Areva MiCOM P441, Technical Manual

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Application Notes P44x/EN AP/E33
MiCOM P441/P442 & P444 Page 69/220
A fault on the system results in the measured impedance rapidly crossing the
∆
R band, en
route to a tripping zone. Power swings follow a much slower impedance locus. A power
swing is detected where all three phase-phase measured impedances have remained within
the
∆
R band for at least 5ms, and have taken longer than 5ms to reach the trip characteristic
(the trip characteristic boundary is defined by zones 3 and 4). PSB is indicated on reaching
zone 3 or zone 4. Typically, the
∆
R and
∆
X band settings are both set with: 0.032 x
∆
f x
Rmin load.
NOTE:
∆
f = Power swing frequency
2.13.2 Unblocking of the Relay for Faults During Power Swings
The relay can operate normally for any fault occurring during a power swing, as there are
three selectable conditions which can unblock the relay:
A biased residual current threshold is exceeded - this allows tripping for earth faults
occurring during a power swing. The bias is set as: Ir> (as a percentage of the highest
measured current on any phase), with the threshold always subject to a minimum of 0.1 x In.
Thus the residual current threshold is:
IN > 0.1 In + ( (IN> / 100) . (I maximum) ).
A biased negative sequence current threshold is exceeded - this allows tripping for phase-
phase faults occurring during a power swing. The bias is set as: I
2
> (as a percentage of the
highest measured current on any phase), with the threshold always subject to a minimum of
0.1 x In. Thus the negative sequence current threshold is:
I
2
> 0.1 In + ( (I
2
> / 100) . (I maximum) ).
A phase current threshold is exceeded - this allows tripping for three-phase faults occurring
during a power swing. The threshold is set as: Imax line> (in A).