Siemens siprotec SJ62, User Manual

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2 Functions
125
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
2.5.2 High-impedance Ground Fault Unit Protection
Application Exam-
ples In the high-impedance procedure, all CT's operate at the limits of the protected zone
parallel on a common, relatively high-resistive resistor R whose voltage is measured.
The CTs must be of the same design and feature at least a separate core for high-im-
pedance protection. In particular, they must have the same transformer ratios and ap-
proximately identical knee-point voltage.
With 7SJ62/63/64, the high-impedance principle is particularly well suited for detecting
ground faults in grounded networks at transformers, generators, motors and shunt re-
actors.
Figure 2-37 shows an application example for a grounded transformer winding or a
grounded motor/generator. The right-hand example depicts an ungrounded transform-
er winding or an ungrounded motor/generator where the grounding of the system is
assumed somewhere else.
Figure 2-37 Ground fault protection according to the high-impedance principle
Function of the
High-Impedance
Principle The high-impedance principle is explained on the basis of a grounded transformer
winding.
No zero sequence current will flow during normal operation, i.e. the starpoint current
is
I
SP
= 0 and the phase currents are 3 I
0
= I
A
+ I
B
+ I
C
= 0.
With an external ground fault (Figure 2-38, left side), whose fault current is supplied
via the grounded starpoint, the same current flows through the transformer starpoint
and the phases. The corresponding secondary currents (all current transformers have
the same transformation ratio) compensate each other; they are connected in series.
Across resistor R only a small voltage is generated. It originates from the inner resis-
tance of the transformers and the connecting cables of the transformers. Even if any
current transformer experiences a partial saturation, it will become low-resistive for the
period of saturation and creates a low-resistive shunt to the high-resistive resistor R.
Thus, the high resistance of the resistor also has a restraining effect (the so-called re-
sistance restraint).