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 75)
All designations relates to figure 83. 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
99) 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 99: 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
100. It presents a zero sequence equivalent
circuit for a fault at B bus of a double circuit line with one circuit disconnected and
grounded 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
Section 8 1MRK 506 369-UUS -
Impedance protection
200 Line distance protection REL670 2.2 ANSI
Application manual
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