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TRANSFORMER
CHAPTER-8
CONTENT
1 Definition, Construction & Principle of operation.
2 Types of Transformers.
3 E.M.F Equation and Voltage Transformation Ratio
4 Tests on Transformers –OC & SC Test.
5 Losses & Efficiency of a Transformer.
6 Three Phase Transformer and connections.
7 Auto Transformers& Instrument transformer –Principle & Working
Transformer - Definition
A transformer is a static piece of apparatus by means of which
electric power in one circuit is transformed into electric power of the
same frequency in another circuit.
It can raise or lower the voltage in a circuit but with
corresponding decrease or increase in current.
Transformer
Construction
Construction
The three main parts of a transformer are,
• Primary Winding of transformer - which produces magnetic flux when it is connected to
electrical source.
• Magnetic Core of transformer - the magnetic flux produced by the primary winding, that
will pass through this low reluctance path linked with secondary winding and create a
closed magnetic circuit.
• Secondary Winding of transformer – the flux, produced by primary winding, passes
through the core, will link with the secondary winding. This winding also wounds on the
same core and gives the desired output of the transformer.
Construction
In all transformers that are used commercially, the core is made out of
transformer sheet steel laminations assembled to provide a continuous magnetic
path with minimum of air-gap included.
The steel should have high permeability and low hysteresis loss. For this to
happen, the steel should be made of high silicon content and must also be heat
treated.
By effectively laminating the core, the eddy-current losses can be reduced.
Construction
The lamination can be done with the help of a light coat of core
plate varnish or lay an oxide layer on the surface.
For a frequency of 50 Hertz, the thickness of the lamination
varies from 0.35mm to 0.5mm for a frequency of 25 Hertz.
Principle of Operation
Transformer works on the principle of mutual induction of two
coils. When current in the primary coil is changed the flux linked to the
secondary coil also changes. Consequently an EMF is induced in the
secondary coil.
Principle of Operation
Types of Transformer
(A) On the basis of construction
1. Core Type Transformer
2. Shell Type Transformer
3. Berry Type Transformer
Core Type & Shell Type Transformer
Berry Type Transformer
Types of Transformer
(B) On the basis of their purpose
1. Step Up Transformer
2. Step Down Transformer
(C) On the basis of type of supply
1. Single Phase Transformer
2. Three Phase Transformer
Three Phase Transformer
Types of Transformer
(D) On the basis of their use
1. Power transformer
2. Distribution transformer
3. Instrument transformer
3.I. Current transformer (CT)
3.II. Potential transformer (PT)
Types of Transformer
(E) On the basis of cooling employed
1. Oil-filled self cooled type
2. Oil-filled water cooled type
3. Air blast type (air cooled)
Difference Between Core & Shell Type
Transformer
Core Type Transformer
Shell type transformer
1. The core has only one magnetic circuit.
1. It has two magnetic circuits.
2. Core has two limbs.
2. Core has three limbs.
3. It has less mechanical protection to coil.
3. It has better mechanical protection to coil.
4. It has better cooling since more surface is exposed to
4. Cooling is not very effective.
atmosphere.
5. Natural cooling is provided.
5. Natural cooling cannot provide
6. This transformer is easy to repair.
6. This transformer is not easy to repair.
7. The winding is surrounded considerable part of core.
7. Core is surrounded considerable part of winding of
transformer.
EMF Equation of a Transformer
EMF Equation of a Transformer
Let,
NA = Number of turns in primary
NB = Number of turns in secondary
Ømax = Maximum flux in the core in webers = Bmax X A
f = Frequency of alternating current input in hertz (HZ)
As shown in figure above, the core flux increases from its zero value to
maximum value Ømax in one quarter of the cycle , that is in ¼ frequency second.
Therefore, average rate of change of flux = Ømax / ¼ f = 4f Ømax Wb/s
EMF Equation of a Transformer
Now, rate of change of flux per turn means induced electro motive
force in volts.
Therefore, average electro-motive force induced/turn = 4f Ømaxvolt.
If flux Ø varies sinusoidally, then r.m.s value of induced e.m.f is obtained by
multiplying the average value with form factor.
Form Factor = r.m.s. value/average value = 1.11
Therefore, r.m.s value of e.m.f/turn = 1.11 X 4f Ømax = 4.44f Ømax
Now, r.m.s value of induced e.m.f in the whole of primary winding
= (induced e.m.f./turn) X Number of primary turns
EMF Equation of a Transformer
Therefore, EA = 4.44f NAØmax = 4.44fNABmA
Similarly, r.m.s value of induced e.m.f in secondary is
EB = 4.44f NB Ømax = 4.44fNBBmA
Voltage Transformation Ratio (K)
From the above equations we get
𝑬𝑩
𝑬𝑨
=
𝑽𝑩
𝑽𝑨
=
𝑵𝑩
=K
𝑵𝑨
This constant K is known as voltage transformation ratio.
(1) If NB>NA , that is K>1 , then transformer is called step-up transformer.
(2) If NB<1, that is K<1 , then transformer is known as step-down transformer.
Test on Transformer
1. Open Circuit Test
2. Short Circuit Test
Open Circuit or No Load test on transformer
Open Circuit test on transformer
The open circuit test on transformer is used to determine core
losses in transformer and parameters of shunt branch of the equivalent
circuit of transformer.
Open Circuit test on transformer
The two components of no load current can be given as,
Iμ=I0sinΦ0and Iw = I0cosΦ0.
cosΦ0 (no load power factor) = W / (V1I0). ... (W = wattmeter reading)
From this, shunt parameters of equivalent circuit parameters of
equivalent circuit of transformer (X0 and R0) can be calculated as
X0 = V1/Iμ and R0 = V1/Iw.
Short circuit or Impedance Test on Transformer
Short Circuit test on transformer
The short circuit test on transformer is used to determine
copper loss in transformer at full load and parameters of approximate
equivalent circuit of transformer.
Short Circuit test on transformer
W = Isc2Req
where Req is the equivalent resistance of transformer
Zeq = Vsc/Isc.
Therefore, equivalent reactance of transformer can be calculated
from the formula Zeq2 = Req2 + Xeq2.
Equivalent Circuit of Transformer
Why Transformers are rated in kVA?
From the above transformer tests, it can be seen that Cu loss of
a transformer depends on current, and iron loss depends on voltage.
Thus, total transformer loss depends on volt-ampere (VA).
It does not depend on the phase angle between voltage and
current, i.e. transformer loss is independent of load power factor. This
is the reason that transformers are rated in kVA.
Three phase transformer connections
Windings of a three phase transformer can be connected in various
configurations as
(i) star-star,
(ii) delta-delta,
(iii) star-delta,
(iv) delta-star,
(v) open delta and
(vi) Scott connection.
Star-star (Y-Y)
Star-star connection is generally used for small, high-voltage
transformers.
Because of star connection, number of required turns/phase is
reduced (as phase voltage in star connection is 1/√3 times of line voltage
only). Thus, the amount of insulation required is also reduced.
The ratio of line voltages on the primary side and the secondary side
is equal to the transformation ratio of the transformers.
Star-star (Y-Y)
Line voltages on both sides are in phase with each other.
This connection can be used only if the connected load is balanced.
Delta-delta (Δ-Δ)
This connection is generally used for large, low-voltage
transformers. Number of required phase/turns is relatively greater than
that for star-star connection.
The ratio of line voltages on the primary and the secondary side
is equal to the transformation ratio of the transformers.
Delta-delta (Δ-Δ)
This connection can be used even for unbalanced loading.
Another advantage of this type of connection is that even if one
transformer is disabled, system can continue to operate in open delta
connection but with reduced available capacity.
Star-delta OR wye-delta (Y-Δ)
The primary winding is star (Y) connected with grounded neutral and
the secondary winding is delta connected.
This connection is mainly used in step down transformer at the
substation end of the transmission line.
The ratio of secondary to primary line voltage is 1/√3 times the
transformation ratio.
There is 30° shift between the primary and secondary line voltages.
Delta-star OR delta-wye (Δ-Y)
The primary winding is connected in delta and the secondary winding
is connected in star with neutral grounded. Thus it can be used to provide 3phase 4-wire service.
This type of connection is mainly used in step-up transformer at the
beginning of transmission line.
The ratio of secondary to primary line voltage is √3 times the
transformation ratio.
There is 30° shift between the primary and secondary line voltages.
Open delta (V-V) connection
Two transformers are used and primary and secondary
connections are made as shown in the figure below.
Open delta connection can be used when one of the
transformers in Δ-Δ bank is disabled and the service is to be continued
until the faulty transformer is repaired or replaced.
Open delta (V-V) connection
It can also be used for small three phase loads where installation
of full three transformer bank is un-necessary.
The total load carrying capacity of open delta connection is
57.7% than that would be for delta-delta connection.
Scott (T-T) connection
Two transformers are used in this type of connection. One of the
transformers has centre taps on both primary and secondary windings
(which is called as main transformer). The other transformer is called as
teaser transformer.
Scott connection can also be used for three phase to two phase
conversion.
Instrument Transformers
How will you measure AC currents and voltages of very high
magnitude?
You will need the measuring instruments having higher range,
which literally mean huge instruments.
Instrument Transformers
Or there's another way, using the transformation property of AC
currents and voltages. You can transform the voltage or current down
with a transformer whose turns ratio is accurately known, then
measuring the stepped down magnitude with a normal range
instrument.
Instrument Transformers
The original magnitude can be determined by just multiplying the
result with the transformation ratio. Such specially constructed transformers
with accurate turns ratio are called as Instrument transformers.
These instruments transformers are of two types –
(i) Current Transformers (CT) and
(ii) Potential Transformers (PT).
Uses of Instrument Transformer
It is used for the following:
1. To insulate the high voltage circuit from the measuring circuit in
order to protect the measuring instruments from burning.
2. To make it possible to measure the high voltage with low range
voltmeter and high current with low range ammeter.
Current Transformers (CT)
Current transformers are generally used to measure currents of high magnitude.
These transformers step down the current to be measured, so that it can be
measured with a normal range ammeter.
A Current transformer has only one or very few number of primary turns.
The primary winding may be just a conductor or a bus bar placed in a hollow core.
The secondary winding has large number turns accurately wound for a specific
turns ratio.
Current Transformers (CT)
Thus the current transformer steps up (increases) the voltage while
stepping down (lowering) the current.
Now, the secondary current is measured with the help of an AC ammeter.
The turns ratio of a transformer is
𝑁𝑃
𝑁𝑆
=
𝐼𝑆
𝐼𝑃
Current Transformer
Potential Transformer (PT)
Potential transformers are also known as voltage transformers and
they are basically step down transformers with extremely accurate turns
ratio.
Potential transformers step down the voltage of high magnitude to a
lower voltage which can be measured with standard measuring instrument.
These transformers have large number of primary turns and smaller
number of secondary turns.
Potential Transformer (PT)
A potential transformer is typically expressed in primary to
secondary voltage ratio.
For example, a 600:120 PT would mean the voltage across
secondary is 120 volts when primary voltage is 600 volts.
Potential Transformer
Auto Transformer
An auto transformer is an electrical transformer having only one winding.
The winding has at least three terminals.
Some of the advantages of auto-transformer are that, they are
• smaller in size,
• cheap in cost,
• low leakage reactance,
• increased kVA rating,
• low exciting current etc.
Autotransformer
Construction
An auto transformer consists of a single copper wire, which is
common in both primary as well as secondary circuit.
The copper wire is wound on a laminated silicon steel core, with
atleast three tappings taken out.
Secondary and primary circuit share the same neutral point of
the winding.
Construction
Variable turns ratio at secondary can be obtained by the tappings
of the winding, or by providing a smooth sliding brush over the
winding.
Primary terminals are fixed.
In an auto transformer, primary and secondary windings are
connected magnetically as well as electrically.
Working
An auto transformer has only one winding which is shared by both
primary and secondary circuit, where number of turns shared by secondary
are variable.
EMF induced in the winding is proportional to the number of turns.
Therefore, the secondary voltage can be varied by just varying
secondary number of turns.
Working
As winding is common in both circuits, most of the energy is
transferred by means of electrical conduction and a small part is
transferred through induction.
Autotransformer
Disadvantage
The considerable disadvantages of an auto transformer are,
any undesirable condition at primary will affect the equipment at secondary
(as windings are not electrically isolated),
Due to low impedance of auto transformer, secondary short circuit
currents are very high,
Harmonics generated in the connected equipment will be passed to
the supply.
Losses in Transformer
In any electrical machine, 'loss' can be defined as the difference
between input power and output power.
An electrical transformer is an static device, hence mechanical
losses (like windage or friction losses) are absent in it.
A transformer only consists of electrical losses (iron losses and
copper losses).
Losses in Transformer
(i) Core loss or Iron loss
• Hysteresis loss
• Eddy current loss
(ii) Copper loss
Core loss or Iron loss
Eddy current loss and hysteresis loss depend upon the magnetic
properties of the material used for the construction of core.
Hence these losses are also known as core losses or iron losses.
Hysteresis loss
Hysteresis loss is due to reversal of magnetization in the transformer
core. This loss depends upon the volume and grade of the iron, frequency of
magnetic reversals and value of flux density. It can be given by,
Steinmetz formula:
Wh= ηBmax1.6f V (watts)
where, η = Steinmetz hysteresis constant
V = volume of the core in m3
Eddy Current loss
In transformer, AC current is supplied to the primary winding which
sets up alternating magnetizing flux.
When this flux links with secondary winding, it produces induced emf
in it.
But some part of this flux also gets linked with other conducting parts
like steel core or iron body or the transformer, which will result in induced
emf in those parts, causing small circulating current in them.
Eddy Current loss
This current is called as eddy current.
Due to these eddy currents, some energy will be dissipated in
the form of heat.
Copper loss
Copper loss is due to ohmic resistance of the transformer windings.
Copper loss for the primary winding is I12R1 and for secondary winding
is I22R2.
Where,
I1 and I2 are current in primary and secondary winding respectively,
R1 and R2 are the resistances of primary and secondary winding respectively.
Copper loss
It is clear that Cu loss is proportional to square of the current,
and current depends on the load.
Hence copper loss in transformer varies with the load.
Efficiency of Transformer
Efficiency ,ŋ=
𝑂𝑢𝑡𝑝𝑢𝑡
𝐼𝑛𝑝𝑢𝑡
Transformers are the most highly efficient electrical devices.
Most of the transformers have full load efficiency between 95% to
98.5% .
Efficiency ŋ=
(𝐼𝑛𝑝𝑢𝑡−𝐿𝑜𝑠𝑠𝑒𝑠)
𝐼𝑛𝑝𝑢𝑡
= 1-
(𝐿𝑜𝑠𝑠𝑒𝑠)
𝐼𝑛𝑝𝑢𝑡
Condition for Maximum Efficiency
Let,
• Copper loss = I21R1
• Iron loss = Wi
Condition for Maximum Efficiency
Condition for Maximum Efficiency
Hence, efficiency of a transformer will be maximum when
copper loss and iron losses are equal.
That is Copper loss = Iron loss.
All Day Efficiency