in figure 86 and a fault occurs behind the capacitor, the resultant reactance becomes
negative 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 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 (Equation 149)
All designations relates to figure 86. 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
102) 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
Figure 102: 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.
1MRK504116-UUS C Section 3
IED application
233
Application manual
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