ABB REG670 Applications Manual page 251

1MRK 502 071-UEN -
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
magnitude of the source impedance and calculations must be made on a case by case
basis, as shown in figure 124. 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 125).
Negative IED impedance, negative fault current (current inversion)
If equation 124 is valid in Figure
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.
X X X
> +
C S 11
EQUATION2036 V2 EN
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:
× > × +
3 X 2 X X
C 1_ 1 L 0 _ 1
EQUATION1920 V1 EN
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
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 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.
-jX
A
C
Z
AC
Z
m0AC
-jX
C
Z
AC
en06000627.vsd
IEC06000627 V1 EN
Figure 126: Double circuit, parallel operating line
112 and a fault occurs behind the capacitor, the
+ × +
2 X X
L 0 _ S 1_ S
B
Z
CB
Z
m0CB
Z
CB
Section 8
Impedance protection
(Equation 124)
(Equation 125)
245