Siemens siprotec 7SA6 User Manual page 226

Functions
Double Earth-
Faults in Earthed
Systems
6-44
Apart from the zone selectivity , the phase selectivity is also important to achieve
correct identification of the faulted phases, required to alarm the faulted phase and
especially to enable single-pole automatic reclosure. Depending on the infeed
conditions, close-in short circuits may cause unfaulted loops to "see" the fault further
away than the faulted loop, but still within the tripping zone. This would cause three-
pole tripping and therefore void the possibility of single-pole automatic reclosure. As a
result power transfer via the line would be lost.
In the 7SA6 this is avoided by the implementation of a loop verification function which
operates in two steps:
Initially, the calculated loop impedances and its components (phase and/or earth) are
used to simulate a replica of the line impedance. If this simulation returns a plausible
line image, the corresponding loop pick-up is designated as a definitely valid loop.
If the impedances of more than one loop are now located within the range of the zone,
the smallest is still declared to be a valid loop. Furthermore, all loops that have an
impedance which does not exceed the smallest loop impedance by more than 50 %
are declared as being valid. Loops with larger impedance are eliminated. Those loops
which were declared as being valid in the initial stage, cannot be eliminated by this
stage, even if they have larger impedances.
In this manner unfaulted "apparent impedances" are eliminated on the one hand, while
on the other hand, unsymmetrical multi-phase faults and multiple short circuits are
recognized correctly.
The loops that were designated as being valid are converted to phase information so
that the fault detection correctly alarms the faulted phases.
In systems with earthed starpoint (effective or low-resistant), each contact of a phase
with earth results in a short-circuit condition which must be isolated immediately by the
closest protection systems. Fault detection occurs in the faulted loop associated with
the faulted phase.
With double earth faults, fault detection is generally in two phase-earth loops. If both
earth loops are in the same direction, a phase-phase loop may also pick-up. It is
possible to restrict the fault detection to particular loops in this case. It is often
desirable to block the phase-earth loop of the leading phase, as this loop tends to
overreach when there is infeed from both ends to a fault with a common earth fault
resistance (address  3K( IDXOWV = %ORFN OHDGLQJ ‘). Alternatively, it is
also possible to block the lagging phase-earth loop (address 3K( IDXOWV = %ORFN
ODJJLQJ ‘). All the affected loops can also be evaluated (address 3K( IDXOWV
= $OO ORRSV), or only the phase-phase loop (address  3K( IDXOWV = ‘‘
ORRSV RQO\) or only the phase-earth loops (address 3K( IDXOWV = ‘( ORRSV
RQO\).
A prerequisite for these restrictions is that the relevant loops indicate fault locations
which are close together and within the reach of the first zone Z1. The loops are
considered to be close together when they have the same direction and do not differ
by more than a factor 1,5 (largest to smallest impedance). This is to avoid in case of
multiple faults with separate fault location the closer fault is eliminated from the
evaluation by configured restrictions. Furthermore, the measurement phase-phase
can only be carried out if two earth faults are close to each other in the determined
direction.
In Table 6-7 the measured values used for the distance measurement in earthed
systems during double earth faults are shown.
7SA6 Manual
C53000-G1176-C133-1