High-Impedance Restricted Ground-Fault Protection with a SIPROTEC 5 Device
Use the I4 measuring input of the SIPROTEC 5 device for the high-impedance restricted ground-fault protec-
tion. This input for this application is to be executed as a sensitive measuring input. Since this is a current
input, the current is detected by this resistor instead of the voltage at the resistor R.
Figure 6-106 shows the connection diagram. The protection device is connected in series with the resistor R
and thus measures its current.
The varistor V limits the voltage in case of an internal fault. The varistor cuts the high instantaneous voltage
peaks in the case of transformer saturation. Simultaneously, a smoothing of the voltage arises without any
relevant reduction of the average value.
[dwanedif-310113-01.tif, 2, en_US]
Figure 6-106 Connection Diagram of the Restricted Ground-Fault Protection According to the High-Impe-
dance Principle
As a protection against overvoltages, it is important that you connect the device directly at the grounded side
of the current transformer. The high voltage at the resistor is thus kept away from the device.
In a similar manner, the high-impedance restricted ground-fault protection for generators, motors, and shunt
reactors is used. With auto transformers, you must connect the upper-voltage side and low-voltage side
current transformers and neutral-point transformer in parallel.
The method can be realized for each protected object. As busbar protection, the device, for example, is
connected via the resistor to the parallel connection of the transformers of all feeders.
Application and Setting Notes
A prerequisite for the application of the high-impedance restricted ground-fault protection is that neutral-
point current detection is possible on the station side (see example in (Figure 6-106). Furthermore, a sensitive
input transformer must be available at device input I4. Set the pickup value for current at input I4 with the
function Overcurrent protection, 1-phase.
Observe the interaction between current-transformer characteristic curve, external resistor R, and the voltage
at R for the overall function of the high-impedance restricted ground-fault protection. Notes on this follow.
Current Transformer Data for High-Impedance Restricted Ground-Fault Protection
All affected current transformers must have the same ratio and approximately the same knee-point voltage.
This is normally the case when the current transformers are of the same type and have the same rated data.
You can calculate the knee-point voltage from the rated data as follows:
[foukniep-310113-01.tif, 1, en_US]
V
KP
Knee-point voltage
Ri Internal resistance of the current transformer
P
rated
Rated power of the current transformer
6.12.7.2
Protection and Automation Functions
6.12 Overcurrent Protection, 1-Phase
SIPROTEC 5, Overcurrent Protection, Manual 537
C53000-G5040-C017-8, Edition 07.2017
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