Siemens 7SC80 Manual page 249

After the voltage element picks up due to detection of a displacement voltage, the grounded phase is identi-
fied, if possible. For this purpose, the individual phase-to-ground voltages are measured or calculated, irre-
spective of the connection type of the voltage transformers. If the voltage magnitude for any given phase falls
below the set threshold VPH MIN, that phase is detected as the grounded phase as long as the remaining
phase-to-ground voltages exceed the set threshold VPH MAX.
[7sj6x_erdschlussbehaftete_phase-150502-kn, 1, en_US]
Figure 2-81
Current Elements
The current elements for ground faults operate with the magnitudes of the ground current. It is sensible to
employ them only where the magnitude of the ground current can be used to specify the ground fault. This
may be the case on grounded systems (solid or low-resistance) or on electrical machines which are directly
connected to the busbar of an isolated power system, when in case of a network ground fault the machine
supplies only a negligible ground fault current across the measurement location, which must be situated
between the machine terminals and the network, whereas in case of a machine ground fault the higher
ground fault current produced by the total network is available. Ground current protection is mostly used as
backup protection for high resistance ground faults in solid or low resistance grounded systems when the
main fault protection does not pickup.
For ground current detection, a two element current/time characteristic can be set. Analogous to the overcur-
rent protection, the high-set current element is designated as 50Ns-2 PICKUP and 50Ns-2 DELAY and is
provided with a definite time characteristic. The overcurrent element 50Ns-1 PICKUP works with a definite
time delay (50Ns-1 PICKUP and 50Ns-1 DELAY). The 51Ns PICKUP element can be operated with either
a user-defined characteristic (51Ns PICKUP and 51NsTIME DIAL) or an inverse time characteristic
according to IEC or ANSI standards. The characteristics of these current elements can be set. Each of these
elements can operate directionally or non-directionally.
The pickup of the definite time overcurrent protection can be stabilized by the configured dropout delay time
(address 3121 50Ns T DROP-OUT).
The dropout delay only works if the current stage is operated independently of the voltage stage. Parameter
3130 PU CRITERIA is set to Vgnd OR INs.
Determination of Direction
When determining the sensitive ground fault direction, it is not the current value that is crucial, but the part of
the current which is perpendicular to a settable directional characteristic (axis of symmetry). As a prerequisite
SIPROTEC Compact, 7SC80, Manual
E50417-G1140-C486-A8, Edition 07.2017
Determination of Grounded Phase
2.13 Ground Fault Protection 64, 67N(s), 50N(s), 51N(s)
Functions
249