Line P-3 Flashcards
What is the standard phase displacement for wye-to-wye and delta-to-delta transformerconnections?
0°
What is the standard phase displacement for wye-to-delta and delta-to-wye transformer connections?
30°
In voltage diagrams for three-phase transformers, in what position is the high-side voltage V(AN) always drawn?
30° to the horizontal
Why is the low-side phasor for any transformer always drawn parallel to the high-side phasor for the same transformer?
Since the same changing flux produces both primary and secondary induced voltages, the two voltages must be in phase
Where is the A-phase transformer normally located in a transformer bank when viewed from the high side?
On the right-hand side
Can an additive-polarity transformer be used in a bank with two subtractive-polarity transformers to make up a three-phase bank?
Yes
Can a subtractive-polarity transformer be used in a bank with two additive-polarity transformers to make up a three-phase bank?
Yes
Before connecting the load, what test should you perform on the secondary of a three-phase transformer bank with a wye-connected secondary?
A voltage test to determine that all line-to-line voltages are equal
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?
The C-phase transformer is connected backward on either the primary or the secondary
For a step-down application, would the primary lines connect to H1, H2 and H3 or to X1, X2 and X3?
H1, H2 and H3
On voltage diagrams for three-phase delta transformers, in what position is the high-side voltage phasor V(AB) always drawn?
60° to the horizontal
What test must be performed on the secondary of a three-phase transformer bank with a delta-connected secondary?
A mesh or delta closure test using a voltmeter
For a delta-to-delta, step-up application, do the primary lines connect to H1, H2 and H3, or to X1, X2 and X3?
X1, X2 and X3
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?
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
What is the relationship between the primary line voltage and the primary transformer-winding voltage in a delta-to-wye, step-down transformer bank?
They are equal to each other
What is the relationship between the secondary line voltage and the secondary transformer-winding voltage in delta-to-wye, step-down transformer bank?
The line voltage is equal to root 3 times the phase voltage
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?
To the centre tap of the B-phase transformer
Which phase does the CEC specify as the high leg?
A phase
Does the CEC permit this high leg to be in the compartment of a panelboard used to supply single-phase loads?
No. The CEC wants to make sure the high leg is not accidentally used instead of one of the other legs.
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.
False
List three applications where an open-delta transformer connection is commonly used
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
State the two major disadvantages the autotransformer has compared with the standard isolated-winding transformer
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
Which type of connection creates a phase displacement between the primary and secondary voltages?
The closed-delta connection
What is the purpose of the tertiary winding included in some wye-connected autotransformer banks?
It permits the flow of third-harmonic currents. This eliminates the third-harmonic voltages in the wye-connected windings
What is the maximum transformer voltage ratio available from a standard delta-connected autotransformer?
2:1 or 1:2, depending upon whether the transformer is step down or step up
What advantage does the open-delta have over the wye-connected compensator type of motor starter?
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
What advantage does the extended-delta autotransformer have over the standard-delta autotransformer?
It permits transformations in ratios higher than 2:1
State four advantages the autotransformer has over the standard isolated-winding transformer
Costs less to manufacture
Much smaller physical size and weight
Higher efficiency
Better voltage regulation
Why are autotransformers not normally used to step down voltages in ratios of more than 2:1 or 3:1?
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
Currents are caused by third-harmonic voltages induced into systems by switching power supplies. Which autotransformer connection is commonly used to filter them out?
The zig-zag connection
Which autotransformer connection would likely be used to achieve a desired phase displacement between the primary and secondary systems?
The closed-delta connection
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?
The secondary winding must be short-circuited to prevent dangerously high voltages from being induced in the secondary circuit due to the open
How do you determine the wattmeter multiplier when fed from a PT and a CT?
wattmeter multiplier = CT ratio × PT ratio
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?
Reverse the potential transformer leads only to the wattmeter so that it will read up-scale.
Why are CTs sometimes used in the line conductors to large three-phase motors?
To supply the overload relay (heaters) protection so that smaller size O/L relays can be used.
What special type of CT is sometimes used for ground-fault detection?
Window or donut-type CTs
