Operation P54x/EN OP/La4
MiCOM P543, P544, P545 & P546
(OP) 5-21
OP
P1006ENa
Flux, j
2j
jm
Magnetising current,
Ie
Figure 7 Transformer magnetizing characteristic
Under normal steady state conditions, the magnetizing current associated with the operating
flux level is relatively small (usually less than 1% of rated current). However, if a transformer
winding is energized at a voltage zero, with no remnant flux, the flux level during the first
voltage cycle (2 x normal max. flux) will result in core saturation and in a high, non-sinusoidal
magnetizing current waveform. This current is commonly referred to as magnetizing inrush
current and may persist for several cycles. The magnitude and duration of magnetizing
inrush current waveforms are dependent upon a number of factors, such as transformer
design, size, system fault level, point on wave of switching, number of banked transformers,
etc. Figure 8 shows typical transformer magnetizin
g currents for steady state and inrush
conditions.
The magnetizing inrush current contains a high percentage of second harmonic. The P543
and P545 relays filter out this component of the waveform and use it as an additional bias
quantity. The total bias used by the relay will therefore be a combination of the average load
current on the line plus a multiple of the second harmonic component of the current. The
multiplying factor is used to ensure stability and is a factory pre-set value.
Where P543 and P545 relays are used and inrush restrain function is enable, it must be
ensure that this function is enabled at each end to avoid possible maloperation.
High set differential setting:
When inrush restrain is enabled, a high set differential protection becomes active. This
unrestrained instantaneous 'Id High Set ' is provided to ensure rapid clearance for heavy
internal faults with saturated CTs. The high set is not restrained by magnetizing inrush. A
setting range 4In -32In (RMS values) is provided on P543 and P545
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