Short Circuit Ppt Latest

8/8/2019 Short Circuit Ppt Latest

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SHORT CIRCUIT PHENOMENON IN

TRANSFORMERS.


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WHAT IS A SHORT CIRCUIT IN A TRANSFORMER?

During the service of a transformer if secondary line to line or line

to ground terminals or overhead lines come into contact accidentally

by means of strong winds, tree branches, reptiles, birds or by anyother means, very high currents will flow due to low resistance

paths. Connected distribution transformers will have to feed these

fault currents.


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WHAT ARE THE EFFECTS OF SHORT CIRCUIT ON A

TRANSFORMER?

1. Basically there are two effects of a short circuit. Thermal andmechanical. Thermal effects cause excessive heat generationwhich can be easily calculated and proper care can be takenbefore commencement of design work by selecting proper currentdensity.

2. IS 2026-1977, PART-I, clause .9.O explains how thermalcalculations can be made.

3. Mechanical or dynamic effects are the critical ones. Evan thoughsome calculation methods are available the same has not beenmentioned in any international standards unlike that of thermal.

4. Customers specifications and even IS 2026 vide clause 9.2 of

part-I,1977 recommends for demonstrating the capability byconducting type tests.

5. This type test is critical and destructive if proper care is not takenin design, material selection, manufacturing and quality controlstages.


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HOW SHORT CIRCUIT FORCES ARE GENERATED IN

A TRANSFORMER?

1. Any current carrying conductor in a magnetic field experiences

forces as already known.

2. These forces which are also termed as electromagnetic forces,

are directly proportional to square of the current.

3. During short circuits as mentioned above, RMS currents of the

order of 12 to 40 times will flow in a distribution transformer,

depending on the transformer rating and impedance.

4. Since forces are proportional to square of the currents large

forces of the order of144 times to 1600 times will be generated.


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5. During type tests impulse short circuit current which is much higherthan RMS value is passed. Consequently this will further generate

enormous forces due to about 2.5 times the RMS value of current.

6. These kind of forces will cause dislocation of the windings, lead

connections, tap changers etc. and many times even create enoughpressure to explode the tank and create fire hazards, if proper

considerations are not taken.


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WHAT ARE THE DIFFERENT FORCES OCCURING IN

SUCH CASES?

1.Two types of forces are generated.

a) Axial

b) Radial

2. Axial forces are generated due to the radial component of themagnetic flux i.e. leakage flux in the region between LV & HVwindings and radial forces are generated due to the axial componentof the leakage flux.

3. Radial forces will have the tendency to push the inner winding on thecore and burst out the outer winding. In other words there is a forceof repulsion between the windings.


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LV HV Radial forces

Core

Fig.1


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4. Axial forces are caused by interaction of current with radial

component of the leakage flux. Due to this a force of attraction

occurs between adjascent turns and hence the entire winding is

subjected to a compressive force.

LV HVMain compressive forces

Core

Fig.2


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5.In case of axial forces additional forces occur due to the following

aspects.

a) Force due to axial displacement of one of the windings.This means that if the electrical centers are not coinciding additional

forces are arising. Fabricational asymmetry can take place during

manufacturing and even minute displacement will give raise to large

forces. The effect of this force is to create further asymmetry

LV HV

CoreFig.3

Centre line of LV

Centre line of HV


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b. Force due to symmetrical shortening of one the windings.

If electrical height of one of the windings is shorter, additionalforces are generated.

This force tends to compress the shorter winding and tends tostretch the longer winding. Thus as in earlier case the tendency isto create further difference in heights.

LV HV

CoreFig.4


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c) Force due to tapping gap in one of the windings.

Generally all transformers will be provided with tapping arrangement

for adjustment towards supply variation. This will create additional

forces due to the absence of the ampere turns in this region. Thisforce will be acting as a tensile force in the winding with tapping gap

and as a compressive force in the other winding as can be seen

clearly in fig.5.

Core

LV HV

Fig.5


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DESIGN CONSIDERATIONS FOR A SHORT CIRCUIT

PROOF TRANSFORMER.

1. From the basic short circuit force calculations it can be deducedthat Axial compressive forces are directly proportional to thesquare of number of turns, directly proportional to windingdiameters and inversely proportional to square of axial height ofcoils.

2. It may not be always economical to design only for aboveconditions since many times we are working for optimization withcapitalization rates. Further, factor of safety, will be inbuilt in many

cases regarding short circuit capability. specially in small range oftransformers.


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3. Hence in critical cases it is advised to work out axial forces as above

where there is every chance that the best short circuit proof designmay not be optimum with respect to loss parameters.

4. At the design stage it is very essential to maintain the centers of both

windings at same level to the mm. This will avoid additional axial

forces as already mentioned above.

5. In distribution transformers due to the difference in HV and LV

voltages and corresponding end insulation, there exists a difference

in electrical heights. This is normally permissible up to 5%. However

in critical cases it is an improvement to maintain same electricalheights to the mm and this practice is being followed now in our

company.


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6. Additional forces due to the ampere turn imbalance due to tapping

gaps can be minimized by following methods.

a) By providing tappings in the middle of winding. This is

practicable in case of cross over coil and continuous disc winding

designs. But in multilayer helical designs tappings are provided in

last layers for convenience though in amorphous core designs

tappings are being arranged in the middle layers in our company.

b) In case of medium sized distribution and small power

transformers it is better to provide tappings in two groups at about

1/4th to 1/3rd distance from top and bottom to get better ampere

turn balance. Ref fig 6.

This will reduce the additional forces due to tapping gap to about

1/4th of that due to single gap.


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C) Providing a compensating gap of about 50% of tapping gap

exactly opposite to the tapping gap will further improve ampere

turn balance.

Following diagrams in fig.7(extract from J&P hand book) shows

ampere turn diagrams in different cases as discussed above.

LV HV

Core

Fig.6


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Fig.7


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7. For proper pressing and clamping of windings perma wood coil

pressing rings of adequate thickness are provided in case of

transformers generally above 500 KVA ratings.

8. Tie rods and core clamps are to be of adequate cross section and

section modulus to withstand predetermined forces and bending

moments.

9. In case of power transformers with continuous disc winding,

failure can take place due to bending of conductors betweenspacers. Hence, number of spacers are to be matched for the

required span of unsupported conductor

10. Against radial forces, adequate number of supports are to be

provided between LV and core to prevent collapse of LV against

core or buckling of LV winding. (ref fig 8 extract from BHEL bookon power transformers)


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Fig.8


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PRECAUTIONS FOR MANUFACTURING SHORT

CIRCUIT PROOF TRANSFORMERS.

1. HV & LV coils are to be wound tight under full tension to meetdimensions as per design requirement.

2. As already emphasized electrical centers are to be exactly matched.

3. Coils are to be thoroughly dried and pre-compressed for designdimension. This is similar to the pre-stressed concrete principle.

4. During assembly of core and coils care should be taken to maintainthe end insulations as per design information. All the three limbs areto be of exactly equal height so that equal pressing is obtained for all

the three limbs.

5. Core bolts and tie rods are to be fully tightened in order to properlysecure the windings.


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6. All the leads are to be properly crimped, bolted or brazed as

applicable so that the joints will not give way due to pulling forcesduring the short circuit.

7. Long lengths of unsupported leads are to be properly secured by

providing wooden cleat supports.

8. CCAs are to be properly secured to the tanks by the manufacturers

standard practice.

9. Tanks are to be of adequate strength to withstand abnormal

pressure created during short circuits. To meet this sheetthicknesses are to be appropriate, welding to be perfect and

adequate stiffeners are to provided depending on the rating of

transformer.


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METHOD OF CONDUCTING DYNAMIC SHORT CIRCUIT

TEST

Brief steps followed as per IS 2026 are listed below.

1. First following routine tests as per IS 2026 are conducted.

a) Measurement of winding resistance.

b) Measurement of voltage ratio.

c) Measurement of impedance voltage/ short circuit impedanceand load losses

d) Measurement of no load loss and current.e) Measurement of insulation resistance.

f) Die-electric tests.- Power frequency voltage & induced overvoltage tests .


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2. Peak value of short circuit current will be calculated.

Distribution transformers up to 3150 KVA will fall in category 1 of

transformers to be tested as per clause 8.1.1 of IS 2026. Hencesystem impedance is not considered for above purpose. Forhigher ratings system impedance us to added to the transformerimpedance for short circuit current calculations.

3. Than the transformer will be subjected to actual shots afterpreliminary calibration shots. There are two methods ofconducting the test.

a) Pre-set method where one of the winding terminals will beshorted before applying the voltage.

b) Post-set method where the terminals will be shorted afterapplying the voltage.


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Normally for distribution transformers Pre- set method will be followed

by CPRI. In this case in order to avoid core saturation supply will be

connected to the winding farther from the core which will be generally

HV. LV terminals will be shorted.

4. Switching on will be controlled to get maximum asymmetry of thecurrent by closing the breakers at instantaneous zero voltage.Duration of the shot will be 0.5 seconds.

5. If the transformer is having taps number of shots will be three oneach limb that is three in highest tap position on one of the outerlimbs, three in nominal switch position on the middle limb andthree in lowest tap position on the other outer limb. After each testreactance measurement is made and tests will be continued onlyif the variation is within 2.0% of the initial value in case ofconcentric circular coils. If the same is above 2.0% it is an

indication of major dislocation of CCA and hence test is notadvisable to be continued. For rectangular coils with impedance ofabout this reference value is 7.5%.


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6. After the tests routine tests as per item 1 above will be repeated

and compared with the initial values.

7. After routine tests transformer will be opened and physical

inspection of CCA is made.

CPRI records the observation and does not declare categorically that

the transformer has passed the test.


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VELs RECENT EXPERIENCE

S.No Transformer details Nature of failure Probable reasons Modification for successful testing

1

EVL 5 MVA

Bangladesh HV winding collapse High axial forces From single group tap arrangement to double

6.0% impedance group tap arrangement.

2

EVL 10 MVA

Bangladesh LV winding Radial forces on LV high LV number of supports increased and LV

6.0% impedance hard drawn conductor with higher tensile strength

used.

3PGCIL 400 kVA

through SPIC

1. After7th shotCore channels and tierods were found bent

High axial forces at

lowest tap

1. As a precautionary measure HV round wire

was replaced by strip conductor.

2. LV winding dislocated

&

Insufficient strength of

clamping structure 2. Electrical heights were exactly maintained

end insulation

damaged 3. LV conductor transposition was removed

4. Higher size core clamp channel was used

5. Common permawood ring was used for LV &

HV

6. Impedance value was raised from 4.0% to

to 4.5% after taking customer approval.


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THANK YOU

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Short Circuit Ppt Latest