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A transformer is a static electrical device used in
electric power systems to transfer power between circuits
through the use of electromagnetic induction.
Transformers are devices that transfer energy from one
circuit to another by means of a common magneticﬁeld.
When an alternating current ﬂows in a conductor,a
magnetic ﬁeld exists around the conductor.If another
conductor is placed in the ﬁeld created by the ﬁrst
conductor such that the ﬂux lines link the second
conductor,then a voltage is induced into the second
conductor.The use of a magnetic ﬁeld from one coil to
induce a voltage into a second coil is the principle on
which transformer theory and application is based.
Transformers range in size from thumbnail-sized used in
microphones to units weighing hundreds of tons
interconnecting the power grid.A wide range of
transformer designs are used in electronic and electric
power applications. Transformers are essential for the
transmission,distributionandutilization of electrical
The transformer is based on two principles:
1. An electric current can produce a magnetic field.
2. A changing magnetic field within a coil of wire induces
a voltage across the ends of the coil (electromagnetic
Referring to the figure here, current passing through the
primary coil creates a magnetic field. The primary and
secondary coils are wrapped around a core of very
high magnetic permeability, so that most of the magnetic
flux passes through both the primary and secondary coils.
Any secondary winding connected load causes current
and voltage induction from primary to secondary circuits
in indicated directions.
The term power transformer is used to refer to those
transformers used in the generatorand the distribution
circuits, and these are usually rated at 500 KVA and
above. Power systems typicallyconsist of a large number
of generation locations, distribution points, and
interconnections within thesystem or with nearby
systems, such as a neighboring utility. The complexity of
the system leads to avariety of transmission and
distribution voltages. Power transformers must be used at
each of these pointswhere there is a transition between
Fig.An example of a power transformer used in electric power system
CAUSE OF FAULTS IN POWER TRANSFORMER
Transformers are prone to variety of faults :
1. The most common type of fault being the winding to
core faults because of weakening of insulation.Phase
faults inside the transformers are rare.However,such faults
may occur on terminals,which fall within the transformer
2. Power transformers are generally provided with on-line
tap changing (OLTC) gear.This is another major area of
occurrence of fault.
3. All large transformers are oil immersed type.There is a
possibility of oil leakage.
4. Transformers experience large inrush currents that are
rich in harmonic content at the time of switching if they
happen to be unloaded.
5. A transformer may develop inter turn faults giving rise
to local hot spots within the winding.
6. Transformers may suffer from over fluxing due to
under frequency operation at rated voltage.Over fluxing
may also be caused when the transformer is subjected to
over voltage at the rated frequency.
7. In case of sustained overload conditions,the
transformer should not be allowed to operate for long
PROTECTION OF POWER TRANSFORMERS
(A) PERCENTAGE DIFFERENTIAL PROTECTION
This scheme is employed for the protection of
transformers against internal short circuits. It provides the
best overall protection for internal faults.However in case
of ungroundedor high impedance grounding it cannot
provide ground fault protection.
The following factors affect the differential current in
transformers and should be consideredwhile applying
differential protection. These factors can result in a
differential current even underbalanced power in & out
1.Magnetizing inrush current– The normal magnetizing
current drawn is 2–5% of therated current. However
during Magnetizing inrush the current can be as high as
8–30times the rated current for typically 10 cycles,
depending upon the transformer andsystem resistance.
2. Overexcitation–This is normally of concern in
generator–transformer units.Transformers are typically
designed to operate just below the flux saturation level.
Anyfurther increase from the max permissible voltage
level (or Voltage/Frequency ratio), could lead to
saturation of the core, in turn leading to substantial
increase in theexcitation current drawn by the
3. CT Saturation – External fault currents can lead to CT
saturation. This can cause relayoperating current to flow
due to distortion of the saturated CT current.
4. Different primary and secondary voltage levels, that is
the primary & secondary CT’s areof different types and
5. Phase displacement in Delta-Wye transformers.
Transformer Differential Relay
To account for the above variables less sensitive
Percentage Differential Relays withpercentage
characteristics in the range of 15 to 60% are applied to
transformers. Additionally, inmodern microprocessor and
numeric relays harmonic restraints can be applied.
Transformer Differential RelayConnections:
Fig. Percentage Differential Relay Connections
The percentage differential scheme tends to maloperate
due to magnetizing inrush.The inrush current waveform is
rich in harmonics whereas the internal fault current
consists of only the fundamental component. So to solve
the problem of inrush current, which is neither an
abnormal condition nor a fault, additional restraint is
developed which comes to picture only during inrush
condition and is ineffective during internal faults.
(B) RESTRICTED EARTH FAULT PROTECTION :
A percentage differential relay has a certain minimum
value of pick up for internal faults.Faults with current
below this value are not detected by the relay.
Winding-to-core faults, which are single phase to
ground type, involving high resistance,fall in this
Therefore for such type of faults RESTRICTED EARTH
FAULT PROTECTION is used.The reach of such a
protection must be restricted to the winding of the
transformer; otherwise it may operate for any ground
fault, anywhere in the system, beyond the transformer,
hence the name of this scheme.
EARTH FAULT PROTECTION FOR THE DELTA
SIDE OF DELTA STAR TRANSFORMER :
(C) OVER CURRENT PROTECTION :
Over current protection is used for the purpose of
providing back up protection for large transformers.
(above 5MVA).Two phase fault and one ground fault
relay is sufficient to provide OC protection to star delta
(D) PROTECTION AGAINST OVERFLUXING :
The magnetic flux increases when voltage increases. This
results in increased iron loss and magnetizing current. The
core and core bolts gets heated and the lamination
insulation is affected. Protection against overfluxing is
required where overfluxing due to sustained overvoltage
can occur.The reduction in frequency also increases the
flux density and thus has the same effect of overfluxing.
The expression for flux ina transformer is given by
Φ = K E/f
Where Φ = flux, f = frequency, E = applied voltage and K
is a constant.
To control flux, the ratio E/ f is controlled. When the ratio
exceeds a threshold value, it has to be detected. Electronic
circuits with suitable relays are available to measure this
ratio. Overfluxing does not require high speed tripping
and hence instantaneous operation is undesirable when
momentary disturbances occur. But the transformer
should be isolated in one or two minutes at the most if
(E) PROTECTION AGAINST OVERHEATING :
The rating of a transformer depends on the temperature
rise above an assumed maximum ambient temperature.
Sustained overload is not allowed if the ambient
temperature is equal to the assumed ambient temperature.
The maximum safe overloading is that which doesnot
overheat the winding.The maximum allowed temperature
is about 95°C. Thus the protection against overload
depends on the winding temperature which is usually
measured by thermal image technique.
In thermal image technique,a temperature sensing device
like silicon resistor is placed in the transformer oil near
the top of the transformer tank. A CT is employed on the
L.V. side to supply current to a small heater. Both the
temperature sensing device and the heater are placed in a
small pocket. The silistor is used as an arm of a resistance
bridge supplied from the stabilized dc source. An
indicating instrument is energized from the out of balance
voltage of the bridge.Also the voltage across the silistor is
applied to a static control circuit which controls cooling
pumps and fans,gives warning of overheating and
ultimately trips the transformer circuit breakers.
(F) PROTECTION AGAINST INCIPIENT FAULTS:
INCIPIENT FAULTS: Faults which are not serious at the
beginning but which slowly develops into serious faults
are known as incipient faults.
It is a gas actuated relay. When a fault develops slowly,it
produces heat, thereby decomposing solid or liquid
insulating material in the transformer. The decomposition
of the insulating material produces inflammable gases.
The Buchholz relay gives an alarm when a specified
amount of gas is formed. The analysis of the gas collected
in the relay chamber indicates the type of the incipient
There is a chamber to accommodate Buchholz relay,in
between the transformer tank and the conservator.The
Buchholz relay is a slow acting device,the minimum
operating time is 0.1 s and the average time is 0.2 s. Too
sensitive settings of the mercury contacts is undesirable
because they are subjected to false operation on shock and
vibration caused by conditions like mechanical shock to
the pipe,tap changer operation and heavy external faults.
Working : When an incipient fault such as a winding-to-
core fault or an inter-turn fault occurs on the transformer
winding, there is severe heating of the oil. This causes
gasesto be liberated from the oil around 350°C. There is a
build-up of oil pressure causing oilto rush into the
conservator. A vane is placed in the path of surge of oil
between thetransformer and the conservator. Aset of
contacts, operated by this vane, is used as tripcontacts of
the Buchholz relay This output of Buchholz relay may be
used to trip thetransformer.
The Buchholz relay also has another set of contacts
operated by a float. Thesecontacts stay open when the
transformer tank is filled with oil. However, in case
ofleakage of oil or decomposition of oil, the float sinks
causing the contacts to close. Lossof oil will no doubt
cause the transformer temperature to rise but does not
warrant immediate tripping. Hence, normally these
contacts are wired to an alarm which alertsthe operator.
GAS ANALYSIS :
The trapped gases in the conservator can give valuable
clue to the type of damage thattakes place inside the
transformer. This is because the insulation between the
windingturns, the insulation between the stampings of the
core and the oil, all liberate specificgases when they get
heated up due to a fault. The presence of these gases can
be usedas a signature of a particular type of damage that
may have taken place inside thetransformer.
PRESSURE RELIEF VALVE : An oil pressure
relief valve is fitted at the top of the transformer tank. It is
a spring controlled valve located at the end of an oil relief
pipe protruding from the top of the tank. Whenever a
surge in the oil is developed, it bursts , thereby allowing
the oil to discharge rapidly. It operates when the pressure
exceeds 10 psi but closes automatically when the pressure
falls below the critical level.This avoids the explosive
rupture of the tank and the risk of fire.
(G) PROTECTION AGAINST FIRE :
Power transformers are subject to fires from many
sources. Theyoften occur because of deterioration of
insulation in the transformer.This produces arcing which
in turn overheats the insulating oil andcauses the tanks to
rupture; further arcing then will start a fire. Firesare also
initiated by lightning and occasionally by dirty insulators
onthe outside of the tanks.
Proper maintenance can reduce these risks. Careful
protection againstfaults by shielding, grounding, lightning
arresters, interrupting devicesand relays can also decrease
the opportunity for a destructive fire.
In spite of protection by these measures and expert
maintenance, therisk of fire remains quite high, and a fire
protection system is alwaysrecommended and often
required. In addition, suppression systemsare frequently
Protection of a power transformer against fire
(H) PROTECTION AGAINST LIGHTNING:
Lightning overvoltage surges originate from atmospheric
discharges and they can reach their peak within a few
microseconds and subsequently decay very rapidly. The
surge voltage can reach up to 10 times the rated
transformer voltage and they pose the greatest threat to
transformers on the distribution networks. The charge
from the surge produces both short duration high current
impulse and long duration continuing current impulse
which affects the transformer insulation system.
Protection against such overvoltage surges can be
achieved by using Lightning Arresters.The distance
between the lightning (surge) arrester and the equipment
to be protected should be as short and straight as
possible. Therefore, integration of the surge arrester in
the equipment to be protected gives almost an ideal
Fig. Lightning Arrester used to protect the
powertransformer against lightning surges.