However, depending upon the setting of the MOV, the fault current will have a resistive
component.
EQUATION2036 V2 EN-US (Equation 119)
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 ground fault can be written as follows:
1_ 1 0 _ 1 0 _ 1_
3 2 2× > × + + × +
C L L S S
X X X X X
EQUATION1920 V1 EN-US (Equation 120)
All designations relates to figure 112. 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
GUID-7A384A24-143F-4695-8214-33E181AD0AB7 v2
T
wo parallel power lines running in electrically close vicinity to each other and ending
at the same busbar at both ends (as shown in figure
126) 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.
en06000627.vsd
-jX
C
-jX
C
Z
AC
Z
CB
ZAC ZCB
A B
Z
m0AC
Z
m0CB
IEC06000627 V1 EN-US
Figure 126: Double circuit, parallel operating line
Zero sequence mutual impedance Z
m0
cannot significantly influence the operation of
distance 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 grounded 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
127. It presents a zero sequence equivalent
circuit for a fault at B bus of a double circuit line with one circuit disconnected and
1MRK 502 071-UUS A Section 8
Impedance protection
Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062 263
Application manual
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