ABB RELION 670 SERIES Applications Manual page 424

Section 8
Impedance protection
418
the source impedance and calculations must be made on a case by case basis, as shown in
figure 97. 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 98).
Negative IED impedance, negative fault current (current inversion)
If equation 74 is valid in Figure
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 ground 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 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.
83 and a fault occurs behind the capacitor, the resultant
+ × +
2 X X
L 0 _ S 1_ S
1MRK 506 369-UUS -
(Equation 378)
(Equation 379)
Line distance protection REL670 2.2 ANSI
Application manual