ABB RET670 - Page 309

Load impedance limitation, without load encroachment function
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:
2
minload
V
Z
S
=
EQUATION1780-ANSI V1 EN (Equation 230)
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:
min
max
3
load
V
Z
I
=
×
EQUATION1781-ANSI V1 EN (Equation 231)
Minimum voltage V
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 ground faults,
consider both: phase-to-phase and phase-to-ground fault operating
characteristics.
To avoid load encroachment for the phase-to-ground measuring elements, the set
resistive reach of any distance protection zone must be less than 80% of the minimum
load impedance.
RFPE 0.8 Z
load
×£
EQUATION792 V1 EN
(Equation 232)
1MRK504116-UUS C Section 3
IED application
303
Application manual