Line P-3

Question 1

What is the standard phase displacement for wye-to-wye and delta-to-delta transformerconnections?

Answer 1

0°

Question 2

What is the standard phase displacement for wye-to-delta and delta-to-wye transformer connections?

Answer 2

30°

Question 3

In voltage diagrams for three-phase transformers, in what position is the high-side voltage V(AN) always drawn?

Answer 3

30° to the horizontal

Question 4

Why is the low-side phasor for any transformer always drawn parallel to the high-side phasor for the same transformer?

Answer 4

Since the same changing flux produces both primary and secondary induced voltages, the two voltages must be in phase

Question 5

Where is the A-phase transformer normally located in a transformer bank when viewed from the high side?

Answer 5

On the right-hand side

Question 6

Can an additive-polarity transformer be used in a bank with two subtractive-polarity transformers to make up a three-phase bank?

Answer 6

Yes

Question 7

Can a subtractive-polarity transformer be used in a bank with two additive-polarity transformers to make up a three-phase bank?

Answer 7

Yes

Question 8

Before connecting the load, what test should you perform on the secondary of a three-phase transformer bank with a wye-connected secondary?

Answer 8

A voltage test to determine that all line-to-line voltages are equal

Question 9

If a voltmeter test on the secondary of a wye-to-wye connected transformer shows V(AB) = 208 V, V(BC) = 120 V and V(CA) = 120 V, what is indicated?

Answer 9

The C-phase transformer is connected backward on either the primary or the secondary

Question 10

For a step-down application, would the primary lines connect to H1, H2 and H3 or to X1, X2 and X3?

Answer 10

H1, H2 and H3

Question 11

On voltage diagrams for three-phase delta transformers, in what position is the high-side voltage phasor V(AB) always drawn?

Answer 11

60° to the horizontal

Question 12

What test must be performed on the secondary of a three-phase transformer bank with a delta-connected secondary?

Answer 12

A mesh or delta closure test using a voltmeter

Question 13

For a delta-to-delta, step-up application, do the primary lines connect to H1, H2 and H3, or to X1, X2 and X3?

Answer 13

X1, X2 and X3

Question 14

If you use the same voltage diagram for a wye-to-delta, step-up application as for a wye-to-delta, step-down application, are you still conforming to the CSA standards for three-phase transformers?

Answer 14

No. You will get the proper phase sequence and a 30° shift, but you will not have the high side leading the low side on one of the applications

Question 15

What is the relationship between the primary line voltage and the primary transformer-winding voltage in a delta-to-wye, step-down transformer bank?

Answer 15

They are equal to each other

Question 16

What is the relationship between the secondary line voltage and the secondary transformer-winding voltage in delta-to-wye, step-down transformer bank?

Answer 16

The line voltage is equal to root 3 times the phase voltage

Question 17

A three-phase, four-wire, delta-connected system is to be used for supplying both three-phase motor loads and single-phase lighting loads. Where do you normally attach the ground?

Answer 17

To the centre tap of the B-phase transformer

Question 18

Which phase does the CEC specify as the high leg?

Answer 18

A phase

Question 19

Does the CEC permit this high leg to be in the compartment of a panelboard used to supply single-phase loads?

Answer 19

No. The CEC wants to make sure the high leg is not accidentally used instead of one of the other legs.

Question 20

T/F
The transformer with the centre tap supplies all of the current to the single-phase load. The other two transformers supply only their share of the balanced three-phase load.

Answer 20

False

Question 21

List three applications where an open-delta transformer connection is commonly used

Answer 21

The temporary supply of three-phase power when one transformer in a three-phase bank fails and is being repaired or replaced

The initial supply of a system that is expected to expand in the future

The supply of loads where the majority of the load is single-phase and only a small amount of three-phase power is required

Question 22

State the two major disadvantages the autotransformer has compared with the standard isolated-winding transformer

Answer 22

There is no electrical isolation between the primary and secondary circuits

The low percent impedance of autotransformers results in much higher available faultcurrents than for transformers with isolated windings

Question 23

Which type of connection creates a phase displacement between the primary and secondary voltages?

Answer 23

The closed-delta connection

Question 24

What is the purpose of the tertiary winding included in some wye-connected autotransformer banks?

Answer 24

It permits the flow of third-harmonic currents. This eliminates the third-harmonic voltages in the wye-connected windings

Question 25

What is the maximum transformer voltage ratio available from a standard delta-connected autotransformer?

Answer 25

2:1 or 1:2, depending upon whether the transformer is step down or step up

Question 26

What advantage does the open-delta have over the wye-connected compensator type of motor starter?

Answer 26

The open-delta connected starter requires the changing of only two tap connections during transition. The wye-connected starter requires the changing of three tap connections

Question 27

What advantage does the extended-delta autotransformer have over the standard-delta autotransformer?

Answer 27

It permits transformations in ratios higher than 2:1

Question 28

State four advantages the autotransformer has over the standard isolated-winding transformer

Answer 28

Costs less to manufacture

Much smaller physical size and weight

Higher efficiency

Better voltage regulation

Question 29

Why are autotransformers not normally used to step down voltages in ratios of more than 2:1 or 3:1?

Answer 29

If an open occurs in the common part of the winding, there is a risk of the high primary voltage being impressed across the secondary lines

Question 30

Currents are caused by third-harmonic voltages induced into systems by switching power supplies. Which autotransformer connection is commonly used to filter them out?

Answer 30

The zig-zag connection

Question 31

Which autotransformer connection would likely be used to achieve a desired phase displacement between the primary and secondary systems?

Answer 31

The closed-delta connection

Question 32

An ammeter connected to the secondary circuit of a current transformer is damaged and needs replacing. What must be done to the secondary terminals of the transformer before the ammeter is removed from the circuit?

Answer 32

The secondary winding must be short-circuited to prevent dangerously high voltages from being induced in the secondary circuit due to the open

Question 33

How do you determine the wattmeter multiplier when fed from a PT and a CT?

Answer 33

wattmeter multiplier = CT ratio × PT ratio

Question 34

In an instrument transformer circuit using the two-wattmeter method for power measurement, what should you do if one of the wattmeters reads down-scale?

Answer 34

Reverse the potential transformer leads only to the wattmeter so that it will read up-scale.

Question 35

Why are CTs sometimes used in the line conductors to large three-phase motors?

Answer 35

To supply the overload relay (heaters) protection so that smaller size O/L relays can be used.

Question 36

What special type of CT is sometimes used for ground-fault detection?

Answer 36

Window or donut-type CTs