The document discusses various transformer protections, including oil temperature indicators (OTI), winding temperature indicators (WTI), Buchholz relays, surge protection relays, pressure relief valves, differential protection, and restricted earth fault protection. These protections monitor factors like temperature, pressure, fluid levels, and current ratios to detect faults and protect the transformer. When they detect issues, they will signal alarms or trip the transformer out of service by opening a circuit breaker.

1. TRANSFORMER
PROTECTIONS
TRANSFORMER PROTECTION SYSTEM
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2. TRANSFORMER PROTECTIONS
What is a Power Transformer?
Power Transformers are the major equipment in the power
system network.
As we know the Transformers are used to deliver the power to
the consumer without changing its power and frequency.
The transformer has numerous kinds of protection in order to
maintain the transformer healthy at all the conditions of the power system
network.
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3. List of Major Transformer Protection:
 OTI
 WTI
 Buchholz Protection
 Surge Protection
 Pressure Relief Valve Protection
 Differential Protection
 Restricted Earth Fault Protection
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TRANSFORMER PROTECTIONS

4. TRANSFORMER PROTECTIONS
OTI:
OTI is abbreviated as Oil Temperature Indicator.
OTI is used to measure the temperature of the Oil inside the
Transformer Main Tank.
OTI has the temperature indicator connected with Thermostat
along with the probe. This thermostat is mounted at the top of the
transformer main tank.
Since hot oil will be at top of the tank, it is mounted anywhere
at the topmost part of the tank.
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5. TRANSFORMER PROTECTIONS
OTI:
Normal OTI indicator will have lobe like structure, inside which
the mercury is filled along with the contacts. It is called a mercury
switch.
This switch is fixed in a circular disk along with a marking scale
in it. Whenever the temperature increases the mercury starts melting
and makes the disk to rotate indicating the temperature.
By rotating, the contacts inside the mercury switch/lobe get
changed from NO/NC position giving signals to the cooling system.
This contacts of OTI is used to control the Air forced/oil forced
cooling system in the transformer.
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6. TRANSFORMER PROTECTIONS
WTI:
WTI is called a Winding Temperature Indicator.
WTI indicator is like an OTI indicator. But here the only change is
the Thermostat.
In WTI, thermostat can’t be used in the winding to measure the
winding temperature.
Instead of using the Thermostat, a rheostat is being used. This
rheostat, having some standard resistance is connected to the control
circuit.
Based on the level of temperature near the winding area, the
resistance of the device will vary. This value is converted into
temperature using a separate Current conversion unit and a transducer
unit.
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7. TRANSFORMER PROTECTIONS
WTI:
The Output from the WTI indicator unit will be utilized for
activating the Cooling unit.
The digital output from the CCU unit will be given to control
devices.
For both OTI and WTI, Alarm temperature and Trip temperature
can be adjusted manually based on the user's needs.
When the temperature reaches the fixed alarm state, the
device will give an alarm indication.
Likewise, if the temperature goes beyond the fixed value of trip
temperature, the command will be initiated by the protection system to
the transformer connected breaker to isolate the transformer from the
grid.
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8. OTI &
WTI
Image of OTI
and WTI
Indicators
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9. TRANSFORMER PROTECTIONS
Buchholz Relay:
Buchholz relay is an internal protection device of a
power transformer.
These relays are used to protect the transformer from internal
faults.
These relays are very fast and sense the internal fault against
the transformer.
Buchholz relay is fixed in-between the Transformer Main tank
and the Conservator Tank.
Buchholz relay has two floats inside it. These floats are used to
sense the internal faults in a transformer.
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10. TRANSFORMER PROTECTIONS
Buchholz Relay Operation:
For example, there is a sudden rise in the temperature
inside the transformer due to some spark or any other burning inside the
tank.
Due to this, there will be the formation of some unwanted gases
inside the tank. These unwanted gases are collected at the top of the
Buchholz relay which is installed at the topmost point of the transformer
main tank.
This results in the activation of the Buchholz relay giving alarm or
trip signals to the protection system.
DGA sample collection device is also installed with the Buchholz
relay point.
Buchholz relay will mal-operate if the level of oil inside the Relay
changes, which disturbs the position of floats under normal condition.
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11. TRANSFORMER PROTECTIONS
Surge Protection:
A surge Protection relay is used in the transformer that
is equipped with OLTC’s.
Surge relay is installed between the OLTC tank and the OLTC
conservator tank.
Each phase of the transformer will have a separate OLTC tank.
Its working is the same as that of a Buchholz relay, but it has only one
float.
It is used to monitor the level of oil in the OLTC’s conservator
tank and how force the oil enters and leaves the chamber of the OLTC
tank.
Surge relay will not operate during the normal transformer tap
changing process. It will operate only when surge develops inside the
OLTC tank.
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12. TRANSFORMER PROTECTIONS
Pressure Relief Valve Relay:
PRD called a Pressure relief device installed in the
transformer main tank.
PRD is used to relieve the high pressure inside the transformer
tank during a fault condition.
PRD has a spring-loaded diaphragm, which is designed to
release a huge amount of pressure rapidly under abnormal conditions.
Based on the amount of oil, and the rating of the transformer,
the number of PRD’s mounted on the transformer tank will differ.
The operation of PRD will trip the transformer/ reactor
considering a sudden rise in temperature and pressure as a fault
occurrence.
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13. TRANSFORMER PROTECTIONS
Differential Protection:
Differential protection is one of the important
protection in the power transformer.
Differential protections are employed in order to measure the
difference in current between the primary and secondary of the power
transformer and take necessary action against the conditions.
Differential protection schemes of any transformer, based on
its conversion ratio, will have a current transformation ratio. Any
difference in this ratio/values can be sensed with the help of neutral CT.
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14. TRANSFORMER PROTECTIONS
Differential Protection:
Under normal conditions, changes in the ratio will be
minimum and current flowing through the Neutral CT will be negligible.
If Fault in any one of the phases, the return current will be
flowing through the Neutral CT measuring fault current.
During this condition, the ratio of current will not be the same
and hence the relay operates.
This protection is main for transformer, reactor, and bus bar
protection.
For this protection in the transformer’s, bay CT’s will be used
based on the electrical scheme.
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15. TRANSFORMER PROTECTIONS
Restricted Earth Fault Protection:
This relay is used in the transformer for
finding the internal earth faults.
This is the same as that of Differential protection, but it is for
sensing earth fault.
Each transformer will have 6+1 CT mounted in the turret and
connected with bushing lead.
This relay is connected along with the neutral CT. The common
phases of Individual phase Ct will be connected to the secondary of NCT
is such a way that secondary unbalanced phase current of phase CT’s
will opposes the secondary current of NCT.
Under normal conditions, both these currents will nearly be the
same and relay will not respond. Under abnormal conditions, the value
will change and the relay operates.
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16. TRANSFORMER PROTECTIONS
Restricted Earth Fault Protection:
In-order to nullify the phase shift at bushings
of the power transformer (Primary will be Delta Connected and
Secondary will be Star connected with Solid Grounding), the CT's are
connected Star side with Delta and vice-versa.
By connecting the Delta side(Primary side) CT’s as Star
connected, 30degree electrical phase shift can be nullified.
This method was employed in olden days while using
Electromagnetic relays.
Nowadays, numerical relays are equipped and it's not necessary
to physically do the wiring as mentioned above. All the numerical relays
have an in-built algorithm to sense the differences.
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