Theory Flashcards
What is the core of a transformer made of?
Laminated iron or steel core about which are wound two or more coils.
What do the 11 transformer components consist of:
Core, coils, ratio, tank, bushings, cooling, losses, taps, impedance, polarity, and nameplate.
What is the purpose of the laminate sheet insulating material coating the core steel sheets?
To reduce the core loss and the heating produced in the core by insulating each sheet from the other.
Define PERMEABILITY
The ease which soft iron conducts magnetic lines of force; the softer the iron, the greater the permeability.
In coils, in comparison to the primary and secondary, What are the size, number of turns, and insulation of the coil wires?
The primary coil has more turns, smaller wire, higher insulation; while the secondary coil has the lesser turns, larger wire and the lesser insulation.
In coils what is the shape of the primary and secondary wires?
Primary wire may be round, rectangular, or sheet. Secondary wire is usually rectangular or sheet.
Askarel is also known as what?
PCB - Polychlorinated Biphenyl
Losses: What are the two main losses of the transformer?
Iron and Copper losses. Iron (core loss) consists of the eddy current and hysteresis losses; Copper losses consist of the heating loss of the conductor I2R loss.
What does the iron loss consist of?
Eddy current and hysteresis losses; that is why the core is laminated instead of being a solid bar.
The amount of the current to fully magnetize the iron core is sometimes knows as the?
Exciting current or charging current.
What is the formula for the copper losses or heating loss?
I2R
What are the standard taps for a transformer?
4 - 2 1/2% below or 2 - (plus/minus) 2 1/2% taps.
When changing taps, what happens to the secondary voltage when you raise and lower the taps?
When you raise the taps, it lowers the customer voltage, when you lower the tap, it raises the customers voltage.
Symbol for IMPEDENCE
%Z
Capacitance is measured in ______ and its symbol is _
Farads, C
Inductance is measured in ______ and its symbol is _
Henries, L
Inductive Reactance is measured in _____ and its symbol is __
Ohms, XL
Capacitive Reactance is measured in ____ and its symbol is __
Ohms, XC
Impedance is measured in ____ and its symbol is _
Ohms, Z
Resistance is measure in ____ and its symbol is _
Ohms, R
The total opposition to current flow in an AC circuit is called ______ and is measured in ____
Impedance, Ohms
Impedance is made of
Resistance, Inductive Reactance, Capacitive Reactance.
In AC circuits with reactance the voltage can lead or lag the current by as much as __ degrees
90
In parallel operation, the transformer with the lower impedance will carry the?
Greater Load
Can transformers of different KVA be paralleled?
Yes, if the impedances are nearly the same, the load will split proportionally.
Define POLARITY
A designation of the relative direction of current flow in the primary and secondary windings.
In the Box, Out of the Box rule for transformers
Transformers rated more than 200 KVA or having a primary voltage over 8600 volts are subtractive. All other transformers are additive polarity.
In order to properly parallel two transformers, what factors have to be checked?
Same voltage ratio, nearly same impedance (plus/minus 10%), must be connected to the same phase, polarity must be connected the same.
The highest tap is called _, _, or _
The lowest tap is called _, _, or _
1, I, or A
5, V, or E
What are the two main enemies of transformers?
Dangerously high voltage surges caused by lightning, and heating caused by overloading.
What are the two basic lightning arresters used on distribution systems?
Expulsion (open gap) and Valve Type
What protection does the CSP transformer have built in?
External tank mounted lightning arrester to protect it from high voltage surges, a secondary circuit breaker to protect it from external short circuits and overloads, an internal high voltage fuse is mounted in the primary bushing to protect the system from internal faults in the transformer.
The primary booster is a convectional transformer connected how?
As an auto transformer, it is not grounded and has no fuse protection.
Primary boosters raise the voltage by how much?
5% or 10%
What does a ground consist of?
An artificial electrical connection to the earth, having a very low resistance to the flow of electric current.
What does it mean to ground a transformer?
To connect it to the earth.
What are the six possible transformer connections?
Open Delta/Open Delta
Open Wye/Open Delta
Wye/Wye
Wye/Delta
Delta/Delta
Delta/Wye
In the wye system the voltage from tip to tip represents the ____ voltage
Line
In the wye system, the voltage from tip to center or the wye represents the _____ voltage
Phase
What is the most popular distribution voltage today?
7200/12470Y volt system
How much more efficient is a wye primary system compared to a delta primary system?
73%
Phasing and Rotation Rule
Phasing ensures rotation, rotation does not ensure phasing.
What unexplainable voltage condition exists in the wye-wye and wye-delta transformer connections?
3rd Harmonic (180 hz). an unwanted voltage on the system.
How much larger can 3rd harmonic voltage be than the phase voltage?
150%
On a wye-delta connection where will the 3rd harmonic voltage be measured?
On the neutral bus floats.
If jumpers are placed between the same identified phases (A to A, B to B, C to C) of two three phase circuits, the circuits would phase together under what conditions?
- That the circuits are electrically in phase. (Angular Displacement) If a circuit is not 30 degrees out of phase due to a wye-wye and delta-wye transformation.
- On a grounded system, the voltage across the gap is near zero.
- On an underground delta system:
(a) Make sure all foreign grounds are removed from the system
(b) connect any two leads together, one lead from each circuit.
(c) Measure voltage across both gaps. They are in phase when the voltage is near zero.
If a customer requests 120/240 volt, three phase, four wire what transformer can be used?
Transformers rated at 120/240 volts can be used. A transformer rated at 120/240 volts must be used as a “lighter”, but transformers rated at 240/480 volts can be used as power pots by paralleling the secondary coils.
If a customer requests 240 volt, three phase, three wire, what transformer can be used?
Transformers rated at 120/240 volt or 240/480 volt can be used.
If a customer request 120/208 volts, three phase, four wire, what transformers can be used?
Only 120/240 volt transformers can be used.
If a customer requests 277/480 volt, three phase, four wire, what transformers can be used?
Only 277/480 transformers can be used.
If a customer requests 480 volt, three phase, three wire transformer, what transformers can be used?
Transformers rated at 277/480 or 240/480 volts can be used.
When using the open wye/open delta connection or an open delta/open delta, in an emergency situation when one transformer in a closed wye/delta or closed delta/closed delta bank fails, the KVA rating of the bank is reduced by what factor?
86.6% of the two remaining transformers or 57.7% of the initial three
What disadvantage of the open wye/open delta connection can be a hazard to lineman working on the bank?
The primary neutral also carries the full load current even though the load is balanced. If the primary neutral connection is disconnected when the bank is energized, one half of the phase to ground voltage will be present.
Can CSP transformers be used on open wye/open delta banks?
Yes, only if future plans do nor call for closing the delta.
On an open wye/open delta bank, what would happen if one of the primary phases were to be switched to the other phase?
Rotation would be reversed, all secondary voltages would be the same.
Can a closed delta and open delta bank be paralled?
Yes when properly phased.
What condition will exist if one primary fuse is blown on an open wye/open delta or open delta/open delta bank, if the three phase load is still connected?
A slight backfeed voltage will exist on the primary lead of the blown fuse transformer.
Rule of Thumb: In order to size the two transformers in an open wye/open delta or open delta/open delta to carry a three phase balanced load, multiply the load by __%.
57%, the single phase load is added only to the lighter transformer.
Can CSP transformers be used in an open delta/open delta connection?
No, must be two bushing transformers.
Can an open wye/open delta bank be paralleled?
No they would be 30 degrees out of phase.
In a wye/delta or delta/delta bank, how is the single phase and three phase load split between the cans?
The lighter carries 2/3 of the single phase load, and each power pot carries 1/3 of the single phase load. The three phase load splits 1/3 of the load on each transformer.
When banking transformers the impedance of the transformers should be matched or no more than __% out of balance?
10%
What will happen if a CSP transformer is used as the lighter transformer on a wye/delta or delta/delta bank and the secondary breaker open or trips off?
The neutral will become isolated causing unequal voltages on the single phase loads. (It would act as an open neutral)
Can CSP transformers be used on a wye/delta bank?
Yes only on 240v and 480v three phase, three wire systems.
When the primary neutral on a wye/delta bank is floated, and the secondary wild leg is opened to the load, and the voltage is measured across the gap, it will read approximately 1 1/2 times the phase voltage. What is this phenomenon called?
3rd Harmonic. When the primary neutral is grounded this voltage will return to zero.
In the wye/delta bank connection, what should be done with the primary neutral?
It should be “floated” or insulated from ground.
In a wye/delta bank, what two things happen if the neutral bus becomes grounded, and a primary transformer fuse is blown?
- The voltage condition remains the same as before, except that the bank capacity is reduced to 57% of the bank rating
- The transformer on the blown fuse line acts as a step-up transformer and provides power to the so called dead line. (This is known as back feed)
On a wye/delta bank, what is the voltage on the floated neutral bus when a transformer fuse is blown?
One half of the phase to ground voltage.
On a wye/delta bank, that is the voltage on the floated neutral bus if two transformer fuses are blown?
Full phase to ground voltage.
What is the voltage on the floated neutral bus on a wye/delta bank every time it is energized or de-energized or a primary line is either energized or de-energized?
One half to full phase to ground voltage will be present on the neutral bus.
What needs to happen every time a wye/delta bank is energized or de-energized?
The floated neutral bus has to be tied down.
Is there a 3rd harmonic voltage in the delta/delta connection?
No.
What are the two common transformer secondary voltage ratings and connections used if a customer required 480v, three phase, three wire service?
- 240/480v Delta (open or closed)
- 277/480v closed wye connected by not grounding the neutral connection.
What are the two common transformer secondary voltage ratings and the connections if a customer requires 240v, three phase, three wire service?
120/240v Delta, Series secondary coils.
240/480v Delta, parallel secondary coils.
In a wye/wye 120/208v bank, how should the transformer KVA ratings be sized?
All should be the same size.
In a wye/wye 120/208v bank, can single bushing cans and CSP cans be used?
Yes.
In a wye/wye 120/208v bank, the primary neutral and secondary neutral connections between the three transformers must have the same what?
Ampacity as the phase conductors.
In a wye/wye 120/208v bank the primary and secondary line neutrals can be what?
Of smaller size, as the wires carries only the unbalance in the three phase lines.
In a wye/wye 120/208 bank, what must be done to the secondary coils?
Must be paralleled
In a wye/wye 120/208v bank, what must be tied, if not what may develop on the secondary side?
Primary neutral must be firmly tied to the system neutral. If not, excessive voltage may develop on the secondary side.
In a wye/wye 120/208v bank, if the primary neutral is disconnected from the system neutral, what condition will exist?
3rd harmonic voltage exists between the primary neutral and the system neutral and ground. This excessive voltage could be hazardous.
In a delta/wye 120/208v closed bank the KVA rating of the three cans should be?
The same size.
In a delta/wye 120/208v closed bank, the secondary neutral connections between the three transformers must have the same ________ as the phase conductors.
Ampacity.
In a delta/wye 120/208v closed bank, the secondary line neutral can be of?
Smaller size, as the wire carries only the unbalance in three phase lines.
In a delta/wye 120/208v closed 120/208 volt bank what must be done to the secondary coils?
Must be paralleled.
In a delta/wye 120/208v closed bank, it is a four wire wye system, the primary neutral should be?
Connected to the secondary neutral.
In a delta/wye 120/208v closed bank, what should be done with the grounding straps and why?
Remove all the transformer grounding straps from all the transformers and ground the secondary neutral only at one location. This keeps tanks and tank ground connections from being in parallel with the neutral bus, and eliminates overheating in the grounding circuit.
Ina wye/wye 277/480v closed bank the three transformers KVA rating should be sized how?
The same.
In a wye/wye 277/480v closed bank the primary and secondary neutral connections between the three transformers must have what?
The same ampacity as the phase conductors.
In a wye/wye 277/480v closed bank the primary and secondary line neutrals can be of what?
Smaller size, as this wire carries only the unbalance in the three phase lines.
In a wye/wye 277/480v closed, the primary neutral must be? If not?
Tied firmly to the system neutral, if not, excessive voltages may develop on the secondary side.
In a wye/wye 277/480v closed bank, if the primary neutral is disconnected from the system neutral what exists?
A 3rd harmonic voltage exists between the primary neutral and the system neutral to ground. This excessive voltage could be hazardous.
In a wye/wye 277/480v closed bank what should be done with the grounding straps and why?
Remove all the transformer grounding straps from all the transformers and ground the secondary neutral only at one location. This keeps tanks and tank ground connections from being in parallel with neutral bus, and eliminates overheating in the grounding circuit.
In a wye/wye 277/480v closed bank, what’s up with the secondary coils?
The transformer has only one secondary coil.
In a delta/wye 277/480v closed bank, the three transformers KVA rating should be sized how?
The same.
In a delta/wye 277/480v closed bank, how many primary bushings are needed per can?
Two.
In a delta/wye 277/480v closed bank, the primary and secondary neutral connections between the three transformers must have what?
The same ampacity as the phase conductors.
