Transformers Flashcards
What are four examples of energy sources?
Water energy via hydro
Wind
Strain
Fuel driven engines
How do we ensure additive effect in a transformer?
We cross cross the H1 and X1 leads (ie H1 is on the left in primary, and the X1 is in the right). If they were in line, the effect would be subtractive.
Why?
Due to instantaneous polarity at any given time, and if current flows H1 to H2, it would be additive connected to H2 to X1, and subtractive connected H2 to X2.
Think of the transformer as a sphere. In that sphere, PHYSICALLY connected leads in a certain way will cause additive or subtractive effect simply because of the DIRECTION of the flux and induced current.
What is magnetizing current? Aka excitation current.
Current that flows in the primary in the absence of a load in the secondary. As a voltage is applied to the primary, and current flows, there is a counter electro magnetic force (cemf) that opposes the direction of the current flow.
When a load is attached, the secondary current makes an emf effect in the primary, cancelling out cemf, and helping to decrease resistance in primary, and helping more current to flow (though this effect accounts for very little in terms of the reason the primary’s current increases; the current increases due to the load causing Kva in secondary to increase, and the primary Kva has to increase to match the secondary, increasing current as it can’t increase resistance or voltage, with voltage being limited by source Ofc).
How does electrical energy move in a transformer system?
Via induced voltage in the secondary by way of flux.
How efficient is a transformer system?
96-99 percent.
True or false; h side is always the primary.
F. The primary is always the circuit with the source in it. The other circuit has the secondary being the source to a load. They don’t have to the high or low side necessarily.
Transformers ratios can be step up , step down, and …
1:1 ratio.
On the line side, what does a higher voltage mean in terms of the effect on the system?
Less line loss.
What’s line loss?
The power that’s lost in a wire conductor in the run due to wire length, wire size, and the current running through the wire. The higher the voltage, the lower the current, so less line loss (remember wye systems give less line loss, and it makes sense, as the current in a wye is the same
Phase and line, whereas in delta, the line current increases in a system of the same Kva compared to a wye.
What is mutual induction?
The method in which transformers don’t have to be electrically connected. It’s the production of voltage in an adjacent coil via current flow in one coil that is linked via magnetic field flux lines to the other coil.
What are the uses of transformers? And how do they do these things?
1) change voltage (via step up or down via mutual induction. The step up or down depends on the number of turns in the coils as the number of volts per turn will always be the same)
2) change current (via voltage change as Kva must be same for both primary and secondary)
3) change impedance. (Higher current side has lower voltage, and so as voltage is equal to current x impedance, the impedance is lower in the higher current lower voltage side, which is the secondary in a step down transformer).
What does the ratio of a transformer tell us?
The ratio of voltage drop, and the ratio of current rise.
How do we use instantaneous polarity to ensure additive or subtractive effect?
Easy way to think of it is When h1 and x1 are both positive and “in line” with one another, the positive h and positive x can be considered opposing forces. When a positive h lines with a negative x, a “line” from h to x will look “healthy” as the forces attract each other than oppose, and so voltage is additive or aided or boosted. So, connecting h1 and x2 in line with each other gives additive polarity.
What are eddy currents and how do we deal with them?
They’re induced in the core of the transformer due to changing magnetic fields and flow in the conductors. We use laminations, an insulating layer, to deal with them.
What is the formula for laminations to determine power loss? And what are layers?
Old power loss/layers^2= new power loss
Layers are the insulating laminations. 1 layer is one lamination.
What are the characteristics of transformer cores?
1) high permeability (to allow more flux)
2) low retentivity in terms of magnetism (so no coercive force needed to demagnetize)
3) high resistance via laminations (to disrupt eddy flow)
What are the ratings of a transformer, and what does it mean against the actual rating?
1) kVA rated is the amount the primary can deliver to secondary. Actual means won’t necessarily get rated amount both side (applies to every rating)
2) max voltage rating (depending on insulation temp rating, and that temp rating is dependant upon the amperage drawn, so at a low amperage, it wouldn’t even matter; can have as much voltage as you want with a million ohm resistance for example. Current draw is small enough so insulation doesn’t get heated)
3) max current rating (limited by wire size).
If a polyphase 3 phase transformer is 120kva, how many single transformers would be needed to
Replace it? And what are the benefits?
3 single phase 40kva transformers.
1) polyphase is bulkier to install
2) there’s less downtime with 3 different transformers as in polyphase, entire thing needs to be removed.
3) easier to store single transformers as backups
4) simpler connections
One con though is polyphase is a bit cheaper per Kva.
All 3 transformers (on each side of a 3 single phase transformers bank) need to be of the same voltage, same Kva, save percent impedance, same ratio; t or f.
T.
What are the CSA standard connections for the 4 different setups possible for transformers in a bank? (Bank Means set). What’s the angle displacement in each system?
1) in a y to y, H1 connects to A, and X1 in secondary connects to A. (H2’s and X2’s are tails in the middle). If in the primary, table design, c is on right bottom, same in secondary (ie c at bottom right, a bottom left, and b at top)
2) in a delta to delta, H1 is the start side of a phase, and X1 is the start of a phase in the secondary. They then all connect in a regular delta (ie x1 at A goes to X2 of A, which is joined to X1 of B). Also, table wise, if a is bottom left of primary triangle, it’ll be bottom left of secondary triangle.
3) in a wyd to delta, Y side connected like a regular Y, and the delta side like a regular delta. Note, if b is top side of y, b is topside of delta.
4) in a delta to wye, only difference is Y is connected with X1’s in the middle due to minimizing displacement angle. Also, if In delta, B at top, A on left , and C on right of table triangle means in the wye, a on left, b at top, and c at bottom (to minimize
Phase displacement or shift)
In a delta to wye, if Eab is at 60 degrees in the delta (Cz of table), Eab on other side is at 30 degree. Vice verse in a wye to delta (due to table).
What are the order of steps in solving transformer problems?
It’s a combo of the 3 phase steps.
1)find va, v, and a
2) Eline primary
3) Ephase in primary
4) Ecoil in primary
- And turns ratio
5) Ecoil in secondary
6) Ephase in secondary
7) Eline in secondary
8) Ephase of the load
9)Iphase of the load (using impendance)
10) Iline in secondary
11) I phase in secondary
12) I coil in secondary
13) I cool in primary
14) I phase in primary
15) I line in primary
16) Kva of system
Get all E’s one way, then I’s on way back.
If given a high side rating of 3600-7200, what does that tell us?
It tells us we can get 7200 in series, with a max of 3600, due to the rating, per coil.
If coil is in parallel and we know the voltage or current value, how do we get phase info?
By formulas.
Why is it important for all transformers in a bank to have same Z?
Easier to determine fault location
What is the formula for a short cct?
iline of secondary / percent Z
I line is same as Kva of total system/ root 3 •e line
In a short cct, why does the secondary determine the extent of the problem?
Because current draw depends on load.
By code, we can use a ___ for overcurrent protection under 10 Kva
Fuse
By code, H1 is always negative; t or f.
False. It isn’t designated positive or negative (that’s why we use instantaneous polarity).
If a wye secondary is misconnected at one phase, what are the new voltages? Ie. If B is misconnected.
If B is misconnected, it flips so to speak. The arrow points down. The x1 tail joins the x2 tails in the center. If drawn out, the b splits the a to c phase. A dotted line showing E ca will be 208 v. This is the base of the triangle that forms between a to b and then to c, closing at c to a. In a triangle like this, 208 is equal to 1.41 • any of the sides. Thus, 208 divided by root 3 gives 120 volts (same as phase voltage per chance).
This means E ca is 208, while AB and BC is 120v.
