Instruction manual
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AQ G3x7 Generator protection IED
148 (211
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network and the frequency is not kept at a “constant” value. If the
generator is excited in this
state and the frequency is below the rated value, then the flux may increase above the tolerated
value. Similar problems may occur in distributed generating stations in case of island operation.
The overexcitation protection is designed to prevent this long-term overexcited state.
The flux is calculated continuously as the integral of the voltage. In case of the supposed
sinusoidal voltage, the shape of the integrated flux will be sinusoidal too, the frequency of
which is identical with that of the voltage. The magnitude of the flux can be found by searching
for the maximum and the minimum values of the sinusoid.
The magnitude can be calculated if at least one positive and one negative peak value have
been found, and the function starts if the calculated flux magnitude is above the setting
value. Accordingly, the starting delay of the function depends on the frequency: if the
frequency is low, more time is needed to reach the opposite peak value. In case of
energizing, the time to find the first peak depends on the starting phase angle of the
sinusoidal flux. If the voltage is increased continuously by increasing the excitation of the
generator, this time delay cannot be measured.
3.2.24.1
Operating characteristics
The most harmful effect of the overexcited state is unwanted overheating. As the heating
effect of the distorted current is not directly proportional to the flux value, the applied
characteristic is of inverse type (so called IEEE type): If the overexcitation increases, the
operating time decreases. To meet the requirements of application, a definite-time
characteristic is also offered in this protection function as an alternative.
The supervised quantity is the calculated U/f value as a percentage of the nominal values
(index N):
The over-dimensioning of generators in this respect is usually about 5%, that of the transformer
about 10%, but for unit transformers this factor can be even higher.
