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7/23/2019 Presentation on Power Transformer & Distribution Transformer Protection
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Presentation on Power Transformer &
Distribution Transformer Protection
Presented by-
Ch. Alamgir Hossain
Deputy General Manager
System Protection & Metering CirclePGCB,Dhaka
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Protection of Transformer
Requirements of Transformer Protection-
-To protect the equipment form external and internal faults
Type of Transformer Faults:A: Internal Faults
1.Earth Fault
2.Phase to Phase Fault
3.Inter-turn Fault
4.Core Fault5.Tank Fault- ex. Loss of Oil
6.Slug Formation
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External Faults:
1. Over Loading Causes I2
R losses2. System Faults- Causes Mechanical stress to transformer
3. Over Voltage-
- Transient Over Voltage- LAs are provided
- System O/V- Causes Over Flexing which increases Iran
losses and damage insulation.
Protection of Transformer: cont.
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Protection of Transformer: cont.
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Protection of Transformer: cont.
Inrush Current A transformer steel core's retains a static magnetic field when power is removed.
When power is then reapplied, the residual field will cause a high inrush
current until the effect of the remnant magnetism is reduced, usually after a
few cycles of the applied alternating current. Transformer protection devices must be selected to allow this harmless inrush to
pass.
When a transformer is energized, the magnetizing inrush currents are estimated as
multiples up to 08 to 10 times of the transformer's rated current.
Factors that affect the Inrush Current
The source impedance
The size of the transformer
The point of wave when the switch closes
The magnetic properties of the core
The remanence magnetism of the core
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Protection of Transformer: cont. Protections provided by external protective relays:
Transformer Differential Protection (87T)
Restricted Earth Fault (REF) Protection (87N)Time Delayed Over Current & Earth Fault Protection (51/51N)
Instantaneous Over Current & Earth Fault Protection (50/50N)
Thermal Overload Protection (49)
Over Fluxing Protection (24)
Mechanical/Self Protection (Provided within the transformer ):
Main Tank Buchholz Protection
OLTC Buchholz Protection
Pressure Relief Device (PRD) Protection
Winding Temperature ProtectionOil Temperature Protection
Oil Level Alarm
Main Tank Buchholz Alarm
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Protection of Transformer: cont.
Conventional Protection Scheme of a Two WindingTransformer
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Protection of Transformer: cont.
Conventional Protection Scheme of a Two Winding TransformerIncluding REF
87N
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Protection of Transformer: cont.
Conventional Protection Scheme of a Three Winding Transformer
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Protection of Transformer: cont.
Conventional Protection Scheme of an Auto-Transformer
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Protection of Transformer: cont.
Grouping of protection:
As transformer protections are not duplicated. So that
protection functions are divided in two groups to obtain some
redundancy.
1. Group A Protection
2. Group B protection
The group A and group B protection are connected to separateDC source.
All the Group-A and Group-B protection functions energize
separate lockout relays (86-1 & 86-2) respectively to trip the
circuit breaker during fault.
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Protection of Transformer: cont.
Generally Group-A protection consists of-
Differential Protection (87T)
Time Delayed Over Current & Earth Fault Protection (51/51N)_HV
Instantaneous Over Current & Earth Fault Protection (50/50N)_HV
Thermal Over Load Protection (49)
Main Tank Buchholz Relay Protection
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Protection of Transformer: cont.
Generally Group-B protection consists of-
Restricted Earth Fault Protection (87N)
Time Delayed Over Current & Earth Fault Protection (51/51N)_LV
Instantaneous Over Current & Earth Fault Protection (50/50N)_LV
OLTC Buchholz Relay Protection
Pressure Relief Device (PRD) Protection
Winding Temperature Protection
Oil Temperature Protection
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Protection of Transformer: cont.
Transformer Differential Protection (87T)Basics:
Transformer differential protection is a unit
scheme that compares the current on the primary
side of a transformer with that of the secondary
side.
If any difference in HV & LV currents exists
(beyond the setting value) it is assumed that the
transformer has developed a fault and the relayinstantaneously trips the relevant circuit
breakers.
The principle of operation is made possible by
virtue of the fact that large transformers are very
efficient and hence under normal operation
power-in equals power-out.
Transformer differential protection detects faults
within the differential protected zone, including
inter-turn short circuits. Fig: Zone of Differential Protection
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Protection of Transformer: cont.
Transformer Differential Protection (87T)
Principle of Operation:
The operating principle employed by transformer differential protection is the Merz-
Price circulating current system as shown in the figure.Under normal conditions I1and I2 are made equal and opposite such that the resultant
current through the relay is zero.
An internal fault produces an unbalance or 'spill' current that is detected by the relay,
leading to operation to isolate the fault.
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Protection of Transformer: cont.
Transformer Differential Protection (87T)
Transformer Differential
Protection (87T) Relay
Types:
High Impedance Type
Low Impedance Type
In a low impedance
protection scheme, thedifferential protection can
have the protection
characteristic set typically
with a two slope restraint
characteristic. 87T relays are generally of
low impedance type. Id min
1 5 10
1
5
10
I
High
Trips
Blocks
Differential current |I1+I2|
Unrestrained
< 25
Irestrain(|I1|+ |I2|)/2
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Protection of Transformer: cont.
Transformer Differential Protection (87T)
Transformer Differential Protection With Matching CT
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Protection of Transformer: cont.
Transformer Differential Protection (87T)
Factors to be considered in selecting 87T relay:
Extremely stable under through fault conditions andmagnetic inrush.
Very fast to operate for an internal fault
Able to restrain second harmonics and block fifthharmonics.
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Protection of Transformer: cont.
Restricted Earth Fault Protection (87N)
Basics:
Conventional earth fault protection using over-current
elements fails to provide adequate protection fortransformer.
Restricted earth fault, or zero-sequence differential
protection is implemented in transformer star winding. Its a unit protection and operation of relay is
instantaneous.
It offers a significant improvement in sensitivity overtraditional differential protection
It does not respond to load current.
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Protection of Transformer: cont.
Restricted Earth Fault Protection (87N)
Basics:
Ground current in the transformer neutral is used as areference and is compared to zero-sequence current at theterminals to determine if a fault is internal to the transformer.
The relay is operative for faults within the region between
current transformers, that is, for faults on the star winding inquestion.
The relay will remain stable for all faults outside this zone.
87N relays are also available high impedance and low
impedance type. Generally 87N relays are of High Impedance type.
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Protection of Transformer: cont.
Restricted Earth Fault Protection (87N)
Basic 87N Scheme:
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Protection of Transformer: cont.
Restricted Earth Fault Protection (87N)
Fig. Restricted earth fault protection : (a) neutral earthed within the
protected zone (b) neutral not earthed within the protected zone
(a)
(b)
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Protection of Transformer: cont.
Time Delayed/Instantaneous Over Current & Earth FaultProtection (50/50N & 51/51N)
Used on all feeding circuits of transformer
Provide back up for internal faults
Also provide back up for system faults
Instantaneous high element51/51N can be definite time or inverse time to
achieve proper relay co-ordination from
upstream to downstream
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The transformer winding hot-spot temperature is
another quantity that should be used for protection of
transformers.
Protection based on winding hot-spot temperature can
potentially prevent short circuits and catastrophic
transformer failure, as excessive winding hot-spot
temperatures cause degradation and eventual failure of
the winding insulation.
During over load conditions excessive load current
through the transformer causes over heating of the
winding and insulating oil.
Protection of Transformer: cont.
Thermal Overload Protection (49)
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Protection of Transformer: cont.
Thermal Overload Protection (49)
To prevent damage of the winding insulation thermal
overload protection is used.
It is basically a over current protection implemented
in both windings of the transformer
Tripping is time delayed. First alarm is generated then
finally tripping occurs.
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Time
Rise of temperature
Trip
Alarm
Protection of Transformer: cont.
Thermal Overload Protection (49)
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Protection of Transformer: cont.
Over Fluxing Protection (24)
Transformer over-fluxing can be a result of
Overvoltage
Low system frequencyA transformer is designed to operate at or below a
maximum magnetic flux density in the transformer core.
Above that design limit the eddy currents in the core andnearby conductive components cause overheating which
within a very short time may cause severe damage.
The magnetic flux in the core is proportional to thevoltage applied to the winding divided by the impedance of
the winding.
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Protection of Transformer: cont.
Over Fluxing Protection (24)
The flux in the core increases with either increasing voltage ordecreasing frequency-
EMF = E = 4.44fmN =>E/f = 4.44 mN
During startup or shutdown of generator-connectedtransformers, or following a load rejection, the transformer mayexperience an excessive ratio of volts to hertz, that is, becomeoverexcited.
Overexcited transformers become overheated and damaged
Over fluxing protection is specially required for GeneratorTransformers
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Protection of Transformer: cont.
Mechanical Protection
There are several mechanical protection relays installed
on transformers .
Transformer mechanical protection relays operate by sensing
operational parameters like oil pressure, oil level, gas evolved,
oil & winding temperature.Generally these relays are built in features of power and
distribution transformers having capacity more than 10MVA.
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Protection of Transformer: cont.
Mechanical Protection: cont.Buchholz Relays (63):
Buchholz relay is a mechanical protection device for monitoring
the gas and oil movements in oil immersed transformers.
It is used on practically all power transformers with the
exception of small distribution transformers.
In addition with the main tank Buchholz relay another Buchholz
relay is mounted on OLTC (On Load Tap Changer).
Location of main tank Buchholz relay is given as follows:
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Fig. Buchholz Relay
Protection of Transformer: cont.
Buchholz Relay: contd.
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The internal mechanism of a Buchholz relay mainly comprises two
floats.
During normal operation, the relay is completely filled with oil
keeping the floats in their top limit or reset position.
The contact mechanisms in the relays respond to:
Slight faults causing a slow evolution of gas in the transformer (e.g.
overheating).
Serious faults creating an immediate surge of oil (e.g. short-
circuits etc.).
Unattended Oil leakage may lead to operation of the Buchholz relay.
Protection of Transformer: cont.
Buchholz Relay: contd.
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Schematic Diagram of a conventional Buchholz Relay Arrangement
Protection of Transformer: cont.
Buchholz Relay: contd.
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Schematic Diagram of a modern Buchholz Relay (Oil Surge) Arrangement
Protection of Transformer: cont. Buchholz Relay: contd.
An oil-surge detection feature of the Buchholz relay will trip the upstream
circuit-breaker instantaneously if a surge of oil occurs in the pipe connecting
the main tank with the conservator tank.
Such a surge can only occur due to the displacement of oil caused by a rapidly
formed bubble of gas, generated by an arc of short-circuit current in the oil.
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Schematic Diagram of a Buchholz Relay Mounting Arrangement
Protection of Transformer: cont.
Buchholz Relay: contd.
= 3-5
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Buchholz Relay Operation:
When a slight fault occurs in the transformer, the small bubbles of gas which
pass upwards towards the oil conservator tank are trapped in the relay housingthis causing its oil level to fall.
As a result, the upper float drops and activates the external alarm switch.
If gas continues to be generated then the second float operates the second
switch that is normally used to isolate (trip) the transformer.
If an arc forms, gas accumulation is rapid, and oil flows rapidly into the
conservator which is called oil surge.
This flow of oil operates a switch attached to a vane located in the path of the
moving oil.
This switch normally will operate a circuit breaker to isolate the apparatus
before the fault causes additional damage
Protection of Transformer: cont.
Buchholz Relay: contd.
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Protection of Transformer: cont.
Pressure Relief Device (PRD)
Basics:
Pressure Relief Device is a safety element of the transformer employs to prevent
heavy damages of the tank in the case of sudden rise of the internal pressure.
These device has been designed in order to remove the excess pressure in a very
short time as soon as the pressure in the tank rises above predetermined safe limit
PRD operates and allows the pressure to drip instantaneously and avoids damage
to transformer body.
During internal faults of a power transformer, there will be an increase of
temperature associated with formation of gases, impurities in oil and thus
increase in pressure.
This pressure is sufficient to damage the transformer.
The pressure relief device is applied to prevent the transformer from this
danger
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Protection of Transformer: cont.
Pressure Relief Device (PRD): cont.
Fig. Pressure Relief Device
Protection of Transformer: cont
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Protection of Transformer: cont.
Pressure Relief Device (PRD): cont.The pressure relief device consists of a spring which normally is uncompressed
and when the pressure increased in the transformer, the spring get compressed and
give a path of gases to go out of the transformer.
Compressing the spring will close an electrical contact, and this contact will give trip to
circuit breakers associated with alarm.Following Figure (shows the pressure relief device in the normal condition (before the
fault occurrence):
Protection of Transformer: cont
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Protection of Transformer: cont.
Pressure Relief Device (PRD): cont.
Following figure shows the fault condition at which the
compressed gases get passage to let the gases out from the
transformer.
Protection of Transformer: cont
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Protection of Transformer: cont.
Winding & Oil Temperature Protection
By making a "Thermal Image" of the
winding the Winding Temperature Indicator,
simulates the winding temperature.The temperature of the winding depends on
the transformer load (i.e. the current through
the winding) and the temperature of the cooling
medium (the oil).
Temperature is measured with a bulb in apocket.
It has a specially designed heating element,
to measure the transformer load.
This heating element is a thermal model of
the winding.The heating element is connected to the
current transformer (CT) via a Matching
Resistance or a Matching Unit, to allow setting
the correct winding temperature gradient.
Winding Temperature Indicator
Protection of Transformer: cont
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Protection of Transformer: cont.
Winding Temperature Protection: cont.
Generally winding temperature indicator consists of four contacts
which are normally open and closes in series according to pre set
closing value (temperature).
These contacts can be assigned as follows:-1.The first contact is used for automatic operation of first fan
group.
2.The second contact is used for automatic operation of second fangroup, this value is higher than the first contact setting.
3.If the cooling fans are not sufficient to retain the transformer
temperature to its normal value, the third contact is applied to feed
alarm circuit.
4.As a last step, the fourth contact is applied for tripping to prevent
the transformer from high temperatures. Normally it trips the load side
breaker, (i.e. the secondary side CB)
Protection of Transformer: cont
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Protection of Transformer: cont.
Oil Temperature Protection
Oil Temperature Indicator:
Oil temperature indicator is
similar to winding temperatureindicator except that it depends only
on the temperature transferred by the
bulb (no current transformer is used).
This consists only of two contacts.
These contacts are similar to the
third and the fourth contacts of the
winding temperature indicator butwith preset values less than
winding temperature indicator by
approximately 5-10 degrees.
Protection of Transformer: cont
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Transformer Dehydrating Breather:
A transformer breather is an
accessory of an oil filled typetransformer which is attached to the
oil conservator tank.
When the insulating oil of the
transformer gets heated up, itexpands and goes back to the
conservator tank and subsequently
pushes the dry air out of the
conservator tank through the
breather
Protection of Transformer: cont.
Transformer Dehydrating Breather
P t ti f T f t T f D h d ti B th
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Protection of Transformer: cont. Transformer Dehydrating Breather
It is filled with some desiccating agent,e.g. silica gel.
When the oil cools down, it retracts andsucks fresh air from the atmospherethrough the breather
the silica gel dries up the moisture
content of the air that goes back in tothe conservator tank.
If the silica gel looses its moistureabsorbing capability then the oil in theconservator gets contaminated andeventually losses the insulatingproperty.
Protection of Transformer: cont.
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Protection of Transformer: cont.
Oil Level Indicator
Oil level indicators with magnetic joint are usually used on
transformers' conservators.
It is mounted on the body of the conservator.
Its function is to give a visual alarm of the oil level contained in
the conservator.
Furthermore they are provided with micro switches to signal the
alarm in case the oil level reaches its minimum and/or maximum.
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Thanks to All