GE P642, Technical Manual

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The pickup of the time delayed overcurrent element can be set to 125-150% of the maximum MVA rating to allow
overloading of the transformer according to IEEE Std. C37.91-2000.
As recommended by IEEE Std. C37.91-2000, you should set the instantaneous overcurrent element to pick up at a
value higher than the maximum asymmetrical through fault current. This is usually the fault current through the
transformer for a low-side three-phase fault. For instantaneous elements, variations in settings of 125–200% are
common. For elements subject to transient overreach, a pickup of 175% of the calculated maximum low-side
three-phase symmetrical fault current generally provides sufficient margin to avoid false tripping for a low-side
bus fault, while still providing protection for severe internal faults. Due to low transient overreach of the third and
fourth overcurrent stages, you may set the instantaneous overcurrent element to 120-130% of the through-fault
level, thus ensuring stability for through faults. The instantaneous pickup setting should also consider the effects of
transformer magnetising inrush current.
In summary, there are a few application considerations to make when applying overcurrent devices to protect a
transformer:
● When applying overcurrent protection to the HV side of a power transformer it is usual to apply a high set
instantaneous overcurrent element in addition to the time delayed low-set, to reduce fault clearance times
for HV fault conditions. Typically, this will be set to approximately 1.3 times the LV fault level, so that it will
only operate for HV faults. A 30% safety margin is sufficient due to the low transient overreach of the third
and fourth overcurrent stages. Transient overreach defines the response of a relay to DC components of
fault current and is quoted as a percentage. A device with a low transient overreach will be largely
insensitive to a DC offset and may therefore be set more closely to the steady state AC waveform.
●
The second requirement for this element is that it should remain inoperative during transformer
energisation, when a large primary current flows for a transient period. In most applications, the
requirement to set the device above the LV fault level will automatically result in settings that will be above
the level of magnetising inrush current.
With the third and fourth overcurrent stages, it is possible to apply settings corresponding to 40% of the peak
inrush current while maintaining stability for the condition.
Where an instantaneous element is required to accompany the time delayed protection, as described above, you
should use the third or fourth overcurrent stages as these have wider setting ranges.
Chapter 9 - Current Protection Functions P64x
208 P64x-TM-EN-1.3