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.
X
X
X
C
S
>
+
11
EQUATION2036 V2 EN-US
(Equation 400)
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:
1_ 1
0 _ 1
0 _
1_
3
2
2
×
> ×
+
+ ×
+
C
L
L
S
S
X
X
X
X
X
EQUATION1920 V1 EN-US
(Equation 401)
All designations relates to figure
. 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
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
) 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
Z
AC
Z
CB
A
B
Z
m0AC
Z
m0CB
IEC06000627 V1 EN-US
Figure 231: 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 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
. 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.
1MRK 505 343-UEN B
Section 7
Impedance protection
351
Application manual
Содержание Relion 670 series
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