ABB REG670 Applications Manual page 228

Section 8
Impedance protection
222
-
Z
Z
0 0m
A
-
Z
Z
0
0m
B
IEC09000253 V1 EN
Figure 100: Equivalent zero sequence impedance circuit of the double-circuit,
parallel, operating line with a single phase-to-earth fault at the remote
busbar
When mutual coupling is introduced, the voltage at the relay point A will be changed
according to equation 42.
æ
Z 0
= × + ×
U Z 1 I 3 I
ç
ph 0
ph L
3
è
IECEQUATION1276 V3 EN
By dividing equation 42
impedance present to the relay at A side as:
æ
×
3 0 I KNm
= +
Z Z 1 1
ç
L
+
I ph 3 0 I
è
EQUATION1277 V3 EN
Where:
KNm = Z0m/(3 · Z1L)
The second part in the parentheses is the error introduced to the measurement of the
line impedance.
If the current on the parallel line has negative sign compared to the current on the
protected line, that is, the current on the parallel line has an opposite direction
compared to the current on the protected line, the distance function will overreach. If
the currents have the same direction, the distance protection will underreach.
Maximum overreach will occur if the fault current infeed from remote line end is
weak. If considering a single phase-to-earth fault at 'p' unit of the line length from A
to B on the parallel line for the case when the fault current infeed from remote line end
is zero, the voltage U in the faulty phase at A side as in equation 44.
A
(
U = ⋅ p ZI I + K
A
L ph N
IECEQUATION1278 V2 EN
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Z
0m
IEC09000253_1_en.vsd
ö
-
Z 1 Z 0
+
L L 3 I m
÷
0 p
× ×
Z 1 3 Z 1
ø
L L
by equation 41 and after some simplification we can write the
ö
÷
×
KN
ø
)
⋅ 3 I + K ⋅ 3 I
0 Nm 0 p
1MRK 502 071-UEN -
C
(Equation 88)
(Equation 89)
(Equation 90)
Application manual