Siemens SIPROTEC 4 User Manual page 201

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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 fault protection, two-step current/time characteristic can be set. Analog to the overcurrent protec-
tion, 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 definite time delay
(50Ns-1 PICKUP and 50Ns-1 DELAY). The 51Ns PICKUP element may be operated with either a user-
defined characteristic (51Ns PICKUP and 51NsTIME DIAL) or an inverse-time characteristic according to
IEC or ANSI standards. Additionally, a current element with logarithmic inverse characteristic or logarithmic
inverse characteristic with knee point is implemented. The characteristics of these current elements can be
configured. Each of these elements may be directional or non-directional.
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
for determining the direction, the displacement voltage V
part influencing the direction (active or reactive component).
The following figure illustrates an example using a complex vector diagram in which the displacement voltage
V is the reference magnitude of the real axis. The active part 3Ι0
0
to the displacement voltage V
SIPROTEC 4, 7SJ62/64, Manual
C53000-G1140-C207-8, Edition 08.2016
Determination of Grounded Phase
and compared with setting value RELEASE DIRECT.. The example is there-
0
2.13 Ground Fault Protection 64, 67N(s), 50N(s), 51N(s)
must be exceeded as well as a configurable current
0
of current 3Ι0 is calculated with reference
real
Functions
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