ABB REG670 Applications Manual page 214

Section 8
Impedance protection
208
2
U
Z = ------ -
loadmin
S
EQUATION571 V1 EN
Where:
U the minimum phase-to-phase voltage in kV
S the maximum apparent power in MVA.
The load impedance [Ω/phase] is a function of the minimum operation voltage and the
maximum load current:
U
min
--------------------- -
Z =
load
×
3 I
max
EQUATION574 V1 EN
Minimum voltage U
min
conditions. Minimum load impedance occurs normally under emergency conditions.
As a safety margin, it is required to avoid load encroachment under
three-phase conditions. To guarantee correct, healthy phase IED
operation under combined heavy three-phase load and earth faults,
both phase-to-phase and phase-to-earth fault operating characteristics
should be considered.
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.
 
RFPEZx 0.8 Z
load
EQUATION792 V2 EN
Equation 74 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 75.

  
RFPEZx 0.8 Z cos


load min
EQUATION578 V5 EN
Where:
∂ is a maximum load-impedance angle, related to the maximum load power.
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
and maximum current I
max
 
2 R Zx R Zx 1 0

 
sin
 
2 X Zx 1 X Zx 0
1MRK 502 071-UEN -
are related to the same operating




Application manual
(Equation 72)
(Equation 73)
(Equation 74)
(Equation 75)