Transformers Finals Equations Flashcards
1
Q
What are the three main insulation classes for dry type?
A
150 Celsius, 185 Celsius, 220 Celsius
2
Q
What is the 150 C insulation class dry type made out of?
A
- Interlayer insulation may be mylar or nomex.
- Conductor insulation may be varnish
3
Q
What is the 185 C insulation class dry type made out of?
A
-Interlayer insulation may be mylar or nomex
-Conductor insulation may be HPTZ
4
Q
What is the 220 C insulation class dry type made out of?
A
-Interlayer insulation may be Nomex
-Conductor insulation may be HML or nomex
5
Q
When does the equilibrium point occur in a transformer with a load?
A
When heat dissipated = the heat generated
6
Q
If the temp is above the equilibrium point, will the insulation start degrading?
Yes or No?
A
Yes
7
Q
Every 10 Celsius over, the temperature rating will be reduced by___?
A
1/2
8
Q
What is the average winding rise?
A
80 Celsius, 115 Celsius, 150 Celsius.
9
Q
What is the standard hotspot rise?
A
30 Celsius
10
Q
What is the average ambient temp over 24 hours? What is the maximum?
A
30 Celsius, 40 Celsius
11
Q
What is the insulation class of most dry types above 5kVa?
A
220 Celsius
12
Q
Advantages of 80 Celsius Average Winding Temp Rise?
A
- More capacity
- Lower Losses
- Increases the trx efficiency
13
Q
Disadvantages of 80 Celsius Average Winding Temp Rise?
A
-Higher Cost
-Uses larger conductors
- Larger physical size of trx
14
Q
Advantages of 150 Celsius Average Winding Temp Rise?
A
- Uses smaller conductors
- Lower Cost
- Smaller physical size of trx
15
Q
Disadvantages of 150 Celsius Average Winding Temp Rise?
A
- No increased capacity
- Higher losses
- Decreases the trx effiency
16
Q
What is the insulation class of an Oil Filled trx?
A
120
17
Q
What makes up the insulation class of an Oil Filled trx (Temps wise)?
A
65 Celsius average winding temp rise
15 Celsius hot spot rise
40 Celsius maximum ambient
18
Q
When is no load loss present?
A
It is always present as long as voltage is applied to the primary
19
Q
When is load loss present?
A
- Only when load is connect
- As long as current flows to the load
20
Q
What are other words for Load Loss?
A
Winding losses or copper (aluminum) losses
21
Q
Where does load loss come from?
A
I^2R and Eddies in both HV and LV windings
+
Stray losses which are due to the stray fluxes in structural steel parts
22
Q
Load losses are specified at rated current and corrected to a temperature.
True or false?
A
True
23
Q
No load losses are specified at rated current only, temperature is negligible.
True or false?
A
False
No load losses are specified at rated voltage only, temperature is negligable.
24
Q
What does resistance depend on (one word answer) ?
A
Material
25
Load Losses are specified at: Rated Rise + 20°C
True or false?
True
26
What are Eddy Losses?
Losses due to eddy currents in the conductors for each winding
Eddy Losses = Eddy Loss AXIAL + Eddy Loss RADIAL
must be account for both HV and LV
27
The eddy losses are a result of the ________ and ________ fluxes.
axial and radial
28
What is the relationship between Eddy Loss and Frequency?
As frequency goes up eddy losses goes up by square
29
Usually the % Eddies are a few percent up to about ___% of the load losses.
10%
30
Want to keep the % Eddies as low as possible, below __%, particularly in the presence of harmonics
because how the frequency changes
3%
31
The most effective way to reduce the Eddy Loss is to reduce the %Eddy Loss by:
Reducing the thickness of conductors by
- Adding more in parallel radially
-Reduce thickness and increase width
- must keep turn area same
32
What causes strays?
Due to the eddy losses in the structural steel of the transformer.
Due to radial and axial fluxes in the steel frame, brackets, enclosures, etc
33
Relating to %X
Increase N would ________ the volts per turn and
increase %X by the square
Decrease
%X = {(0.126 * f * kVA) / (V/T)2 }* (DR / heff)
34
Relating to %X
higher f results in _______ %X
Higher
%X = {(0.126 * f * kVA) / (V/T)2 }* (DR / heff)
35
Relating to %X
increase heff, results in a _______ %X would also increase the window height
Decrease
%X = {(0.126 * f * kVA) / (V/T)2 }* (DR / heff)
36
Relating to %X
Increase DR, results in an ________ of %X
Increase
%X = {(0.126 * f * kVA) / (V/T)2 }* (DR / heff)
37
Does %X vary with temperature?
No it does not
38
How to increase DR?
Must increase the build of the LV, MG, or HV
Increase by putting more or larger ducts. As a result, this would increase the window width, which increases the size of the core and increases the cost.
39
During testing we are able to measure and calculate these factors
o Measure V Imp
Calculate %Z = (V Imp / VRated ) * 100
o Measure P LL = W @ °C and VA
Calculate %R = ( P LL / S Rated ) * 100
o Calculate %X = √(%Z2 - %R 2 )
True or false?
True
40
What is impedance?
The impedance voltage is the voltage required to circulate rated current in one winding while the other winding is shorted.
41
Why is load loss important?
Efficiency!
Lower load loss means higher efficiency
Lower no load loss also means higher efficiency
42
4 reasons why impedance is important?
- Voltage regulation
- Stray losses
- Fault Current
- Paralleling transformers
43
Why is voltage regulation important relating to impedance?
Lower impedance causes lower internal voltage drops which results in a lower voltage regulation
As current increases the voltage drops gets larger. This means the voltage output is not exactly the same as the turns ratio
44
Why is stray losses important relating to impedance?
A lower impedance causes a lower %X which results in a stray losses
On larger units in particular, impedance is mostly %X.
%X is really a measure of how closely the flux from one winding links to the other winding
Better flux linkage (less stray flux) results in a lower %X
45
Why is fault current important relating to impedance?
A higher impedance results in a lower maximum fault current
46
Why is paralleling transformers important relating to impedance?
Require that the impedance be the same so that:
o Circulating currents will not flow between the transformers.
o Transformers will be equally loaded
Because of the voltage regulation, if the two transformers do not have the same impedance, it would
be like paralleling to sources of different voltage.
47
What makes up a single phase auto transformer?
- Made up of two windings that are electrically connected
- Common winding is the winding that is common to both the LV and HV
- The series winding is connected to the common winding to add the extra turns for the voltage of the HV winding
- The current flows from the HV through the series winding to the LV without being transformed.
- The additional current comes from the common winding
48
What is the standard rating for a single phase auto transformer?
125kVA 600 - 480V
49
What current makes up the series winding?
High Voltage winding
I series = I HV
50
What current makes up the common winding?
The difference between the HV and LV current
ICommon = ILV – IHV
51
What voltage makes up the series winding?
VSerries = VHV - VLV = 600 - 480 = 120V
52
What voltage makes up the common winding?
VCommon = VLV = 480V
53
What makes up the apparent power in the common winding?
S Common = VCommon * ICommon
54
Advantages of a single phase auto transformer?
Smaller physical size (due to equivalent kVA) – Electrical output VA rating is still the same
o Only a size advantage when the ratio is close.
o EX 100kVA 13800V – 480V
o S Equiv = S Isolation Rating * (VHV – VLV) / VHV = 100 * (13800 – 480) / 13800 = 96.52kVA
* Less expensive
* Can build these out of existing transformers and use as buck-boost
55
Disadvantages of a single phase auto transformer?
- No isolation - If common winding opens, you would get full primary voltage on the secondary
- Impedance is very low
56
Where is a single phase auto transformer used?
- To buck or boost the voltage
- Motor starting auto
57
What makes up a three phase auto transformer?
- Connected in a wye configuration
- Made up of two windings that are electrically connected per phase
- Common winding is the winding the is common to both the LV and HV per phase
- The series winding is connected to the common winding to add the extra turns for the voltage of the HV winding per phase
- The current flows from the HV through the series winding to the LV without being transformed
- Additional current comes from the common winding
58
What makes up the Vphase of a three phase auto transformer?
Vphase LV = Vline LV / root3
Vphase HV = Vline LV root3
59
What makes up the Iphase of a three phase auto transformer?
Iphase LV = S / (root3 x VlineLV)
Iphase HV = S / (root3 x VlineHV)
60
What makes up the series winding current of a three phase auto transformer?
Iseries = ILHV
61
What makes up the common winding current of a three phase auto transformer?
Icommon = ILLV-ILHV
62
What makes up the common voltage of a three phase auto transformer?
Vcommon = Vphase LV
63
What makes up the series voltage of a three phase auto transformer?
Vseries = Vphase HV - Vphase LV
64
What makes up the common apparent power of a three phase auto transformer?
Scommon = Vcommon x Icommon X 3
65
What makes up the series apparent power of a three phase auto transformer?
Sseries = Vseries x I series x 3
66
What makes up the equivalent apparent power of a three phase auto transformer?
S equivalent = KVA isolation rated (KVA on nameplate) x (VHV - VLV) / VHV
67
What do taps do in a transformer?
- Taps are introduced to the transformers to compensate for the voltage drop in the line
- Taps are used to match the transformer primary winding voltage to the actual line voltage to keep the secondary at rated voltage
- Taps are introduced to adjust that voltage, by adding or subtracting turns to the winding
68
Where are taps usually located in a transformer?
Taps are always found on the primary.
69
What the two types of taps in a transformer?
One type - taps at the end of winding, these are
usually found when the voltage of the primary is less than 10000V (or layer wound).
Second type - typically found on windings where the primary voltage is greater than 10000V (or when disc wound) are taps in the middle of the winding
70
What is the conventional way of numbering the taps?
To have the lowest tap number giving the highest voltage
and highest number of turns
71
What is Off Circuit Tap Changers?
the transformer must be de-energized in order to change the taps
72
What can Load Tap Changer (LTC) that Off circuit tap changers cant?
LTC’s can change taps while under full load without de-energizing.
73
What are the two classifications of transformers?
Power and Measurement
74
What are the four Power transformer types?
Control Transformer
Distribution Transformers
Power transformers
Auto-Transformer
75
Characteristics of a control transformer:
Control Transformer:
typically use 120V-24V for control voltage (i.e. building automation, furnace, doorbell,
control voltage for relays).
o 1φ and typically less than 5 kVA.
o most have added turns on the secondary, to compensate for poorer regulation.
% Voltage regulation = (VNL - VFL ) / VFL *100
i.e. at no load secondary is 28V (even though on nameplate it is 24V), but at
full load it is 24V
76
Characteristics of a distribution transformer:
Dry Type Oil Filled
o Indoors in a small distribution electrical
room
o Typically, less than 750kVA
Oil Filled
o Outdoors in a city distribution substation
o Typically, less than 50MVA
77
Characteristics of a power transformer:
Dry Type
o Indoors in the main electrical room
o Maximum 25MVA
Oil Filled
o Outdoors in a transmission substation
o Maximum 1000MVA
78
Characteristics of a auto transformer:
Single winding (2 windings that are electrically connected) for adjusting voltages (Boost-
raise, Buck –lower).
o not isolating.
o can be used for motor starting.
79
What are the two types of measuring transformers?
Potential Transformer (PT) or Voltage Transformer (VT)
Current Transformer (CT)
80
Characteristics of a Potential Transformer (PT):
operation is the same as conventional transformer
81
Characteristics of a Current Transformer (CT) :
o operation is that one or more turns are in series with the line.
o toroid core.
o remember to short terminals of CT when not in use
82
Definition of transformer:
A transformer is a static (no moving parts) device for transferring energy from one system to another at
very high efficiency.
* Can also change one voltage to another.
* Can also isolate one electrical system from another.
83
Types of transformers?
Air Insulated (Dry type)
Oil Insulated (Liquid Filled)
Gas insulated
84
Characteristics of Air Insulated:
Smaller sizes (up to 25MVA) 80000lbs, ship truck.
Up to 45kV voltage class (most common though is 15kV and below).
used mainly in factories, shopping plazas, schools
85
Characteristics of Oil Insulated:
Large MVA, up to 1,000,000kVA (1000MVA) 1200 tons, ship train.
* high voltage class up to 800,000V
86
Characteristics of Gas Insulated:
* SF6 gas.
* high voltage.
* special instrument transformers, breakers.
* 3 times the insulation of oil.
* usually, space is a constriction
87
What is flash point?
temperature at which a liquid can ignite, typically will only burn if the heat source is still present).
88
What is fire point?
firepoint – temperature at which a liquid can ignite, will continue to burn if the heat source is taken away.
89
Characteristics of Standard Mineral Oil:
Lowest cost
Flashpoint 160°C
Breakdown 350kV/cm
90
Characteristics of Silicon Oil:
Highest cost
Flashpoint 300°C.
Breakdown 250kV/cm
91
Characteristics of Biodegradable Oil:
Medium cost
Flashpoint 275°C
Breakdown 375kV/cm
92
Characteristics of PCB :
Fire retardant
