Alstom MiCOM P546 - 1.11 Mho element polarization and expansion (Distance option only)

P54x/EN OP/La4 Operation
(OP) 5-34 MiCOM P543, P544, P545 & P546
OP
Under conditions on load with high levels of sub-synchronous frequencies, it is necessary to
increase the I phase selector threshold from its default (4% In) to prevent sporadic
operation. This is automatically performed by the relay, which will self-adjust the threshold to
prevent operation upon the noise signals, whilst still maintaining a high sensitivity to faults.
In order to facilitate testing of the Distance elements using test sets which do not provide a
dynamic model to generate true fault delta conditions, a Static Test Mode setting is provided.
This setting is found in the COMMISSIONING TESTS menu column. When set, this
disables phase selector control and forces the relay to use a conventional (non-delta)
directional line.
1.11 Mho element polarization and expansion (Distance option only)
To en
sure coverage for close-up faults, distance protection always includes a proportion of
voltage memory. Therefore when each zone characteristic is determined, the phase
comparator used in the zone decision will use a mix of vectors “V” (the directly measured
phase/line voltage), “IZ” (a voltage constructed from the fault current and zone impedance
reach setting) and “Vpol” (a polarizing voltage). The MiCOM P54x allows the user to specify
the composition of Vpol, deciding on how to mix the proportion of two voltage selections:
The amount of directly measured (“self”) polarizing in the mix;
The amount of clean memory stored from before the fault inception.
One of the additional benefits in adding memory into the polarizing mix is that mho
characteristics will offer dynamic expansion in the event of a forward fault. This
phenomenon is shown in Figure 18 for the default setting Vpol=1, where a Zone 1
characteri
stic with reach Z will grow to cover 50% of Zs to cover more fault arc resistance.
Z
Vpol/I
V/I-Z
Zs/2
P1171ENb
P1134ENa
jx
R
V/I
Figure 18 Expansion of zone 1 for the default polarizing setting Vpol=1
(Distance option only)