case basis, as shown in figure 95. Distance IEDs with separate impedance and
directional measurement offer additional setting and operational flexibility when it
comes to measurement of negative apparent impedance (as shown in figure 96).
Negative IED impedance, negative fault current (current inversion)
SEMOD168320-408 v2
If equation
71
EQUATION2036 V2 EN-US (Equation 71)
in figure 81 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 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 72)
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
SEMOD168320-417 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
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.
1MRK 506 369-UEN B Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 187
Application manual
To Next Page
Loading...
