ABB REL670 2.2 IEC, Applications Manual

Page: 408 / 908

Z
loadmin
U
2
S
------- =
EQUATION571 V1 EN-US
(Equation 393)
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 394)
Minimum voltage U
min
and maximum current I
max
are related to the same
operating conditions. Minimum load impedance occurs normally under emergency
conditions.
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.
0.8
load
RFPEZx Z
 
EQUATION792 V2 EN-US
(Equation 395)
This equation is applicable only when the loop characteristic angle for the single
phase-to-earth faults is more than three times as large as the maximum expected
load-impedance angle. For the case when the loop characteristic angle is less than
three times the load-impedance angle, more accurate calculations are necessary
according to equation
396 .
min
2 1 0
0.8 cos sin
2 1 0
load
R Zx R Zx
RFPEZx Z
X Zx X Zx
 
 
    
 
 
 
 
EQUATION578 V5 EN-US
(Equation 396)
Where:
ϑ
is a maximum load-impedance angle, related to the maximum load power.
To avoid load encroachment for the phase-to-phase measuring elements, the set
resistive reach of any distance protection zone must be less than 160% of the
minimum load impedance.
Section 8
1MRK 506 369-UEN B
Impedance protection
402 Line distance protection REL670 2.2 IEC
Application manual