Application - ABB RELION 620 Series Technical Manual

1MAC504801-IB E
4.7.1.5
620 series ANSI
Technical Manual
X
Z reach zone x
1
Z angle zone x
1
R
Z
1 source
GUID-078EC6D0-5760-4D6D-AE60-9FCB97535BE1 V1 EN
Figure 236: Mho (circular) operating characteristics with cross polarization
In the case of positive-sequence polarization, the polarization voltage is the positive-
sequence voltage. The voltage phase angle is rotated to match the original fault loop
voltage. Positive-sequence polarization expands the tripping characteristic according to
the prevailing system conditions, that is, in the case of short circuit faults the circle
expands as a function of source impedance magnitude Z1source/2 as shown in the curve
for mho (circular) operating characteristics with a positive-sequence polarization. This
provides a variable mho feature insuring protection near the origin of the RX-diagram.
X
Z reach zone x
1
Z angle zone x
1
R
Z /2
1 source
GUID-AE3DE744-7803-45B4-84C4-A97E764571DC V1 EN
Figure 237: Mho (circular) operating characteristics with positive-sequence
polarization
In case of cross- and positive-sequence polarization; the expansion of the mho circle is a
positive feature as the fault resistance coverage is increased. The reach of the zone is not
affected by the expansion. The greatest expansion is gained with the cross-polarization.

Application

21P provides a fast and reliable protection for overhead lines and power cables. 21P is
applied in distribution and sub-transmission networks where three-phase tripping is
allowed for phase-to-phase or three-phase faults.
Typically, these networks are operated in ring or meshed type of configurations. It is also
a characteristic for these networks that the switching state is changed frequently due to
daily operation and load flow considerations. The networks also include varying
capacities of a distributed generation. This makes it impossible to apply simple
Section 4
Protection functions
497