Siemens siprotec SJ62 User Manual page 206

Application Instruc-
tions
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
In systems with isolated starpoint, ground fault current flows as capacitive current from
healthy lines to the location of the ground fault via the measuring point. The capacitive
reactive power is thus relevant for the direction.
In networks with arc suppression coils, the Petersen coil superimposes a correspond-
ing inductive current on the capacitive ground fault current when a ground fault occurs,
so that the capacitive current at the point of fault is compensated. Depending on the
measuring point in the system the resultant measured current may be inductive or ca-
pacitive. Therefore, the reactive current is not suited for direction determination of the
ground current. In this case, only the ohmic (active) residual current which results from
the losses of the Petersen coil can be used for directional determination. The residual
current of the ground fault is only about some per cent of the capacitive ground fault
current.
Please note that depending on the mounting location of the device, the real compo-
nent of the current may only be a small fraction of the reactive current component (in
extreme cases down to 1/50 th). The accuracy of the calculation algorithm which is ex-
tremely high is not sufficient if the instrument transformer is not able to transmit the
primary values accuratetly.
The measuring input of the protection relay for high-sensitive ground fault detection is
especially calibrated to these concerns and allows an extremely high sensitivity for the
direction determination of the residual wattmetric current. In order to make use of this
sensitivity, we recommend cable core balance current transformers for ground fault
detection in resonant grounded systems. Furthermore, the angle error of the cable
core balance current transformer can be compensated in the device. Since the angle
error is non-linear, this is achieved by entering two operating points of the angle error
curve of the transformer. The device then calculates the error curve with sufficient ac-
curacy.
2.12 Ground Fault Protection 64, 67N(s), 50N(s), 51N(s)
204