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Voltage

Frequency

Volts

  Flux

Over Excitation:

May result in thermal damage to core due to excessively high flux inmagnetic circuits.

Under Over Excitation condition excessive flux saturates core causinghigh Eddy current losses in core and adjacent conducting material.

Flux Voltage

Flux (Frequecy!" 

Hertztransformer#s

$henever

%ated transformer#s  Volts

Hertz> Over Excitation

&s

Over Excitation can occur under no load and on load conditions.

('.U ('.U

 

Over Excitation deends on the flux flo) in the transformer core.

Volts

*ert+

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Over Excitation rotection:

Instantaneous tripping is undesirable for Over Excitation. As systemmay get overexcited momentary due to some momentary disturbance.

Momentary system disturbances are those which can be borne safely,and within a very short specific time  system restores its normalconditions. 

Over Excitation protection is time dependent function.

For each transformer there exists an Over Excitation curve!.

Over Excitation curve! defines over excitation characteristics of a particulartransformer versus time.

Overfluxing protection relay follow Over Excitation curve! to protecttransformer from over excitation.

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Over Excitation rotection: (,ontOver Excitation curve:

It follows Inverse time characteristics and definite time characteristics.

Every transformer possesses its own excitation curve.

"ransformer Over Excitation curve

Danger zone

#elow is the Over Excitation curve of a typical $%&'()*+ substation transformer.

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If transformer will not be isolated from supply within

one or two minutes

A transformer blast can occur !.

ue to some external fault, transformer-s operating freuency fallsdown to %/01. ue to which Flux inside the core has been exceeded.A relay connected at the primary side of transformer through +"

having ratio 2%$'3% will read the information as follows. 

(%./'3%

f4 /& 01

5

6

ip7t8 is7t8

+p7t84 2%$*+

5

6

+s7t84 22*+

Over Excitation: (,ont

f4 /& 01

9onsider the above transformer under normaloperation and having rating as follows

-asic concet:

  24 MVA  132 kV / 11 kV  50 Hz  3-phase  FOA

f4 %/ 01 f4 %/ 01

Over Excitation will occur

If transformer-s :+'01; exceeds transformer-s rated :+'01; !. i.e<2.$= +'01.50Hz

Volts

Hertz

63.5V= =

transformer#s%ated

Under normal Oeration

1.27 V/Hz35Hz

Volts

Hertz

63.5V= =

transformer#s 

Fault condition

1.81 V/Hz   %ated 1.27 V/HzVolts

Hertz=>

Over Excitation

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>elay Over Excitation curve

Over Excitation relay follows the same transformer curve.

Over Excitation %elay:

Over Excitation relay tripping should be well coordinated with transformer-s

over excitation curve.

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>elay "ime to initiate "rip

"ime from transformer curve

An Over flux relay tripping should be well coordinated with transformer-sover excitation characteristics.

Over Excitation /olution: (,ont

Danger zone