Over fluxing or over excitation of a transformer connected
to the terminals of a generator, can occur if the ratio of voltage to frequency
exceeds certain limits. High voltage or low frequency, causing a rise in the
V/Hz ratio, will produce high flux densities in the magnetic core of the
transformer. This could cause the core of the transformer to saturate and stray
flux to be induced in un-laminated components that have not been designed to
carry flux. The resulting eddy currents in solid components (core bolts and
clamps) and end of core laminations can cause rapid overheating and damage.
Transformer over fluxing might arise for the following
reasons:
High system voltage
Generator full load rejection
Ferranti effect with light loading transmission lines
Low system frequency
Generator excitation at low speed with AVR in service Geo-magnetic
disturbance
Low frequency earth current circulation through a
transmission system
The initial effects of over fluxing will be to increase the
magnetizing current of a transformer. This current will be seen as a
differential current. If it reaches a high level there would be a risk of differential
protection tripping. Persistent over fluxing may result in thermal damage or
degradation of a transformer as a result of heating caused by eddy currents
that may be induced in non-laminated metalwork of a transformer. The flux
levels in such regions would normally be low, but excessive flux may be passed
during over fluxed operation of a transformer.
The following protection strategy is proposed to address potential
over-fluxing conditions:
Maintain protection stability during transient over-fluxing
Ensure tripping for persistent over-fluxing
By measuring the 5th Harmonic component in the differential
current, the over-fluxing can be identified. The ratio of fifth to fundamental
component ic compared with set value. If exceeds the set value, the transformer
can be isolated. Further, where there is any risk of persistent geomagnetic
over-fluxing, with normal system voltage and frequency, the 5th harmonic
differential current facility could be used to initiate tripping after a long
time delay. To ensure tripping for persistent over-fluxing, due to high system
voltage or low system frequency, time delayed Volts per Hertz protection is
provided. Protection against damage due to prolonged over-fluxing is offered by
a V/f protection element with a variable time tripping characteristic. The
setting flexibility of this element, by adjustment of the time delay at various
V/f values, makes it suitable for various applications. The manufacturer of the
transformer or generator should be able to supply information about the short-time
over-excitation capabilities, which can be used to determine appropriate
settings for the V/f tripping element. The variable time over-fluxing
protection would be used to trip the transformer directly.
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