Presentation on Power Transformer & Distribution Transformer Protection

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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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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Conventional Protection Scheme of a Two WindingTransformer


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Conventional Protection Scheme of a Two Winding TransformerIncluding REF

87N


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Conventional Protection Scheme of a Three Winding Transformer


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Conventional Protection Scheme of an Auto-Transformer


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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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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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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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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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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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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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Transformer Differential Protection With Matching CT


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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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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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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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Basic 87N Scheme:


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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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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.




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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




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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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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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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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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


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.




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Schematic Diagram of a conventional Buchholz Relay Arrangement




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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



= 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




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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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Pressure Relief Device (PRD): cont.

Fig. Pressure Relief Device

Protection of Transformer: cont


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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):



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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.



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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



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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)



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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.



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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


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.



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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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Presentation on Power Transformer & Distribution Transformer Protection