Siemens SIPROTEC 7SA522 Manual page 74

2 Functions
Double Earth Faults
in Effectively
Earthed Systems
Double Earth Faults
in Non-earthed
Systems
Switching onto a
Fault
74
The coupling factors were already set as part of the general protection data (Subsec-
tion 2.1.5.1), 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 is only possible via DIGSI
ditional Settings". In most cases, %ORFN OHDGLQJ ‘ (blocking of the leading phase,
default setting) is favourable because the leading phase-earth loop tends to over-
reach, 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 evaluation of all affected
loops with the setting $OO ORRSV allows a maximum degree of redundancy. It is also
possible to evaluate as loop ‘‘ ORRSV RQO\. This ensures the most accuracy for
two phase to earth faults. Ultimately it is possible to declare the phase-to-earth loops
as valid (setting ‘( ORRSV RQO\).
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.
7SA522 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 / / $&<&/,& is consis-
tently valid for the remainder of the system.
If the earth fault detection threatens to pick up due to fault inception transients follow-
ing the occurrence of a single earth fault, the detection can be delayed via parameter
7, 3+$6 (address ). 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 pickup is possible any-
more. Double earth faults are, however, detected correctly and evaluated according to
the preference mode.
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
triple zero-sequence voltage 3U
will be √3 times the phase-to-phase voltage. Afterwards the delay 7, 3+$6 is not
active anymore: an earth fault occurring now in a different phase can only be a double
earth fault.
To determine the reaction of the distance protection during closure of the circuit
breaker 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 (in the direction specified for these zones) to
be cleared without delay following closure of the circuit breaker. If set to =%
XQGLUHFW, zone Z1B is also decisive, but it operates in both directions regardless
of the direction set at address  2S PRGH =%. The setting 3,&.83 implies that
the non-delayed tripping following line energization is activated for all recognized
faults in any zone (i.e. with general fault detection of the distance protection).
is relevant here. With a full displacement its value
0
®
at "Ad-
7SA522 Manual
C53000-G1176-C155-3