ABB REL670 2.2 IEC, Applications Manual

Page: 286 / 908

Setting of the resistive reach for the underreaching zone1 should follow the
condition:
4.5 1 RFPE X
£ ×
EQUATION569 V2 EN-US
(Equation 215)
8.5.3.7 Load impedance limitation, without load encroachment function
SEMOD154704-91 v2
The following instructions is valid when the load encroachment function is not
activated (
OperationLdCmp is set to Off). If the load encroachment 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
IED boundary and the minimum load impedance. The minimum load impedance
(Ω/phase) is calculated as:
Z
loadmin
U
2
S
------- =
EQUATION571 V1 EN-US
(Equation 216)
Where:
U
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:
Z
load
U
min
3 I
max
×
----------------------
=
EQUATION574 V1 EN-US
(Equation 217)
Minimum voltage U
min
and maximum current Imax are related to the same
operating conditions. Minimum load impedance occurs normally under emergency
conditions.
Because a safety margin is required to avoid load encroachment
under three-phase conditions and to guarantee correct healthy phase
IED operation under combined heavy three-phase load and earth
faults, consider both: phase-to-phase and phase-to-earth fault
operating characteristics.
To avoid load encroachment for the phase-to-earth measuring elements, the set
resistive reach of any distance protection zone must be less than 80% of the
minimum load impedance.
Section 8
1MRK 506 369-UEN B
Impedance protection
280 Line distance protection REL670 2.2 IEC
Application manual