Mho Characteristics - ABB REL650 Applications Manual

Section 6
Impedance protection
6.1.3.4
138

Mho characteristics

Load impedance limitation, with load encroachment function activated
The parameters for load encroachment shaping of the characteristic are found in the
description of Faulty phase identification with load encroachment for mho (FMPSPDIS).
Load impedance limitation, without load encroachment function
The following instruction is valid when the load encroachment function or blinder
function is not activated (BlinderMode=Disabled). If the load encroachment or blinder
function is to be used for all or some of the measuring zones, the load limitation for those
zones according to this chapter can be omitted. Check the maximum permissible resistive
reach for any zone to ensure that there is a sufficient setting margin between the relay
boundary and the minimum load impedance. The minimum load impedance (Ω/phase) is
calculated as:
2
V
=
Zload min
S
EQUATION1753-ANSI V1 EN
Where:
V is the minimum phase-to-phase voltage in kV
S is the maximum apparent power in MVA.
The load impedance [Ω/phase] is a function of the minimum operation voltage and the
maximum load current:
V
min
Zload =
×
3 I
max
EQUATION1754-ANSI V1 EN
Minimum voltage V and maximum current I
min
conditions. Minimum load impedance occurs normally under emergency conditions.
To avoid load encroachment for the phase-to-ground measuring elements, the set
impedance reach of any distance protection zone must be less than 80% of the minimum
load impedance.
1MRK 506 334-UUS A
are related to the same operating
max
(Equation 57)
(Equation 58)
Application manual