Current Elements 50Ns, 51Ns - Siemens siprotec SJ62 User Manual

2 Functions
Determination of
the Grounded
Phase
Figure 2-67 Determination of Grounded Phase

2.12.2 Current Elements 50Ns, 51Ns

201
After the voltage element picks up due to detection of a displacement voltage, the
grounded phase is identified, if possible. To do this, the individual phase-to-ground
voltages are measured. Of course, this is only possible if three phase-to-ground volt-
ages are obtained from voltage transformers connected in a grounded-wye configura-
tion. If the voltage magnitude for any given phase is below the setting value V
phase is detected as the grounded phase as long as the remaining phase-ground volt-
ages are simultaneously above the setting value V
The following figure shows the logic for determining the grounded phase.
The current elements for ground faults operate with the magnitudes of the ground cur-
rent. They only make sense 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 re-
sistance ground faults in solid or low resistance grounded systems when main fault
protection does not pickup.
For ground fault detection, a two-step current/time characteristic can be set. Analog to
the time overcurrent 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 may be operated with either a definite time delay (50Ns-1
PICKUP and 50Ns-1 DELAY) or with a user-defined characteristic (51Ns PICKUP
.
Ph max
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
that
Ph min