Siemens siprotec 7SA6 User Manual page 231

Double Earth Faults
in Earthed Systems
Double Earth Faults
in Non-earthed
Systems
Line Energization
onto a Dead Fault
7SA6 Manual
C53000-G1176-C133-1
parallel line is connected to the I
the configuration settings.
In this case, the setting 3DUDO/LQH &RPS = <(6 must be set in address ;
otherwise the presetting 12 remains.
The coupling factors were already set as part of the general protection data (Sub-
section 6.1.3), as was the reach of the parallel line compensation.
The loop selection for double earth faults is set in address  3K( IDXOWV
(Phase–Phase–Earth–fault detection). This setting can only be changed with DIGSI
4 under "Additional Settings". In general the %ORFN OHDGLQJ ‘ (blocking of the
leading phase, presetting) is favourable, because the leading phase-earth loop tends
to overreach, especially in conjunction with large earth fault resistance. In certain
cases (fault resistance phase-phase larger than phase-earth) the setting %ORFN
ODJJLQJ
(blocking of the lagging phase) may be more favourable. The selection
Ø
of all affected loops with the setting $OO ORRSV allows a maximum degree of
redundancy. Alternatively, ‘‘ ORRSV RQO\ may be evaluated. This ensures the
most accuracy for two phase to earth faults. Ultimately it is possible to declare the
Ø-E loops only as valid.
In isolated or resonant-earthed systems it must be guaranteed that the preference for
double earth faults in whole galvanically-connected systems is consistent. The double
earth fault preference is set in address  3+$6( 35()SKH.
The 7SA6 enables the user to detect all foot points of a multiple earth fault. 3+$6(
35()SKH = $OO ORRSV means that each earth fault point on a protected line is
switched off independent of the preference. It can also be combined with a different
preference. For a transformer feeder, for example, any foot point can be switched off
following occurrence of a double earth fault, whereas / / DF\FOLF is
consistently valid for the remainder of the system.
If the earth fault detection threatens to pick up due to fault inception transients
following the occurrence of a single earth fault, the detection can be delayed with
setting address  7, 3+$6 Usually the presetting (0.04 s) is sufficient. For
large resonant-earthed systems the time delay should be increased. Set parameter
7, 3+$6 to ∞ if the earth current threshold can also be exceeded during steady-
state conditions. Then, even with high earth current, no single-phase pick up is
possible anymore. Double earth faults are, however, detected correctly and evaluated
according to the preference mode (Section 6.2.3.1, paragraph "Double Earth Faults in
Non-earthed Systems").
If a double earth fault occurs right after a single earth fault, it is detected and evaluated
according to the preference scheme. The already existing earth fault is detected by
the zero-sequence voltage (address  8! &203,62/). Please note that
three times the zero-sequence voltage 3U
value will be √3-times the phase-to-phase voltage. After detection of 3U
delay 7, 3+$6 is not active anymore: an earth fault ocurring then in a different
phase can only be a double earth fault.
To determine the reaction of the distance protection during closure of the circuit break-
er onto a dead fault, the parameter in address  627) ]RQH is used. The setting
,QDFWLYH specifies that there is no special reaction, i.e. all distance stages operate
according to their set zone parameters. The setting =RQH =% causes all faults inside
the overreaching zone Z1B to be cleared without delay following closure of the circuit
breaker. The setting 3LFNXS implies that the non-delayed tripping following line ener-
measuring input of the 7SA6 and this is entered in
4
is relevant. With a full displacement its
0
Functions
®
, the time
0
6-49