Setting Guidelines - ABB RER670 Applications Manual

Section 8
Current protection
8.5.3
176
Directional and power functionality uses IN and UN. If a connection is made to
GRPxN this signal is used, else if connection is made only to inputs GRPxL1 and
GRPxL2 the internally calculated sum of these inputs (2I0 and 2U0) will be used.

Setting guidelines

The sensitive earth fault protection is intended to be used in high impedance
earthed systems, or in systems with resistive earthing where the neutral point
resistor gives an earth fault current larger than what normal high impedance gives
but smaller than the phase-to-phase short circuit current.
In a high impedance system the fault current is assumed to be limited by the system
zero sequence shunt impedance to earth and the fault resistance only. All the series
impedances in the system are assumed to be zero.
In the setting of earth fault protection, in a high impedance earthed system, the
neutral point voltage (zero sequence voltage) and the earth fault current will be
calculated at the desired sensitivity (fault resistance). The complex neutral point
voltage (zero sequence) can be calculated as:
U

phase
U

0
2 R

f
1
Z
0
IECEQUATION16029 V1 EN-US
Where
U is the phase voltage in the fault point before the fault,
phase
R is the resistance to earth in the fault point and
f
Z is the system zero sequence impedance to earth
0
The fault current, in the fault point, can be calculated as:

2 U
 
phase
I 2 I
 
j 0
Z 2 R
0 f
IECEQUATION16030 V1 EN-US
The impedance Z is dependent on the system earthing. In an isolated system
0
(without neutral point apparatus) the impedance is equal to the capacitive coupling
between the phase conductors and earth:

2 U
   
phase
Z jX j
0 c
I
j
IECEQUATION16031 V1 EN-US
1MRK 506 375-UEN A
SEMOD171961-4 v10
(Equation 42)
(Equation 43)
(Equation 44)
Railway application RER670 2.2 IEC
Application manual