Transformer Protection( Power Grid Ballabhgarh

8/3/2019 Transformer Protection( Power Grid Ballabhgarh

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TRANSFORMER PROTECTION

Prof. M.L.Kothari

Deptt of Electrical Engineering

Indian Institute of TechnologyDelhi

New Delhi INDIA

Visiting Professor, HelsinkiUniversity of Technology


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OUTLINE OF THE LESSON

1. Introduction

2.Types of faults affecting Power Transformers

3.Buchholz Relay

4.Nature and effect of transformer faults

5.Magnetising inrush current of a transformer

6.Transformer Differential protection

7.Harmonic Restraint Differential Relay

8.Restricted Earth Fault Protection


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The power transformer is one of the

most important links in a power

transmission and distribution system.

INTRODUCTION


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It is a highly reliable piece of equipment.

This reliability depends on

adequate design

careful erection

proper maintenance

application of protection

system.


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1. Surge diverters

2. Gas relay:

It gives early warning of a slowlydeveloping fault, permitting shutdown

and repair before severe damage

can occur.3. Electrical relays.

PROTECTION EQUIPMENT

INCLUDES


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The choice of suitable protection is also

governed by economic considerations.

Although this factor is not unique to power

transformers, it is brought in prominence

by the wide range of transformer ratings

used( few KVA to several hundreds MVA)


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Only the simplest protection such as

fuses can be justified for transformers of

lower ratings.

for large transformers best protection

should be provided.


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THROUGH FAULTS

a) Overload conditions.

b) External short-circuit conditions.

TYPES OF FAULTS AFFECTINGPOWER TRANSFORMER


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The transformer must be disconnected

when such faults occur only after allowing

a predetermined time during which otherprotective gears should have operated.


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The primary protection of a power

transformer is intended for conditions which

arises as a result of faults inside the

protection zone.

INTERNAL FAULTS


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Internal faults are very serious and there

is always a risk of fire; these internal

faults are classifieds into two groups.1. GROUP-A

2. GROUP-B


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Electrical faults which cause immediate

serious damage but are generally

detectable by unbalance of current or

voltage such as:

GROUP A


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1. Phase-to-earth fault or phase- to-

phase fault on HV and LV external

terminals

2. Phase-to-earth fault or phase-to- phase

fault on HV and LV windings.

3. Interturn faults of HV and LV windings.

4. Earth fault on tertiary winding, or short

circuit between turns of a tertiary

windings.


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For Group(A) faults, it is important that

the faulted equipment should be isolated

as quickly as possible


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So called incipient faults which are

initially minor faults, causing slowly

developing damage. These are NOTDETECTABLE at the winding terminals by

unbalance current or voltage.

GROUP B


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A poor electrical connection of conductors

or core faults (due to breakdown of the

insulation of laminations, bolts or

clamping rings which cause limited arcing

under the oil.)


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Coolant failure, which will cause a rise oftemperature even below full load operation

Related to above is the possibility of low oil

content or clogged oil flow, which can readilycause local hot spot on the windings.


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Regulator faults and bad load sharingbetween transformers in parallel, which cancause overheating due to circulating

currents The Group(B) faults, though not serious in

their incipient stage, may cause major faults

in due course of time and should thus be

cleared as soon as possible.


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It should be emphasized that the means

adopted for protection against faults in

Groups (A) are not capable of detecting the

faults of Groups of (B), where as the means

applicable to detect the Group (B) faults can

not necessarily detect the terminal faults and

are not quick enough to clear other faults in

Group (A)


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These ideas are basic to transformer

protection, & the means of protection

against the Group (A) & (B) should not be

treated as alternatives but as supplements

to each other.


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BUCHHOLZ RELAY

All faults below the oil in transformer result inthe localized heating & breakdown of the oil,some degree of arcing will always take place in

a winding fault & the resulting decomposition ofit will release gases such as hydrogen, carbonmonoxide & hydrocarbons.


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BUCHHOLZ RELAY

When the fault is of a very minor type, suchas hot joints gas is released slowly, but amajor fault involving severe arcing causesrapid release of large volumes of gas aswell as oil vapour.

Recognition of the above action byBUCHHOLZ led to the development of theprotective device known as BUCHHOLZRELAY .


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BUCHHOLZ RELAY

A buchholz is contained in a cast housingwhich is connected as shown below


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BUCHHOLZ RELAY

A typical buchholz relay comprises twopivoted aluminum brackets, each counterbalance so that when empty or completely

full of oil, the bucket is in high position.


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BUCHHOLZ RELAY

Each pivoted bucket assembly carries amercury switch.

In the normal condition the casing is filled

with oil, so that mercury switches are open. If gas bubbles pass up the piping, they will

be trapped in the relay casing, so displacingthe oil .


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BUCHHOLZ RELAY

As the oil level falls the upper bucket willfollow, since the weight of the bucket filledwith oil exceeds that of the counterbalance

when the buoyancy from the surrounding oil islost.

As the bucket falls, the mercury switch tilts &closes the alarm circuit.


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BUCHHOLZ RELAY

A similar operation will occurs if tank leakscauses the oil level to fall.

A major winding faults causes a surge of oilwhich displaces the lower bucket & thusisolates the transformer.


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BUCHHOLZ RELAY

Relay is usually provided with an inspectionwindow on each side of the gas collectionspace, through which the oil level can be

observed. This may also helps in diagnosingthe fault.


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BUCHHOLZ RELAY

Because of its universal response to faultswithin the transformer, some of which aredifficult to detect by other means, the

BUCHHOLZ RELAY is invaluable, whetherregarded as a main protection or assupplement to other protection schemes


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BUCHHOLZ RELAY

The minimum operating time of the

BUCHHOLZ RELAY is about 0.1 sec & the

average operating time is 0.2 sec

Transformer without conservator can not

be provided with BUCHHOLZ RELAY


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NATURE & EFFECT OF

TRANSFRMER FAULTS

A faults on transformer winding is

controlled in magnitude by

a) Source & neutral earthing impedance

b) Leakage reactance of the transformer

c) Position of the fault on the winding.


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NATURE & EFFECT OFTRANSFORMER FAULTS

Following distinct cases are examined

below

(1) Star connected winding with neutral pointearthed through an impedance


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If

If

Rnp

Earth fault on resistance earthed star winding


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An earth fault on resistance earthed starwinding will give rise to a fault current (IF) depends on the value of the earthing

impedance. is proportional to the distance of the

fault from the neutral point since thefault voltage will be directly

proportional to the distance from theneutral


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The ratio of transformation between theprimary winding and short circuited turnsalso varies with the position of the fault, so

that the current which flows into thetransformer primary terminals will be inproportion to the square of the fraction ofthe winding which is short circuited.


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The variation of magnitude of the faultcurrent (% of respective maximum singlephase earth fault current) with the distance

of the fault measured from the neutral ( % ofthe winding) is shown


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Star connected winding withNeutral point solidly earthed

The fault current in this case is controlledmainly by the leakage reactance of the

transformer which varies in a complexmanner ( approximately, proportional to thesquare of the number of turns involved)


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Further, the voltage is not proportional tothe turns involved by the fault near theneutral because of the increased magnetic

leakage. The variation of the fault current with fault

position is shown in the fig


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The fault current reflected to the input sideis modified as before by the variabletransformation ratio.

The fault current magnitude remains highthroughout the winding.

Further, the general current scale is high inthe absence of current limiting resistance.


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Delta Connected Winding

No part of the delta-connected windingoperates with a voltage to earth less than50% of the phase voltage.

The range of fault current magnitude forsuch a winding is therefore less than for astar winding

The actual value of the fault current will still

depend on the way the system is earthed


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The impedance of the deltawinding isparticularly high to the fault currents flowingto a centrally placed fault on one leg


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Phase to Phase faults

Faults between phases within a transformer

are relatively rare; if such a fault occurs it

will give rise to substantial current

comparable to the earth fault currents.


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Interturn Faults

A high voltage transformer connected to anoverhead transmission system is very likelyto be subjected to steep fronted impulse

voltage A line surge, which may be of several times

the rated system voltage, will concentrateon the end turns of the winding because of

the high equivalent frequency of the surgefront


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The interturn insulation of the end turns isreinforced,but can not be increased inproportion to the insulation to earth,which is

relatively great.

The risk of the partial winding flashovercompared to that of the breakdown to earthis comparatively high. It is claimed that 70-80% of all transformer failures arise fromfaults between turns.


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A short circuit of few turns of the windingwill give rise to heavy fault currents in theshort circuited loop, but the terminal

currents will be very small, because of thehigh ratio of the transformation between thewhole winding and short circuited turns


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Externally Applied Conditions

Sources of abnormal stress in a transformer

are :

a) Overload

b) System faults

c) Over voltage

d) Reduced System Frequency


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Overload causes increased copper loss

and a consequent temperature rise.Overloads can be carried for a limited

periods, depending on the initialtemperature and cooling conditions

System short-circuits produce a relativelyintense rate of heating of the feeding

transformer, the copper loss increasingproportional to the square of the per unitfault current.


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The duration of the external short circuitthat a transformer can withstand withoutdamage if the current is limited only by self

reactance is shown in Table on the nextslide


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Transformerreactance(%)

Faultcurrent(multiple of

rating)

Permittedfaultduration

(seconds)4 25 2

5 20 3

6 16.6 4

7 14.2 5


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Large fault currents produce severemechanical stresses in the transformers;the maximum stress occurs during the first

cycle of the asymmetric fault current and socannot be averted by automatic tripping ofthe circuit


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Overvolatges

Transient surge voltages

Power frequency voltages


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Transient Surge Overvoltages

Transient over voltages arise from switchingand lightning disturbances and are liable tocause inter-turn faults

These voltages are usually limited byproviding lightening arrester (Metal ZincOxide).


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Power Frequency Overvoltages

Causes increase in stress on insulation

Increase in working flux

Increase in iron loss

Disproportionality increase in magnetizingcurrent

Flux is diverted from the laminated core tothe steel structure

Increase in heating and temperature rise


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Reduction in Frequency

It has an effect with regard to flux densitysimilar to that of power frequencyovervolatges.

Operation must not be continued with a highvoltage input at a low frequency

V/f > 1.1 is not permissible where V and fare expressed in p.u. of their rated value


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MAGNETISING INRUSH CURRENT


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MAGNETISING INRUSH CURRENTOF A TRANSFORMER

When a transformer is first energized,a

transient exciting current flows to bridge the

gap between the conditions existing before the

transformer is energized and the conditions

dictated by the steady state requirements


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For any given transformer this transientcurrent depends upon:

The point on the voltage wave at which

the switch is closed.

The value and direction of the residualcore flux.

The shape of the saturation curve ,andthe normal flux density used.


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Often the magnitude of this transient

current exceeds full load current and may

reach 8-10 times full load current.


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In studying the phenomena that occur when

a transformer is energized, it is more

satisfactory to determine the flux in themagnetic circuit first and then derive the

current from the flux.


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This procedure is preferred because the

flux does not depart much from the sine

wave even though the current wave isdistorted.


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If the secondary of the transformer is open ,

the transformer can be treated as an iron-

core reactor, the differential equation for

the circuit consisting of the supply and

transformer can be written as

1de Ri ndt

(1)


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Normally the resistance R is small and can

be ignored for simplicity.

Under this condition the equation (1) can

be written as

1

de n

dt (2)


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If the supply voltage is sine wave voltage,

where,

= rms value of the supply voltage

e= instantaneous voltage applied to transformer

2 sin( )e E t

E

2 f

.(3)


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Substituting in equation (2)

1 2 sin( )

dn E t

dt

Solving the above equation

1

2cos( )

t

Et

n

..(4)


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Equation (4) can be used to determine the

transient flux in the core immediately after

the transformer is energized.


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Amplitude of Harmonics in a typical


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Component Typical Value(%)offundamental

DC 55

2nd Harmonic 633rd Harmonic 26.8

4th Harmonic 5.1

5th

Harmonic 4.16th Harmonic 3.7

7th Harmonic 2.4

Amplitude of Harmonics in a typicalMagnetizing Inrush current wave

TRANSFORMER DIFFERENTIAL


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TRANSFORMER DIFFERENTIALPROTECTION

BASIC CONSIDERATIONS


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The nominal currents in the primary and

secondary sides of the transformer vary in

inverse ratio to the corresponding voltages.

This should be compensated for by usingdifferent transformation ratios for the CTs on

the primary and secondary sides of the

transformer.

a. Transformation ratio

b Current Transformer


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b. Current TransformerConnections

When a transformer is connected in

star/delta, the secondary current has a

phase shift of 300 relative to the primary

This phase shift can be offset by suitable

secondary CT connections


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The zero-sequence currents flowing on

the star-side of the transformer will not

produce current outside the delta on theother side. The zero sequence current

must therefore be eliminated from the star-

side by connecting the CTs in delta.


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The CTs on delta side should be

connected in star in order to give 300

phase shift.


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When CTs are connected in delta, their

secondary ratings must be reduced to

1/3 times the secondary ratings of the

star-connected transformer, in order that

the currents outside the delta may

balance with the secondary currents of

the star-connected CTs.


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If transformers were connected in

star/star, the CTs on both sides would

need be connected in delta-delta.

c. Bias to cover tap-changing


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If the transformer has the benefit of a tap

changer, it is possible to vary its

transformation ratio for voltage control.

c as to co e tap c a g g

facility and CT mismatch


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The differential protection system should

be able to cope with this variation.


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This is because if the CTs are chosen to

balance for the mean ratio of the power

transformer, a variation in ratio from the

mean will create an unbalance

proportional to the ratio change. At

maximum through fault current, the spill

output produced by the small percentageunbalance may be substantial.


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Differential protection should be provided

with a proportional bias of an amount

which exceeds in effect the maximum ratio

deviation. This stabilizes the protection

under through fault conditions while still

permitting the system to have good basic

sensitivity.

d Magnetization Inrush


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The magnetizing inrush produces a

current flow into the primary winding that

does not have any equivalent in the

secondary winding. The net effect is thus

similar to the situation when there is an

internal fault on the transformer.

d. Magnetization Inrush

Contd


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Since the differential relay sees the

magnetizing current as an internal fault, it

is necessary to have some method ofdistinguishing between the magnetizing

current and the fault current, These

methods include:


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Using a differential relay with a suitable

sensitivity to cope with the magnetizing

current, usually obtained by a unit thatintroduces a time delay to cover the

period of the initial inrush peak.


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Using a harmonic-restraint unit, or a

supervisory unit, in conjunction with adifferential unit.

Inhibiting the differential relay during the

energizing the transformer.


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Example:

A 3-phase, 33000/6600V transformer is

connected star/delta an the protecting CTs

on the low voltage side have a ratio of

300/5. What will be ratio of the CTS on the

H.V. side?

[20:8:1]

RESTRICTED EARTH FAULT


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PROTECTION

Fig : Restricted earth fault protection for starconnected winding


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Fig Amount of winding protected when transformer isresistance earthed and ratings of transformer andresistor are equal

Percentage of winding protected

Primaryoperating

current


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Degree of protection is very much improved

with the application of a restricted earth fault(REF) protection

OVERFLUXING PROTECTION


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OVERFLUXING PROTECTION


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The system voltage, as measured by a

voltage transformer, is applied to a resistance

to produce a proportionate current; this

current , on being passed through a

capacitor, produces a voltage drop which is

proportional to the function in question, E/f,

and hence to the flux in the powertransformer.

HARMONIC RESTRAINT


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DIFFERENTIAL RELAY


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Transformer Protection( Power Grid Ballabhgarh