This is just a sample view how flux will behave in core & corresponding current wave form.


Overexcitation of a transformer means that the magnetic flux in the core is increased above the normal design level.

When power transformers are over-excited, the leakage flux increases and this results in heavy hysteresis and eddy current losses in non-laminated parts of the transformers. If the temperature rise, due to these losses, is excessive, the insulation and core laminations can be damaged and flashovers may occur.

There are many causes of over-excitation:
− Power transformers which are directly connected to a generator can be subject to over-excitation during the start-up or shut-down of the generator.
− Load shedding in systems supplied by overhead lines or cables, can cause excessive voltage rises which result in over-excitation of power transformers connected to these systems.Voltage rise in conjunction with low loads can occur in both generator and distribution stations.

Generally, modern power transformers withstand relatively low over-excitation since the flux density is already high at rated values. To protect transformers against dangerous overheating during over-excitation, the excitation must be kept under strict control.

The ( ex: RXLK 2H) over-excitation relay monitors the magnitude of excitation by measuring the relationship between the voltage and the frequency. When the set operate value is exceeded, an alarm and delayed tripping is obtained. The inverse time tripping is settable with different characteristics in order to suit different types of power transformers.

Power Transformer Overexcitation condition caused by decreased frequency; Flux (Green), Iron Core's Magnetic characteristics (Red) and Magnetizing Current (Blue)


Source: [ Ссылка ]

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