system between a phase and grounded neutral

Question 12

What kind of legs do transformers connected WYE-WYE on the secondary side have?

Answer 12

One Hot Leg on each transformer to deliver three-phase power

Question 13

What is the X3 Lug on a WYE-WYE connection?

Answer 13

Idle

Question 14

What are the two main advantages to WYE connected banks?

Answer 14

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

Question 15

What is the disadvantage to WYE connected banks?

Answer 15

If one transformer fails, the customer is down to one phase until the defective transformer is fixed or replaced

Question 16

How is a WYE-DELTA bank connected?

Answer 16

Each transformer is connected to the system and the WYE point is floated not grounded.

Question 17

What would happen if you grounded the WYE point in a WYE-DELTA?

Answer 17

Create a ground source for the primary system, which could subject the transformer to severe overloading during a primary system disturbance or load imbalance

Question 18

With the WYE being floated what does it allow this connection to do?

Answer 18

Can be used on both three- and four-wire systems

Question 19

How are the secondary coils connected in a WYE-DELTA bank?

Answer 19

In Series

Question 20

What is the voltage between the neutral and the power leg?

Answer 20

The power leg is 1.73x higher than the neutral

Question 21

What are some different terms for hot legs?

Answer 21

• High Leg
• Wild Leg
• Stinger Leg

Question 22

What do three-phase motors see?

Answer 22

Only the h-h voltage and does not “notice” the power leg

Question 23

How are the connections in a DELTA-WYE connection?

Answer 23

The H2’s are connected to the H1’s creating a symmetrical pattern, and then connected to each phase of the system

Question 24

What is the rating of a DELTA bank if one of the single phase transformers is out of service when supplying three-phase load only?

Answer 24

57.7%

Question 25

How do DELTA secondaries share single-phase load?

Answer 25

One transformer is the “lighting” while the other two are “power” transformer

Question 26

How is three-phase power supplied on underground systems?

Answer 26

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

Question 27

What must the lineworker rely on when working on three-phase underground transformers?

Answer 27

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

Question 28

How is a WYE - Open DELTA Bank installed?

Answer 28

The banking is performed with the secondary cables through short ducts

Question 29

How is a Three-Phase 120/208 installed from a V-Phase Primary?

Answer 29

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 30

How does a corner ground work?

Answer 30

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.