[dwhimpef-310113-01.tif, 2, en_US]
Figure 6-104 Restricted Ground-Fault Protection According to the High-Impedance Principle
Function of the High-Impedance Principle
The high-impedance principle is explained using the example of a grounded transformer winding.
In normal state, no residual currents flow, that is, in the transformer neutral point I
Y
= 0 and in the phases
3I
0
= I
A
+ I
B
+ I
C
= 0.
With an external ground fault (on the left in Figure 6-105), the short-circuit current of which is fed via a
grounded neutral point, the same current flows in the transformer neutral point and in the phases. The
respective secondary currents (with the same transfer ratio of all current transformers) draw each other off.
They are connected in series. At the resistor R, only a little voltage arises, which results from the internal resis-
tances of the transformers and those of the transformer connection lines. Even if a current transformer is
briefly saturated, it becomes a low-impedance during the time of the saturation and forms a low-impedance
shunt to the high-impedance resistor R. The high resistance of the resistor thus has a stabilizing effect (so-
called resistor stabilization).
[dwprhimp-310113-01.tif, 2, en_US]
Figure 6-105 Principle of the Restricted Ground-Fault Protection According to the High-Impedance Principle
With a ground-fault in the protection range (on the right in Figure 6-105), a neutral-point current I
Y
flows in
any case. The magnitude of the residual current in the phase currents depends on the grounding conditions in
the rest of the network. A secondary current corresponding to the entire short-circuit current attempts to flow
via the resistor R. But since this resistor is high-impedance, a high voltage arises there which causes the satura-
tion of the current transformers. The effective voltage at the resistor therefore corresponds approximately to
the knee-point voltage of the current transformers.
The resistor R is thus dimensioned in such a way that even the smallest ground-fault current to be detected
leads to a secondary voltage that corresponds to half of the knee-point voltage of the current transformers
(see chapter 2.5.4).
Further information can be found at Sensitivity view for high-impedance ground-fault differential protection in
chapter 6.12.7.2 Application and Setting Notes .
Protection and Automation Functions
6.12 Overcurrent Protection, 1-Phase
536 SIPROTEC 5, Overcurrent Protection, Manual
C53000-G5040-C017-8, Edition 07.2017
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