ABB RELION 670 Series Applications Manual page 184

Section 7
Impedance protection
in figure
and the fault current will have an opposite direction compared with fault current in a power
line without a capacitor (current inversion). The negative direction of the fault current will
persist until the spark gap has flashed. Sometimes there will be no flashover at all, because the
fault current is less than the setting value of the spark gap. The negative fault current will
cause a high voltage on the network. The situation will be the same even if a MOV is used.
However, depending upon the setting of the MOV, the fault current will have a resistive
component.
The problems described here are accentuated with a three phase or phase-to-phase fault, but
the negative fault current can also exist for a single-phase fault. The condition for a negative
current in case of an earth fault can be written as follows:
×
3 X
C
EQUATION1920 V1 EN-US
All designations relates to figure 81. A good protection system must be able to cope with both
positive and negative direction of the fault current, if such conditions can occur. A distance
protection cannot operate for negative fault current. The directional element gives the wrong
direction. Therefore, if a problem with negative fault current exists, distance protection is not
a suitable solution. In practice, negative fault current seldom occurs. In normal network
configurations the gaps will flash in this case.
Double circuit, parallel operating series compensated lines
Two parallel power lines running in electrically close vicinity to each other and ending at the
same busbar at both ends (as shown in figure 97) causes some challenges for distance
protection because of the mutual impedance in the zero sequence system. The current reversal
phenomenon also raises problems from the protection point of view, particularly when the
power lines are short and when permissive overreach schemes are used.
A
Z
Z
IEC06000627 V1 EN-US
Figure 97: Double circuit, parallel operating line
Zero sequence mutual impedance Z
protection as long as both circuits are operating in parallel and all precautions related to
settings of distance protection on series compensated line have been considered. Influence of
disconnected parallel circuit, which is earthed at both ends, on operation of distance
protection on operating circuit is known.
Series compensation additionally exaggerates the effect of zero sequence mutual impedance
between two circuits, see figure 98. It presents a zero sequence equivalent circuit for a fault at
B bus of a double circuit line with one circuit disconnected and earthed at both IEDs. The
effect of zero sequence mutual impedance on possible overreaching of distance IEDs at A bus
is increased compared to non compensated operation, because series capacitor does not
compensate for this reactance. The reach of underreaching distance protection zone 1 for
phase-to-earth measuring loops must further be decreased for such operating conditions.
178
81 and a fault occurs behind the capacitor, the resultant reactance becomes negative
> × + + ×
2 X X 2
1_ 1 L 0 _ 1 L
-jX
C
Z
AC CB
Z Z
m0AC m0CB
-jX
C
Z
AC CB
en06000627.vsd
+
X X
0 _ S 1_ S
B
cannot significantly influence the operation of distance
m0
1MRK 504 152-UEN B
(Equation 87)
SEMOD168320-417 v2
Application manual