Transformer Theory Flashcards
How are primary and secondary coils of a transformer connected?
Magnetic coupled
What are the two main enemies of transformers?
Heat and over voltage
What are the three ratios in transformers? And which two are the same?
Current, Turns, and Voltage. Turns and Voltage are the same.
What coil is a tap changer located in?
Primary coil
When installing a new transformer with a tap changer, what tap do you install it on?
100%
How many cutouts, and arrestors are needed for a open delta, open delt transformer bank?
3 cutouts / 3 LA’s
Why do you need to tie your primary, and secondary neutrals together on a WYE, WYE transformer bank?
Excess voltage
What is a transformer?
Converts high voltage to low voltage
How do you protect a transformer from overcurrent, and overvoltage?
Lighting Arrester
What are the 2 main losses in transformers?
current, voltage
What connections need to be made to parallel coils in a transformer?
ACBD, Open can and make your connection
If you need to climb above a de-energized transformer, what needs to be done to the secondaries?
Ground secondaries
What is backfeed?
NEED TO FIND RIGHT ANSWER
What is the purpose of a case ground?
Reference to ground, make the can not energized
What is a C.S.P. transformer?
Completely self protect transformer
What are the components that make it C.S.P.?
LA, Secondary breaker, Fuse link
What are the 2 reasons tap changers are used?
To correct primary voltage AND to control voltage to a customer
What is impedance?
Sum of opposition found in some AC current
What is impedance made up of?
Induction reactance, capacitive reactance, and resistance
When banking or paralleling transformers, how close must the impedance be?
Nearly equal
What baking connections can be made with 2 or 3 transformers?
OPEN - delta delta, open wide delta, CLOSED BANKS - delta delta, wye wye, delta wye, wye delta
When testing a new transformer, what must be worn?
Primary gloves
On a loop fed padmount transformer, how are the primary bushings connected?
Buss bar
How many primary phases are needed for a open delta, open delta transformer bank?
3 phases
What types of transformers are needed for primary metering?
Current transformer (CT), Potential Transformer (PT), AND Voltage Transformer (VT)
The copper loss in transformers is cause by _____.
a. Eddy currents in the transformer core.
b. The resistance of the coil conductors.
c. The impedance of the core and coils.
d. The reactance of the core and coils.
b. The resistance of the coil conductors.
Transformer iron losses are caused by ____.
a. The impedance of the primary and secondary windings.
b. The reactance of the iron core.
c. the opposition to current flow in the core.
d. Hysteresis and eddy currents.
d. Hysteresis and eddy currents.
A primary metering installation has a current transformer with a 30:5 ration and a potential transformer with a 7200:120 ration. To find the KWH consumed you must ______.
a. Multiply the meter dial reading by 360.
b. Multiply the meter dial reading by 5 times 120.
c. Multiply the meter dial reading by 30, then divide by 120.
d. Divide the meter reading by 30 then multiply by 120.
NEED TO FIND CORRECT ANSWER.
Wye delta and delta wye banks must be connected with one of two possible angular displacements. These angular displacements are either _______.
a. 30 or 210
b. 0 or 180
c. 90 or 270
d. 120 or 300
a. 30 or 210
What is a Auto Transformer?
What are they used for?
Where are they found?
A transformer in which at least two windings have a shared section, they are used to either buck or boost the incoming line voltage. They are found in substations, transmission substations, distribution substations, all substations.
What is a Booster Connection?
NEED TO FIND ANSWER.
_____ is a phenomenon which occurs around a conductor when the conductor potential is raised above the dielectric strength of the surrounding air.
a. Ozone
b. Corona
c. Induction
d. Capacitive reactance
c. Induction
A low power factor due to a _____ current condition can be improved by the addition of ______ to the line.
a. leading / voltage regulators
b. lagging / capacitors
c. lagging / reactors
d. leading / capacitors
b. lagging / capacitors
The strength of the magnetic field surrounding a conductor is determined by the _____.
a. voltage of the conductor
b. cross sectional area of the conductor
c. proximity of other conductors
d. current through the conductor
a. voltage of the conductor
Review battery connection….
?
Impedance is the total opposition to current flow in an AC circuit. The letter symbol is Z and it is measured in ohms. It is made up of _______.
a. reactance, henries, and farads
b. resistance, induction, and current
c. counter electromotive force, flux, and farads
d. resistance, inductive reactance and capacitive reactance
d. resistance, inductive reactance, and capacitive reactance
The RMS voltage value of a line is 480 volts. This means that the ______ voltage is 480.
a. effective
b. average
c. peak
d. mean
a. effective
A copper atom has an atomic number of 29. This means there are 29 ______ in the nucleus and 29 _____.
a. neutrons / protons in the shells
b. protons / neutrons in the shells
c. (CUT OFF ON PAPER)
d. electrons / protons in the valence orbit.
NEED TO FIND CORRECT ANSWER.
Why is a close Y close Δ bank primary neutral floated?
Third harmonics, if the primary fuse blows on a Y and Δ bank it is grounded the rest of the pots only run at 57% efficiency.
Why is a fourth cutout install on the primary neutral of a close Y close Δ bank?
NEED TO FIND CORRECT ANSWER
On a open transformer bank what kva capacity do you have of the two transformers being used?
86.6 %
What must be done before climbing above a de-energized transformer?
Grounded
What do you need to do on a closed Y closed Y transformer bank?
NEED TO FIND CORRECT ANSWER
What purpose does the case ground serve on a transformer?
If can becomes energized it has a reference to ground.
Name the types of transformer banks you can make with two or three transformers.
Open -
Y Δ
Δ Δ
Closed - Y Y Δ Δ Y Δ Δ Y
What does CSP mean?
Completely self protected transformer
On a CSP transformer what coil is the circuit breaker on?
Secondary side
TAPS are used for two purposes. Name them…
1) correct low primary voltage
2) correct voltage for the customer
What determines the amount of voltage drop when a transformer is fully loaded?
Wire size and how far.
Explain Polarity of a transformer….
It is the direction of current flow in the primary and secondary side.
Impedance must be how close when paralleling or banking transformers?
Near equal
What capacity would you have if you concert a closed delta bank to a open delta bank?
57.7 %
How much capacity would you gain from using a closed delta bank instead of a open delta bank?
86.6 %
TRUE or FALSE? Two cutouts and arresters are needed on a wye connected transformer.
FALSE
TRUE or FALSE? Series can happen when you contact two current carrying sources at the same time.
TRUE
TRUE or FALSE? If you have high primary voltage on a transformer your secondary voltage will be low.
TRUE
TRUE or FALSE? Name plates are only needed if you don’t know your phase to phase voltage.
FALSE
TRUE or FALSE? Low voltage rubber gloves are needed if you are checking voltage on a new transformer.
FALSE
TRUE or FALSE? A 250 KVA transformer would have a additive polarity.
FALSE
TRUE or FALSE? You can connect the H 2 bushing on a single bushing transformer only to the neutral.
FALSE
TRUE or FALSE? A transformer can usually only be connected to one circuit voltage on the primary side.
FALSE
TRUE or FALSE? A delta connection will allow you to balance the system voltage better than a wye connection.
FALSE
TRUE or FALSE? The unlike bushing are connected together to make a wye connection.
FALSE
TRUE or FALSE? You must always use the same phase and bushing when paralleling transformers.
TRUE
TRUE or FALSE? A transformer whose primary voltage rating is 4160/7200 volts can be used on a 7200/12470 volt system.
TRUE
TRUE or FALSE? A transformer whose secondary voltage rating is 277/480 has two secondary bushings.
TRUE
TRUE or FALSE? A 250 KVA transformer whose secondary voltage rating is a 120/240 volt has three secondary bushing.
FALSE
What are the two basic functions of oil in a transformer?
a. cooling and lubrication
b. prevents rust and condensation
c. lubrication and insulation
d. cooling and insulation
d. cooling and insulation
Underground loop fed transformer are connected through?
a. a buss bar
b. a radial feeder
c. a conductor loop in the cabinet
d. the primary coil
a. a buss bar
How many phases of a feeder must be present to connect the open delta-delta bank?
a. two phases
b. two phases and a neutral
c. three phases and a neutral
d. three phases
d. three phases
Choose the three most important things that should be considered the nameplate of a transformer, before installing it in a transformer bank.
a. KVA
b. impedance
c. polarity
d. manufacture
e. voltage
a. KVA
b. impedance
e. voltage
How are the internal secondary leads connected in parallel on a conventional transformer?
a. AC-BD
b. AD-BC
c. AB-CD
d. BC-AD
a. AC-BD
In common distribution transformers the tap changer is located in which coil?
a. primary coil
b. secondary coil
c. tap changers are not in the coils
d. between the primary and secondary coils
a. primary coil
What is the main advantage of a wye secondary over a delta secondary?
a. efficiency
b. balanced loads
c. less voltage fluctuation
d. equal phase voltages
a. efficiency
Two 50 KVA transformers are connected open-delta, open-delta. What is the KVA capacity of the bank?
a. 100 KVA
b. 50 KVA
c. 57.7 KVA
d. 86.6 KVA
a. 86.6 KVA
What types of transformers are used for primary meeting?
a . insulating and isolating
b. single and three phase
c. dry and oil filled
d. current and potential
d. current and potential
Transformers are additive polarity if their KVA rating is less than _____ ?
a. 175
b. 200
c. 250
d. 500
b. 200
Transformer tap changers are?
a. located in the oil on the primary coil
b. located in the oil on the secondary coil
c. located in the oil on the primary coil and the control handle is located above the oil
d. located in the oil on the primary coil and the control handle is located on the lid.
c. located in the oil on the primary coil and the control handle is located above the oil
Explain back-feed when paralleling transformers in an energized circuit…..
You make and break on secondary side, when paralleling cans you can have backfeed if you hook up secondary last.
What could happen if the case grounds were removed from a single bushing transfomer?
Tank could become hot.
Explain the procedure for disconnecting an energized transformer from a circuit where the secondary is paralleled….
Make and break on secondary side. Open up cutouts. Lift high sides. Can should be isolated.
Three transformers are banked together connected closed wye-closed delta, why is the primary neutral floated?
Third harmonics.
How can an additive and subtractive transformer be paralleled?
Make sure you connect 3 to 3, 2 to 2, 1 to 1.
Briefly describe how a transformer works.
Voltage is induced in the coil that have copper wire wrapped around in circles that induces a voltage on secondary side.
Explain how a tap changer works.
It add or takes away coils in a transformer, stepping up a transformer or stepping down.
Draw a diagram and explain method for determining the polarity of a transformer…..
You gotta do something.
(See diagram drawn on packet…..)
Connect 120 volts across the H1 and H2 bushing, a jumper is placed between the two adjacent bushings. So A jumper is placed from H1 A secondary side and a volt meter is connected across the opposite adjacent bushing. So H2 to B.
When 3 transformers are banked together connected closed wye-closed wye, why is the mid-point on the primary side grounded?
Needs reference to ground.
What determines the secondary voltage capability of a transformer?
Name plate.
TRUE OR FALSE? In an open delta - delta bank, two cutouts are required.
FALSE
TRUE OR FALSE? Changing the taps of a transformer from tap 5 to tap 2 raises the secondary voltage.
TRUE
TRUE OR FALSE? A manual tap changer in normally operated under load.
FALSE
TRUE OR FALSE? If voltage is transformed in a 10:1 ratio, then current is transformed in a 1:10 ratio.
TRUE
TRUE OR FALSE? Primary current can be metered at the neutral bushing of a transformer.
TRUE
TRUE OR FALSE? “turns ratio” is the relationship between the number of turns of wire in the coils, and the number of layers in the core.
TRUE
TRUE OR FALSE? Due to larger current carrying requirements, the high voltage coils of a transformer are usually wound with heavier wire than the low voltage.
FALSE*** (double check?)
TRUE OR FALSE? A distribution transformer used in a 2400 volt, 3 wire system can be used in a 3 phase, 4 wire 4160 volt system.
TRUE
TRUE OR FALSE? Only one lightning arrester and cutout is needed when a transformer is connected across two phases.
FALSE.
TRUE OR FALSE? The “Wild Leg” on a delta secondary is generally used for lightning.
TRUE
TRUE OR FALSE? One leg of a Delta power bank can be grounded, and it will continue to operate.
TRUE
TRUE OR FALSE? When the secondary breaker is open on a CSP the primary becomes de-energized.
FALSE
TRUE OR FALSE? A transformer bank whose voltage to ground reads 120, 120, 208, is connected Wye
FALSE
TRUE OR FALSE? A bank of transformers fed by a wye connected feeder cannot be delta connected delta on the primary side
FALSE
TRUE OR FALSE? The nameplate of the transformer identifies tap positions in terms of percentage of the line voltage to the rated transformer.
TRUE
