Unit-2
1.
Transformer:
A transformer is a device that transfers electric energy from one alternating-current circuit to one
or more other circuits, either increasing (stepping up) or reducing (stepping down) the voltage.
1.2.
Types of Transformers
There are various types of transformers used in the electrical power system for different
purposes, like generation, distribution and transmission and utilization of electrical power.
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Step up and Stepdown Transformer
Power Transformer
Distribution Transformer
Instrument Transformer
Current Transformer
Potential Transformer
Single Phase Transformer
Three Phase Transformer
Fig. Various Types of Transformer
The various types of transformer shown in the figure above are explained in detail below.
1.2.1.
Step up and Step down Transformer
This type of transformer is categorized on the basis of a number of turns in the primary and
secondary windings and the induced emf.
Step-up transformer transforms a low voltage, high current AC into a high voltage, low current
AC system In this type of transformer the number of turns in the secondary winding is greater than
the number of turns in the primary winding. If (V2 > V1) the voltage is raised on the output side
and is known as Step-up transformer
Step down transformer converts a high primary voltage associated with the low current into a low
voltage, high current. With this type of transformer, the number of turns in the primary winding is
greater than the number of turns in the secondary winding. If (V2 < V1) the voltage level is lowered
on the output side and is known as Step down transformer
1.2.2.
Power Transformer
The power transformers are used in the transmission networks of higher voltages. The ratings of
the power transformer are as follows 400 KV, 200 KV, 110 KV, 66 KV, 33 KV. They are mainly
rated above 200 MVA. Mainly installed at the generating stations and transmission substations.
They are designed for maximum efficiency of 100%. They are larger in size as compared to the
distribution transformer.
At a very high voltage, the power cannot be distributed to the consumer directly, so the power is
stepped down to the desired level with the help of step-down power transformer. The transformer
is not loaded fully hence the core loss takes place for the whole day, but the copper loss is based
on the load cycle of the distribution network.
If the power transformer is connected in the transmission network, the load fluctuation will be very
less as they are not connected at the consumer end directly, but if connected to the distribution
network there will be fluctuations in the load.
The transformer is loaded for 24 hours at the transmission station, thus, the core and copper loss
will occur for the whole day. The power transformer is cost-effective when the power is generated
at low voltage levels. If the level of voltage is raised, then the current of the power transformer is
reduced, resulting in I2R losses and the voltage regulation is also increased.
1.2.3.
Distribution Transformer
This type of transformer has lower ratings like 11 KV, 6.6 KV, 3.3 KV, 440 V and 230 V. They
are rated less than 200 MVA and used in the distribution network to provide voltage transformation
in the power system by stepping down the voltage level where the electrical energy is distributed
and utilized at the consumer end.
The primary coil of the distribution transformer is wound by enamel coated copper or aluminum
wire. A thick ribbon of aluminum and copper is used to make secondary of the transformer which
is a high current, low voltage winding. Resin impregnated paper and oil is used for the insulation
purpose.
The oil in the transformer is used for
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•
•
1.2.3.1.
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•
•
Cooling
Insulating the windings
Protecting from the moisture
The various types of the distribution transformer are categorized on the following
basis and are shown in the figure below
Mounting location
Type of insulation
Nature of supply
The distribution transformer less than 33 KV is used in industries and 440, 220 V is used for the
domestic purpose. It is smaller in size, easy to install and has low magnetic losses and is not always
loaded fully.
As it does not work for constant load throughout 24 hours as in the daytime its load is at its peak,
and during the night hours it is very lightly loaded thus the efficiency depends on load cycle and
is calculated as All Day Efficiency. The distribution transformers are designed for maximum
efficiency of 60 to 70%
1.2.3.2.
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•
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Uses of Distribution Transformer
Used in pumping stations where the voltage level is below 33 KV
Power supply for the overhead wires railways electrified with AC
In urban areas, many houses are fed with single-phase distribution transformer and in
rural areas, it may be possible that one house requires one single transformer
depending upon the loads.
Multiple distribution transformers are used for industrial and commercial areas.
•
1.2.4.
Used in wind farms where the electrical energy is generated by the windmills. There
it is used as a power collector to connect the substations which are away from the
wind energy generation system.
Instrument Transformer
They are generally known as an isolation transformer. Instrument transformer is an electrical
device used to transform current as well as a voltage level. The most common use of instrument
transformer is to safely isolate the secondary winding when the primary has high voltage and high
current supply so that the measuring instrument, energy meters or relays which are connected to
the secondary side of the transformer will not get damaged. The instrument transformer is further
divided into two types
•
Current Transformer (CT)
•
Potential Transformer (PT)
The current and potential transformer is explained below in detail
1.2.4.1. Current Transformer
The current transformer is used for measuring and also for the protection. When the current in the
circuit is high to apply directly to the measuring instrument, the current transformer is used to
transform the high current into the desired value of the current required in the circuit.
The primary winding of the current transformer is connected in series to the main supply and the
various measuring instruments like ammeter, voltmeter, wattmeter or protective relay coil. They
have accurate, current ratio and phase relation to enable the meter accurately on the secondary
side. The term ratio has a great significance in CT.
For example, if its ratio is 2000:5, it means a CT has an output of 5 Ampere when the input current
is 2000 amp on the primary side. The accuracy of the Current Transformer depends upon many
factors like Burden, load, temperature, phase change, rating, saturation, etc.
In the current transformer, the total primary current is the vector sum of the excitation current and
the current equal to the reversal of secondary current multiplied by turn ratio.
Where, Ip – primary current
Is – secondary or reversal current
I0 – excitation current
KT – turn ratio
1.2.4.2. Potential Transformer
The potential transformer is also called as the voltage transformer. The primary winding is
connected across the High voltage line whose voltage is to be measured, and all the measuring
instruments and meters are connected to the secondary side of the transformer.
The main function of the Potential transformer is to step down the voltage level to a safe limit or
value. The primary winding of the potential transformer is earthed or grounded as a safety point.
For example, the voltage ratio primary to secondary is given as 500:120, it means the output
voltage is of 120 V when the 500 V is applied to the primary. The different types of potential
transformer are shown below in the figure
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•
•
The
Electromagnetic (it is a wire wound transformer)
Capacitor (capacitor voltage transformer CVT uses capacitor voltage divider)
Optical (works on the electrical property if optical materials)
percentage
voltage
error
is
given
by
the
equation
shown
below
Potential Transformer (PT)
The potential transformer may be defined as an instrument transformer used for the transformation
of voltage from a higher value to the lower value. This transformer step down the voltage to a safe
limit value which can be easily measured by the ordinary low voltage instrument like a voltmeter,
wattmeter and watt-hour meters, etc.
Construction of Potential Transformer
The potential transformer is made with high-quality core operating at low flux density so that the
magnetising current is small. The terminal of the transformer should be designed so that the
variation of the voltage ratio with load is minimum and the phase shift between the input and
output voltage is also minimum.
The primary winding has a large number of turns, and the secondary winding has a much small
number of turns. For reducing the leakage reactance, the co-axial winding is used in the potential
transformer. The insulation cost is also reduced by dividing the primary winding into the sections
which reduced the insulation between the layers.
Connection of Potential Transformer
The potential transformer is connected in parallel with the circuit. The primary windings of the
potential transformer are directly connected to the power circuit whose voltage is to be measured.
The secondary terminals of the potential transformer are connected to the measuring instrument
like the voltmeter, wattmeter, etc.The secondary windings of the potential transformer are
magnetically coupled through the magnetic circuit of the primary windings.
The primary terminal of the transformer is rated for 400V to several thousand volts, and the
secondary terminal is always rated for 400V. The ratio of the primary voltage to the secondary
voltage is termed as transformation ratio or turn ratio.
Types of Potential Transformer
The potential transformer is mainly classified into two types, i.e., the conventional wound types
(electromagnetic types) and the capacitor voltage potential transformers.
Conventional wound type transformer is very expensive because of the requirement of the
insulations. Capacitor potential transformer is a combination of capacitor potential divider and a
magnetic potential transformer of relatively small ratio.
The circuit diagram of the capacitor potential transformer is shown in the figure below. The stack
of high voltage capacitor from the potential divider, the capacitors of two sections become C1 and
C2, and the Z is the burden.
The voltage applied to the primary of the intermediate transformer is usually of the order 10kV.
Both the potential divider and the intermediate transformer have the ratio and insulation
requirement which are suitable for economical construction.
The intermediate transformer must be of very small ratio error, and phase angle gives the
satisfactory performance of the complete unit. The secondary terminal voltage is given by the
formula shown below.
Ratio and Phase Angle Errors of Potential Transformer
In an ideal potential transformer, the primary and the secondary voltage is exactly proportional to
the primary voltage and exactly in phase opposition. But this cannot be achieved practically due
to the primary and secondary voltage drops. Thus, both the primary and secondary voltage is
introduced in the system.
Voltage Ratio Error – The voltage ratio error is expressed in regarding measured voltage, and it
is given by the formula as shown below.
Where Kn is the nominal ratio, i.e., the ratio of the rated primary voltage and the rated secondary
voltage.
Phase Angle Error – The phase angle error is the error between the secondary terminal voltage
which is exactly in phase opposition with the primary terminal voltage.
The increases in the number of instruments in the relay connected to the secondary of the potential
transformer will increase the errors in the potential transformers.
Burden of a Potential Transformer
The burden is the total external volt-amp load on the secondary at rated secondary voltage. The
rated burden of a PT is a VA burden which must not be exceeded if the transformer is to operate
with its rated accuracy. The rated burden is indicated on the nameplate.
The limiting or maximum burden is the greatest VA load at which the potential transformer will
operate continuously without overheating its windings beyond the permissible limits. This burden
is several times greater than the rated burden.
Phasor Diagram of a Potential Transformer
The phasor diagram of the potential transformer is shown in the figure below.
Where,
Is –
secondary
Es –
secondary
induced
Vs –
secondary
terminal
Rs –
secondary
winding
Xs –
secondary
winding
Ip –
Primary
Ep –
primarily
induced
Vp –
primary
terminal
Rp –
primary
winding
Xp –
primary
winding
Kt –
turn
Io –
excitation
Im –
magnetising
component
Iw –
core
loss
component
Φm –
main
Β- phase angle error
current
emf
voltage
resistance
reactance
current
emf
voltage
resistance
reactance
ratio
current
of
Io
of
Io
flux
The main flux is taken as a reference. In instrument transformer, the primary current is the vector
sum of the excitation current Io and the current equal to the reversal secondary current Is multiplied
by the ratio of 1/kt. The Vp is the voltage applied to the primary terminal of the potential
transformer.
The voltage drops due to resistance and reactance of primary winding due to primary current is
given by IpXp and IpRp. When the voltage drop subtracts from the primary voltage of the potential
transformer, the primarily induced emf will appear across the terminals.
This primary emf of the transformer will transform into secondary winding by mutual induction
and converted into secondary induced emf Es. This emf will drop by the secondary winding
resistance and reactance, and the resultant voltage will appear across the secondary terminal
voltage, and it is denoted by Vs.
Applications of Potential Transformer
1.
2.
3.
4.
It is used for a metering purpose.
For the protection of the feeders.
For protecting the impedance of the generators.
For synchronising the generators and feeders.
The potential transformers are used in the protecting relaying scheme because the potential coils
of the protective device are not directly connected to the system in case of the high voltage.
Therefore, it is necessary to step down the voltage and also to insulate the protective equipment
from the primary circuit.
1.2.5.
Single Phase Transformer
A single-phase transformer is a static device, works on the principle of Faraday’s law of mutual
Induction. At a constant level of frequency and variation of voltage level, the transformer transfers
AC power from one circuit to the other circuit. There are two types of windings in the transformer.
The winding to which AC supply is given is termed as Primary winding and in the secondary
winding, the load is connected.
1.2.6.
Three Phase Transformer
If the three single-phase transformer is taken and connected together with their all the three primary
winding connected to each other as one and all the three secondary windings to each other, forming
as one secondary winding, the transformer is said to behave as a three-phase transformer, that
means a bank of three single-phase transformer connected together which acts as a three-phase
transformer.
Three-phase supply is mainly used for electric power generation, transmission and distribution for
industrial purpose. It is less costly to assemble three single-phase transformer to form a three-phase
transformer than to purchase one single three-phase transformer. The three-phase transformer
connection can be done by Star (Wye) and Delta (Mesh) type.
The connection of primary and secondary winding can be done by various combinations shown
below
Primary Winding
Secondary Winding
Star (Wye)
Star
Delta (mesh)
Delta
Star
Delta
Delta
Star
The combination of the primary winding and the secondary winding is done as star-star, deltadelta, star-delta and delta-star.
Related Terms:
1. Difference Between Power Transformer and Distribution Transformer
2. Difference Between Current Transformer (CT) & Potential Transformer (PT)
3. Potential Transformer (PT)
4. Construction of Potential Transformer
5. What is a Transformer
Harmonics in Three Phase Transformers
The harmonic is the distortion in the waveform of the voltage and current. It is the integral multiple
of some reference waves. The harmonic wave increases the core and copper loss of the transformer
and hence reduces their efficiency. It also increases the dielectric stress on the insulation of the
transformer.
In a three-phase transformer, the non-sinusoidal nature of magnetising current produces sinusoidal
flux which gives rise to the undesirable phenomenon. The phase magnetising currents in
transformer should contain third harmonics and higher harmonics necessary to produce a
sinusoidal flux.
If the phase voltage across each phase is to remain sinusoidal, then the phase magnetising currents
must be of the following form.
It is seen from equation (1),
(2), and (3) that the third harmonics in the three currents are co-phase, that is they have the same
phase. The fifth harmonics have different phases.
Delta Connection
Let the IAO, IBO and the ICO represent the phase magnetising current in a delta connection. The line
currents can be found by subtracting two phases current. For examples,
The third harmonic present in the phase magnetising current of three phase transformer is not
present in the line current. The third harmonic components are co-phase and hence cancel out in
the line. The third harmonic components are flows rounds the closed loop of the delta.
The delta connection only allows a sinusoidal flux and voltage with no third harmonic current in
the transmission line. For this reason majority of the 3-phase transformer has delta connected
windings and in places where it is not convenient to have an either primary or secondary connected
in delta, a tertiary winding is provided. The tertiary windings carry the circulating third harmonics
current required by the sinusoidal flux in each limb of the core.
In a delta connection, the voltage acting around the closed delta is,
This is a third harmonic voltage and it will circulate a third harmonic current round the closed loop
of the delta
Star connection
If IAO, IBO and ICO, represents the phase magnetising current in a star connection,
Where In is the current in the neutral wire.
The harmonics above the seventh be neglected. The equation (6) shows that under the balanced
condition the current flow in the neutral wire is the third harmonic current. The magnitude of the
third harmonics current is thrice the magnitude of each third phase current. The thirds harmonic
current produced inductive interference with communication circuit. If the supply to the star
connection is three wires, neutral current must be zero and therefore
Thus, it is seen that the three wire star connection suppresses the flow of harmonic and magnetising
currents. For a four wire star-connected system, the in phase third harmonic current flow in the
neutral wire.
Similarly, the third balance phase voltage containing harmonics can be written as
The equation (7), (8) and (9) shows that the third harmonics in the three phase voltage have the
same phase. The line voltage in a star connection can be obtained by subtracting two phase
voltages. For example
From equation (10) it is seen that the third harmonics is not present in the line to line voltage of a
star connection. This applies to all triplers harmonics.
Three-Phase Transformer Connections
The three phase transformer consists three transformers either separate or combined with one core.
The primary and secondary of the transformer can be independently connected either in star or
delta. There are four possible connections for a 3-phase transformer bank.
1.
2.
3.
4.
Δ – Δ (Delta – Delta) Connection
Υ – Υ (Star – Star) Connection
Δ – Υ (Delta – Star) Connection
Υ – Δ (Star – Delta ) Connection
The choice of connection of three phase transformer depends on the various factors likes the
availability of a neutral connection for grounding protection or load connections, insulation to
ground and voltage stress, availability of a path for the flow of third harmonics, etc. The various
types of connections are explained below in details.
1. Delta-Delta (Δ-Δ) Connection
The delta-delta connection of three identical single phase transformer is shown in the figure below.
The secondary winding a1a2 is corresponding to the primary winding A1A2, and they have the same
polarity. The polarity of the terminal a connecting a1 and c2 is same as that connecting A1 and C2.
The figure below shows the phasor diagram for lagging power factor cosφ.
The magnetising current and voltage drops in impedances have been neglected. Under the balanced
condition, the line current is √3 times the phase winding current. In this configuration, the
corresponding line and phase voltage are identical in magnitude on both primary and secondary
sides.
The secondary line-to-line voltage is in phase with the primary line-to-line voltage with a voltage
ratio equal to the turns ratio.
If the connection of the phase windings is reversed on either side, the phase difference of 180° is
obtained between the primary and the secondary system. Such a connection is known as an 180º
connection.
The delta-delta connection with 180º phase shift is shown in the figure below. The phasor diagram
of a three phase transformer shown that the secondary voltage is in phase opposition with the
primary voltage.
The delta-delta transformer has no phase shift associated with it and problems with unbalanced
loads or harmonics.
Advantages of delta–delta connection of transformer
The following are the advantages of the delta-delta configuration of transformers.
1. The delta-delta transformer is satisfactory for a balanced and unbalanced load.
2. If one transformer fails, the remaining two transformers will continue to supply the
three-phase power. This is called an open delta connection.
3. If third harmonics present, then it circulates in a closed path and therefore does not
appear in the output voltage wave.
The only disadvantage of the delta-delta connection is that there is no neutral. This connection is
useful when neither primary nor secondary requires a neutral and the voltage are low and moderate.
2. Star-Star (Υ-Υ) Connection of Transformer
The star-star connection of three identical single phase transformer on each of the primary and
secondary of the transformer is shown in the figure below. The phasor diagram is similar as in
delta-delta connection.
The phase current is equal to the line current, and they are in phase. The line voltage is three times
the phase voltage. There is a phase separation of 30º between the line and phase voltage.The 180º
phase shift between the primary and secondary of the transformer is shown in the figure above.
Problems Associated With Star-Star Connection
The star-star connection has two very serious problems. They are
1. The Y-Y connection is not satisfactory for the unbalance load in the absence of a
neutral connection. If the neutral is not provided, then the phase voltages become
severely unbalance when the load is unbalanced.
2. The Y-Y connection contains a third harmonics, and in balanced conditions, these
harmonics are equal in magnitude and phase with the magnetising current. Their sum
at the neutral of star connection is not zero, and hence it will distort the flux wave
which will produce a voltage having a harmonics in each of the transformers
The unbalanced and third harmonics problems of Y-Y connection can be solved by using the solid
ground of neutral and by providing tertiary windings.
3. Delta-Star (Δ-Υ) Connection
The ∆-Y connection of the three winding transformer is shown in the figure below. The primary
line voltage is equal to the secondary phase voltage. The relation between the secondary voltages
is VLS= √3 VPS.
The phasor diagram of the ∆-Y connection of the three phase transformer is shown in the figure
below. It is seen from the phasor diagram that the secondary phase voltage Van leads the primary
phase voltage VAN by 30°. Similarly, Vbn leads VBN by 30º and Vcn leads VCN by 30º.This
connection is also called +30º connection.
By reversing the connection on either side, the secondary system voltage can be made to lag the
primary system by 30°. Thus, the connection is called -30° connection.
4. Star-Delta (Υ-Δ) Connection
The star-delta connection of three phase transformer is shown in the figure above. The primary
line voltage is √3 times the primary phase voltage. The secondary line voltage is equal to the
secondary phase voltage. The voltage ratio of each phase is
Therefore line-to-line voltage ratio of Y-∆ connection is
The phasor diagram of the configuration is shown in the figure above. There is a phase shift of 30
lead exists between respective phase voltage. Similarly, 30° leads exist between respective phase
voltage. Thus the connection is called +30º connection.
The phase shows the star-delta connection of transformer for a phase shift of 30° lag. This
connection is called – 30° connection. This connection has no problem with the unbalanced
load and thirds harmonics. The delta connection provided balanced phase on the Y side and
provided a balanced path for the circulation of third harmonics without the use of the neutral wire.
Open delta or V-V Connection
If one transformer of delta-delta connection is damaged or accidentally opened, then the defective
transformer is removed, and the remaining transformer continues to work as a three phase bank.
The rating of the transformer bank is reduced to 58% of that of the actual bank. This is known as
the open delta or V-V delta. Thus, in open winding transformer, two transformers are used instead
of three for the 3-phase operation.
Let the Vab, Vbc and Vca be the voltage applied to the primary winding of the transformer. The
voltage induced in the transformer secondary or on winding one is Vab. The voltage induced on
the low voltage winding two is Vbc. There is no winding between points a and c. The voltage may
be found by applying KVL around a closed path made up of point a, b, and c. Thus,
Let,
Where Vp is the magnitude of the line on the primary side.
On substituting the value of Vab and Vbc in equation, we get
The Vca is equal in magnitude from the secondary terminal voltage and 120º apart in time from
both of them. The balanced three phase line voltage produced balanced 3-phase voltage on the
secondary side.
If the three transformers are connected in delta-delta configuration and are supplying rated load
and if the connection becomes V-V transformer, the current in each phase winding is increased by
√3 times. The full line current flows in each of the two phase windings of the transformer. Thus
the each transformer in the V-V system is overloaded by 73.2%.
It should be noticed that the load should be reduced by √3 times in case of an open delta connected
transformer. Otherwise, serious overheating and breakdown of the two transformers may take
place.
Scott-T Transformer Connection
The Scott-T Connection is the method of connecting two single phase transformer to perform the
3-phase to 2-phase conversion and vice-versa. The two transformers are connected electrically but
not magnetically. One of the transformers is called the main transformer, and the other is called
the auxiliary or teaser transformer.
The figure below shows the Scott-T transformer connection. The main transformer is centre tapped
at D and is connected to the line B and C of the 3-phase side. It has primary BC and secondary
a1a2. The teaser transformer is connected to the line terminal A and the centre tapping D. It has
primary AD and the secondary b1b2
The identical, interchangeable transformers are used for Scott-T connection in which each
transformer has a primary winding of Tp turns and is provided with tapping at 0.289Tp , 0.5Tpand
0.866 Tp.
Phasor Diagram of Scott Connection Transformer
The line voltages of the 3-phase system VAB, VBC, and VCA which are balanced are shown in the
figure below. The same voltage is shown as a closed equilateral triangle. The figure below shows
the primary windings of the main and the teaser transformer.
The D divides the primary BC of the main transformers into two halves and hence the number of
turns in portion BD = the number of turns in portion DC = Tp/2.The voltage VBD and VDC are
equal, and they are in phase with VBC.
The voltage between A and D is
The teaser transformer has the primary voltage rating that is √3/2 or 0.866 of the voltage ratings
of the main transformer. Voltage VAD is applied to the primary of the teaser transformer and
therefore the secondary of the voltage V2t of the teaser transformer will lead the secondary terminal
voltage V2m of the main transformer by 90º as shown in the figure below.
Then,
For keeping the voltage per turn same in the primary of the main transformer and the primary of
the teaser transformer, the number of turns in the primary of the teaser transformer should be equal
to √3/2Tp.
Thus, the secondaries of both transformers should have equal voltage ratings. The V2t and V2m are
equal in magnitude and 90º apart in time; they result in the balanced 2-phase system.
Position of Neutral Point N
The primary of the two transformers may have a four wire connection to a 3-phase supply if the
tapping N is provided on the primary of the teaser transformer such that
The voltage across AN = VAN = phase voltage = Vl/√3.
Since the voltage across the portion AD.
the voltage across the portion ND
The same voltage turn in portion AN, ND and AD are shown by the equations,
The equation above shows that the neutral point N divides the primary of the teaser transformer in
ratio.
AN : ND = 2 : 1
Applications of Scott Connection:
The following are the applications of the Scott-T connection.
1. The Scott-T connection is used in an electric furnace installation where it is desired
to operate two single-phase together and draw the balanced load from the three-phase
supply.
2. It is used to supply the single phase loads such as electric train which are so scheduled
as to keep the load on the three phase system as nearly as possible.
3. The Scott-T connection is used to link a 3-phase system with a two–phase system with
the flow of power in either direction.
The Scott-T connection permits conversions of a 3-phase system to a two-phase system and vice
versa. But since 2-phase generators are not available, the converters from two phases to three
phases are not used in practice.
Grounding Transformer
The grounding transformer is used to provide a path to an ungrounded system or when the system
neutral is not available for some reason, for example, when a system is delta connected. It provides
a low impedance path to the neutral and also limits the transient overvoltage when the ground
faults occur in the system. The grounding of the system can be done in the following way
•
By using a delta-star grounding transformer.
• By using a zig-zag grounding transformer
Delta-Star Grounding Transformer
In a case of delta-star grounding transformer, the delta side is closed to provide a path for zerosequence current. The star winding must be of the same voltage rating as the circuit that is to
be grounded, whereas the delta voltage rating can be chosen to be any standard voltage level.
The selection of the type of grounding depends on the type of the system and its voltage levels.
The following considerations are made for the selection of the grounding
•
•
•
Transient overvoltage developed.
The Magnitude of ground-fault current as a percentage of 3-phase fault current.
Dip in line voltage due to fault conditions.
Generally, solid grounding is used for a low-voltage system up to 600V. For voltages up to 11KV
resistance grounding is used.
Difference Between Power and Distribution Transformer:
BASIS
OF
POWER TRANSFORMER
DIFFERENCE
DISTRIBUTION
TRANSFORMER
Type of network
It is used in transmission It is used in the distribution
network of higher voltages
network for lower voltages.
Availability
ratings
400 kV, 200 kV, 110 kV , 66
kV, 33 kV.
of
11 Kv, 6.6 Kv, 3.3 Kv, 440
V,230 V
BASIS
OF
POWER TRANSFORMER
DIFFERENCE
DISTRIBUTION
TRANSFORMER
Maximum rating
of usage
Power transformers are used
for rating above 200 MVA
Distribution transformers are
used for rating less than 200
MVA
Size
Larger in size as compared of
distribution transformers
Smaller in size
Designed
Efficiency
Designed
for
maximum
efficiency of 100%
Designed for 50-70% efficiency
Efficiency
formula
Efficiency is measured as the Here All Day Efficiency is
ratio of output to the input considered. It is the ratio of
power
output in kilowatt hour (kWh)
or watt hour (Wh) to the input
in kWh or Wh of a transformer
over 24 hours.
Application
Used in generating stations
and transmission substations
Used in distribution stations,
also for industrial and domestic
purposes
Losses
Copper and iron losses take
place throughout the day
Iron losses take place for 24
hours and copper losses are
based on load cycle
Load fluctuation
In power transformer the load
fluctuations are very less
Load fluctuations are very high
Operating
condition
Always operated at full load
Operated at load less than full
load as load cycle fluctuates
Considering time
It is independent of time
It is time dependent
BASIS
OF
POWER TRANSFORMER
DIFFERENCE
DISTRIBUTION
TRANSFORMER
Flux density
In power transformer flux
density is higher
As compared to power
transformer the flux density is
lower
in
distribution
transformer
Designing of the
core
Designed to utilize the core for
maximum and will operate
near to the saturation point of
the B-H curve, which helps to
bring down the mass of core
As compared to power
transformer the flux density is
lower
in
distribution
transformer
Usage
Used to step up and step down
voltages
Used as an
connectivity
end
user
Parallel Operation of a Transformer
The Transformer is said to be in Parallel Operation when its primary winding is connected to a
common voltage supply, and the secondary winding is connected to a common load.
The connection diagram of the parallel operation of a transformer is shown in the figure below.
The parallel operation of a transformer has some advantages likes it increases the efficiency of the
system, makes the system more flexible and reliable. But it increases the short-circuit current of
the transformers.
Reasons For Parallel Operation
Parallel operation of a transformer is necessary because of the following reasons are given below:
•
•
•
•
•
It is impractical and uneconomical to have a single large transformer for heavy and
large loads. Hence, it will be a wise decision to connect a number of transformers in
parallel.
In substations, the total load required may be supplied by an appropriate number of
the transformer of standard size. As a result, this reduces the spare capacity of the
substation.
If the transformers are connected in parallel, so there will be scope in future, for
expansion of a substation to supply a load beyond the capacity of the transformer
already installed.
If there will be any breakdown of a transformer in a system of transformers connected
in parallel, there will be no interruption of power supply, for essential services.
If any of the transformer from the system is taken out of service for its maintenance
and inspection, the continuity of the supply will not get disturbed.
Necessary Conditions For Parallel Operation
For the satisfactory parallel operation of the transformer, the two main conditions are
necessary. One is that the Polarities of the transformers must be the same. Another
condition is that the Turn Ratio of the transformer should be equal.
The other two desirable conditions are as follows:
•
•
The voltage at full load across the transformer internal impedance should be equal.
The ratio of their winding resistances to reactances should be equal for both the
transformers. This condition ensures that both transformers operate at the same power
factor, thus sharing their active power and reactive volt-amperes according to their
ratings.
Parallel Operation of a Single Phase Transformer
Parallel Operation of a Single Phase Transformer means that the two or more
transformers having the same polarities, same turn ratios, same phase sequence and
the same voltage ratio are connected in parallel with each other.
The circuit diagram of two single-phase transformer A and B connected in parallel
are shown below:
Let,
a1 is the turn ratio of the transformer A
a2 is the turn ratio of transformer B
ZA is the equivalent impedance of the transformer A referred to secondary
ZB is the equivalent impedance of the transformer B referred to secondary
ZL is the load impedance across the secondary
IA is the current supplied to the load by the secondary of the transformer A
IB is the current supplied to the load by the secondary of the transformer B
VL is the secondary load voltage
IL is the load current
Applying Kirchhoff’s Current Law,
By Kirchhoff’s Voltage Law,
Now putting the value of IB from the equation (1) in equation (3) we will get,
Solving equations (2) and (4) we will get,
The current IA and IB have two components. The first component represents the
transformers share of the load currents and the second component is a circulating
current in the secondary windings of the single-phase transformer.
The undesirable effects of the circulating currents are as follows
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They increase the copper loss.
The circulating current overloads the one transformer and reduce the
permissible load kVA.
Equal Voltage Ratio
In order to eliminate circulating currents, the voltage ratios must be identical. That
is a1=a2
Under this condition,
Equating equation (7) and (8) we will get:
From the above equation (9), it is clear that the transformer currents are inversely
proportional to the transformer impedance. Thus, for the efficient parallel operation
of the two single-phase transformers, the potential differences at full load across the
transformer internal impedance should be equal.
This condition ensures that the load sharing between the two single-phase
transformer is according to the rating of each transformer. If the per-unit equivalent
impedance are not equal, then the transformer will not share the load in proportion
to their kVA ratings. As a result, the overall rating of the transformer bank will be
reduced.
Equation (9) can also be written as
The current in the equations (7) and (8) is changed into volt-amperes by multiplying
the two equations by the common load voltage VL.
Therefore, we know that,
The total load in volt-ampere (VA) is
The volt-ampere of transformer A is
Similarly, the volt-ampere of transformer B is
Hence, the various equations will be written as shown below
Equating the equation (11) and (12) we will get
Equation (13) tells that the volt-ampere load on each single-phase transformer is
inversely proportional to its impedance.
Hence, to share the load in proportion to their ratings, the transformers should have
the impedance which is inversely proportional to their ratings.
Cooling of Transformer and Methods of Cooling
Cooling of Transformer is the process by which heat generated in the transformer is dissipated
or treated to the safe value. This is achieved by various cooling methods of transformer available.
The major factor for the generation of heat in the transformer is the various losses like hysteresis,
eddy current, iron, and copper loss. Among all the various losses the major contributor of the heat
generation is the copper loss or I2R loss.
If the temperature of the transformer will continue to increase rapidly, it will result in the
degradation of the insulation used in the transformer resulting in the damaging of the various parts
and hence the failure of the transformer. Thus, proper removal or treatment of heat is necessary
for the efficient working, longer life and higher efficiency of the transformer.
The various coolants used for the cooling purpose of the transformer are air, synthetic oils, mineral
oils, gas, water.
Basically, there are two types of transformer one is the dry type, and another one is oilimmersed type. For the cooling of transformers, the following cooling methods listed below are
used.
1.
2.
3.
4.
5.
6.
7.
Air Natural
Air Blast or forced
Oil Natural Air Natural
Oil Natural Air Forced
Oil Forced Air Forced
Oil Natural Water Forced
Oil Forced Water Forced
Methods of Cooling of Transformer
The detailed description of cooling methods, one by one is given below.
Dry Type Transformer is cooled by the following two methods given below:
Air Natural (AN)
By Air Natural method the generated heat in the transformer is cooled by the
circulation of natural air. When the temperature of the transformer becomes higher as
compared to the temperature of the surrounding air, thus by the process of natural
convection, heated air is replaced by the cool air. This method is also known as a selfcooled method. This method is used for cooling the smaller output transformer rating
that is up to 1.5 MVA.
Air Forced (AF) or Air Blast
In this method, the heat generated is cooled by the forced air circulation method.
With the help of fans and blowers, high velocity of air is forced on the core and the
windings of the transformer. As the temperature inside the transformer goes beyond
the standard safe level, an alarm is activated, and the fans and blowers are switched
ON automatically. This method is used for transformer rating up to 15MVA.
Oil-immersed type transformer is cooled by the oil-air cooling method and oilwater cooling method.
Oil Natural Air Natural (ONAN)
Natural convection process is used for this type of cooling. The assembly of the core
and windings are placed in the oil-immersed tank. As the core and the windings heat
up the temperature of the oil in the transformer rises. As a result, the oil moves upward
and flows from the upper portion of the transformer tank. This hot oil dissipates heat
in the air by natural convection and conduction process, the oil gets cooled by the
circulation of natural air and passes through the radiator again for the use of the
transformer. This type of cooling is used for the transformer rating up to 30 MVA.
Fig. Oil Natural Air Natural Cooling of Transformer
Oil Natural Air Forced (ONAF)
ONAF method is used for the cooling of the transformer of rating up to 60 Mega volts
ampere. As discussed above that in ONAN method, the dissipation of heat is taking
place by the convection process in which air is naturally circulated to cool down, but
in this type, the forced air is used for the purpose of cooling the transformer.
The cooling of oil will be faster if the area of the tank of the transformer is increased
finally, which result in the increase in heat dissipation level. As the fans and blowers
are installed, a high velocity of air is forcefully applied to the radiator and cooling
towers which will help in cooling oil more quickly and efficiently.
Its cost is higher as compared to another process where the circulation of oil and air
is done naturally because a fan and blowers are attached as extra cooling equipment,
in this method.
Fig. Oil Natural Air Forced Cooling of Transformer
Oil Forced Air Forced (OFAF)
As the name itself says that both the oil and the air are applied by force for cooling of
a transformer. The Heat Exchanger is installed through which hot oil is circulated with
the help of a pump. Air is forced to pass on the heat exchanger with the help of highspeed fans.
This method is similar to ONAN, as when there is low load on the transformer the
cooling is done by a simple ONAN method. However, as soon as the load is increased,
the generated heat will also be more and therefore the sensor gives an alarm that the
dissipation of heat has exceeded the safe value and as a result, the fans and pumps are
switched on automatically. Thus, the cooling takes place by OFAF method.
Oil Forced Air Forced Cooling of Transformer
Oil Natural Water Forced (ONWF)
In Oil Natural Water Force cooling method, the transformer core and the windings are
immersed in the oil tank. A radiator is installed outside the tank, as the temperature
rises and the oil heats up and moves upward, the heat is dissipated by the natural
process of convection and oil is passed through the radiator, but the water is pumped
and passed through the heat exchanger for cooling of the oil.
Oil Forced Water Forced (OFWF)
A heat exchanger is installed through which both oil and water are passed with the
help of a pump. The level and pressure of the oil are always kept higher than that of
water so that if any leakage occurs in the system the oil mixes with the water, but
water does not get mixed up with the oil.
This type of method is suitable for large capacity of the transformer having rating as
several hundred MVA or where banks of transformers are installed. Mainly this type
of cooling is done for the transformer installed at the hydropower plant.
Fig. Oil Forced Water Forced Cooling of Transformer